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Volume I
Table of Contents
Volume I – Stormwater
Site Planning
Table of Contents
Purpose of this Volume .................................................................................................................. 7
Content and Organization of this Volume........................................................................................ 7
Chapter 1 Development and Redevelopment Impacts ........................................................8
1.1 Hydrologic Changes.............................................................................................................. 8
1.2 Water Quality Changes ......................................................................................................... 8
1.3 Biological Changes ............................................................................................................... 9
Chapter 2 Areas with Special Development Requirements ..............................................10
2.1 Groundwater Protection Area 2 ........................................................................................... 10
2.2 Impaired Water Bodies ....................................................................................................... 10
2.3 Floodplains ......................................................................................................................... 10
Chapter 3 Minimum Requirements for New Development and Redevelopment..............11
3.1 Overview of the Minimum Requirements ............................................................................. 11
3.2 Exemptions ......................................................................................................................... 12
3.2.1 Road Maintenance ....................................................................................................... 12
3.2.2 Parking Lots and Parking Lot Maintenance ................................................................... 12
3.2.3 Underground Utility Projects ......................................................................................... 13
3.2.4 Minor Clearing and Grading .......................................................................................... 13
3.2.5 Emergencies ................................................................................................................ 13
3.2.6 Key Terms .................................................................................................................... 13
3.3 Applicability of the Minimum Requirements ......................................................................... 14
3.3.1 New Development ........................................................................................................ 17
3.3.2 Redevelopment ............................................................................................................ 17
3.3.3 Assessed Value ............................................................................................................ 18
3.3.4 Roads........................................................................................................................... 18
3.3.5 Cumulative Impact Mitigation Requirement ................................................................... 19
3.4 Description of Minimum Requirements ................................................................................ 21
3.4.1 Minimum Requirement #1: Preparation of a Stormwater Site Plan ................................. 21
3.4.1.1 Objective ............................................................................................................. 21
3.4.2 Minimum Requirement #2: Construction Stormwater Pollution Prevention (SWPP) ....... 21
3.4.2.1 Objective ............................................................................................................. 22
3.4.3 Minimum Requirement #3: Source Control of Pollution.................................................. 22
Volume I
Table of Contents
3.4.3.1 Objective ............................................................................................................. 22
3.4.4 Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls....... 23
3.4.4.1 Objective ............................................................................................................. 23
3.4.5 Minimum Requirement #5: On-Site Stormwater Management ....................................... 23
3.4.5.1 Objective ............................................................................................................. 23
3.4.6 Minimum Requirement #6: Runoff Treatment ................................................................ 24
3.4.6.1 Thresholds .......................................................................................................... 24
3.4.6.2 Treatment Facility Selection, Design, and Maintenance ....................................... 24
3.4.6.3 Additional Requirements ..................................................................................... 24
3.4.6.4 Objective ............................................................................................................. 24
3.4.6.5 Supplemental Guidelines..................................................................................... 24
3.4.7 Minimum Requirement #7: Flow Control ....................................................................... 25
3.4.7.1 Applicability ......................................................................................................... 25
3.4.7.2 Thresholds .......................................................................................................... 25
3.4.7.3 Standard Requirement ........................................................................................ 26
3.4.7.4 Infrastructure Protection Requirement ................................................................. 26
3.4.7.5 Objective ............................................................................................................. 27
3.4.7.6 Modeling Requirements ...................................................................................... 27
3.4.8 Minimum Requirement #8: Wetlands Protection ............................................................ 27
3.4.8.1 Applicability ......................................................................................................... 27
3.4.8.2 Thresholds .......................................................................................................... 28
3.4.8.3 Standard Requirement ........................................................................................ 28
3.4.8.4 Additional Requirements ..................................................................................... 28
3.4.8.5 Objective ............................................................................................................. 29
3.4.8.6 Supplemental Guidelines..................................................................................... 29
3.4.9 Minimum Requirement #9: Operation and Maintenance ................................................ 29
3.4.9.1 Objective ............................................................................................................. 29
3.4.9.2 Supplemental Guidelines..................................................................................... 29
3.4.10 Minimum Requirement #10: Off-Site Analysis and Mitigation ......................................... 29
3.4.10.1 Qualitative Analysis: ............................................................................................ 30
3.4.10.2 Quantitative Analysis ........................................................................................... 30
3.4.10.3 Objective ............................................................................................................. 30
3.5 Exceptions .......................................................................................................................... 30
Chapter 4 Preparation of Stormwater Site Plans ...............................................................33
4.1 Stormwater Site Plan Outline .............................................................................................. 33
4.2 Plans Required After Stormwater Site Plan Approval........................................................... 40
4.3 Land Use Submittal Requirements ...................................................................................... 40
Appendix A Regulatory Requirements ...............................................................................41
Appendix B Stormwater Site Plan Submittal Requirements Checklist .............................46
Appendix C Hydraulic Analysis Worksheet ........................................................................53
Appendix D Maintenance Standards for Drainage Facilities ............................................55
Appendix E Wetlands and Stormwater Management Guidelines......................................85
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Purpose Volume I
Content and Organization Introduction 7
Volume I:
Stormwater Site Planning
Purpose of this Volume
This volume provides a discussion of the minimum requirements for stormwater management, and
information and guidance for preparing a Stormwater Site Plan. This includes an overview of the
impacts of development on water flow and quality, an overview of the affected watershed areas,
procedures for preparing the plan, and information helpful for selecting BMPs and facilities for
permanent stormwater management.
Content and Organization of this Volume
Volume I contains four chapters and five appendices.
• Chapter 1 describes the impacts of development and redevelopment on water flow
and quality.
• Chapter 2 describes areas with special development requirements
• Chapter 3 defines the minimum requirements for stormwater management for
development and redevelopment projects.
• Chapter 4 describes the Stormwater Site Plan, and provides step-by-step guidance
for preparing the plan.
• Appendix A provides information about regulatory requirements.
• Appendix B provides a checklist of stormwater site plan submittal requirements.
• Appendix C provides a hydraulic analysis worksheet.
• Appendix D describes maintenance standards for drainage facilities.
• Appendix E describes guidelines for wetlands and stormwater management.
Volume
I
SURFACE WATER MANAGEMENT MANUAL
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Development and Redevelopment Impacts Volume I
Chapter 1 8
Chapter 1 Development and Redevelopment Impacts
1.1 Hydrologic Changes
As settlement occurs and the population grows, trees are logged and land is cleared for the addition
of impervious surfaces such as rooftops, roads, parking lots, and sidewalks. Maintained landscapes
that have much higher runoff characteristics typically replace the natural vegetation. The natural soil
structure is also changed due to grading and compaction during construction. Roads are cut through
slopes and low spots are filled. Drainage patterns are irrevocably altered. All of this can result in
drastic changes in the natural hydrology, including:
• Increased volumetric flow rates of runoff
• Increased volume of runoff
• Decreased time for runoff to reach a natural receiving water
• Reduced groundwater recharge
• Increased frequency and duration of high stream flows and wetlands inundation
during and after wet weather
• Reduced stream flows and wetlands water levels during the dry season
• Greater stream velocities
• Adverse impacts on existing City infrastructure and capacity
1.2 Water Quality Changes
Urbanization also can cause an increase in the types and quantities of pollutants in surface and
groundwaters. Runoff from urban areas has been shown to contain many different types of
pollutants, depending on the nature of the activities in those areas. The runoff from roads and
highways can be contaminated with pollutants from vehicles. Oil and grease, polynuclear aromatic
hydrocarbons (PAHs), lead, zinc, copper, cadmium, as well as sediments (soil particles) and road
salts can be typical pollutants in road runoff. Runoff from industrial areas can contain many types of
heavy metals, sediments, and a broad range of man-made organic pollutants, including phthalates,
PAHs, and other petroleum hydrocarbons. Residential areas can contribute the same road-based
pollutants to runoff, as well as herbicides, pesticides, nutrients (from fertilizers), bacteria, and viruses
(from animal waste). All of these contaminants can seriously impair beneficial uses of receiving
waters.
Regardless of the eventual land use conversion, the sediment load produced by a construction site
can turn the receiving waters turbid and be deposited over the natural sediments of the receiving
water. The addition of sediment loads also impacts existing City systems, causing localized flooding
and increases in the cost and frequency of maintenance.
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Development and Redevelopment Impacts Volume I
Chapter 1 9
Urbanization can cause changes in water temperature. Heated stormwater from impervious surfaces
and exposed treatment and detention ponds may discharge to streams with less riparian vegetation
for shade. Urbanization also reduces groundwater recharge, which reduces sources of cool
groundwater inputs to streams. In winter, stream temperatures may lower due to loss of riparian
cover. There is also concern that the replacement of warmer groundwater inputs with colder surface
runoff during colder periods may have biological impacts.
1.3 Biological Changes
Hydrologic and water quality changes can result in changes to the biological systems that were
supported by the natural hydrologic system. In particular, aquatic life is greatly affected by
urbanization. Habitats are altered when a stream changes its physical configuration and substrate
due to increased flows. Natural riffles, pools, gravel bars and other areas can be altered or destroyed.
These and other alterations produce a habitat structure that is very different from the one in which the
resident aquatic life evolved.
The biological communities in wetlands also can be severely impacted and altered by the
hydrological changes. Relatively small changes in the natural water elevation fluctuations can cause
dramatic shifts in vegetative and animal species composition.
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Areas with Special Development Volume I
Requirements Chapter 2 10
Chapter 2 Areas with Special Development Requirements
This chapter identifies geographic areas within the City of Auburn and the requirements specific
to those areas. Theses requirements shall be in addition to the minimum requirements found in
Chapter 3 of this volume unless the text in this chapter specifically indicates that the area-
specific requirement supersedes or replaces a minimum requirement.
2.1 Groundwater Protection Area 2
In 2005, the City of Auburn adopted the Critical Areas Ordinance, ACC 16.10 which formally
designates Groundwater Protection Areas within the City of Auburn. Groundwater Protection
Zone 2 represents the land area in the central part of the city beneath which the principal aquifer
used by the city for water supply is overlain by highly permeable sand and gravel deposits.
These geologic conditions provide a direct pathway for contaminants that may be released to
the soil to reach the aquifer.
Private infiltration systems used in Groundwater Protection Zone 2 that receive stormwater from
any pollution-generating surfaces including streets, parking areas, or galvanized roofs are
prohibited unless in the opinion of the Public Works Department no other reasonable alternative
exists. In such case, the Public Works Department may approve a private disposal system.
Design shall meet all requirements of the Public Works Department. Additional water quality
measures may also be required.
To request infiltration of pollution-generating surfaces in Groundwater Protection Zone 2, a
formal request for exception shall be submitted per Section 3.5 for review and approval.
2.2 Impaired Water Bodies
Section 305(b) of the Clean Water Act (CWA) requires the Department of Ecology to prepare a
report every two years on the status of the overall condition of the state’s waters. Section 303(d)
of the CWA requires Ecology to prepare a list every two years containing water bodies not
expected to meet state surface water quality standards after implementation of technology-
based controls. The State is then required to complete a Total Maximum Daily Load (TMDL) for
all water on that list. The existing list and other related information is available on Ecology’s
water quality website:
http://www.ecy.wa.gov/programs/wq/links/wq_assessments.html
If a project site discharges to one of these listed waterbodies, additional treatment or flow
control requirements may apply.
2.3 Floodplains
Floodplains are not regulated through the Surface Water Management Manual. However,
surface water facilities proposed within flood plains will be reviewed on a case-by-case basis to
determine if the facilities are acceptable. Additional analysis and requirements may be needed
for surface water facilities located within flood plains.
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 11
Chapter 3 Minimum Requirements for New Development
and Redevelopment
This Chapter identifies the minimum requirements for stormwater management applicable to new
development and redevelopment sites. These requirements are codified in Chapter 13.48 of the
Auburn City Code (ACC). New development and redevelopment projects also may be subject to
other City code requirements, depending on the nature and location of the project. These code
requirement may include, but are not limited to, the subdivision and land use permit procedures in
Titles 17 and 14 ACC; excavation and grading and off-site improvement Chapter 15.74 ACC;
driveway control Chapter 12.20 ACC; groundwater protection, Chapter 8.08 ACC; shoreline
regulation, Chapter 16.08 ACC; and critical areas preservation Chapter 16.10 ACC.
These requirements are intended to provide for and promote the health, safety and welfare of the
general public, and are not intended to create or otherwise establish or designate any particular class
or group of persons who will or should be especially protected or benefited by the provisions of this
chapter.
3.1 Overview of the Minimum Requirements
The Minimum Requirements are:
1. Preparation of Stormwater Site Plans
2. Construction Stormwater Pollution Prevention
3. Source Control of Pollution
4. Preservation of Natural Drainage Systems and Outfalls
5. On-site Stormwater Management
6. Runoff Treatment
7. Flow Control
8. Wetlands Protection
9. Operation and Maintenance
The City also has one additional requirement beyond those required in Ecology’s 2005 manual:
10. Off-Site Analysis and Mitigation
Depending on the type and size of the proposed project, different combinations of these minimum
requirements apply. In general, small sites are required to control erosion and sedimentation from
construction activities and to apply simpler approaches to treatment and flow control of stormwater
runoff from the developed site. Large sites must provide erosion and sedimentation control during
construction and permanent control of stormwater runoff from the developed site.
Section 3.4 provides additional information on applicability of the Minimum Requirements to different
types of sites.
This manual is designed to be equivalent to Ecology’s 2005 Stormwater Management Manual for
Western Washington. Ecology considers its manual to include all known, available, and reasonable
methods of prevention, control, and treatment (AKART). Ecology’s manual has no independent
SURFACE WATER MANAGEMENT MANUAL
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 12
regulatory authority. However, Ecology has required as a condition of the City’s General Permit for
Discharges from Municipal Separate Storm Sewers, the adoption of stormwater program
components that are the substantial equivalent to the minimum requirements found in Ecology’s
2005 manual.
The minimum requirements of this Chapter are conditions of the City’s stormwater NPDES permit,
and are required under Auburn City Code, Chapter 13.48 Storm Drainage Utility.
3.2 Exemptions
The following classes of projects have exemption from the minimum requirements. All other new
development or redevelopment projects are subject to one or more of the Minimum Requirements
(see Section 3.4).
3.2.1 Road Maintenance
The following road maintenance practices are exempt:
• pothole and square cut patching
• overlaying existing asphalt or concrete pavement with asphalt or concrete without
expanding the area of coverage
• shoulder grading
• reshaping/regrading drainage systems
• crack sealing
• resurfacing with in-kind material without expanding the road prism
• vegetation maintenance
3.2.2 Parking Lots and Parking Lot Maintenance
Parking lots are considered pollution generating impervious surfaces and must comply with all
relevant BMPs per the Minimum Requirements. Parking lot surfacing material requirements are
regulated through the City’s Land Use code. Parking lots must provide a design to control and
manage surface water per the minimum requirements. No special consideration will be given to
“temporary” parking areas as the impacts resulting from the proposed impervious surface must be
mitigated as part of the construction.
The following parking lot maintenance practices are exempt:
• pothole and square cut patching
• overlaying existing asphalt or concrete pavement with asphalt or concrete without
expanding the area of coverage
• crack sealing
• catch basin, pipe and vegetation maintenance
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 13
3.2.3 Underground Utility Projects
Underground utility projects that replace the ground surface with in-kind material or materials with
similar runoff characteristics are only subject to Minimum Requirement #2, Construction Stormwater
Pollution Prevention.
3.2.4 Minor Clearing and Grading
The following minor clearing and grading activities are exempt from all the Minimum Requirements
except for Minimum Requirement #2; unless located within a critical or sensitive area governed by
the City’s Critical Areas Ordinance. Information on Critical Areas is available through the City of
Auburn Planning Department.
• Excavation for wells, except that fill made with the material from such excavation
shall not be exempt;
• Exploratory excavations under the direction of soil engineers or engineering
geologists, except that fill made with the material from such excavation shall not be
exempt;
• Removal of hazardous trees;
• Removal of trees or other vegetation which cause sight distance obstructions at
intersections;
• Minor clearing and grading associated with cemetery graves;
• Land clearing associated with routine maintenance by public utility agencies, as long
as appropriate vegetation management practices are followed as described in the
Best Management Practices of the Regional Road Maintenance Endangered
Species Act Program Guidelines located at
http://www.wsdot.wa.gov/maintenance/roadside/esa.htm
3.2.5 Emergencies
Emergency projects which, if not performed immediately would substantially endanger life or
property, are exempt only to the extent necessary to meet the emergency. Emergency activities may
include but are not limited to: sandbagging, diking, ditching, filling, or similar work during or after
periods of extreme weather. Permits authorizing the emergency work may be required after
completion of the emergency project.
3.2.6 Key Terms
A few key words to be aware of pertaining to the requirements that follow are:
• Arterial
• Effective Impervious Surface
• Impervious Surface
• Land Disturbing Activity
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 14
• Maintenance
• Native Vegetation
• New Development
• Pollution-Generating Impervious Surface (PGIS)
• Pollution Generating Pervious Surfaces (PGPS)
• Pre-Developed Conditions
• Project Site
• Receiving Waters
• Redevelopment
• Replaced Impervious Surface
• Site
• Source Control BMP
• Threshold Discharge Area.
The definition of these and other stormwater-related words and phrases used in this manual are
provided in the Glossary.
3.3 Applicability of the Minimum Requirements
NOTE: Throughout this section, requirements are written in bold print. Supplemental guidelines
that serve as advice and other materials are not bolded.
Not all of the Minimum Requirements apply to every development or redevelopment project. The
applicability varies depending on the type and size of the project. This section Identifies thresholds
that determine the applicability of the Minimum Requirements to different projects. The thresholds
shall be determined using the proposed improvements for the entire project site.
The flow charts in Figure I-3-1, Figure I-3-2, and Figure I-3-3, can be used to determine which
requirements apply. The Minimum Requirements themselves are presented in Section 3.4.
Flow credits as outlined in Volume VI are used when determining project thresholds.
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Minimum Requirements for Volume I
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Figure I-3-1. Determining Minimum Requirements for New and Redevelopment Project Sites
Do the new, replaced or new plus replaced impervious surfaces total
2,000 square feet or more?
OR
Does the project disturb 7,000 square feet or more of land?
Comply with Minimum Requirements
#1-#5 and #10
Continue to next questions
Comply with Minimum
Requirement #2
Does the project add 5,000 square feet or more of new impervious surface?
OR
Convert ¾ acres or more of native vegetation to lawn/landscaped?
OR
Convert 2.5 acres or more of native vegetation to pasture?
Minimum Requirements #1-#10 apply
to new impervious and converted
surfaces.
Continue to next questions
Is the total of new plus replaced impervious
surfaces 5,000 square feet or more and does the
value of the proposed improvements, including
interior improvements, exceed 50% of the
assessed value of the existing site improvements?
Minimum Requirements #1-#10 apply to
new impervious and replaced surfaces.
Continue to Fig. I-3-2 Flow Control chart No additional requirements
YES
YES
YES
NO
NO
NO
SURFACE WATER MANAGEMENT MANUAL
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Minimum Requirements for Volume I
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Figure I-3-2. Determining Minimum Requirements for Flow Control
Does the project total 10,000 square feet or more of effective impervious surfaces?
OR
Convert 2.5 acres or more of native vegetation to pasture?
OR
Convert ¾ acres or more of native vegetation to lawn or landscaped?
OR
Cause a 0.1 ft3/s increase in the 100-year flood frequency?
(Must use the WWHM model)
Does the project discharge directly
or indirectly into freshwater?
Flow control is not required. Provide
on-site stormwater management per
Minimum Requirement #5
Provide flow control per
Minimum Requirement #7
Flow control is not required.
Provide on-site stormwater
management per Minimum
Requirement #5
YES
YES
NO
NO
SURFACE WATER MANAGEMENT MANUAL
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 17
3.3.1 New Development
All new development shall be required to comply with Minimum Requirement #2.
The following new development shall comply with Minimum Requirements #1 through #5 for
the new and replaced impervious surfaces and the land disturbed:
• Creates or adds 2,000 square feet, or greater, of new, replaced, or new plus
replaced impervious surface area, or
• Has land disturbing activity of 7,000 square feet or greater.
The following new development shall comply with Minimum Requirements #1 through #10 for
the new impervious surfaces and the converted pervious surfaces.
• Creates or adds 5,000 square feet , or more, of new impervious surface area, or
• Converts ¾ acres, or more, of native vegetation to lawn or landscaped areas,
or
• Converts 2.5 acres, or more, of native vegetation to pasture.
3.3.2 Redevelopment
Redevelopment is development on a site that is already substantially developed (i.e., has 35% or
more existing impervious surface coverage). See the Glossary at the back of this manual for
definitions.
Redevelopment projects have the same requirements as new development projects in order to
minimize the impacts from new surfaces. To encourage redevelopment projects, replaced surfaces
are not required to be brought up to new stormwater standards unless the thresholds noted in
Section 3.3.3 are exceeded. As long as the replaced surfaces have similar pollution-generating
potential, the amount of pollutants discharged should not be significantly different. However, if the
redevelopment project scope is sufficiently large such that the thresholds noted in Section 3.3.3 are
exceeded, it is reasonable to require the replaced surfaces to be brought up to current stormwater
standards. This is consistent with other utility standards. When a structure or a property undergoes
significant remodeling, local governments often require the site to be brought up to new building code
requirements (e.g., onsite sewage disposal systems, fire systems).
All redevelopment shall be required to comply with Minimum Requirement #2. In addition, all
redevelopment that exceeds certain thresholds shall be required to comply with additional
Minimum Requirements as follows.
The following redevelopment shall comply with Minimum Requirements #1 through #5 for the
new and replaced impervious surfaces and the land disturbed:
• The new, replaced, or total of new plus replaced impervious surfaces is
2,000 square feet or more, or
• 7,000 square feet or more of land disturbing activities.
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Minimum Requirements for Volume I
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In addition to meeting Minimum Requirements #1 through #5, the following redevelopment
shall comply with Minimum Requirements #6 through #10 for the new impervious surfaces
and converted pervious areas:
• Adds 5,000 square feet or more of new impervious surfaces or,
• Converts ¾ acres, or more, of native vegetation to lawn or landscaped areas,
or
• Converts 2.5 acres, or more, of native vegetation to pasture.
If the runoff from the new impervious surfaces and converted pervious surfaces is not
separated from runoff from other surfaces on the project site, the stormwater treatment
facilities must be sized for the entire flow that is directed to them. The City may allow the
Minimum Requirements to be applied to an equivalent area (flow and pollution
characteristics) within the same site. For public road projects, the equivalent area does not
have to be within the project limits, but must drain to the same receiving water within the
watershed.
3.3.3 Assessed Value
Other types of redevelopment projects shall comply with all the Minimum Requirements for
the new and replaced impervious surfaces if the total of new plus replaced impervious
surfaces is 5,000 square feet or more, and the valuation of proposed improvements (materials
plus labor to construct) – including interior improvements – exceeds 50% of the assessed
value of the existing site improvements as determined from the latest available building
valuation data published by the International Code Council, available at
http://www.iccsafe.org/cs/techservices/index.html .
3.3.4 Roads
For road-related projects, runoff from the replaced and new impervious surfaces (including
pavement, shoulders, curbs, driveways, and sidewalks) shall meet all the Minimum
Requirements if the new impervious surfaces total 5,000 square feet or more and total 50% or
more of the existing impervious surfaces within the site (see Figure I-3-3). The site shall be
defined by the length of the project and the width of the right-of-way. For the purposes of this
manual, public roads (off-site improvements) required as part of a private project will be
considered part of the threshold area determination for the minimum requirements.
The following road maintenance practices are considered redevelopment. The extent to which
the manual applies is explained for each circumstance.
• Removing and replacing a paved surface to base course or lower, or repairing the
roadway base: If impervious surfaces are not expanded, Minimum Requirements #1
- #5 apply. However, in most cases, only Minimum Requirement #2, Construction
Stormwater Pollution Prevention, will be germane. Where appropriate, project
proponents are encouraged to look for opportunities to use permeable and porous
pavements.
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Minimum Requirements for Volume I
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• Extending the pavement edge without increasing the size of the road prism, or
paving graveled shoulders: These are considered new impervious surfaces and are
subject to the minimum requirements that are triggered when the thresholds
identified for redevelopment projects are met.
• Resurfacing by upgrading from dirt to gravel, asphalt, or concrete; upgrading from
gravel to asphalt or concrete; or upgrading from a bituminous surface treatment
(“chip seal”) to asphalt or concrete. These are considered new impervious surfaces
and are subject to the minimum requirements that are triggered when the thresholds
identified for redevelopment projects are met.
3.3.5 Cumulative Impact Mitigation Requirement
The determination of thresholds for a project site shall be based on the total increase or
replacement of impervious surfaces that occurred after adoption of the 2009 SWMM. Under
this provision, the City will consider the cumulative impacts of all permits issued on or after
February 16, 2010. The combined total of new or replaced surfaces will be applied to the
thresholds that determine applicability of the Minimum Requirements.
The intent of this Cumulative Impact Mitigation Requirement is to adequately mitigate the stormwater
from improvements on a project site that are submitted under separate permits. The separate
submittals could have project areas that do not meet the thresholds, but would meet the thresholds if
the projects were combined as one project.
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 20
Figure I-3-3. Determining Minimum Requirements for Road-Related Projects
Do the new, replaced, or new plus replaced impervious surfaces total
2,000 square feet or more?
OR
Does the project disturb 7,000 square feet or more of land?
Comply with Minimum Requirements
#1 through #5 and # 10
Continue to next questions
Comply with Minimum
Requirement #2
Does the project add 5,000 square feet or more of new impervious surface?
OR
Convert ¾ acres or more of native vegetation to lawn/landscape?
OR
Convert 2.5 acres or more of native vegetation to pasture?
Minimum Requirements #1 through
#10 apply to new impervious and
converted surfaces.
Continue to next questions
Does the project add 5,000 square feet or more
of new impervious surfaces?
AND
Do the new impervious surfaces add 50% or
more to the existing impervious surfaces within
the project limits?
Minimum Requirements #1 through
#10 apply to new impervious and
replaced surfaces. No additional requirements
YES
YES
YES
NO
NO
NO
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Minimum Requirements for Volume I
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3.4 Description of Minimum Requirements
NOTE: Throughout this Section, guidance to meet requirements is written in BOLD.
Supplemental guidelines that serve as advice and other materials are not written in bold.
This section describes the minimum requirements for stormwater management at new development
and redevelopment sites. Consult Section 3.3 to determine which requirements apply to any given
project.
Volumes II through VI of this manual present Best Management Practices (BMPs) for use in meeting
the Minimum Requirements.
3.4.1 Minimum Requirement #1: Preparation of a Stormwater Site Plan
All projects meeting the thresholds in Section 3.3 shall prepare a Stormwater Site Plan for
local government review. Stormwater Site Plans shall be prepared in accordance with
Chapter 4 of this volume.
A Stormwater Site Plan consists of an assessment of both temporary and permanent
stormwater and drainage impacts and may include a construction stormwater pollution
prevention plan, when required by Minimum Requirement #2.
3.4.1.1 Objective
To outline the existing and post-developed conditions of the project site, describe the proposed
stormwater facilities, and present the stormwater analysis.
3.4.2 Minimum Requirement #2: Construction Stormwater Pollution
Prevention (SWPP)
All new development and redevelopment shall comply with Construction SWPP Elements #1
through #12. A full description of these elements can be found in Volume II, Chapter 2.
Projects which meet or exceed the thresholds of Volume I, Section 3.3 must prepare a
Construction Stormwater Pollution Prevention Plan (SWPPP) as part of the Stormwater Site
Plan (see Section 3.4.1). Each of the twelve elements must be considered and included in the
Construction SWPPP unless site conditions render the element unnecessary and the
exemption from that element is clearly justified in the narrative of the SWPPP.
The City has developed a Construction SWPPP Short Form for projects that:
• Add or replace between 2,000 and 5,000 square feet of impervious surface, or
• Clear or disturb between 7,000 square feet and 1 acre of land.
The SWPPP Short Form is intended to take the place of the Construction SWPPP. A Certified
Erosion and Sediment Control Lead (CESCL) is not required for those projects using the
City’s Construction SWPPP Short Form.
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For all other projects requiring a Construction SWPPP, a CESCL shall be identified in the
Construction SWPPP and shall be on-site or on-call at all times. CESCLs must be trained
through an Ecology approved training program found at:
http://www.ecy.wa.gov/programs/wq/stormwater/cescl.htm
Unless located in a Critical Area, projects that add or replace less than 2,000 square feet of
impervious surface or disturb less than 7,000 square feet of land are not required to prepare a
Construction SWPPP, but must consider all of the twelve Elements of Construction
Stormwater Pollution Prevention (SWPP) and develop controls for all elements that pertain to
the project site.
SWPP Elements are:
Element 1: Mark Clearing Limits
Element 2: Establish Construction Access
Element 3: Control Flow Rates
Element 4: Install Sediment Controls
Element 5: Stabilize Soils
Element 6: Protect Slopes
Element 7: Protect Drain Inlets
Element 8: Stabilize Channels and Outlets
Element 9: Control Pollutants
Element 10: Control De-Watering
Element 11: Maintain BMPs
Element 12: Manage the Project
These Elements are described in detail in Volume II.
3.4.2.1 Objective
The purpose of construction SWPP is to control erosion and prevent sediment and other pollutants
from leaving the site during the construction phase of a project.
3.4.3 Minimum Requirement #3: Source Control of Pollution
All known, available, and reasonable source control BMPs shall be applied to all projects.
Source control BMPs shall be selected, designed, and maintained according to this manual.
Structural source control BMPs shall be identified in the stormwater site plan and shall be
shown on construction plans submitted for City review.
Source Control BMPs include Operational BMPs and Structural Source Control BMPs. See
Volume IV for design details of these BMPs. For construction sites, see Volume II, Chapter 3.
3.4.3.1 Objective
The intent of source control BMPs is to prevent stormwater from coming in contact with pollutants.
They are a cost-effective means of reducing pollutants in stormwater, and, therefore, should be a first
consideration in all projects.
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3.4.4 Minimum Requirement #4: Preservation of Natural Drainage Systems
and Outfalls
Natural drainage patterns shall be maintained, and discharges from the project site shall
occur at the natural location, to the maximum extent practicable. The manner by which runoff
is discharged from the project site must not cause a significant adverse impact to
downstream receiving waters and downgradient properties. All outfalls require energy
dissipation.
As part of a submittal, the applicant shall identify the location of natural drainage,
topography, historic drainage information and any potential impacts.
3.4.4.1 Objective
To preserve and utilize natural drainage systems to the fullest extent because of the multiple
stormwater benefits these systems provide; and to prevent erosion at and downstream of the
discharge location.
3.4.5 Minimum Requirement #5: On-Site Stormwater Management
Projects shall employ, where feasible and appropriate, On-site Stormwater Management
BMPs to infiltrate, disperse, and retain stormwater runoff onsite to the maximum extent
feasible without causing flooding, erosion, water quality or groundwater impacts. All projects
required to comply with Minimum Requirement #5 shall employ all of the following BMPs as
applicable:
• Roof Downspout Control BMPs, functionally equivalent to those described in
Volume III, Section 2.1, and
• Dispersion, functionally equivalent to those described in Volume VI, Section 2.2, and
• Soil Quality BMPs, functionally equivalent to those in Volume VI, Section 2.2.1.4.
Where roof downspout controls are planned, the following three types shall be considered in
descending order of preference:
• Downspout infiltration systems including rain gardens (Volume III, Section 2.1.2 and
Section 2.1.4, and Volume VI, Section 2.2.3).
• Downspout dispersion systems (Volume III, Section 2.1.3), only if infiltration is not
feasible.
• Collect and convey to City system (Volume III, Section 2.1.5) if other alternatives are
not feasible.
3.4.5.1 Objective
To use inexpensive practices on individual properties to reduce the amount of disruption of the
natural hydrologic characteristics of the site
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3.4.6 Minimum Requirement #6: Runoff Treatment
3.4.6.1 Thresholds
The following require construction of stormwater treatment facilities:
• Projects in which the total of effective pollution-generating impervious surface
(PGIS) is 5,000 square feet or more in a threshold discharge area of the project,
or
• Projects in which the total of pollution-generating pervious surfaces (PGPS) is
three-quarters (3/4) of an acre or more in a threshold discharge area, and from
which there is a surface discharge in a natural or man-made conveyance
system from the site.
Total effective pollution-generating impervious surface shall include all new plus replaced
PGIS. That portion of any development project in which the above PGIS or PGPS thresholds
are not exceeded in a threshold discharge area shall apply On-site Stormwater Management
BMPs, where feasible, in accordance with Minimum Requirement #5.
3.4.6.2 Treatment Facility Selection, Design, and Maintenance
Stormwater treatment facilities shall be:
• Selected in accordance with the process identified in Volume V, Chapter 1;
• Designed in accordance with the design criteria in Volume V; and
• Maintained in accordance with the maintenance standards in Volume I,
Appendix D that shall be incorporated in the design as part of a facility
operation and maintenance manual.
3.4.6.3 Additional Requirements
• Direct discharge of untreated stormwater from pollution-generating surfaces
above the thresholds given in Section 3.4.6.1 to groundwater is prohibited.
• Infiltration of any amount of PGS is not allowed within the Groundwater
Protection Zone 2 unless approved in writing per Volume I, Section 2.1.
3.4.6.4 Objective
The purpose of runoff treatment is to reduce pollutant loads and concentrations in stormwater runoff
using physical, biological, and chemical removal mechanisms so that beneficial uses of receiving
waters are maintained and, where applicable, restored. When site conditions are appropriate,
infiltration can potentially be the most effective BMP for runoff treatment.
3.4.6.5 Supplemental Guidelines
The above thresholds apply to both a project’s on-site and off-site improvements. Once the project is
required to meet this minimum requirement, all new and replaced pollution generating impervious
surfaces are required to provide treatment. No net or average is permitted between non-pollution
generating surfaces and pollution generating.
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NOTE: With respect to Water Quality, a “net” total of pollution generating impervious surface will not
be considered when dealing with replaced impervious surfaces. Construction of new surfaces that do
not generate pollution does not balance the environmental impacts of newly created pollution
generating surfaces. All new or redeveloped pollution generating surfaces that meet the thresholds
for new and redevelopment and create, add and/or replace 5,000 square feet pollution generating
impervious surface shall provide water quality.
See Volume V for more detailed guidance on selection, design, and maintenance of treatment
facilities.
3.4.7 Minimum Requirement #7: Flow Control
3.4.7.1 Applicability
Projects must provide flow control to reduce the impacts of stormwater runoff from
impervious surfaces and land cover conversions. Portions of projects discharging to a
wetland shall also be subject to Minimum Requirement #8.
The flow control requirement thresholds apply to projects that discharge directly or
indirectly:
Through a conveyance system, into fresh water; or
Through a conveyance system into a gulch; or
To a City identified capacity problem existing downstream of the development;
or
To a manmade conveyance system (ditch, swale, etc.) which has not been
adequately stabilized to prevent erosion; or
To a conveyance system without capacity to convey the fully developed
design event as defined in Volume III, Chapter 3.
3.4.7.2 Thresholds
Projects that meet or exceed the following thresholds require construction of flow control
facilities and/or land use management BMPs.
Project sites in which the total of effective impervious surfaces is
10,000 square feet or more in a threshold discharge area, or
Projects that convert ¾ acres or more of native vegetation to lawn or
landscape, or convert 2.5 acres or more of native vegetation to pasture in a
threshold discharge area, and from which there is a surface discharge in
natural or man-made conveyance system from the site, or
Projects that, through a combination of effective impervious surfaces and
converted pervious surfaces, cause a 0.1 cfs increase in the 100-year flow
frequency from a threshold discharge area as estimated using the Western
Washington Hydrology Model or other approved model. Comparison will be
between existing and proposed site conditions.
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That portion of any development project in which the thresholds listed above are not
exceeded in a threshold discharge area, shall apply Onsite Stormwater Management BMPs in
accordance with Minimum Requirement #5. Refer to Figure I-3-1, Figure I-3-2 and Figure I-3-3
to aid in determining project requirements.
3.4.7.3 Standard Requirement
Using WWHM for design, stormwater discharges shall match developed discharge durations
to pre-developed durations for the range of pre-developed discharge rates from 50% of the
2-year peak flow up to the full 50-year peak flow. The pre-developed condition to be matched
shall be a forested land cover. The pre-developed soil types shall be assumed as either
outwash (Hydrologic Soil Group A/B) or till (Hydrologic Soil Group C/D) soils, depending on
supporting geotechnical information. Saturated soil conditions shall only be considered
when determining existing wetland hydrology.
This standard requirement is waived for sites that will reliably infiltrate all the runoff from
impervious surfaces and converted pervious surfaces.
Any areas for which the minimum thresholds are not exceeded must still meet the following
criteria:
The project must be drained by a conveyance system with capacity to convey
the fully developed design event as defined in Volume III, Chapter 3. The
conveyance system must consist entirely of manmade conveyance elements
(e.g., pipes, ditches, outfall protection, etc.) and extend to the ordinary high
water line of the receiving water; and
Any erodible elements of the manmade conveyance system must be
adequately stabilized to prevent erosion under future build-out conditions from
areas that contribute flow to the system; and
No City identified capacity problems may exist downstream of the
development; and
Surface water flows from the area must not be diverted from or increased to an
existing wetland, stream, or near-shore habitat sufficient to cause a significant
adverse impact.
3.4.7.4 Infrastructure Protection Requirement
The infrastructure protection requirement is intended to mitigate stormwater impacts from
projects that are not required to provide flow control, but discharge to a system with capacity
limitations such as projects with the following characteristics:
Inadequate capacity in downstream conveyance.
Applicant may resolve the downstream capacity problem or may provide on-site detention.
Where detention is provided, stormwater discharges for the developed condition shall match
the discharges under existing conditions.
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3.4.7.5 Objective
To prevent increases in the stream channel erosion rates that are characteristic of natural conditions
(i.e., prior to the European settlement). The standard intends to maintain the total amount of time that
a receiving stream exceeds an erosion-causing threshold based upon historic rainfall and natural
land cover conditions. That threshold is assumed to be 50% of the 2-year peak flow. Maintaining the
naturally occurring erosion rates within streams is vital, though by itself insufficient, to protect fish
habitat and production.
3.4.7.6 Modeling Requirements
To meet the Standard Requirement, the applicant shall use the most current software
version of the Department of Ecology’s Western Washington Hydrology Model
(WWHM) model (see Volume III). Alternative models for sizing flow control and water
quality facilities may be considered, provided they are Washington State Department
of Ecology equivalent, and approved by the City of Auburn. Approval from the City
shall be obtained prior to submittal of design documents.
To meet the Downstream Analysis requirements, piped conveyance systems shall be
modeled using either continuous simulation or single event methods. Stream
systems shall be modeled using only continuous simulation methods.
The designer shall provide a copy of the completed hydrology analysis worksheet (Appendix
C) and a copy of the electronic project files.
NOTE: Hand-calculated hydrographs and flow routing will no longer be accepted because of the
wide availability of various software programs.
3.4.8 Minimum Requirement #8: Wetlands Protection
Wetlands are regulated by the City of Auburn through this requirement and the Critical Areas Code,
Auburn City Code 16.10. For more information about wetlands, wetland permits and development
close to wetlands, please contact the Planning, Building & Community Department at (253) 931-
3090.
3.4.8.1 Applicability
Stormwater discharges to wetlands are regulated under the City’s Critical Areas Ordinance
(ACC 16.10).
The requirements below are in addition to requirements given in ACC 16.10 and apply only to
projects whose stormwater discharges into a wetland, either directly or indirectly through a
conveyance system. These requirements must be met in addition to meeting Minimum
Requirement #6, Runoff Treatment. All pollution generating surfaces discharging to wetlands
shall require water quality treatment prior to discharge to the wetlands. Streams may also be
regulated under this requirement as part of the wetland permit.
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3.4.8.2 Thresholds
When either of the thresholds identified in Minimum Requirement #6 – Runoff Treatment, or
Minimum Requirement #7 – Flow Control are met or exceeded, this requirement shall also be
applied.
3.4.8.3 Standard Requirement
Discharges to wetlands shall maintain the hydrologic conditions, hydrophytic vegetation, and
substrate characteristics necessary to support existing and designated uses. The hydrologic
analysis shall use the existing land cover condition to determine the existing hydrologic
conditions unless directed otherwise by a regulatory agency with jurisdiction. A wetland can
be considered for hydrologic modification and/or stormwater treatment in accordance with
Guidesheet 1B in Appendix E. Modeling shall be completed with a continuous simulation
model. Model calibration and pre- and post-development monitoring of wetland levels,
groundwater levels, and water quality may be required by the City.
3.4.8.4 Additional Requirements
The standard requirement does not excuse any discharge from the obligation to apply
whatever technology is necessary to comply with state water quality standards,
Chapter 173-201A WAC, or state groundwater standards, Chapter 173-200 WAC. Additional
treatment requirements to meet those standards may be required by federal, state, or local
governments.
Stormwater treatment and flow control facilities shall not be constructed within a natural
vegetated buffer, except for:
Necessary conveyance systems as approved by the City; or
As allowed in wetlands approved for hydrologic modification and/or treatment
in accordance with Guidesheet 1B in Appendix E of this Volume.
Flow splitting devices or drainage BMPs must be applied to route natural runoff
volumes from the project site to any downstream stream or wetland.
Design of flow splitting devices or drainage BMPs will be based on continuous
hydrologic modeling analysis. The design will assure that flows delivered to stream
reaches will approximate, but in no case exceed, durations ranging from 50% of the
2-year to the 50-year peak flow.
Flow splitting devices or drainage BMPs that deliver flow to wetlands shall be designed using
continuous hydrologic modeling to preserve pre-project wetland hydrologic conditions
unless specifically waived or exempted by regulatory agencies with permitting jurisdiction;
An adopted and implemented basin plan, or a Total Maximum Daily Load (TMDL, also known
as a Water Clean-up Plan) may be used to develop requirements for wetlands that are tailored
to a specific basin.
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3.4.8.5 Objective
To ensure that wetlands receive the same level of protection as any other waters of the state.
Wetlands are extremely important natural resources which provide multiple stormwater benefits,
including groundwater recharge, flood control, and stream channel erosion protection. They are
easily impacted by development unless careful planning and management are conducted. Wetlands
can be severely degraded by stormwater discharges from urban development due to pollutants in the
runoff and also due to disruption of natural hydrologic functioning of the wetland system. Changes in
water levels and the frequency and duration of inundations are of particular concern.
3.4.8.6 Supplemental Guidelines
Appendix E contains guidance for wetlands when interacting with stormwater. The City of Auburn
may require applicants to utilize portions or all of the guidance in analyzing and mitigating
wetland impacts.
3.4.9 Minimum Requirement #9: Operation and Maintenance
An operation and maintenance manual that is consistent with the provisions in Section 4.1 of
this Volume shall be provided for all proposed stormwater facilities and BMPs at the time
construction plans are submitted for review, and the party (or parties) responsible for
maintenance and operation shall be identified.
For private facilities, a copy of the manual shall be retained onsite or within reasonable
access to the site, and shall be transferred with the property to the new owner. For private
systems serving multiple lots within residential developments or other developments, a
separate covenant or other guarantee of proper maintenance that can be recorded on title
shall be provided and recorded. For public facilities, a copy of the manual shall be retained in
the appropriate department.
For all facilities (public and private), a log of maintenance activity that indicates what actions
were taken shall be kept and be available for inspection by the City.
3.4.9.1 Objective
To ensure that stormwater control facilities are adequately maintained and operated properly.
3.4.9.2 Supplemental Guidelines
Inadequate maintenance is a common cause of failure for stormwater control facilities. The
description of each BMP in Volumes II, III, V, and VI includes a section on maintenance. Appendix D
of Volume I includes a schedule of maintenance standards for drainage facilities.
3.4.10 Minimum Requirement #10: Off-Site Analysis and Mitigation
As required by the Minimum Requirements of this Chapter, development projects that
discharge stormwater offsite shall submit as part of their Stormwater Site Plan and Report an
off-site analysis that assesses the potential off-site impacts of stormwater discharge.
All projects shall perform a qualitative analysis downstream from the site.
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The City may require a quantitative analysis for any project deemed to need additional
downstream information.
3.4.10.1 Qualitative Analysis:
Project applicants shall submit a qualitative analysis of each upstream system entering a site
(run-on) and each downstream system leaving a site (run-off). The qualitative analysis shall
extend downstream for the entire flow path, from the project site to the receiving water, or up
to one-quarter mile, whichever is less. The upstream analysis shall identify and describe
points where water enters the site and the tributary area. A basin map defining the onsite and
offsite basins tributary to the site shall be provided. The basin map shall be to a defined
scale.
Upon review of this analysis, the City may require a qualitative analysis further downstream,
mitigation measures deemed adequate to address the problems, or a quantitative analysis,
depending upon the presence of existing or predicted flooding, erosion, or water quality
problems, and on the proposed design of the onsite drainage facilities. Details on how to
perform this analysis are located in Volume I, Chapter 4 and Volume I, Appendix B.
3.4.10.2 Quantitative Analysis
The City may require a quantitative analysis for any project deemed to need additional
downstream information. Details on how to perform this analysis are located in Volume III,
Section 3.1.2.
3.4.10.3 Objective
To identify and evaluate offsite water quality, erosion, slope stability, and drainage impacts that may
be caused or aggravated by a proposed project, and to determine measures for preventing impacts
and for not aggravating existing impacts. Aggravated shall mean increasing the frequency of
occurrence and/or severity of a problem. Some of the most common and potentially destructive
impacts of land development are erosion of downgradient properties, localized flooding, and slope
failures. These are caused by increased surface water volumes and changed runoff patterns. The
City believes taking the precautions of offsite analysis could prevent substantial property damage and
public safety risks. In addition the applicant will evaluate types and locations of surface run-on to the
project site. These must be safely conveyed across the project site.
3.5 Exceptions
NOTE: Throughout this Section, guidance to meet the requirements is written in BOLD.
Supplemental guidelines that serve as advice and other materials are not written in bold.
Deviations from the Minimum Requirements may be requested, in writing, in accordance with
ACC 13.48.226 to allow a waiver of a requirement, a reduction or modification of a
requirement, or to permit an alternative requirement. Public notice of application for a
deviation, draft decision, and written findings will be published in accordance with ACC
13.48.226, with an opportunity for public comment. Deviations must meet the following
criteria:
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The minimum requirements would impose a severe and unexpected economic
hardship; and
The deviation will not increase risk to the public health and welfare, nor injurious to
other properties in the vicinity and/or downstream, and to the quality of waters of the
state; and
The deviation is the least possible exception that could be granted to comply with the
intent of the Minimum Requirements.
In accordance with ACC 13.48.226, the City Engineer may grant a deviation following a
documented finding that:
The deviation is likely to be equally protective of public health, safety and welfare,
the environment, and public and private property, as the requirement from
which an exception is sought.
OR
Substantial reasons exist under ACC 13.48.226 C., for approving the requested
deviation and the deviation will not cause significant harm. The substantial
reasons may include, but are not limited to:
o The requirement to be imposed is not technically feasible; or
o An emergency situation necessitates approval of the deviation; or
o No reasonable use of the property is possible unless the deviation is
approved; or
o The requirement would cause significant harm or a significant threat of
harm to public health, safety and welfare, the environment, or to public
and private property, or would cause extreme financial hardship which
substantially outweighs its benefits.
The decision to grant a deviation is within the sole discretion of the City, and the City Engineer shall
only approve a deviation to the extent it is necessary. The City Engineer may impose new or
additional requirements to offset or mitigate harm that may be caused by approving the deviation.
The City Engineer may require the applicant to submit a licensed engineer’s report or analysis along
with a request, in writing, for a deviation. Deviations are intended to maintain necessary flexible
working relationship between the City and applicants.
The approval of a deviation shall not be construed to be an approval of any violation of any of the
other provisions of the City’s Municipal Code, or of any other valid law of any governmental entity
having jurisdiction.
Applications for a deviation from the Minimum Requirements of ACC13.48.225 must be in
writing and include the following information:
The current (pre-project) use of the site, and
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How the application of the minimum requirement(s) restricts the proposed use of the
site compared to the restrictions that existed prior to the adoption of the minimum
requirements; and
The possible remaining uses of the site if the deviation were not granted; and
The uses of the site that would have been allowed prior to the adoption of the
minimum requirements; and
A comparison of the estimated amount and percentage of value loss as a result of the
minimum requirements versus the estimated amount and percentage of value loss as
a result of requirements that existed prior to adoption of the minimum requirements;
and
The feasibility for the owner to alter the project to apply the minimum requirements.
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Chapter 4 Preparation of Stormwater Site Plans
The Stormwater Site Plan is the comprehensive report containing all of the technical information and
analysis necessary for the City to evaluate a proposed new development or redevelopment project
for compliance with stormwater requirements. Contents of the Stormwater Site Plan will vary with the
type and size of the project, and individual site characteristics. The scope of the Stormwater Site Plan
also varies depending on the applicability of Minimum Requirements (see Section 3.4). However,
typical Stormwater Site Plans will contain both a report and detailed plans.
This chapter describes the contents of a Stormwater Site Plan and provides a general procedure for
how to prepare the plan. The goal of this chapter is to provide a framework for uniformity in plan
preparation. Such uniformity will promote predictability and help secure prompt review. Properly
drafted engineering plans and supporting documents will also facilitate the operation and
maintenance of the proposed system long after construction is complete.
To aid the design engineer, a checklist containing submittal requirements is located in Appendix B
and a hydraulic analysis worksheet is provided in Appendix C. These appendices should be
completed and provided by the design engineer. These documents will be utilized by the City during
the project review.
Stormwater Site Plans shall be prepared by a licensed Professional Engineer. All Stormwater Site
Plans and drawings shall be signed, stamped, and dated prior to review by the City.
4.1 Stormwater Site Plan Outline
The Stormwater Site Plan (SSP) encompasses the entire submittal to the City for drainage review.
This section provides an outline for a SSP and details drawing requirements.
Chapter 1 - Project Overview
The project overview must provide a general description of the project, pre-developed and developed
conditions of the site, site area, and size of the improvements, and the pre- and post-developed
stormwater runoff conditions. The overview shall summarize difficult site parameters, the natural
drainage system, and drainage to and from adjacent properties, including bypass flows.
The vicinity map shall clearly locate the property, identify all roads bordering the site, show the route
of stormwater off-site to the local natural receiving water, and show significant geographic features
and sensitive/critical areas (streams, wetlands, lakes, steep slopes, etc.).
Include a list of other necessary permits and approvals as required by other regulatory agencies, if
those permits or approvals include conditions that affect the drainage plan, or contain more restrictive
drainage-related requirements.
Chapter 2 – Existing Conditions Summary
Collect and review information on the existing site conditions, including topography, drainage
patterns, soils, ground cover, presence of any critical areas, adjacent areas, existing development,
existing stormwater facilities, and adjacent on- and off-site utilities. Analyze data to determine site
limitations including:
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• Areas with high potential for erosion and sediment deposition (based on soil
properties, slope, etc.); and
• Locations of sensitive and critical areas (e.g. vegetative buffers, wetlands, steep
slopes, floodplains, geologic hazard areas, streams, etc.).
• Points where existing surface water enters and exits the project site.
Delineate these areas on the vicinity map and/or a site map. Prepare an Existing Conditions
Summary that will be submitted as part of the Site Plan. Part of the information collected in this step
should be used to help prepare the Construction Stormwater Pollution Prevention Plan.
Chapter 3 – Off-Site Analysis – Minimum Requirement # 10
The existing or potential impacts to be evaluated and mitigated as part of any off-site/downstream
analysis shall include:
• Conveyance system capacity problems;
• Localized flooding;
• Aquatic habitat (wetlands) impacts;
• FEMA flood plain;
• Upland erosion impacts, including landslide hazards;
• Stream channel erosion at the outfall location;
• Impacts to surface water, groundwater, or sediment quality as identified in a Basin
Plan or TMDL (Water Clean-up Plan);
• Locations where surface water enters and exits the site.
Qualitative Analysis:
Project applicants shall submit a qualitative analysis of each upstream system entering a site (run-on)
and each downstream system leaving a site (run-off). The qualitative analysis shall extend
downstream for the entire flow path, from the project site to the receiving water, or up to one-quarter
mile, whichever is less. The upstream analysis shall identify and describe points where water enters
the site and the tributary area. A basin map defining the onsite and offsite basins tributary to the site
shall be provided. The basin map shall be to a defined scale.
Upon review of this analysis, the City may require a qualitative analysis further downstream,
mitigation measures deemed adequate to address the problems, or a quantitative analysis,
depending upon the presence of existing or predicted flooding, erosion, or water quality problems,
and on the proposed design of the onsite drainage facilities. Details on how to perform this analysis
are located in Volume I, Appendix B.
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Quantitative Analysis:
The City may require a quantitative analysis for any project deemed to need additional downstream
information. Details on how to perform this analysis are located in Volume III, Section 3.1.2.
The off-site analysis shall extend downstream of the site for a minimum of ¼ mile from the point of
connection to the existing public drainage system, or until a trunk main is reached.
Chapter 4 – Permanent Stormwater Control Plan
The Permanent Stormwater Control Plan consists of those stormwater control BMPs and facilities
that will serve the project site in its developed condition.
A preliminary design of the BMPs and facilities is necessary to determine how they will fit within and
serve the entire preliminary development layout. After a preliminary design is developed, the designer
may want to reconsider the site layout to reduce the need for construction of facilities, or the size of
the facilities by reducing the amount of impervious surfaces created and increasing the areas to be
left undisturbed. After the designer is satisfied with the BMP and facilities selections, the information
must be presented within a Permanent Stormwater Control Plan.
Where modeling is completed, the City may require the model files be provided electronically.
The Permanent Stormwater Control Plan should contain the following sections:
1. Threshold Discharge Areas and Applicable Requirements for Treatment, Flow Control
and Wetlands Protection
Complete the following tasks:
A. Read the definitions in the Glossary located at the back of this manual for the
following terms: effective impervious surface, impervious surface, pollution-
generating impervious surface (PGIS), pollution-generating pervious surface
(PGPS), threshold discharge area, project site, and replaced impervious surfaces.
B. Outline the threshold discharge areas for your project site.
C. Determine the amount of effective pollution-generating impervious surfaces and
pollution –generating pervious surfaces in each threshold discharge area. Compare
those totals to the categories in Section 3.4.6 to determine where treatment facilities
are necessary. Note that On-site Stormwater Management BMPs are always
applicable.
D. Determine the amount of effective impervious surfaces and converted pervious
surfaces in each threshold discharge area. Using an approved continuous runoff
simulation model, estimate the increase in the 100-year flow frequency within each
threshold discharge area.
E. Compare those totals to the categories in Section 3.4.7 to determine where flow
control facilities are necessary. Note that On-site Stormwater Management BMPs
may alter the calculation of effective impervious surface. See Volume VI for WWHM
flow credit information.
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2. Pre-developed Site Hydrology
The acreage, soil types, and land covers used to determine the pre-developed flow characteristics,
along with basin maps, graphics, and exhibits for each sub-basin affected by the project should be
included.
Provide a topographic map, of sufficient scale and contour intervals to determine basin boundaries
accurately, and show:
• Delineation and acreage of areas contributing runoff to the site;
• Flow control facility location;
• Outfall;
• Overflow route; and
• All natural streams and drainage features.
The direction of flow, acreage of areas contributing drainage, and the limits of development should be
indicated. Each basin within or flowing through the site should be named and model input parameters
referenced, as appropriate.
If stormwater facilities that require sizing are proposed, provide a listing of assumptions and site
parameters used in analyzing the pre-developed site hydrology.
For projects requiring flow control, the pre-developed condition to be matched shall be a forested
land cover unless reasonable, historic information is provided that indicates the site was prairie prior
to settlement.
3. Developed Site Hydrology
All Projects:
Total of impervious surfaces, total pollution-generating impervious surfaces, total pollution-generating
pervious surfaces, and total disturbed area must be tabulated for each threshold discharge area.
These are needed to verify which minimum requirements apply to a project.
Projects and Threshold Discharge Areas within Projects That Require Treatment and Flow Control
Facilities:
Provide narrative, mathematical, and graphic presentations of model input parameters selected for
the developed site condition, including acreage, soil types, and land covers, road layout, and all
drainage facilities. The applicant shall reference sources for all variables and equations. All
submissions shall be in typed format with a table of contents and labels for all figures and
calculations. If calculations are used from other sections of the submittal, they shall be referenced
with the appropriate, section and page number to the point of their original derivation.
Previous stormwater reports may be referenced. The City may request submission of all reference
reports in their entirety.
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Developed basin areas and flows shall be shown on a map and cross-referenced to computer
printouts or calculation sheets. Developed basin flows should be listed and tabulated.
Any documents used to determine the developed site hydrology should be included. Maintain the
same basin name as used for the pre-developed site hydrology. If the boundaries of a basin have
been modified by the project, that should be clearly shown on a map and the name modified to
indicate the change.
Final grade topographic maps shall be provided including finished floor elevations, where
appropriate.
4. Performance Standards and Goals
If treatment facilities are proposed, provide a listing of the water quality menus used (Chapter 2 of
Volume V). If flow control facilities are proposed, provide a confirmation of the flow control standard
being achieved (e.g., the Ecology flow duration standard).
5. Flow Control System
Provide a drawing of the flow control facility and its appurtenances. This drawing must show basic
measurements necessary to calculate the storage volumes available from zero to the maximum
head, all orifice/restrictor sizes and head relationships, control structure/restrictor placement, and
placement on the site.
Include computer printouts, calculations, equations, references, storage/volume tables, graphs as
necessary to show results and methodology used to determine the storage facility volumes. Where
the Western Washington Hydrology Model is used, its documentation files shall be submitted
electronically.
6. Water Quality System
Provide a drawing of the proposed treatment facilities, and any structural source control BMPs. The
drawing must show overall measurements and dimensions, placement on the site, location of inflow,
bypass, and discharge systems.
Include computer printouts, calculations, equations, references, and graphs as necessary to show
the facilities are designed in accordance with the requirements and design criteria in Volume V.
If using a manufactured system provide a specification from the manufacturer as well as all design
specific parameters.
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7. Conveyance System Analysis and Design
Present an analysis of any existing conveyance systems, and the analysis and design of the
proposed stormwater conveyance system for the project. Portions of this analysis may include the
criteria established in Item 3 above. This information should be presented in a clear, concise manner
that can be easily followed, checked, and verified. All pipes, culverts, catch basins, channels, swales,
and other stormwater conveyance appurtenances must be clearly labeled and correspond directly to
the engineering plans. The analysis should be based on the design elements within the City of
Auburn Engineering Design and Construction Standards and Volume III, Chapter 3 of this manual.
Chapter 5 – Discussion of Minimum Requirements
Provide a list of the minimum requirements that apply to the project site. Indicate where in the
Stormwater Site Plan the documentation showing how the minimum requirements are satisfied can
be found.
Appendix A – Operation and Maintenance (O & M) Manual
The O&M manual shall be designed as a stand-alone document, including all necessary figures and
maps. The document may be submitted as either an Appendix to the SSP or bound separately.
Submit an operations and maintenance manual for each permanent stormwater facility. The manual
shall contain a description of the facility, what it does, and how it works. The manual must identify and
describe the maintenance tasks, and the required frequency of each task. The maintenance tasks
and frequencies must meet the standards established in this manual.
Include a recommended format for a maintenance activity log. The log will have space to list
maintenance activities.
The manual must prominently indicate where it shall be kept, and that it must be made available for
inspection by the City. Specifically the manual will include:
Statements:
• Where the O&M manual shall be kept.
• That the O&M manual must be made available for inspection by the City.
• Name of the person or organization responsible for maintenance of the on-site storm
system, including the phone number of the current responsible party.
Descriptions of:
• Each flow control and treatment facility, what it does, how it works, and maintenance
tasks and frequency
• Operation and Maintenance Guidelines from the manufacturer of any proprietary flow
control and treatment facility.
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Sample forms:
• A summary sheet of the required inspection and maintenance frequencies for each
specific facility (catch basins, ponds, vaults)
• A recommended format for a maintenance activity log that will indicate what
maintenance actions have been taken for each flow control and treatment facility
• Relevant maintenance checklists from Appendix D of Volume 1 of the SWMM.
Figures and/or maps:
• An 11” x 17” map of the site, with the locations of the flow control and treatment
facilities prominently noted.
Appendix B – Construction Stormwater Pollution Prevention Plan
This is the plan described in Section 3.4.2 and Volume II.
Appendix C – Submittal Requirements Checklist
A copy of the checklist can be found in Volume I, Appendix B and shall be completed by the
engineer.
Appendix D – Hydraulic Analysis Worksheet
A copy of the worksheet can be found in Volume I, Appendix C and shall be completed by the
engineer.
Appendix E – Other Special Reports
In this Appendix, include any special reports and studies conducted to prepare the Stormwater Site
Plan (e.g. soil testing, wetlands delineation).
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4.2 Plans Required After Stormwater Site Plan Approval
Follow the plan approval process given in the Chapters 2 and 3 of the City of Auburn Engineering
Design Standards Manual.
4.3 Land Use Submittal Requirements
A reference to the subdivision checklists will be inserted here.
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Appendix A 41
Appendix A Regulatory Requirements
This appendix contains the regulatory requirements that apply to applicable sites and their
stormwater discharges.
Relationship of this Manual to Federal, State, and Local Regulatory Requirements
This manual is modeled after Ecology’s 2005 Stormwater Management Manual for Western
Washington. Ecology considers its manual to include all known, available, and reasonable methods
of prevention, control, and treatment (AKART; RCW 90.48.010). Within Auburn, Ecology’s manual
has no independent regulatory authority except where Ecology directly requires or issues permits.
The City of Auburn currently is regulated under a General Permit for Discharges from Municipal
Separate Storm Sewers, effective February 16, 2007. Under federal regulations, Auburn is required
to obtain coverage under this permit, and the permit is expected to require the adoption of stormwater
program components that are the substantial equivalent to the minimum requirements found in
Ecology’s 2005 stormwater manual for western Washington. Upon adoption, Auburn will use this
manual in issuing permits and other authorizations for development.
The Puget Sound Water Quality Management Plan
The current Puget Sound Water Quality Management Plan (the Plan), adopted in 2000 by the Puget
Sound Action Team (PSAT), is a voluntary plan that calls for every city and county in the Puget
Sound Basin to develop and implement a comprehensive stormwater management program. The
Plan recognizes that stormwater programs will vary among jurisdictions, depending on the
jurisdiction’s population, density, threats posed by stormwater, and results of watershed planning
efforts. Under the Plan, cities and counties are encouraged to form intergovernmental cooperative
agreements in order to pool resources and carry out program activities more efficiently. More
information about what the Plan contains can be found in Chapter 1 of Ecology’s Manual, and a
complete copy of the Plan can be downloaded from the PSAT website.
Phase II - Ecology’s NPDES and State Waste Discharge Stormwater Permits for
Municipalities
Auburn is subject to permitting under the U.S. Environmental Protection Agency (EPA) Phase II
Stormwater Regulations (40 CFR Part 122) under the Clean Water Act National Pollutant Discharge
Elimination System (NPDES) provisions. In Washington State, administration of the NPDES program
is delegated to the Department of Ecology. In Western Washington, Ecology has issued joint NPDES
and State Waste Discharge permits to regulate the discharges of stormwater from the municipal
separate storm sewer systems operated by small municipal permittees.
Requirements arising out of Auburn’s municipal stormwater permit are incorporated into this manual.
Ecology’s State Waste Discharge Permits for Direct Discharges
The requirements imposed under the Phase II EPA Stormwater Regulations apply to discharges to
Auburn’s municipal stormwater system. However, the regulations do not apply to “direct discharges,”
that is, discharges that do not enter the City’s system but go directly into receiving waters such as
creeks or rivers.
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Direct discharges are subject to permitting under Ecology’s State Waste Discharge Permit program
in Chapter 90.48 RCW.
Ecology’s Industrial Stormwater Permit (i.e. NPDES and State Waste Discharge
Baseline General Permit for Stormwater Discharges Associated With Industrial
Activities)
This is a statewide permit for facilities conducting industrial activities. Most industrial facilities that
discharge stormwater to a surface water body or to a municipal storm sewer system require permit
coverage. Existing and new facilities for private entities, state, and local governments are required to
have coverage. For a complete list of industrial categories identified for coverage, see Ecology’s
website or the permit itself. Ecology can also require permit coverage of any facility on a case-by-
case basis in order to protect waters of the state. As above, direct discharges from industrial activities
are subject to permitting under Ecology’s State Waste Discharge Permit program in Chapter 90.48
RCW.
Ecology’s Construction Stormwater Permit (i.e. NPDES and State Waste Discharge
General Permit for Stormwater Discharges Associated With Construction Activity)
Coverage under Ecology’s Construction General Permit is required for any clearing, grading, or
excavating that will disturb one or more acres of land area and that will discharge stormwater from
the site into surface water(s), or into storm drainage systems that discharge to a surface water. The
permit requires:
• Application of stabilization and structural practices to reduce the potential for erosion
and the discharge of sediments from the site. The stabilization and structural
practices cited in the permit are similar to the minimum requirements for
sedimentation and erosion control in Volume II of this manual.
• Construction sites within the Puget Sound basin to select from BMPs described in
Volume 2 of the most recent edition of Ecology’s Stormwater Management Manual
(SWMM) that has been available at least 120 days prior to the BMP selection.
If local government requirements for construction sites are at least as stringent as Ecology’s, Ecology
will accept compliance with the local requirements. Accordingly, projects subject to Auburn’s
permitting authority that are also required to obtain coverage under Ecology’s NPDES Construction
Permit should be designed in accordance with Auburn’s manual.
The permit is also required for projects or construction activities that disturb less than one acre of land
area, if the project or activity is part of a larger common plan of development or sale that will
ultimately disturb one or more acres of land area. The "common plan" in a common plan of
development or sale is broadly defined as any announcement or piece of documentation (including a
sign, public notice or hearing, sales pitch, advertisement, drawing, permit application, zoning request,
computer design, etc.) or physical demarcation (including boundary signs, lot stakes, surveyor
markings, etc.) indicating construction activities may occur on a specific plot.
The permit is not required for routine maintenance that is performed to maintain the original line and
grade, hydraulic capacity, or original purpose of the site. For example, re-grading a dirt road or
cleaning out a roadside drainage ditch to maintain its "as built" state does not require permit
coverage.
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Appendix A 43
Any construction activity discharging stormwater that Ecology and/or the City determine to be a
“significant contributor of pollutants” to waters of the state may also be required to apply for and
obtain permit coverage regardless of project size.
Applicants for coverage under the Construction General Permit must do the following:
• File a Notice of Intent (application for coverage). The permit application, called a
Notice of Intent (NOI), shall be submitted to Ecology before the date of the first public
notice and at least 38 days prior to the start of construction.
• Publish a Public Notice. At the time of application, the applicant must publish a notice
that they are seeking coverage under Ecology’s general stormwater permit for
construction activities. This notice must be published at least once each week for two
consecutive weeks in a single newspaper that has general circulation in the county in
which the construction is to take place. Refer to the NOI instructions for public notice
language requirements. State law requires a 30-day public comment period prior to
permit coverage; therefore, permit coverage will not be granted sooner than 31 days
after the date of the last public notice. Applicants who discharge surface water
associated with construction activity to a storm drain operated by the City of Auburn
are also required to submit a copy of the NOI to the municipality.
• Prepare a Construction Stormwater Pollution Prevention Plan. Permit coverage will
not be granted until the permittee has indicated completion of the SWPPP or certified
that development of a SWPPP in accordance with Special Condition S9 of the permit
will occur prior to the commencement of construction. The construction SWPPP
prepared using the City’s manual will satisfy both the Ecology permit and City of
Auburn permits.
Endangered Species Act
With the listing of multiple species of salmon as threatened or endangered across much of
Washington state, and the probability of more listings in the future, implementation of the
requirements of the Endangered Species Act will have a dramatic effect on urban stormwater
management. The manner in which that will occur is still evolving. Provisions of the Endangered
Species Act that may apply directly to stormwater management include the Section 4(d) rules,
Section 7 consultations, and Section 10 Habitat Conservation Plans (50 CFR).
Section 401 Water Quality Certifications
For projects that require a fill or dredge permit under Section 404 of the Clean Water Act, Ecology
must certify to the permitting agency, the U.S. Army Corps of Engineers, that the proposed project
will not violate state water quality standards. In order to make such a determination, Ecology may do
a more specific review of the potential impacts of a stormwater discharge from the construction
phase of the project and from the completed project. As a result of that review, Ecology may
condition its certification to require:
• Application of the minimum requirements and BMPs in Ecology’s manual; or
• Application of more stringent requirements.
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Hydraulic Project Approvals (HPAs)
Under Chapter 77.55 RCW, the Hydraulics Act, the Washington State Department of Fish and
Wildlife has the authority to require actions when stormwater discharges related to a project would
change the natural flow or bed of state waters. The implementing mechanism is the issuance of a
Hydraulic Project Approval (HPA) permit.
Aquatic Lands Use Authorizations
The Washington State Department of Natural Resources (DNR), as the steward of public aquatic
lands, may require a stormwater outfall to have a valid use authorization, and to avoid or mitigate
resource impacts under authority of Chapter 79.90 through 96 RCW, and in accordance with Chapter
332-30 WAC.
Requirements Identified through Watershed/Basin Planning or Total Maximum Daily
Loads
A number of the requirements of this manual can be superseded or modified by the adoption of
ordinances and rules to implement the recommendations of watershed plans or basin plans.
Requirements in this manual can also be superseded or added to through the adoption of specific
actions and requirements identified in a Waste Load Allocation or cleanup plan that implements a
Total Maximum Daily Load (TMDL) approved by the EPA.
Underground Injection Control Authorizations
Congress passed the Safe Drinking Water Act in 1974 and required the Environmental Protection
Agency (EPA) to create the Underground Injection Control (UIC) Program as on of the key programs
for protecting drinking water sources. The UIC program is administered under 40 CFR Part 144. In
1984, Ecology received the authority from EPA to regulate UIC wells and adopted the UIC rule,
Chapter 173-218 WAC. Ecology adopted revisions to Chapter 173-218 WAC rules on January 3,
2006 and the new rule went into affect on February 3, 2006.
The program requires:
• A non-endangerment performance standard be met, prohibiting injection that allows
the movement of fluids containing any contaminant into groundwater.
• All well owners must provide inventory information by registering their wells with
Ecology.
More information on the UIC program and how to register your well is available at:
http://www.ecy.wa.gov/programs/wq/grndwtr/uic/index.html.
It is the responsibility of applicants/owners to contact Ecology and determine if their facilities are
regulated under this program. If regulated, the applicant/owner is responsible to fulfill the program
requirements properly.
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Appendix A 45
Other City Requirements
The Planning, Building and Community Development Department is responsible for all land use
permitting activities, including permits for buildings, grading, paving, shoreline activities, critical areas,
short plats, formal subdivisions, etc.
Title 13 of the Auburn City Code (ACC) governs wastewater and surface water and gives the City its
authority to regulate water quality control of surface waters, the stormwater system, and the sanitary
sewer system. This Title also provides inspection authority, and enforcement authority for illegal
discharges to the stormwater system.
New development and redevelopment projects also may be subject to other city code requirements,
depending upon the nature and location of the project. These code requirements may include, but
are not limited to the subdivision and land use permit procedures in Titles 17 and 14 ACC; excavation
and grading in ACC Chapter 15.74; off-site improvements that include storm drainage in ACC
13.48.330; driveway control in Chapter 12.20 ACC; groundwater protection in Chapter 8.08 ACC;
shoreline regulation in Chapter 16.08 ACC; and critical areas protection in Chapter 16.10 ACC.
The City of Auburn’s Permit Center assists customers through every aspect of the permitting
process, from initial questions and pre-application meetings through inspections and final
certificate of occupancy. Applicants are encouraged to meet with City staff prior to plan
submittal. Contact the City’s Permit Counter at 253-591-5030 for more information.
Under the Growth Management Act, Chapter 36.70A RCW, the City has developed utilities and
capital facilities plans to help ensure the provision of adequate utilities, including storm drainage.
Depending upon the type of projects, new development and redevelopment may be required to
contribute to the construction of facilities necessary to accommodate impacts created by that
development.
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Appendix B 46
Appendix B Stormwater Site Plan Submittal
Requirements Checklist
The Submittal Requirements Checklist is intended to aid the design engineer in preparing a
Stormwater Site Plan. All items included in the following checklist must be addressed as part of any
stormwater site plan. The City recommends the design engineer follow the order and structure of the
checklist to facilitate review, which in turn will expedite permit issuance.
Chapter 1 – Project Overview
The project overview is intended to be a summary of detailed information contained in the body of the
Stormwater Site Plan.
Identify type of permit requested and permit number
Identify other permits required (e.g. hydraulic permits, Army Corps 404 permits, wetlands, etc.).
Identify the project location (including address, legal description, and parcel number).
Brief description of project to include the following:
Current and proposed condition/land-use
Size of parcel
Acreage developed, redeveloped, replaced or converted by the project
Current assessed value and cost of proposed improvements (for redevelopment projects)
Watershed
Proposed flow control improvements
Proposed runoff treatment improvements
Proposed conveyance improvements
Proposed discharge location and improvements
Downstream condition, impacts and problem
Locations of surface water run-on to the property
Reference appropriate Sections/Chapters/Appendices of the document for detailed
descriptions.
Chapter 2 – Existing Condition Summary
The Existing Condition Summary is intended to provide a complete understanding of the project site
and must be based on thorough site research and investigation.
Describe, discuss and identify the following for the project site:
Topography
Land use and ground cover
Natural and man-made drainage patterns
Points of entry and exit for existing drainage to and from the site
Any known historical drainage problems such as flooding, erosion, etc.
Existing utilities (storm, water, sewer)
Areas with high potential for erosion and sediment deposition
Locations of sensitive and critical areas (i.e. vegetative buffers, wetlands, steep slopes,
floodplains, geologic hazard areas, streams, creeks, ponds, ravines, springs, etc).
Existing fuel tanks
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Groundwater wells on-site and within 100 feet of site
Septic systems on-site and/or within 100 feet of the site
Identify difficult site conditions.
State whether the project is located in an aquifer recharge area or wellhead protection area as
defined by the Washington State Health Department, the Environmental Protection Agency or
by the City.
Identify any Superfund areas in the vicinity, and state whether they are tributary to, or receive
drainage from, the project site.
Identify any specific requirements included in a basin plan for the area.
Include references to relevant reports such as basin plans, flood studies, groundwater studies,
wetland designations, sensitive area designations, environmental impact statements,
environmental checklists, lake restoration plans, water quality reports, etc. Where such reports
impose additional conditions on the Proponent, state these conditions, and describe any
proposed mitigation measures.
Grading Plan per requirements.
A soil report to identify the following:
Soil types
Hydrologic soil group classification
Groundwater elevation
Presence of perched aquifers, acquitters and confined aquifers
Location of test pits
Infiltration rates determined per the requirements of Volume III (where applicable)
Discussion of critical areas or geologic hazards where present
Soil reports should be contained in an Appendix of the report or as a separate document.
Describe the 100-year flood hazard zone.
Chapter 3 – Off-Site Analysis (Minimum Requirement #10)
The City requires a qualitative discussion of the off-site upstream and downstream system for all
projects. The City may require a quantitative analysis for any project deemed to need additional
downstream information. Detailed calculations shall be contained in an Appendix of the report.
Volume I, Chapter 4 describes the Off-site Analysis. In addition, a list of elements to be included is
provided as follows.
Qualitative Analysis
Review all available plans, studies, maps pertaining to the off-site study area.
Investigate the drainage system ¼ mile downstream from the project by site visit, including the
following items:
Problems reported or observed during the resource review
Existing/potential constrictions or capacity deficiencies in the drainage system
Existing/potential flooding problems
Existing/potential overtopping, scouring, bank sloughing, or sedimentation
Significant destruction of aquatic habitat (e.g., siltation, stream incision)
Existing public and private easements through the project site and their corresponding
widths
Qualitative data on features such as land use, impervious surface, topography, soils,
presence of streams, and wetlands
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Information on pipe sizes, channel characteristics and drainage structures
Verification of tributary drainage areas
Date and weather at the time of the inspection
Describe the drainage system and its existing and predicted problems through observations,
reports, and hydraulic modeling (as necessary) of the City-specified design storm event
described in Chapter 3 of Volume III. Describe all existing or potential problems as listed above
(e.g. pooling water or erosion). The following information shall be provided for each existing or
potential problem:
Magnitude of or damage caused by the problem
General frequency and duration
Return frequency of storm or flow when the problem occurs (may require quantitative
analysis)
Water elevation when the problem occurs
Names and concerns of the parties involved
Current mitigation of the problem
Possible cause of the problem
Whether the project is likely to aggravate the problem or create a new one
Properly include off-site areas in drainage calculations.
Quantitative Analysis (see Volume III, Section 3.1.2)
Clearly describe tail water assumptions.
Summarize results in text.
Include calculations in Appendix B of the report.
Discuss potential fixes for capacity problems.
Provide profiles where appropriate.
Chapter 4 – Permanent Stormwater Control Plan
Chapter 4 will contain the information used to select, size and locate permanent stormwater control
BMPs for the project site.
Pre-Developed Site Hydrology
Provide a list of assumptions and site parameters for the pre-developed condition.
Identify all sub-basins within, or flowing through, the site. Use consistent labeling for all sub-
basins throughout figures, calculations, and text.
For each sub-basin, identify current land use, acreage, hydrologic soil group and land use to be
modeled under pre-developed conditions. The format used in Example Table I-B-1 show below
is recommended.
Provide justification for land uses other than forest.
The pre-developed soil types shall be assumed as either outwash (Hydrologic Soil Group A/B)
or till (Hydrologic Soil Group C/D) soils, depending on supporting geotechnical information.
Saturated soil conditions shall only be considered when determining existing wetland hydrology.
Summarize output data from the pre-developed condition. Example Tables I-B-2a or I-B-2b are
recommended formats.
Include completed Hydraulic Analysis worksheet (see Appendix C in this volume) and
hydrologic calculations in Appendix D of the report.
For WWHM models, provide model files electronically.
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Example Table I-B-1
Sub-
Basin ID
Land Use and
Cover Condition
Acreage Soil
Group
Modeled as:
(List CN)
Comments
Example Table I-B-2a
Pre-Developed Condition Event Output: SBUH
Basin ID:
Peak Flow (cfs) Volume (ac-ft) Area (ac)
2-year existing
10-year existing
25-year existing
100-year existing
Example Table I-B-2b
Pre-Developed Condition Event Output: WWHM
Basin ID:
Peak Flow (cfs) Area (ac)
2-year existing
10-year existing
25-year existing
100-year existing
Developed Site Hydrology
Provide a list of assumptions and site parameters for the developed condition.
Identify all sub-basins within, or flowing through, the site. Use consistent labeling for all sub-
basins throughout figures, calculations, and text.
For each sub-basin, identify current land use, acreage, hydrologic soil group and land use to be
modeled under developed conditions. The format used in Example Table I-B-1 is
recommended.
Summarize output data from the developed condition. The formats used in Example
Tables I-B-2a or I-B-2b are recommended.
Include completed Hydraulic Analysis worksheet (see Appendix C in this volume) and
hydrologic calculations in Appendix D of the report.
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Performance Goals and Standards
Indicate total acreage of impervious surfaces, pollution-generating impervious surfaces and
pollution-generating pervious surfaces for each Threshold Discharge Area (TDA). The format
used in Example Table I-B-3 is recommended.
Include applicable decision chart (Figure I-3-1, Figure I-3-2, or Figure I-3-3) with treatment
requirements clearly marked and supported.
Include applicable decision chart (Figure I-3-2) with flow control requirements clearly marked
and supported. If flow control facilities are required, indicate that they are required.
State conclusions from decision and flow charts.
Example Table I-B-3
Threshold Discharge Area ID:
Total pollution generating pervious surface (PGPS) acres
Total pollution generating impervious surface ((PGIS) acres
Native vegetation converted to lawn/landscape acres
Total effective impervious surface acres
Increase in 100-yer storm peak cfs
Flow Control System (where required)
Identify sizing system used.
Summarize model results.
Describe proposed flow control system and appurtenances, including size, type, and
characteristics of storage facility and control structure.
Provide a drawing of the flow control facility and its appurtenances, including:
Include Hydraulic Analysis Worksheet, calculations, and computer printouts (including stage
storage tables) for the flow control system to be included in Appendix D of the report.
Water Quality System (where required)
Identify the sizing method used.
Summarize model results.
Identify treatment methods used, including size, type, and characteristics of treatment facility
and appurtenances.
Provide a drawing of the treatment facility and its appurtenances, including:
Dimensions
Inlet/outlet sizes and elevations
Location of the facility on the project site
Appurtenances/fittings
Calculations for the water quality design storm and facility sizing calculations must be included in
an Appendix of the report.
Where appropriate, include manufacturer’s specifications in an Appendix of the report.
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Appendix B 51
Conveyance System Analysis and Design
Illustrate the proposed conveyance system on a project site plan.
Identify pipe sizes, types, and slopes.
Describe capacities, design flows, and velocities for each reach.
Include conveyance calculations in an Appendix of the report.
Chapter 5 – Discussion of Minimum Requirements
Chapter 5 is intended as a checklist for the applicant and reviewer to verify that the applicable
Minimum Requirements have been met within the project submittal.
Include applicable flowcharts for determining minimum requirements (Figure I-3-1, Figure I-3-2,
or Figure I-3-3) with decision path clearly marked.
List the minimum requirements that apply to the project.
Discuss how the project satisfies each minimum requirement.
Indicate where in the project documentation each minimum requirement is satisfied.
Chapter 6 – Operation and Maintenance Manual
The Operation and Maintenance Manual may be included in the Stormwater Site Plan, however it
shall be written with the intention of becoming a stand-alone document for the project owner once the
project is complete. The Operation and Maintenance Manual must include:
A narrative description of the on-site storm system.
An 11 x 17 inch map of the site, with the locations of the treatment/detention/infiltration/etc.
facilities prominently noted. This is needed to enable the Operation and Maintenance manual to
be a stand-alone document.
The person or organization responsible for maintenance of the on-site storm system, including
the phone number and current responsible party.
Where the Operation and Maintenance manual is to be kept. Note that it must be made
available to the City for inspection.
A description of each flow control and treatment facility, including what it does and how it works.
Include any manufacturer’s documentation.
A description of all maintenance tasks and the frequency of each task for each flow control and
treatment facility. Include any manufacturer’s recommendations.
A sample maintenance activity log indicating emergency and routine actions to be taken.
Chapter 7 – Construction Stormwater Pollution Prevention Plan
Short-Form – Please refer to Volume II, Appendix C for a complete checklist, or
Formal/Long-Form – Please refer to Volume II, Chapter 2 for a complete checklist.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Submittal Requirements Checklist Volume I
Appendix B 52
Appendices
Appendix A – Operations and Maintenance Manual
Appendix B – Construction Stormwater Pollution Prevention Plan
Appendix C – Submittal Requirements Checklist
Appendix D – Hydraulic Analysis Worksheet
Appendix E – Other reports, as required
Required Drawings
Project drawings shall be provided as required in Chapter 4, and shall include the following:
Vicinity Map
Site Map and Grading Plan
Basin Map
Storm Plan and Profile
Erosion Control Plan
Detail Sheets
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Hydraulic Analysis Volume I
Worksheet Appendix C 53
Appendix C Hydraulic Analysis Worksheet
Provide the following information for all projects, as applicable.
Name/Project: ___________________
Address
Parcel Number __________ __ Permit Number: ______ ______
Watershed: ____________
WWHM or Continuous Models Input
Model files must be provided electronically. Include both on-site and off-site quantities.
Amount of new impervious (square feet):
Amount of replaced impervious (square feet):
Amount of new plus replaced (square feet):
Amount of land disturbed (square feet):
Native vegetation to lawn/landscaped (acres):
Native vegetation to pasture (acres):
Value of proposed improvements ($):
Assessed value of existing site improvements ($):
Amount to be graded/filled (cubic feet):
Existing impervious:
Amount of new pgis (square feet):
Amount of existing pgis (square feet):
Amount of new pgs (square feet):
Amount of existing pgs (square feet):
SBUH Input
Rainfall Type:
Hydraulic Method:
Hydraulic Interval:
Peak Factor:
Tp Factor:
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Hydraulic Analysis Volume I
Worksheet Appendix C 54
Complete the following tables for sub-basins tributary to the project site (on-site and off-site).
Pre-Developed Conditions
Sub-basin Name Acreage Land Use/
Ground Cover*
Hydrologic Soil
Group*
Curve Number
* Where more than one land use or soil group are present within a sub-basin, a line item must be
shown for each to support calculation of the composite pervious and impervious Curve Numbers.
Developed Conditions
Sub-basin Name Acreage Land Use/
Ground Cover*
Hydrologic Soil
Group*
Curve Number
* Where more than one land use or soil group are present within a sub-basin, a line item must be
shown for each to support calculation of the composite pervious and impervious Curve Numbers.
Provide pervious and impervious Tc data for each sub-basin including the flow path shown on an
attached figure.
Flow Control Facilities
For the flow control facility, provide the following:
• Bottom length:
• Bottom width:
• Side slopes:
• Stage/ Storage Table with units:
For the control structure, provide the following:
• Outlet pipe size:
• Orifice elevation: Diameter:
• Orifice elevation: Diameter:
• Orifice elevation: Diameter:
• Riser elevation: Diameter:
• V-notch weir data (alternate):
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 55
Appendix D Maintenance Standards for
Drainage Facilities
The facility-specific maintenance standards contained in this section are intended to be conditions for
determining if maintenance actions are required as identified through inspection. They are not
intended to be measures of the facility's required condition at all times between inspections. In other
words, exceeding these conditions at any time between inspections and/or maintenance does not
automatically constitute a violation of these standards. However, based upon inspection
observations, the inspection and maintenance schedules shall be adjusted to minimize the length of
time that a facility is in a condition that requires a maintenance action.
Table I-D-1. Maintenance Standards
No. 1 – Detention Ponds
Maintenance
Component
Defect Conditions When Maintenance Is
Needed
Results Expected When Maintenance
Is Performed
Trash & Debris Any trash and debris which exceed 5
cubic feet per 1,000 square feet (this is
about equal to the amount of trash it
would take to fill up one standard size
garbage can). In general, there should
be no visual evidence of dumping.
If less than threshold all trash and
debris will be removed as part of next
scheduled maintenance.
Trash and debris cleared from site.
Poisonous
Vegetation and
noxious weeds
Any poisonous or nuisance vegetation
which may constitute a hazard to
maintenance personnel or the public.
Any evidence of noxious weeds as
defined by State or local regulations.
(Apply requirements of adopted IPM
policies for the use of herbicides).
No danger of poisonous vegetation
where maintenance personnel or the
public might normally be. (Coordinate
with local health department)
Complete eradication of noxious weeds
may not be possible. Compliance with
State or local eradication policies
required
Contaminants
and Pollution
Any evidence of oil, gasoline,
contaminants or other pollutants
(Coordinate removal/cleanup with local
water quality response agency).
No contaminants or pollutants present
General
Rodent Holes Any evidence of rodent holes if facility
is acting as a dam or berm, or any
evidence of water piping through dam
or berm via rodent holes.
Rodents destroyed and dam or berm
repaired. (Coordinate with local health
department; coordinate with Ecology
Dam Safety Office if pond exceeds 10
acre-feet.)
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 56
No. 1 – Detention Ponds
Maintenance
Component
Defect Conditions When Maintenance Is
Needed
Results Expected When Maintenance
Is Performed
Beaver Dams Dam results in change or function of
the facility.
Facility is returned to design function.
(Coordinate trapping of beavers and
removal of dams with appropriate
permitting agencies)
Insects When insects such as wasps and
hornets interfere with maintenance
activities.
Insects destroyed or removed from site.
Apply insecticides in compliance with
adopted IPM policies
General
Tree Growth
and Hazard
Trees
Tree growth does not allow
maintenance access or interferes
with maintenance activity (i.e., slope
mowing, silt removal, vactoring, or
equipment movements). If trees are
not interfering with access or
maintenance, do not remove
If trees are dead, diseased, or dying.
(Use a certified Arborist to determine
health of tree or removal
requirements)
Trees do not hinder maintenance
activities. Harvested trees should be
recycled into mulch or other beneficial
uses (e.g., alders for firewood).
Remove hazard trees
Side Slopes
of Pond
Erosion Eroded damage over 2 inches deep
where cause of damage is still
present or where there is potential for
continued erosion.
Any erosion observed on a
compacted berm embankment.
Slopes should be stabilized using
appropriate erosion control measure(s);
e.g., rock reinforcement, planting of
grass, compaction.
If erosion is occurring on compacted
berms a licensed civil engineer should
be consulted to resolve source of
erosion.
Sediment Accumulated sediment that exceeds
10% of the designed pond depth
unless otherwise specified or affects
inletting or outletting condition of the
facility.
Sediment cleaned out to designed pond
shape and depth; pond reseeded if
necessary to control erosion.
Storage Area
Liner (If
Applicable)
Liner is visible and has more than
three 1/4-inch holes in it.
Liner repaired or replaced. Liner is fully
covered.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 57
No. 1 – Detention Ponds
Maintenance
Component
Defect Conditions When Maintenance Is
Needed
Results Expected When Maintenance
Is Performed
Settlements Any part of berm which has settled 4
inches lower than the design
elevation.
If settlement is apparent, measure
berm to determine amount of
settlement.
Settling can be an indication of more
severe problems with the berm or
outlet works. A licensed civil engineer
should be consulted to determine the
source of the settlement.
Dike is built back to the design
elevation.
Pond Berms
(Dikes)
Piping Discernable water flow through pond
berm. Ongoing erosion with potential
for erosion to continue.
(Recommend a Geotechnical
engineer be called in to inspect and
evaluate condition and recommend
repair of condition.
Piping eliminated. Erosion potential
resolved.
Tree Growth Tree growth on emergency spillways
creates blockage problems and may
cause failure of the berm due to
uncontrolled overtopping.
Tree growth on berms over 4 feet in
height may lead to piping through the
berm which could lead to failure of
the berm.
Trees should be removed. If root
system is small (base less than 4
inches) the root system may be left in
place. Otherwise the roots should be
removed and the berm restored. A
licensed civil engineer should be
consulted for proper berm/spillway
restoration.
Emergency
Overflow/
Spillway and
Berms over 4
feet in height.
Piping Discernable water flow through pond
berm. Ongoing erosion with potential
for erosion to continue.
(Recommend a Geotechnical
engineer be called in to inspect and
evaluate condition and recommend
repair of condition.
Piping eliminated. Erosion potential
resolved.
Emergency
Overflow/
Spillway
Only one layer of rock exists above
native soil in area five square feet or
larger, or any exposure of native soil
at the top of out flow path of spillway.
(Rip-rap on inside slopes need not be
replaced.)
Rocks and pad depth are restored to
design standards.
Emergency
Overflow/
Spillway
Erosion See “Side Slopes of Pond”
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 58
No. 2 – Infiltration
Maintenance
Component
Defect Conditions When Maintenance
Is Needed
Results Expected When Maintenance
Is Performed
Trash & Debris See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1).
Poisonous/Noxious
Vegetation
See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1).
Contaminants and
Pollution
See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1).
General
Rodent Holes See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1)
Storage Area Sediment Water ponding in infiltration pond
after rainfall ceases and
appropriate time allowed for
infiltration.
(A percolation test pit or test of
facility indicates facility is only
working at 90% of its designed
capabilities. If two inches or
more sediment is present,
remove).
Sediment is removed and/or facility is
cleaned so that infiltration system works
according to design.
Filter Bags (if
applicable)
Filled with
Sediment and
Debris
Sediment and debris fill bag
more than 1/2 full.
Filter bag is replaced or system is
redesigned.
Rock Filters Sediment and
Debris
By visual inspection, little or no
water flows through filter during
heavy rain storms.
Gravel in rock filter is replaced.
Side Slopes of
Pond
Erosion See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1).
Tree Growth See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1). Emergency
Overflow
Spillway and
Berms over 4
feet in height.
Piping See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1).
Rock Missing See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1). Emergency
Overflow
Spillway Erosion See "Detention Ponds" (No. 1). See "Detention Ponds" (No. 1).
Pre-settling
Ponds and
Vaults
Facility or sump
filled with sediment
and/or debris
6" or designed sediment trap
depth of sediment.
Sediment is removed.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 59
No. 3 – Closed Detention Systems (Tanks/Vaults)
Maintenance
Component
Defect Conditions When Maintenance is Needed Results Expected When
Maintenance is Performed
Plugged Air Vents One-half of the cross section of a vent is
blocked at any point or the vent is
damaged.
Vents open and functioning.
Debris and
Sediment
Accumulated sediment depth exceeds
10% of the diameter of the storage area
for 1/2 length of storage vault or any
point depth exceeds 15% of diameter.
(Example: 72-inch storage tank would
require cleaning when sediment reaches
depth of 7 inches for more than 1/2
length of tank.)
All sediment and debris removed
from storage area.
Joints Between
Tank/Pipe Section
Any openings or voids allowing material
to be transported into facility.
(Will require engineering analysis to
determine structural stability).
All joint between tank/pipe
sections are sealed.
Tank Pipe Bent
Out of Shape
Any part of tank/pipe is bent out of shape
more than 10% of its design shape.
(Review required by engineer to
determine structural stability).
Tank/pipe repaired or replaced to
design.
Cracks wider than 1/2-inch and any
evidence of soil particles entering the
structure through the cracks, or
maintenance/inspection personnel
determines that the vault is not
structurally sound.
Vault replaced or repaired to
design specifications and is
structurally sound.
Storage
Area
Vault Structure
Includes Cracks in
Wall, Bottom,
Damage to Frame
and/or Top Slab
Cracks wider than 1/2-inch at the joint of
any inlet/outlet pipe or any evidence of
soil particles entering the vault through
the walls.
No cracks more than 1/4-inch
wide at the joint of the inlet/outlet
pipe.
Cover Not in
Place
Cover is missing or only partially in
place. Any open manhole requires
maintenance.
Manhole is closed.
Locking
Mechanism Not
Working
Mechanism cannot be opened by one
maintenance person with proper tools.
Bolts into frame have less than 1/2 inch
of thread (may not apply to self-locking
lids).
Mechanism opens with proper
tools.
Cover Difficult to
Remove
One maintenance person cannot remove
lid after applying normal lifting pressure.
Intent is to keep cover from sealing off
access to maintenance.
Cover can be removed and
reinstalled by one maintenance
person.
Manhole
Ladder Rungs
Unsafe
Ladder is unsafe due to missing rungs,
misalignment, not securely attached to
structure wall, rust, or cracks.
Ladder meets design standards.
Allows maintenance person safe
access.
Catch
Basins
See “Catch
Basins”
(No. 5)
See “Catch Basins” (No. 5). See “Catch Basins” (No. 5).
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 60
No. 4 – Control Structure/Flow Restrictor
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance
is Performed
Trash and
Debris (Includes
Sediment)
Material exceeds 25% of sump
depth or 1 foot below orifice plate.
Control structure orifice is not blocked.
All trash and debris removed.
Structure is not securely attached
to manhole wall.
Structure securely attached to wall and
outlet pipe.
Structure is not in upright position
(allow up to 10% from plumb).
Structure in correct position.
Connections to outlet pipe are not
watertight and show signs of rust.
Connections to outlet pipe are water
tight; structure repaired or replaced and
works as designed.
General
Structural
Damage
Any holes--other than designed
holes--in the structure.
Structure has no holes other than
designed holes.
Cleanout gate is not watertight, is
missing, or is left open.
Gate is watertight, works as designed,
and is left closed.
Gate cannot be moved up and
down by one maintenance person.
Gate moves up and down easily and is
watertight.
Chain/rod leading to gate is
missing or damaged.
Chain is in place and works as
designed.
Cleanout
Gate
Damaged or
Missing
Gate is rusted over 50% of its
surface area.
Gate is repaired or replaced to meet
design standards.
Damaged or
Missing
Control device is not working
properly due to missing, out of
place, or bent orifice plate.
Plate is in place and works as designed. Orifice Plate
Obstructions Any trash, debris, sediment, or
vegetation blocking the plate.
Plate is free of all obstructions and
works as designed.
Overflow Pipe Obstructions Any trash or debris blocking (or
having the potential of blocking) the
overflow pipe.
Pipe is free of all obstructions and
works as designed.
Manhole See “Closed
Detention
Systems” (No.
3).
See “Closed Detention Systems”
(No. 3).
See “Closed Detention Systems” (No.
3).
Catch Basin See “Catch
Basins”
(No. 5).
See “Catch Basins” (No. 5). See “Catch Basins” (No. 5).
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 61
No. 5 – Catch Basins
Maintenance
Component
Defect Conditions When Maintenance is Needed Results Expected When
Maintenance is performed
Trash or debris which is located
immediately in front of the catch basin
opening or is blocking inletting capacity
of the basin by more than 10%.
No Trash or debris located
immediately in front of catch
basin or on grate opening.
Trash or debris (in the basin) that
exceeds 60 percent of the sump depth
as measured from the bottom of basin to
invert of the lowest pipe into or out of the
basin, but in no case less than a
minimum of six inches clearance from
the debris surface to the invert of the
lowest pipe.
No trash or debris in the catch
basin.
Trash or debris in any inlet or outlet pipe
blocking more than 1/3 of its height.
Inlet and outlet pipes free of
trash or debris.
Trash & Debris
Dead animals or vegetation that could
generate odors that could cause
complaints or dangerous gases (e.g.,
methane).
No dead animals or vegetation
present within the catch basin.
Sediment Sediment (in the basin) that exceeds 60
percent of the sump depth as measured
from the bottom of basin to invert of the
lowest pipe into or out of the basin, but in
no case less than a minimum of 6 inches
clearance from the sediment surface to
the invert of the lowest pipe.
No sediment in the catch basin
Top slab has holes larger than 2 square
inches or cracks wider than 1/4 inch
(Intent is to make sure no material is
running into basin).
Top slab is free of holes and
cracks.
Structure Damage
to Frame and/or
Top Slab
Frame not sitting flush on top slab, i.e.,
separation of more than 3/4 inch of the
frame from the top slab. Frame not
securely attached
Frame is sitting flush on the
riser rings or top slab and firmly
attached.
Maintenance person judges that
structure is unsound.
Basin replaced or repaired to
design standards.
Fractures or
Cracks in Basin
Walls/ Bottom Grout fillet has separated or cracked
wider than 1/2 inch and longer than 1
foot at the joint of any inlet/outlet pipe or
any evidence of soil particles entering
catch basin through cracks.
Pipe is regrouted and secure at
basin wall.
Settlement/
Misalignment
If failure of basin has created a safety,
function, or design problem.
Basin replaced or repaired to
design standards.
Vegetation growing across and blocking
more than 10% of the basin opening.
No vegetation blocking opening
to basin.
General
Vegetation
Vegetation growing in inlet/outlet pipe
joints that is more than six inches tall and
less than six inches apart.
No vegetation or root growth
present.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 62
No. 5 – Catch Basins (continued)
Maintenance
Component
Defect Conditions When Maintenance is
Needed
Results Expected When Maintenance is
performed
General Contamination
and Pollution
See "Detention Ponds" (No. 1). No pollution present.
Cover Not in
Place
Cover is missing or only partially in
place. Any open catch basin
requires maintenance.
Catch basin cover is closed
Locking
Mechanism Not
Working
Mechanism cannot be opened by
one maintenance person with
proper tools. Bolts into frame have
less than 1/2 inch of thread.
Mechanism opens with proper tools.
Catch Basin
Cover
Cover Difficult
to Remove
One maintenance person cannot
remove lid after applying normal
lifting pressure.
(Intent is keep cover from sealing
off access to maintenance.)
Cover can be removed by one
maintenance person.
Ladder Ladder Rungs
Unsafe
Ladder is unsafe due to missing
rungs, not securely attached to
basin wall, misalignment, rust,
cracks, or sharp edges.
Ladder meets design standards and
allows maintenance person safe
access.
Grate opening
Unsafe
Grate with opening wider than 7/8
inch.
Grate opening meets design
standards.
Trash and
Debris
Trash and debris that is blocking
more than 20% of grate surface
inletting capacity.
Grate free of trash and debris.
Metal Grates
(If Applicable)
Damaged or
Missing.
Grate missing or broken member(s)
of the grate.
Grate is in place and meets design
standards.
No. 6 – Debris Barriers (e.g., Trash Racks)
Maintenance
Components
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance is
Performed
General Trash and
Debris
Trash or debris that is plugging
more than 20% of the openings in
the barrier.
Barrier cleared to design flow capacity.
Bars are bent out of shape more
than 3 inches.
Bars in place with no bends more than
3/4 inch.
Bars are missing or entire barrier
missing.
Bars in place according to design.
Damaged/
Missing Bars.
Bars are loose and rust is causing
50% deterioration to any part of
barrier.
Barrier replaced or repaired to design
standards.
Metal
Inlet/Outlet Pipe Debris barrier missing or not
attached to pipe
Barrier firmly attached to pipe
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 63
No. 7 – Energy Dissipaters
Maintenance
Components
Defect Conditions When Maintenance is Needed Results Expected When
Maintenance is Performed
External:
Missing or Moved
Rock
Only one layer of rock exists above
native soil in area five square feet or
larger, or any exposure of native soil.
Rock pad replaced to design
standards.
Rock Pad
Erosion Soil erosion in or adjacent to rock pad. Rock pad replaced to design
standards.
Pipe Plugged with
Sediment
Accumulated sediment that exceeds
20% of the design depth.
Pipe cleaned/flushed so that it
matches design.
Not Discharging
Water Properly
Visual evidence of water discharging at
concentrated points along trench (normal
condition is a “sheet flow” of water along
trench). Intent is to prevent erosion
damage.
Trench redesigned or rebuilt to
standards.
Perforations
Plugged.
Over 1/2 of perforations in pipe are
plugged with debris and sediment.
Perforated pipe cleaned or
replaced.
Water Flows Out
Top of
“Distributor” Catch
Basin.
Maintenance person observes or
receives credible report of water flowing
out during any storm less than the design
storm or its causing or appears likely to
cause damage.
Facility rebuilt or redesigned to
standards.
Dispersion
Trench
Receiving Area
Over-Saturated
Water in receiving area is causing or has
potential of causing landslide problems.
No danger of landslides.
Internal:
Worn or Damaged
Post, Baffles, Side
of Chamber
Structure dissipating flow deteriorates to
1/2 of original size or any concentrated
worn spot exceeding one square foot
which would make structure unsound.
Structure replaced to design
standards.
Manhole/Ch
amber
Other Defects See “Catch Basins” (No. 5). See “Catch Basins” (No. 5).
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 64
No. 8 – Typical Biofiltration Swale
Maintenance
Component
Defect or Problem Condition When
Maintenance is Needed
Recommended Maintenance to Correct Problem
Sediment
Accumulation on
Grass
Sediment depth
exceeds 2 inches.
Remove sediment deposits on grass treatment area
of the bio-swale. When finished, swale should be
level from side to side and drain freely toward outlet.
There should be no areas of standing water once
inflow has ceased.
Standing Water When water stands in
the swale between
storms and does not
drain freely.
Any of the following may apply: remove sediment or
trash blockages, improve grade from head to foot of
swale, remove clogged check dams, add
underdrains or convert to a wet biofiltration swale.
Flow spreader Flow spreader uneven
or clogged so that
flows are not uniformly
distributed through
entire swale width.
Level the spreader and clean so that flows are
spread evenly over entire swale width.
Constant
Baseflow
When small quantities
of water continually
flow through the swale,
even when it has been
dry for weeks, and an
eroded, muddy
channel has formed in
the swale bottom.
Add a low-flow pea-gravel drain the length of the
swale or by-pass the baseflow around the swale.
Poor Vegetation
Coverage
When grass is sparse
or bare or eroded
patches occur in more
than 10% of the swale
bottom.
Determine why grass growth is poor and correct that
condition. Re-plant with plugs of grass from the
upper slope: plant in the swale bottom at 8-inch
intervals. Or re-seed into loosened, fertile soil.
Vegetation When the grass
becomes excessively
tall (greater than 10-
inches); when
nuisance weeds and
other vegetation starts
to take over.
Mow vegetation or remove nuisance vegetation so
that flow not impeded. Grass should be mowed to a
height of 3 to 4 inches. Remove grass clippings.
Excessive
Shading
Grass growth is poor
because sunlight does
not reach swale.
If possible, trim back over-hanging limbs and
remove brushy vegetation on adjacent slopes.
Inlet/Outlet Inlet/outlet areas
clogged with sediment
and/or debris.
Remove material so that there is no clogging or
blockage in the inlet and outlet area.
Trash and Debris
Accumulation
Trash and debris
accumulated in the
bio-swale.
Remove trash and debris from bioswale.
General
Erosion/Scouring Eroded or scoured
swale bottom due to
flow channelization, or
higher flows.
For ruts or bare areas less than 12 inches wide,
repair the damaged area by filling with crushed
gravel. If bare areas are large, generally greater
than 12 inches wide, the swale should be re-graded
and re-seeded. For smaller bare areas, overseed
when bare spots are evident, or take plugs of grass
from the upper slope and plant in the swale bottom
at 8-inch intervals.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 65
No. 9 – Wet Biofiltration Swale
Maintenance
Component
Defect or Problem Condition When Maintenance is
Needed
Recommended Maintenance to Correct
Problem
Sediment
Accumulation
Sediment depth exceeds 2-
inches in 10% of the swale
treatment area.
Remove sediment deposits in
treatment area.
Water Depth
Water not retained to a depth of
about 4 inches during the wet
season.
Build up or repair outlet berm so that
water is retained in the wet swale.
Wetland
Vegetation
Vegetation becomes sparse
and does not provide adequate
filtration, OR vegetation is
crowded out by very dense
clumps of cattail, which do not
allow water to flow through the
clumps.
Determine cause of lack of vigor of
vegetation and correct. Replant as
needed. For excessive cattail growth,
cut cattail shoots back and compost
off-site. Note: normally wetland
vegetation does not need to be
harvested unless die-back is causing
oxygen depletion in downstream
waters.
Inlet/Outlet Inlet/outlet area clogged with
sediment and/or debris.
Remove clogging or blockage in the
inlet and outlet areas.
Trash and
Debris
Accumulation
See "Detention Ponds" (No. 1). Remove trash and debris from wet
swale.
General
Erosion/Scouring Swale has eroded or scoured
due to flow channelization, or
higher flows.
Check design flows to assure swale is
large enough to handle flows. By-pass
excess flows or enlarge swale. Replant
eroded areas with fibrous-rooted
plants such as Juncus effusus (soft
rush) in wet areas or snowberry
(Symphoricarpos albus) in dryer areas.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 66
No. 10 – Filter Strips
Maintenance
Component
Defect or Problem Condition When
Maintenance is Needed
Recommended Maintenance to Correct
Problem
Sediment
Accumulation on
Grass
Sediment depth exceeds 2
inches.
Remove sediment deposits, re-level so
slope is even and flows pass evenly through
strip.
Vegetation When the grass becomes
excessively tall (greater
than 10-inches); when
nuisance weeds and other
vegetation starts to take
over.
Mow grass, control nuisance vegetation,
such that flow not impeded. Grass should be
mowed to a height between 3-4 inches.
Trash and Debris
Accumulation
Trash and debris
accumulated on the filter
strip.
Remove trash and Debris from filter.
Erosion/Scouring Eroded or scoured areas
due to flow channelization,
or higher flows.
For ruts or bare areas less than 12 inches
wide, repair the damaged area by filling with
crushed gravel. The grass will creep in over
the rock in time. If bare areas are large,
generally greater than 12 inches wide, the
filter strip should be re-graded and re-
seeded. For smaller bare areas, overseed
when bare spots are evident.
General
Flow spreader Flow spreader uneven or
clogged so that flows are
not uniformly distributed
through entire filter width.
Level the spreader and clean so that flows
are spread evenly over entire filter width.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 67
No. 11 – Wetponds
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Water level
First cell is empty, does not hold
water.
Line the first cell to maintain at least 4 feet of
water. Although the second cell may drain, the
first cell must remain full to control turbulence of
the incoming flow and reduce sediment
resuspension.
Trash and
Debris
Accumulation that exceeds 1 CF
per 1000-SF of pond area.
Trash and debris removed from pond.
Inlet/Outlet
Pipe
Inlet/Outlet pipe clogged with
sediment and/or debris material.
No clogging or blockage in the inlet and outlet
piping.
Sediment
Accumulati
on in Pond
Bottom
Sediment accumulations in pond
bottom that exceeds the depth of
sediment zone plus 6-inches,
usually in the first cell.
Sediment removed from pond bottom.
Oil Sheen
on Water
Prevalent and visible oil sheen. Oil removed from water using oil-absorbent
pads or vactor truck. Source of oil located and
corrected. If chronic low levels of oil persist,
plant wetland plants such as Juncus effusus
(soft rush) which can uptake small
concentrations of oil.
Erosion Erosion of the pond’s side slopes
and/or scouring of the pond
bottom, that exceeds 6-inches, or
where continued erosion is
prevalent.
Slopes stabilized using proper erosion control
measures and repair methods.
Settlement
of Pond
Dike/Berm
Any part of these components
that has settled 4-inches or lower
than the design elevation, or
inspector determines dike/berm
is unsound.
Dike/berm is repaired to specifications.
Internal
Berm
Berm dividing cells should be
level.
Berm surface is leveled so that water flows
evenly over entire length of berm.
General
Overflow
Spillway
Rock is missing and soil is
exposed at top of spillway or
outside slope.
Rocks replaced to specifications.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 68
No. 12 – Wetvaults
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Trash/Debris
Accumulation
Trash and debris accumulated in
vault, pipe or inlet/outlet
(includes floatables and non-
floatables).
Remove trash and debris from vault.
Sediment
Accumulation in
Vault
Sediment accumulation in vault
bottom exceeds the depth of the
sediment zone plus 6-inches.
Remove sediment from vault.
Damaged Pipes Inlet/outlet piping damaged or
broken and in need of repair.
Pipe repaired and/or replaced.
Access Cover
Damaged/Not
Working
Cover cannot be opened or
removed, especially by one
person.
Pipe repaired or replaced to proper
working specifications.
Ventilation Ventilation area blocked or
plugged.
Blocking material removed or cleared from
ventilation area. A specified % of the vault
surface area must provide ventilation to
the vault interior (see design
specifications).
Maintenance/inspection
personnel determine that the
vault is not structurally sound.
Vault replaced or repairs made so that
vault meets design specifications and is
structurally sound.
Vault Structure
Damage -
Includes Cracks
in Walls Bottom,
Damage to
Frame and/or
Top Slab
Cracks wider than 1/2-inch at the
joint of any inlet/outlet pipe or
evidence of soil particles entering
through the cracks.
Vault repaired so that no cracks exist
wider than 1/4-inch at the joint of the
inlet/outlet pipe.
Baffles Baffles corroding, cracking,
warping and/or showing signs of
failure as determined by
maintenance/inspection staff.
Baffles repaired or replaced to
specifications.
General
Access Ladder
Damage
Ladder is corroded or
deteriorated, not functioning
properly, not attached to
structure wall, missing rungs, has
cracks and/or misaligned.
Confined space warning sign
missing.
Ladder replaced or repaired to
specifications, and is safe to use as
determined by inspection personnel.
Replace sign warning of confined space
entry requirements. Ladder and entry
notification complies with OSHA
standards.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 69
No. 13 – Sand Filters (above ground/open)
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Sediment
Accumulatio
n on top
layer
Sediment depth exceeds 1/2-
inch.
No sediment deposit on grass
layer of sand filter that would
impede permeability of the filter
section.
Trash and
Debris
Accumulatio
ns
Trash and debris accumulated
on sand filter bed.
Trash and debris removed from
sand filter bed.
Sediment/
Debris in
Clean-Outs
When the clean-outs become
full or partially plugged with
sediment and/or debris.
Sediment removed from clean-
outs.
Sand Filter
Media
Drawdown of water through the
sand filter media takes longer
than 24-hours, and/or flow
through the overflow pipes
occurs frequently.
Top several inches of sand are
scraped. May require
replacement of entire sand filter
depth depending on extent of
plugging (a sieve analysis is
helpful to determine if the lower
sand has too high a proportion of
fine material).
Prolonged
Flows
Sand is saturated for prolonged
periods of time (several weeks)
and does not dry out between
storms due to continuous base
flow or prolonged flows from
detention facilities.
Low, continuous flows are limited
to a small portion of the facility by
using a low wooden divider or
slightly depressed sand surface.
Short
Circuiting
When flows become
concentrated over one section
of the sand filter rather than
dispersed.
Flow and percolation of water
through sand filter is uniform and
dispersed across the entire filter
area.
Erosion
Damage to
Slopes
Erosion over 2-inches deep
where cause of damage is
prevalent or potential for
continued erosion is evident.
Slopes stabilized using proper
erosion control measures.
Rock Pad
Missing or
Out of Place
Soil beneath the rock is visible. Rock pad replaced or rebuilt to
design specifications.
Flow
Spreader
Flow spreader uneven or
clogged so that flows are not
uniformly distributed across
sand filter.
Spreader leveled and cleaned so
that flows are spread evenly over
sand filter.
Above Ground
(open sand
filter)
Damaged
Pipes
Any part of the piping that is
crushed or deformed more than
20% or any other failure to the
piping.
Pipe repaired or replaced.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 70
No. 14 –Sand Filters (below ground/enclosed)
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When
Maintenance is Performed
Sediment
Accumulation on
Sand Media
Section
Sediment depth exceeds 1/2-inch. No sediment deposits on sand filter
section that which would impede
permeability of the filter section.
Sediment
Accumulation in
Pre-Settling
Portion of Vault
Sediment accumulation in vault bottom
exceeds the depth of the sediment zone
plus 6-inches.
No sediment deposits in first
chamber of vault.
Trash/Debris
Accumulation
Trash and debris accumulated in vault,
or pipe inlet/outlet, floatables and non-
floatables.
Trash and debris removed from
vault and inlet/outlet piping.
Sediment in Drain
Pipes/Cleanouts
When drain pipes, cleanouts become
full with sediment and/or debris.
Sediment and debris removed.
Short Circuiting When seepage/flow occurs along the
vault walls and corners. Sand eroding
near inflow area.
Sand filter media section re-laid
and compacted along perimeter of
vault to form a semi-seal. Erosion
protection added to dissipate force
of incoming flow and curtail
erosion.
Damaged Pipes Inlet or outlet piping damaged or broken
and in need of repair.
Pipe repaired and/or replaced.
Access Cover
Damaged/Not
Working
Cover cannot be opened,
corrosion/deformation of cover.
Maintenance person cannot remove
cover using normal lifting pressure.
Cover repaired to proper working
specifications or replaced.
Ventilation Ventilation area blocked or plugged Blocking material removed or
cleared from ventilation area. A
specified % of the vault surface
area must provide ventilation to the
vault interior (see design
specifications).
Cracks wider than 1/2-inch or evidence
of soil particles entering the structure
through the cracks, or
maintenance/inspection personnel
determine that the vault is not
structurally sound.
Vault replaced or repairs made so
that vault meets design
specifications and is structurally
sound.
Vault Structure
Damaged;
Includes Cracks
in Walls, Bottom,
Damage to Frame
and/or Top Slab.
Cracks wider than 1/2-inch at the joint
of any inlet/outlet pipe or evidence of
soil particles entering through the
cracks.
Vault repaired so that no cracks
exist wider than 1/4-inch at the
joint of the inlet/outlet pipe.
Baffles/Internal
walls
Baffles or walls corroding, cracking,
warping and/or showing signs of failure
as determined by
maintenance/inspection person.
Baffles repaired or replaced to
specifications.
Below Ground
Vault.
Access Ladder
Damaged
Ladder is corroded or deteriorated, not
functioning properly, not securely
attached to structure wall, missing
rungs, cracks, and misaligned
Ladder replaced or repaired to
specifications, and is safe to use
as determined by inspection
personnel.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 71
No. 15 – STORMFILTERTM
Maintenance
Component
Defect Condition When Maintenance is Needed Results Expected When
Maintenance is Performed
Sediment
Accumulation on
Media.
Sediment depth exceeds 0.25-inches. No sediment deposits which
would impede permeability of
the compost media.
Sediment
Accumulation in
Vault
Sediment depth exceeds 6-inches in first
chamber.
No sediment deposits in vault
bottom of first chamber.
Trash/Debris
Accumulation
Trash and debris accumulated on
compost filter bed.
Trash and debris removed from
the compost filter bed.
Sediment in
Drain
Pipes/Clean-
Outs
When drain pipes, clean-outs, become
full with sediment and/or debris.
Sediment and debris removed.
Damaged Pipes Any part of the pipes that are crushed or
damaged due to corrosion and/or
settlement.
Pipe repaired and/or replaced.
Access Cover
Damaged/Not
Working
Cover cannot be opened; one person
cannot open the cover using normal
lifting pressure, corrosion/deformation of
cover.
Cover repaired to proper
working specifications or
replaced.
Cracks wider than 1/2-inch or evidence
of soil particles entering the structure
through the cracks, or
maintenance/inspection personnel
determine that the vault is not structurally
sound.
Vault replaced or repairs made
so that vault meets design
specifications and is structurally
sound.
Vault Structure
Includes Cracks
in Wall, Bottom,
Damage to
Frame and/or
Top Slab
Cracks wider than 1/2-inch at the joint of
any inlet/outlet pipe or evidence of soil
particles entering through the cracks.
Vault repaired so that no cracks
exist wider than 1/4-inch at the
joint of the inlet/outlet pipe.
Baffles Baffles corroding, cracking warping,
and/or showing signs of failure as
determined by maintenance/inspection
person.
Baffles repaired or replaced to
specifications.
Below Ground
Vault
Access Ladder
Damaged
Ladder is corroded or deteriorated, not
functioning properly, not securely
attached to structure wall, missing rungs,
cracks, and misaligned.
Ladder replaced or repaired and
meets specifications, and is
safe to use as determined by
inspection personnel.
Media Drawdown of water through the media
takes longer than 1 hour, and/or overflow
occurs frequently.
Media cartridges replaced. Below Ground
Cartridge Type
Short Circuiting Flows do not properly enter filter
cartridges.
Filter cartridges replaced.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 72
No. 16 – Baffle Oil/Water Separators (API Type)
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance
is Performed
Monitoring Inspection of discharge water for
obvious signs of poor water
quality.
Effluent discharge from vault should
be clear with out thick visible sheen.
Sediment
Accumulation
Sediment depth in bottom of vault
exceeds 6-inches in depth.
No sediment deposits on vault
bottom that would impede flow
through the vault and reduce
separation efficiency.
Trash and Debris
Accumulation
Trash and debris accumulation in
vault, or pipe inlet/outlet,
floatables and non-floatables.
Trash and debris removed from
vault, and inlet/outlet piping.
Oil Accumulation Oil accumulations that exceed 1-
inch, at the surface of the water.
Extract oil from vault by vactoring.
Disposal in accordance with state
and local rules and regulations.
Damaged Pipes Inlet or outlet piping damaged or
broken and in need of repair.
Pipe repaired or replaced.
Access Cover
Damaged/Not
Working
Cover cannot be opened,
corrosion/deformation of cover.
Cover repaired to proper working
specifications or replaced.
See “Catch Basins” (No. 5)
Vault replaced or repairs made so
that vault meets design
specifications and is structurally
sound.
Vault Structure
Damage - Includes
Cracks in Walls
Bottom, Damage to
Frame and/or Top
Slab Cracks wider than 1/2-inch at the
joint of any inlet/outlet pipe or
evidence of soil particles entering
through the cracks.
Vault repaired so that no cracks
exist wider than 1/4-inch at the joint
of the inlet/outlet pipe.
Baffles Baffles corroding, cracking,
warping and/or showing signs of
failure as determined by
maintenance/inspection person.
Baffles repaired or replaced to
specifications.
General
Access Ladder
Damaged
Ladder is corroded or
deteriorated, not functioning
properly, not securely attached to
structure wall, missing rungs,
cracks, and misaligned.
Ladder replaced or repaired and
meets specifications, and is safe to
use as determined by inspection
personnel.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 73
No. 17 – Coalescing Plate Oil/Water Separators
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance
is Performed
Monitoring Inspection of discharge water for
obvious signs of poor water
quality.
Effluent discharge from vault
should be clear with no thick visible
sheen.
Sediment
Accumulation
Sediment depth in bottom of vault
exceeds 6-inches in depth and/or
visible signs of sediment on
plates.
No sediment deposits on vault
bottom and plate media, which
would impede flow through the
vault and reduce separation
efficiency.
Trash and Debris
Accumulation
Trash and debris accumulated in
vault, or pipe inlet/outlet,
floatables and non-floatables.
Trash and debris removed from
vault, and inlet/outlet piping.
Oil Accumulation Oil accumulation that exceeds 1-
inch at the water surface.
Oil is extracted from vault using
vactoring methods. Coalescing
plates are cleaned by thoroughly
rinsing and flushing. Should be no
visible oil depth on water.
Damaged
Coalescing Plates
Plate media broken, deformed,
cracked, and/or showing signs of
failure.
A portion of the media pack or the
entire plate pack is replaced
depending on severity of failure.
Damaged Pipes Inlet or outlet piping damaged or
broken and in need of repair.
Pipe repaired and or replaced.
Baffles Baffles corroding, cracking,
warping and/or showing signs of
failure as determined by
maintenance/inspection person.
Baffles repaired or replaced to
specifications.
Cracks wider than 1/2-inch or
evidence of soil particles entering
the structure through the cracks,
or maintenance/inspection
personnel determine that the vault
is not structurally sound.
Vault replaced or repairs made so
that vault meets design
specifications and is structurally
sound.
Vault Structure
Damage -
Includes Cracks in
Walls, Bottom,
Damage to Frame
and/or Top Slab
Cracks wider than 1/2-inch at the
joint of any inlet/outlet pipe or
evidence of soil particles entering
through the cracks.
Vault repaired so that no cracks
exist wider than 1/4-inch at the joint
of the inlet/outlet pipe.
General
Access Ladder
Damaged
Ladder is corroded or
deteriorated, not functioning
properly, not securely attached to
structure wall, missing rungs,
cracks, and misaligned.
Ladder replaced or repaired and
meets specifications, and is safe to
use as determined by inspection
personnel.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 74
No. 18 – Catchbasin Inserts
Maintenance
Component
Defect Conditions When Maintenance is Needed Results Expected When
Maintenance is Performed
Sediment
Accumulation
When sediment forms a cap over the
insert media of the insert and/or unit.
No sediment cap on the insert
media and its unit.
Trash and
Debris
Accumulation
Trash and debris accumulates on insert
unit creating a blockage/restriction.
Trash and debris removed
from insert unit. Runoff freely
flows into catch basin.
Media Insert Not
Removing Oil
Effluent water from media insert has a
visible sheen.
Effluent water from media
insert is free of oils and has no
visible sheen.
Media Insert
Water Saturated
Catch basin insert is saturated with water
and no longer has the capacity to
absorb.
Remove and replace media
insert
Media Insert-Oil
Saturated
Media oil saturated due to petroleum spill
that drains into catch basin.
Remove and replace media
insert.
General
Media Insert Use
Beyond Normal
Product Life
Media has been used beyond the typical
average life of media insert product.
Remove and replace media at
regular intervals, depending on
insert product.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 75
No. 19 – Ecology Embankment
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Erosion, scour, or
vehicular damage
No vegetation zone uneven or
clogged so that flows are not
uniformly distributed
Level the area and clean so that flows
are spread evenly
No Vegetation
Zone adjacent
to pavement
Sediment
accumulation on
edge of pavement
Flows no longer sheeting off of
roadway. Sediment accumulation
on pavement edge exceeds top of
pavement elevation.
Remove sediment deposits such that
flows can sheet off of roadway.
Sediment
accumulation on
grass
Sediment depth exceeds two
inches
Remove sediment deposits, re-level
so slope is even and flows pass
evenly through Ecology Embankment.
Excessive vegetation
or undesirable
species
When grass becomes excessively
tall; when nuisance weeds and
other vegetation starts to take
over or shades out desirable
vegetation growth characteristics.
See also Pierce County Noxious
Weeds list at:
piercecountyweedboard.wsu.edu/
weedlist.html
Mow grass, control nuisance
vegetation such that flow is not
impeded. Grass should be mowed to a
height that encourages dense, even
herbaceous growth.
Vegetated
Filter
Erosion, scour, or
vehicular damage
Eroded or scoured areas due to
flow channelization, high flows, or
vehicular damage.
For ruts or bare areas less than 12
inches wide, repair the damaged area
by filling with suitable topsoil. The
grass will creep in over the rock in
time. If bare areas are large, generally
greater than 12 inches wide, the filter
strip should be re-graded and re-
seeded. For smaller bare areas,
overseed when bare spots are
evident.
Erosion, scour, or
vehicular damage
Eroded or scoured areas due to
flow channelization, high flows, or
vehicular damage.
For ruts or bare areas less than 12
inches wide, repair the damaged area
by filling with suitable media. If bare
areas are large, generally greater than
12 inches wide, the media bed should
be re-graded.
Media Bed
Sediment
accumulation on
media bed
Sediment depth inhibits free
infiltration of water
Remove sediment deposits, re-level
so slope is even and flows pass freely
through the media bed.
Underdrains Sediment Depth of sediment within
perforated pipe exceeds one-half
inch
Flush underdrains through access
ports and collect flushed sediment.
General Trash and debris
accumulation
Trash and debris which exceed 5
cubic feet per 1,000 square feet
(this is about equal to the amount
of trash it would take to fill up one
32-gallow garbage can). In
general, there should be no visual
evidence of dumping. If less than
threshold, all trash and debris will
be removed as part of the next
scheduled maintenance
Remove trash and debris.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 76
No. 19 – Ecology Embankment
Maintenance
Component
Defect Condition When Maintenance is
Needed
Results Expected When Maintenance is
Performed
General Flows are bypassing
Ecology
Embankment
Evidence of significant flows
downslope (rills, sediment,
vegetation damage, etc.) of
Ecology Embankment
Remove sediment deposits, relevel so
slope is even and flows pass evenly
through Ecology Embankment. If
Ecology Embankment is completely
clogged, it may require more extensive
repair or replacement.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Maintenance Standards for Volume I
Drainage Facilities Appendix D 77
No. 20 – Bioretention Rain Gardens
Maintenance
Component
Defect Conditions When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Cracks or failure
in concrete
planter reservoir
Cracks wider than ½ inch or
maintenance/inspection personnel
determine that the vault is not
structurally sound
Vault repaired or replaced so that it
meets design specification and is
structurally sound
Erosion (gullies/rills) greater than 2
inches around inlets, outlet, and
along side slopes
Eliminate source of erosion and stabilize
damaged area (regrade, rock,
vegetation, erosion control blanket)
Settlement greater than 4 inches
(relative to undisturbed sections of
the berm)
Restore to design height
Downstream face of the berm or
embankment wet, seeps or leaks
evident
Plug holes. Contact geotechnical
engineer ASAP.
Failure in
earthen reservoir
(embankments,
dikes, berms,
and side slopes)
Any evidence of rodent holes or
water piping around holes if facility
acts as a dam or berm
Eradicate rodents and repair holes (fill
and compact)
Sediment or
debris
accumulation
Accumulation of sediment or debris Remove excess sediment or debris.
Identify and control the sediment
source, if feasible. Facility should be
free of material. May contain standing
water.
Rockery
reservoir or walls
Rock walls are insecure Stabilize walls
Basin inlet via
surface flow
Soil is exposed or signs of erosion
are visible.
Repair and control erosion sources.
Basin inlet via
concentrated
flow (e.g. curb
cuts)
Sediment, vegetation, or debris
partially or fully blocking inlet
structure
Clear the blockage. Identify the source
of the blockage and take actions to
prevent future blockages.
Water splashes adjacent buildings Basin inlet
splash block
failure Water disrupts soil media.
Reconfigure/repair blocks.
Pipe is damaged. Repair/replace pipe. Inlet/outlet pipe
failure Pipe is clogged. Remove roots or debris.
Outlet pipe/
structure failure
Sediment, vegetation, or debris is
partially or fully blocking the outlet
structure.
Clear the blockage. Identify the source
of the blockage and take actions to
prevent future blockages.
Trash or debris present on trash
rack.
Clean and dispose of trash. Trash rack
failure
Bar screen damaged or missing. Replace bar screen.
Ponding Area
Check dams and
weirs failures
Sediment, vegetation, or debris is
partially or fully blocking the check
dam or weir.
Clear the blockage. Identify the source
of the blockage and take actions to
prevent future blockages.
Ponding Area Check dams and
weirs failures
Erosion and/or undercutting is
present.
Repair and take preventative measures
to prevent future erosion and/or
undercutting.
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Drainage Facilities Appendix D 78
No. 20 – Bioretention Rain Gardens
Maintenance
Component
Defect Conditions When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Sediment blocks 35% or more of
ports/notches or, sediment fills 35%
or more of sediment trap.
Remove and dispose of sediment. Flow spreader
problems
Grade board/baffle damaged or not
level.
Remove and reinstall to level position.
Overflow spillway is partially or fully
plugged with sediment or debris.
Remove and dispose of sediment.
Native soil is exposed, or other
signs of erosion are present.
Repair erosion and stabilize surface of
spillway.
Overflow/
emergency
spillway
Spillway armament is missing. Replace armament.
Bioretention soil Water remains in the basin 48 hours
or longer after the end of a storm.
Ensure that underdrain (if present) is not
clogged. If necessary, clear underdrain.
If this is not the problem, the
bioretention soil is likely clogged.
Remove the upper 2 to 3 inches of soil
and replace with imported bioretention
soil. Identify sources of clogging and
correct.
Bottom swale
vegetation
Less than 80% of swale bottom is
covered with healthy wetland
vegetation.
Upland slope
vegetation
Less than 70% of upland slopes are
covered with healthy vegetation.
Plant additional vegetation. Ideally,
planting should be performed in the fall
or winter.
Large trees and shrubs interfere
with operation of the basin or
access for maintenance
Prune or remove large trees and
shrubs.
Trees and
shrubs
Standing dead vegetation is present Remove standing dead vegetation when
covering greater than 10% of the basin
area. Replace dead vegetation annually
or immediately if necessary to control
erosion (e.g. on a steep slope).
Mulch Bare spots (without much cover) are
present or mulch covers less than 3
inches deep for compost or 4 inches
deep for coarse, woody mulch.
Replenish with the appropriate type of
mulch to cover bare spots and augment
to minimum depth.
Vegetation
Clippings Grass or other vegetation clippings
accumulate to 2 inches or greater in
depth.
Remove clippings.
Noxious weeds Listed noxious vegetation is present.
See Pierce County noxious weed
list.
By law, noxious weeds must be
removed and disposed immediately.
Herbicides and pesticides shall not be
used in order to protect water quality.
Vegetation
Weeds Weeds are present (unless on edge
and providing erosion control).
Remove and dispose of weed material.
Herbicides and pesticides shall not be
used in order to protect water quality.
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Drainage Facilities Appendix D 79
No. 20 – Bioretention Rain Gardens
Maintenance
Component
Defect Conditions When Maintenance is
Needed
Results Expected When Maintenance is
Performed
Irrigation system
(if any)
Irrigation system present Follow manufacturer’s instructions for
O&M
Plant establishment period (1-3
years)
Water weekly during periods of no rain
to ensure plant establishment
Irrigation
Plant watering
Longer term period (3+ years) Water during drought conditions or more
often if necessary to maintain plant
cover.
Spill prevention Storage or use of potential
contaminants in the vicinity of the
facility.
Exercise spill prevention measures
whenever handling or storing potential
contaminants.
Spill Prevention
and Response
Spill response Release of pollutants. Call to report
any spill to the Washington Dept. of
Emergency Management
1-800-258-5990
Cleanup spills as soon as possible to
prevent contamination of stormwater.
Training and
Documentation
Training/written
guidance
Training/written guidance is required
for proper O&M
Provide property owners and tenants
with proper training and a copy of the
O&M manual and Landscape and
Maintenance Manual.
Safety (slopes) Erosion of sides causes slope to
exceed 1:4 or otherwise become a
hazard.
Take actions to eliminate the hazard.
Safety (hydraulic
structures)
Hydraulic structures (pipes, culverts,
vaults, etc.) become a hazard to
children playing in and around the
facility.
Take actions to eliminate the hazard
(such as covering and securing any
openings).
Safety
Line of sight Vegetation causes some visibility
(line of sight) or driver safety issues.
Prune.
Aesthetics Damage/vandalism/debris
accumulation
Restore facility to original aesthetic
conditions.
Grass/vegetation Less than 75% of planted vegetation
is healthy with a generally good
appearance.
Take appropriate maintenance actions
(e.g. remove/replace plants, amend soil,
etc.)
Aesthetics
Edging Grass is starting to encroach on
swale.
Repair edging.
Mosquitoes Standing water remains in the basin
for more than three days following
storms.
Identify the cause of the standing water
and take appropriate actions to address
the problem (see Bioretention Soil
above)
Pest Control
Rodents Rodent holes are present near the
facility.
Fill and compact the soil around the
holes (refer to Integrated Pest
Management).
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No. 21 - Cistern
Maintenance
Component
Defect Conditions When Maintenance Is Needed Results Expected When
Maintenance Is Performed
Roof Debris has accumulated Remove debris
Gutter Debris has accumulated. Clean gutters (the most
critical cleaning is mid- to
late-spring to flush the
pollen deposits from
surrounding trees).
Screen has deteriorated. Replace Screens at the top
of the downspout
and cistern inlet Preventative maintenance Clear screen of any
accumulated debris.
Low flow orifice Preventative maintenance. Clean low flow orifice.
Pipe is damaged. Repair/replace Overflow pipe
Pipe is clogged. Remove debris.
Collection
Facilities
Cistern Debris has accumulated in the bottom of
the tank.
Remove debris.
Training and
Documentation
Training/written
guidance
Training/written guidance is required for
proper O&M.
Provide property owners
and tenants with proper
training and a copy of the
O&M manual.
Safety Access and safety Access to cistern required for maintenance
or cleaning.
Any cistern detention
systems opening that
could allow the entry of
people must be marked:
“DANGER – CONFINED
SPACE”.
Pest Control Mosquitoes Standing water remains for more than three
days following storms.
Ensure cause of standing
water is corrected. Also
ensure all inlets,
overflows, and other
openings are protected
with mosquito screens.
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Drainage Facilities Appendix D 81
No. 22 – Compost Amended Soil
Maintenance
Component
Defect Conditions When Maintenance
Is Needed
Results Expected When Maintenance Is
Performed
Vegetation not fully covering
ground surface
Re-mulch landscape beds with 2-3 inches of
mulch until the vegetation fully closes over the
ground surface
Return leaf fall and shredded woody materials
from the landscape to the site as mulch.
On turf areas, “grasscycle” ((mulch-mow or
leave the clippings) to build turf health.
Avoid broadcast use of pesticides (bug and
weed killers) like “weed & feed”, which
damage the soil life.
Soil media
(maintain high
organic soil
content) Preventative maintenance
Where fertilization is needed (mainly turf and
annual flower beds), use a moderate
fertilization program that relies on natural
organic fertilizers (like compost) or slow-
release synthetic balanced fertilizers.
Compaction Soils become waterlogged, do
not appear to be infiltrating.
To remediate, aerate soil, till or further amend
soil. If drainage is still slow, consider
investigating alternative causes (e.g. high wet-
season groundwater levels, low-permeability
soils). Also consider land use and protection
from compacting activities. If areas are turf,
aerate compacted areas and top dress them
with ¼ to ½ inch of compost to renovate them.
General
Facility
Requirements
Erosion/scourin
g
Areas of potential erosion are
visible.
Take steps to repair or prevent erosion.
Identify and address the causes of erosion.
Grass/vegetatio
n
Less than 75% of planted
vegetation is healthy with a
generally good appearance.
Take appropriate maintenance actions (e.g.
remove/replace plants).
Noxious weeds Listed noxious vegetation is
present. See Pierce County
noxious weed list.
By law, noxious weeds must be removed and
disposed immediately. Herbicides and
pesticides shall not be used in order to protect
water quality.
General
Facility
Requirements
Weeds Weeds are present. Remove and dispose of weed material.
Herbicides and pesticides shall not be used in
order to protect water quality.
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Drainage Facilities Appendix D 82
No. 23 – Vegetated Roof
Maintenance
Component
Defect Conditions When Maintenance
Is Needed
Results Expected When Maintenance Is
Performed
Growth medium Water does not permeate
growth media (runs off soil
surface)
Aerate or replace media
Fallen
leaves/debris
Fallen leaves or debris are
present.
Remove/dispose
Soil/Growth
Medium
Erosion/scouring Areas of potential erosion are
visible.
Take steps to repair or prevent erosion.
Stabilize with additional soil substrate/growth
medium and additional plants.
General Structural components are
present.
Inspect structural components for deterioration
or failure. Repair/replace as necessary.
Sediment, vegetation, or
debris blocks 35% or more of
inlet structure
Clear blockage. Identify and correct any
problems that led to blockage.
Inlet pipe is in poor condition. Repair/replace.
System
Structural
Components Inlet pipe
Inlet pipe is clogged. Remove roots or debris.
Coverage Vegetative coverage falls
below 75% (unless design
specifications stipulate less
than 75% coverage).
Install more vegetation.
Noxious weeds Listed noxious vegetation is
present. See Pierce County
noxious weed list.
By law, noxious weeds must be removed and
disposed immediately. Herbicides and
pesticides shall not be used in order to protect
water quality.
Weeds Weeds are present. Remove and dispose of weed material.
Herbicides and pesticides shall not be used in
order to protect water quality.
Vegetation
Plants Dead vegetation is present. Remove dead vegetation when covering
greater than 10% of basin area. Replace dead
vegetation annually or immediately if
necessary to control erosion.
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No. 23 – Vegetated Roof
Maintenance
Component
Defect Conditions When Maintenance
Is Needed
Results Expected When Maintenance Is
Performed
Irrigation system
(if any)
Irrigation system present Follow manufacturer’s instructions for O&M.
Plant establishment period
(1-3 years)
Water weekly during periods of no rain to
ensure plant establishment
Irrigation
Plant watering
Longer term period (3+
years)
Water during drought conditions or more often
if necessary to maintain plant cover.
Spill prevention Storage or use of potential
contaminants in the vicinity of
the facility.
Exercise spill prevention measures whenever
handling or storing potential contaminants.
Spill Prevention
and Response
Spill response Release of pollutants. Call to
report any spill to the
Washington Dept. of
Emergency Management
1-800-258-5990
Cleanup spills as soon as possible to prevent
contamination of stormwater.
Training and
Documentation
Training/written
guidance
Training/written guidance is
required for proper O&M.
Provide property owners and tenants with
proper training and a copy of the O&M manual
and Landscape and Maintenance manual.
Safety Access and
Safety
Egress and ingress routes Maintain egress and ingress routes to design
standards and fire codes.
Aesthetics Damage/vandalism/debris
accumulation
Restore facility to original aesthetic conditions. Aesthetics
Grass/vegetation Less than 75% of planted
vegetation is healthy with a
generally good appearance.
Take appropriate maintenance actions (e.g.
remove/replace plants, amend soils, etc.)
Pest Control Mosquitoes Standing water remains for
more than three days
following a storm.
Remove standing water. Identify the cause of
the standing water and take appropriate action
to address the problem (improve drainage).
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Drainage Facilities Appendix D 84
No. 24 – Pervious Pavement
Maintenance
Component
Defect Conditions When Maintenance Is
Needed
Results Expected When Maintenance
Is Performed
Use conventional street sweepers
equipped with vacuums.
Maintenance to prevent clogging with
fine sediment.
Prohibit use of sand and sealant
application and protect from
construction runoff.
Major cracks or trip hazards Fill with patching mixes. Large cracks
and settlement may require cutting
and replacing the pavement section.
Pervious asphalt
or cement
concrete
Utility cuts Any damage or change due to utility
cuts must be replaced in kind.
Fallen
leaves/debris
Fallen leaves or debris Remove/dispose
Interlocking paving block missing or
damaged.
Replace paver block
Settlement of surface May require resettling
Sediment or debris accumulation
between paver blocks
Remove/dispose
Loss of void material between paver
blocks
Refill per manufacturer’s
recommendations.
Surface
Interlocking
concrete paver
blocks
Varied conditions Perform O&M per manufacturer’s
recommendations.
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Appendix E Wetlands and Stormwater Management
Guidelines
As Amended from Chapter 14 of “Wetlands and Urbanization, Implications for the Future,” by Richard
R. Horner, Amanda A. Azous, Klaus O. Richter, Sarah S. Cooke, Lorin E. Reinelt and Kern Ewing
If you are unfamiliar with these guidelines, read the description of the approach and organization that
follows. If you are familiar, proceed directly to the appropriate guide sheet(s) for guidelines covering
your issue(s) or objective(s):
Guide Sheet 1: Comprehensive Landscape Planning for Wetlands and Stormwater
Management
Guide Sheet 2: Wetlands Protection Guidelines
Approach and Organization of the Management Guidelines
Introduction
The Puget Sound Wetlands and Stormwater Management Research Program performed
comprehensive research with the goal of deriving strategies that protect wetland resources in urban
and urbanizing areas, while also benefiting the management of urban stormwater runoff that can
affect those resources. The research primarily involved long-term comparisons of wetland ecosystem
characteristics before and after their watersheds urbanized, and between a set of wetlands that
became affected by urbanization (treatment sites) and a set whose watersheds did not change
(control sites). This work was supplemented by shorter term and more intensive studies of pollutant
transport and fate in wetlands, several laboratory experiments, and ongoing review of relevant work
being performed elsewhere. These research efforts were aimed at defining the types of impacts that
urbanization can cause and the degree to which they develop under different conditions, in order to
identify means of avoiding or minimizing impacts that impair wetland structure and functioning. The
program's scope embraced both situations where urban drainage incidentally affects wetlands in its
path, as well as those in which direct stormwater management actions change wetlands' hydrology,
water quality or both.
This document presents preliminary management guidelines for urban wetlands and their stormwater
discharges based on the research results. The set of guidelines is the principal vehicle to implement
the research findings in environmental planning and management practice.
Guidelines Scope and Underlying Principles.
NOTE: For terms in boldface type see item 1 under Support Materials.
1. These provisions currently have the status of guidelines rather than requirements.
Application of these guidelines does not fulfill assessment and permitting requirements that
may be associated with a project. It is, in general, necessary to follow the stipulations of the
State Environmental Policy Act and to contact such agencies as the local planning agency;
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the Washington Departments of Ecology, Fisheries, and Wildlife; the U. S. Environmental
Protection Agency; and the U. S. Army Corps of Engineers.
2. Using the guidelines should be approached from a problem-solving viewpoint. The
“problem” is regarded to be accomplishing one or more particular planning or management
objectives involving a wetland potentially or presently affected by stormwater drainage from
an urban or urbanizing area. The objectives can be broad, specific, or both. Broad
objectives involve comprehensive planning and subsequent management of a drainage
catchment or other landscape unit containing one or more wetlands. Specific objectives
pertain to managing a wetland having particular attributes to be sustained. Of course, the
prospect for success is greater with ability to manage the whole landscape influencing the
wetland, rather than just the wetland itself.
3. The guidelines are framed from the standpoint that some change in the landscape has the
potential to modify the physical and chemical structure of the wetland environment, which
in turn could alter biological communities and the wetland’s ecological functions. The
general objective in this framework would be to avoid or minimize negative ecological
change. This view is in contrast to one in which a wetland has at some time in the past
experienced negative change, and consequent ecological degradation, and where the
general objective would be to recover some or all of the lost structure and functioning
through enhancement or restoration actions. Direct attention to this problem was outside
the scope of the Puget Sound Wetlands and Stormwater Management Research Program.
However, the guidelines do give information that applies to enhancement and restoration.
For example, attempted restoration of a diverse amphibian community would not be
successful if the water level fluctuation limits consistent with high amphibian species
richness are not observed.
4. The guidelines can be applied with whatever information concerning the problem is
available. Of course, the comprehensiveness and certainty of the outcome will vary with the
amount and quality of information employed. The guidelines can be applied in an iterative
fashion to improve management understanding as the information improves. Wetlands
Guidance Appendix 1 lists the information needed to perform basic analyses, followed by
other information that can improve the understanding and analysis.
5. These guidelines emphasize avoiding structural, hydrologic, and water quality
modifications of existing wetlands to the extent possible in the process of urbanization and
the management of urban stormwater runoff.
6. In pursuit of this goal, the guidelines take a systematic approach to management problems
that potentially involve both urban stormwater (quantity, quality, or both) and wetlands. The
consideration of wetlands involves their area extent, values, and functions. This approach
emphasizes a comprehensive analysis of alternatives to solve the identified problem. The
guidelines encourage conducting the analysis on a landscape scale and considering all of
the possible stormwater management alternatives, which may or may not involve a wetland.
They favor source control best management practices (BMPs) and pre-treatment of
stormwater runoff prior to release to wetlands.
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7. Furthermore, the guidelines take a holistic view of managing wetland resources in an urban
setting. Thus, they recognize that urban wetlands have the potential to be affected
structurally and functionally whether or not they are formally designated for stormwater
management purposes. Even if an urban wetland is not structurally or hydrologically
engineered for such purposes, it may experience altered hydrology (more or less water),
reduced water quality, and a host of other impacts related to urban conditions. It is the
objective of the guidelines to avoid or reduce the negative effects on wetland resources from
both specific stormwater management actions and incidental urban impacts.
Support Materials
1. The guidelines use certain terms that require definition to ensure that the intended meaning
is conveyed to all users. Such terms are printed in boldface the first time that they appear in
each guide sheet, and are defined in Wetlands Guidance Appendix B.
2. The guideline provisions were drawn principally from the available results of the Puget
Sound Wetlands and Stormwater Management Research Program, as set forth in Sections
2 and 3 of the program’s summary publication, Wetlands and Urbanization, Implications for
the Future (Horner et al. 1996). Where the results in this publication are the basis for a
numerical provision, a separate reference is not given. Numerical provisions based on other
sources are referenced. See Wetlands Guidance References at the end of this appendix.
3. Appendix 3 presents a list of plant species native to wetlands in the Puget Sound Region.
This appendix is intended for reference by guideline users who are not specialists in wetland
botany. However, non-specialists should obtain expert advice when making decisions
involving vegetation.
4. Appendix 4 compares the water chemistry characteristics of Sphagnum bog and fen
wetlands (termed priority peat wetlands in these guidelines) with more common wetland
communities. These bogs and fens appear to be the most sensitive among the Puget Sound
lowland wetlands to alteration of water chemistry, and require special water quality
management to avoid losses of their relatively rare communities.
Guide Sheet 1: Comprehensive Landscape Planning for Wetlands and
Stormwater Management
Wetlands in newly developing areas will receive urban effects even if not specifically "used" in
stormwater management. Therefore, the task is proper overall management of the resources and
protection of their general functioning, including their role in storm drainage systems. Stormwater
management in newly developing areas is distinguished from management in already developed
locations by the existence of many more feasible stormwater control options prior to development.
The guidelines emphasize appropriate selection among the options to achieve optimum overall
resource protection benefits, extending to downstream receiving waters and ground water aquifers,
as well as to wetlands.
The comprehensive planning guidelines are based on two principles that are recognized to create the
most effective environmental management: (1) the best management policies for the protection of
wetlands and other natural resources are those that prevent or minimize the development of impacts
at potential sources; and (2) the best management strategies are self-perpetuating, that is they do not
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require periodic infusions of capital and labor. To apply these principles in managing wetlands in a
newly developing area, carry out the following steps.
Guide Sheet 1A: Comprehensive Planning Steps
1. Define the landscape unit subject to comprehensive planning. Refer to the definition of
landscape unit in Appendix 2 for assistance in defining it.
2. Begin the development of a plan for the landscape unit with attention to the following
general principles:
• Formulate the plan on the basis of clearly articulated community goals. Carefully
identify conflicts and choices between retaining and protecting desired resources and
community growth.
• Map and assess land suitability for urban uses. Include the following landscape
features in the assessment: forested land, open unforested land, steep slopes,
erosion-prone soils, foundation suitability, soil suitability for waste disposal, aquifers,
aquifer recharge areas, wetlands, floodplains, surface waters, agricultural lands, and
various categories of urban land use. When appropriate, the assessment can
highlight outstanding local or regional resources that the community determines
should be protected (e. g., a fish run, scenic area, recreational area, threatened
species habitat, farmland). Mapping and assessment should recognize not only
these resources but also additional areas needed for their sustenance.
3. Maximize natural water storage and infiltration opportunities within the landscape unit and
outside of existing wetlands, especially:
• Promote the conservation of forest cover. Building on land that is already deforested
affects basin hydrology to a lesser extent than converting forested land. Loss of forest
cover reduces interception storage, detention in the organic forest floor layer, and water
losses by evapotranspiration, resulting in large peak runoff increases and either their
negative effects or the expense of countering them with structural solutions.
• Maintain natural storage reservoirs and drainage corridors, including depressions, areas
of permeable soils, swales, and intermittent streams. Develop and implement policies
and regulations to discourage the clearing, filling, and channelization of these features.
Utilize them in drainage networks in preference to pipes, culverts, and engineered
ditches.
• In evaluating infiltration opportunities refer to the stormwater management manual for
the jurisdiction and pay particular attention to the selection criteria for avoiding
groundwater contamination and poor soils and hydrogeological conditions that cause
these facilities to fail. If necessary, locate developments with large amounts of
impervious surfaces or a potential to produce relatively contaminated runoff away from
groundwater recharge areas. Relatively dense developments on glacial outwash soils
may require additional runoff treatment to protect groundwater quality.
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4. Establish and maintain buffers surrounding wetlands and in riparian zones as required by
local regulations or recommended by the Puget Sound Water Quality Authority's wetland
guidelines. Also, maintain interconnections among wetlands and other natural habitats to
allow for wildlife movements.
5. Determine whether the wetland has a breeding, native amphibian population. A survey
should be conducted in the spring.
6. Take specific management measures to avoid general urban impacts on wetlands and other
water bodies (e. g., littering, vegetation destruction, human and pet intrusion harmful to
wildlife).
7. To support management of runoff water quantity, perform a hydrologic analysis of the
contributing drainage catchment to define the type and extent of flooding and stream
channel erosion problems associated with existing development, redevelopment, or new
development that require control to protect the beneficial uses of receiving waters, including
wetlands. This analysis should include assembly of existing flow data and hydrologic
modeling as necessary to establish conditions limiting to attainment of beneficial uses.
Modeling should be performed as directed by the stormwater management manual in effect
in the jurisdiction.
8. In wetlands previously relatively unaffected by human activities, manage stormwater
quantity to attempt to match the pre-development hydroperiod and hydrodynamics. In
wetlands whose hydrology has been disturbed, consider ways of reducing hydrologic
impacts. This provision involves not only management of high runoff volumes and rates of
flow during the wet season, but also prevention of water supply depletion during the dry
season. The latter guideline may require flow augmentation if urbanization reduces existing
surface or groundwater inflows. Refer to Guide Sheet 2, Wetland Protection Guidelines, for
detail on implementing these guidelines.
9. Assess alternatives for the control of runoff water quantities as follows:
a. Define the runoff quantity problem subject to management by analyzing the proposed
land development action.
b. For existing development or redevelopment, assess possible alternative solutions that
are applicable at the site of the problem occurrence, including:
– Protect health, safety, and property from flooding by removing habitation from the
flood plain.
– Prevent stream channel erosion by stabilizing the eroding bed and/or bank area with
bioengineering techniques, preferably, or by structurally reinforcing it, if this solution
would be consistent with the protection of aquatic habitats and beneficial uses of the
stream (refer to Chapter 173-201A of the Washington Administrative Code (WAC) for
the definition of beneficial uses).
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c. For new development or redevelopment, assess possible regulatory and incentive land
use control alternatives, such as density controls, clearing limits, impervious surface
limits, transfer of development rights, purchase of conservation areas, etc.
d. If the alternatives considered in Steps 9a or 9b cannot solve an existing or potential
problem, perform an analysis of the contributing drainage catchment to assess possible
alternative solutions that can be applied on-site or on a regional scale. The most
appropriate solution or combination of alternatives should be selected with regard to the
specific opportunities and constraints existing in the drainage catchment. For new
development or redevelopment, on-site facilities that should be assessed include, in
approximate order of preference:
– Infiltration basins or trenches;
– Retention/detention ponds;
– Below-ground vault or tank storage;
– Parking lot detention.
Regional facilities that should be assessed for solving problems associated with new
development, redevelopment, or existing development include:
– Infiltration basins or trenches;
– Detention ponds;
– Constructed wetlands;
– Bypassing a portion of the flow to an acceptable receiving water body, with treatment
as required to protect water quality and other special precautions as necessary to
prevent downstream impacts.
e. Consider structurally or hydrologically engineering an existing wetland for water quantity
control only if upland alternatives are inadequate to solve the existing or potential
problem. To evaluate the possibility, refer to the Storm-water Wetland Assessment
Criteria in Guide Sheet 1B.
10. Place strong emphasis on water resource protection during construction of new
development. Establish effective erosion control programs to reduce the sediment loadings
to receiving waters to the maximum extent possible. No preexisting wetland or other water
body should ever be used for the sedimentation of solids in construction-phase runoff.
11. In wetlands previously relatively unaffected by human activities, manage stormwater quality
to attempt to match pre-development water quality conditions. To support management of
runoff water quality, perform an analysis of the contributing drainage catchment to define the
type and extent of runoff water quality problems associated with existing development,
redevelopment, or new development that require control to protect the beneficial uses of
receiving waters, including wetlands. This analysis should incorporate the hydrologic
assessment performed under step 7 and include identification of key water pollutants, which
may include solids, oxygen-demanding substances, nutrients, metals, oils, trace organics,
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and bacteria, and evaluation of the potential effects of water pollutants throughout the
drainage system.
12. Assess alternatives for the control of runoff water quality as follows:
a. Perform an analysis of the contributing drainage catchment to assess possible
alternative solutions that can be applied on-site or on a regional scale. The most
appropriate solution or combination of alternatives should be selected with regard to the
specific opportunities and constraints existing in the drainage catchment. Consider both
source control BMPs and treatment BMPs as alternative solutions before considering
use of existing wetlands for quality improvement according to the following
considerations:
– Implementation of source control BMPs prevent the generation or release of
water pollutants at potential sources. These alternatives are generally both more
effective and less expensive than treatment controls. They should be applied to
the maximum extent possible to new development, redevelopment, and existing
development.
– Treatment BMPs capture water pollutants after their release. This alternative
often has limited application in existing developments because of space
limitations, although it can be employed in new development and when
redevelopment occurs in already developed areas. Refer to Minimum
Requirement #6 in Volume 1 of the Stormwater Management Manual for Western
Washington to determine whether a treatment facility is necessary for your site. If
a facility is required, refer to Chapter 4 of Volume I, or Chapter 2 of Volume V to
determine which treatment requirement – basic, enhanced, phosphorus, or oil
control - applies to your site. Then refer to the corresponding BMP menu for that
requirement in Chapter 3 of Volume V. From the menu select a BMP that fits with
your project site.
b. Consider structurally or hydrologically engineering an existing wetland for water quality
control only if upland alternatives are inadequate to solve the existing or potential
problem. Use of Waters of the State and Waters of the United States, including
wetlands, for the treatment or conveyance of wastewater, including stormwater, is
prohibited under state and federal law. Discussions with federal and state regulators
during the research program led to development of a statement concerning the use of
existing wetlands for improving stormwater quality (polishing), as follows. Such use is
subject to analysis on a case-by-case basis and may be allowed only if the following
conditions are met:
– If restoration or enhancement of a previously degraded wetland is required,
and if the upgrading of other wetland functions can be accomplished along with
benefiting runoff quality control, and
– If appropriate source control and treatment BMPs are applied in the contributing
catchment on the basis of the analysis in Step 12a, and any legally adopted
water quality standards for wetlands are observed.
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If these circumstances apply, refer to the Stormwater Wetland Assessment Criteria in
Guide Sheet 1B to evaluate further.
13. Stimulate public awareness of and interest in wetlands and other water resources in order to
establish protective attitudes in the community. This program should include:
• Education regarding the use of fertilizers and pesticides, automobile maintenance,
the care of animals to prevent water pollution, and the importance of retaining
buffers;
• Descriptive signboards adjacent to wetlands informing residents of the wetland type,
its functions, the protective measures being taken, etc.
• If beavers are present in a wetland, educate residents about their ecological role and
value and take steps to avoid human interference with beavers.
Guide Sheet 1B: Stormwater Wetland Assessment Criteria
This guide sheet gives criteria that disqualify a natural wetland from being structurally or
hydrologically engineered for control of stormwater quantity, quality, or both. These criteria should be
applied only after performing the alternatives analysis outlined in Guide Sheet 1A.
1. A wetland should not be structurally or hydrologically engineered for runoff quantity or
quality control and should be given maximum protection from overall urban impacts (see
Guide Sheet 2, Wetland Protection Guidelines) under any of the following circumstances:
• In its present state it is primarily an estuarine or forested wetland or a priority peat
system.
• It is a rare or irreplaceable wetland type, as identified by the Washington Natural
Heritage Program, the Puget Sound Water Quality Preservation Program, or local
government.
• It provides rare, threatened, or endangered species habitat that could be impaired
by the proposed action. Determining whether or not the conserved species will be
affected by the proposed project requires a careful analysis of its requirements in
relation to the anticipated habitat changes.
In general, the wetlands in these groups are classified in Categories I and II in the Puget
Sound Water Quality Authority's draft wetland guidelines.
2. A wetland can be considered for structural or hydrological modification for runoff quantity or
quality control if most of the following circumstances exist:
• It is classified in Category IV in the Puget Sound Water Quality Authority's draft
wetland guidelines. In general, Category IV wetlands have monotypic vegetation of
similar age and class, lack special habitat features, and are isolated from other
aquatic systems.
• The wetland has been previously disturbed by human activity, as evidenced by
agriculture, fill, ditching, and/or introduced or invasive weedy plant species.
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• The wetland has been deprived of a significant amount of its water supply by
draining or previous urbanization (e. g., by loss of groundwater supply), and
stormwater runoff is sufficient to augment the water supply. A particular candidate is
a wetland that has experienced an increased summer dry period, especially if the
drought has been extended by more than two weeks.
• Construction for structural or hydrologic modification in order to provide runoff
quantity or quality control will disturb relatively little of the wetland.
• The wetland can provide the required storage capacity for quantity or quality control
through an outlet orifice modification to increase storage of water, rather than
through raising the existing overflow. Orifice modification is likely to require less
construction activity and consequent negative impacts.
• Under existing conditions the wetland's experiences a relatively high degree of water
level fluctuation and a range of velocities (i.e., a wetland associated with
substantially flowing water, rather than one in the headwaters or entirely isolated
from flowing water).
• The wetland does not exhibit any of the following features:
- Significant priority peat system or forested zones that will experience substantially
altered hydroperiod as a result of the proposed action;
- Regionally unusual biological community types;
- Animal habitat features of relatively high value in the region (e. g., a protected,
undisturbed area connected through undisturbed corridors to other valuable habitats,
an important breeding site for protected species);
- The presence of protected commercial or sport fish;
- Configuration and topography that will require significant modification that may
threaten fish stranding;
- A relatively high degree of public interest as a result of, for example, offering valued
local open space or educational, scientific, or recreational opportunities, unless the
proposed action would enhance these opportunities;
• The wetland is threatened by potential impacts exclusive of stormwater management,
and could receive greater protection if acquired for a stormwater management project
rather than left in existing ownership.
• There is good evidence that the wetland actually can be restored or enhanced to
perform other functions in addition to runoff quantity or quality control.
• There is good evidence that the wetland lends itself to the effective application of the
Wetland Protection Guidelines in Guide Sheet 2.
• The wetland lies in the natural routing of the runoff. Local regulations often prohibit
drainage diversion from one basin to another.
• The wetland allows runoff discharge at the natural location.
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Guide Sheet 2: Wetland Protection Guidelines
This guide sheet provides information about likely changes to the ecological structure and
functioning of wetlands that are incidentally subject to the effects of an urban or urbanizing
watershed or are modified to supply runoff water quantity or quality control benefits. The guide sheet
also recommends management actions that can avoid or minimize deleterious changes in these
wetlands.
Guide Sheet 2A: General Wetland Protection Guidelines
1. Consult regulations issued under federal and state laws that govern the discharge of
pollutants. Wetlands are classified as "Waters of the United States" and "Waters of the
State" in Washington.
2. Maintain the wetland buffer required by local regulations or recommended by the Puget
Sound Water Quality Authority's draft wetland guidelines.
3. Retain areas of native vegetation connecting the wetland and its buffer with nearby wetlands
and other contiguous areas of native vegetation.
4. Avoid compaction of soil and introduction of exotic plant species during any work in a
wetland.
5. Take specific site design and maintenance measures to avoid general urban impacts (e. g.,
littering and vegetation destruction). Examples are protecting existing buffer zones;
discouraging access, especially by vehicles, by plantings outside the wetland; and
encouragement of stewardship by a homeowners' association. Fences can be useful to
restrict dogs and pedestrian access, but they also interfere with wildlife movements. Their
use should be very carefully evaluated on the basis of the relative importance of intrusive
impacts versus wildlife presence. Fences should generally not be installed when wildlife
would be restricted and intrusion is relatively minor. They generally should be used when
wildlife passage is not a major issue and the potential for intrusive impacts is high. When
wildlife movements and intrusion are both issues, the circumstances will have to be weighed
to make a decision about fencing.
6. If the wetland inlet will be modified for the stormwater management project, use a diffuse
flow method, such as a spreader swale, to discharge water into the wetland in order to
prevent flow channelization.
Guide Sheet 2B: Guidelines for Protection from Adverse Impacts of Modified Runoff
Quantity Discharged to Wetlands
1. Protection of wetland plant and animal communities depends on controlling the wetland’s
hydroperiod, meaning the pattern of fluctuation of water depth and the frequency and
duration of exceeding certain levels, including the length and onset of drying in the summer.
A hydrologic assessment is useful to measure or estimate elements of the hydroperiod
under existing pre-development and anticipated post-development conditions. This
assessment should be performed with the aid of a qualified hydrologist. Post-development
estimates of watershed hydrology and wetland hydroperiod must include the cumulative
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effect of all anticipated watershed and wetland modifications. Provisions in these guidelines
pertain to the full anticipated build-out of the wetland’s watershed.
This analysis hypothesizes a fluctuating water stage over time before development that
could fluctuate more, both higher and lower after development; these greater fluctuations
are termed stage excursions. The guidelines set limits on the frequency and duration of
excursions, as well as on overall water level fluctuation, after development.
To determine existing hydroperiod use one of the following methods, listed in order of
preference:
• Estimation by a continuous simulation computer model--The model should be
calibrated with at least one year of data taken using a continuously recording level
gage under existing conditions and should be run for the historical rainfall period.
The resulting data can be used to express the magnitudes of depth fluctuation, as
well as the frequencies and durations of surpassing given depths. [Note: Modeling
that yields high quality information of the type needed for wetland hydroperiod
analysis is a complex subject. Providing guidance on selecting and applying
modeling options is beyond the scope of these guidelines but is being developed by
King County Surface Water Management Division and other local jurisdictions. An
alternative possibility to modeling depths, frequencies, and durations within the
wetland is to model durations above given discharge levels entering the wetland over
various time periods (e. g., seasonal, monthly, weekly). This option requires further
development.]
• Measurement during a series of time intervals (no longer than one month in length)
over a period of at least one year of the maximum water stage, using a crest stage
gage, and instantaneous water stage, using a staff gage--The resulting data can be
used to express water level fluctuation (WLF) during the interval as follows:
Average base stage = (Instantaneous stage at beginning of interval + Instantaneous
stage at end of interval)/2
WLF = Crest stage - Average base stage
Compute mean annual and mean monthly WLF as the arithmetic averages for each year
and month for which data are available.
To forecast future hydroperiod use one of the following methods, listed in order of
preference:
• Estimation by the continuous simulation computer model calibrated during pre-
development analysis and run for the historical rainfall period--The resulting data can
be used to express the magnitudes of depth fluctuation, as well as the frequencies
and durations of surpassing given depths. [Note: Post-development modeling results
should generally be compared with pre-development modeling results, rather than
directly with field measurements, because different sets of assumptions underlie
modeling and monitoring. Making pre- and post-development comparisons on the
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basis of common assumptions allows cancellation of errors inherent in the
assumptions.]
• Estimation according to general relationships developed from the Puget Sound
Wetlands and Stormwater Management Program Research Program, as follows (in
part adapted from Chin 1996):
- Mean annual WLF is very likely (100% of cases measured) to be < 20 cm (8 inches
or 0.7 ft) if total impervious area (TIA) cover in the watershed is < 6% (roughly
corresponding to no more than 15% of the watershed converted to urban land use).
- Mean annual WLF is very likely (89% of cases measured) to be > 20 cm if TIA in the
watershed is > 21% (roughly corresponding to more than 30% of the watershed
converted to urban land use).
- Mean annual WLF is somewhat likely (50% of cases measured) to be > 30 cm
(1.0 ft) if TIA in the watershed is > 21% (roughly corresponding to more than 30% of
the watershed converted to urban land use).
- Mean annual WLF is likely (75% of cases measured) to be > 30 cm, and somewhat
likely (50% of cases measured) to be 50 cm (20 inches or 1.6 ft) or higher, if TIA in
the watershed is > 40% (roughly corresponding to more than 70% of the watershed
converted to urban land use).
- The frequency of stage excursions greater than 15 cm (6 inches or 0.5 ft) above or
below pre-development levels is somewhat likely (54% of cases measured) to be
more than six per year if the mean annual WLF increases to > 24 cm (9.5 inches or
0.8 ft).
- The average duration of stage excursions greater than 15 cm above or below pre-
development levels is likely (69% of cases measured) to be more than 72 hours if the
mean annual WLF increases to > 20 cm.
2. The following hydroperiod limits characterize wetlands with relatively high vegetation
species richness and apply to all zones within all wetlands over the entire year. If these
limits are exceeded, then species richness is likely to decline. If the analysis described
above forecasts exceedences, one or more of the management strategies listed in step 5
should be employed to attempt to stay within the limits.
• Mean annual WLF (and mean monthly WLF for every month of the year) does not
exceed 20 cm. Vegetation species richness decrease is likely with: (1) a mean
annual (and mean monthly) WLF increase of more than 5 cm (2 inches or 0.16 ft) if
pre-development mean annual (and mean monthly) WLF is greater than 15 cm, or
(2) a mean annual (and mean monthly) WLF increase to 20 cm or more if pre-
development mean annual (and mean monthly) WLF is 15 cm or less.
• The frequency of stage excursions of 15 cm above or below pre-development stage
does not exceed an annual average of six. Note: A short-term lagging or
advancement of the continuous record of water levels is acceptable. The 15 cm limit
applies to the temporary increase in maximum water surface elevations (hydrograph
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peaks) after storm events and the maximum decrease in water surface elevations
(hydrograph valley bottoms) between events and during the dry season.
• The duration of stage excursions of 15 cm above or below pre-development stage
does not exceed 72 hours per excursion. Note: A short-term lagging or
advancement of the continuous record of water levels is acceptable. However, the 15
cm limit applies throughout the entire hydrograph, not just the peaks and valleys.
• The total dry period (when pools dry down to the soil surface everywhere in the
wetland) does not increase or decrease by more than two weeks in any year.
• Alterations to watershed and wetland hydrology that may cause perennial wetlands
to become vernal are avoided.
3. The following hydroperiod limit characterizes priority peat wetlands (bogs and fens as
more specifically defined by the Washington Department of Ecology) and applies to all
zones over the entire year. If this limit is exceeded, then characteristic bog or fen wetland
vegetation is likely to decline. If the analysis described above forecasts exceedance, one or
more of the management strategies listed in step 5 should be employed to attempt to stay
within the limit.
• The duration of stage excursions above the pre-development stage does not exceed
24 hours in any year.
NOTE: This guideline is in addition to the guidelines in #2 directly above. To apply this
guideline a continuous simulation computer model needs to be employed. The model should
be calibrated with data taken under existing conditions at the wetland being analyzed and
then used to forecast post-development duration of excursions.
4. The following hydroperiod limits characterize wetlands inhabited by breeding native
amphibians and apply to breeding zones during the period 1 February through 31 May. If
these limits are exceeded, then amphibian breeding success is likely to decline. If the
analysis described above forecasts exceedences, one or more of the management
strategies listed in step 5 should be employed to attempt to stay within the limits.
• The magnitude of stage excursions above or below the pre-development stage
should not exceed 8 cm for more than 24 hours in any 30-day period.
NOTE: To apply this guideline a continuous simulation computer model needs to be
employed. The model should be calibrated with data taken under existing conditions at the
wetland being analyzed and then used to forecast post-development magnitude and duration
of excursions.
5. If it is expected that the hydroperiod limits stated above could be exceeded, consider
strategies such as:
• Reduction of the level of development;
• Increasing runoff infiltration
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NOTE: Infiltration is prone to failure in many Puget Sound Basin locations with glacial till soils
and generally requires pretreatment to avoid clogging. In other situations infiltrating urban
runoff may contaminate groundwater. Consult the stormwater management manual adopted
by the jurisdiction and carefully analyze infiltration according to its prescriptions.
• Increasing runoff storage capacity; and
• Selective runoff bypass.
6. After development, monitor hydroperiod with a continuously recording level gauge or staff
and crest stage gauges. If the applicable limits are exceeded, consider additional
applications of the strategies in step 5 that may still be available. It is also recommended
that goals be established to maintain key vegetation species, amphibians, or both, and that
these species be monitored to determine if the goals are being met.
Guide Sheet 2C: Guidelines for Protection from Adverse Impacts of Modified Runoff
Quality Discharged to Wetlands
1. Require effective erosion control at any construction sites in the wetland's drainage
catchment.
2. Institute a program of source control BMPs to minimize the generation of pollutants that
will enter storm runoff that drains to the wetland.
3. Provide a water quality control facility consisting of one or more treatment BMPs to treat all
urban runoff entering the wetland. Refer to Chapter 4 of Volume 1 or Chapter 2 of Volume 5
of the Stormwater Management Manual for Western Washington to determine treatment
requirements. Then refer to the corresponding BMP menu for that requirement in Chapter 3
of Volume V. From the menu select a BMP that fits with the project site.
• If the wetland is a priority peat wetland (bogs and fens as more specifically defined
by the Washington Department of Ecology), the facility should include a BMP with
the most advanced ability to control nutrients (e. g., an infiltration device, a wet pond
or constructed wetland with residence time in the pooled storage of at least two
weeks). [Note: Infiltration is prone to failure in many Puget Sound Basin locations
with glacial till soils and generally requires pretreatment to avoid clogging. In other
situations infiltrating urban runoff may contaminate groundwater. Consult the
stormwater management manual adopted by the jurisdiction and carefully analyze
infiltration according to its prescriptions.] Refer to Appendix 4 for a comparison of
water chemistry conditions in priority peat versus more typical wetlands.
Refer to the stormwater management manual to select and design the facility. Generally, the
facility should be located outside and upstream of the wetland and its buffer.
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4. Design and perform a water quality monitoring program for priority peat wetlands and for
other wetlands subject to relatively high water pollutant loadings. The research results
(Horner 1989) identified such wetlands as having contributing catchments exhibiting either
of the following characteristics:
• More than 20 percent of the catchment area is committed to commercial, industrial,
and/or multiple family residential land uses; or
• The combination of all urban land uses (including single family residential) exceeds
30 percent of the catchment area.
A recommended monitoring program, consistent with monitoring during the research
program, is:
• Perform pre-development baseline sampling by collecting water quality grab
samples in an open water pool of the wetland for at least one year, allocated through
the year as follows: November 1-March 31--4 samples, April 1-May 31--1 sample,
June 1-August 31--2 samples, and September 1-October 31--1 sample (if the
wetland is dry during any period, reallocate the sample(s) scheduled then to another
time). Analyze samples for pH; dissolved oxygen (DO); conductivity (Cond); total
suspended solids (TSS); total phosphorus (TP); nitrate + nitrite-nitrogen (N); fecal
coliforms (FC); and total copper (Cu), lead (Pb), and zinc (Zn). Find the median and
range of each water quality variable.
• Considering the baseline results, set water quality goals to be maintained in the post-
development period. Example goals are: (1) pH--no more than “x” percent (e. g.,
10%) increase (relative to baseline) in annual median and maximum or decrease in
annual minimum; (2) DO--no more than “x” percent decrease in annual median and
minimum concentrations; (3) other variables --no more than “x” percent increase in
annual median and maximum concentrations; (4) no increase in violations of the
Washington Administrative Code (WAC) water quality criteria.
• Repeat the sampling on the same schedule for at least one year after all
development is complete. Compare the results to the set goals.
If the water quality goals are not met, consider additional applications of the source and
treatment controls described in steps 2 and 3. Continue monitoring until the goals are
met at least two years in succession.
NOTE: Wetland water quality was found to be highly variable during the research, a fact
that should be reflected in goals. Using the maximum (or minimum), as well as a
measure of central tendency like the median, and allowing some change from pre-
development levels are ways of incorporating an allowance for variability. Table I-E-2
presents data from the wetlands studied during the research program to give an
approximate idea of magnitudes and degree of variability to be expected. Non-urbanized
watersheds (N) are those that have both < 15% urbanization and < 6% impervious
cover. Highly urbanized watersheds (H) are those that have both lost all forest cover and
have > 20% impervious cover. Moderately urbanized watersheds (M) are those that fit
neither the N nor H category.
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Table I-E-2. Water Quality Ranges Found in Study Wetlands
N M H
Metric Median Mean Std.Dev./na Median Mean Std.Dev./na Median Mean Dev./na
pHb 6.4 6.4 0.5/162 6.7 6.5 0.8/132 6.9 6.7 0.6/52
DO (mg/L) 5.9 5.7 2.6/205 5.1 5.53.6/173 6.3 5.4 2.9/67
Cond. (S/cm) 46 73 64/190 160 142 73/161 132 151 86/61
TSS (g/L) 2.0 4.6 8.5/204 2.8 9.2 22/175 4.0 9.2 15/66
TP (g/L) 29 52 87/206 70 93 92/177 69 110 234/67
N (g/L) 112 368 485/206 304 598 847/177 376 395 239/67
FC (no./100mL) 9.0 271 1000/206 46 2665 27342/173 61 969 4753/66
Cu (g/L) <5.0 <3.3 >2.7/93 <5.0 <3.7 >1.9/78 <5.0 <4.1 <2.5/29
Pb (g/L) 1.0 <2.7 >2.8/136 3.0 <3.4 >2.7/122 5.0 <4.5 >4.0/44
Zn (g/L) 5.0 8.4 8.3/136 8.0 9.8 7.2/122 20 20 17/44
a Std. Dev.--standard deviation; n--number of observations. b Values do not apply to priority peat wetlands. The program did not specifically study these wetlands but measured
pH in three wetlands with “bog-like” characteristics. The minimum value measured in these wetlands was 4.5, and
the lowest median was 4.8; but pH can be approximately 1 unit lower in wetlands of this type.
Guide Sheet 2D: Guidelines for the Protection of Specific Biological Communities
1. For wetlands inhabited by breeding native amphibians:
• Refer to step 4 of Guide Sheet 2B for hydroperiod limit.
• Avoid decreasing the sizes of the open water and aquatic bed zones.
• Avoid increasing the channelization of flow. Do not form channels where none exist,
and take care that inflows to the wetland do not become more concentrated and do
not enter at higher velocities than accustomed. If necessary, concentrated flows can
be uniformly distributed with a flow-spreading device such as a shallow weir, stilling
basin, or perforated pipe. Velocity dissipation can be accomplished with a stilling
basin or rip-rap pad.
• Limit the post-development flow velocity to < 5 cm/s (0.16 ft/second) in any location
that had a velocity in the range 0-5 cm/s in the pre-development condition.
• Avoid increasing the gradient of wetland side slopes.
2. For wetlands inhabited by forest bird species:
• Retain areas of coniferous forest in and around the wetland as habitat for forest
species.
• Retain shrub or woody debris as nesting sites for ground-nesting birds and downed
logs and stumps for winter wren habitat.
• Retain snags as habitat for cavity-nesting species, such as woodpeckers.
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• Retain shrubs in and around the wetland for protective cover. If cover is insufficient
to protect against domestic pet predation, consider planting native bushes such as
rose species in the buffer.
3. For wetlands inhabited by wetland obligate bird species:
• Retain forested zones, sedge and rush meadows, and deep open water zones, both
without vegetation and with submerged and floating plants.
• Retain shrubs in and around the wetland for protective cover. If cover is insufficient
to protect against domestic pet predation, consider planting native bushes such as
rose species in the buffer.
• Avoid introducing invasive weedy plant species, such as purple loosestrife and
reed canary grass.
• Retain the buffer zone. If it has lost width or forest cover, consider re-establishing
forested buffer area at least 30 meters (100 ft) wide.
• If human entry is desired, establish paths that permit people to observe the wetland
with minimum disturbance to the birds.
4. For wetlands inhabited by fish:
• Protect fish habitats by avoiding water velocities above tolerated levels (selected with
the aid of a qualified fishery biologist to protect fish in each life stage when they are
present), siltation of spawning beds, etc. Habitat requirements vary substantially
among fish species. If the wetland is associated with a larger water body, contact the
Department of Fisheries and Wildlife to determine the species of concern and the
acceptable ranges of habitat variables.
• If stranding of protected commercial or sport fish could result from a structural or
hydrologic modification for runoff quantity or quality control, develop a strategy to
avoid stranding that minimizes disturbance in the wetland (e. g., by making
provisions for fish return to the stream as the wetland drains, or avoiding use of the
facility for quantity or quality control during fish presence).
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Wetlands Guidance Appendix 1:
Information Needed to Apply Guidelines
The following information listed for each guide sheet is most essential for applying the Wetlands and
Stormwater Management Guidelines. As a start, obtain the relevant soil survey; the National Wetland
Inventory, topographic and land use maps, and the results of any local wetland inventory.
Guide Sheet 1
1. Boundary and area of the contributing watershed of the wetland or other landscape unit
2. A complete definition of goals for the wetland and landscape unit subject to planning and
management
3. Existing management and monitoring plans
4. Existing and projected land use in the landscape unit in the categories commercial,
industrial, multi-family residential, single-family residential, agricultural, various categories of
undeveloped, and areas subject to active logging or construction (expressed as percentages
of the total watershed area)
5. Drainage network throughout the landscape unit
6. Soil conditions, including soil types, infiltration rates, and positions of seasonal water table
(seasonally) and restrictive layers
7. Groundwater recharge and discharge points
8. Wetland category (I - IV in draft Puget Sound Water Quality Authority wetland protection
guidelines); designation as rare or irreplaceable. Refer to the Washington Natural Heritage
Program database. If the needed information is not available, a biological assessment will
be necessary.
9. Watershed hydrologic assessment
10. Watershed water quality assessment
11. Wetland type and zones present, with special note of estuarine, priority peat system,
forested, sensitive scrub-shrub zone, sensitive emergent zone and other sensitive or critical
areas designated by state or local government (with dominant plant species)
12. Rare, threatened, or endangered species inhabiting the wetland
13. History of wetland changes
14. Relationship of wetland to other water bodies in the landscape unit and the drainage
network
15. Flow pattern through the wetland
16. Fish and wildlife inhabiting the wetland
17. Relationship of wetland to other wildlife habitats in the landscape unit and the corridors
between them
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Guide Sheet 2
1. Existing and potential stormwater pollution sources
2. Existing and projected landscape unit land use (see number 4 under Guide Sheet 1)
3. Existing and projected wetland hydroperiod characteristics
4. Wetland bathymetry
5. Inlet and outlet locations and hydraulics
6. Landscape unit soils, geologic and hydrogeologic conditions
7. Wetland type and zones present (see number 11 under Guide Sheet 1)
8. Presence of breeding populations of native amphibian species
9. Presence of forest and wetland obligate bird species
10. Presence of fish species
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Wetlands Guidance Appendix 2: Definitions
Baseline sampling Sampling performed to define an existing state before any
modification occurs that could change the state.
Bioengineering Restoration or reinforcement of slopes and stream banks with
living plant materials.
Buffer The area that surrounds a wetland and that reduces adverse
impacts to it from adjacent development.
Constructed wetland A wetland intentionally created from a non-wetland site for the
sole purpose of wastewater or stormwater treatment. These
wetlands are not normally considered Waters of the United States
or Waters of the State.
Degraded (disturbed)
wetland (community)
A wetland (community) in which the vegetation, soils,
and/or hydrology have been adversely altered,
resulting in lost or reduced functions and values;
generally, implies topographic isolation; hydrologic
alterations such as hydroperiod alteration (increased or
decreased quantity of water), diking, channelization,
and/or outlet modification; soils alterations such as
presence of fill, soil removal, and/or compaction;
accumulation of toxicants in the biotic or abiotic
components of the wetland; and/or low plant species
richness with dominance by invasive weedy species
Enhancement Actions performed to improve the condition of an existing
degraded wetland, so that functions it provides are of a higher
quality.
Estuarine wetland Generally, an eelgrass bed; salt marsh; or rocky, sandflat, or
mudflat intertidal area where fresh and salt water mix.
(Specifically, a tidal wetland with salinity greater than 0.5 parts per
thousand, usually semi-enclosed by land but with partly obstructed
or sporadic access to the open ocean).
Forested communities
(wetlands)
In general terms, communities (wetlands) characterized by
woody vegetation that is greater than or equal to 6 meters in
height; in these guidelines the term applies to such communities
(wetlands) that represent a significant amount of tree cover
consisting of species that offer wildlife habitat and other values
and advance the performance of wetland functions overall.
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Functions The ecological (physical, chemical, and biological) processes or
attributes of a wetland without regard for their importance to
society (see also Values). Wetland functions include food chain
support, provision of ecosystem diversity and fish and wildlife
habitat, flood flow alteration, groundwater recharge and discharge,
water quality improvement, and soil stabilization.
Hydrodynamics The science involving the energy and forces acting on water and
its resulting motion.
Hydroperiod The seasonal occurrence of flooding and/or soil saturation;
encompasses the depth, frequency, duration, and seasonal
pattern of inundation.
Invasive weedy plant
species
Opportunistic species of inferior biological value that tend to out-
compete more desirable forms and become dominant; applied to
non-native species in these guidelines.
Landscape unit An area of land that has a specified boundary and is the locus of
interrelated physical, chemical, and biological processes.
Modification, Modified
(wetland)
A wetland whose physical, hydrological, or water quality
characteristics have been purposefully altered for a management
purpose, such as by dredging, filling, forebay construction, and
inlet or outlet control.
On-site An action (here, for stormwater management purposes) taken
within the property boundaries of the site to which the action
applies.
Polishing Advanced treatment of a waste stream that has already received
one or more stages of treatment by other means.
Pre-development, post-
development
Respectively, the situation before and after a specific stormwater
management project (e. g., raising the outlet, building an outlet
control structure) will be placed in the wetland or a land use
change occurs in the landscape unit that will potentially affect the
wetland.
Pre-treatment An action taken to remove pollutants from runoff before it is
discharged into another system for additional treatment.
Priority peat systems Unique, irreplaceable fens that can exhibit water pH in a wide
range from highly acidic to alkaline, including fens typified by
Sphagnum species, Rhododendron groenlandicum (Labrador tea),
Drosera rotundifolia (sundew), and Vaccinium oxycoccos (bog
cranberry); marl fens; estuarine peat deposits; and other moss
peat systems with relatively diverse, undisturbed flora and fauna.
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Bog is the common name for peat systems having the Sphagnum
association described, but this term applies strictly only to systems
that receive water income from precipitation exclusively.
Rare, threatened, or
endangered species
Plant or animal species that are regional relatively uncommon,
are nearing endangered status, or whose existence is in
immediate jeopardy and is usually restricted to highly specific
habitats. Threatened and endangered species are officially listed
by federal and state authorities, whereas rare species are
unofficial species of concern that fit the above definitions.
Redevelopment Conversion of an existing development to another land use, or
addition of a material improvement to an existing development.
Regional An action (here, for stormwater management purposes) that
involves more than one discrete property.
Restoration Actions performed to reestablish wetland functional characteristics
and processes that have been lost by alterations, activities, or
catastrophic events in an area that no longer meets the definition
of a wetland.
Source control best
management practices
(BMPs)
Actions that are taken to prevent the development of a problem
(e. g., increase in runoff quantity, release of pollutants) at the
point of origin.
Stage excursion A post-development departure, either higher or lower, from the
water depth existing under a given set of conditions in the pre-
development state.
Structure The components of an ecosystem, both the abiotic (physical and
chemical) and biotic (living).
Treatment best
management practices
(BMPs)
Actions that remove pollutants from runoff through one or more
physical, chemical, biological mechanisms.
Unusual biological
community types
Assemblages of interacting organisms that are relatively
uncommon regionally.
Values Wetland processes or attributes that are valuable or beneficial to
society (also see Functions). Wetland values include support of
commercial and sport fish and wildlife species, protection of life
and property from flooding, recreation, education, and aesthetic
enhancement of human communities.
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Vernal wetland A wetland that has water above the soil surface for a period of
time during and/or after the wettest season but always dries to or
below the soil surface in warmer, drier weather.
Wetland obligate A biological organism that absolutely requires a wetland habitat for
at least some stage of its life cycle.
Wetlands Those areas that are inundated or saturated by surface or ground
water at a frequency and duration sufficient to support, and that
under normal circumstances do support, a prevalence of
vegetation typically adapted for life in saturated soil conditions.
Wetlands generally include swamps, marshes, bogs, and similar
areas. Wetlands do not include those artificial wetlands
intentionally created from non-wetland sites, including, but not
limited to, irrigation and drainage ditches, grass-lined swales,
canals, detention facilities, wastewater treatment facilities, farm
ponds, and landscape amenities, or those wetlands created after
July 1, 1990, that were unintentionally created as a result of the
construction of a road, street, or highway. Wetlands may include
those artificial wetlands intentionally created from non-wetland
areas to mitigate the conversion of wetlands. (Waterbodies not
included in the definition of wetlands as well as those mentioned
in the definition are still waters of the state.)
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Wetlands Guidance Appendix 3: Native and Recommended
Noninvasive Plant Species for Wetlands in the Puget Sound Basin
CAUTION: Extracting plants from an existing wetland donor site can cause a significant negative
effect on that site. It is recommended that plants be obtained from native plant nursery stocks
whenever possible. Collections from existing wetlands should be limited in scale and undertaken with
care to avoid disturbing the wetland outside of the actual point of collection. Plant selection is a
complex task, involving matching plant requirements with environmental conditions. It should be
performed by a qualified wetlands botanist. Refer to Restoring Wetlands in Washington by the
Washington Department of Ecology for more information.
The following plants are preferred in Puget Sound Basin freshwater wetlands:
Open water zone Potamogeton species (pondweeds)
Nymphaea odorata (pond lily)
Brasenia schreberi (watershield)
Nuphar luteum (yellow pond lily)
Polygonum hydropiper (smartweed)
Alisma plantago-aquatica (broadleaf water plantain)
Ludwigia palustris (water purslane)
Menyanthes trifoliata (bogbean)
Utricularia minor, U. vulgaris (bladderwort)
Emergent zone Carex obnupta, C. utriculata, C. arcta, C. stipata, C. vesicaria C. aquatilis,
C. comosa, C. lenticularis (sedge)
Scirpus atricinctus (woolly bulrush)
Scirpus microcarpus (small-fruited bulrush)
Eleocharis palustris, E. ovata (spike rush)
Epilobium watsonii (Watson's willow herb)
Typha latifolia (common cattail) (Note: This native plant can be aggressive
but has been found to offer certain wildlife habitat and water quality
improvement benefits; use with care.)
Veronica americana, V. scutellata (American brookline, marsh speedwell)
Mentha arvensis (field mint)
Lycopus americanus, L. uniflora (bugleweed or horehound)
Angelica species (angelica)
Oenanthe sarmentosa (water parsley)
Heracleum lanatum (cow parsnip)
Glyceria grandis, G. elata (manna grass)
Juncus acuminatus (tapertip rush)
Juncus ensifolius (daggerleaf rush)
Juncus bufonius (toad rush)
Mimulus guttatus (common monkey flower)
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Scrub-shrub zone Salix lucida, S. rigida, S. sitchensis, S. scouleriana, S. pedicellaris (willow)
Lysichiton americanus (skunk cabbage)
Athyrium filix-femina (lady fern)
Cornus sericea (redstem dogwood)
Rubus spectabilis (salmonberry)
Physocarpus capitatus (ninebark)
Ribes species (gooseberry)
Rhamnus purshiana (cascara)
Sambucus racemosa (red elderberry) (occurs in wetland-upland transition)
Loniceria involucrata (black twinberry)
Oemleria cerasiformis (Indian plum)
Stachys cooleyae (Stachy's horsemint)
Prunus emarginata (bitter cherry)
Forested zone Populus balsamifera, ssp. trichocarpa (black cottonwood)
Fraxinus latifolia (Oregon ash)
Thuja plicata (western red cedar)
Picea sitchensis (Sitka spruce)
Alnus rubra (red alder)
Tsuga heterophylla (hemlock)
Acer circinatum (vine maple)
Maianthemum dilatatum (wild lily-of-the-valley)
Ivzula parviflora (small-flower wood rush)
Torreyochloa pauciflora (weak alkaligrass)
Ribes species (currants)
Bog Sphagnum species (sphagnum mosses)
Rhododendron groenlandicum (Labrador tea)
Vaccinium oxycoccos (bog cranberry)
Kalmia microphylla, ssp. occidentalis (bog laurel)
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The following exotic plants should not be introduced to existing, created, or constructed
Puget Sound Basin freshwater wetlands:
Hedera helix (English ivy)
Phalaris arundinacea (reed canarygrass)
Lythrum salicaria (purple loosestrife)
Iris pseudacorus (yellow iris)
Ilex aquifolia (holly)
Impatiens glandulifera (policeman’s helmet)
Lotus corniculatus (birdsfoot trefoil)
Lysimachia thyrsiflora (tufted loosestrife)
Myriophyllum species (water milfoil, parrot’s feather)
Polygonum cuspidatum (Japanese knotweed)
Polygonum sachalinense (giant knotweed)
Rubus discolor (Himalayan blackberry)
Tanacetum vulgare (common tansy)
The following native plants should not be introduced to existing, created, or constructed
Puget Sound Basin freshwater wetlands
Potentilla palustris (Pacific silverweed)
Solarum dulcimara (bittersweet nightshade)
Juncus effusus (soft rush)
Conium maculatum (poison hemlock)
Ranunculus repens (creeping buttercup)
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Wetlands Guidance Appendix 4: Comparison of Water Chemistry
Characteristics in Sphagnum Bog and Fen versus More Typical
Wetlands
Water Quality Variable Typical Wetlands Sphagnum Bogs and Fens
PH 6 - 7 3.5 - 4.5
Dissolved oxygen (mg/L) 4 - 8 Shallow surface layer
oxygenated, anoxic below
Cations Divalent Ca, Mg common Divalent Ca, Mg uncommon;
Univalent Na, K predominant
Anions HCO3
-, CO3
2- predominant Cl-, SO4
2- predominant; almost
no HCO3-, CO32- (organic acids
form buffering system)
Hardness Moderate Very low
Total phosphorus (g/L) 50 - 500 5 - 50
Total Kjeldahl nitrogen (g/L) 500 - 1000 ~ 50
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Management Guidelines Appendix E 112
Wetland Protection Guidelines References
Chin, N. T. 1996. Watershed Urbanization Effects on Palustrine Wetlands: A Study of the
Hydrologic, Vegetative, and Amphibian Community Response over Eight Years. M. S. C. E.
Thesis, University of Washington, Seattle, WA.
Clymo, R. S. 1963. Ion exchange in Sphagnum and its relation to bog ecology. Annals of
Botany 27 (106):310-324.
Cooke, S. S., Puget Sound Wetlands and Stormwater Management Research Program,
unpublished Queen’s Bog data.
Horner, R. R. 1989. Long-term effects of urban runoff on wetlands. Pp. in L. A. Roesner, B.
Urbonas, and M. B. Sonnen (eds.), Design of Urban Runoff Controls, American Society of
Civil Engineers, New York, NY.
Horner, R. R., J. J. Skupien, E. H. Livingston, and H. E. Shaver. 1994. Fundamentals of
Urban Runoff Management: Technical and Institutional Issues. Terrene Institute,
Washington, D. C.
Horner, R. R., S. S. Cooke, K. O. Richter, A. L. Azous, L. E. Reinelt, B. L. Taylor, K. A.
Ludwa, and M. Valentine. 1996. Wetlands and Urbanization, Implications for the Future.
Puget Sound Wetlands and Stormwater Management Research Program, Engineering
Professional Programs, University of Washington, Seattle, WA.
Meyer, J., L. Vogel, and T. Duebendorfer, East Lake Sammamish wetland no. 21
unpublished data, submitted to L. Kulzer, King County Surface Water Management Division.
Moore, P. D. and D. J. Bellamy. 1974. Chapter 3, The Geochemical template. Peatlands.
Elek Science, London, U. K.
Thurman, E. M. 1985. Organic Geochemistry of Natural Waters. Martinus Nijhoff/Dr W.
Junk Publishers, Dordrecht, The Netherlands.
Vitt, D. H., D. G. Horton, N. G. Slack, and N. Malmer. 1990. Sphagnum-dominated
peatlands of the hyperoceanic British Columbia coast: Patterns in surface water chemistry
and vegetation. Canadian Journal of Forest Research 20:696-711.
Volume II
i Table of Contents
Volume II – Stormwater
Management for Construction Sites
Table of Contents
Purpose of this Volume...................................................................................................................113
Content and Organization of this Volume.......................................................................................113
Chapter 1 The 12 Elements of Construction Stormwater Pollution Prevention............114
Chapter 2 Developing a Construction Stormwater Pollution Prevention
Plan (SWPPP)...................................................................................................123
2.1 General Requirements and Guidelines.................................................................................123
2.1.1 BMP Standards and Specifications.................................................................................123
2.1.2 General Principles...........................................................................................................124
2.2 Step-by-Step Procedure........................................................................................................124
2.2.1 Step 1 – Data Collection..................................................................................................124
2.2.2 Step 2 – Data Analysis....................................................................................................125
2.2.3 Step 3 – Construction SWPPP Development and Analysis............................................127
2.2.3.1 Construction SWPPP Narrative............................................................................127
2.2.3.2 Erosion and Sediment Control Drawings..............................................................128
2.3 Construction SWPPP Checklists...........................................................................................128
Chapter 3 Standards and Specifications for Best Management Practices (BMPs).......134
3.1 Source Control BMPs............................................................................................................135
3.1.1 BMP C101: Preserving Natural Vegetation.....................................................................135
3.1.1.1 Purpose.................................................................................................................135
3.1.1.2 Conditions of Use..................................................................................................135
3.1.1.3 Design and Installation Specifications...................................................................135
3.1.1.4 Maintenance Standards........................................................................................137
3.1.2 BMP C102: Buffer Zone...................................................................................................138
3.1.2.1 Purpose.................................................................................................................138
3.1.2.2 Conditions of Use..................................................................................................138
3.1.2.3 Design and Installation Specifications...................................................................138
3.1.2.4 Maintenance Standards........................................................................................138
3.1.3 BMP C103: High Visibility Plastic or Metal Fence...........................................................139
3.1.3.1 Purpose.................................................................................................................139
3.1.3.2 Conditions of Use..................................................................................................139
3.1.3.3 Design and Installation Specifications...................................................................139
3.1.3.4 Maintenance Standards........................................................................................139
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3.1.4 BMP C104: Stake and Wire Fence..................................................................................140
3.1.4.1 Purpose.................................................................................................................140
3.1.4.2 Conditions of Use..................................................................................................140
3.1.4.3 Design and Installation Specifications...................................................................140
3.1.4.4 Maintenance Standards........................................................................................140
3.1.5 BMP C105: Stabilized Construction Entrance.................................................................141
3.1.5.1 Purpose.................................................................................................................141
3.1.5.2 Conditions of Use..................................................................................................141
3.1.5.3 Design and Installation Specifications...................................................................141
3.1.5.4 Maintenance Standards........................................................................................141
3.1.6 BMP C106: Wheel Wash.................................................................................................144
3.1.6.1 Purpose.................................................................................................................144
3.1.6.2 Conditions of Use..................................................................................................144
3.1.6.3 Design and Installation Specifications...................................................................144
3.1.6.4 Maintenance Standards........................................................................................144
3.1.7 BMP C107: Construction Road/Parking Area Stabilization.............................................146
3.1.7.1 Purpose.................................................................................................................146
3.1.7.2 Conditions of Use..................................................................................................146
3.1.7.3 Design and Installation Specifications...................................................................146
3.1.7.4 Maintenance Standards........................................................................................147
3.1.8 BMP C120: Temporary and Permanent Seeding............................................................148
3.1.8.1 Purpose.................................................................................................................148
3.1.8.2 Conditions of Use..................................................................................................148
3.1.8.3 Design and Installation Specifications...................................................................148
3.1.8.4 Maintenance Standards........................................................................................151
3.1.9 BMP C121: Mulching.......................................................................................................154
3.1.9.1 Purpose.................................................................................................................154
3.1.9.2 Conditions of Use..................................................................................................154
3.1.9.3 Design and Installation Specifications...................................................................154
3.1.9.4 Maintenance Standards........................................................................................154
3.1.10 BMP C122: Nets and Blankets........................................................................................156
3.1.10.1 Purpose.................................................................................................................156
3.1.10.2 Conditions of Use..................................................................................................156
3.1.10.3 Design and Installation Specifications...................................................................157
3.1.10.4 Maintenance Standards........................................................................................158
3.1.11 BMP C123: Plastic Covering...........................................................................................161
3.1.11.1 Purpose.................................................................................................................161
3.1.11.2 Conditions of Use..................................................................................................161
3.1.11.3 Design and Installation Specifications...................................................................161
3.1.11.4 Maintenance Standards........................................................................................162
3.1.12 BMP C124: Sodding........................................................................................................164
3.1.12.1 Purpose.................................................................................................................164
3.1.12.2 Conditions of Use..................................................................................................164
3.1.12.3 Design and Installation Specifications...................................................................164
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3.1.12.4 Maintenance Standards........................................................................................164
3.1.13 BMP C125: Compost.......................................................................................................165
3.1.13.1 Purpose.................................................................................................................165
3.1.13.2 Conditions of Use..................................................................................................165
3.1.13.3 Design and Installation Specifications...................................................................165
3.1.13.4 Maintenance Standards........................................................................................166
3.1.14 BMP C126: Topsoiling.....................................................................................................167
3.1.14.1 Purpose.................................................................................................................167
3.1.14.2 Conditions of Use..................................................................................................167
3.1.14.3 Design and Installation Specifications...................................................................167
3.1.14.4 Maintenance Standards........................................................................................169
3.1.15 BMP C127: Polyacrylamide for Soil Erosion Protection..................................................170
3.1.15.1 Purpose.................................................................................................................170
3.1.15.2 Conditions of Use..................................................................................................170
3.1.15.3 Design and Installation Specifications...................................................................170
3.1.15.4 Maintenance Standards........................................................................................172
3.1.16 BMP C130: Surface Roughening....................................................................................173
3.1.16.1 Purpose.................................................................................................................173
3.1.16.2 Conditions for Use.................................................................................................173
3.1.16.3 Design and Installation Specifications...................................................................173
3.1.16.4 Maintenance Standards........................................................................................173
3.1.17 BMP C131: Gradient Terraces........................................................................................175
3.1.17.1 Purpose.................................................................................................................175
3.1.17.2 Conditions of Use..................................................................................................175
3.1.17.3 Design and Installation Specifications...................................................................175
3.1.17.4 Maintenance Standards........................................................................................176
3.1.18 BMP C140: Dust Control.................................................................................................177
3.1.18.1 Purpose.................................................................................................................177
3.1.18.2 Conditions of Use..................................................................................................177
3.1.18.3 Design and Installation Specifications...................................................................177
3.1.18.4 Maintenance Standards........................................................................................178
3.1.19 BMP C150: Materials On Hand.......................................................................................179
3.1.19.1 Purpose.................................................................................................................179
3.1.19.2 Conditions of Use..................................................................................................179
3.1.19.3 Design and Installation Specifications...................................................................179
3.1.19.4 Maintenance Standards........................................................................................179
3.1.20 BMP C151: Concrete Handling.......................................................................................180
3.1.20.1 Purpose.................................................................................................................180
3.1.20.2 Conditions of Use..................................................................................................180
3.1.20.3 Design and Installation Specifications...................................................................180
3.1.20.4 Maintenance Standards........................................................................................180
3.1.21 BMP C152: Sawcutting and Surfacing Pollution Prevention...........................................181
3.1.21.1 Purpose.................................................................................................................181
3.1.21.2 Conditions of Use..................................................................................................181
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3.1.21.3 Design and Installation Specifications...................................................................181
3.1.21.4 Maintenance Standards........................................................................................181
3.1.22 BMP C153: Material Delivery, Storage and Containment...............................................182
3.1.22.1 Purpose.................................................................................................................182
3.1.22.2 Conditions of Use..................................................................................................182
3.1.22.3 Design and Installation Specifications...................................................................182
3.1.22.4 Material Storage Areas and Secondary Containment Practices:..........................183
3.1.23 BMP C154: Concrete Washout Area...............................................................................184
3.1.23.1 Purpose.................................................................................................................184
3.1.23.2 Conditions of Use..................................................................................................184
3.1.23.3 Implementation......................................................................................................184
3.1.23.4 Education..............................................................................................................185
3.1.23.5 Contracts...............................................................................................................185
3.1.23.6 Location and Placement Considerations:.............................................................185
3.1.23.7 Onsite Temporary Concrete Washout Facility, Transit Truck
Washout Procedures:...........................................................................................185
3.1.23.8 Inspection and Maintenance.................................................................................186
3.1.23.9 Removal of Temporary Concrete Washout Facilities............................................187
3.1.24 BMP C160: Certified Erosion and Sediment Control Lead..............................................190
3.1.24.1 Purpose.................................................................................................................190
3.1.24.2 Conditions of Use..................................................................................................190
3.1.24.3 Specifications........................................................................................................190
3.1.25 BMP C161: Payment of Erosion Control Work................................................................192
3.1.25.1 Purpose.................................................................................................................192
3.1.25.2 Conditions of Use..................................................................................................192
3.1.26 BMP C162: Scheduling....................................................................................................193
3.1.26.1 Purpose.................................................................................................................193
3.1.26.2 Conditions of Use..................................................................................................193
3.1.26.3 Design Considerations..........................................................................................193
3.1.27 BMP C180: Small Project Construction Stormwater Pollution Prevention......................194
3.1.27.1 Purpose.................................................................................................................194
3.1.27.2 Conditions of Use..................................................................................................194
3.1.27.3 Design and Installation Specifications...................................................................194
3.2 Runoff, Conveyance and Treatment BMPs...........................................................................196
3.2.1 BMP C200: Interceptor Dike and Swale..........................................................................196
3.2.1.1 Purpose.................................................................................................................196
3.2.1.2 Conditions of Use..................................................................................................196
3.2.1.3 Design and Installation Specifications...................................................................196
3.2.2 BMP C201: Grass-Lined Channels.................................................................................199
3.2.2.1 Purpose.................................................................................................................199
3.2.2.2 Conditions of Use..................................................................................................199
3.2.2.3 Design and Installation Specifications...................................................................199
3.2.2.4 Maintenance Standards........................................................................................200
3.2.3 BMP C202: Channel Lining.............................................................................................203
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3.2.3.1 Purpose.................................................................................................................203
3.2.3.2 Conditions of Use..................................................................................................203
3.2.3.3 Design and Installation Specifications...................................................................203
3.2.4 BMP C203: Water Bars...................................................................................................205
3.2.4.1 Purpose.................................................................................................................205
3.2.4.2 Conditions of Use..................................................................................................205
3.2.4.3 Design and Installation Specifications...................................................................205
3.2.4.4 Maintenance Standards........................................................................................206
3.2.5 BMP C204: Pipe Slope Drains........................................................................................207
3.2.5.1 Purpose.................................................................................................................207
3.2.5.2 Conditions of Use..................................................................................................207
3.2.5.3 Design and Installation Specifications...................................................................207
3.2.5.4 Maintenance Standards........................................................................................208
3.2.6 BMP C205: Subsurface Drains........................................................................................210
3.2.6.1 Purpose.................................................................................................................210
3.2.6.2 Conditions of Use..................................................................................................210
3.2.6.3 Design and Installation Specifications...................................................................210
3.2.6.4 Maintenance Standards........................................................................................212
3.2.7 BMP C206: Level Spreader.............................................................................................213
3.2.7.1 Purpose.................................................................................................................213
3.2.7.2 Conditions of Use..................................................................................................213
3.2.7.3 Design and Installation Specifications...................................................................213
3.2.7.4 Maintenance Standards........................................................................................214
3.2.8 BMP C207: Check Dams.................................................................................................215
3.2.8.1 Purpose.................................................................................................................215
3.2.8.2 Conditions of Use..................................................................................................215
3.2.8.3 Design and Installation Specifications...................................................................215
3.2.8.4 Maintenance Standards........................................................................................216
3.2.9 BMP C208: Triangular Silt Dike (Geotextile-Encased Check Dam)................................218
3.2.9.1 Purpose.................................................................................................................218
3.2.9.2 Conditions of Use..................................................................................................218
3.2.9.3 Design and Installation Specifications...................................................................218
3.2.9.4 Maintenance Standards........................................................................................219
3.2.10 BMP C209: Outlet Protection..........................................................................................222
3.2.10.1 Purpose.................................................................................................................222
3.2.10.2 Conditions of Use..................................................................................................222
3.2.10.3 Design and Installation Specifications...................................................................222
3.2.10.4 Maintenance Standards........................................................................................223
3.2.11 BMP C220: Storm Drain Inlet Protection.........................................................................224
3.2.11.1 Purpose.................................................................................................................224
3.2.11.2 Conditions of Use..................................................................................................224
3.2.11.3 Design and Installation Specifications...................................................................225
3.2.11.4 Maintenance Standards........................................................................................232
3.2.12 BMP C231: Brush Barrier................................................................................................234
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3.2.12.1 Purpose.................................................................................................................234
3.2.12.2 Conditions of Use..................................................................................................234
3.2.12.3 Design and Installation Specifications...................................................................234
3.2.12.4 Maintenance Standards........................................................................................234
3.2.13 BMP C232: Gravel Filter Berm........................................................................................236
3.2.13.1 Purpose.................................................................................................................236
3.2.13.2 Conditions of Use..................................................................................................236
3.2.13.3 Design and Installation Specifications...................................................................236
3.2.13.4 Maintenance Standards........................................................................................236
3.2.14 BMP C233: Silt Fence.....................................................................................................237
3.2.14.1 Purpose.................................................................................................................237
3.2.14.2 Conditions of Use..................................................................................................237
3.2.14.3 Design and Installation Specifications...................................................................237
3.2.14.4 Maintenance Standards........................................................................................240
3.2.15 BMP C234: Vegetated Strip.............................................................................................243
3.2.15.1 Purpose.................................................................................................................243
3.2.15.2 Conditions of Use..................................................................................................243
3.2.15.3 Design and Installation Specifications...................................................................243
3.2.15.4 Maintenance Standards........................................................................................243
3.2.16 BMP C235: Straw Wattles...............................................................................................244
3.2.16.1 Purpose.................................................................................................................244
3.2.16.2 Conditions of Use..................................................................................................244
3.2.16.3 Design Criteria.......................................................................................................244
3.2.16.4 Maintenance Standards........................................................................................245
3.2.17 BMP C240: Sediment Trap..............................................................................................247
3.2.17.1 Purpose.................................................................................................................247
3.2.17.2 Conditions of Use..................................................................................................247
3.2.17.3 Design and Installation Specifications...................................................................248
3.2.17.4 Maintenance Standards........................................................................................250
3.2.18 BMP C241: Temporary Sediment Pond..........................................................................251
3.2.18.1 Purpose.................................................................................................................251
3.2.18.2 Conditions of Use..................................................................................................251
3.2.18.3 Design and Installation Specifications...................................................................251
3.2.18.4 Maintenance Standards........................................................................................254
3.2.19 BMP C250: Construction Stormwater Chemical Treatment............................................258
3.2.19.1 Purpose.................................................................................................................258
3.2.19.2 Conditions of Use..................................................................................................258
3.2.19.3 Design and Installation Specifications...................................................................258
3.2.19.4 Monitoring..............................................................................................................263
3.2.20 BMP C251: Construction Stormwater Filtration...............................................................265
3.2.20.1 Purpose.................................................................................................................265
3.2.20.2 Conditions of Use..................................................................................................265
3.2.20.3 Background Information........................................................................................265
3.2.20.4 Design and Installation Specifications...................................................................265
Volume II
vii Table of Contents
3.2.20.5 Maintenance Standards........................................................................................268
3.2.21 BMP C252: High pH Neutralization using CO2................................................................269
3.2.21.1 Description............................................................................................................269
3.2.21.2 Treatment Procedures...........................................................................................270
3.2.21.3 Safety and Materials Handling..............................................................................271
3.2.21.4 Operator Records..................................................................................................271
3.2.22 BMP C253: pH Control for High pH Water......................................................................272
3.2.22.1 Description............................................................................................................272
3.2.22.2 Disposal Methods..................................................................................................272
Appendix A Standard Notes for Erosion Control Plans.....................................................273
Appendix B Background Information on Chemical Treatment..........................................274
Appendix C Construction SWPPP Short Form...................................................................277
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Purpose Volume II
Content and Organization Introduction 113
Volume II:
Stormwater Management for
Construction Sites
Purpose of this Volume
This volume of the Surface Water Management Manual discusses stormwater impacts and controls
associated with construction activities. It addresses the planning, design, and implementation of
stormwater management activities prior to and during the construction phase of projects.
The purpose of this volume is to provide guidance to prevent construction activities from adversely
impacting downstream resources and on-site stormwater flows. Prevention of soil erosion, capture of
water-borne sediment that has been unavoidably released from exposed soils, and protection of
water quality from on-site pollutant sources are all readily achievable when the proper Best
Management Practices (BMPs) are planned, installed, and properly maintained.
Content and Organization of this Volume
Volume II consists of three chapters that address the preparation and implementation of Construction
Stormwater Pollution Prevention Plans (SWPPPs).
• Chapter 1 describes the 12 elements of stormwater pollution prevention.
• Chapter 2 presents a step-by-step method for developing a Construction SWPPP. It
encourages examination of all possible conditions that could reasonably affect a
particular project’s stormwater control systems during the construction phase of the
project.
• Chapter 3 contains BMPs for construction stormwater control and site management.
The first section of Chapter 3 contains BMPs for Source Control. The second section
addresses runoff, conveyance, and treatment BMPs. Various combinations of these
BMPs should be used in the Construction SWPPP to satisfy each of the 12 elements
applying to the project.
Volume
II
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Chapter 1 The 12 Elements of Construction
Stormwater Pollution Prevention
The 12 elements of construction stormwater pollution prevention cover the general water quality
protection strategies of limiting site impacts, preventing erosion and sedimentation, and managing
activities and sources. The applicant is required to address the following 12 elements in the
construction stormwater pollution prevention plan (SWPPP). If an element is considered
unnecessary, the Construction SWPPP must describe why that element is not needed.
The 12 elements are:
• Element 1 – Mark Clearing Limits
• Element 2 – Establish Construction Access
• Element 3 – Control Flow Rates
• Element 4 – Install Sediment Controls
• Element 5 – Stabilize Soils
• Element 6 – Protect Slopes
• Element 7 – Protect Drain Inlets
• Element 8 – Stabilize Channels and Outlets
• Element 9 – Control Pollutants
• Element 10 – Control Dewatering
• Element 11 – Maintain BMPs
• Element 12 – Manage the Project
Element #1: Mark Clearing Limits
• Before beginning any land disturbing activities, including clearing and grading, clearly
mark all clearing limits, sensitive areas and their buffers, and trees that are to be
preserved within the construction area to prevent damage and offsite impacts. Mark
clearing limits both in the field and on the plans.
• Plastic, metal, or stake wire fence may be used to mark the clearing limits.
• Suggested BMPs:
o BMP C101: Preserving Natural Vegetation
o BMP C102: Buffer Zones
o BMP C103: High Visibility Plastic or Metal Fence
o BMP C104: Stake and Wire Fence
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Element #2: Establish Construction Access
• Construction vehicle ingress and egress shall be limited to one route. Additional
routes may be allowed for very large projects or linear projects.
• Access points shall be stabilized per BMP C105 – Stabilized Construction Entrance.
• Wheel wash or tire baths shall be located on site, if applicable. Wheel washes shall
be required if other measures fail to control sediment from leaving the site.
• No tracking of sediment onto the roadway is allowed. If sediment is tracked onto the
road, the road shall be thoroughly and immediately cleaned by shoveling or pickup
sweeping. Transport sediment to a controlled sediment disposal area.
• Keep streets clean at ALL times. Clean tracked sediment immediately.
• Street washing of sediment to the storm drain system is not allowed.
• Suggested BMPs:
o BMP C105: Stabilized Construction Entrance
o BMP C106: Wheel Wash
o BMP C107: Construction Road/Parking Area Stabilization
Element #3: Control Flow Rates
• Protect properties and waterways downstream of development sites from erosion
due to increases in the volume, velocity, and peak flow rate of stormwater runoff from
the project site.
• Conduct a downstream analysis if changes to offsite flows could impair or alter
conveyance systems, stream banks, bed sediment, or aquatic habitat. See Volume I,
Chapter 3 – Minimum Requirement #11 for offsite analysis guidelines.
• Construct stormwater detention facilities as one of the first steps in grading.
Detention facilities shall be functional prior to construction of site improvements (e.g.
impervious surfaces).
• During construction, the City may require non-standard temporary sediment control
pond designs in order to provide additional flow control necessary to address local
conditions or to protect properties and waterways downstream from erosion due to
construction activities.
• Permanent infiltration ponds shall not be used for flow control during construction
unless specifically allowed in writing by the City. If allowed, these facilities shall be
protected from siltation during the construction phase as required by the City. A liner
may be required. The ponds shall be excavated to final grade after the site is
stabilized.
• Suggested BMPs:
o BMP C240: Sediment Trap
o BMP C241: Temporary Sediment Pond
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Element #4: Install Sediment Controls
• Retain the duff layer, native topsoil, and natural vegetation in an undisturbed state to
the maximum extent practicable. If it is not practicable to retain the duff layer in
place, it should be stockpiled on-site, covered to prevent erosion, and replaced
immediately upon completion of the ground disturbing activities.
• Prior to leaving a construction site or prior to discharge to an infiltration facility,
surface water runoff from disturbed areas shall pass through a sediment pond or
other appropriate sediment removal BMP.
• Construct sediment ponds, vegetated buffer strips, sediment barriers or filters, dikes,
and other BMPs intended to trap sediment on site as one of the first steps in grading.
These BMPs shall be functional before other land disturbing activities take place.
• Locate BMPs in a manner to avoid interference with the movement of juvenile
salmonids attempting to enter off-channel areas or drainages.
• Seed and mulch earthen structures such as dams, dikes, and diversions according to
the timing indicated in Element #5.
• Suggested BMPs:
o BMP C231: Brush Barrier
o BMP C232: Gravel Filter Berm
o BMP C233: Silt Fence
o BMP C234: Vegetated Strip
o BMP C235: Straw Wattles
o BMP C240: Sediment Trap
o BMP C241: Temporary Sediment Pond
o BMP C250: Construction Stormwater Chemical Treatment
o BMP C251: Construction Stormwater Filtration
• Proprietary technologies exist that can be used for sediment control. Ecology to
determine if the temporary sediment control device is equivalent to an existing BMP
or requires Ecology approval via the Technology Assessment Protocol Ecology
program.
Element #5: Stabilize Soils
• Stabilize exposed and unworked soils by application of effective BMPs that protect
the soil from the erosive forces of raindrop impact, flowing water, and wind.
• From October 1 through April 30, no soils shall remain exposed and unworked for
more than 2 days. From May 1 to September 30, no soils shall remain exposed and
unworked for more than 7 days. This stabilization requirement applies to all soils on
site, whether at final grade or not. (See City’s Clearing and Grading Code
ACC 15.74).
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• Stabilize soils at the end of the shift, before a holiday or weekend, if needed, based
on the weather forecast.
• Select appropriate soil stabilization measures for the time of year, site conditions,
estimated duration of use, and the potential water quality impacts that stabilization
agents may have on downstream waters or groundwater.
• Stabilize soil stockpiles from erosion, protect stockpiles with sediment trapping
measures, and where possible, locate piles away from storm drain inlets, waterways,
and drainage channels.
• Suggested BMPs:
o BMP C120: Temporary and Permanent Seeding
o BMP C121: Mulching
o BMP C122: Nets and Blankets
o BMP C123: Plastic Covering
o BMP C124: Sodding
o BMP C125: Compost
o BMP C126: Topsoiling
o BMP C127: Polyacrylamide for Soil Erosion Protection
o BMP C130: Surface Roughening
o BMP C131: Gradient Terraces
o BMP C140: Dust Control
o BMP C180: Small Project Construction Stormwater Pollution Prevention
Element #6: Protect Slopes
• Reduce slope runoff velocities by reducing continuous length of slope with terracing
and diversions, reducing slope steepness, and/or roughing slope surface.
• Divert off-site stormwater (sometimes called run-on) away from slopes and disturbed
areas with interceptor dikes and/or swales. Manage off-site stormwater separately
from stormwater generated on the site.
• At the top of slopes, collect drainage in pipe slope drains or protected channels to
prevent erosion. Size temporary pipe slope drains for the peak flow from a 10-year,
24-hour event. Alternatively, the 10-year and 25-year, 1-hour flow rates indicated by
WWHM, increased by a factor of 1.6, may be used. Size permanent pipe slope
drains for the 25-year, 24-hour event. Use the existing land cover condition for
predicting flow rates from tributary areas outside the project limits for the hydrologic
analysis. For tributary areas on the project site, use the temporary or permanent
project land cover condition, whichever will produce the higher flows for the analysis.
If using WWHM to predict flows, model bare soils and landscaped areas.
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• Provide drainage to remove groundwater seepage from the slope surface of exposed
soil areas.
• Place excavated material on the uphill side of trenches, consistent with safety and
space considerations.
• Place check dams at regular intervals within channels that are cut down a slope.
• Stabilize soils on slopes, as specified in Element #5.
• Suggested BMPs:
o BMP C120: Temporary and Permanent Seeding
o BMP C130: Surface Roughening
o BMP C131: Gradient Terraces
o BMP C200: Interceptor Dike and Swale
o BMP C201: Grass-Lined Channels
o BMP C204: Pipe Slope Drains
o BMP C205: Subsurface Drains
o BMP C206: Level Spreader
o BMP C207: Check Dams
o BMP C208: Triangular Silt Dike (Geotextile-Encased
Check Dam)
Element #7: Protect Drain Inlets
• Protect all storm drain inlets that are operable during construction so that stormwater
runoff does not enter the conveyance system without first being filtered or treated to
remove sediment.
• Keep all approach roads clean. Do not allow sediment to enter storm drains.
• Inspect inlets weekly at a minimum and after each storm events. Clean or remove
and replace inlet protection devices when sediment has filled one-third of the
available storage (unless a different standard is specified by the product
manufacturer).
• Suggested BMPs:
o BMP C220: Storm Drain Inlet Protection
Element #8: Stabilize Channels and Outlets
• Design, construct, and stabilize all temporary on-site conveyance channels to
prevent erosion from the expected peak 10-minute velocity of a 10-year, 24-hour
frequency storm for the developed condition. Alternatively, the 10-year, 1-hour flow
rate indicated by an approved continuous runoff model, increased by a factor of 1.6,
may be used. For tributary areas outside the project limits, use the existing land
cover conditions for predicting flow rates from tributary areas outside the project
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limits for the hydrologic analysis. For tributary areas on the project site, use the
temporary or permanent project land cover condition, whichever will produce the
highest flow rates, for the hydrologic analysis. If using WWHM, model bare soils as
landscaped.
• Provide stabilization, including armoring material, adequate to prevent erosion of
outlets, adjacent stream banks, slopes, and downstream reaches at the outlets of all
conveyance systems.
• Suggested BMPs:
o BMP C202: Channel Lining
o BMP C209: Outlet Protection
Element #9: Control Pollutants
• All discharges to the City sewer system (storm or sanitary sewers) require City
approval.
• Handle and dispose of all pollutants, including waste materials and demolition debris
that occur on site during construction in a manner that does not cause contamination
of stormwater. Woody debris may be chopped and spread on site.
• Provide cover, containment, and protection for all chemicals, liquid products,
petroleum products, and other materials that have the potential to pose a threat to
human health and the environment. Include secondary containment for on-site
fueling tanks.
• Use spill prevention and control measures, such as drip pans, when conducting
maintenance and repair of heavy equipment and vehicles involving oil changes,
hydraulic system drain down, solvent and de-greasing cleaning operations, fuel tank
drain down and removal, and other activities which may result in discharge or
spillage of pollutants to the ground or into stormwater runoff. Clean contaminated
surfaces immediately following any discharge or spill incident. Emergency repairs
may be performed on-site using temporary plastic placed beneath and, if raining,
over the vehicle.
• Discharge wheel wash or tire bath wastewater to a separate on-site treatment
system or to the sanitary sewer.
• Only apply agricultural chemicals, including fertilizers and pesticides, when
absolutely necessary and only in a manner and at application rates that will not result
in loss of chemical to stormwater runoff. Follow manufacturers’ recommendations for
application rates and procedures.
• Use BMPs to prevent or treat contamination of stormwater runoff by pH modifying
sources. These sources include, but are not limited to, bulk cement, cement kiln
dust, fly ash, new concrete washing and curing waters, waste streams generated
from concrete grinding and sawing, exposed aggregate processes, and concrete
pumping and mixer washout waters. Construction site operators must adjust the pH
of stormwater to prevent violations of water quality standards.
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• Written approval from the Department of Ecology is required prior to using chemical
treatment other than CO2 or dry ice to adjust pH.
• Suggested BMPs:
o BMP C151: Concrete Handling
o BMP C152: Sawcutting and Surfacing Pollution Prevention
o BMP C154: Concrete Washout Area
o Source Control BMPs from Volume IV, as appropriate
Element #10: Control Dewatering
• All discharges to the City sewer system (storm or sanitary sewers) require City
approval.
• Discharge foundation, vault, and trench dewatering water that has similar
characteristics to site stormwater runoff into a controlled conveyance system prior to
discharge to a sediment pond or sediment tank/vault. Stabilize channels as specified
in Element #8.
• Clean, non-turbid dewatering water, such as well-point groundwater, can be
discharged to systems tributary to state surface waters, as specified in Element #8,
provided the dewatering flow does not cause erosion or flooding of receiving waters.
These clean waters should not be routed through stormwater sediment ponds/tanks.
• Handle highly turbid or contaminated dewatering water from construction equipment
operation, clamshell digging, concrete tremie pour, or work inside a cofferdam
separately from stormwater at the site.
• Other disposal options, depending on site constraints, may include:
o Infiltration
o Transport off-site in vehicle, such as a vacuum flush truck, for legal disposal
in a manner that does not pollute state waters
o Ecology approved on-site chemical treatment or other suitable treatment
technologies
o Use of a sedimentation bag with outfall to a ditch or swale for small volumes
of localized dewatering
Element #11: Maintain BMPs
• Maintain and repair as needed all temporary and permanent erosion and sediment
control BMPs to assure continued performance of their intended function. Conduct
maintenance and repair in accordance with BMP specifications.
• Remove temporary erosion and sediment control BMPs within 30 days after final site
stabilization is achieved or after the temporary BMPs are no longer needed. Trapped
sediment shall be removed or stabilized on site. Permanently stabilize disturbed soil
resulting from removal of BMPs or vegetation.
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Element #12: Manage the Project
• Phasing of Construction – Phase development projects in order to prevent soil
erosion and the transport of sediment from the project site during construction,
unless the project engineer can demonstrate that construction phasing is infeasible.
Revegetation of exposed areas and maintenance of that vegetation shall be an
integral part of the clearing activities for any phase.
• Seasonal Work Limitations – From October 1 through April 30, clearing, grading,
and other soil disturbing activities shall only be permitted if shown to the satisfaction
of the City that silt-laden runoff will be prevented from leaving the site through a
combination of the following:
o Site conditions including existing vegetative coverage, slope, soil type, and
proximity to receiving waters;
o Limitations on activities and the extent of disturbed areas; and
o Proposed erosion and sediment control measures.
Based on the information provided and local weather conditions, the City may expand or
restrict the seasonal limitation on site disturbance. The following activities are exempt from
the seasonal clearing and grading limitations:
o Routine maintenance and necessary repair of erosion and sediment control
BMPs
o Routine maintenance of public facilities or existing utility structures that do not
expose the soil or result in the removal of the vegetative cover to soil
o Activities where there is one hundred percent infiltration of surface water
runoff within the site in approved and installed erosion and sediment control
facilities
• Coordination with Utilities and Other Contractors – Include surface water
management requirements for the entire project, including the utilities and other
contractors, in the Construction SWPPP.
• Inspection and Monitoring – Inspect, maintain, and repair all BMPs as needed to
assure continued performance of their intended function. At a minimum, inspect all
BMPs after each storm event. Site inspections shall be conducted by a person who
is knowledgeable in the principles and practices of erosion and sediment control. The
person must have the skills to 1) assess the site conditions and construction
activities that could impact the quality of stormwater, and 2) assess the effectiveness
of erosion and sediment control measures used to control the quality of stormwater
discharges.
For construction sites one acre or larger that discharge to surface waters of the state, a
Certified Erosion and Sediment Control Lead (CESCL) shall be identified in the Construction
SWPPP and shall be on-site or on-call at all times. Certification must be obtained through an
Ecology-approved training program.
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Sampling and analysis of the surface water discharges from a construction site may be
necessary on a case-by-case basis to ensure compliance with standards. Ecology or the City
will establish these monitoring and associated reporting requirements.
Whenever inspection and/or monitoring reveals that the BMPs identified in the Construction
SWPPP are inadequate, due to the actual discharge of or potential to discharge a significant
amount of any pollutant, the appropriate BMPs or design changes shall be implemented as
soon as possible.
• Reporting – Report spillage or discharge of pollutants within 24-hours to the City of
Auburn Spill Hotline 24-hour phone number at (253) 931-3048.
• Maintenance of the Construction SWPPP – Keep the Construction SWPPP on-site
or within reasonable access to the site. Modify the SWPPP whenever there is a
change in the design, construction, operation, or maintenance at the construction site
that has, or could have, a significant effect on the discharge of pollutants to waters of
the state.
Modify the SWPPP if, during inspections or investigations conducted by the owner/operator,
City staff, or by local or state officials, it is determined that the SWPPP is ineffective in
eliminating or significantly minimizing pollutants in stormwater discharges from the site.
Modify the SWPPP as necessary to include additional or modified BMPs designed to correct
problems identified. Complete revisions to the SWPPP within seven (7) days following the
inspection.
The inspector may require that a modification to the SWPPP go through additional City
review.
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Chapter 2 Developing a Construction Stormwater
Pollution Prevention Plan (SWPPP)
This chapter provides an overview of the important components of, and the process for, developing
and implementing a Construction Stormwater Pollution Prevention Plan (SWPPP).
2.1 General Requirements and Guidelines
The Construction SWPPP is a document that describes the potential for pollution problems on a
construction project. The Construction SWPPP explains and illustrates the measures to be taken on
the construction site to control those problems.
All sites are required to comply with elements #1-#12.
Unless located in a critical area, a SWPPP is not required for projects that:
• Add or replace less than 2000 square feet of impervious surface, or,
• Disturb less than 7000 square feet of land
The Construction Stormwater Pollution Prevention Plan Short Form (Appendix C) may be used for
projects that:
• Add or replace between 2000 square feet and 5000 square feet of impervious
surface, or,
• Disturb between 7000 square feet and 1 acre
A complete SWPPP is required for projects that:
• Add or replace 5000 square feet or greater of impervious surface, or,
• Disturb greater than1 acre, or,
• Grade/Fill greater than 500 cubic yards of material.
The Construction SWPPP shall be prepared as a separate stand-alone document. Keep the
Construction SWPPP on the construction site or within reasonable access to the site for construction
and inspection personnel. As site work progresses, the plan must be modified to reflect changing site
conditions, subject to the rules for plan modification by the City.
Include all 12 elements described in Volume II, Chapter 1 in the Construction SWPPP unless an
element is determined not to be applicable to the project and the exemption is justified in the
narrative.
2.1.1 BMP Standards and Specifications
Chapter 3 of this volume contains standards and specifications for the BMPs referred to in this
chapter. Wherever any of these BMPs are to be employed on a site, clearly reference the specific title
and number of the BMP in the narrative and mark it on the drawings.
Where appropriate BMPs do not exist, experimental practices may be considered or minor
modifications to standard practices may be employed. Such practices must be approved by the City
before implementation.
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2.1.2 General Principles
The following general principles should be applied to the development of any Construction SWPPP.
• Retain the duff layer, native topsoil, and natural vegetation in an undisturbed state to
the maximum extent practicable.
• Prevent pollutant release. Select source control BMPs as a first line of defense.
Prevent erosion rather than treat turbid runoff.
• Select BMPs depending on site characteristics (topography, drainage, soil type,
ground cover, and critical areas) and the construction plan.
• Divert runoff away from exposed areas wherever possible. Keep clean water clean.
• Limit the extent of clearing operations and phase construction operations.
• Before reseeding a disturbed soil area, amend all soils with compost wherever
topsoil has been removed.
• Incorporate natural drainage features whenever possible, using adequate buffers
and protecting areas where flow enters the drainage system.
• Minimize slope length and steepness.
• Reduce runoff velocities to prevent channel erosion.
• Prevent the tracking of sediment off-site.
• Select appropriate BMPs for the control of pollutants in addition to sediment.
• Be realistic about the limitations of BMPs specified and the operation and
maintenance of those BMPs. Anticipate what may go wrong, how you can prevent it
from happening, and what will need to be done to fix it.
2.2 Step-by-Step Procedure
There are three basic steps in producing a Construction SWPPP:
• Step 1 - Data Collection
• Step 2 - Data Analysis
• Step 3 - Construction SWPPP Development and Implementation
Steps 1 and 2, described in more detail below, are intended for projects that must complete a full
SWPPP. Smaller projects below the thresholds indicated in Section 2.1 may prepare a short form
Construction SWPPP, consisting of a checklist and a plan view (see Appendix C).
2.2.1 Step 1 – Data Collection
Evaluate existing site conditions and gather information that will help develop the most effective
Construction SWPPP. The information gathered should be explained in the narrative and shown on
the drawings. Appendix A provides standard notes required on the drawing.
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• Topography - Prepare a topographic drawing of the site to show the existing contour
elevations at intervals of 1 to 5 feet depending upon the slope of the terrain.
• Drainage - Locate and clearly mark existing drainage ditches, swales, and patterns
on the drawing, including existing storm drain pipe systems. Mark location of site
runon and runoff on drawing.
• Soils - Identify and label soil type(s) and erodibility (low, medium, high). A
geotechnical investigation may be required since published soils information in the
City is very limited. Regardless of the availability of published soils information, the
project proponent is responsible for characterizing site soils for erosive potential.
• Ground Cover - Label existing vegetation on the drawing. Show such features as
tree clusters, grassy areas, and unique or sensitive vegetation. Unique vegetation
may include existing trees above a given diameter. The City of Auburn encourages
tree preservation where possible. In addition, indicate existing denuded or exposed
soil areas.
• Critical Areas - Delineate critical areas adjacent to or within the site on the drawing.
Such features as steep slopes, streams, floodplains, lakes, wetlands, sole source
aquifers, and geologic hazard areas, etc., should be shown. Delineate setbacks and
buffer limits for these features on the drawings. Other related jurisdictional
boundaries such as Shorelines Management and the Federal Emergency
Management Agency (FEMA) base floodplain should also be shown on the
drawings.
• Adjacent Areas - Identify existing buildings, roads, and facilities adjacent to or within
the project site on the drawings. Identify existing and proposed utility locations,
construction clearing limits, and erosion and sediment control BMPs on the drawings.
• Existing Encumbrances - Identify wells, existing and abandoned septic drain fields,
utilities, easements, and site constraints.
• Precipitation Records - Determine the average monthly rainfall and rainfall intensity
for the required design storm events.
2.2.2 Step 2 – Data Analysis
Consider the data collected in Step 1 to visualize potential problems and limitations of the site.
Determine those areas that have critical erosion hazards. The following are some important factors to
consider in data analysis:
• Topography - The primary topographic considerations are slope steepness and
slope length. The longer and steeper the slope, the greater the erosion potential.
Erosion potential should be determined by a qualified engineer, soil professional, or
certified erosion control specialist. Measures to decrease erosion potential shall be
considered.
• Drainage - Natural drainage patterns that consist of overland flow, swales, and
depressions should be used to convey runoff through the site to avoid construction of
an artificial drainage system. Man-made ditches and waterways will become part of
the erosion problem if they are not properly stabilized. Care should be taken to
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ensure that increased runoff from the site will not erode or flood the existing natural
drainage system. Possible sites for temporary surface water retention and detention
should be considered at this point.
• Direct construction site runoff away from saturated soil areas where groundwater
may be encountered and critical areas where drainage will concentrate. Preserve
natural drainage patterns on the site.
• Soils - Evaluate soil properties such as surface and subsurface runoff
characteristics, depth to impermeable layer, depth to seasonal groundwater table,
permeability, shrink-swell potential, texture, settleability, and erodibility. Develop the
Construction SWPPP based on known soil characteristics. Infiltration sites should be
properly protected from clay and silt which will reduce infiltration capacities.
• Ground Cover - Ground cover is the most important factor in terms of preventing
erosion. Existing vegetation that can be saved will prevent erosion better than
constructed BMPs. Trees and other vegetation protect the soil structure. Disturb as
little of the site as required to construct proposed improvements. If the existing
vegetation cannot be saved, consider such practices as phasing of construction,
temporary seeding, and mulching. Phasing of construction involves stabilizing one
part of the site before disturbing another. In this way, the entire site is not disturbed
at once.
• Critical Areas - Critical areas may include flood hazard areas, mine hazard areas,
slide hazard areas, sole source aquifers, wetlands, stream banks, fish-bearing
streams, and other water bodies. Any critical areas within or adjacent to the
development shall be a key consideration on land development decisions. Critical
areas and their buffers shall be delineated on the drawings and clearly flagged in the
field. Critical areas identified by the City of Auburn are available from the Planning,
Building & Community Department. Orange plastic fencing may be more useful than
flagging to assure that equipment operators stay out of critical areas. Only
unavoidable work should take place within critical areas and their buffers. Such
unavoidable work will require special BMPs, permit restrictions, and mitigation plans.
• Adjacent Areas - An analysis of adjacent properties should focus on areas upslope
and down slope from the construction project. Water bodies that will receive direct
runoff from the site are a major concern. Investigate and identify runon to the site.
The types, values, and sensitivities of and risks to downstream resources, such as
private property, stormwater facilities, public infrastructure, or aquatic systems,
should be evaluated. Develop a plan to route runon around areas disturbed by
construction. Erosion and sediment controls should be selected accordingly.
• Precipitation Records - Refer to Volume III to determine the required rainfall
records and the method of analysis for design of BMPs.
• Timing of the Project - An important consideration in selecting BMPs is the timing
and duration of the project. Projects that will proceed during the wet season and
projects that will last through several seasons must take all necessary precautions to
remain in compliance with the water quality standards.
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2.2.3 Step 3 – Construction SWPPP Development and Analysis
The Construction SWPPP consists of two parts: a narrative and the drawings. This section describes
the contents of the narrative and the drawings. The Department of Ecology has prepared a SWPPP
template that offers a quick and convenient means for developing a SWPPP for development and
redevelopment projects in the City of Auburn. This template can be found on Ecology’s website at:
http://www.ecy.wa.gov/programs/wq/stormwater/construction/
NOTE: Ensure that BMP numbers and references match the City SWMM when using the Ecology
template.
2.2.3.1 Construction SWPPP Narrative
The following topic headings shall be used, at a minimum, when preparing the Construction SWPPP
narrative.
• Project Description – Describe the nature and purpose of the construction project.
Include the total size of the area, any increase in existing impervious area; the total
area expected to be disturbed by clearing, grading, excavation or other construction
activities, including off-site borrow and fill areas; and the volumes of grading, cut and
fill that are proposed.
• Existing Site Conditions – Describe the existing topography, vegetation, and
drainage (including runon and runoff). Include a description of any structures or
development on the parcel including the area of existing impervious surfaces.
• Adjacent Areas – Describe adjacent areas, including streams, lakes, wetlands,
residential areas, and roads that might be affected by the construction project.
Provide a description of the downstream drainage leading from the site to the
receiving body of water.
• Critical Areas – Describe areas on or adjacent to the site that are classified as
critical areas. Critical areas that receive runoff from the site shall be described up to
¼ mile away. The distance may be increased by the City if special downstream
critical areas exist. Describe special requirements for working near or within these
areas. Critical areas identified by the City of Auburn from the Planning, Building &
Community Department. Critical areas not identified on the website still require
consideration.
• Soils – Describe the soils on the site, giving such information as soil names,
mapping unit, erodibility, settleability, permeability, depth, texture, and soil structure.
• Potential Erosion Problem Areas – Describe areas on the site that have potential
erosion problems.
• Construction Stormwater Pollution Prevention Elements – Describe how the
Construction SWPPP addresses each of the 12 required elements. Include the type
and location of BMPs used to satisfy the required element. If an element is not
applicable to a project, provide a written justification for why it is not necessary.
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• Construction Phasing – Describe the intended sequence and timing of construction
activities.
• Construction Schedule – Describe the construction schedule. If the schedule
extends into the wet season, describe what activities will continue during the wet
season and how the transport of sediment from the construction site to receiving
waters will be prevented.
• Financial/Ownership Responsibilities – Describe ownership and obligations for
the project. Include bond forms and other evidence of financial responsibility for
environmental liabilities associated with construction.
• Engineering Calculations – Attach any calculations made for the design of BMPs
such as sediment ponds, diversions, and waterways, as well as calculations for
runoff and stormwater detention design (if applicable). Engineering calculations must
bear the signature and stamp of an engineer licensed in the state of Washington.
Provide references for all variables used and clearly state any assumptions.
2.2.3.2 Erosion and Sediment Control Drawings
See the City of Auburn Engineering Design Standard 3.04 for plan sheet requirements.
2.3 Construction SWPPP Checklists
The following checklists provide a tool to the applicant to determine if all the major items are included
in the Construction SWPPP. The checklist will be used by reviewers to determine that SWPPPs meet
all requirements and are complete. Applicants are encouraged to complete and submit this form with
their application.
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Construction Stormwater Pollution Prevention Plan Checklist
Project Name:
Address:
Parcel No.: ______________ Section: _________ Township: __________ Range: ________
City Reference/Permit No.:
Responsible Parties: Owner:__________________ Engineer:_____________________
Section I – Construction SWPPP Narrative
1. Project Description
A. Total project area.
B. Total proposed impervious area.
C. Total proposed area to be disturbed, including off-site borrow and fill areas.
D. Total volumes of proposed cut and fill.
2. Existing Site Conditions
A. Description of the existing topography.
B. Description of the existing vegetation.
C. Description of the existing drainage.
3. Adjacent Areas
A. Description of adjacent areas which may be affected by site disturbance
1. Streams
2. Lakes
3. Wetlands
4. Residential areas
5. Roads
6. Ditches, pipes, culverts
7. Other
B. Description of the downstream drainage path leading from the site to the receiving
body
of water (minimum distance of ¼ mile.)
4. Critical Areas
A. Description of critical areas that are on or adjacent to the site.
B. Description of special requirements for working in or near critical areas.
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Project Name:
Address: Parcel No:
City Reference/Permit No.:
5. Soils
Description of on-site soils.
1. Soil name(s)
2. Soil mapping unit
The following information may be required:
• Erodibility
• Settleability
• Permeability
• Depth
• Texture
• Soil structure
6. Potential Erosion Problem Areas
Description of potential erosion problems on site.
7. Construction Stormwater Pollution Prevention Elements
A. Describe how each of the Construction Stormwater Pollution Prevention Elements
has
been addressed though the Construction SWPPP.
B. Identify the type and location of BMPs used to satisfy the required element.
C. Written justification identifying the reason an element is not applicable to the proposal.
12 Required Elements - Construction Stormwater Pollution Prevention Plan:
1. Mark Clearing Limits
2. Establish Construction Access
3. Control Flow Rates
4. Install Sediment Controls
5. Stabilize Soils
6. Protect Slopes
7. Protect Drain Inlets
8. Stabilize Channels and Outlets
9. Control Pollutants
10. Control Dewatering
11. Maintain BMPs
12. Manage the Project
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Project Name:
Address: Parcel No:
City Reference/Permit No.:
8. Construction Phasing
A. Construction sequence
B. Construction phasing (if proposed)
9. Construction Schedule
A. Provide a proposed construction schedule.
B. Wet Season Construction Activities
1. Proposed wet season construction activities.
2. Proposed wet season construction restraints for environmentally
sensitive/critical areas.
10. Financial/Ownership Responsibilities
A. Identify the property owner responsible for the initiation of bonds and/or other financial
securities.
B. Describe bonds and/or other evidence of financial responsibility for liability associated
with erosion and sedimentation impacts.
C. Maintenance bond.
11. Engineering Calculations
Provide Design Calculations.
1. Sediment ponds/traps
2. Diversions
3. Waterways
4. Runoff/stormwater detention calculations
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Construction Stormwater Pollution Prevention Plan Checklist
Project Name:
Address: Parcel No.:
City Reference/Permit No.:
Responsible Parties: Owner:___________________ Engineer:_________________
Section II - Erosion and Sediment Control Drawings
1. General
A. Vicinity map with roads and waters of the state within one mile of the site.
B. Address, Parcel Number, and Street names labels
C. Erosion and Sediment Control Notes
2. Site Plan
A. Legal description of subject property.
B. North Arrow
C. Indicate boundaries of existing vegetation, e.g. tree lines, pasture areas, etc.
D. Identify and label areas of potential erosion problems.
E. Identify any on-site or adjacent surface waters, critical areas and associated buffers.
F. Identify FEMA base flood boundaries and Shoreline Management boundaries
(if applicable).
G. Show existing and proposed contours.
H. Indicate drainage basins and direction of flow for individual drainage areas.
I. Label final grade contours and identify developed condition drainage basins.
J. Delineate areas that are to be cleared and graded.
K. Show all cut and fill slopes indicating top and bottom of slope catch lines.
3. Conveyance Systems
A. Designate locations for swales, interceptor trenches, or ditches.
B. Show all temporary and permanent drainage pipes, ditches, or cut-off trenches required
for erosion and sediment control.
C. Provide minimum slope and cover for all temporary pipes or call out pipe inverts.
D. Show grades, dimensions, and direction of flow in all ditches, swales, culverts and
pipes.
E. Provide details for bypassing offsite runoff around disturbed areas.
F. Indicate locations and outlets of any dewatering systems.
4. Location of Detention BMPs
Identify location of detention BMPs.
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Project Name:
Address: Parcel No.:
City Reference/Permit No.:
5. Erosion and Sediment Control Facilities
Show the locations of all ESC facilities with dimensions and details as appropriate.
6. Detailed Drawings
Any best management practices used that are not referenced in the SWMM should be
explained and illustrated with detailed drawings.
7. Other Pollutant BMPs
Indicate on the site plan the location of BMPs to be used for the control of pollutants
other than sediment, e.g. concrete wash water.
8. Monitoring Locations
Indicate on the site plan the water quality sampling locations to be used for monitoring
water quality on the construction site, if applicable.
Describe inspection reporting responsibility, documentation, and filing.
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Chapter 3 Standards and Specifications for Best
Management Practices (BMPs)
BMPs are defined as schedules of activities, prohibitions of practices, maintenance procedures, and
structural and/or managerial practices, that when used singly or in combination, prevent or reduce the
release of pollutants to waters of Washington State. This chapter contains standards and
specifications for temporary BMPs to be used as applicable during the construction phase of a
project.
Section 3.1 contains the standards and specifications for Source Control BMPs specific to
construction operations.
Section 3.2 contains the standards and specifications for Runoff Conveyance and Treatment BMPs.
The standards for each individual BMP are divided into four sections:
1. Purpose
2. Conditions of Use
3. Design and Installation Specifications
4. Maintenance Standards
Note that the “Conditions of Use” always refers to site conditions. As site conditions change, BMPs
must be changed to remain in compliance.
Information on stream bank stabilization is available in the Integrated Streambank Protection
Guidelines, Washington State Department of Fish and Wildlife, 2003.
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3.1 Source Control BMPs
3.1.1 BMP C101: Preserving Natural Vegetation
3.1.1.1 Purpose
The purpose of preserving natural vegetation is to reduce erosion wherever practicable. Limiting site
disturbance is the single most effective method for reducing erosion. For example, conifers can hold
up to about 50 percent of all rain that falls during a storm. Up to 20-30 percent of this rain may never
reach the ground but is taken up by the tree or evaporates. Another benefit is that the rain held in the
tree can be released slowly to the ground after the storm.
3.1.1.2 Conditions of Use
Natural vegetation should be preserved on steep slopes, near perennial and intermittent
watercourses or swales, in wooded areas, and any other location practicable.
3.1.1.3 Design and Installation Specifications
Natural vegetation can be preserved in natural clumps or as individual trees, shrubs and vines.
The preservation of individual plants is more difficult because heavy equipment is generally used to
remove unwanted vegetation. The points to remember when attempting to save individual plants are:
• Is the plant worth saving? Consider the location, species, size, age, vigor, and the
work involved. The City of Auburn encourages the preservation of natural vegetation
and trees.
• Fence or clearly mark areas around trees that are to be saved. Keep ground
disturbance away from the trees as far out as the dripline (at a minimum).
Plants need protection from three kinds of injuries:
• Construction Equipment - This injury can be above or below the ground level.
Damage results from scarring, cutting of roots, and compaction of the soil. Placing a
fenced buffer zone around plants to be saved prior to construction can prevent
construction equipment injuries.
• Grade Changes - Changing the natural ground level will alter grades, which affects
the plant's ability to obtain the necessary air, water, and minerals. Minor fills usually
do not cause problems although sensitivity between species does vary and should
be checked. Trees can tolerate fill of 6 inches or less. For shrubs and other plants,
the fill should be less.
When there are major changes in grade, it may become necessary to supply air to the roots
of plants. This can be done by placing a layer of gravel and a tile system over the roots
before the fill is made. A tile system protects a tree from a raised grade. The tile system
should be laid out on the original grade leading from a dry well around the tree trunk. The
system should then be covered with small stones to allow air to circulate over the root area.
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Lowering the natural ground level can seriously damage trees and shrubs. The highest
percentage of the plant roots are in the upper 12 inches of the soil and cuts of only 2-3 inches
can cause serious injury. To protect the roots, it may be necessary to terrace the immediate
area around the plants to be saved. If roots are exposed, construction of retaining walls may
be needed to keep the soil in place. Plants can also be preserved by leaving them on an
undisturbed, gently sloping mound. To increase the chances for survival, it is best to limit
grade changes and other soil disturbances to areas outside the dripline of the plant.
• Excavations - Protect trees and other plants when excavating for drainfields, power,
water, and sewer lines. Where possible, route the trenches around trees and large
shrubs. When this is not possible, it is best to tunnel under them. This can be done
with hand tools or power augers. If it is not possible to route the trench around plants
to be saved, then the following methods should be observed:
o Cut as few roots as possible. When you have to cut, cut clean. Paint cut root
ends with a wood dressing like asphalt base paint.
o Backfill the trench as soon as possible.
o Tunnel beneath root systems as close to the center of the main trunk as
possible to preserve most of the important feeder roots.
Some problems that can be encountered with a few specific trees are:
• Maple, Dogwood, Red alder, Western hemlock, Western red cedar, and Douglas fir
do not readily adjust to changes in environment and special care should be taken to
protect these trees.
• The windthrow hazard of Pacific silver fir and madrona is high, while that of Western
hemlock is moderate. The danger of windthrow increases where dense stands have
been thinned. Other species (unless they are in shallow, wet soils less than
20 inches deep) have a low windthrow hazard.
• Cottonwoods, maples, and willows have water-seeking roots. These species thrive in
high moisture conditions that other trees would not. Roots of these plants can cause
problems in sewer lines and infiltration fields.
• Thinning operations in pure or mixed stands of Grand fir, Pacific silver fir, Noble fir,
Sitka spruce, Western red cedar, Western hemlock, Pacific dogwood, and Red alder
can cause serious disease problems. Disease can become established through
damaged limbs, trunks, roots, and freshly cut stumps. Diseased and weakened trees
are also susceptible to insect attack.
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3.1.1.4 Maintenance Standards
• Inspect flagged and/or fenced areas regularly to make sure flagging or fencing has
not been removed or damaged. If the flagging or fencing has been damaged or
visibility reduced, it shall be repaired or replaced immediately and visibility restored.
• If tree roots have been exposed or injured, “prune” cleanly with an appropriate
pruning saw or loppers directly above the damaged roots and recover with native
soils. Treatment of sap flowing trees (fir, hemlock, pine, soft maples) is not advised
as sap forms a natural healing barrier.
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3.1.2 BMP C102: Buffer Zone
3.1.2.1 Purpose
An undisturbed area or strip of natural vegetation or an established suitable planting that will provide
a living filter to reduce soil erosion and runoff velocities.
3.1.2.2 Conditions of Use
Natural buffer zones are used along streams, wetlands and other bodies of water that need
protection from erosion and sedimentation. Vegetative buffer zones can be used to protect natural
swales and can be incorporated into the natural landscaping of an area.
Critical-areas buffer zones should not be used as sediment treatment areas. Do not disturb critical
area buffers. The City may expand the buffer widths temporarily to allow the use of the expanded
area for removal of sediment.
3.1.2.3 Design and Installation Specifications
• Preserve natural vegetation or plantings in clumps, blocks, or strips as this is
generally the easiest and most successful method. However, single specimen trees
and plants should also be preserved.
• Leave all unstable slopes in their natural, undisturbed state.
• Mark clearing limits and keep all equipment and construction debris out of the natural
areas. Steel construction fencing is the most effective method of protecting sensitive
areas and buffers. Alternatively, wire-backed silt fence on steel posts is marginally
effective. Flagging alone is typically not effective and will not be allowed.
• Keep all excavations outside the dripline of trees and shrubs.
• Do not push debris or extra soil into the buffer zone area because it will cause
damage from burying and smothering.
• Vegetative buffer zones for streams, lakes or other waterways shall be established
by the City or other state or federal permits or approvals.
3.1.2.4 Maintenance Standards
• Inspect the area frequently to make sure flagging remains in place and the area
remains undisturbed.
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3.1.3 BMP C103: High Visibility Plastic or Metal Fence
3.1.3.1 Purpose
Fencing is intended to:
• Restrict clearing to approved limits.
• Prevent disturbance of sensitive areas, their buffers, and other areas required to be
left undisturbed.
• Limit construction traffic to designated construction entrances or roads.
• Protect areas where marking with survey tape may not provide adequate protection.
3.1.3.2 Conditions of Use
To establish clearing limits, plastic or metal fence may be used:
• At the boundary of sensitive areas, their buffers, and other areas required to be left
uncleared.
• As necessary to control vehicle access to and on the site.
3.1.3.3 Design and Installation Specifications
• High visibility plastic fence shall be composed of a high-density polyethylene material
and shall be at least four feet in height. Posts for the fencing shall be steel or wood
and placed every 6 feet on center (maximum) or as needed to ensure rigidity. The
fencing shall be fastened to the post every six inches with a polyethylene tie. On long
continuous lengths of fencing, a tension wire or rope shall be used as a top stringer
to prevent sagging between posts. See City of Auburn Construction Standards for
high visibility fence specifications.
• Design and install metal fences according to the manufacturer's specifications.
• Metal fences shall be at least 3 feet high and must be highly visible.
• Do not wire or staple fences to trees.
3.1.3.4 Maintenance Standards
• If the fence has been damaged or visibility reduced, it shall be repaired or replaced
immediately and visibility restored.
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3.1.4 BMP C104: Stake and Wire Fence
3.1.4.1 Purpose
Fencing is intended to:
• Restrict clearing to approved limits.
• Prevent disturbance of sensitive areas, their buffers, and other areas required to be
left undisturbed.
• Limit construction traffic to designated construction entrances or roads.
• Protect areas where marking with survey tape may not provide adequate protection.
3.1.4.2 Conditions of Use
To establish clearing limits, stake or wiring fence may be used:
• At the boundary of sensitive areas, their buffers, and other areas required to be left
uncleared.
• As necessary to control vehicle access to and on the site.
3.1.4.3 Design and Installation Specifications
• See Figure II-3-1 for details.
• Use more substantial fencing if the fence does not prevent encroachment into those
areas that are not to be disturbed.
3.1.4.4 Maintenance Standards
• If the fence has been damaged or visibility reduced, it shall be repaired or replaced
immediately and visibility restored.
Figure II-3-1. Stake and Wire Fence
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3.1.5 BMP C105: Stabilized Construction Entrance
3.1.5.1 Purpose
Construction entrances are stabilized to reduce the amount of sediment transported onto paved
roads by vehicles or equipment by constructing a stabilized pad of quarry spalls at entrances to
construction sites.
3.1.5.2 Conditions of Use
Construction entrances shall be stabilized wherever traffic will be leaving a construction site and
traveling on paved roads or other paved areas within 1,000 feet of the site.
On large commercial, highway, and road projects, the designer should include enough extra
materials in the contract to allow for additional stabilized entrances not shown in the initial
Construction SWPPP. It is difficult to determine exactly where access to these projects will take
place; additional materials will enable the contractor to install them where needed.
3.1.5.3 Design and Installation Specifications
• See Figure II-3-2 for details.
NOTE: Reduce the length of the entrance to the maximum practicable size when the size or
configuration of the site does not allow the full 100-foot length.
• Place a separation geotextile under the spalls to prevent fine sediment from pumping
up into the rock pad. The geotextile shall meet the following standards:
o Grab Tensile Strength (ASTM D4751) – 200 psi min.
o Grab Tensile Elongation (ASTM D4632) – 30% max.
o Mullen Burst Strength (ASTM D3786-80a) – 400 psi min.
o AOS (ASTM D4751) – 20 to 45 (U.S. standard sieve size)
• Consider early installation of the first lift of asphalt in areas that will be paved; this
can be used as a stabilized entrance. Also consider the installation of excess
concrete as a stabilized entrance. During large concrete pours, excess concrete is
often available for this purpose.
• Install fencing (see BMPs C103 and C104) as necessary to restrict traffic to the
construction entrance.
• Whenever possible, construct the entrance on a firm, compacted subgrade. This can
substantially increase the effectiveness of the pad and reduce the need for
maintenance.
3.1.5.4 Maintenance Standards
• Add quarry spalls if the pad is no longer in accordance with the specifications.
• If the entrance is not preventing sediment from being tracked onto pavement, then
alternative measures to keep the streets free of sediment shall be used. This may
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include street sweeping, an increase in the dimensions of the entrance, or the
installation of a wheel wash.
• No tracking of sediment onto the roadway is allowed. If sediment is tracked onto the
road, clean the road thoroughly by shoveling or pickup sweeping. Transport
sediment to a controlled sediment disposal area.
• Keep streets clean at ALL times. Clean tracked sediment immediately.
• Street washing of sediment to the storm drain system is not allowed.
• Immediately remove any quarry spalls that are loosened from the pad and end up on
the roadway.
• Install fencing (BMPs C103 and C104) to control traffic if vehicles are entering or
exiting the site at points other than the construction entrance(s).
• Upon project completion and site stabilization, permanently stabilize all construction
accesses intended as permanent access for maintenance.
Figure II-3-2. Stabilized Construction Entrance
Figure II-3-3 shows a small site, stabilized construction entrance.
1
0
0'
MI
N
.
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Figure II-3-3. Small-Site Stabilized Construction Entrance
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3.1.6 BMP C106: Wheel Wash
3.1.6.1 Purpose
Wheel washes reduce the amount of sediment transported onto paved roads by motor vehicles.
3.1.6.2 Conditions of Use
Can be used when a stabilized construction entrance (see BMP C105) is not preventing sediment
from being tracked onto pavement.
• Wheel washing is generally an effective BMP when installed with careful attention to
topography. For example, a wheel wash can be detrimental if installed at the top of a
slope abutting a right-of-way where the water from the dripping truck can run
unimpeded into the street.
• Pressure washing combined with an adequately sized and surfaced pad with direct
drainage to a large 10-foot x 10-foot sump can be very effective.
3.1.6.3 Design and Installation Specifications
Suggested details are shown in Figure II-3-4. The City may allow other designs. A minimum of
6 inches of asphalt treated base (ATB) over crushed base material or 8 inches over a good subgrade
is recommended to pave the wheel wash.
Use a low clearance truck to test the wheel wash before paving. Either a belly dump or lowboy will
work well to test clearance.
Keep the water level from 12 to 14 inches deep to avoid damage to truck hubs and filling the truck
tongues with water.
Midpoint spray nozzles are only needed in extremely muddy conditions.
Design wheel wash systems with a small grade change, 6 to 12 inches for a 10-foot-wide pond, to
allow sediment to flow to the low side of pond to help prevent re-suspension of sediment. A drainpipe
with a 2- to 3-foot riser should be installed on the low side of the pond to allow for easy cleaning and
refilling. Polymers may be used to promote coagulation and flocculation in a closed-loop system.
Polyacrylamide (PAM) added to the wheel wash water at a rate of 0.25 - 0.5 pounds per
1,000 gallons of water increases effectiveness and reduces cleanup time. If PAM is already being
used for dust or erosion control and is being applied by a water truck, the same truck can be used to
change the wash water.
3.1.6.4 Maintenance Standards
The wheel wash should start out the day with fresh water.
The wash water should be changed a minimum of once per day. On large earthwork jobs where
more than 10 to 20 trucks per hour are expected, the wash water will need to be changed more
often.
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Wheel wash or tire bath wastewater shall be discharged to a separate on-site treatment system, such
as closed-loop recirculation or land application, or to the sanitary sewer with a King County – Metro
wastewater discharges from construction sites permit.
Figure II-3-4. Wheel Wash
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3.1.7 BMP C107: Construction Road/Parking Area Stabilization
3.1.7.1 Purpose
Stabilizing subdivision roads, parking areas and other onsite vehicle transportation routes
immediately after grading reduces erosion caused by construction traffic or runoff.
3.1.7.2 Conditions of Use
• Stabilize roads or parking areas wherever they are constructed, whether permanent
or temporary, for use by construction traffic.
• Install fencing (see BMPs C103 and C104), if necessary, to limit the access of
vehicles to only those roads and parking areas that are stabilized.
3.1.7.3 Design and Installation Specifications
• On areas that will receive asphalt as part of the project, install the first lift as soon as
possible.
• Apply a 6-inch depth of 2- to 4-inch crushed rock, gravel base, or crushed surfacing
base course immediately after grading or utility installation. A 4-inch course of
asphalt treated base (ATB) may also be used, or the road/parking area may be
paved. It may also be possible to use cement or calcium chloride for soil stabilization.
If cement or cement kiln dust is used for roadbase stabilization, pH monitoring and
BMPs are necessary to evaluate and minimize the effects on stormwater. If the area
will not be used for permanent roads, parking areas, or structures, a 6-inch depth of
hog fuel may also be used, but this is likely to require more maintenance. Whenever
possible, place construction roads and parking areas on a firm, compacted
subgrade.
• Temporary road gradients shall not exceed 15 percent. Carefully grade roadways to
drain. Provide drainage ditches on each side of the roadway in the case of a
crowned section, or on one side in the case of a super-elevated section. Direct
drainage ditches to a sediment control BMP.
• Rather than relying on ditches, it may also be possible to grade the road so that
runoff sheet-flows into a heavily vegetated area with a well-developed topsoil.
Landscaped areas are not adequate. If this area has at least 50 feet of vegetation,
then it is generally preferable to use the vegetation to treat runoff, rather than a
sediment pond or trap. The 50 feet shall not include wetlands. If runoff is allowed to
sheetflow through adjacent vegetated areas, it is vital to design the roadways and
parking areas so that no concentrated runoff is created.
• Protect storm drain inlets to prevent sediment-laden water entering the storm drain
system (see BMP C220).
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3.1.7.4 Maintenance Standards
• Inspect stabilized areas regularly, especially after large storm events.
• Add crushed rock, gravel base, hog fuel, etc. as required to maintain a stable driving
surface and to stabilize any eroded areas.
• Following construction, restore all areas to preconstruction condition or better to
prevent future erosion.
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3.1.8 BMP C120: Temporary and Permanent Seeding
3.1.8.1 Purpose
Seeding is intended to reduce erosion by stabilizing exposed soils. A well-established vegetative
cover is one of the most effective methods of reducing erosion.
3.1.8.2 Conditions of Use
• Seeding may be used throughout the project on disturbed areas that have reached
final grade or that will remain unworked for more than 30 days.
• Channels that will be vegetated should be installed before major earthwork and
hydroseeded with a Bonded Fiber Matrix. The vegetation should be well established
(i.e., 75 percent cover) before water is allowed to flow in the ditch. With channels that
will have high flows, install erosion control blankets over the hydroseed. If vegetation
cannot be established from seed before water is allowed in the ditch, sod should be
installed in the bottom of the ditch over hydromulch and blankets.
• Seed retention/detention ponds as required.
• Mulch is required at all times because it protects seeds from heat, moisture loss, and
transport due to runoff.
• All disturbed areas shall be reviewed in late August to early September and all
seeding shall be completed by the end of September. Otherwise, vegetation will not
establish itself enough to provide more than average protection.
• At final site stabilization, seed and mulch all disturbed areas not otherwise vegetated
or stabilized. Final stabilization means the completion of all soil disturbing activities at
the site and the establishment of a permanent vegetative cover, or equivalent
permanent stabilization measures (such as pavement, riprap, gabions, or
geotextiles) which will prevent erosion.
3.1.8.3 Design and Installation Specifications
• Seed during seasons most conducive to plant growth. The optimum seeding
windows for western Washington are April 1 through June 30 and September 1
through October 1. Seeding that occurs between July 1 and August 30 will require
irrigation until 75 percent grass cover is established. Seeding that occurs between
October 1 and March 30 will require a mulch or plastic cover until 75 percent grass
cover is established.
• Deviation from these specifications shall be allowed if alternatives are developed by
a licensed Landscape Professional and approved by the City.
• To prevent seed from being washed away, confirm that all required surface water
control measures have been installed.
• The seedbed should be firm and rough. All soil should be roughened no matter what
the slope. If compaction is required for engineering purposes, track walk slopes
before seeding. Backblading or smoothing of slopes greater than 4:1 is not allowed if
they are to be seeded.
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• New and more effective restoration-based landscape practices rely on deeper
incorporation than that provided by a simple single-pass rototilling treatment.
Wherever practical, the subgrade should be initially ripped to improve long-term
permeability, infiltration, and water inflow qualities. At a minimum for permanent
areas, use soil amendments to achieve organic matter and permeability performance
defined in engineered soil/landscape systems. For systems that are deeper than
8 inches, complete the rototilling process in multiple lifts, or prepare the soil system
properly and then place it to achieve the specified depth.
• Organic matter is the most appropriate form of “fertilizer” because it provides
nutrients (including nitrogen, phosphorus, and potassium) in the least water-soluble
form. A natural system typically releases 2-10 percent of its nutrients annually.
Chemical fertilizers have since been formulated to simulate what organic matter does
naturally.
• In general, 10-4-6 N-P-K (nitrogen-phosphorus-potassium) fertilizer can be used at a
rate of 90 pounds per acre. Always use slow-release fertilizers because they are
more efficient and have fewer environmental impacts. It is recommended that soils
tests are conducted in areas being seeded for final landscaping to determine the
exact type and quantity of fertilizer needed. This will prevent the over-application of
fertilizer. Fertilizer should not be added to the hydromulch machine and agitated
more than 20 minutes before it is to be used. If agitated too much, the slow-release
coating is destroyed.
• There are numerous products available on the market that take the place of chemical
fertilizers. These include several with seaweed extracts that are beneficial to soil
microbes and organisms. If 100 percent cottonseed meal is used as the mulch in
hydroseed, chemical fertilizer may not be necessary. Cottonseed meal is a good
source of long-term, slow-release, available nitrogen.
• Hydroseed applications shall include a minimum of 1,500 pounds per acre of mulch
with 3 percent tackifier. Mulch may be made up of 100 percent: cottonseed meal;
fibers made of wood, recycled cellulose, hemp, and kenaf; compost; or blends of
these. Tackifier shall be plant-based, such as guar or alpha plantago, or chemical-
based such as polyacrylamide or polymers. Any mulch or tackifier product used shall
be installed per manufacturer’s instructions. Generally, mulches come in 40-
50 pound bags. Seed and fertilizer are added at time of application.
• Mulch is always required for seeding. Mulch can be applied on top of the seed or
simultaneously by hydroseeding.
• On steep slopes, Bonded Fiber Matrix (BFM) or Mechanically Bonded Fiber Matrix
(MBFM) products should be used. BFM/MBFM products are applied at a minimum
rate of 3,000 pounds per acre of mulch with approximately 10 percent tackifier.
Application is made so that a minimum of 95 percent soil coverage is achieved.
Numerous products are available commercially and should be installed per
manufacturer’s instructions. Most products require 24 to 36 hours to cure before a
rainfall and cannot be installed on wet or saturated soils. Generally, these products
come in 40 to 50 pound bags and include all necessary ingredients except for seed
and fertilizer.
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• BFMs and MBFMs have some advantages over blankets:
o No surface preparation required;
o On slopes steeper than 2.5:1, blanket installers may need to be roped and
harnessed for safety;
• In most cases, the shear strength of blankets is not a factor when used on slopes,
only when used in channels. BFMs and MBFMs are good alternatives to blankets in
most situations where vegetation establishment is the goal.
• When installing seed via hydroseeding operations, only about 1/3 of the seed
actually ends up in contact with the soil surface. This reduces the ability to establish
a good stand of grass quickly. One way to overcome this is to increase seed
quantities by up to 50 percent.
• Vegetation establishment can also be enhanced by dividing the hydromulch
operation into two phases:
o Phase 1- Install all seed and fertilizer with 25 to 30 percent mulch and
tackifier onto soil in the first lift;
o Phase 2- Install the rest of the mulch and tackifier over the first lift.
• An alternative is to install the mulch, seed, fertilizer, and tackifier in one lift. Then,
spread or blow straw over the top of the hydromulch at a rate of about 800 to
1,000 pounds per acre. Hold straw in place with a standard tackifier. Both of these
approaches will increase cost moderately but will greatly improve and enhance
vegetative establishment. The increased cost may be offset by the reduced need for:
o Irrigation
o Reapplication of mulch
o Repair of failed slope surfaces
o This technique works with standard hydromulch (1,500 pounds per acre
minimum) and BFM/MBFMs (3,000 pounds per acre minimum).
• Provide a healthy topsoil to areas to be permanently landscaped. This will reduce the
need for fertilizers, improve overall topsoil quality, provide for better vegetal health
and vitality, improve hydrologic characteristics, and reduce the need for irrigation.
See the Post-Construction Soil Quality and Depth BMP in Volume VI for more
information. Areas that will be seeded only and not landscaped may need compost
or meal-based mulch included in the hydroseed in order to establish vegetation.
Replace native topsoil on the disturbed soil surface before application.
• Seed that is installed as a temporary measure may be installed by hand if it will be
covered by straw, mulch, or topsoil. Seed that is installed as a permanent measure
may be installed by hand on small areas (usually less than 1 acre) that will be
covered with mulch, topsoil, or erosion blankets. The seed mixes listed below include
recommended mixes for both temporary and permanent seeding. These mixes, with
the exception of the wetland mix, shall be applied at a rate of 120 pounds per acre.
This rate can be reduced if soil amendments or slow-release fertilizers are used.
Local suppliers or the local conservation district should be consulted for their
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recommendations because the appropriate mix depends on a variety of factors,
including location, exposure, soil type, slope, and expected foot traffic. Alternative
seed mixes approved by the City of Auburn may be used.
• Table II-3-1 represents the standard mix for those areas where just a temporary
vegetative cover is required.
• Table II-3-2 provides just one recommended possibility for landscaping seed.
• The turf seed mix in Table II-3-3 is for dry situations. The advantage is that this mix
requires very little maintenance.
• Table II-3-4 presents a mix recommended for bioswales and other intermittently wet
areas.
• The seed mix shown in Table II-3-5 is a recommended low-growing, relatively non-
invasive seed mix appropriate for very wet areas that are not regulated wetlands.
Other mixes may be appropriate, depending on the soil type and hydrology of the
area. Recent research suggests that bentgrass (agrostis sp.) should be emphasized
in wet-area seed mixes. Apply this mixture at a rate of 60 pounds per acre.
• The meadow seed mix in Table II-3-6 is recommended for areas that will be
maintained infrequently or not at all and where colonization by native plants is
desirable. Likely applications include rural road and utility right-of-way. Seeding
should take place in September or very early October in order to obtain adequate
establishment prior to the winter months. The appropriateness of clover in the mix
may need to be considered, as this can be a fairly invasive species. If the soil is
amended, the addition of clover may not be necessary.
3.1.8.4 Maintenance Standards
• Reseed any seeded areas that fail to establish at least 80 percent cover within
6 weeks from the initial seeding (100 percent cover for areas that receive sheet or
concentrated flows). If reseeding is ineffective, use an alternate method, such as
sodding, mulching, or nets/blankets. If winter weather prevents adequate grass
growth, this time limit may be relaxed at the discretion of the City.
• After adequate cover is achieved, reseed and protect with mulch any areas that
experience erosion. If the erosion problem is drainage related, the problem shall be
fixed and the eroded area reseeded and protected by mulch.
• Water seeded areas if necessary. Watering shall not cause runoff.
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Table II-3-1. Temporary Erosion Control Seed Mix
% Weight % Purity % Germination
Chewings or annual bluegrass
Festuca rubra var. commutate or Poa anna 40 98 90
Perennial rye
Lolium perenne 50 98 90
Redtop or colonial bentgrass
Agrostis alba or Agrostis tenuis 5 92 85
White Dutch clover
Trifolium repens 5 98 90
Table II-3-2. Landscaping Seed Mix
% Weight % Purity % Germination
Perennial rye
Lolium perenne 70 98 90
Chewings and red fescue blend
Festuca rubra var commutate or Festuca rubra 30 98 90
Table II-3-3. Low-Growing Turf Seed Mix
% Weight % Purity % Germination
Dwarf tall fescue (several varieties)
Festuca arundinacea var. 45 98 90
Dwarf perennial rye (Barclay)
Lolium perenne var. barclay 30 98 90
Red fescue
Festuca rubra 20 98 90
Colonial bentgrass
Agrostis tenuis 5 98 90
Table II-3-4. Bioswale Seed Mix1
% Weight % Purity % Germination
Tall or meadow fescue
Festuca arundinacea or Festuca elatior 75-80 98 90
Seaside/Creeping bentgrass
Agrostis palustriis 1-15 92 85
Redtop bentgrass
Agrostis alba or Agrostis gigantea 5-10 90 80
1Modified Briargreen, Inc. Hydroseeding Guide Wetlands Seed Mix
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Table II-3-5. Wet Area Seed Mix
% Weight % Purity % Germination
Tall or meadow fescue
Festuca arundinacea or Festuca elatior 60-70 98 90
Seaside/Creeping bentgrass
Agrostis palustriis 10-15 98 85
Meadow foxtail
Alepocurus pratensis 10-15 90 80
Alsike clover
Trifolium hybridium 1-6 98 90
Redtop bentgrass
Agrostis alba or Agrostis gigantea 106 92 85
Table II-3-6. Meadow Seed Mix
% Weight % Purity % Germination
Redtop or Oregon bentgrass
Agrostis alba or Agrostis oregonensis 20 92 85
Red fescue
Festuca rubra 70 98 90
White Dutch clover
Trifolium repens 10 98 90
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3.1.9 BMP C121: Mulching
3.1.9.1 Purpose
The purpose of mulching soils is to provide immediate temporary protection from erosion. Mulch also
enhances plant establishment by conserving moisture, holding fertilizer, seed, and topsoil in place,
and moderating soil temperatures. There is an enormous variety of mulches that can be used. Only
the most common types are discussed in this section.
3.1.9.2 Conditions of Use
As a temporary cover measure, mulch should be used:
• On disturbed areas that require cover measures for less than 30 days.
• As a cover for seed during the wet season and during the hot summer months.
• During the wet season on slopes steeper than 3H:1V with more than 10 feet of
vertical relief.
• Mulch may be applied at any time of the year and must be refreshed periodically.
3.1.9.3 Design and Installation Specifications
For mulch materials, application rates, and specifications, see Table II-3-7.
NOTE: Thicknesses may be increased for disturbed areas in or near sensitive areas or other areas
highly susceptible to erosion.
Mulch used within the ordinary high-water mark of surface waters should be selected to minimize
potential flotation of organic matter. Composted organic materials have higher specific gravities
(densities) than straw, wood, or chipped material.
3.1.9.4 Maintenance Standards
• The thickness of the cover must be maintained.
• Re-mulch and/or protect with a net or blanket any areas that experience erosion. If
the erosion problem is drainage related, then fix the problem and remulch the eroded
area.
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Table II-3-7. Mulch Standards and Guidelines
Mulch
Material
Quality
Standards
Application
Rates
Remarks
Straw Air-dried; free from
undesirable seed
and coarse material.
3” thick; 5
bales per
1000 sf or 2
to 3 tons per
acre.
Cost-effective protection when applied with adequate
thickness. Hand-application generally requires greater
thickness than blown straw. The thickness of straw may
be reduced by half when used in conjunction with seeding.
In windy areas, straw must be held in place by crimping,
using a tackifier, or covering with netting. Blown straw
always has to be held in place with a tackifier as even light
winds will blow it away. Straw, however, has several
deficiencies that should be considered when selecting
mulch materials. If often introduces and/or encourages the
propagation of weed species and it has no significant long-
term benefits. Straw should be used only if mulches with
long-term benefits are unavailable locally. It should also
not be used within the ordinary high-water elevation of
surface waters (due to flotation).
Hydro-
mulch
No growth inhibiting
factors.
Approx. 25-30
lbs per 1000
sf or 1500-
2000 lbs per
acre.
Shall be applied with hydromulcher. Shall not be used
without seed and tackifier unless the application rate is at
least doubled. Fibers longer than about ¾ - 1 inch clog
hydromulch equipment. Fibers should be kept to less than
¾ inch.
Composte
d Mulch
and
Compost
No visible water or
dust during
handling. Must be
purchased from
supplier with a Solid
Waste Handling
permit (unless
exempt)
3” thick, min.;
approx. 100
tons per acre
(approx. 800
lbs. per yard).
Mulch is excellent for protecting final grades until
landscaping because it can be directly seeded or tilled into
soil as an amendment. Composted mulch has a coarser
size gradation than compost. It is more stable and
practical to use in wet areas and during rainy weather
conditions.
Chipped
Site
Vegetation
Average size shall
be several inches.
Gradations from fine
to 6-inches in length
for texture, variation,
and interlocking
properties.
3” minimum
thickness
This is a cost-effective way to dispose of debris from
clearing and grubbing, and it eliminates the problems
associated with burning. Generally, it should not be used
on slopes above approx. 10% because of its tendency to
be transported by runoff. It is not recommended within 200
feet of surface waters. If seeding is expected shortly after
mulch, the decomposition of the chipped vegetation may
tie up nutrients important to grass establishment.
Wood-
based
mulch
No visible water or
dust during
handling. Must be
purchased from a
supplier with a Solid
Waste Handling
permit or one
exempt from solid
waste regulations.
3” thick;
approx. 100
tons per acre
(approx. 800
lbs. per yard).
This material is often called “hog” or “hogged fuel”. It is
usable as a material for Stabilized Construction Entrances
(BMP C105) and as a mulch. The use of mulch ultimately
improves the organic matter in the soil. Special caution is
advised regarding the source and composition of wood-
based mulches. Its preparation typically does not provide
any weed seed control, so evidence of residual vegetation
in its composition or known inclusion of weed plants or
seeds should be monitored and prevented (or minimized).
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3.1.10 BMP C122: Nets and Blankets
3.1.10.1 Purpose
Erosion control nets and blankets are intended to prevent erosion and hold seed and mulch in place
on steep slopes and in channels so that vegetation can become well established. In addition, some
nets and blankets can be used to reinforce turf permanently to protect drainage ways during high
flows. Nets (commonly called matting) are strands of material woven into an open, but high-tensile
strength net (for example, coconut fiber matting). Blankets are strands of material that are not tightly
woven, but instead form a layer of interlocking fibers, typically held together by a biodegradable or
photodegradable netting (for example, excelsior or straw blankets). They generally have lower tensile
strength than nets, but cover the ground more completely. Coir (coconut fiber) fabric comes as both
nets and blankets.
3.1.10.2 Conditions of Use
Erosion control nets and blankets should be used:
• To aid permanent vegetated stabilization of slopes 2H:1V or greater and with more
than 10 feet of vertical relief.
• For drainage ditches and swales (highly recommended). The application of
appropriate netting or blanket to drainage ditches and swales can protect bare soil
from channelized runoff while vegetation is established. Nets and blankets also can
capture a great deal of sediment due to their open, porous structure. Synthetic nets
and blankets can be used to stabilize channels permanently and may provide a cost-
effective, environmentally preferable alternative to riprap. 100 percent synthetic
blankets manufactured for use in ditches may be easily reused as temporary ditch
liners.
• Disadvantages of blankets include:
o Surface preparation required;
o On slopes steeper than 2.5:1, blanket installers may need to be roped and
harnessed for safety;
• Advantages of blankets include:
o Can be installed without mobilizing special equipment;
o Can be installed by anyone with minimal training;
o Can be installed in stages or phases as the project progresses;
o Seed and fertilizer can be hand-placed by the installers as they progress
down the slope;
o Can be installed in any weather;
o There are numerous types of blankets that can be designed with various
parameters in mind. Those parameters include: fiber blend, mesh strength,
longevity, biodegradability, cost, and availability.
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3.1.10.3 Design and Installation Specifications
• See Figure II-3-5 and Figure II-3-6 for typical orientation and installation of blankets
used in channels and as slope protection. Note: these are typical only; all blankets
must be installed per manufacturer’s installation instructions.
• Installation is critical to the effectiveness of these products. If good ground contact is
not achieved, runoff can concentrate under the product, resulting in significant
erosion.
Installation of Blankets on Slopes:
• Complete final grade and track walk up and down the slope.
• Install hydromulch with seed and fertilizer.
• Dig a small trench, approximately 12 inches wide by 6 inches deep along the top of
the slope.
• Install the leading edge of the blanket into the small trench and staple approximately
every 18 inches.
NOTE: Staples are metal. ”U”-shaped, and a minimum of 6 inches long. Longer staples are
used in sandy soils. Biodegradable stakes are also available and should be used where
applicable.
• Roll the blanket slowly down the slope as the installer walks backwards.
• NOTE: The blanket rests against the installer’s legs. Staples are installed as the blanket is
unrolled. It is critical that the proper staple pattern in used for the blanket being installed. The
blanket should not be allowed to roll down the slope on its own as this stretches the blanket,
making it impossible to maintain soil contact. In addition, no one should be allowed to walk on
the blanket after it is in place.
• If the blanket is not long enough to cover the entire slope length, the trailing edge of
the upper blanket should overlap the leading edge of the lower blanket and be
stapled. On steeper slopes, this overlap should be installed in a small trench,
stapled, and covered with soil.
• With the variety of products available, it is impossible to cover all the details of
appropriate use and installation. Therefore, it is critical that the design engineer
consults the manufacturer's information and that a site visit takes place in order to
insure that the product specified is appropriate. Information is also available at the
following websites:
o WSDOT: http://www.wsdot.wa.gov/eesc/environmental/
o Texas Transportation Institute:
http://www.dot.state.tx.us/insdtdot/orgchart/cmd/erosion/contents.htm
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• Jute matting must be used in conjunction with mulch (BMP C121). Excelsior, woven
straw blankets, and coir (coconut fiber) blankets may be installed without mulch.
There are many other types of erosion control nets and blankets on the market that
may be appropriate in certain circumstances.
• In general, most nets (e.g., jute matting) require mulch in order to prevent erosion
because they have a fairly open structure. Blankets typically do not require mulch
because they usually provide complete protection of the surface.
• Extremely steep, unstable, wet, or rocky slopes are often appropriate candidates for
use of synthetic blankets, as are riverbanks, beaches, and other high-energy
environments. If synthetic blankets are used, the soil should be hydromulched first.
• 100 percent biodegradable blankets are available for use in sensitive areas. These
organic blankets are usually held together with a paper or fiber mesh and stitching
which may last up to a year.
• Most netting used with blankets is photodegradable, meaning it will break down
under sunlight (not UV stabilized). However, this process can take months or years
even under bright sun. Once vegetation is established, sunlight does not reach the
mesh. It is not uncommon to find non-degraded netting still in place several years
after installation. This can be a problem if maintenance requires the use of mowers
or ditch cleaning equipment. In addition, birds and small animals can become
trapped in the netting.
3.1.10.4 Maintenance Standards
• Good contact with the ground must be maintained, and erosion must not occur
beneath the net or blanket.
• Repair or staple any areas of the net or blanket that are damaged or not in close
contact with the ground.
• If erosion occurs due to poorly controlled drainage, fix the problem and protect the
eroded area.
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Figure II-3-5. Nets and Blankets – Slope Installation
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Figure II-3-6. Nets and Blankets – Channel Installation
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3.1.11 BMP C123: Plastic Covering
3.1.11.1 Purpose
Plastic covering provides immediate, short-term erosion protection to slopes and disturbed areas.
3.1.11.2 Conditions of Use
See Figure II-3-7.
• Plastic covering may be used on disturbed areas that require cover measures for
less than 30 days, except as stated below.
• Plastic is particularly useful for protecting cut and fill slopes and stockpiles. Note: The
relatively rapid breakdown of most polyethylene sheeting makes it unsuitable for
long-term (greater than six months) applications.
• Clear plastic sheeting can be used over newly-seeded areas to create a greenhouse
effect and encourage grass growth if the hydroseed was installed too late in the
season to establish 75 percent grass cover, or if the wet season started earlier than
normal. Clear plastic should not be used for this purpose during the summer months
because the resulting high temperatures can kill the grass.
• Due to rapid runoff caused by plastic sheeting, this method shall not be used upslope
of areas that might be adversely impacted by concentrated runoff. Such areas
include steep and/or unstable slopes.
• While plastic is inexpensive to purchase, the added cost of installation, maintenance,
removal, and disposal can make this an expensive material.
• Whenever plastic is used to protect slopes, water collection measures must be
installed at the base of the slope. These measures include plastic-covered berms,
channels, and pipes used to convey clean rainwater away from bare soil and
disturbed areas. At no time is clean runoff from a plastic covered slope to be mixed
with dirty runoff from a project.
• Other uses for plastic include:
o Temporary ditch liner;
o Pond liner in temporary sediment pond;
o Liner for bermed temporary fuel storage area if plastic is not reactive to the
type of fuel being stored;
o Emergency slope protection during heavy rains; and
o Temporary drainpipe (“elephant trunk”) used to direct water.
3.1.11.3 Design and Installation Specifications
Plastic slope cover must be installed as follows:
• Run plastic up and down slope, not across slope.
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• Plastic may be installed perpendicular to a slope if the slope length is less than
10 feet.
• Minimum of 8-inch overlap at seams.
• On long or wide slopes, or slopes subject to wind, all seams should be taped.
• Place plastic into a small (12-inch wide by 6-inch deep) slot trench at the top of the
slope and backfill with soil to keep water from flowing underneath.
• Place sand filled burlap or geotextile bags every 3 to 6 feet along seams and pound
a wooden stake through each to hold them in place. Alternative options for holding
plastic in place exist and may be considered with City of Auburn approval.
• Inspect plastic for rips, tears, and open seams regularly and repair immediately. This
prevents high velocity runoff from contacting bare soil, which causes extreme
erosion;
• Sandbags may be lowered into place tied to ropes. However, all sandbags must be
staked in place.
NOTE: Methods other than staking down plastic with sandbags may be used with City of
Auburn approval.
• Plastic sheeting shall have a minimum thickness of 0.06 millimeters.
• If erosion at the toe of a slope is likely, a gravel berm, riprap, or other suitable
protection shall be installed at the toe of the slope in order to reduce the velocity of
runoff.
3.1.11.4 Maintenance Standards
• Torn sheets must be replaced and open seams repaired.
• If the plastic begins to deteriorate due to ultraviolet radiation, it must be completely
removed and replaced.
• When the plastic is no longer needed, it shall be completely removed.
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Figure II-3-7. Soil Erosion Protection – Plastic Covering
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3.1.12 BMP C124: Sodding
3.1.12.1 Purpose
The purpose of sodding is to establish permanent turf for immediate erosion protection and to
stabilize drainage ways where concentrated overland flow will occur.
3.1.12.2 Conditions of Use
Sodding may be used in the following areas:
• Disturbed areas that require short-term or long-term cover.
• Disturbed areas that require immediate vegetative cover.
• All waterways that require vegetative lining. Waterways may also be seeded rather
than sodded, and protected with a net or blanket.
3.1.12.3 Design and Installation Specifications
Sod shall be free of weeds, of uniform thickness (approximately 1-inch thick), and shall have a dense
root mat for mechanical strength.
The following steps are recommended for sod installation:
• Shape and smooth the surface to final grade in accordance with the approved
grading plan. Overexcavate the swale 4 to 6 inches below design elevation to allow
room for placing soil amendment and sod.
• Amend 4 inches (minimum) of compost into the top 8 inches of the soil if the organic
content of the soil is less than ten percent or the permeability is less than 0.6 inches
per hour. Compost used should meet Ecology specifications for Grade A quality
compost. See http://www.ecy.wa.gov/programs/swfa/compost/
• Fertilize according to the supplier's recommendations.
• Work lime and fertilizer 1 to 2 inches into the soil, and smooth the surface.
• Lay strips of sod beginning at the lowest area to be sodded and perpendicular to the
direction of water flow. Wedge strips securely into place. Square the ends of each
strip to provide for a close, tight fit. Stagger joints at least 12 inches. Staple on slopes
steeper than 3H:1V. Staple the upstream edge of each sod strip.
• Roll the sodded area and irrigate.
• When sodding is carried out in alternating strips or other patterns, seed the areas
between the sod immediately after sodding.
3.1.12.4 Maintenance Standards
If the grass is unhealthy, the cause shall be determined and appropriate action taken to reestablish a
healthy groundcover. If it is impossible to establish a healthy groundcover due to frequent saturation,
instability, or some other cause, the sod shall be removed, the area seeded with an appropriate mix,
and protected with a net or blanket.
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3.1.13 BMP C125: Compost
3.1.13.1 Purpose
The purpose of compost is to help establish vegetation and filter stormwater thus removing fine
sediment and other contaminants. Compost can be used alone as a compost blanket, as a berm, or
inside a sock.
3.1.13.2 Conditions of Use
• Do not use if stormwater will discharge to a nutrient sensitive waterbody.
• Do not use as a storm drain inlet protection measure.
3.1.13.3 Design and Installation Specifications
Compost Blankets
Compost blankets are simply compost blanketed over an area.
• Place compost 3” thick.
• Compost can be blown onto slopes up to 2:1 or spread by hand on shallower slopes.
• Compost must be ¾ to 1 inch-minus screened compost meeting Ecology’s
requirements for Grade A quality compost. See
http://www.ecy.wa.gov/programs/swfa/compost for more information on compost
quality.
• Compost can be mixed with a seed mix to ensure rapid vegetation.
• Compost does not need to be removed after construction phase unless required by
the project engineer or geotechnical professional.
Compost Berms
Compost berms are a perimeter sediment control that can be used instead of silt fence.
• Do not use compost berms on steep slopes.
• Berm width shall be a minimum of 2 feet.
• Berm height shall be a minimum of 12 inches.
• Berm width shall be twice the berm height.
Compost can be blown in place or placed by front-end loader. Compost must be ¾ to 1 inch-minus
screened compost meeting Ecology’s requirements for Grade A quality compost. See
http://www.ecy.wa.gov/programs/swfa/compost for more information on compost quality.
Compost should be spread over proposed landscaped section when construction is complete to aid
in revegetation.
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Compost Socks
Compost socks are similar to straw wattles.
• Sock material that is biodegradable will last up to 6 months and can be used for soil
amendment after 6 months.
• Sock material that is non-biodegradable must be removed after construction is
complete.
• Place socks perpendicular to flow.
• Walk socks in place to ensure good soil contact.
• Install wooden stakes every 12” on steep slopes or every 24” on shallow slopes
3.1.13.4 Maintenance Standards
Compost Blankets
• Inspect compost regularly.
• Ensure a 3” thick blanket.
Compost Berms
• Inspect compost berm regularly.
• Ensure vehicular traffic does not cross berm and track compost off-site. If this
occurs, sweep compost immediately.
Compost Socks
• Do not allow erosion or concentrated runoff under or around the barrier.
• Inspect the socks after each rainfall and repair any socks that tear or are not abutting
the ground.
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3.1.14 BMP C126: Topsoiling
3.1.14.1 Purpose
To provide a suitable growth medium for final site stabilization with vegetation. While not a permanent
cover practice in itself, topsoiling is an integral component of providing permanent cover in those
areas where there is an unsuitable soil surface for plant growth. Native soils and disturbed soils that
have been organically amended not only retain much more stormwater, but they also serve as
effective biofilters for urban pollutants and, by supporting more vigorous plant growth, reduce the
amount of water, fertilizer, and pesticides needed to support installed landscapes. Topsoil does not
include any subsoils, only the material from the top several inches, including organic debris.
3.1.14.2 Conditions of Use
Native soils should be left undisturbed to the maximum extent practicable. Native soils disturbed
during clearing and grading should be restored, to the maximum extent practicable, to a condition
where moisture-holding capacity is equal to or better than the original site conditions. This criterion
can be met by using on-site native topsoil, incorporating amendments into on-site soil, or importing
blended topsoil.
• Topsoiling is a required procedure when establishing vegetation on shallow soils,
and soils of critically low pH (high acid) levels.
• Stripping of the existing, properly functioning soil system and vegetation for the
purpose of topsoiling during construction is not acceptable. If an existing soil system
is functioning properly, it shall be preserved in its undisturbed and uncompacted
condition.
• Depending on where the topsoil comes from, or what vegetation was on site before
disturbance, invasive plant seeds may be included and could cause problems for
establishing native plants, landscaped areas, or grasses.
• Topsoil from the site will contain mycorrhizal bacteria that are necessary for healthy
root growth and nutrient transfer. These native mycorrhiza are acclimated to the site
and will provide optimum conditions for establishing grasses. Commercially available
mycorrhiza products should be used when topsoil is brought in from off-site.
3.1.14.3 Design and Installation Specifications
If topsoiling is to be done, the following items should be considered:
• Maximize the depth of the topsoil wherever possible to provide the maximum
possible infiltration capacity and beneficial growth medium. Topsoil depth shall be at
least 8 inches with a minimum organic content of 10 percent dry weight and pH
between 6.0 and 8.0 or matching the pH of the undisturbed soil. This can be
accomplished either by returning native topsoil to the site and/or incorporating
organic amendments. Organic amendments should be incorporated to a minimum 8-
inch depth except where tree roots or other natural features limit the depth of
incorporation. Subsoils below the 12-inch depth should be scarified at least 4 inches
to avoid stratified layers, where feasible. The decision to either layer topsoil over a
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subgrade or incorporate topsoil into the underlying layer may vary depending on the
planting specified.
• If blended topsoil is imported, fines should be limited to 25 percent passing through a
200 sieve.
• The final composition and construction of the soil system will result in a natural
selection or favoring of certain plant species over time. For example, recent practices
have shown that incorporation of topsoil may favor grasses, while layering with mildly
acidic, high-carbon amendments may favor more woody vegetation.
• Locate the topsoil stockpile so it meets specifications and does not interfere with
work on the site. It may be possible to locate more than one pile in proximity to areas
where topsoil will be used.
• Allow sufficient time in scheduling for topsoil to be spread prior to seeding, sodding,
or planting.
• Care must be taken not to apply topsoil over subsoil if the two soils have contrasting
textures. Sandy topsoil over clayey subsoil is a particularly poor combination, as
water creeps along the junction between the soil layers and causes the topsoil to
slough.
• If topsoil and subsoil are not properly bonded, water will not infiltrate the soil profile
evenly and it will be difficult to establish vegetation. The best method to prevent a
lack of bonding is to work the topsoil into the layer below for a depth of at least 6
inches.
• Ripping or re-structuring the subgrade may also provide additional benefits regarding
the overall infiltration and interflow dynamics of the soil system.
• Field exploration of the site shall be made to determine if there is surface soil of
sufficient quantity and quality to justify stripping. Topsoil shall be friable and loamy
(loam, sandy loam, silt loam, sandy clay loam, clay loam). Areas of natural
groundwater recharge should be avoided.
• Confine stripping to the immediate construction area. A 4- to 6- inch stripping depth
is common, but depth may vary depending on the particular soil. Place all surface
runoff control structures in place prior to stripping.
Stockpile topsoil in the following manner:
• Side slopes of the stockpile shall not exceed 2:1.
• Surround all topsoil stockpiles between October 1 and April 30 with an interceptor
dike with gravel outlet and silt fence. Between May 1 and September 30, install an
interceptor dike with gravel outlet and silt fence if the stockpile will remain in place for
a longer period of time than active construction grading.
• Complete erosion control seeding or covering with clear plastic or other mulching
materials of stockpiles within 2 days (October 1 through April 30) or 7 days (May 1
through September 30) of the formation of the stockpile. Do not cover native topsoil
stockpiles with plastic.
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• Topsoil shall not be placed while in a frozen or muddy condition, when the subgrade
is excessively wet, or when conditions exist that may otherwise be detrimental to
proper grading or proposed sodding or seeding.
• Maintain previously established grades on the areas to be topsoiled according to the
approved plan.
• When native topsoil is to be stockpiled and reused, the following should apply to
ensure that the mycorrhizal bacterial, earthworms, and other beneficial organisms
will not be destroyed:
o Topsoil is to be re-installed within 4 to 6 weeks;
o Topsoil is not to become saturated with water;
o Plastic cover is not allowed.
3.1.14.4 Maintenance Standards
Inspect stockpiles regularly, especially after large storm events. Stabilize any areas that have eroded.
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3.1.15 BMP C127: Polyacrylamide for Soil Erosion Protection
3.1.15.1 Purpose
Polyacrylamide (PAM) is used on construction sites to prevent soil erosion.
Applying PAM to bare soil in advance of a rain event significantly reduces erosion and controls
sediment in two ways. PAM helps maintain soil structure, which increases the ability to infiltrate.
3.1.15.2 Conditions of Use
Do not apply PAM directly to water or allow it to enter a water body. In areas that drain to a
sediment pond, PAM can be applied to bare soil under the following conditions:
• During rough grading operations.
• Staging areas.
• Balanced cut and fill earthwork.
• Haul roads prior to placement of crushed rock surfacing.
• Compacted soil roadbase.
• Stockpiles.
• After final grade and before paving or final seeding and planting.
• Pit sites.
• Sites having a winter shut down. In the case of winter shut down, or where soil will
remain unworked for several months, PAM should be used together with mulch.
3.1.15.3 Design and Installation Specifications
PAM may be applied in dissolved form with water, or it may be applied in dry, granular or powdered
form. The preferred application method is the dissolved form.
PAM is to be applied at a maximum rate of 2/3 pound PAM per 1,000 gallons water (80 mg/L) per 1
acre of bare soil. Higher concentrations of PAM do not provide any additional effectiveness.
The Preferred Method:
• Pre-measure the area where PAM is to be applied and calculate the amount of
product and water necessary to provide coverage at the specified application rate
(2/3 pound PAM per 1,000 gallons per acre).
• PAM has infinite solubility in water, but dissolves very slowly. Dissolve pre-measured
dry granular PAM with a known quantity of clean water in a bucket several hours or
overnight. Mechanical mixing will help dissolve the PAM. Always add PAM to water -
not water to PAM.
• Pre-fill the water truck about 1/8 full with water. The water does not have to be
potable, but it must have relatively low turbidity – in the range of 20 NTU or less.
• Add PAM and water mixture to the truck.
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• Completely fill the water truck to specified volume.
• Spray PAM and water mixture onto dry soil until the soil surface is uniformly and
completely wetted.
An Alternate Method:
PAM may also be applied as a powder at the rate of 5 pounds per acre. This must be applied on a
day that is dry. For areas less than 5 to 10 acres, a hand-held “organ grinder” fertilizer spreader set to
the smallest setting will work. Tractor-mounted spreaders will work for larger areas.
Benefits and Limitations:
The following benefits and limitations should be considered:
• PAM shall be used in conjunction with other BMPs and not in place of other BMPs.
• The steeper the slope, the less benefit PAM will provide and the more critical it is to
use proper groundcover for erosion control.
• Do not use PAM on a slope that flows directly into a stream or wetland or any other
waterbody.
• PAM has little to no effect on sandy soils with little clay content.
• Do not add PAM to water discharging from site.
• When the total drainage area is greater than or equal to 5 acres, PAM treated areas
shall drain to a sediment pond.
• Areas less than 5 acres shall drain to sediment control BMPs, such as a minimum of
3 check dams per acre. The total number of check dams used shall be maximized to
achieve the greatest amount of settlement of sediment prior to discharging from the
site. Each check dam shall be spaced evenly in the drainage channel through which
stormwater flows are discharged off-site.
• On all sites, use silt fences to limit the discharges of sediment from the site.
• Cover and protect all areas not being actively worked from rainfall. PAM shall not be
the only cover BMP used.
• PAM can be applied to wet soil, but dry soil is preferred due to less sediment loss.
• PAM will work when applied to saturated soil but is not as effective as applications to
dry or damp soil.
• Keep the granular PAM supply out of the sun. Granular PAM loses its effectiveness
in three months after exposure to sunlight and air.
• Proper application and re-application plans are necessary to ensure total
effectiveness of PAM usage.
• PAM, combined with water, is very slippery and can be a safety hazard. Care must
be taken to prevent spills of PAM powder onto paved surfaces. During an application
of PAM, prevent over-spray from reaching pavement, as pavement will become
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slippery. If PAM powder gets on skin or clothing, wipe it off with a rough towel rather
than washing with water, which makes cleanup messier and take longer.
• Some PAMs are more toxic and carcinogenic than others. Only the most
environmentally safe PAM products should be used.
• The specific PAM copolymer formulation must be anionic. Cationic PAM shall not
be used in any application because of known aquatic toxicity problems. Only
the highest drinking water grade PAM, certified for compliance with ANSI/NSF
Standard 60 for drinking water treatment, will be used for soil applications. PAM use
shall be reviewed and approved by the City. The Washington State Department of
Transportation (WSDOT) has listed approved PAM products on its web page.
• PAM designated for these uses should be "water soluble", "linear", or "non-
crosslinked". Cross-linked or water absorbent PAM, polymerized in highly acidic
(pH<2) conditions, are used to maintain soil moisture content.
• The PAM anionic charge density may vary from 2 to 30 percent; a value of 18
percent is typical. Studies conducted by the United States Department of Agriculture
(USDA)/ARS demonstrated that soil stabilization was optimized by using very high
molecular weight (12-15 mg/mole), highly anionic (>20% hydrolysis) PAM.
• PAM tackifiers are available and being used in place of guar and alpha plantago.
Typically, PAM tackifiers should be used at a rate of no more than 0.5 to 1 pounds
per 1,000 gallons of water in a hydromulch machine. Some tackifier product
instructions say to use at a rate of 3 to 5 pounds per acre, which can be too much. In
addition, pump problems can occur at higher rates due to increased viscosity.
3.1.15.4 Maintenance Standards
• PAM may be reapplied on actively worked areas after a 48-hour period.
• Reapplication is not required unless PAM treated soil is disturbed or turbidity levels
show the need for an additional application. If PAM treated soil is left undisturbed, a
reapplication may be necessary after two months. When PAM is applied first to bare
soil and then covered with straw, a reapplication may not be necessary for several
months.
• Loss of sediment and PAM may be a basis for penalties per RCW 90.48.080.
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3.1.16 BMP C130: Surface Roughening
3.1.16.1 Purpose
Surface roughening aids in the establishment of vegetative cover, reduces runoff velocity, increases
infiltration, and provides for sediment trapping through the provision of a rough soil surface.
Horizontal depressions are created by operating a tiller or other suitable equipment on the contour or
by leaving slopes in a roughened condition by not fine grading them.
3.1.16.2 Conditions for Use
All slopes steeper than 3H:1V and greater than 5 vertical feet require surface roughening.
• Areas with grades steeper than 3H:1V should be roughened to a depth of 2 to 4
inches prior to seeding.
• Areas that will not be stabilized immediately may be roughened to reduce runoff
velocity until seeding takes place.
• Slopes with a stable rock face do not require roughening.
• Slopes where mowing is planned should not be excessively roughened.
3.1.16.3 Design and Installation Specifications
There are different methods for achieving a roughened soil surface on a slope, and the selection of
an appropriate method depends upon the type of slope. Roughening methods include stair-step
grading, grooving, contour furrows, and tracking. See Figure II-3-8 for tracking and contour furrows.
Factors to be considered in choosing a method are slope steepness, mowing requirements, and
whether the slope is formed by cutting or filling.
• Graded areas with slopes greater than 3:1 but less than 2:1 should be roughened
before seeding. This can be accomplished in a variety of ways, including "track
walking," or driving a crawler tractor up and down the slope, leaving a pattern of cleat
imprints parallel to slope contours.
• Tracking is done by operating equipment up and down the slope to leave horizontal
depressions in the soil.
3.1.16.4 Maintenance Standards
• Areas that are graded in this manner should be seeded as quickly as possible.
• Regular inspections should be made of the area. If rills appear, they should be re-
graded and re-seeded immediately.
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Figure II-3-8. Surface Roughening by Tracking and Contour Furrows
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3.1.17 BMP C131: Gradient Terraces
3.1.17.1 Purpose
Gradient terraces reduce erosion damage by intercepting surface runoff and conducting it to a stable
outlet at a non-erosive velocity.
3.1.17.2 Conditions of Use
Gradient terraces normally are limited to denuded land having a water erosion problem. They should
not be constructed on deep sands or on soils that are too stony, steep, or shallow to permit practical
and economical installation and maintenance. Gradient terraces may be used only where suitable
outlets are or will be made available. See Figure II-3-9 for gradient terraces.
3.1.17.3 Design and Installation Specifications
The maximum spacing of gradient terraces should be determined by the following method:
VI = (0.8)s + y
Where:
VI = vertical interval in feet
s = land rise per 100 feet, expressed in feet
y = a soil and cover variable with values from 1.0 to 4.0
Values of “y” are influenced by soil erodibility and cover practices. The lower values are applicable to
erosive soils where little to no residue is left on the surface. The higher value is applicable only to
erosion-resistant soils where a large amount of residue (1½ tons of straw/acre equivalent) is on the
surface.
• The minimum constructed cross-section should meet the design dimensions.
• The top of the constructed ridge should not be lower at any point than the design
elevation plus the specified overfill for settlement. The opening at the outlet end of
the terrace should have a cross section equal to that specified for the terrace
channel.
• Channel grades may be either uniform or variable with a maximum grade of 0.6 feet
per 100 feet length. For short distances, terrace grades may be increased to improve
alignment. The channel velocity should not exceed that which is non-erosive for the
soil type with the planned treatment.
• All gradient terraces should have adequate outlets. Such an outlet may be a grassed
waterway, vegetated area, or tile outlet. In all cases, the outlet must convey runoff
from the terrace or terrace system to a point where the outflow will not cause
damage. Vegetative cover should be used in the outlet channel.
• The design elevation of the water surface of the terrace should not be lower than the
design elevation of the water surface in the outlet at their junction, when both are
operating at design flow.
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• Vertical spacing determined by the above methods may be increased as much as
0.5 feet or 10 percent, whichever is greater, to provide better alignment or location,
avoid obstacles, adjust for equipment size, or reach a satisfactory outlet.
• The drainage area above the top should not exceed the area that would be drained
by a terrace with normal spacing.
• The terrace should have enough capacity to handle the peak runoff expected from a
2-year, 24-hour design storm without overtopping.
• The terrace cross-section should be proportioned to fit the land slope. The ridge
height should include a reasonable settlement factor. The ridge should have a
minimum top width of 3 feet at the design height. The minimum cross-sectional area
of the terrace channel should be 8 square feet for land slopes of 5 percent or less, 7
square feet for slopes from 5 to 8 percent, and 6 square feet for slopes steeper than
8 percent. The terrace can be constructed wide enough to be maintained using a
small cat.
3.1.17.4 Maintenance Standards
Maintenance should be performed as needed. Terraces should be inspected regularly, at least once
a year, and after large storm events.
Figure II-3-9. Gradient Terraces
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3.1.18 BMP C140: Dust Control
3.1.18.1 Purpose
Dust control prevents wind transport of dust from disturbed soil surfaces onto roadways, drainage
ways, and surface waters.
3.1.18.2 Conditions of Use
Use dust control practices in areas (including roadways) subject to surface and air movement of dust
where on-site and off-site impacts to roadways, drainage ways, or surface waters are likely.
3.1.18.3 Design and Installation Specifications
• Vegetate or mulch areas that will not receive vehicle traffic. In areas where planting,
mulching, or paving is impractical, apply gravel or landscaping rock.
• Limit dust generation by clearing only to those areas where immediate activity will
take place, leaving the remaining area(s) in the original condition, if stable. Maintain
the original ground cover as long as practical.
• Construct natural or artificial windbreaks or windscreens. These may be designed as
enclosures for small dust sources.
• Sprinkle the site with water until surface is wet. Repeat as needed. To prevent
carryout of mud onto street, refer to Stabilized Construction Entrance (BMP C105).
• Irrigation water can be used for dust control. Install irrigation systems as a first step
on sites where dust control is a concern.
• Spray exposed soil areas with a dust palliative, following the manufacturer’s
instructions and cautions regarding handling and application. Used oil is prohibited
from use as a dust suppressant. The City may approve other dust palliatives such as
calcium chloride or PAM.
• PAM (BMP C127) added to water at a rate of 2/3 pounds per 1,000 gallons of water
per acre and applied from a water truck is more effective than water alone. This is
due to the increased infiltration of water into the soil and reduced evaporation. In
addition, small soil particles are bonded together and are not as easily transported by
wind. Adding PAM may actually reduce the quantity of water needed for dust control.
There are concerns with the proper use of PAM, refer to BMP C127 for more
information on PAM application. PAM use requires City approval.
• Lower speed limits. High vehicle speed increases the amount of dust stirred up from
unpaved roads and lots.
• Upgrade the road surface strength by improving particle size, shape, and mineral
types that make up the surface and base materials.
• Add surface gravel to reduce the source of dust emission. Limit the amount of fine
particles to 10 to 20 percent.
• Use geotextile fabrics to increase the strength of new roads or roads undergoing
reconstruction.
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• Encourage the use of alternate, paved routes, if available.
• Restrict use by tracked vehicles and heavy trucks to prevent damage to road
surfaces and bases.
• Apply chemical dust suppressants using the admix method, blending the product
with the top few inches of surface material. Suppressants may also be applied as
surface treatments.
• Pave unpaved permanent roads and other trafficked areas.
• Use vacuum street sweepers.
• Remove mud and other dirt promptly so it does not dry and then turn into dust.
• Limit dust-causing work on windy days.
• Contact the Puget Sound Clean Air Agency for guidance and training on other dust
control measures. Compliance with the Puget Sound Clean Air Agency’s
recommendations/requirements constitutes compliance with this BMP.
3.1.18.4 Maintenance Standards
Evaluate the potential for dust generation frequently during dry periods. Complete the actions outlines
above as needed to limit the dust.
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3.1.19 BMP C150: Materials On Hand
3.1.19.1 Purpose
Quantities of erosion prevention and sediment control materials should be kept on the project site at
all times to be used for emergency situations such as unexpected heavy summer rains. Having these
materials on-site reduces the time needed to implement BMPs when inspections indicate that
existing BMPs are not meeting the Construction SWPPP requirements. In addition, it may be more
economical to buy some materials in bulk and store them at the office or yard for future use.
3.1.19.2 Conditions of Use
Construction projects of any size or type can benefit from having materials on hand. A small
commercial development project could have a roll of plastic and some gravel available for immediate
protection of bare soil and temporary berm construction. A large earthwork project, such as highway
construction, might have several tons of straw, several rolls of plastic, flexible pipe, sandbags,
geotextile fabric, and steel “T” posts.
• Materials are stockpiled and readily available before any site clearing, grubbing, or
earthwork begins. A large contractor or developer could keep a stockpile of materials
that are available to be used on several projects.
• If storage space at the project site is at a premium, the contractor could maintain the
materials at their office or yard. The office or yard must be less than an hour from the
project site.
3.1.19.3 Design and Installation Specifications
Depending on project type, size, complexity, and length, materials and quantities will vary. Table II-
3-8 provides a good minimum that will cover numerous situations.
Table II-3-8. Materials on Hand
Material Measure Quantity
Clear Plastic, 6 mil 100 foot roll 1-2
Drainpipe, 6 or 8 inch diameter 25 foot section 4-6
Sandbags, filled each 25-50
Straw Bales for mulching, approx. 50# each 10-20
Quarry Spalls ton 2-4
Washed Gravel cubic yard 2-4
Geotextile Fabric 100 foot roll 1-2
Catch Basin Inserts each 2-4
Steel “T” Posts each 12-24
3.1.19.4 Maintenance Standards
• All materials with the exception of the quarry spalls, steel “T” posts, and gravel
should be kept covered and out of both sun and rain.
• Re-stock materials used as needed.
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3.1.20 BMP C151: Concrete Handling
3.1.20.1 Purpose
Concrete work can generate process water and slurry that contain fine particles and high pH, both of
which can violate water quality standards in the receiving water. This BMP is intended to minimize
and eliminate concrete process water and slurry from entering waters of the state.
3.1.20.2 Conditions of Use
Utilize these management practices any time concrete is used.
Concrete construction projects include, but are not limited to, the following:
• Curbs
• Sidewalks
• Roads
• Bridges
• Foundations
• Floors
• Runways
3.1.20.3 Design and Installation Specifications
• Concrete truck chutes, pumps, and internals shall be washed out only into formed
areas awaiting installation of concrete or asphalt.
• When no formed areas are available, contain washwater and leftover product in a
lined container. Dispose of washwater in a manner that does not violate groundwater
or surface water quality standards.
• Unused concrete remaining in the truck and pump shall be returned to the originating
batch plant for recycling.
• Hand tools including, but not limited to, screeds, shovels, rakes, floats, and trowels
shall be washed off only into formed areas awaiting installation of concrete or
asphalt.
• Equipment that cannot be easily moved, such as concrete pavers, shall only be
washed in areas that do not directly drain to natural or constructed stormwater
conveyances.
• Washdown from areas such as concrete aggregate driveways shall not drain directly
to natural or constructed stormwater conveyances.
3.1.20.4 Maintenance Standards
Containers shall be checked for holes in the liner daily during concrete pours and repaired the same
day.
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3.1.21 BMP C152: Sawcutting and Surfacing Pollution Prevention
3.1.21.1 Purpose
Sawcutting and surfacing operations generate slurry and process water that contains fine particles
and high pH (concrete cutting), both of which can violate water quality standards in the receiving
water. This BMP is intended to minimize and eliminate process water and slurry from entering waters
of the State
3.1.21.2 Conditions of Use
Anytime sawcutting or surfacing operations take place, use these management practices. Sawcutting
and surfacing operations include, but are not limited to, the following:
• Sawing
• Coring
• Grinding
• Roughening
• Hydro-demolition
• Bridge and road surfacing
3.1.21.3 Design and Installation Specifications
• Vacuum slurry and cuttings during cutting and surfacing operations.
• Do not leave slurry and cuttings on permanent concrete or asphalt pavement
overnight.
• Do not drain slurry and cuttings to any natural or constructed drainage conveyance.
• Dispose of collected slurry and cuttings in a manner that does not violate
groundwater or surface water quality standards.
• Do not drain process water that is generated during hydro-demolition, surface
roughening, or similar operations to any natural or constructed drainage conveyance
and dispose of it in a manner that does not violate groundwater or surface water
quality standards.
• Handle and dispose of cleaning waste material and demolition debris in a manner
that does not cause contamination of water. If the area is swept with a pick-up
sweeper, haul the material out of the area to an appropriate disposal site.
3.1.21.4 Maintenance Standards
Continually monitor operations to determine whether slurry, cuttings, or process water could enter
waters of the state. If inspections show that a violation of water quality standards could occur, stop
operations and immediately implement preventive measures such as berms, barriers, secondary
containment, and vacuum trucks.
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3.1.22 BMP C153: Material Delivery, Storage and Containment
3.1.22.1 Purpose
Prevent, reduce, or eliminate the discharge of pollutants from material delivery and storage to the
stormwater system or watercourses by minimizing the storage of hazardous materials onsite, storing
materials in a designated area, and installing secondary containment.
3.1.22.2 Conditions of Use
These procedures are suitable for use at all construction sites with delivery and storage of the
following materials:
• Petroleum products such as fuel, oil, and grease
• Soil stabilizers and binders (e.g. Polyacrylamide)
• Fertilizers, pesticides, and herbicides
• Detergents
• Asphalt and concrete compounds
• Hazardous chemicals such as acids, lime, adhesives, paints, solvents, and curing
compounds
• Any other material that may be detrimental if released to the environment
3.1.22.3 Design and Installation Specifications
The following steps should be taken to minimize risk:
• Locate temporary storage area away from vehicular traffic, near the construction
entrance(s), and away from waterways or storm drains.
• Supply Material Safety Data Sheets (MSDS) for all materials stored. Keep chemicals
in their original labeled containers.
• Surrounding materials with earth berms is an option for temporary secondary
containment.
• Minimize hazardous material storage on-site.
• Handle hazardous materials as infrequently as possible.
• During the wet weather season (October 1 through April 30), consider storing
materials in a covered area.
• Store materials in secondary containment, such as an earthen dike, a horse trough,
or a children’s wading pool for non-reactive materials such as detergents, oil, grease,
and paints. “Bus boy” trays or concrete mixing trays may be used as secondary
containment for small amounts of material.
• Do not store chemicals, drums, or bagged materials directly on the ground. Place
these items on a pallet and, when possible, in secondary containment.
• If drums cannot be stored under a roof, domed plastic covers are inexpensive and
snap to the top of drums, preventing water from collecting.
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3.1.22.4 Material Storage Areas and Secondary Containment Practices:
• Store liquids, petroleum products, and substances listed in 40 CFR Parts 110, 117,
or 302 in approved containers and drums and do not overfill the containers or drums.
Store containers and drums in temporary secondary containment facilities.
• Temporary secondary containment facilities shall provide for a spill containment
volume able to contain precipitation from a 25 year, 24 hour storm event plus 10% of
the total enclosed container volume of all containers, or 110% of the capacity of the
largest container within its boundary, whichever is greater.
• Secondary containment facilities shall be impervious to the materials stored therein
for a minimum contact time of 72 hours.
• Secondary containment facilities shall be maintained free of accumulated rainwater
and spills. In the event of spills or leaks, collect accumulated rainwater and spills and
place into drums. Handle these liquids as hazardous waste unless testing
determines them to be non-hazardous.
• Provide sufficient separation between stored containers to allow for spill cleanup and
emergency response access.
• During the wet weather season (October 1 through April 30), cover each secondary
containment facility during non-working days, prior to and during rain events.
• Keep material storage areas clean, organized, and equipped with an ample supply of
appropriate spill clean-up material.
• The spill kit should include, at a minimum:
o 1 water resistant nylon bag
o 3 oil absorbent socks (3-inches by 4-feet)
o 2 oil absorbent socks (3-inches by 10-feet)
o 12 oil absorbent pads (17-inches by 19-inches)
o 1 pair splash resistant goggles
o 3 pairs nitrile gloves
o 10 disposable bags with ties
o Instructions
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3.1.23 BMP C154: Concrete Washout Area
3.1.23.1 Purpose
Prevent or reduce the discharge of pollutants to stormwater from concrete waste by conducting
washout offsite, or performing onsite washout in a designated area to prevent pollutants from
entering surface waters or groundwater.
3.1.23.2 Conditions of Use
Use concrete washout best management practices on construction projects where:
• Concrete is used as a construction material.
• It is not possible to dispose of all concrete wastewater and washout offsite (ready
mix plant, etc.)
• Concrete trucks, pumpers, or other concrete coated equipment are washed onsite.
NOTE: If less than 3 concrete trucks or pumpers need to be washed on-site, the washwater may be
disposed of in a formed area awaiting concrete or an upland disposal area where it cannot
contaminate surface or groundwater. The upland disposal area must be at least 50 feet from
sensitive areas such as storm drains, open ditches, or waterbodies, including wetlands. Do not allow
dirty water to enter storm drains, open ditches, or any waterbody.
3.1.23.3 Implementation
The following steps will help reduce stormwater pollution from concrete wastes:
• Perform washout of concrete trucks offsite or in designated concrete washout areas
only.
• Do not wash out concrete trucks onto the ground, or into storm drains, open ditches,
streets, or streams.
• Do not allow excess concrete to be dumped onsite, except in designated concrete
washout areas.
• Concrete washout areas may be prefabricated concrete washout containers, or self-
installed structures (above-grade or below-grade).
o Prefabricated containers are most resistant to damage and protect against
spills and leaks. Companies may offer delivery service and provide regular
maintenance and disposal of solid and liquid waste.
o If self-installed concrete washout areas are used, below-grade structures are
preferred over above-grade structures because they are less prone to spills
and leaks.
o Self-installed above-grade structures should only be used if excavation is not
practical.
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3.1.23.4 Education
• Discuss the concrete management techniques described in this BMP with the ready-
mix concrete supplier before any deliveries are made.
• Educate employees and subcontractors on the concrete waste management
techniques described in this BMP.
• Arrange for contractor’s superintendent or Certified Erosion and Sediment Control
Lead (CESCL) to oversee and enforce concrete waste management procedures.
• Install a sign adjacent to each temporary concrete washout facility to inform concrete
equipment operators to utilize the proper facilities.
3.1.23.5 Contracts
Incorporate requirements for concrete waste management into concrete supplier and subcontractor
agreements.
3.1.23.6 Location and Placement Considerations:
• Locate washout area at least 50 feet from sensitive areas such as storm drains, open
ditches, or water bodies, including wetlands.
• Allow convenient access for concrete trucks, preferably near the area where the
concrete is being poured.
• If trucks need to leave a paved area to access washout, prevent track-out with a pad
of rock or quarry spalls (BMP C105). These areas should be far enough away from
other construction traffic to reduce the likelihood of accidental damage and spills.
• The number of facilities you install should depend on the expected demand for
storage capacity.
• On large sites with extensive concrete work, washouts should be placed in multiple
locations for ease of use by concrete truck drivers.
3.1.23.7 Onsite Temporary Concrete Washout Facility, Transit Truck Washout
Procedures:
• Locate temporary concrete washout facilities a minimum of 50 ft from sensitive areas
including storm drain inlets, open drainage facilities, and watercourses.
• Construct and maintain concrete washout facilities in order to contain all liquid and
concrete waste generated by washout operations.
o Approximately 7 gallons of wash water are used to wash one truck chute.
o Approximately 50 gallons are used to wash out the hopper of a concrete
pump truck.
• Washout of concrete trucks shall be performed in designated areas only.
• Concrete washout from concrete pumper bins can be washed into concrete pumper
trucks and discharged into designated washout area or properly disposed of offsite.
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• Once concrete wastes are washed into the designated area and allowed to harden,
the concrete should be broken up, removed, and disposed of per applicable solid
waste regulations. Dispose of hardened concrete on a regular basis.
Temporary Above-Grade Concrete Washout Facility
• Temporary concrete washout facility (type above grade) should be constructed as
shown on the details at the end of this BMP, with a recommended minimum length
and minimum width of 10 ft, but with sufficient quantity and volume to contain all
liquid and concrete waste generated by washout operations.
• Plastic lining material should be a minimum of 10 mil polyethylene sheeting and
should be free of holes, tears, or other defects that compromise the impermeability of
the material.
Temporary Below-Grade Concrete Washout Facility
• Temporary concrete washout facilities (type below grade) should be constructed as
shown on the details at the end of this BMP, with a recommended minimum length
and minimum width of 10 ft. The quantity and volume should be sufficient to contain
all liquid and concrete waste generated by washout operations.
• Lath and flagging should be commercial type.
• Plastic lining material shall be a minimum of 10 mil polyethylene sheeting and should
be free of holes, tears, or other defects that compromise the impermeability of the
material.
• Liner seams shall be installed in accordance with manufacturers’ recommendations.
• Soil base shall be prepared free of rocks or other debris that may cause tears or
holes in the plastic lining material.
3.1.23.8 Inspection and Maintenance
• Inspect and verify that concrete washout BMPs are in place prior to the
commencement of concrete work.
• During periods of concrete work, inspect daily to verify continued performance.
o Check overall condition and performance.
o Check remaining capacity (% full).
o If using self-installed washout facilities, verify plastic liners are intact and
sidewalls are not damaged.
o If using prefabricated containers, check for leaks.
• Maintain washout facilities to provide adequate holding capacity with a minimum
freeboard of 12 inches.
• Washout facilities must be cleaned, or new facilities must be constructed and ready
for use once the washout is 75% full.
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• If the washout is nearing capacity, vacuum and dispose of the waste material in an
approved manner.
o Do not discharge liquid or slurry to waterways, storm drains or directly onto
ground.
o Do not use sanitary sewer without local approval.
o Place a secure, non-collapsing, non-water collecting cover over the concrete
washout facility prior to predicted wet weather to prevent accumulation and
overflow of precipitation.
o Remove and dispose of hardened concrete and return the structure to a
functional condition. Concrete may be reused onsite or hauled away for
disposal or recycling.
• When you remove materials from the self-installed concrete washout, build a new
structure; or, if the previous structure is still intact, inspect for signs of weakening or
damage, and make any necessary repairs. Re-line the structure with new plastic
after each cleaning.
3.1.23.9 Removal of Temporary Concrete Washout Facilities
• When temporary concrete washout facilities are no longer required for the work,
remove and properly dispose of the hardened concrete, slurries and liquids.
• Remove materials used to construct temporary concrete washout facilities from the
site of the work and dispose of or recycle it.
• Holes, depressions or other ground disturbance caused by the removal of the
temporary concrete washout facilities shall be backfilled, repaired, and stabilized to
prevent erosion.
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Figure II-3-10. Temporary Concrete Washout Facility
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Figure II-3-11. Prefabricated Concrete Washout Container with Ramp
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3.1.24 BMP C160: Certified Erosion and Sediment Control Lead
3.1.24.1 Purpose
The project proponent designates at least one person as the responsible representative in charge of
erosion and sediment control (ESC) and water quality protection. The designated person shall be the
Certified Erosion and Sediment Control Lead (CESCL), who is responsible for ensuring compliance
with all local, state, and federal erosion and sediment control and water quality requirements.
3.1.24.2 Conditions of Use
A CESCL should be made available on project types that include, but are not limited to, the following:
• Construction activity that disturbs one acre of land or more.
• Construction activity that disturbs less than one acre of land, but is part of a larger
common plan of development or sale that will ultimately disturb one acre of land or
more.
• Heavy construction of roads, bridges, highways, airports, buildings.
• Projects near wetlands and sensitive or critical areas.
• Projects in or over water.
3.1.24.3 Specifications
The CESCL shall:
• Have a current certificate proving attendance in an erosion and sediment control
training course that meets the minimum ESC training and certification requirements
established by Ecology. Ecology will maintain a list of ESC training and certification
providers at: www.ecy.wa.gov/programs/wq/stormwater.
OR
• Be a Certified Professional in Erosion and Sediment Control (CPESC). For additional
information go to: www.cpesc.net
The CESCL shall have authority to act on behalf of the contractor or developer and shall be available,
on call, 24 hours per day throughout the period of construction.
The Construction SWPPP shall include the name, telephone number, fax number, and address of
the designated CESCL.
A CESCL may provide inspection and compliance services for multiple construction projects in the
same geographic region.
Duties and responsibilities of the CESCL shall include, but are not limited to, the following:
• Maintaining a permit file on site at all times which includes the SWPPP and any
associated permits and plans.
• Directing BMP installation, inspection, maintenance, modification, and removal.
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• Updating all project drawings and the Construction SWPPP with changes made.
• Keeping daily logs and inspection reports. Inspection reports should include:
o Inspection date/time.
o Weather information, general conditions during inspection, and approximate
amount of precipitation since the last inspection.
o A summary or list of all BMPs implemented, including observations of all
erosion/sediment control structures or practices. The following shall be noted:
Locations of BMPs inspected,
Locations of BMPs that need maintenance,
Locations of BMPs that failed to operate as designed or intended, and
Locations where additional or different BMPs are required.
o Visual monitoring results, including a description of discharged stormwater.
The presence of suspended sediment, turbid water, discoloration, and oil
sheen shall be noted, as applicable.
o Any water quality monitoring performed during inspection.
o General comments and notes, including a brief description of any BMP
repairs, maintenance, or installations made as a result of the inspection.
• Facilitate, participate in, and take corrective actions resulting from inspections
performed by outside agencies or the owner.
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3.1.25 BMP C161: Payment of Erosion Control Work
3.1.25.1 Purpose
As with any construction operation, the contractor should be paid for erosion control work. Address
payment for erosion control during project development and design. Identify the method of payment
in the SWPPP.
3.1.25.2 Conditions of Use
Erosion control work should never be “incidental” to the contract as it is extremely difficult for the
contractor to bid the work. Work that is incidental to the contract is work where no separate
measurement or payment is made. The cost for incidental work is included in payments made for
applicable bid items in the Schedule of Unit Prices. For example, any erosion control work associated
with an item called “Clearing and Grubbing” is bid and paid for as part of that item, not separately.
Two effective means for payment of erosion control work are described below. These include:
• TESC-Force Account
• Unit Prices
TESC Force Account
One good method for ensuring that contingency money is available to address unforeseen erosion
and sediment control problems is to set up an item called “TESC-Force Account”. For example, an
amount such as $15,000 is written in both the Unit Price and Amount columns for the item. This
requires all bidders to bid $15,000 for the item.
The Force Account is used only at the discretion of the contracting agency or developer. If there are
no unforeseen erosion problems, the money is not used. If there are unforeseen erosion problems,
the contracting agency would direct the work to be done and pay an agreed upon amount for the
work (such as predetermined rates under a Time and Materials setting).
Contract language for this item could look like this:
Measurement and Payment for “TESC-Force Account” will be on a Force Account basis in
accordance with_________(include appropriate section of the Contract Specifications). The amount
entered in the Schedule of Unit Prices is an estimate.
Unit Prices
When the material or work can be quantified, it can be paid by Unit Prices. For example, the project
designer knows that 2 acres will need to be hydroseeded and sets up an Item of Work for
Hydroseed, with a Bid Quantity of 2, and a Unit for Acre. The bidder writes in the unit Prices and
Amount.
Unit Price items can be used in conjunction with TESC-Force Account and TESC-Lump Sum.
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3.1.26 BMP C162: Scheduling
3.1.26.1 Purpose
Sequencing a construction project reduces the amount and duration of soil exposed to erosion by
wind, rain, runoff, and vehicle tracking.
3.1.26.2 Conditions of Use
The construction sequence schedule is an orderly listing of all major land-disturbing activities together
with the necessary erosion and sedimentation control measures planned for the project. This type of
schedule guides the contractor on work to be done before other work is started so serious erosion
and sedimentation problems can be avoided.
Following a specified work schedule that coordinates the timing of land-disturbing activities and the
installation of control measures is perhaps the most cost-effective way of controlling erosion during
construction. The removal of surface ground cover leaves a site vulnerable to accelerated erosion.
Construction procedures that limit land clearing, provide timely installation of erosion and
sedimentation controls, and restore protective cover quickly can significantly reduce the erosion
potential of a site.
3.1.26.3 Design Considerations
• Avoid rainy periods.
• Schedule projects to disturb only small portions of the site at any one time. Complete
grading as soon as possible. Immediately stabilize the disturbed portion before
grading the next portion. Practice staged seeding in order to revegetate cut and fill
slopes as the work progresses.
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3.1.27 BMP C180: Small Project Construction Stormwater Pollution
Prevention
3.1.27.1 Purpose
To prevent the discharge of sediment and other pollutants to the maximum extent practicable from
small construction projects.
3.1.27.2 Conditions of Use
Can be used on small construction projects that:
• Add or replace between 2,000 and 5,000 square feet of impervious surfaces, or
• Clear/disturb between 7,000 square feet and 1 acre of land, or
• Grade/fill less than 500 cubic yards of material.
3.1.27.3 Design and Installation Specifications
• Plan and implement proper clearing and grading of the site. It is most important to
clear only the areas needed, thus keeping exposed areas to a minimum. Phase
clearing so that only those areas actively being worked are uncovered.
NOTE: Clearing limits should be flagged in the lot or area prior to initiating clearing.
• Manage soil in a manner that does not permanently compact or deteriorate the final
soil and landscape system. If disturbance and/or compaction occur, the impact must
be corrected at the end of the construction activity. This shall include restoration of
soil depth, soil quality, permeability, and percent organic matter. Construction
practices must not cause damage to or compromise the design of permanent
landscape or infiltration areas.
• Locate excavated basement soil a reasonable distance behind the curb, such as in
the backyard or side yard area. This will increase the distance eroded soil must
travel to reach the storm sewer system. Cover soil piles until the soil is either used or
removed. Situate piles so sediment does not run into the street or adjoining yards.
• Backfill basement walls as soon as possible and rough grade the lot. This will
eliminate large soil mounds, which are highly erodible, and prepares the lot for
temporary cover, which will further reduce erosion potential.
• Remove excess soil from the site as soon as possible after backfilling. This will
eliminate any sediment loss from surplus fill.
• If a lot has a soil bank higher than the curb, install a trench or berm, moving the bank
several feet behind the curb. This will reduce the occurrence of gully and rill erosion
while providing a storage and settling area for stormwater.
• Stabilize the construction entrance where traffic will be leaving the construction site
and traveling on paved roads or other paved areas within 1,000 feet of the site.
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• Provide for periodic street cleaning to remove any sediment that may have been
tracked out. Remove sediment by shoveling or sweeping and carefully move it to a
suitable disposal area where it will not be re-eroded.
• Backfill utility trenches that run up and down slopes within seven days. Cross-slope
trenches may remain open throughout construction to provide runoff interception and
sediment trapping, provided that they do not convey turbid runoff off site.
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3.2 Runoff, Conveyance and Treatment BMPs
3.2.1 BMP C200: Interceptor Dike and Swale
3.2.1.1 Purpose
Provide a ridge of compacted soil, or a ridge with an upslope swale, at the top or base of a disturbed
slope or along the perimeter of a disturbed construction area to convey stormwater. Use the dike
and/or swale to intercept the runoff from unprotected areas and direct it to areas where erosion can
be controlled. This can prevent storm runoff from entering the work area or sediment-laden runoff
from leaving the construction site.
3.2.1.2 Conditions of Use
Where the runoff from an exposed site or disturbed slope must be conveyed to an erosion control
facility that can safely convey the stormwater.
• Locate upslope of a construction site to prevent runoff from entering disturbed area.
• When placed horizontally across a disturbed slope, it reduces the amount and
velocity of runoff flowing down the slope.
• Locate downslope to collect runoff from a disturbed area and direct it to a sediment
basin.
3.2.1.3 Design and Installation Specifications
• Stabilize dike and/or swale and channel with temporary or permanent vegetation or
other channel protection during construction.
• Channel requires a positive grade for drainage; steeper grades require channel
protection and check dams.
• Review construction for areas where overtopping may occur.
• Can be used at the top of new fill before vegetation is established.
• May be used as a permanent diversion channel to carry the runoff.
• Sub-basin tributary area should be one acre or less.
• Design capacity for the peak flow from a 10-year, 24-hour storm assuming a Type 1A
rainfall distribution (3-inches) for temporary facilities. Alternatively, use 1.6 times the
10-year, 1-hour flow indicated by WWHM. Design capacity for the peak flow from a
25-year, 24-hour storm for permanent facilities.
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Interceptor Dikes
• Interceptor dikes shall meet the following criteria:
Top Width 2 feet minimum.
Height 1.5 feet minimum on berm.
Side Slope 2:1 or flatter.
Grade Depends on topography, however, dike system minimum is
0.5% and maximum is 1%
Compaction Minimum of 90 percent ASTM D698 standard proctor.
• Horizontal Spacing of Interceptor Dikes:
Average Slope Slope Percent Flowpath Length
20H:1V or less 3-5% 300 feet
(10 to 20)H:1V 5-10% 200 feet
(4 to 10)H:1V 10-25% 100 feet
(2 to 4)H:1V 25-50% 50 feet
• Stabilization depends on velocity and reach.
Slopes <5% Seed and mulch applied within 5 days of dike construction (see
BMP C121, Mulching).
Slopes 5 - 40% Dependent on runoff velocities and dike materials. Stabilization
should be done immediately using either sod or riprap or other
measures to avoid erosion.
• The upslope side of the dike shall provide positive drainage to the dike outlet. No
erosion shall occur at the outlet. Provide energy dissipation measures as necessary.
Sediment-laden runoff must be released through a sediment trapping facility.
• Minimize construction traffic over temporary dikes. Use temporary cross culverts for
channel crossing.
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Interceptor Swales
• Interceptor swales shall meet the following criteria:
Bottom Width 2 feet minimum; the bottom shall be level.
Depth 1-foot minimum.
Side Slope 2H:1V or flatter
Grade Maximum 5 percent, with positive drainage to a suitable outlet
(such as a sediment pond).
Stabilization Seed as per BMP C120, Temporary and Permanent Seeding, or
BMP C202, Channel Lining, 12 inches thick of riprap pressed
into the bank and extending at least 8 inches vertical from the
bottom.
• Inspect diversion dikes and interceptor swales once a week and after every rainfall.
Immediately remove sediment from the flow area.
• Repair damage caused by construction traffic or other activity before the end of each
working day.
• Check outlets and make timely repairs as needed to avoid gully formation. When the
area below the temporary diversion dike is permanently stabilized, remove the dike
and fill and stabilize the channel to blend with the natural surface.
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3.2.2 BMP C201: Grass-Lined Channels
3.2.2.1 Purpose
To provide a channel with a vegetative lining for conveyance of runoff. See Figure II-3-12 for typical
grass-lined channels.
3.2.2.2 Conditions of Use
This practice applies to construction sites where concentrated runoff needs to be contained to
prevent erosion or flooding.
• When a vegetative lining can provide sufficient stability for the channel cross section
and lower velocities of water (normally dependent on grade). This means that the
channel slopes are generally less than 5 percent and space is available for a
relatively large cross section.
• Typical uses include roadside ditches, channels at property boundaries, outlets for
diversions, and other channels and drainage ditches in low areas.
• Channels that will be vegetated should be installed before major earthwork and
hydroseeded with a bonded fiber matrix (BFM). The vegetation should be well
established (i.e., 75 percent cover) before water is allowed to flow in the ditch. With
channels that will have high flows, erosion control blankets should be installed over
the hydroseed. If vegetation cannot be established from seed before water is allowed
in the ditch, sod should be installed in the bottom of the ditch in lieu of hydromulch
and blankets.
3.2.2.3 Design and Installation Specifications
Locate the channel where it can conform to the topography and other features such as roads.
• Locate them to use natural drainage systems to the greatest extent possible.
• Avoid sharp changes in alignment or bends and changes in grade.
• Do not reshape the landscape to fit the drainage channel.
• Base the maximum design velocity on soil conditions, type of vegetation, and method
of revegetation, but at no times shall velocity exceed 5 feet/second. The channel
shall not be overtopped by the peak runoff from a 10-year, 24–hour storm, assuming
a type 1A rainfall distribution (3.0-inches). Alternatively, use 1.6 times the 10-year, 1-
hour flow indicated by an approved continuous runoff model to determine a flow rate
which the channel must contain.
• An established grass or vegetated lining is required before the channel can be used
to convey stormwater, unless stabilized with nets or blankets.
• If design velocity of a channel to be vegetated by seeding exceeds 2 ft/sec, a
temporary channel liner is required. Geotextile or special mulch protection, such as
fiberglass roving or straw and netting, provides stability until the vegetation is fully
established. See Figure II-3-13.
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• Remove check dams when the grass has matured sufficiently to protect the ditch or
swale unless the slope of the swale is greater than 4 percent. Seed and mulch the
area beneath the check dams immediately after dam removal.
• If vegetation is established by sodding, the permissible velocity for established
vegetation may be used and no temporary liner is needed.
• Do not subject grass-lined channel to sedimentation from disturbed areas. Use
sediment-trapping BMPs upstream of the channel.
• V-shaped grass channels generally apply where the quantity of water is small, such
as in short reaches along roadsides. The V-shaped cross section is least desirable
because it is difficult to stabilize the bottom where velocities may be high.
• Trapezoidal grass channels are used where runoff volumes are large and slope is
low so that velocities are non-erosive to vegetated linings. (Note: it is difficult to
construct small parabolic shaped channels.)
• Subsurface drainage, or riprap channel bottoms, may be necessary on sites that are
subject to prolonged wet conditions due to long duration flows or a high water table.
• Provide outlet protection at culvert ends and at channel intersections.
• Grass channels, at a minimum, should carry peak runoff for temporary construction
drainage facilities from the 10-year, 24-hour storm (3.0 inches) without eroding.
Where flood hazard exists, increase the capacity according to the potential damage.
• Grassed channel side slopes generally are constructed 3:1 or flatter to aid in the
establishment of vegetation and for maintenance.
• Construct channels a minimum of 0.2 foot larger around the periphery to allow for
soil bulking during seedbed preparations and sod buildup.
3.2.2.4 Maintenance Standards
During the establishment period, check grass-lined channels after every rainfall.
• After grass is established, periodically check the channel; check the channel after
every heavy rainfall event. Immediately make repairs.
• It is particularly important to check the channel outlet and all road crossings for bank
stability and evidence of piping or scour holes.
• Remove all significant sediment accumulations to maintain the designed carrying
capacity. Keep the grass in a healthy, vigorous condition at all times, since it is the
primary erosion protection for the channel.
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Figure II-3-12. Typical Grass-Lined Channels
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Figure II-3-13. Temporary Channel Liners
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3.2.3 BMP C202: Channel Lining
3.2.3.1 Purpose
To protect erodible channels by providing a channel liner using either blankets or riprap.
3.2.3.2 Conditions of Use
• When natural soils or vegetated stabilized soils in a channel are not adequate to
prevent channel erosion.
• When a permanent ditch or pipe system is to be installed and a temporary measure
is needed.
• In almost all cases, synthetic and organic coconut blankets are more effective than
riprap for protecting channels from erosion. Blankets can be used with and without
vegetation. Blanketed channels can be designed to handle any expected flow and
longevity requirement. Some synthetic blankets have a predicted life span of 50
years or more, even in sunlight.
• The Federal Highway Administration recommends not using flexible liners whenever
the slope exceeds 10 percent or the shear stress exceeds 8 pounds per square foot.
3.2.3.3 Design and Installation Specifications
See BMP C122 for information on blankets.
Since riprap is used where erosion potential is high, construction must be sequenced so the riprap is
put in place with the minimum possible delay (see Figure II-3-14).
• Only disturb areas where riprap is to be placed if final preparation and placement of
the riprap can immediately follow the initial disturbance. Where riprap is used for
outlet protection, place the riprap before or in conjunction with the construction of the
pipe or channel so it is in place when the pipe or channel begins to operate.
• The designer, after determining the appropriate riprap size for stabilization, shall
consider that size to be a minimum size and then, based on riprap gradations
actually available in the area, select the size or sizes that equal or exceed the
minimum size. Consider the possibility of drainage structure damage by children
when selecting a riprap size, especially if there is nearby water or a gully in which to
toss the stones.
• Use field stone or quarry stone of approximately rectangular shape for the riprap.
The stone shall be hard and angular and of such quality that it will not disintegrate on
exposure to water or weathering and shall be suitable in all respects for the purpose
intended.
• Rubble concrete may be used, provided it has a density of at least 150 pounds per
cubic foot and otherwise meets the requirement of this standard and specification.
• Place a lining of engineering filter fabric (geotextile) between the riprap and the
underlying soil surface to prevent soil movement into or through the riprap. The
geotextile should be keyed in at the top of the bank.
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• Do not use filter fabric on slopes steeper than 1-1/2H:1V as slippage may occur. It
should be used in conjunction with a layer of coarse aggregate (granular filter
blanket) when the riprap to be placed is 12 inches and larger.
Figure II-3-14. Soil Erosion Protection – Rip Rap Protection
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3.2.4 BMP C203: Water Bars
3.2.4.1 Purpose
A small ditch or ridge of material is constructed diagonally across a road or right-of-way to divert
stormwater runoff from the road surface, wheel tracks, or a shallow road ditch.
3.2.4.2 Conditions of Use
Clearing right-of-way and construction of access for power lines, pipelines, and other similar
installations often require long, narrow right-of-ways over sloping terrain. Disturbance and
compaction promotes gully formation in these cleared strips by increasing the volume and velocity of
runoff. Gully formation may be especially severe in tire tracks and ruts. To prevent gullying, runoff can
often be diverted across the width of the right-of-way to undisturbed areas by using small
predesigned diversions.
Give special consideration to each individual outlet area, as well as to the cumulative effect of added
diversions. Use gravel to stabilize the diversion where significant vehicular traffic is anticipated.
3.2.4.3 Design and Installation Specifications
Height: 8-inch minimum measured from the channel bottom to the top of the ridge.
• Side slope of channel: 2H:1V maximum; 3H:1V or flatter when vehicles will cross.
• Base width of ridge: 6-inch minimum.
• Locate them to use natural drainage systems and to discharge into well vegetated
stable areas.
• Guideline for Spacing:
Slope % Spacing (ft)
< 5 125
5 - 10 100
10 - 20 75
20 – 35 50
> 35 Use rock lined ditch
• Grade of water bar and angle: Select angle that results in ditch slope of less than 2
percent.
• Install as soon as clearing and grading is complete. Reconstruct when construction is
complete on a section when utilities are being installed.
• Compact the ridge when installed.
• Stabilize, seed, and mulch portions that are not subject to traffic. Gravel areas
crossed by vehicles.
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3.2.4.4 Maintenance Standards
Periodically inspect right-of-way diversions for wear and after every heavy rainfall inspect for erosion
damage.
• Immediately remove sediment from the flow area and repair the dike.
• Check outlet areas and make timely repairs as needed.
• When permanent road drainage is established and the area above the temporary
right-of-way diversion is permanently stabilized, remove the dike and fill the channel
to blend with the natural ground, and appropriately stabilize the disturbed area.
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3.2.5 BMP C204: Pipe Slope Drains
3.2.5.1 Purpose
To use a pipe to convey stormwater anytime water needs to be diverted away from or over bare soil
to prevent gullies, channel erosion, and saturation of slide-prone soils.
3.2.5.2 Conditions of Use
Pipe slope drains should be used when a temporary or permanent stormwater conveyance is
needed to move the water down a steep slope to avoid erosion (Figure II-3-15).
On highway projects, pipe slope drains should be used at bridge ends to collect runoff and pipe it to
the base of the fill slopes along bridge approaches. These can be designed into a project and
included as bid items. Another use on road projects is to collect runoff from pavement and pipe it
away from side slopes. These are useful because there is generally lag time between having the first
lift of asphalt installed and the curbs, gutters, and permanent drainage installed. Used in conjunction
with sand bags or other temporary diversion devices, these will prevent massive amounts of
sediment from leaving a project.
Water can be collected; channeled with sand bags, Triangular Silt Dikes, berms, or other material;
and piped to temporary sediment ponds.
Pipe slope drains can be:
• Connected to new catch basins and used temporarily until all permanent piping is
installed;
• Used to drain water collected from aquifers exposed on cut slopes and convey it to
the base of the slope;
• Used to collect clean runoff from plastic sheeting and direct it away from exposed
soil;
• Installed in conjunction with silt fence to drain collected water to a controlled area;
• Used to divert small seasonal streams away from construction. They have been used
successfully on culvert replacement and extension jobs. Large flex pipe can be used
on larger streams during culvert removal, repair, or replacement; and,
• Connected to existing down spouts and roof drains and used to divert water away
from work areas during building renovation, demolition, and construction projects.
There are now several commercially available collectors that are attached to the pipe inlet and help
prevent erosion at the inlet.
3.2.5.3 Design and Installation Specifications
Size the pipe to convey the flow. The capacity for temporary drains shall be sufficient to handle the
peak flow from a 10-year, 24-hour storm event assuming a Type 1A rainfall distribution (3.0-inches).
Alternatively, use 1.6 times the 10-year, 1-hour flow indicated by WWHM. Size permanent pipe slope
drains for the 25-year, 24-hour peak flow.
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• Use care in clearing vegetated slopes for installation.
• Re-establish cover immediately on areas disturbed by installation.
• Use temporary drains on new cut or fill slopes.
• Use diversion dikes or swales to collect water at the top of the slope.
• Ensure that the entrance area is stable and large enough to direct flow into the pipe.
• Piping of water through the berm at the entrance area is a common failure mode.
• The entrance shall consist of a standard flared end section for culverts 12 inches and
larger with a minimum 6-inch metal toe plate to prevent runoff from undercutting the
pipe inlet. The slope of the entrance shall be at least 3 percent. Sand bags may also
be used at pipe entrances as a temporary measure.
• Thoroughly compact the soil around and under the pipe and entrance section to
prevent undercutting.
• Securely connect the flared inlet section to the slope drain and have watertight
connecting bands.
• Securely fasten, fuse or have gasketed, watertight fittings for the slope drain
sections, and securely anchor them into the soil.
• Install thrust blocks anytime 90 degree bends are utilized. Depending on size of pipe
and flow, these can be constructed with sand bags, straw bales staked in place, “T”
posts and wire, or ecology blocks.
• Pipe needs to be secured along its full length to prevent movement. This can be
done with steel “T” posts and wire. A post is installed on each side of the pipe and
the pipe is wired to them. This should be done every 10-20 feet of pipe length,
depending on the size of the pipe and quantity of water to be diverted.
• Use interceptor dikes to direct runoff into a slope drain. Ensure the height of the dike
is at least 1 foot higher at all points than at the top of the inlet pipe.
• Stabilize the area below the outlet with a riprap apron (see BMP C209 Outlet
Protection for the appropriate outlet material).
• If the pipe slope drain is conveying sediment-laden water, direct all flows into the
sediment trapping facility.
• Materials specifications for any permanent piped system shall be set by the local
government.
3.2.5.4 Maintenance Standards
Check inlet and outlet points regularly, especially after storms.
The inlet should be free of undercutting, and no water should be going around the point of entry. If
there are problems, the headwall should be reinforced with compacted earth or sand bags.
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• The outlet point should be free of erosion and installed with appropriate outlet
protection.
• For permanent installations, inspect pipe periodically for vandalism and physical
distress such as slides and wind-throw.
• Normally the pipe slope is so steep that clogging is not a problem with smooth wall
pipe; however, debris may become lodged in the pipe.
Figure II-3-15. Pipe Slope Drains
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3.2.6 BMP C205: Subsurface Drains
3.2.6.1 Purpose
To intercept, collect, and convey groundwater to a satisfactory outlet, using a perforated pipe or
conduit below the ground surface. Subsurface drains are also known as “French drains.” The
perforated pipe provides a dewatering mechanism to drain excessively wet soils, provide a stable
base for construction, improve stability of structures with shallow foundations, or to reduce hydrostatic
pressure to improve slope stability.
3.2.6.2 Conditions of Use
Use when excessive water must be removed from the soil. The soil permeability, depth to water
table, and impervious layers are all factors which may govern the use of subsurface drains.
3.2.6.3 Design and Installation Specifications
• Relief drains
o Are used either to lower the water table in large, relatively flat areas, improve
the growth of vegetation, or to remove surface water.
o Are installed along a slope and drain in the direction of the slope.
o Can be installed in a grid pattern, a herringbone pattern, or a random pattern.
• Interceptor drains
o Are used to remove excess groundwater from a slope, stabilize steep slopes,
and lower the water table immediately below a slope to prevent the soil from
becoming saturated.
o Are installed perpendicular to a slope and drain to the side of the slope.
o Usually consist of a single pipe or series of single pipes instead of a
patterned layout.
• Depth and spacing considerations for interceptor drains
o The depth of an interceptor drain is determined primarily by the depth to
which the water table is to be lowered or the depth to a confining layer. For
practical reasons, the maximum depth is usually limited to 6 feet, with a
minimum cover of 2 feet to protect the conduit.
o The soil should have depth and sufficient permeability to permit installation of
an effective drainage system at a depth of 2 to 6 feet.
o An adequate outlet for the drainage system must be available either by
gravity or pumping.
o The quantity and quality of discharge needs to be accounted for in the
receiving stream (additional detention may be required).
o This standard does not apply to subsurface drains for building foundations or
deep excavations.
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• The capacity of an interceptor drain is determined by calculating the maximum rate
of groundwater flow to be intercepted. Therefore, it is good practice to make
complete subsurface investigations, including hydraulic conductivity of the soil,
before designing a subsurface drainage system.
• Drain sizing considerations
o Size subsurface drains to carry the required capacity without pressure flow.
The minimum diameter for a subsurface drain is 4 inches.
o The minimum velocity required to prevent silting is 1.4 feet per second. Grade
the line to achieve this velocity at a minimum. The maximum allowable
velocity using a sand-gravel filter or envelope is 9 feet per second.
• Use filter material and fabric around all drains for proper bedding and filtration of fine
materials. Envelopes and filters should surround the drain to a minimum of 3-inch
thickness.
• Empty the outlet of the subsurface drain into a sediment pond through a catch basin.
If free of sediment, it can then empty into a receiving channel, swale, or stable
vegetated area adequately protected from erosion and undermining.
• Construct the trench on a continuous grade with no reverse grades or low spots.
• Stabilize soft or yielding soils under the drain with gravel or other suitable material.
• Backfill immediately after placement of the pipe. Do not allow sections of pipe to
remain uncovered overnight or during a rainstorm. Place backfill material in the
trench in such a manner that the drain pipe is not displaced or damaged.
• Do not install permanent drains near trees as tree roots may clog the lines. Use solid
pipe with watertight connections where necessary to pass a subsurface drainage
system through a stand of trees.
• Outlet considerations
o Ensure that the outlet of a drain empties into a channel or other watercourse
above the normal water level.
o Secure an animal guard to the outlet end of the pipe to keep out rodents.
o Use at least 10 feet of corrugated metal, cast iron, or heavy-duty plastic
without perforations outlet pipe. Do not use an envelope or filter material
around the outlet pipe, and bury at least two-thirds of the pipe length.
o When outlet velocities exceed those allowable for the receiving stream,
provide outlet protection.
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3.2.6.4 Maintenance Standards
Check the subsurface drains periodically to ensure that they are free-flowing and not clogged with
sediment or roots.
• Keep the outlet clean and free of debris.
• Keep surface inlets open and free of sediment and other debris.
• Trees located too close to a subsurface drain often clog the system with their roots. If
a drain becomes clogged, relocate the drain or remove the trees as a last resort.
Plan the placement of the drain to minimize this problem.
• Where drains are crossed by heavy vehicles, check the line to ensure that it is not
crushed and use pipe material that can handle traffic loads.
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3.2.7 BMP C206: Level Spreader
3.2.7.1 Purpose
To provide a temporary outlet for dikes and diversions consisting of an excavated depression
constructed at zero grade across a slope. To convert concentrated runoff to sheet flow and release it
onto areas stabilized by existing vegetation or an engineered filter strip.
3.2.7.2 Conditions of Use
Used when a concentrated flow of water needs to be dispersed over a large area with existing stable
vegetation.
• Items to consider are:
o What is the risk of erosion or damage if the flow may become concentrated?
o Is an easement required if discharged to adjoining property?
o Most of the flow should be as groundwater and not as surface flow.
o Is there an unstable area downstream that cannot accept additional
groundwater?
• Use only where the slopes are gentle, the water volume is relatively low, and the soil
will adsorb most of the low flow events.
3.2.7.3 Design and Installation Specifications
Use above undisturbed areas that are stabilized by existing vegetation.
If the level spreader has any low points, flow will concentrate, create channels and may cause
erosion.
• Discharge area below the outlet must be uniform with a slope of less than 5H:1V.
• Construct outlet level in a stable, undisturbed soil profile (not on fill).
• Do not allow the runoff to reconcentrate after release unless intercepted by another
downstream measure.
• The grade of the channel for the last 20 feet of the dike or interceptor entering the
level spreader shall be less than or equal to 1 percent. The grade of the level
spreader shall be 0 percent to ensure uniform spreading of storm runoff.
• A 6-inch high gravel berm placed across the level lip shall consist of washed crushed
rock, 2- to 4-inch or 3/4-inch to 1½-inch size.
• Calculate the spreader length by estimating the peak flow expected from the 10-year,
24-hour design storm (3.0-inches). The length of the spreader shall be a minimum of
15 feet for 0.1 cubic feet per second and shall be 10 feet for each 0.1 cubic feet per
second there after to a maximum of 0.5 cubic feet per second per spreader. Use
multiple spreaders for higher flows.
• The width of the spreader should be at least 6 feet.
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• The depth of the spreader as measured from the lip should be at least 6 inches and it
should be uniform across the entire length.
• Level spreaders shall be setback from the property line unless there is an easement
for flow.
• Level spreaders, when installed every so often in grassy swales, keep the flows from
concentrating. Materials that can be used include sand bags, lumber, logs, concrete,
and pipe. To function properly, the material needs to be installed level and on
contour. Figure II-3-16 and Figure II-3-17 provide a cross-section and a detail of a
level spreader.
3.2.7.4 Maintenance Standards
The spreader should be inspected after every runoff event to ensure proper function.
• The contractor should avoid the placement of any material on the structure and
should prevent construction traffic from crossing over the structure.
• If the spreader is damaged by construction traffic, immediately repair it.
Figure II-3-16. Cross-Section of a Level Spreader
Figure II-3-17. Detail of a Level Spreader
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3.2.8 BMP C207: Check Dams
3.2.8.1 Purpose
Construction of small dams across a swale or ditch reduces the velocity of concentrated flow and
dissipates energy at the check dam.
3.2.8.2 Conditions of Use
Where temporary channels or permanent channels are not yet vegetated, channel lining is infeasible,
and velocity checks are required.
• Do not place check dams in streams unless approved by the State Department of
Fish and Wildlife. Do not place check dams in wetlands without approval from a
permitting agency.
• Do not place check dams below the expected backwater from any salmonid bearing
water between September 15 and June 15 to ensure that there is no loss of high flow
refuge habitat for overwintering juvenile salmonids and emergent salmonid fry.
3.2.8.3 Design and Installation Specifications
Whatever material is used, the dam should form a triangle when viewed from the side. This prevents
undercutting as water flows over the face of the dam rather than falling directly onto the ditch bottom.
Check dams in association with sumps work more effectively at slowing flow and retaining sediment
than just a check dam alone. A deep sump should be provided immediately upstream of the check
dam.
• In some cases, if carefully located and designed, check dams can remain as
permanent installations with very minor regrading. They may be left as either
spillways, in which case accumulated sediment would be graded and seeded, or as
check dams to prevent further sediment from leaving the site.
• Check dams can be constructed of either rock or pea-gravel filled bags. Numerous
new products are also available for this purpose. They tend to be re-usable, quick
and easy to install, effective, and cost efficient.
• Check dams should be placed perpendicular to the flow of water.
• The maximum spacing between the dams shall be such that the toe of the upstream
dam is at the same elevation as the top of the downstream dam.
• Keep a maximum height of 2 feet at the center of the dam.
• Keep the center of the check dam at least 12 inches lower than the outer edges at
natural ground elevation.
• Keep the side slopes of the check dam at 2H:1V or flatter.
• Key the stone into the ditch banks and extend it beyond the abutments a minimum of
18 inches to avoid washouts from overflow around the dam.
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• Use filter fabric foundation under a rock or sand bag check dam. If a blanket ditch
liner is used, this is not necessary. A piece of organic or synthetic blanket cut to fit
will also work for this purpose.
• Construct rock check dams of appropriately sized rock. Place the rock by hand or by
mechanical means (no dumping of rock to form dam) to achieve complete coverage
of the ditch or swale and to ensure that the center of the dam is lower than the
edges. The rock used must be large enough to stay in place given the expected
design flow through the channel.
• In the case of grass-lined ditches and swales, remove all check dams and
accumulated sediment when the grass has matured sufficiently to protect the ditch or
swale - unless the slope of the swale is greater than 4 percent. Seed and mulch the
area beneath the check dams immediately after dam removal.
• Ensure that channel appurtenances, such as culvert entrances below check dams,
are not subject to damage or blockage from displaced stones. Figure II-3-18 depicts
a typical rock check dam.
3.2.8.4 Maintenance Standards
Monitor check dams for performance and sediment accumulation during and after each runoff
producing rainfall. Remove sediment when it reaches one half the sump depth.
• Anticipate submergence and deposition above the check dam and erosion from high
flows around the edges of the dam.
• If significant erosion occurs between dams, install a protective riprap liner in that
portion of the channel.
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Figure II-3-18. Check Dams
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3.2.9 BMP C208: Triangular Silt Dike (Geotextile-Encased Check Dam)
3.2.9.1 Purpose
Triangular silt dikes (TSDs) may be used as check dams, for perimeter protection, for temporary soil
stockpile protection, for drop inlet protection, or as a temporary interceptor dike (see Figure II-3-19
and Figure II-3-20).
3.2.9.2 Conditions of Use
May be used in place of straw bales for temporary check dams in ditches of any dimension.
• May be used on soil or pavement with adhesive or staples.
• TSDs have been used to build temporary:
o sediment ponds
o diversion ditches
o concrete wash out facilities
o curbing
o water bars
o level spreaders
o berms
3.2.9.3 Design and Installation Specifications
Made of urethane foam sewn into a woven geosynthetic fabric.
It is triangular, 10 inches to 14 inches high in the center, with a 20-inch to 28-inch base. A 2–foot
apron extends beyond both sides of the triangle along its standard section of 7 feet. A sleeve at one
end allows attachment of additional sections as needed.
• Install with ends curved up to prevent water from flowing around the ends.
• The fabric flaps and check dam units are attached to the ground with wire staples.
Wire staples should be No. 11 gauge wire and should be 200 mm to 300 mm in
length.
• When multiple units are installed, the sleeve of fabric at the end of the unit shall
overlap the abutting unit and be stapled.
• Check dams should be located and installed as soon as construction will allow.
• Check dams should be placed perpendicular to the flow of water.
• When used as check dams, the leading edge must be secured with rocks, sandbags,
or a small key slot and staples.
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3.2.9.4 Maintenance Standards
Monitor triangular silt dikes for performance and sediment accumulation during and after each runoff
producing rainfall. Remove sediment when it reaches one half the height of the dam.
Anticipate submergence and deposition above the triangular silt dam and erosion from high flows
around the edges of the dam. Immediately repair any damage or undercutting of the dam.
In the case of grass-lined ditches and swales, remove check dams and accumulated sediment when
the grass has matured sufficiently to protect the ditch or swale, unless the slope of the swale is
greater than 4 percent. Seed and mulch the area beneath the check dams immediately after dam
removal.
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Figure II-3-19. Sediment Barrier – Triangular Sediment Filter Dikes
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Figure II-3-20. Sediment Barrier – Geosynthetic Dike
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3.2.10 BMP C209: Outlet Protection
3.2.10.1 Purpose
Outlet protection prevents scour at conveyance outlets and minimizes the potential for downstream
erosion by reducing the velocity of concentrated stormwater flows.
3.2.10.2 Conditions of Use
Outlet protection is required at the outlets of all ponds, pipes, ditches, or other conveyances, and
where runoff is conveyed to a natural or manmade drainage feature such as a stream, wetland, lake,
or ditch.
3.2.10.3 Design and Installation Specifications
Protect the receiving channel at the outlet of a culvert from erosion by rock lining a minimum of 6 feet
downstream and extending rock lining up the channel sides a minimum of 1–foot above the
maximum tailwater elevation or 1-foot above the crown, whichever is higher. For large pipes (more
than 18 inches in diameter), the outlet protection lining of the channel is lengthened to four times the
diameter of the culvert.
• Standard wingwalls, and tapered outlets and paved channels should also be
considered when appropriate for permanent culvert outlet protection. (See the
WSDOT Hydraulic Manual, available through WSDOT Engineering Publications).
• Organic or synthetic erosion blankets, with or without vegetation, may be, cheaper,
and easier to install than rock. Materials can be chosen using manufacturer product
specifications. ASTM test results are available for most products and the designer
can choose the correct material for the expected flow.
• With low flows, vegetation (including sod) can be effective.
• Use the following guidelines for riprap outlet protection:
o If the discharge velocity at the outlet is less than 5 feet per second (pipe
slope less than 1 percent), use 2-inch to 8-inch riprap. Minimum thickness is
1-foot.
o For 5 to 10 feet per second discharge velocity at the outlet (pipe slope less
than 3 percent), use 24-inch to 4-foot riprap. Minimum thickness is 2 feet.
o For outlets at the base of steep slope pipes (pipe slope greater than 10
percent), an engineered energy dissipater shall be used.
• Always use filter fabric or erosion control blankets under riprap to prevent scour and
channel erosion.
• New pipe outfalls can provide an opportunity for low-cost fish habitat improvements.
For example, an alcove of low-velocity water can be created by constructing the pipe
outfall and associated energy dissipater back from the stream edge and digging a
channel, over-widened to the upstream side, from the outfall. Overwintering juvenile
and migrating adult salmonids may use the alcove as shelter during high flows. Bank
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stabilization, bioengineering, and habitat features may be required for disturbed
areas. See Volume V for more information on outfall system design.
3.2.10.4 Maintenance Standards
• Inspect and repair as needed.
• Add rock as needed to maintain the intended function.
• Clean energy dissipater if sediment builds up.
Figure II-3-21. No Figure Placeholder
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3.2.11 BMP C220: Storm Drain Inlet Protection
3.2.11.1 Purpose
To prevent coarse sediment from entering drainage systems prior to permanent stabilization of the
disturbed area.
3.2.11.2 Conditions of Use
Where storm drain inlets are to be made operational before permanent stabilization of the disturbed
drainage area.
Provide protection for all storm drain inlets downslope and within 500 feet of a disturbed or
construction area, unless the runoff that enters the catch basin will be conveyed to a sediment pond
or trap. Inlet protection may be used anywhere to protect the drainage system. It is likely that the
drainage system will still require cleaning.
Table II-3-9 lists several options for inlet protection. All of the methods for storm drain inlet protection
are prone to plugging and require a high frequency of maintenance. Drainage areas should be limited
to 1 acre or less. Emergency overflows may be required where stormwater ponding would cause a
hazard. If an emergency overflow is provided, additional end-of-pipe treatment may be required.
Only bag filter type catch basin filters (per Section 3.2.11.3) are allowed within the right of way.
Table II-3-9. Storm Drain Inlet Protection
Type of Inlet
Protection
Emergency
Overflow
Applicable for
Paved/Earthen
Surfaces
Conditions of Use
Excavated drop inlet
protection
Yes, temporary
flooding will
occur
Earthen Applicable for heavy flows. Easy to
maintain. Large area requirement:
30’ x 30’ per acre.
Block and gravel drop
filter
Yes Paved or earthen Applicable for heavy concentrated
flows. Will not pond.
Gravel and mesh
filter
No Paved Applicable for heavy concentrated
flows. Will pond. Can withstand
traffic.
Catch basin filters Yes Paved or earthen Frequent maintenance required.
Curb inlet protection
with a wooden weir
Small capacity
overflow
Paved Used for sturdy, more compact
installation.
Block and gravel curb
inlet protection
Yes Paved Sturdy, but limited filtration.
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3.2.11.3 Design and Installation Specifications
Excavated Drop Inlet Protection
An excavated impoundment around the storm drain. Sediment settles out of the stormwater prior to
entering the storm drain.
• Depth 1 to 2 feet, as measured from the crest of the inlet structure.
• Side slopes of excavation no steeper than 2H:1V.
• Minimum volume of excavation 35 cubic yards.
• Shape basin to fit site with longest dimension oriented toward the longest inflow
area.
• Install provisions for draining to prevent standing water problems.
• Clear the area of all debris.
• Grade the approach to the inlet uniformly.
• Drill weep holes into the side of the inlet.
• Protect weep holes with screen wire and washed aggregate.
• Seal weep holes when removing structure and stabilizing area.
• It may be necessary to build a temporary dike to the down slope side of the structure
to prevent bypass flow.
Block and Gravel Filter
A barrier formed around the storm drain inlet with standard concrete blocks and gravel. See Figure II-
3-22.
• Height 1 to 2 feet above inlet.
• Recess the first row 2 inches into the ground for stability.
• Support subsequent courses by placing a piece of 2x4 lumber through the block
opening.
• Do not use mortar.
• Lay some blocks in the bottom row on their side for dewatering the pool.
• Place hardware cloth or comparable wire mesh with ½-inch openings over all block
openings.
• Place gravel just below the top of blocks on slopes of 2H:1V or flatter.
• An alternative design is a gravel donut.
• Inlet slope of 3H:1V.
• Outlet slope of 2H:1V.
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• 1-foot wide level stone area between the structure and the inlet.
• Inlet slope stones 3 inches in diameter or larger.
• Outlet slope use gravel ½- to ¾-inch at a minimum thickness of 1-foot.
Gravel and Wire Mesh Filter
A gravel barrier placed over the top of the inlet (see Figure II-3-23). This structure does not provide
an overflow.
• Hardware cloth or comparable wire mesh with ½-inch openings.
• Coarse aggregate.
• Height 1-foot or more, 18 inches wider than inlet on all sides.
• Place wire mesh over the drop inlet so that the wire extends a minimum of 1-foot
beyond each side of the inlet structure.
• If more than one strip of mesh is necessary, overlap the strips.
• Place coarse aggregate over the wire mesh.
• The depth of the gravel should be at least 12 inches over the entire inlet opening and
extend at least 18 inches on all sides.
Catchbasin Filters
Inserts (Figure II-3-24) shall be designed by the manufacturer for use at construction sites. The
limited sediment storage capacity increases the frequency of inspection and maintenance required,
which may be daily for heavy sediment loads. The maintenance requirements can be reduced by
combining a catchbasin filter with another type of inlet protection. This type of inlet protection provides
flow bypass without overflow and therefore may be a better method for inlets located along active
rights-of-way. See Figure II-C-46 for one example.
• Should have a minimum of 5 cubic feet of storage.
• Dewatering provisions.
• High-flow bypass that will not clog under normal use at a construction site.
• The catchbasin filter is inserted in the catchbasin just below the grating.
• Only bag filter type catch basin filters are allowed in the City right-of-way.
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Figure II-3-22. Drop Inlet with Block and Gravel Filter
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Figure II-3-23. Gravel and Wire Mesh Filter
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Figure II-3-24. Catchbasin Filter
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Curb Inlet Protection with Wooden Weir
Barrier formed around a curb inlet with a wooden frame and gravel.
• Wire mesh with ½-inch openings.
• Extra strength filter cloth.
• Construct a frame.
• Attach the wire and filter fabric to the frame.
• Pile coarse washed aggregate against the wire and fabric.
• Place weight on frame anchors.
Block and Gravel Curb Inlet Protection
Barrier formed around an inlet with concrete blocks and gravel. See Figure II-3-25.
• Wire mesh with ½-inch openings.
• Place two concrete blocks on their sides abutting the curb at either side of the inlet
opening. These are spacer blocks.
• Place a 2x4 stud through the outer holes of each spacer block to align the front
blocks.
• Place blocks on their sides across the front of the inlet and abutting the spacer
blocks.
• Place wire mesh over the outside vertical face.
• Pile coarse aggregate against the wire to the top of the barrier.
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Figure II-3-25. Block and Gravel Curb Inlet Protection
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Curb and Gutter Sediment Barrier
Sandbag or rock berm (riprap and aggregate) 3 feet high and 3 feet wide in a horseshoe shape. See
Figure II-3-26.
• Construct a horseshoe shaped berm, faced with coarse aggregate if using riprap,
3 feet high and 3 feet wide, at least 2 feet from the inlet.
• Construct a horseshoe shaped sedimentation trap on the outside of the berm sized
to sediment trap standards for protecting a culvert inlet.
3.2.11.4 Maintenance Standards
Inspect catch basin filters frequently, especially after storm events. If the insert becomes clogged,
clean or replace it.
• For systems using stone filters: If the stone filter becomes clogged with sediment, the
stones must be pulled away from the inlet and cleaned or replaced. Since cleaning of
gravel at a construction site may be difficult, an alternative approach would be to use
the clogged stone as fill and put fresh stone around the inlet.
• Do not wash sediment into storm drains while cleaning. Spread all excavated
material evenly over the surrounding land area or stockpile and stabilize as
appropriate.
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Figure II-3-26. Curb and Gutter Sediment Barrier
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3.2.12 BMP C231: Brush Barrier
3.2.12.1 Purpose
The purpose of brush barriers is to reduce the transport of coarse sediment from a construction site
by providing a temporary physical barrier to sediment and reducing the runoff velocities of overland
flow.
3.2.12.2 Conditions of Use
• Brush barriers may be used downslope of all disturbed areas of less than one-
quarter acre.
• Brush barriers are not intended to treat concentrated flows, nor are they intended to
treat substantial amounts of overland flow. Any concentrated flows must be
conveyed through the drainage system to a sediment pond. The only circumstance in
which overland flow can be treated solely by a barrier, rather than by a sediment
pond, is when the area draining to the barrier is small.
• Only install brush barriers on contours.
3.2.12.3 Design and Installation Specifications
• Height 2 feet (minimum) to 5 feet (maximum).
• Width 5 feet at base (minimum) to 15 feet (maximum).
• Filter fabric (geotextile) may be anchored over the brush berm to enhance the
filtration ability of the barrier. Ten-ounce burlap is an adequate alternative to filter
fabric.
• Chipped site vegetation, composted mulch, or wood-based mulch (hog fuel) can be
used to construct brush barriers.
• A 100 percent biodegradable installation can be constructed using 10-ounce burlap
held in place by wooden stakes. Figure II-3-27 depicts a typical brush barrier.
3.2.12.4 Maintenance Standards
• Do not allow erosion or concentrated runoff under or around the barrier. If
concentrated flows are bypassing the barrier, it must be expanded or augmented by
toed-in filter fabric.
• Maintain the dimensions of the barrier.
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Figure II-3-27. Brush Barrier
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3.2.13 BMP C232: Gravel Filter Berm
3.2.13.1 Purpose
A gravel filter berm is constructed on rights-of-way or traffic areas within a construction site to retain
sediment by using a filter berm of gravel or crushed rock.
3.2.13.2 Conditions of Use
Where a temporary measure is needed to retain sediment from rights-of-way or in traffic areas on
construction sites.
3.2.13.3 Design and Installation Specifications
Berm material shall be ¾ to 3 inches in size, washed well-graded gravel or crushed rock, with less
than 5 percent fines.
• Space berms:
o Every 300 feet on slopes less than 5 percent
o Every 200 feet on slopes between 5 percent and 10 percent
o Every 100 feet on slopes greater than 10 percent
• Berm dimensions:
o 1 foot high with 3:1 side slopes
o 8 linear feet per 1 cubic foot per second runoff based on the 10-year, 24-hour
design storm (3.0-inches)
3.2.13.4 Maintenance Standards
Regular inspection is required. Remove sediment and replace filter material as needed.
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3.2.14 BMP C233: Silt Fence
3.2.14.1 Purpose
Use of a silt fence reduces the transport of coarse sediment from a construction site by providing a
temporary physical barrier to sediment and reducing the runoff velocities of overland flow. See Figure
II-3-28 for details on silt fence construction.
3.2.14.2 Conditions of Use
Silt fence may be used downslope of all disturbed areas.
• Silt fence is not intended to treat concentrated flows, nor is it intended to treat
substantial amounts of overland flow. Convey any concentrated flows through the
drainage system to a sediment pond. The only circumstance in which overland flow
can be treated solely by a silt fence, rather than by a sediment pond, is when the
area draining to the fence is one acre or less and flow rates are less than 0.5 cfs.
• Do not construct silt fences in streams or use them in V-shaped ditches. They are
not an adequate method of silt control for anything deeper than sheet or overland
flow.
3.2.14.3 Design and Installation Specifications
Drainage area of 1 acre or less or in combination with sediment basin on a larger site.
Maximum slope steepness (perpendicular to fence line) 1H:1V.
• Maximum sheet or overland flow path length to the fence of 100 feet.
• No flows greater than 0.5 cubic feet per second.
• The geotextile used shall meet the following standards. All geotextile properties listed
below are minimum average roll values (i.e., the test result for any sampled roll in a
lot shall meet or exceed the values shown in Table II-3-10).
Table II-3-10. Geotextile Standards
Polymeric Mesh AOS (ASTM D4751) 0.60 mm maximum for slit film wovens
(#30 sieve). 0.30 mm maximum for all other
geotextile types (#50 sieve). 0.15 mm minimum
for all fabric types (#100 sieve).
Water Permittivity (ASTM D4491) 0.02 sec-1 minimum
Grab Tensile Strength (ASTM D4632) 180 lbs. minimum for extra strength fabric.
100 lbs. minimum for standard strength fabric.
Grab Tensile Strength (ASTM D4632) 30% maximum
Ultraviolet Resistance (ASTM D4355) 70% minimum
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• Support standard strength fabrics with wire mesh, chicken wire, 2-inch x 2-inch wire,
safety fence, or jute mesh to increase the strength of the fabric. Silt fence materials
are available that have synthetic mesh backing attached.
• Filter fabric material shall contain ultraviolet ray inhibitors and stabilizers to provide a
minimum of six months of expected usable construction life at a temperature range
of 0° to 120° Fahrenheit.
• 100 percent biodegradable silt fence is available that is strong and long lasting.
• The following are standard design and installation methods. Refer to Figure II-3-28
for standard silt fence details.
o Install and maintain temporary silt fences at the locations shown in the plans.
Install the silt fences in the areas of clearing, grading, or drainage prior to
starting those activities. Do not consider a silt fence temporary if the silt fence
must function beyond the life of the contract. The silt fence shall prevent soil
carried by runoff water from going beneath, through, or over the top of the silt
fence, but shall allow the water to pass through the fence.
o The minimum height of the top of silt fence shall be 2 feet and the maximum
height shall be 2½ feet above the original ground surface.
o Sew the geotextile together at the point of manufacture, or at an approved
location as determined by the Engineer, to form geotextile lengths as
required. Locate all sewn seams at a support post. Alternatively, two sections
of silt fence can be overlapped, provided the Contractor can demonstrate, to
the satisfaction of the Engineer, that the overlap is long enough and adjacent
fence sections are close enough together to prevent silt laden water from
escaping through the fence at the overlap.
o Attach the geotextile on the up-slope side of the posts and support system
with staples, wire, or in accordance with the manufacturer's
recommendations. Attach the geotextile to the posts in a manner that reduces
the potential for geotextile tearing at the staples, wire, or other connection
device. Silt fence back-up support for the geotextile in the form of a wire or
plastic mesh is dependent on the properties of the geotextile selected for use.
If wire or plastic back-up mesh is used, fasten the mesh securely to the up-
slope of the posts with the geotextile being up-slope of the mesh back-up
support.
o Bury the geotextile at the bottom of the fence in a trench to a minimum depth
of 4 inches below the ground surface. Backfill the trench and tamp the soil in
place over the buried portion of the geotextile, such that no flow can pass
beneath the fence and scouring can not occur. When wire or polymeric back-
up support mesh is used, the wire or polymeric mesh shall extend into the
trench a minimum of 3 inches.
o Drive fence posts in to a minimum depth of 18 inches. A minimum depth of
12 inches is allowed if topsoil or other soft subgrade soil is not present and a
minimum depth of 18 inches cannot be reached. Increase fence post depths
by 6 inches if the fence is located on slopes of 3H:1V or steeper and the
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slope is perpendicular to the fence. If required post depths cannot be
obtained, adequately secure the posts by bracing or guying to prevent
overturning of the fence due to sediment loading.
o Locate the silt fences on contour as much as possible, except at the ends of
the fence, where the fence shall be turned uphill such that the silt fence
captures the runoff water and prevents water from flowing around the end of
the fence.
o If the fence must cross contours, with the exception of the ends of the fence,
place gravel check dams perpendicular to the back of the fence to minimize
concentrated flow and erosion along the back of the fence. The gravel check
dams shall be approximately 1-foot deep at the back of the fence and be
perpendicular to the fence at the same elevation until the top of the check
dam intercepts the ground surface behind the fence. The gravel check dams
shall consist of crushed surfacing base course, gravel backfill for walls, or
shoulder ballast. Locate the gravel check dams every 10 feet along the fence
where the fence must cross contours. The slope of the fence line where
contours must be crossed shall not be steeper than 3H:1V.
o Use wood, steel or equivalent posts. Wood posts shall have minimum
dimensions of 2 inches by 2 inches by 3 feet minimum length, and shall be
free of defects such as knots, splits, or gouges. Steel posts shall consist of
either size No. 6 rebar or larger; ASTM A120 steel pipe with a minimum
diameter of 1-inch; U, T, L, or C shape steel posts with a minimum weight of
1.35 pounds per foot; or other steel posts having equivalent strength and
bending resistance to the post sizes listed. The spacing of the support posts
shall be a maximum of 6 feet.
o Fence back-up support, if used, shall consist of steel wire with a maximum
mesh spacing of 2 inches, or a prefabricated polymeric mesh. The strength of
the wire or polymeric mesh shall be equivalent to or greater than 180 pounds
grab tensile strength. The polymeric mesh must be as resistant to ultraviolet
radiation as the geotextile it supports.
• Specification details for silt fence installation using the slicing method follow. Refer to
Figure II-3-29 for slicing method details.
o The base of both end posts must be at least 2 to 4 inches above the top of
the silt fence fabric on the middle posts for ditch checks to drain properly. Use
a hand level or string level, if necessary, to mark base points before
installation.
o Install posts 3 to 4 feet apart in critical retention areas and a minimum of
6 feet apart in standard applications.
o Install posts 24 inches deep on the downstream side of the silt fence, and as
close as possible to the fabric, enabling posts to support the fabric from
upstream water pressure.
o Install posts with the nipples facing away from the silt fence fabric.
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o Attach the fabric to each post with three ties, all spaced within the top
8 inches of the fabric. Attach each tie diagonally 45 degrees through the
fabric, with each puncture at least 1 inch vertically apart. In addition, each tie
should be positioned to hang on a post nipple when tightening to prevent
sagging.
o Wrap approximately 6 inches of fabric around the end posts and secure with
3 ties.
o No more than 24 inches of a 36-inch fabric is allowed above ground level.
o The rope lock system must be used in all ditch check applications.
o The installation should be checked and corrected for any deviation before
compaction. Use a flat-bladed shovel to tuck fabric deeper into the ground, if
necessary.
o Compaction is vitally important for effective results. Compact the soil
immediately next to the silt fence fabric with the front wheel of a tractor, skid
steer, or roller exerting at least 60 pounds per square inch. Compact the
upstream side first and then each side twice for a total of four trips.
3.2.14.4 Maintenance Standards
• Repair any damage immediately.
• If concentrated flows are evident uphill of the fence, intercept and convey them to a
sediment pond.
• It is important to check the uphill side of the fence for signs of the fence clogging,
acting as a barrier to flow, and then causing channelization of flows parallel to the
fence. If this occurs, replace the fence or remove the trapped sediment.
• Remove sediment deposits when the deposit reaches approximately one-third the
height of the silt fence, or install a second silt fence.
• If the filter fabric (geotextile) has deteriorated due to ultraviolet breakdown, replace it.
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Figure II-3-28. Silt Fence
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Figure II-3-29. Silt Fence Installation by Slicing
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3.2.15 BMP C234: Vegetated Strip
3.2.15.1 Purpose
Vegetated strips reduce the transport of coarse sediment from a construction site by providing a
temporary physical barrier to sediment and reducing the runoff velocities of overland flow.
3.2.15.2 Conditions of Use
Vegetated strips may be used downslope of all disturbed areas.
Vegetated strips are not intended to treat concentrated flows, nor are they intended to treat
substantial amounts of overland flow. Convey any concentrated flows through the drainage system to
a sediment pond. The only circumstance in which overland flow can be treated solely by a strip,
rather than by a sediment pond, is when the criteria shown in Table II-3-11 are met.
Table II-3-11. Vegetated Strips
Average Slope Slope Percent Flowpath Length
1.5H:1V or less 67% or less 100 feet
2H:1V or less 50% or less 115 feet
4H:1V or less 25% or less 150 feet
6H:1V or less 16.7% or less 200 feet
10H:1V or less 10% or less 250 feet
3.2.15.3 Design and Installation Specifications
The vegetated strip shall consist of a minimum of a 25-foot wide continuous strip of dense vegetation
with permeable topsoil. Grass-covered, landscaped areas are generally not adequate because the
volume of sediment overwhelms the grass. Ideally, vegetated strips shall consist of undisturbed
native growth with a well-developed soil that allows for infiltration of runoff.
• The slope within the strip shall not exceed 4H:1V.
• Delineate the uphill boundary of the vegetated strip with clearing limits.
3.2.15.4 Maintenance Standards
• Seed any areas damaged by erosion or construction activity immediately and
protected with mulch.
• If more than 5 feet of the original vegetated strip width has had vegetation removed
or is being eroded, install sod.
• If there are indications that concentrated flows are traveling across the vegetated
strip, surface water controls must be installed to reduce the flows entering the
vegetated strip, or install additional perimeter protection.
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3.2.16 BMP C235: Straw Wattles
3.2.16.1 Purpose
Straw wattles are temporary erosion and sediment control barriers consisting of straw that is wrapped
in biodegradable tubular plastic or similar encasing material. They reduce the velocity and can spread
the flow of rill and sheet runoff, and can capture and retain sediment. Straw wattles are typically 8 to
10 inches in diameter and 25 to 30 feet in length. The wattles are placed in shallow trenches and
staked along the contour of disturbed or newly constructed slopes. See Figure II-3-30 for typical
construction details.
3.2.16.2 Conditions of Use
• Disturbed areas that require immediate erosion protection.
• Exposed soils during the period of short construction delays.
• On slopes requiring stabilization until permanent vegetation can be established.
• Straw wattles are effective for one to two seasons.
• If conditions are appropriate, wattles can be staked to the ground using live cuttings
for added revegetation.
3.2.16.3 Design Criteria
• It is critical that wattles are installed perpendicular to the flow direction and parallel to
the slope contour.
• Dig narrow trenches across the slope on contour to a depth of 3 to 5 inches on clay
soils and soils with gradual slopes. On loose soils, steep slopes, and areas with high
rainfall, dig the trenches to a depth of 5 to 7 inches, or 1/2 to 2/3 of the thickness of
the wattle.
• Start building trenches and installing wattles from the base of the slope and work up.
Excavated material should be spread evenly along the uphill slope and compacted
using hand tamping or other methods.
• Construct trenches at contour intervals of 3 to 30 feet apart depending on the
steepness of the slope, soil type, and rainfall. The steeper the slope, the closer
together the trenches shall be.
• Install the wattles snugly into the trenches and abut tightly end to end. Do not overlap
the ends. Rilling can occur beneath wattles if not properly entrenched, and water can
pass between wattles if not tightly abutted.
• Install stakes at each end of the wattle, and at 4-foot centers along entire length of
wattle.
• If required, install pilot holes for the stakes using a straight bar to drive holes through
the wattle and into the soil.
• At a minimum, wooden stakes should be approximately 3/4 x 3/4 x 24 inches. Live
cuttings or 3/8-inch rebar can also be used for stakes.
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• Stakes should be driven through the middle of the wattle, leaving 2 to 3 inches of the
stake protruding above the wattle.
3.2.16.4 Maintenance Standards
• Wattles may require maintenance to ensure they are in contact with soil and
thoroughly entrenched, especially after significant rainfall on steep sandy soils.
• Inspect the slope after significant storms and repair any areas where wattles are not
tightly abutted or water has scoured beneath the wattles.
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Figure II-3-30. Straw Wattles
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3.2.17 BMP C240: Sediment Trap
3.2.17.1 Purpose
A sediment trap is a small temporary ponding area with a gravel outlet used to collect and store
sediment from sites cleared and/or graded during construction. Install sediment traps, along with
other perimeter controls, before any land disturbance takes place in the drainage area.
3.2.17.2 Conditions of Use
Prior to leaving a construction site, stormwater runoff must pass through a sediment pond or trap or
other appropriate sediment removal best management practice. Non-engineered sediment traps may
be used on-site prior to an engineered sediment trap or sediment pond to provide additional sediment
removal capacity.
Sediment traps are intended for use on sites where the tributary drainage area is less than 3 acres,
with no unusual drainage features, and a projected build-out time of six months or less. The sediment
trap is a temporary measure (with a design life of approximately 6 months) and shall be maintained
until the site area is permanently protected against erosion by the installation of vegetation and/or
structures.
Sediment traps and ponds are only effective in removing sediment down to about the medium silt
size fraction. Runoff with sediment of finer grades (fine silt and clay) will pass through untreated,
emphasizing the need to control erosion to the maximum extent first.
Whenever possible, discharge sediment-laden water into onsite, relatively level, vegetated areas
(see BMP C234 – Vegetated Strip). Do not use vegetated wetlands for this purpose. All projects that
are constructing permanent detention facilities for runoff quantity control should use the rough-graded
or final-graded permanent facilities for traps and ponds. This includes combined facilities and
infiltration facilities. When permanent facilities are used as temporary sedimentation facilities, the
surface area requirement of a sediment trap or pond must be met. If the surface area requirements
are larger than the surface area of the permanent facility, then the trap or pond shall be enlarged to
comply with the surface area requirement. The permanent pond shall also be divided into two cells as
required for sediment ponds.
Use of infiltration facilities for sedimentation basins during construction tends to clog the soils and
reduce their capacity to infiltrate. If infiltration facilities are to be used, the sides and bottom of the
facility must only be rough excavated to a minimum of 2 feet above final grade. Final grading of the
infiltration facility shall occur only when all contributing drainage areas are fully stabilized. The
infiltration pretreatment facility should be fully constructed and used with the sedimentation basin to
help prevent clogging.
Either a permanent control structure or the temporary control structure described in BMP C241 -
Temporary Sediment Pond can be used. If a permanent control structure is used, it may be advisable
to partially restrict the lower orifice with gravel to increase residence time while still allowing
dewatering of the pond. A shut-off valve may be added to the control structure to allow complete
retention of stormwater in emergency situations. In this case, add an emergency overflow weir.
A skimmer may be used for the sediment trap outlet if approved by the City.
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3.2.17.3 Design and Installation Specifications
See Figure II-3-31 and Figure II-3-32 for details.
If permanent runoff control facilities are part of the project, they should be used for sediment
retention.
• To determine the sediment trap geometry, first calculate the design surface area
(SA) of the trap, measured at the invert of the weir. Use the following equation:
SA = FS(Q2/Vs)
Where:
SA = Design surface area, in square feet, of the sediment trap measured at the
invert of the weir.
Q2 = Design inflow, in cubic feet per second, based on the peak discharge from the
developed 2-year runoff event from the contributing drainage area as
computed in the hydrologic analysis. The 10-year peak flow shall be used if
the project size, expected timing and duration of construction, or downstream
conditions warrant a higher level of protection. If no hydrologic analysis is
required, the Rational Method may be used.
Alternatively, Q2 = Design inflow (cfs) based on the 2-year, 1-hour flowrate
predicted by WWHM for the developed (unmitigated site) multiplied by 1.3.
Use the 10-year peak flow if the project size, expected timing and duration of
construction, or downstream conditions warrant a higher level of protection.
Q10 is the 10-year, 1-hour flowrate predicted by WWHM multiplied by 1.6.
Vs = The settling velocity of the soil particle of interest. The 0.02 millimeter
(medium silt) particle with an assumed density of 2.65 grams per cubic
centimeter has been selected as the particle of interest and has a settling
velocity (Vs) of 0.00096 feet per second.
FS = A safety factor of 2 to account for non-ideal settling.
Therefore, the equation for computing surface area becomes:
SA = 2 x Q2/0.00096 or
= 2080 (Q2)
NOTE: Even if permanent facilities are used, they must still have a surface area that is at least as
large as that derived from the above formula. If they do not, the pond must be enlarged.
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• Smaller sites may use the minimum pond sizes in Table II-3-12 instead of providing
calculations.
Table II-3-12. Sediment Trap Sizing
Contributing Area (Acres) Required Surface Area
of Pond (sq. ft.)
1/8 acre or less 130
¼ acre or less 260
½ acre or less 520
¾ acre or less 780
1 acre or less 1040
• To aid in determining sediment depth, all sediment traps shall have a staff gauge
with a prominent mark 1-foot above the bottom of the trap.
• Sediment traps may not be feasible on utility projects due to the limited work space
or short-term nature of the work. Portable tanks may be used in place of sediment
traps for utility projects.
• The basic geometry of the pond can now be determined using the following design
criteria:
o Required surface area SA (from the equation above) at top of riser.
o Minimum 3.5-foot depth from top of riser to bottom of pond.
o Maximum 3H:1V interior side slopes and maximum 2H:1V exterior slopes.
The interior slopes can be increased to a maximum of 2H:1V if fencing is
provided at or above the maximum water surface.
o One foot of freeboard between the top of the riser and the crest of the
emergency spillway.
o Flat bottom.
o Minimum 1-foot deep spillway.
o Length-to-width ratio between 3:1 and 6:1.
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3.2.17.4 Maintenance Standards
• Remove sediment from the trap when it reaches 1-foot in depth.
• Repair any damage to the pond embankments or slopes.
Figure II-3-31. Cross-Section of a Sediment Trap
Figure II-3-32. Sediment Trap Outlet
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3.2.18 BMP C241: Temporary Sediment Pond
3.2.18.1 Purpose
Sediment ponds remove sediment from runoff originating from disturbed areas of the site. Sediment
ponds are typically designed to remove sediment no smaller than medium silt (0.02 mm).
Consequently, they usually reduce turbidity only slightly.
3.2.18.2 Conditions of Use
Prior to leaving a construction site, stormwater runoff must pass through a sediment pond or other
appropriate sediment removal best management practice.
Use a sediment pond where the contributing drainage area is 3 acres or more. Ponds must be used
in conjunction with erosion control practices to reduce the amount of sediment flowing into the basin.
3.2.18.3 Design and Installation Specifications
Only install sediment basins on sites where failure of the structure would not result in loss of life,
damage to homes or buildings, or interruption of use or service of public roads or utilities. Also,
sediment traps and ponds are attractive to children and can be very dangerous. Compliance with
local ordinances regarding health and safety must be addressed. If fencing of the pond is required,
show the type of fence and its location on the ESC plan.
• Structures having a maximum storage capacity at the top of the dam of 10 acre-feet
(435,600 cubic feet) or more are subject to the Washington Dam Safety Regulations
(Chapter 173-175 WAC).
• See Figure II-3-33, Figure II-3-34 and Figure II-3-35 for details.
• If permanent detention facilities are part of the project, they may be used for
sediment retention. The surface area requirements of the sediment basin must be
met. This may require enlarging the permanent basin to comply with the surface area
requirements. If a permanent control structure is used, it may be advisable to
partially restrict the lower orifice with gravel to increase residence time while still
allowing dewatering of the basin.
• Use of infiltration facilities for sedimentation basins during construction tends to clog
the soils and reduce their capacity to infiltrate. If infiltration facilities are to be used,
the sides and bottom of the facility must only be rough excavated to a minimum of
2 feet above final grade. Final grading of the infiltration facility shall occur only when
all contributing drainage areas are fully stabilized. The infiltration pretreatment facility
should be fully constructed and used with the sedimentation basin to help prevent
clogging.
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Determining Pond Geometry
• Determine the required surface area at the top of the riser pipe with the equation:
SA = 2 x Q2/0.00096 or
SA = 2080 (Q2)
Where:
SA = Design surface area, in square feet, of the sediment trap measured at the
invert of the weir.
Q2 = Design inflow, in cubic feet per second, based on the peak discharge from the
developed 2-year runoff event from the contributing drainage area as
computed in the hydrologic analysis. The 10-year peak flow shall be used if
the project size, expected timing and duration of construction, or downstream
conditions warrant a higher level of protection. If no hydrologic analysis is
required, the Rational Method may be used.
Alternatively, Q2 = Design inflow (cfs) based on the 2-year, 15-minute flowrate
predicted by WWHM for the developed (unmitigated site). Use the 10-year
peak flow if the project size, expected timing and duration of construction, or
downstream conditions warrant a higher level of protection. Q10 is the 10-year,
15-minute flowrate predicted by WWHM. Note: WWHM 2 and 3 do not use 15
minute time steps for 2 or 10 year flow rates, they use 1-hour time steps. The
2-year flowrate predicted by WWHM 2 or 3 must be multiplied by 1.3 and the
10-year flowrate predicted by WWHM 2 or 3 must be multiplied by 1.6.
Currently it is unknown what time steps future versions of WWHM will use.
• See BMP C240 for more information on the derivation of the surface area
calculation.
• The basic geometry of the pond can now be determined using the following design
criteria:
o Required surface area SA (from the equation above) at top of riser.
o Minimum 3.5-foot depth from top of riser to bottom of pond.
o Maximum 3H:1V interior side slopes and maximum 2H:1V exterior slopes.
The interior slopes can be increased to a maximum of 2H:1V if fencing is
provided at or above the maximum water surface.
o One foot of freeboard between the top of the riser and the crest of the
emergency spillway.
o Flat bottom.
o Minimum 1-foot deep spillway.
o Length-to-width ratio between 3:1 and 6:1.
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Sizing of Discharge Mechanisms
The outlet for the basin consists of a combination of principal and emergency spillways. These outlets
must pass the peak runoff expected from the contributing drainage area for a 100-year storm. If, due
to site conditions and basin geometry, a separate emergency spillway is not feasible, the principal
spillway must pass the entire peak runoff expected from the 100-year storm. However, an attempt to
provide a separate emergency spillway should always be made. The runoff calculations shall be
based on the site conditions during construction. The flow through the dewatering orifice cannot be
utilized when calculating the 100-year storm elevation because of its potential to become clogged;
therefore, available spillway storage must begin at the principal spillway riser crest.
The principal spillway designed by the procedures contained in this standard will result in some
reduction in the peak rate of runoff. However, the riser outlet design will not adequately control the
basin discharge to the predevelopment discharge limitations as stated in Minimum Requirement #7:
Flow Control. However, if the basin for a permanent stormwater detention pond is used for a
temporary sedimentation basin, the control structure for the permanent pond can be used to maintain
predevelopment discharge limitations. The size of the basin, the expected life of the construction
project, the anticipated downstream effects, and the anticipated weather conditions during
construction should be considered to determine the need of additional discharge control. See Figure
II-3-36 for riser inflow curves.
Principal Spillway: Determine the required diameter for the principal spillway (riser pipe). The
diameter shall be the minimum necessary to pass the pre-developed 10-year peak flow (Q10). Use
Figure II-3-36 to determine this diameter (h = 1-foot).
NOTE: A permanent control structure may be used instead of a temporary riser.
Emergency Overflow Spillway: Determine the required size and design of the emergency overflow
spillway for the developed 100-year peak flow using the method contained in Volume III.
Alternatively, the 100-year peak flow as determined by WWHM multiplied by 1.6 can be used to size
the emergency overflow.
Dewatering Orifice: Determine the size of the dewatering orifice(s) (minimum 1-inch diameter) using
a modified version of the discharge equation for a vertical orifice and a basic equation for the area of
a circular orifice. Determine the required area of the orifice with the following equation:
5.0
5.0
3600x6.0
)2(
Tg
hAAs
o =
Where:
Ao = orifice area (square feet)
As = pond surface area (square feet)
h = head of water above orifice (height of riser in feet)
T = dewatering time (24 hours)
g = acceleration of gravity (32.2 feet per second squared)
D = orifice diameter (inches)
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Convert the required surface area to the required diameter D of the orifice:
o
o AADx54.13x24==
The vertical, perforated tubing connected to the dewatering orifice must be at least 2 inches larger in
diameter than the orifice to improve flow characteristics. The size and number of perforations in the
tubing shall be large enough so the tubing does not restrict flow. The orifice shall control the flow rate.
Additional Design Specifications
The pond shall be divided into two roughly equal volume cells by a permeable divider that will
reduce turbulence while allowing movement of water between cells. The divider shall be at least one-
half the height of the riser and a minimum of one foot below the top of the riser. Wire-backed, 2- to 3-
foot high, extra strength filter fabric supported by treated 4"x4"s can be used as a divider. If the pond
is more than 6 feet deep, a different mechanism must be proposed. A riprap embankment is one
acceptable method of separation for deeper ponds. Other designs that satisfy the intent of this
provision are allowed as long as the divider is permeable, structurally sound, and designed to prevent
erosion under or around the barrier.
To aid in determining sediment depth, prominently mark one-foot intervals on the riser.
If an embankment height of more than 6 feet is proposed, the pond must comply with the criteria
contained in Volume III regarding dam safety for detention BMPs.
The most common structural failure of sedimentation basins is caused by piping. Piping refers to two
phenomena: (1) water seeping through fine-grained soil, eroding the soil grain by grain and forming
pipes or tunnels and (2) water under pressure flowing upward through a granular soil with a head of
sufficient magnitude to cause soil grains to lose contact and capability for support.
The most critical construction sequences to prevent piping will be:
• Tight connections between the riser and barrel and other pipe connections.
• Adequate anchoring of the riser.
• Proper soil compaction of the embankment and riser footing.
• Proper construction of anti-seep devices.
3.2.18.4 Maintenance Standards
• Remove sediment from the pond when it reaches 1–foot in depth.
• Repair any damage to the pond embankments or slopes.
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Figure II-3-33. Sediment Pond
Figure II-3-34. Sediment Pond Cross Section
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Figure II-3-35. Sediment Pond Riser Detail
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Figure II-3-36. Riser Inflow Curves
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3.2.19 BMP C250: Construction Stormwater Chemical Treatment
3.2.19.1 Purpose
This BMP applies when using stormwater chemicals in batch treatment or flow-through treatment.
Turbidity is difficult to control once fine particles are suspended in stormwater runoff from a
construction site. Sedimentation ponds are effective at removing larger particulate matter by gravity
settling, but are ineffective at removing smaller particulates such as clay and fine silt. Traditional
erosion and sediment control BMPs may not be adequate to ensure compliance with the water
quality standards for turbidity in the receiving water.
3.2.19.2 Conditions of Use
Formal written approval from Ecology and the City is required for the use of chemical
treatment regardless of site size. When approved, include the chemical treatment system in
the Stormwater Pollution Prevention Plan (SWPPP).
3.2.19.3 Design and Installation Specifications
See Appendix B for background information on chemical treatment.
Criteria for Chemical Treatment Product Use
Chemically treated stormwater discharged from construction sites must be nontoxic to aquatic
organisms. The Chemical Technology Assessment Protocol (CTAPE) must be used to evaluate
chemicals proposed for stormwater treatment. Only chemicals approved by Ecology under the
CTAPE may be used for stormwater treatment. The approved chemicals, their allowable
application techniques (batch treatment or flow-through treatment), allowable application rates, and
conditions of use can be found at the Department of Ecology Emerging Technologies website:
http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html
Treatment System Design Considerations
The design and operation of a chemical treatment system should take into consideration the factors
that determine optimum, cost-effective performance. It is important to recognize the following:
• Only Ecology approved chemicals may be used and must follow approved dose
rates.
• The pH of the stormwater must be in the proper range for the polymers to be
effective, which is typically 6.5 to 8.5.
• The coagulant must be mixed rapidly into the water to ensure proper dispersion.
• A flocculation step is important to increase the rate of settling, to produce the lowest
turbidity, and to keep the dosage rate as low as possible.
• Too little energy input into the water during the flocculation phase results in flow that
are too small and/or insufficiently dense. Too much energy can rapidly destroy floc
as it is formed.
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• Care must be taken in the design of the withdrawal system to minimize outflow
velocities and to prevent floc discharge. Discharge from a batch treatment system
should be directed through a physical filter such as a vegetated swale that would
catch any unintended floc discharge. Currently, flow-through systems always
discharge through the chemically enhanced sand filtration system.
• System discharge rates must take into account downstream conveyance integrity.
Polymer Batch Treatment Process Description
A batch chemical treatment system consists of the stormwater collection system (either temporary
diversion or the permanent site drainage system), an untreated stormwater storage pond, pumps, a
chemical feed system, treatment cells, and interconnecting piping.
The batch treatment system shall use a minimum of two lined treatment cells in addition to the
untreated stormwater storage pond. Multiple treatment cells allow for clarification of treated water
while other cells are being filled or emptied. Treatment cells may be ponds or tanks. Ponds with
constructed earthen embankments greater than six feet high require special engineering analyses.
Stormwater is collected at interception point(s) on the site and is diverted by gravity or by pumping to
an untreated stormwater storage pond or other untreated stormwater holding area. The stormwater is
stored until treatment occurs. It is important that the holding pond be large enough to provide
adequate storage.
The first step in the treatment sequence is to check the pH of the stormwater in the untreated
stormwater storage pond. The pH is adjusted by the application of carbon dioxide or a base until the
stormwater in the storage pond is within the desired pH range, 6.5 to 8.5. When used, carbon dioxide
is added immediately downstream of the transfer pump. Typically sodium bicarbonate (baking soda)
is used as a base, although other bases may be used. When needed, base is added directly to the
untreated stormwater storage pond. The stormwater is recirculated with the treatment pump to
provide mixing in the storage pond. Initial pH adjustments should be based on daily bench tests.
Further, pH adjustments can be made at any point in the process.
Once the stormwater is within the desired pH range (dependant on polymer being used), the
stormwater is pumped from the untreated stormwater storage pond to a treatment cell as polymer is
added. The polymer is added upstream of the pump to facilitate rapid mixing.
After polymer addition, the water is kept in a lined treatment cell for clarification of the sediment-floc.
In a batch mode process, clarification typically takes from 30 minutes to several hours. Prior to
discharge samples are withdrawn for analysis of pH and turbidity. If both are acceptable, the treated
water is discharged.
Several configurations have been developed to withdraw treated water from the treatment cell. The
original configuration is a device that withdraws the treated water from just beneath the water surface
using a float with adjustable struts that prevent the float from settling on the cell bottom (Figure II-
3-37). This reduces the possibility of picking up sediment-floc from the bottom of the pond. The struts
are usually set at a minimum clearance of about 12 inches; that is, the float will come within 12
inches of the bottom of the cell. Other systems have used vertical guides or cables which constrain
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the float, allowing it to drift up and down with the water level. More recent designs have an H-shaped
array of pipes, set on the horizontal.
Figure II-3-37. Floating Platform with Struts
This scheme provides for withdrawal from four points rather than one. This configuration reduces the
likelihood of sucking settled solids from the bottom. It also reduces the tendency for a vortex to form.
Inlet differs, a long floating or fixed pipe with many small holes in it, are also an option.
Safety is a primary concern. Design should consider the hazards associated with operations, such as
sampling. Facilities should be designed to reduce slip hazards and drowning. Tanks and ponds
should have life rings, ladder, or steps extending from the bottom to the top.
Polymer Flow-Through Treatment Process Description
At a minimum, a flow-through chemical treatment system consists of the stormwater collection
system (either temporary diversion or the permanent site drainage system), an untreated stormwater
storage pond, and the chemically enhanced sand filtration system.
Stormwater is collected at interception point(s) on the site and is diverted by gravity or by pumping to
an untreated stormwater storage pond or other untreated stormwater holding area. The stormwater is
stored until treatment occurs. It is important that the holding pond be large enough to provide
adequate storage.
Stormwater is then pumped from the untreated stormwater storage pond to the chemically enhanced
sand filtration system where polymer is added. Adjustments to pH may be necessary before chemical
addition. The sand filtration system continually monitors the stormwater for turbidity and pH. If the
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discharge water is ever out of an acceptable range for turbidity or pH, the water is recycled to the
untreated stormwater pond where it can be retreated.
Equipment
For batch treatment and flow-through treatment, the following equipment should be located in a
lockable shed:
• The chemical injector
• Secondary non-corrosive containment for acid, caustic, buffering compound, and
treatment chemical
• Emergency shower and eyewash
• Monitoring equipment
System Sizing
Certain sites are required to implement flow control for the developed sites. These sites must also
control stormwater release rates during construction. Generally, these are sites that discharge
stormwater directly or indirectly, through a conveyance system, into a freshwater. System sizing is
dependent on flow control requirements.
Sizing Criteria for Batch Treatment Systems for Flow Control Exempt Water Bodies
• The total volume of the untreated stormwater storage pond and treatment ponds or
tanks must be large enough to treat the volume of stormwater that is produced during
multiple day storm events. At a minimum, size the untreated storage pond to hold
1.5 times the runoff volume of the 10-year, 24-hour storm event. Provide bypass
around the chemical treatment system to accommodate extreme storm events.
Calculate runoff volumes using the methods in Volume III, Chapter 3. Use worst-case
land cover conditions (i.e., producing the most runoff) for analyses (in most cases, this
would be the land cover conditions just prior to final landscaping).
• Primary settling should be encouraged in the untreated stormwater storage pond. A
forebay with access for maintenance is beneficial.
• There are two opposing considerations in sizing the treatment cells. A larger cell is
able to treat a larger volume of water each time a batch is processed. However, the
larger the cell the longer the time is required to empty the cell. A larger cell may also
be less effective at flocculation and therefore require a longer settling time. The
simplest approach to sizing the treatment cell is to multiply the allowable discharge
flowrate times the desired drawdown time. A 4-hour drawdown time allow one batch
per cell per 8-hour work period, given 1 hour of flocculation followed by 2 hours of
settling.
• If the discharge is directly to a lake, flow control exempt receiving water, or to an
infiltration system, there is no discharge flow limit.
• Ponds sized for flow control water bodies must at a minimum meet the sizing criteria
for flow control exempt waters.
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Sizing Criteria for Flow-Through Treatment Systems for Flow Control Exempt Water Bodies:
• Sites that must implement flow control for the developed site condition must also
control stormwater release rates during construction. Construction site stormwater
discharges shall not exceed the discharge durations of the predeveloped condition for
the range of predeveloped discharge rates from ½ of the 2-year flow through the
10-year flow as predicted by WWHM. The predeveloped condition to be matched shall
be the land cover condition immediately prior to the development project. This
restriction on release rates can affect the size of the storage pond and treatment cells.
• The following is how WWHM can be used to determine the release rates from the
chemical treatment systems:
1. Determine the predeveloped flow durations to be matched by entering the land use
area under the “Predeveloped” scenario in WWHM. The default flow range is from ½
of the 2-year flow through the 10-year flow.
2. Enter the post developed land use area in the “Developed Unmitigated” scenario in
WWHM.
3. Copy the land use information for the “Developed Unmitigated” to “Developed
Mitigated” scenario.
4. While in the “Developed Mitigated” scenario, add a pond element under the basin
element containing the post-developed land use areas. This pond element represents
information on the available untreated stormwater storage and discharge from the
chemical treatment system. In cases where the discharge from the chemical
treatment is controlled by a pump, a stage/storage/discharge (SSD) table
representing the pond must be generated outside WWHM and imported into WWHM.
WWHM can route the runoff from the post-developed condition through this SSD
table (the pond) and determine compliance with the flow duration standard. This
would be an iterative design procedure where if the initial SSD table proved to be
inadequate, the designer would have to modify the SSD table outside WWHM and
reimport in WWHM and route the runoff through it again. The iteration will continue
until a pond that complies with the flow duration standard is correctly sized.
Notes on SSD table characteristics:
• The pump discharge rate would likely be initially set at just below ½ of the 2-year flow
from the pre-developed condition. As runoff coming into the untreated stormwater
storage pond increases and the available untreated stormwater storage volume gets
used up, it would be necessary to increase the pump discharge rate above ½ of the
2-year. The increase(s) above ½ of the 2-year must be such that they provide some
relief to the untreated stormwater storage needs but at the same time will not cause
violations of the flow duration standard at the higher flows. The final design SSD table
will identify the appropriate pumping rates and the corresponding stage and storages.
• When building such a flow control system, the design must ensure that any automatic
adjustments to the pumping rates will be as a result of the changes to the available
storage in accordance with the final design SSD table.
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• It should be noted that the above procedures would be used to meet the flow control
requirements. The chemical treatment system must be able to meet the runoff
treatment requirements. It is likely that the discharge flowrate of ½ of the 2-year or
more may exceed the treatment capacity of the system. If that is the case, the
untreated stormwater discharge rate(s) (i.e., influent to the treatment system) must be
reduced to allow proper treatment. Any reduction in the flows will likely result in the
need for a larger untreated stormwater storage volume.
• If the discharge is to a municipal storm drainage system, the allowable discharge rate
may be limited by the capacity of the public system. It may be necessary to clean the
municipal storm drainage system prior to the start of the discharge to prevent scouring
solids from the drainage system. If the municipal storm drainage system discharges to
a water body that is not flow control exempt, the project site is subject to flow control
requirements.
• If system design does not allow you to discharge at the slower rates as described
above and if the site had a retention or detention pond that will serve the planned
development, the discharge from the treatment system may be directed to the
permanent retention/detention pond to comply with the flow control requirement. In this
case, the untreated stormwater storage pond and treatment system will be sized
according to the sizing criteria for flow-through system for flow control exempt water
bodies described earlier except all discharge (water passing through the treatment
system and stormwater bypassing the treatment system) will be directed into the
permanent retention/detention pond. If site constraints make locating the untreated
stormwater storage pond difficult, the permanent retention/detention pond may be
divided to serve as the untreated stormwater storage pond and the post-treatment flow
control pond. A berm or barrier must be used in this case so the untreated water does
not mix with the treated water. Both untreated stormwater storage requirements, and
adequate post-treatment flow control must be achieved. The post-treatment flow
control pond’s revised dimensions must be entered into the WWHM and the WWHM
must be run to confirm compliance with the flow control requirements.
3.2.19.4 Monitoring
Conduct the following monitoring. Record test results on a daily log kept on site. Additional testing
may be required by the NPDES permit based on site conditions.
Operational Monitoring:
• Total volume treated and discharged
• Flow must be continuously monitored and recorded at not greater than 15-minute
intervals
• Type and amount of chemical used for pH adjustment, if any
• Quantity of chemical used for treatment
• Settling time
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Compliance Monitoring
• Influent and effluent pH and turbidity must be continuously monitored and recorded
at not greater than 15-minute intervals.
• pH and turbidity of the receiving water
Biomonitoring
• Treated stormwater must be non-toxic to aquatic organisms. Treated stormwater must
be tested for aquatic toxicity or residual chemical content. Frequency of biomonitoring
will be determined by Ecology.
• Residual chemical tests must be approved by Ecology prior to their use.
• If testing treated stormwater for aquatic toxicity, you must test for acute (lethal) toxicity.
Bioassays shall be conducted by a laboratory accredited by Ecology, unless otherwise
approved by Ecology. Acute toxicity tests shall be conducted per the CTAPE protocol.
Discharge Compliance
• Prior to discharge, treated stormwater must be sampled and tested for
compliance with pH and turbidity limits. These limits may be established by the
Construction Stormwater General Permit, or a site-specific discharge permit. Sampling
and testing for other pollutants may also be necessary at some sites. pH must be
within the range of 6.5 to 8.5 standard units and not cause a change in the pH of the
receiving water of more than 0.2 standard units.
• Treated stormwater samples and measurements shall be taken from the discharge
pipe or another location representative of the nature of the treated stormwater
discharge. Samples used for determining compliance with the water quality standards
in the receiving water shall not be taken from the treatment pond prior to decanting.
Compliance with the water quality standards is determined in the receiving water.
Operator Training
• Each contractor who intends to use chemical treatment shall be trained by an
experienced contractor on an active site.
Standard BMPs
• Surface stabilization BMPs should be implemented on site to prevent significant
erosion. All sites shall use a truck wheel wash to prevent tracking of sediment off site.
Sediment Removal and Disposal:
• Remove sediment from the storage or treatment cells as necessary. Typically,
sediment removal is required at least once during a wet season and at the
decommissioning of the cells. Sediment remaining in the cells between batches may
enhance the settling process and reduce the required chemical dosage.
• Sediment that is known to be non-toxic may be incorporated into the site away from
drainages.
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3.2.20 BMP C251: Construction Stormwater Filtration
3.2.20.1 Purpose
Filtration removes sediment from runoff originating from disturbed areas of the site.
3.2.20.2 Conditions of Use
Traditional BMPs used to control soil erosion and sediment loss from sites under development may
not be adequate to ensure compliance with the water quality standard for turbidity in the receiving
water. Filtration may be used in conjunction with gravity settling to remove sediment as small as fine
silt (0.5 m). The reduction in turbidity will be d ependent on the particle size distribution of the
sediment in the stormwater. In some circumstances, sedimentation and filtration may achieve
compliance with the water quality standard for turbidity.
The use of construction stormwater filtration does not require approval from Ecology as long as
treatment chemicals are not used. Filtration in conjunction with polymer treatment requires testing
under the Chemical Technology Assessment Protocol – Ecology (CTAPE) before it can be initiated.
Approval from the appropriate regional Ecology office must be obtained at each site where polymers
use is proposed prior to use. For more guidance on stormwater chemical treatment see BMP C250.
3.2.20.3 Background Information
Filtration with sand media has been used for over a century to treat water and wastewater. The use
of sand filtration for treatment of stormwater has developed recently, generally to treat runoff from
streets, parking lots, and residential areas. The application of filtration to construction stormwater is
currently under development.
3.2.20.4 Design and Installation Specifications
Two types of filtration systems may be applied to construction stormwater treatment: rapid and slow.
Rapid sand filters are the typical system used for water and wastewater treatment. They can achieve
relatively high hydraulic flow rates, on the order of 2 to 20 gpm/sf, because they have automatic
backwash systems to remove accumulated solids. In contrast, slow sand filters have very low
hydraulic rates, on the order of 0.02 gpm/sf, because they do not have backwash systems. To date,
slow sand filtration has generally been used to treat stormwater. Slow sand filtration is mechanically
simple in comparison to rapid sand filtration but requires a much larger filter area.
Filtration Equipment
Sand media filters are available with automatic backwashing features that can filter to 50 m particle
size. Screen or bag filters can filter down to 5 m . Fiber wound filters can remove particles down to
0.5 m. Filters should be sequenced from the larges t to the smallest pore opening. Sediment
removal efficiency will be related to particle size distribution in the stormwater.
Treatment Process Description
Stormwater is collected at interception point(s) on the site and is diverted to an untreated stormwater
sediment pond or tank for removal of large sediment and storage of the stormwater before it is
treated by the filtration system. The stormwater is pumped from the trap, pond, or tank through the
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filtration system in a rapid sand filtration system. Slow sand filtration systems are designed as flow
through systems using gravity.
Sizing Criteria for Flow-Through Treatment Systems for Flow Control Exempt Water Bodies
When sizing storage ponds or tanks for flow-through systems for flow control exempt water bodies,
the treatment system capacity should be a factor. The untreated stormwater storage pond or tank
should be sized to hold 1.5 times the runoff volume of the 10-year, 24-hour storm event minus the
treatment system flowrate for an 8-hour period. For a chitosan-enhanced sand filtration system, the
treatment flowrate should be sized using a hydraulic loading rate between 6-8 gpm/ft². Other
hydraulic loading rates may be more appropriate for other systems. Bypass should be provided
around the chemical treatment system to accommodate extreme storms. Runoff volumes shall be
calculated using the methods presented in Volume III, Chapter 3. Worst-case conditions (i.e.,
producing the most runoff) should be used for analyses (most likely conditions present prior to final
landscaping).
Sizing Criteria for Flow Control Waters:
Sites that must implement flow control for the developed site condition must also control stormwater
release rates during construction. Construction site stormwater discharges shall not exceed the
discharge durations of the pre-developed condition for the range of pre-developed discharge rates
from ½ of the 2-year flow through the 10-year flow as predicted by WWHM. The pre-developed
condition to be matched shall be the land cover condition immediately prior to the development
project. This restriction on release rates will affect the size of the sediment pond, the filtration system,
and the flow rate through the filter system.
The following is how WWHM can be used to determine the release rates from the filtration systems:
1. Determine the pre-developed flow durations to be matched by entering the land use area
under the “Pre-developed” scenario in WWHM. The default flow range is from ½ of the 2-year
flow through the 10-year flow.
2. Enter the post developed land use area in the “Developed Unmitigated” scenario in WWHM.
3. Copy the land use information from the “Developed Unmitigated” to “Developed Mitigated”
scenario.
4. There are two possible ways to model stormwater filtration systems:
a. The stormwater filtration system uses a storage pond/tank and the discharge from
this pond/tank is pumped to one or more filters. In-line filtration chemicals would be
added to the flow right after the pond/tank and before the filter(s). Because the
discharge is pumped, WWHM cannot generate a stage/storage/discharge (SSD)
table for this system. This system is modeled the same way as described in BMP
C250 and is as follows:
While in the “Developed Mitigated” scenario, add a pond element under
the basin element containing the post-developed land use areas. This
pond element represents information on the available storage and
discharge from the filtration system. In cases where the discharge from
the filtration system is controlled by a pump, a stage/storage/discharge
(SSD) table representing the pond must be generated outside WWHM
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and imported into WWHM. WWHM can route the runoff from the post-
developed condition through this SSD table (the pond) and determine
compliance with the flow duration standard. This would be an iterative
design procedure where if the initial SSD table proved to be out of
compliance, the designer would have to modify the SSD table outside
WWHM and re-import in WWHM and route the runoff through it again.
The iteration will continue until a pond that enables compliance with the
flow duration standard is designed.
Notes on SSD Table Characteristics
• The pump discharge rate would likely be initially set at just below ½ if the 2-year flow
from the pre-developed condition. As runoff coming to the storage pond increases
and the available storage volume gets used up, it would be necessary to increase the
pump discharge rate above ½ of the 2-year. The increase(s) above ½ of the 2-year
must be such that they provide some relief to the storage needs but at the same time
they will not cause violations of the flow duration standard at the higher flows. The
final design SSD table will identify the appropriate pumping rates and the
corresponding stage and storages.
• When building such a flow control system, the design must ensure that any automatic
adjustments to the pumping rates will be as a result of changes to the available
storage in accordance with the final design SSD table.
b. The stormwater filtration system uses a storage pond/tank and the discharge from
this pond/tank gravity flows to the filter. This is usually a slow sand filter system
and it is possible to model it in WWHM as a Filter element or as a combination of
Pond and Filter element placed in series. The stage/storage/discharge table(s)
may then be generated within WWHM as follows:
(i) While in the “Developed Mitigated” scenario, add a Filter element under the basin
element containing the post-developed land use areas. The length and width of
this filter element would have to be the same as the bottom length and width of
the upstream storage pond/tank.
(ii) In cases where the length and width of the filter is not the same as those for the
bottom of the upstream storage tank/pond, the treatment system may be
modeled as a Pond element followed by a Filter element. By having these two
elements, WWHM would then generate a SSD table for the storage pond which
then gravity flows to the Filter element. The Filter element downstream of the
storage pond would have a storage component through the media, and an
overflow component for when the filtration capacity is exceeded.
WWHM can route the runoff from the post-developed condition through the treatment
systems in 4b and determine compliance with the flow duration standard. This would be an
iterative design procedure where if the initial sizing estimates for the treatment system proved
to be inadequate, the designer would have to modify the system and route the runoff through
it again. The iteration would continue until compliance with the flow duration standard is
achieved.
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5. It should be noted that the above procedures would be used to meet the flow control
requirements. The filtration system must be able to meet the runoff treatment requirements. It
is likely that the discharge flow rate of ½ of the 2-year or more may exceed the treatment
capacity of the system. If that is the case, the discharge rate(s) must be reduced to allow
proper treatment. Any reduction in the flows would likely result in the need for a larger storage
volume.
If the system does not allow you to discharge at the slower rate as described above and if the
site has a retention or detention pond that will serve the planned development, the discharge
from the treatment system may be directed to the permanent retention/detention pond to
comply with the flow control requirements. In this case, the untreated stormwater storage
pond and treatment system will be sized according to the sizing criteria for flow-through
treatment systems for flow control exempt waterbodies except all discharges (water passing
through the treatment system and stormwater bypassing the treatment system) will be
directed into the permanent retention/detention pond. If site constraints make locating the
untreated stormwater storage pond difficult, the permanent retention/detention pond may be
divided to serve as the untreated stormwater discharge pond and the post-treatment flow
control pond. A berm or barrier must be used in this case so the untreated water does not mix
with the treated water. Both untreated stormwater storage requirements, and adequate post-
treatment flow control must be achieved. The post-treatment flow control pond’s revised
dimensions must be entered into the WWHM and the WWHM must be run to confirm
compliance with the flow control requirements.
3.2.20.5 Maintenance Standards
Rapid sand filters typically have automatic backwash systems that are triggered by a pre-set
pressure drop across the filter. If the backwash water volume is not large or substantially more turbid
than the stormwater stored in the holding pond or tank, backwash return to the pond or tank may be
appropriate. However, land application or another means of treatment and disposal may be
necessary.
• Clean and/or replace screen, bag, and fiber filters when they become clogged.
• Remove sediment from the storage and/or treatment ponds as necessary. Typically,
sediment removal is required once or twice during a wet season and at the
decommissioning of the ponds.
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3.2.21 BMP C252: High pH Neutralization using CO2
3.2.21.1 Description
When pH levels in stormwater rise above 8.5 it is necessary to lower the pH levels to the acceptable
range of 6.5 to 8.5, this process is called pH neutralization. pH neutralization involves the use of solid
or compressed carbon dioxide gas in water requiring neutralization. Neutralized stormwater may be
discharged to surface waters under the General Construction NPDES permit but neutralized process
wastewater must be managed to prevent discharge to surface waters. Process wastewater includes
wastewaters such as concrete truck wash-out, hydro-demolition, or saw-cutting slurry.
Reason for pH neutralization
A pH level range of 6.5 to 8.5 is typical for most natural watercourses, and this neutral pH is required
for the survival of aquatic organisms. Should the pH rise or drop out of this range, fish and other
aquatic organisms may become stressed and may die.
Calcium hardness can contribute to high pH values and cause toxicity that is associated with high pH
conditions. A high level of calcium hardness in waters of the state is not allowed.
The water quality standard for pH in Washington State is in the range of 6.5 to 8.5.
Groundwater standard for calcium and other dissolved solids in Washington State is less than 500
mg/l.
Causes of high pH
High pH at construction sites is most commonly caused by the contact of stormwater with poured or
recycled concrete, cement, mortars, and other Portland cement or lime-containing construction
materials. (See BMP C151: Concrete Handling for more information on concrete handling
procedures). The principal caustic agent in cement is calcium hydroxide (free lime).
Advantages of CO2 Sparging
• Rapidly neutralizes high pH water.
• Cost effective and safer to handle than acid compounds.
• CO2 is self-buffering. It is difficult to overdose and create harmfully low pH levels.
• Material is readily available.
The Chemical Process
When carbon dioxide (CO2) is added to water (H2O), carbonic acid (H2CO3) is formed which can
further dissociate into a proton (H+) and a bicarbonate anion (HCO3-) as shown below:
CO2 + H2O H2CO3 H+ + HCO3
-
The free proton is a weak acid that can lower the pH.
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Water temperature has an effect on the reaction as well. The colder the water temperature is the
slower the reaction occurs and the warmer the water temperature is the quicker the reaction occurs.
Most construction applications in Washington State have water temperatures in the 50°F or higher
range so the reaction is almost simultaneous.
3.2.21.2 Treatment Procedures
High pH water may be treated using continuous treatment, continuous discharge systems. These
manufactured systems continuously monitor influent and effluent pH to ensure that pH values are
within an acceptable range before being discharged. All systems must have fail safe automatic shut
off switches in the event that pH is not within the acceptable discharge range. Only trained operators
may operate manufactured systems. System manufacturers often provide trained operators or
training on their devices.
The following procedure may be used when not using a continuous discharge system:
• Prior to treatment, the appropriate jurisdiction should be notified in accordance with the
regulations set by the jurisdiction.
• Every effort should be made to isolate the potential high pH water in order to treat it
separately from other stormwater on-site.
• Water should be stored in an acceptable storage facility, detention pond, or
containment cell prior to treatment.
• Transfer water to be treated to the treatment structure. Ensure that treatment structure
size is sufficient to hold the amount of water that is to be treated. Do not fill tank
completely, allow at least 2 feet of freeboard.
• The operator samples the water for pH and notes the clarity of the water. As a rule of
thumb, less CO2 is necessary for clearer water. This information should be recorded.
• In the pH adjustment structure, add CO2 until the pH falls in the range of 6.9-7.1.
Remember that pH water quality standards apply so adjusting pH to within 0.2 pH units
of receiving water (background pH) is recommended. It is unlikely that pH can be
adjusted to within 0.2 pH units using dry ice. Compressed carbon dioxide gas should
be introduced to the water using a carbon dioxide diffuser located near the bottom of
the tank, this will allow carbon dioxide to bubble up through the water and diffuse more
evenly.
• Slowly release the water to discharge making sure water does not get stirred up in the
process. Release about 80% of the water from the structure leaving any sludge
behind.
• Discharge treated water through a pond or drainage system.
• Excess sludge needs to be disposed of properly as concrete waste. If several batches
of water are undergoing pH treatment, sludge can be left in treatment structure for the
next batch treatment. Dispose of sludge when it fills 50% of tank volume.
Sites that must implement flow control for the developed site must also control stormwater release
rates during construction. All treated stormwater must go through a flow control facility before being
released to surface waters which require flow control.
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3.2.21.3 Safety and Materials Handling
• All equipment should be handled in accordance with OSHA rules and regulations.
• Follow manufacturer guidelines for materials handling.
3.2.21.4 Operator Records
Each operator should provide:
• A diagram of the monitoring and treatment equipment
• A description of the pumping rates and capacity the treatment equipment is capable
of treating.
Each operator should keep a written record of the following:
• Client name and phone number
• Date of treatment
• Weather conditions
• Project name and location
• Volume of water treated
• pH of untreated water
• Amount of CO2 needed to adjust water to a pH range of 6.9-7.1
• pH of treated water
• Discharge point location and description
A copy of this record should be given to the client/contractor who should retain the record for three
years.
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3.2.22 BMP C253: pH Control for High pH Water
3.2.22.1 Description
When pH levels in stormwater rise above 8.5 it is necessary to lower the pH levels to the acceptable
range of 6.5 to 8.5, this process is called pH neutralization. Stormwater with pH levels exceeding
water quality standards may be treated by infiltration, dispersion in vegetation or compost, pumping
to a sanitary sewer, disposal at a permitted concrete batch plant with pH neutralization capabilities, or
carbon dioxide sparging. BMP C252 provides guidance for carbon dioxide sparging.
Reason for pH neutralization
A pH level between 6.5 and 8.5 is typical for most natural watercourses, and this pH range is required
for the survival of aquatic organisms. Should the pH rise or drop out of this range, fish and other
aquatic organisms may become stressed and may die.
Causes of high pH
High pH levels at construction sites are most commonly caused by the contact of stormwater with
poured or recycled concrete, cement, mortars, and other Portland cement or lime-containing
construction materials. (See BMP C151: Concrete Handling for more information on concrete
handling procedures). The principal caustic agent in cement is calcium hydroxide (free lime).
3.2.22.2 Disposal Methods
Infiltration
• Infiltration is only allowed if soil type allows all water to infiltrate (no surface runoff) without
causing or contributing to a violation of surface or groundwater quality standards.
• Infiltration techniques should be consistent with Volume V, Chapter 5.
Dispersion
• Use BMP L614 Full Dispersion
Sanitary Sewer Disposal
• Local sewer authority approval is required prior to disposal via the sanitary sewer.
Concrete Batch Plant Disposal
• Only permitted facilities may accept high pH water.
• Facility should be contacted before treatment to ensure they can accept the high pH water.
Stormwater Discharge
Any pH treatment options that generate treated water that must be discharged off site are subject to
flow control requirements. Sites that must implement flow control for the developed site must also
control stormwater release rates during construction. All treated stormwater must go through a flow
control facility before being released to surface waters which require flow control.
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Appendix A Standard Notes for Erosion Control Plans
Use the following standard notes on project Stormwater Pollution Prevention Plan (SWPPP) and
associated drawings. Other mandatory notes for construction plans may be applicable. Plans shall
identify the name and phone number of the person or firm responsible for the preparation and
maintenance of the erosion control plan.
Standard Notes
Approval of this erosion/sedimentation control (ESC) plan does not constitute an approval of
permanent road or drainage design (e.g. size and location of roads, pipes, restrictors, channels,
retention facilities, utilities, etc.).
The implementation of these ESC plans and the construction, maintenance, replacement, and
upgrading of these ESC facilities is the responsibility of the applicant/contractor until all construction
is completed and approved and vegetation/landscaping is established.
The boundaries of the clearing limits shown on this plan shall be clearly flagged in the field prior to
construction. During the construction period, no disturbance beyond the flagged clearing limits shall
be permitted. The flagging shall be maintained by the applicant/contractor for the duration of
construction.
The ESC facilities shown on this plan must be constructed in conjunction with all clearing and grading
activities, and in such a manner as to ensure that sediment and sediment laden water do not enter
the drainage system or roadways, or violate applicable water standards.
The ESC facilities shown on this plan are the minimum requirements for anticipated site conditions.
During the construction period, these ESC facilities shall be upgraded as needed for unexpected
storm events and to ensure that sediment and sediment-laden water do not leave the site.
The ESC facilities shall be inspected daily by the applicant/contractor and maintained as necessary
to ensure their continued functioning.
The ESC facilities on inactive sites shall be inspected and maintained a minimum of once a month or
within the 48 hours following a major storm event.
At no time shall more than one foot of sediment be allowed to accumulate within a catch basin
sediment trap. All catch basins and conveyance lines shall be cleaned prior to paving. The cleaning
operation shall not flush sediment-laden water into the downstream system.
Stabilized construction entrances shall be installed at the beginning of construction and maintained
for the duration of the project. Additional measures may be required to ensure that all paved areas
are kept clean for the duration of the project.
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Appendix B Background Information on Chemical
Treatment
Coagulation and flocculation have been used for over a century to treat water. It is used less
frequently for the treatment of wastewater. The use of coagulation and flocculation for treating
stormwater is a very recent application. Experience with the treatment of water and wastewater has
resulted in a basic understanding of the process, in particular factors that affect performance. This
experience can provide insights as to how to most effectively design and operate similar systems in
the treatment of stormwater.
Fine particles suspended in water give it a milky appearance, measured as turbidity. Their small size,
often much less than 1 m in diameter, give them a very large surface area relative to their volume.
These fine particles typically carry a negative surface charge. Largely because of these two factors,
small size and negative charge, these particles tend to stay in suspension for extended periods of
time. Thus, removal is not practical by gravity settling. These are called stable suspensions.
Polymers, as well as inorganic chemicals such as alum, speed the process of clarification. The added
chemical destabilizes the suspension and causes the smaller particles to agglomerate. The process
consists of three steps: coagulation, flocculation, and settling or clarification. Each step is explained
below, as well as the factors that affect the efficiency of the process.
Coagulation: Coagulation is the first step. It is the process by which negative charges on the fine
particles that prevent their agglomeration are disrupted. Chemical addition is one method of
destabilizing the suspension, and polymers are one class of chemicals that are generally effective.
Chemicals that are used for this purpose are called coagulants. Coagulation is complete when the
suspension is destabilized by the neutralization of the negative charges. Coagulants perform best
when they are thoroughly and evenly dispersed under relatively intense mixing. This rapid mixing
involves adding the coagulant in a manner that promotes rapid dispersion, followed by a short time
period for destabilization of the particle suspension. The particles are still very small and are not
readily separated by clarification until flocculation occurs.
Flocculation: Flocculation is the process by which fine particles that have been destabilized bind
together to form larger particles that settle rapidly. Flocculation begins naturally following coagulation,
but is enhanced by gentle mixing of the destabilized suspension. Gentle mixing helps to bring
particles in contact with one another such that they bind and continually grow to form "flocs." As the
size of the flocs increases, they become heavier and tend to settle more rapidly.
Clarification: The final step is the settling of the particles. Particle density, size, and shape are
important during settling. Dense, compact flocs settle more readily than less dense, fluffy flocs.
Because of this, flocculation to form dense, compact flocs is particularly important during water
treatment. Water temperature is important during settling. Both the density and viscosity of water are
affected by temperature; these in turn affect settling. Cold temperatures increase viscosity and
density, thus slowing down the rate at which the particles settle.
The conditions under which clarification is achieved can affect performance. Currents can affect
settling. Currents can be produced by wind, by differences between the temperature of the incoming
water and the water in the clarifier, and by flow conditions near the inlets and outlets.
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Quiescent water, such as that which occurs during batch clarification, provides a good environment
for effective performance, as many of these factors become less important in comparison to typical
sedimentation basins. One source of currents that is likely important in batch systems is movement of
the water leaving the clarifier unit. Given that flocs are relatively small and light, the exit velocity of the
water must be as low as possible. Sediment on the bottom of the basin can be resuspended and
removed by fairly modest velocities.
Coagulants: Polymers are large organic molecules that are made up of subunits linked together in a
chain-like structure. Attached to these chain-like structures are other groups that carry positive or
negative charges, or have no charge. Polymers that carry groups with positive charges are called
cationic, those with negative charges are called anionic, and those with no charge (neutral) are called
nonionic.
Cationic polymers can be used as coagulants to destabilize negatively charged turbidity particles
present in natural waters, wastewater, and stormwater. Aluminum sulfate (alum) can also be used as
this chemical becomes positively charged when dispersed in water. In practice, the only way to
determine whether a polymer is effective for a specific application is to perform preliminary or on-site
testing.
Polymers are available as powders, concentrated liquids, and emulsions (which appear as milky
liquids). The latter are petroleum based, which are not allowed for construction stormwater treatment.
Polymer effectiveness can degrade with time and from other influences. Thus, manufacturers'
recommendations for storage should be followed. Manufacturers’ recommendations usually do not
provide assurance of water quality protection or safety to aquatic organisms. Consideration of water
quality protection is necessary in the selection and use of all polymers.
Application Considerations: Application of coagulants at the appropriate concentration or dosage
rate for optimum turbidity removal is important for management of chemical cost, for effective
performance, and to avoid aquatic toxicity. The optimum dose in a given application depends on
several site-specific features. Turbidity of untreated water can be important with turbidities greater
than 5,000 NTU. The surface charge of particles to be removed is also important. Environmental
factors that can influence dosage rate are water temperature, pH, and the presence of constituents
that consume or otherwise affect polymer effectiveness. Laboratory experiments indicate that mixing
previously settled sediment (floc sludge) with untreated stormwater significantly improves clarification,
therefore reducing the effective dosage rate. Preparation of working solutions and thorough dispersal
of polymers in water to be treated is also important to establish the appropriate dosage rate.
For a given water sample, there is generally an optimum dosage rate that yields the lowest residual
turbidity after settling. When dosage rates below this optimum value (underdosing) are applied, there
is an insufficient quantity of coagulant to react with, and therefore destabilize, all of the turbidity
present. The result is residual turbidity (after flocculation and settling) that is higher than with the
optimum dose. Overdosing, application of dosage rates greater than the optimum value, can also
negatively impact performance. Again, the result is higher residual turbidity than that with the
optimum dose.
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on Chemical Treatment Appendix B 276
Mixing in Coagulation/Flocculation: The G-value, or just "G", is often used as a measure of the
mixing intensity applied during coagulation and flocculation. The symbol G stands for “velocity
gradient”, which is related in part to the degree of turbulence generated during mixing. High G-values
mean high turbulence, and vice versa. High G-values provide the best conditions for coagulant
addition. With high Gs, turbulence is high and coagulants are rapidly dispersed to their appropriate
concentrations for effective destabilization of particle suspensions.
Low G-values provide the best conditions for flocculation. Here, the goal is to promote formation of
dense, compact flocs that will settle readily. Low Gs provide low turbulence to promote particle
collisions so that flocs can form. Low Gs generate sufficient turbulence such that collisions are
effective in floc formation, but do not break up flocs that have already formed.
Design engineers wishing to review more detailed presentations on this subject are referred to the
following textbooks:
• Fair, G., J. Geyer and D. Okun, Water and Wastewater Engineering, Wiley and
Sons, NY, 1968.
• American Water Works Association, Water Quality and Treatment, McGraw-Hill, NY,
1990.
• Weber, W.J., Physiochemical Processes for Water Quality Control, Wiley and Sons,
NY, 1972.
Adjustment of pH and Alkalinity: The pH must be in the proper range for the polymers to be
effective, which is 6.5 to 8.5 for Calgon CatFloc 2953, the most commonly used polymer. As
polymers tend to lower the pH, it is important that the stormwater have sufficient buffering capacity.
Buffering capacity is a function of alkalinity. Without sufficient alkalinity, the application of the polymer
may lower the pH to below 6.5. A pH below 6.5 not only reduces the effectiveness of the polymer, it
may create a toxic condition for aquatic organisms. Stormwater may not be discharged without
readjustment of the pH to above 6.5. The target pH should be within 0.2 standard units of the
receiving water pH.
Experience gained at several projects in the City of Redmond has shown that the alkalinity needs to
be at least 50 mg/L to prevent a drop in pH to below 6.5 when the polymer is added.
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Appendix C Construction SWPPP Short Form
Projects falling within the thresholds listed below may use this short form instead of preparing a
professionally-designed Construction Stormwater Pollution Prevention Plan (SWPPP). If your project
meets the following thresholds and includes or may impact a critical area, please contact the City to
determine if the SWPPP short form may be used.
The thresholds for using this form are projects that propose to:
• Add or replace between 2,000 and 5,000 square feet of impervious surface.
OR
• Clear or disturb between 7,000 square feet and 1 acre of land.
OR
• Grade/fill less than 500 cubic yards.
If project quantities exceed either of these thresholds, prepare a formal Construction SWPPP as
described in Chapter 2 of this volume.
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City of Auburn
Construction Stormwater Pollution Prevention Plan
Short Form
Project Name: __ ______
Address:
Contact/Owner: Phone:
Erosion Control Supervisor:
Phone: Cell: Pager:
Emergency (After hour) contact: Phone:
Permit No:
Parcel No.:
Required Submittals
1. Project Narrative
The Construction Stormwater Pollution Prevention Plan (SWPPP) Short-Form Narrative must be
completed as part of this packet. Any information described, as part of the narrative, should be shown
on the site plan.
NOTE: From October 1 thru April 30, clearing, grading, and other soil disturbing activities shall only
be permitted by special authorization from the City of Auburn.
A. Project Description (Check all that apply)
New Structure Building Addition Grading/Excavation Paving
Utilities Other: ______________________________________________
1. Total project area__________ (square feet)
2. Total proposed impervious area__________(square feet)
3. Total existing impervious area _________(square feet)
4. Total proposed area to be disturbed __________(square feet)
5. Total volumes of proposed cuts/fill_________(cubic yards)
Additional Project Information:
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B. Existing Site Conditions (Check all that apply)
• Describe the existing vegetation on the site. (Check all that apply)
Forest Pasture/prairie grass Pavement Landscaping Brush
Trees Other_____________________________________________________
• Describe how surface water (stormwater) drainage flows across/from the site. (Check all
that apply)
Sheet Flow Gutter Catch Basin Ditch/Swale Storm sewer
Stream Other __________________________________________________
• Describe any unusual site condition(s) or other features of note.
Steep Grades Large depression Underground tanks Springs
Easements Existing Structures Existing Utilities
Other___________________________________________________________
C. Adjacent Areas (Check all that apply)
1. Check any adjacent areas that may be affected by site disturbance and describe in fully
describe in item 2 below:
Streams* Lakes* Wetlands* Steep Slopes*
Residential Areas Roads Ditches, pipes, culverts
Other __________________________________________________________
* If site is on or adjacent to a critical area, the City of Auburn may require additional information, engineering, and
other permits to be submitted with this short-form.
2. Describe how and where surface water enters the site from upstream properties:
3. Describe the downstream drainage path leading from the site to the receiving body of
water. (Minimum distance of ¼-mile (1320 feet)) {E.g. water flows from site, into curb-
line to catch basin at intersection of X and Y streets. A 10-inch pipe system conveys
water another 1000 feet to a ravine/wetland.}
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D. Soils (Check all that apply)
The intent of this section is to identify when additional soils information may be required for applicants
using this short form. There are other site-specific issues that may necessitate a soils investigation or
more extensive erosion control practices. The City will determine these situations on a case-by-case
basis as part of their review.
1. Does the project propose infiltration? Infiltration systems require prior City approval.
Yes No Groundwater Protection Zone 2
2. Does the project propose construction near or on steep slopes?
Yes No
If infiltration is proposed for the site or steep slopes have been identified, the City will require soils
information as part of the project design. The applicant must contact a soil professional or civil
engineer specializing in soil analysis to perform an in-depth soils investigation. If yes is checked
for either question, the City may not permit the use of this short-form.
E. Construction Sequencing/Phasing
1. Construction sequence: The standard construction sequence is as follows:
• Mark clearing/grading limits.
• Call Building Inspector to inspect clearing/grading limits.
• Install initial erosion control practices (construction entrance, silt fence, catch basin
inserts).
• Contact Building Inspector to inspect initial erosion control practices.
• Clear, grade, and fill site as outlined in the site plan while implementing and
maintaining temporary erosion and sediment control practices at the same time.
• Install permanent erosion protection (impervious surface, landscaping, etc.).
• Contact Building Inspector for approval of permanent erosion protection and site
grades.
• Remove erosion control methods as permitted by the Building Inspector and repair
permanent erosion protection as necessary.
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• Monitor and maintain permanent erosion protection until fully established.
List any changes from the standard construction sequence outlined above.
2. Construction phasing: If construction is going to occur in separate phases, describe:
F. Construction Schedule
1. Provide a proposed construction schedule (dates construction starts and ends, and
dates for any construction phasing).
Start Date: End Date:
Interim Phasing Dates:
Wet Season Construction Activities: Wet season occurs from October 1 to April 30. Describe
construction activities that will occur during this time period.
NOTE: Additional erosion control methods may be required during periods of increased surface
water runoff.
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2. Site Plan (See attached example)
A site plan, to scale, shall be included with this checklist that shows the following items:
___ a. Address, Parcel Number, Permit Number and Street names
___ b. North Arrow
___ c. Indicate boundaries of existing vegetation (e.g. tree lines, grassy areas, pasture
areas, fields, etc.)
___ d. Identify any on-site or adjacent critical areas and associated buffers (e.g. wetlands,
steep slopes, streams, etc.).
___ e. Identify any FEMA base flood boundaries and Shoreline Management boundaries.
___ f. Show existing and proposed contours.
___ g. Delineate areas that are to be cleared and graded.
___ h. Show all cut and fill slopes, indicating top and bottom of slope catch lines
___ i. Show locations where upstream runon enters the site and locations where runoff
leaves the site.
___ j. Indicate existing surface water flow direction(s).
___ k. Label final grade contours and indicate proposed surface water flow direction and
surface water conveyance systems (e.g. pipes, catch basins, ditches, etc.).
___ l. Show grades, dimensions, and direction of flow in all (existing and proposed) ditches,
swales, culverts, and pipes.
__ m. Indicate locations and outlets of any dewatering systems (usually to sediment trap).
___ n. Identify and locate all erosion control techniques to be used during and after
construction.
See attached: Guidelines for Erosion Control Practices and sample Site Plan.
Onsite field verification of actual conditions is required.
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Nov 4, 2002-c:\projects\storm\small site erosion control.dwg
Figure II-C-38. Sample Erosion and Sediment Control Plan
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Guidelines for Erosion Control Practices
As required by Ecology, this SWPPP must contain the 12 required elements. Check off each
element as it is addressed in the SWPPP Short Form and/or on your site plan.
___ 1. Mark Clearing Limits (orange construction fence, staking with ribbon).
___ 2. Establish Construction Access (gravel entrance, tire wash area).
___ 3. Control Flow Rates (using pipe, drainage swales, berms).
___ 4. Install Sediment Controls (silt fence, sediment traps).
___ 5. Stabilize Soils (mulch, hydroseed, straw).
___ 6. Protect Slopes (divert water from top of slope, cover with plastic or erosion
control blanket).
___ 7. Protect Drain Inlets (catch basin inserts).
___ 8. Stabilize Channels and Outlets (cover with grass, riprap).
___ 9. Control Pollutants (maintain equipment to prevent leaks).
___ 10. Control Dewatering (pump to sediment trap).
___ 11. Maintain BMPs (weekly maintenance/replacement, preparation for storm events).
___ 12. Manage the Project (establish construction schedule, phasing, contact numbers).
Several common erosion control techniques are explained and described in this section. Standard
details for installation of these methods are included in this document. The applicant does not need to
reproduce these drawings, but must indicate where each BMP will be used on a site plan and
indicate which detail will be used. An example site plan and symbols list is provided to assist the
applicant in preparation of their own site plan.
Only those erosion and sediment control techniques most pertinent to small construction sites are
included here. More detailed information on construction BMPs can be found in Volume II of the City
of Auburn Surface Water Management Manual. The BMP numbers referenced are BMPs located in
the City of Auburn SWMM.
For phased construction plans, clearly indicate erosion control methods to be used for each phase of
construction.
Mark Clearing Limits
All construction projects must clearly mark any clearing limits, sensitive areas and their buffers, and
any trees that will be preserved prior to beginning any land disturbing activities, including clearing and
grading. Clearly mark limits both in the field and on the plans. Plastic, metals, or stake wires may be
used to mark the clearing limits. Do not staple or wire fences to trees. See Figure II-3-1 for Stake and
Wire fencing
Applicable BMPs include:
• BMP C101: Preserving Natural Vegetation
• BMP C102: Buffer Zones
• BMP C103: High Visibility Plastic and Wire Fence
• BMP C104: Stake and Wire Fence
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Construction Entrance
All construction projects subject to vehicular traffic shall provide a means of preventing vehicle
“tracking” of soil from the site onto City streets. At a minimum, there shall be a rock pad construction
entrance at every construction access point. Note: The applicant should consider placing the
entrance in the area for future driveway(s), as the rock can be used for driveway base material. The
entrance(s) shall be inspected weekly and if excessive sediment is found, more rock shall be added
to ensure proper functioning.
If sediment is tracked off site, it shall be swept or shoveled from the paved surface on a daily basis.
Washing of the streets to remove the sediment is not permitted because wash water can transport
sediments to streams and other water courses via the City storm drainage system.
The entrance must be identified on the site plan and must conform to Figure II-C-39.
Applicable BMPs include:
• BMP C105: Stabilized Construction Entrance
• BMP C106: Wheel Wash
• BMP C107: Construction Road/Parking Area Stabilization
Sediment Barriers (Figure II-C-41 through Figure II-C-45)
Sediment barriers should be used downslope of disturbed areas. Sediment barriers are intended to
create a barrier to slow the “sheet” flow of stormwater and allow the sediment to settle out behind the
barrier. Do not use sediment barriers in streams, channels, ditches or around inlets/outlets of
culverts. Sediment barriers selected shall be identified on the site plan and must conform to those
shown in Figure II-C-41 through Figure II-C-45.
1. Silt fence
A silt fence is a temporary sediment barrier consisting of filter fabric, attached to supporting posts and
entrenched into the soil. See Figure II-C-41.
2. Berm Barriers
A continuous berm is a temporary diversion dike or sediment barrier. It may be constructed with:
• Soil, sand, or aggregate encased within a geosynthetic fabric
(see Figure II-C-42 and Figure II-C-43).
• Straw wattles (see Figure II-C-44).
• Sand bags (see Figure II-C-45).
Applicable BMPs include:
• BMP C231: Brush Barrier
• BMP C232: Gravel Filter Berm
• BMP C233: Silt Fence
• BMP C234: Vegetated Strip
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• BMP C235: Straw Wattles
Catch Basin Protection (Figure II-C-46 and Figure II-C-47)
To prevent sediment from entering drainage systems prior to site stabilization, install catch basin
protection within onsite and nearby downstream catch basins. Figure II-C-46 and Figure II-C-47 are
acceptable methods of catch basin protection.
NOTE: Only Figure II-C-46 is approved for use in City of Auburn right of way.
Applicable BMPs include:
• BMP C220: Storm Drain Inlet Protection
Water Runoff Containment/Control
As an alternative to or in conjunction with sediment barriers, a combination of drainage swales and
possibly a sediment trap may be used to control runoff and trap sediment before it leaves the
construction site.
1. Sediment traps (Figure II-C-48 and Figure II-C-49)
Sediment traps are small temporary ponds (typically less than 3 feet deep) used to trap sediment
suspended in site runoff before it leaves a construction site. As concentrated surface water pools
within the pond, sediment is allowed to settle out of the water. Typically, a sediment trap will not be
required for small sites as long as concentrated stormwater runoff (swales or ditches) does not occur.
Use Table II-C-13 for sizing your sediment trap.
Table II-C-13. Sediment Trap Sizing
Contributing Area (Acres) Required Surface Area of Pond
(sq. ft.)
1/8 acre or less 130
¼ acre or less 260
½ acre or less 520
¾ acre or less 780
1 acre or less 1040
If expected time of construction or downstream conditions warrant more protection, see BMP C240
for sizing information.
NOTE: If dewatering or significant stormwater runoff is expected, a sediment trap should be used to
settle out solids before discharging to the City system.
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2. Drainage Swales (Figure II-C-50)
Drainage swales are temporary ditches (minimum slope of 0.5% and a maximum of 10%) used to
convey concentrated stormwater flows away from construction activities into a temporary sediment
trap. Drainage swales carrying concentrated flows must discharge into a sediment trap or pond.
Swales should be stabilized with erosion protection (see below). Note: Swales should be completely
stabilized before directing concentrated flows or they themselves will erode.
Applicable BMPs include:
• BMP C240: Sediment Trap
• BMP C201: Grass-Lined Channels
• BMP C202: Channel Lining
• BMP C207: Check Dams
Soil Erosion Protection
Soil erosion protection is applied over the soil surface to reduce erosion from rainfall and wind. It can
also be used to aid the establishment of vegetation. Between October 1st and April 30th, no soils shall
remain exposed for more than 2 days unless they are being actively worked. From April 1st to
September 30th, no soils shall remains exposed for more than 7 days unless they are being actively
worked. See Table II-C-14, Table II-C-15 and Figure II-C-51 through Figure II-C-54.
1. Mulches/Seeding/Hydroseeding (Table II-C-14 and Table II-C-15)
Mulching is the application of a protective layer of straw or other suitable material to the soil surface.
Mulch can be applied to any site where soil has been disturbed and the protective vegetation has
been removed. Materials that may be used for mulching include:
• Straw or hay
• Compost material
• Wood or bark chips
• Hydraulically applied grass seed (Hydroseed)
• Bonded Fiber Matrix
Applicable BMPs include:
• BMP C121: Mulching
• BMP C120: Temporary and Permanent Seeding
• BMP C124: Sodding
• BMP C125: Compost
• BMP C126: Topsoiling
• BMP C130: Surface Roughening
• BMP C140: Dust Control
NOTE: The applicant may wish to mix in grass seed with the above practices to further aid in soil
stabilization. Please refer to Table II-C-14 and Table II-C-15.
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2. Erosion Control Blankets/ Mats (Figure II-C-51)
Erosion control blankets are suited for post-construction site stabilization, but may be used for
temporary stabilization of highly erosive soils. Erosion control blankets are suitable for steep slopes,
stream banks, and areas where vegetation will be slow to establish. These blankets are typically
made from straw, coconut fiber, excelsior, or synthetic material that is enveloped in plastic,
biodegradable netting, jute, polypropylene, or nylon.
Applicable BMPs include:
• BMP C122: Nets and Blankets
3. Gravel/Riprap (Figure II-C-52 and Figure II-C-53)
Gravel and Riprap are used to protect hillsides, drainage channels, stream banks, and pipe outlets
from erosion due to surface water flow.
4. Plastic Sheeting (Figure II-C-54)
Plastic sheeting is a temporary method of erosion control. Plastic covering provides immediate, short-
term erosion protection to slopes, soil stockpiles, and other disturbed areas. Unlike the other erosion
protection techniques mentioned above, plastic sheeting shall be removed prior to applying
permanent erosion protection.
Applicable BMPs include:
• BMP C123: Plastic Covering
Protect Slopes
Design, construct and phase projects in a manner that will minimize erosion. Protect slopes by
diverting water at the top of the slope. Reduce slope velocities by minimizing the continuous length of
slope. This can be accomplished by terracing and roughening slope sides. Seeding and establishing
vegetation on slopes will help protect slopes as well.
Applicable BMPs include:
• BMP C120: Temporary and Permanent Seeding
• BMP C130: Surface Roughening
• BMP C131: Gradient Terraces
• BMP C200: Interceptor Dike and Swale
• BMP C204: Pipe Slope Drains
Control Pollutants Other Than Sediment
All pollutants must be disposed of in a manner that does not cause contamination of surface waters.
Do not maintain or repair any heavy equipment or vehicles onsite. Clean any spills immediately.
Handle concrete and concrete waste appropriately.
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Applicable BMPs include:
• BMP C150: Materials on Hand
• BMP C151: Concrete Handling
• BMP C152: Sawcutting and Surfacing Pollution Prevention
• BMP C153: Materials Delivery, Storage and Containment
• BMP C154: Concrete Washout Area
Control Dewatering
All discharges to the City sewer system require City and King County approval. This approval
process may be initiated by contacting the City. The City will coordinate the request for a letter of
authorization from the King County Wastewater Treatment Division.
Any dewatering water must be discharged through a stabilized channel to a sediment pond.
Maintain BMPs
Maintain and repair temporary erosion and sediment control BMPs as needed. Inspect all BMPs at
least weekly and after every storm event. Remove all temporary erosion and sediment control BMPs
within 30 days after final site stabilization.
Table II-C-14. Temporary Erosion Control Seed Mix
% Weight % Purity % Germination
Chewings or annual bluegrass
Festuca rubra var. commutate or Poa anna 40 98 90
Perennial rye
Lolium perenne 50 98 90
Redtop or colonial bentgrass
Agrostis alba or Agrostis tenuis 5 92 85
White Dutch clover
Trifolium repens 5 98 90
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Table II-C-15. Mulch Standards and Guidelines
Mulch
Material
Quality
Standards
Application
Rates
Remarks
Straw Air-dried; free from
undesirable seed and
coarse material.
3” thick; 5 bales
per 1000 sf or 2
to 3 tons per
acre.
Cost-effective protection when applied with adequate thickness.
Hand-application generally requires greater thickness than blown
straw. The thickness of straw may be reduced by half when used
in conjunction with seeding. In windy areas, straw must be held in
place by crimping, using a tackifier, or covering with netting.
Blown straw always has to be held in place with a tackifier as
even light winds will blow it away. Straw, however, has several
deficiencies that should be considered when selecting mulch
materials. If often introduces and/or encourages the propagation
of weed species and it has no significant long-term benefits.
Straw should be used only if mulches with long-term benefits are
unavailable locally. It should also not be used within the ordinary
high-water elevation of surface waters (due to flotation).
Hydro-
mulch
No growth inhibiting
factors.
Approx. 25-30
lbs per 1000 sf
or 1500-2000
lbs per acre.
Shall be applied with hydromulcher. Shall not be used without
seed and tackifier unless the application rate is at least doubled.
Fivers longer than about ¾ - 1 inch clog hydromulch equipment.
Fibers should be kept to less than ¾ inch.
Composted
Mulch and
Compost
No visible water or
dust during handling.
Must be purchased
from supplier with a
Solid Waste Handling
permit (unless
exempt)
3” thick, min.;
approx. 100
tons per acre
(approx. 800
lbs. per yard).
More effective control can be obtained by increasing thickness to
3”. Excellent mulch for protecting final grades until landscaping
because it can be directly seeded or tilled into soil as an
amendment. Composted mulch has a coarser size gradation
than compost. It is more stable and practical to use in wet areas
and during rainy weather conditions.
Chipped
Site
Vegetation
Average size shall be
several inches.
Gradations from fine to
6-inches in length for
texture, variation, and
interlocking properties.
3” minimum
thickness
This is a cost-effective way to dispose of debris from clearing and
grubbing, and it eliminates the problems associated with burning.
Generally, it should not be used on slopes above approx. 10%
because of its tendency to be transported by runoff. It is not
recommended within 200 feet of surface waters. If seeding is
expected shortly after mulch, the decomposition of the chipped
vegetation may tie up nutrients important to grass establishment.
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Figure II-C-39. Construction Entrance
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Figure II-C-40. Stake and Wire Fence
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Figure II-C-41. Sediment Barrier – Silt Fence
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Figure II-C-42. Sediment Barrier – Triangular Sediment Filter Dikes
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Figure II-C-43. Sediment Barrier – Geosynthetic Dike
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Figure II-C-44. Sediment (Berm) Barrier – Straw Wattle Rolls
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Figure II-C-45. Sediment (Berm) Barrier – Sandbag Berm
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Figure II-C-46. Catch Basin Protection – Bag Filter
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Figure II-C-47. Catch Basin Protection – Inlet Gravel and Filter Fabric
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Figure II-C-48. Water Runoff Containment/Control – Sediment Trap Cross-Section
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Figure II-C-49. Water Runoff Containment/Control – Sediment Trap Outlet
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Figure II-C-50. Water Runoff Containment/Control – Drainage Swale Cross-Sections
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Figure II-C-51. Soil Erosion Protection – Erosion Blankets and Turf Reinforcement Mats
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Figure II-C-52. Soil Erosion Protection – Rip Rap Protection
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Figure II-C-53. Soil Erosion Protection – Pipe Slope Drains
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Figure II-C-54. Soil Erosion Protection – Tarp Covering
Volume III
i Table of Contents
Volume III – Surface Water
Quantity Control and Conveyance
Table of Contents
Purpose of this Volume...................................................................................................................307
Content and Organization of this Volume.......................................................................................307
Chapter 1 Hydrologic Analysis......................................................................................308
1.1 Minimum Computational Standards......................................................................................308
1.2 Western Washington Hydrology Model.................................................................................308
1.3 Single-Event Hydrograph Method.........................................................................................309
1.3.1 Design Storm...................................................................................................................309
1.3.2 Curve Number.................................................................................................................309
1.4 Closed Depression Analysis..................................................................................................310
Chapter 2 Flow Control Design......................................................................................312
2.1 Roof Downspout Controls......................................................................................................312
2.1.1 Selection of Roof Downspout Controls............................................................................312
2.1.1.1 Roof Downspout Controls in Potential Landslide Hazard Areas...........................313
2.1.2 Downspout Infiltration Systems.......................................................................................313
2.1.2.1 Application.............................................................................................................313
2.1.2.2 Flow Credit for Roof Downspout Infiltration..........................................................313
2.1.2.3 Procedure for Evaluating Feasibility......................................................................313
2.1.2.4 Design Criteria for Infiltration Trenches.................................................................314
2.1.3 Downspout Dispersion Systems......................................................................................319
2.1.3.1 Application.............................................................................................................319
2.1.3.2 Flow Credit for Roof Downspout Dispersion.........................................................319
2.1.3.3 General Design Criteria.........................................................................................319
2.1.4 Bioinfiltration “Rain Gardens”..........................................................................................321
2.1.5 Collect and Convey..........................................................................................................326
2.2 Infiltration Facilities for Stormwater Flow Control..................................................................327
2.2.1 Purpose............................................................................................................................327
2.2.2 Description.......................................................................................................................327
2.2.3 Application.......................................................................................................................327
2.2.4 Design Methodology........................................................................................................329
2.2.5 Simplified Approach.........................................................................................................329
2.2.6 Detailed Approach...........................................................................................................330
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2.2.7 Site Characterization Criteria...........................................................................................334
2.2.7.1 Surface Features Characterization.......................................................................335
2.2.7.2 Subsurface Characterization.................................................................................335
2.2.7.3 Infiltration Rate Determination...............................................................................335
2.2.7.4 Soil Testing............................................................................................................336
2.2.7.5 Infiltration Receptor...............................................................................................337
2.2.8 Design Infiltration Rate Determination – Guidelines and Criteria....................................338
2.2.9 Three Methods for Determining Long-term Infiltration Rates for Sizing Infiltration
Facilities...........................................................................................................................339
2.2.10 Site Suitability Criteria (SSC)...........................................................................................343
2.2.10.1 SSC-1 Setback Criteria.........................................................................................344
2.2.10.2 SSC-2 Groundwater Protection Areas..................................................................344
2.2.10.3 SSC-3 High Vehicle Traffic Areas.........................................................................344
2.2.10.4 SSC-4 Drawdown Time.........................................................................................345
2.2.10.5 SSC-5 Depth to Bedrock, Water Table, or Impermeable Layer............................345
2.2.10.6 SSC-6 Seepage Analysis and Control..................................................................345
2.2.10.7 SSC-7 Cold Climate and Impact of Roadway Deicers..........................................345
2.2.10.8 SSC-8 Verification Testing of the Completed Facility...........................................345
2.2.11 Design Criteria for Infiltration Facilities............................................................................345
2.2.12 Construction Criteria........................................................................................................346
2.2.13 Maintenance Criteria........................................................................................................347
2.2.14 Verification of Performance.............................................................................................347
2.2.15 Infiltration Basins.............................................................................................................347
2.2.16 Infiltration Trenches.........................................................................................................349
2.2.16.1 Description:...........................................................................................................349
2.2.16.2 Design Criteria.......................................................................................................349
2.2.16.3 Construction Criteria..............................................................................................350
2.2.16.4 Maintenance Criteria.............................................................................................351
2.3 Detention Facilities................................................................................................................351
2.3.1 Detention Ponds..............................................................................................................351
2.3.1.1 Dam Safety for Detention BMPs...........................................................................351
2.3.1.2 Design Criteria.......................................................................................................352
2.3.1.3 Methods of Analysis..............................................................................................366
2.3.2 Detention Tanks...............................................................................................................367
2.3.2.1 Design Criteria.......................................................................................................370
2.3.3 Detention Vaults..............................................................................................................371
2.3.3.1 Design Criteria.......................................................................................................373
2.3.3.2 Methods of Analysis..............................................................................................375
2.3.4 Control Structures............................................................................................................375
2.3.4.1 Design Criteria.......................................................................................................375
2.3.4.2 Maintenance..........................................................................................................376
2.3.4.3 Methods of Analysis..............................................................................................377
2.3.5 Other Detention Options..................................................................................................386
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Chapter 3 Conveyance System Design and Hydraulic Analysis.................................387
3.1 Conveyance System Analysis Requirements........................................................................387
3.1.1 On-site Analysis...............................................................................................................387
3.1.2 Offsite Analysis (1/4 mile Downstream Analysis)............................................................388
3.2 Design Event.........................................................................................................................388
3.2.1 Additional Design Criteria................................................................................................389
3.3 Methods of Analysis..............................................................................................................389
3.3.1 Rational Method...............................................................................................................390
3.3.1.1 Rational Method Equation.....................................................................................390
3.4 Pipes, Culverts and Open Channels.....................................................................................396
3.4.1 Pipe Systems...................................................................................................................396
3.4.1.1 Design Flows.........................................................................................................396
3.4.1.2 Conveyance Capacity...........................................................................................397
3.4.1.3 Backwater Analysis...............................................................................................402
3.4.1.4 Inlet Grate Capacity...............................................................................................406
3.4.1.5 Pipe Materials........................................................................................................406
3.4.1.6 Pipe Sizes.............................................................................................................406
3.4.1.7 Changes in Pipe Sizes..........................................................................................406
3.4.1.8 Pipe Alignment and Depth....................................................................................406
3.4.1.9 Pipe Slopes and Velocities....................................................................................407
3.4.1.10 Pipes on Steep Slopes..........................................................................................407
3.4.1.11 Structures..............................................................................................................408
3.4.1.12 Pipe Clearances....................................................................................................410
3.4.1.13 Pipe Cover.............................................................................................................411
3.4.1.14 System Connections.............................................................................................411
3.4.1.15 Debris Barriers......................................................................................................412
3.4.2 Culverts............................................................................................................................413
3.4.2.1 Design Event.........................................................................................................413
3.4.2.2 Design Flows.........................................................................................................413
3.4.2.3 Headwater.............................................................................................................413
3.4.2.4 Conveyance Capacity...........................................................................................413
3.4.2.5 Inlet Control Analysis.............................................................................................413
3.4.2.6 Outlet Control Analysis..........................................................................................418
3.4.2.7 Inlets and Outlets..................................................................................................424
3.4.3 Open Channels................................................................................................................424
3.4.3.1 Natural Channels...................................................................................................424
3.4.3.2 Constructed Channels...........................................................................................424
3.4.3.3 Design Flows.........................................................................................................426
3.4.3.4 Conveyance Capacity...........................................................................................431
3.4.3.5 Manning’s Equation for Preliminary Sizing............................................................432
3.4.3.6 Direct Step Backwater Method..............................................................................432
3.4.3.7 Standard Step Backwater Method........................................................................436
3.4.3.8 Computer Applications..........................................................................................436
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3.4.3.9 Riprap Design........................................................................................................436
3.5 Outfalls Systems....................................................................................................................440
3.5.1 Outfall Design Criteria......................................................................................................440
3.5.1.1 Energy dissipation.................................................................................................441
3.5.1.2 Flow dispersion.....................................................................................................441
3.5.2 Tightline Systems............................................................................................................448
3.5.3 Habitat Considerations....................................................................................................448
3.6 Pump Systems.......................................................................................................................448
3.6.1 Design Criteria.................................................................................................................449
3.6.2 Pump Requirements........................................................................................................449
3.6.3 Additional Requirements..................................................................................................449
3.6.4 Sump Pumps...................................................................................................................450
3.7 Easements and Access........................................................................................................450
3.7.1 Public Easements............................................................................................................450
3.7.2 Private Easements...........................................................................................................451
3.7.3 Maintenance Access........................................................................................................451
Appendix A Auburn Design Storm....................................................................................453
Appendix B Procedure for Conducting a Pilot Infiltration Test.......................................454
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Purpose Volume III
Content and Organization Introduction 307
Volume III:
Surface Water Quantity
Control and Conveyance
Purpose of this Volume
The purpose of this volume is to outline methods for calculating and designing methods to control the
quantity of surface water runoff at developed sites. Quantity controls and on-site management for
roof downspouts are described. Design criteria and methods of analysis for flow control BMPs are
presented. Conveyance system requirements and design methods are also presented
Content and Organization of this Volume
Volume III of this manual contains three chapters and two appendices.
Chapter 1 reviews methods of hydrologic analysis.
Chapter 2 describes flow control design.
Chapter 3 describes the requirements for analysis and design of surface water
conveyance systems.
Appendix A provides the Auburn Design Storm precipitation values.
Appendix B describes the procedure for a Pilot Infiltration Test.
Volume
III
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Chapter 1 Hydrologic Analysis
The purpose of this chapter is to define the minimum computational standards required, and outline
how these computational standards may be applied.
1.1 Minimum Computational Standards
The minimum computational standards depend on the type of information required and the size of
the drainage area to be analyzed, as follows:
The most current software version of the Department of Ecology’s Western
Washington Hydrology Model (WWHM) model shall be used. Alternative models for
sizing flow control and water quality facilities may be considered, provided they are
Washington State Department of Ecology equivalent, and approved by the City of
Auburn. Approval from the City shall be obtained prior to submittal of design
documents.
Model calibration shall be required for basins greater than 320 acres.
Exception: The Santa Barbara Urban Hydrograph method (SBUH) may be used to determine a
water quality design storm volume for wetpond treatment facilities only.
Table III-1-1 summarizes the circumstances under which different design methodologies apply.
Table III-1-1. BMP Designs in Western Washington
Method Treatment Flow Control
Standard Continuous Runoff
Model (WWHM or
approved equivalent)
Method applies to all
BMPs.
Method applies throughout
Auburn where flow control is
required.
Alternative SBUH Wetpool water quality
treatment facilities only.
Acceptable for City storm
drainage system capacity
problems.
1.2 Western Washington Hydrology Model
For most flow control design purposes, a continuous runoff model, such as the Western Washington
Hydrology Model (WWHM), must be used. Information on the WWHM is provided in the Stormwater
Management Manual for Western Washington (Washington State Department of Ecology, 2005).
The software can be downloaded at the following website:
http://www.ecy.wa.gov/programs/wq/stormwater/wwhmtraining/index.html
More WWHM information is available at http://www.clearcreeksolutions.com
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Note: Pre-developed conditions shall be modeled as a forested land cover with either outwash
(Hydrologic Soil Group A/ B) or till (Hydrologic Soil Group C/D) soils. Saturated soil conditions shall
only be considered when determining existing wetland hydrology.
1.3 Single-Event Hydrograph Method
Hydrograph analysis with a single event hydrograph method utilizes the standard plot of runoff flow
versus time for a given design storm, allowing the key characteristics of runoff such as peak, volume,
and phasing to be considered in the design of drainage facilities. Single event methods are only
acceptable for sizing wetpool treatment facilities or for determining pipe capacity.
All storm event hydrograph methods require input of parameters that describe physical drainage
basin characteristics. These parameters provide the basis from which the runoff hydrograph is
developed.
1.3.1 Design Storm
The total depth of rainfall for storms of 24-hour duration and 2, 5, 10, 25, 50, and 100-year recurrence
intervals are published by the National Oceanic and Atmospheric Administration (NOAA). The
information is presented in the form of “isopluvial” maps for each state. Isopluvial maps are maps
where the contours represent total inches of rainfall for a specific duration. Isopluvial maps for the 2,
5, 10, 25, 50, and 100-year recurrence interval and 24-hour duration storm events can be found in
the NOAA Atlas 2, “Precipitation - Frequency Atlas of the Western United States, Volume IX-
Washington.” Based on these isopluvials, the following design storms shall be used for the City of
Auburn:
6-month, 24-hour design storm: 1.44 inches
2-year, 24-hour design storm: 2.0 inches
10-year, 24-hour design storm: 3.0 inches
100-year, 24-hour design storm: 4.0 inches
1.3.2 Curve Number
Surface soils are classified by the National Resource Conservation Service into four hydrologic soil
groups based on the soil’s runoff potential: A, B, C, and D. Group A soils generally have the lowest
runoff potential while Group D soils have the highest. In Auburn the valley floor is mostly Group D
soils, which typically have very low infiltration rates and high runoff potential. The West Hill, Lea Hill,
and Lakeland Hills areas are predominately Group C soils, which have low infiltration rates and
moderate to high runoff potential. The southeast area, Bowman Creek area, and valley area located
between Highway 18 and the White River contain some Group A soils, which are characterized by
high infiltration rates and low runoff potential. Soils within the City limits shall be assumed to fall in the
Hydrologic Soils Groups as shown in figure 4-4 of the City of Auburn Comprehensive Drainage Plan
unless grain size distribution and/or permeability testing indicate otherwise. Refer to Section 2.2.7.4
for details on appropriate soil testing methods.
Table III-1-2 shows the curve numbers (CNs), by land use description, for the four hydrologic soil
groups. These numbers are for a 24-hour duration storm and the typical antecedent soil moisture
condition preceding 24-hour storms.
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The following are important criteria/considerations for selection of CN values.
Many factors may affect the CN value for a given land use. For example, the movement of heavy
equipment over bare ground may compact the soil so that it has a lesser infiltration rate and greater
runoff potential than would be indicated by strict application of the CN value to developed site
conditions.
CN values can be area weighted when they apply to pervious areas of similar CNs (within 20 CN
points). However, high CN areas should not be combined with low CN areas. In this case, separate
estimates of S (potential maximum natural detention) and Qd (runoff depth) should be generated and
summed to obtain the cumulative runoff volume unless the low CN areas are less than 15 percent of
the sub-basin.
Separate CN values must be selected for the pervious and impervious areas of an urban basin or
sub-basin. For residential districts, the percent impervious area given in Table III-1-2 must be used to
compute the respective pervious and impervious areas. For proposed commercial areas, plats, etc.,
the percent impervious area must be computed from the site plan. For all other land uses, the
percent impervious area must be estimated from best available aerial topography and/or field
reconnaissance. The pervious area CN value must be a weighted average of all the pervious area
CNs within the sub-basin. The impervious area CN value shall be 98.
1.4 Closed Depression Analysis
The analysis of closed depressions requires careful assessment of the existing hydrologic
performance in order to evaluate the impacts of a proposed project. A calibrated continuous
simulation hydrologic model must be used for closed depression analysis and design of mitigation
facilities. The applicable requirements of this manual (see Minimum Requirement #7 and #8) and the
City’s Critical Areas Ordinance and Rules should be thoroughly reviewed prior to proceeding with the
analysis.
Closed depressions generally facilitate infiltration of runoff. If a closed depression is classified as a
wetland, then Minimum Requirement #8 for wetlands applies. If there is an outflow from the wetland
to a surface water (such as a creek), then the flow from the wetland must also meet Minimum
Requirement #7 for flow control. If a closed depression is not classified as a wetland, the ponding
area at the bottom of the closed depression should be modeled as an infiltration pond.
Guidance for modeling closed depressions and model calibration shall be provided by the
Department of Public Works.
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Table III-1-2. Runoff Curve Numbers for Selected Agricultural, Suburban and Urban Areas
(Sources: TR 55, 1986, and Stormwater Management Manual, 1992. See Section 2.1.1 for explanation)
CNs for hydrologic soil group
Cover type and hydrologic condition. A B C D
Curve Numbers for Pre-Development Conditions
Pasture, grassland, or range-continuous forage for grazing:
Fair condition (ground cover 50% to 75% and not heavily grazed). 49 69 79 84
Good condition (ground cover >75% and lightly or only occasionally grazed) 39 61 74 80
Woods:
Fair (Woods are grazed but not burned, and some forest litter covers the soil). 36 60 73 79
Good (Woods are protected from grazing, and litter and brush adequately cover the soil). 30 55 70 77
Curve Numbers for Post-Development Conditions
Open space (lawns, parks, golf courses, cemeteries, landscaping, etc.)1
Fair condition (grass cover on 50% - 75% of the area). 77 85 90 92
Good condition (grass cover on >75% of the area) 68 80 86 90
Impervious areas:
Open water bodies: lakes, wetlands, ponds etc. 100 100 100 100
Paved parking lots, roofs2, driveways, etc. (excluding right-of-way) 98 98 98 98
Permeable Pavement (see Appendix C of 2005 Ecology Manual to decide which condition applies)
Landscaped area 77 85 90 92
50% landscaped area/50% impervious 87 91 94 96
100% impervious area 98 98 98 98
Paved 98 98 98 98
Gravel (including right-of-way) 76 85 89 91
Dirt (including right-of-way) 72 82 87 89
Pasture, grassland, or range-continuous forage for grazing:
Poor condition (ground cover <50% or heavily grazed with no mulch). 68 79 86 89
Fair condition (ground cover 50% to 75% and not heavily grazed). 49 69 79 84
Good condition (ground cover >75% and lightly or only occasionally grazed) 39 61 74 80
Woods:
Poor (Forest litter, small trees, brush are destroyed by heavy grazing or regular burning). 45 66 77 83
Fair (Woods are grazed but not burned, and some forest litter covers the soil). 36 60 73 79
Good (Woods are protected from grazing, and litter and brush adequately cover the soil). 30 55 70 77
Single family residential3: Should only be used for Average Percent
Dwelling Unit/Gross Acre subdivisions > 50 acres impervious area3,4
1.0 DU/GA 15 Separate curve number
1.5 DU/GA 20 shall be selected for
2.0 DU/GA 25 pervious & impervious
2.5 DU/GA 30 portions of the site or
3.0 DU/GA 34 basin
3.5 DU/GA 38
4.0 DU/GA 42
4.5 DU/GA 46
5.0 DU/GA 48
5.5 DU/GA 50
6.0 DU/GA 52
6.5 DU/GA 54
7.0 DU/GA 56
7.5 DU/GA 58
PUDs, condos, apartments, commercial %impervious Separate curve numbers shall
businesses, industrial areas & must be be selected for pervious and
& subdivisions < 50 acres computed impervious portions of the site
For a more detailed and complete description of land use curve numbers refer to chapter two (2) of the Soil Conservation Service’s
Technical Release No. 55, (210-VI-TR-55, Second Ed., June 1986).
1 Composite CNs may be computed for other combinations of open space cover type. 2Where roof runoff and driveway runoff are infiltrated or dispersed according to the requirements in Chapter 2, the average percent
impervious area may be adjusted in accordance with the procedure described under “Flow Credit for Roof Downspout Infiltration”
and “Flow Credit for Roof Downspout Dispersion” in Volume 6, Chapter 2. 3Assumes roof and driveway runoff is directed into street/storm system. 4All the remaining pervious area (lawn) is considered to be in good condition for these curve numbers.
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Chapter 2 Flow Control Design
2.1 Roof Downspout Controls
This section presents the criteria for design and implementation of roof downspout controls. Roof
downspout controls are simple pre-engineered designs for infiltrating and/or dispersing runoff from
roof areas for the purposes of increasing opportunities for groundwater recharge and reduction of
runoff volumes from new development or redevelopment.
For roof areas below 10,000 square feet, these designs may typically be implemented with a single
test pit, unless directed otherwise by the City. For designs other than those presented in Section 2.1,
the requirements of Section 2.2 shall apply.
Roof downspout controls are used in conjunction with, and in addition to, any additional flow control
facilities that may be necessary to mitigate stormwater impacts from the overall development.
Implementation of roof downspout controls may reduce the total effective impervious area and result
in less runoff from these surfaces. Flow credits for implementing infiltration and dispersion for controls
are available as follows:
• If all the roof runoff is infiltrated according to the requirements of this section, the roof
• area may be discounted from the total project area used for determining project
thresholds and sizing stormwater facilities.
• If roof runoff is dispersed according to the requirements of this section on lots greater
than 22,000 square feet and the vegetative flow path is 50 feet or longer through
undisturbed native landscape or lawn/landscape area that meets BMP L613, the roof
area may be modeled as grassed surface.
Additional information on flow credits is available in Volume VI, Chapter 2.
2.1.1 Selection of Roof Downspout Controls
Large lots in rural areas (5 acres or greater) typically have enough area to disperse or infiltrate roof
runoff. Lots created in urban areas will typically be smaller and have a limited amount of area
in which to infiltrate or disperse stormwater. Downspout infiltration may be used in those soils
that readily infiltrate (coarse sands and cobbles to medium sands). Dispersion BMPs may be used
for urban lots located in less permeable soils, where infiltration is not feasible. Where infiltration
and/or dispersion is not feasible because of very small lot size, or where there is a potential
for creating drainage problems on adjacent lots, downspouts shall be connected to the street
storm drain system, which directs the runoff to a regional facility or receiving water.
Where roof downspout controls are planned, the following methods should be considered in
descending order of preference:
• Rain gardens (Section 2.1.4)
• Downspout infiltration systems (Section 2.1.2)
• Downspout dispersion systems (Section 2.1.3)
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• Collect and convey to the City stormwater system – only allowed if it can be
demonstrated that infiltration and dispersion are not feasible.
2.1.1.1 Roof Downspout Controls in Potential Landslide Hazard Areas
If or where the City has identified “geologically hazardous areas” (WAC 365-195-410), lots
immediately adjacent to or within the hazard area shall collect roof runoff in a tightline system which
conveys the runoff to the City system or to the base of the slope and then into the City system.
Easements across adjacent properties may be necessary to convey drainage to the City system.
2.1.2 Downspout Infiltration Systems
Downspout infiltration systems are trenches designed for flow control and are intended only for use in
infiltrating runoff from roof downspout drains. They are not designed to infiltrate directly runoff from
pollutant-generating impervious surfaces. Volume V, Chapter 5 contains a discussion of infiltration
trenches for water quality treatment.
2.1.2.1 Application
Use downspout infiltration on all sites that meet feasibility and setback requirements.
2.1.2.2 Flow Credit for Roof Downspout Infiltration
If roof runoff is infiltrated according to the requirements of this section, the roof area may be
discounted from the project area used for determining project thresholds and sizing stormwater
facilities.
2.1.2.3 Procedure for Evaluating Feasibility
A soils report to determine if soils suitable for infiltration are present on the site shall be prepared by a
professional soil scientist certified by the Soil Science Society of America (or an equivalent national
program), a locally licensed onsite sewage designer, other suitably trained professional engineer,
geologist, hydrogeologist, or engineering geologist registered in the State of Washington, or persons
working under the supervision of one of the soils professional listed here.
NOTE: On sites where soils are insufficient for infiltration, a downspout dispersion system per
Section 2.1.3 may be feasible in lieu of infiltration.
1. Where downspout infiltration is being proposed, additional site-specific testing must be done.
For single lots, at least one soils log at the location of the infiltration system is required. It
must be a minimum of 4 feet deep (from proposed grade). Identify the SCS series of the soil
and the USDA textural class of the soil horizon through the depth of the log, and note any
evidence of high groundwater level, such as mottling.
2. If site-specific tests indicate less than 3 feet of permeable soil from the proposed final grade
to the seasonal high groundwater table, a downspout infiltration system is not feasible and a
downspout dispersion system per Section 2.1.3 may be feasible in lieu of infiltration.
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3. On lots or sites with more than 3 feet of permeable soil from the proposed final grade to the
seasonal high groundwater table, downspout infiltration is considered feasible if the soils are
outwash type soils and the infiltration trench can be designed to meet the minimum design
criteria specified below. Under no circumstances shall the seasonal high groundwater table
be less then 1 foot from the bottom of the infiltration trench.
2.1.2.4 Design Criteria for Infiltration Trenches
Figure III-2-1 shows a typical downspout infiltration trench system, and Figure III-2-2 presents an
alternative infiltration trench system for sites with coarse sand and cobble soils. These systems are
designed as specified below. Alternate trench lengths require modeling per Section 2.2.
General
1. The following minimum lengths (in linear feet [LF]) per 1,000 square feet of roof area based
on soil type may be used for sizing downspout infiltration trenches.
Coarse sands and cobbles 20 LF
Medium sand 30 LF
Fine sand, loamy sand 75 LF
Sandy loam 125 LF
Loam 190 LF
2. Maximum length of trench must not exceed 100 feet from the inlet sump.
3. Minimum spacing between trenches shall be 4 feet measured from the edge of trench.
4. Filter fabric must be placed over the drain rock as shown on Figure III-2-1 prior to backfilling.
5. Three feet of permeable soil, measured from the proposed final grade to the seasonal high
groundwater table is required.
6. A minimum of 1 foot of separation is required from the bottom of the infiltration trench to the
seasonal high groundwater table.
7. Infiltration trenches may be placed in fill material if the fill is placed and compacted under the
direct supervision of a geotechnical engineer or professional civil engineer with geotechnical
expertise, and if the measured infiltration rate is at least 8 inches per hour. Trench length in fill
must be 60 linear feet per 1,000 square feet of roof area. Infiltration rates can be tested using
the methods described in Section 2.2.7.3.
8. Infiltration trenches shall not be built on slopes steeper than 20 percent (5H:1V). A
geotechnical analysis and report shall be required on slopes over 15 percent or if located
within 200 feet of the top of steep slope (40% or greater) or landslide hazard area. More
stringent setbacks may be required as described in Auburn City Code.
9. Trenches may be located under pavement if a small yard drain or catch basin with grate
cover is placed at the end of the trench pipe such that overflow would occur out of the catch
basin at an elevation at least one foot below that of the pavement, and in a location which can
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accommodate the overflow without creating a significant adverse impact to downhill
properties or drainage systems. This is intended to prevent saturation of the pavement in the
event of system failure.
Setbacks
The City requires specific setbacks for sites with steep slopes, landslide areas, open water features,
springs, wells, and septic tank drain fields. Adequate room for maintenance access and equipment
shall also be considered. Project proponents should consult Auburn City Code to determine all
applicable setback requirements. Where a conflict between setbacks occurs, the City shall require
compliance with the most stringent of the setback requirements from the various codes/regulations.
Required setbacks are as follows or as determined by a qualified geotechnical engineer:
• Minimum spacing between trenches shall be 4 feet measured from the edge of
trench.
• Stormwater infiltration facilities shall be set back at least 100 feet from drinking water
wells and springs used for public drinking water supplies. Stormwater infiltration
facilities shall be set back at least 10 feet from septic tanks and septic drainfields.
Additional setbacks from DOH publication 333-117 On-Site Sewage Systems,
Chapter 246-272A WAC may apply. Infiltration facilities upgradient of drinking water
supplies and within 1, 5, and 10-year time of travel zones must comply with Health
Department requirements (Washington Wellhead Protection Program, DOH,
Publication # 331-018).
• All infiltration systems shall be at least 10 feet from any structure. If necessary,
setbacks shall be increased from the minimum 10 feet in order to maintain a 1H:1V
side slope for future excavation and maintenance.
• All infiltration systems shall be placed at least 5 feet from any property line. If
necessary, setbacks shall be increased from the minimum 5 feet in order to maintain
a 1H:1V side slope for future excavation and maintenance.
• Infiltration systems shall be setback from sensitive areas, steep slopes, landslide
hazard areas, and erosion hazard areas as governed by Auburn City Code. Runoff
discharged near landslide hazard areas must be evaluated by a geotechnical
engineer or qualified geologist licensed in Washington State. The discharge point
shall not be placed on or above slopes greater than 20% (5H:1V) or above erosion
hazard areas without evaluation by a geotechnical engineer or qualified geologist
and City approval. Infiltration trenches should not be built on slopes steeper than
20%.
• For sites with septic systems, infiltration systems shall be downgradient of the
drainfield unless the site topography clearly prohibits surface flows from intersecting
the drainfield.
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Figure III-2-1. Typical Downspout Infiltration Trench
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Figure III-2-2. Alternative Downspout Infiltration Trench System for Coarse Sand and Gravel
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Figure III-2-3. Typical Downspout Dispersion Trench
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2.1.3 Downspout Dispersion Systems
Downspout dispersion systems are splash blocks or dispersion facilities that spread roof runoff over
vegetated pervious areas. Dispersion attenuates peak flows by slowing entry of the runoff into the
conveyance system, allowing for some infiltration, and providing some water quality benefits. Also
refer to BMP L610, Downspout Dispersion, in Volume VI.
2.1.3.1 Application
Downspout dispersion may be used on all sites that cannot infiltrate roof runoff and that meet the
feasibility and setback requirements.
2.1.3.2 Flow Credit for Roof Downspout Dispersion
If roof runoff is dispersed according to the requirements of this section, and the vegetative flow1 path
of the roof runoff is 50 feet or greater through undisturbed native landscape or lawn/landscape area
that meets BMP L613, the roof area may be modeled as a grassed surface for both threshold
determination and modeling.
2.1.3.3 General Design Criteria
• Downspout dispersion trenches designed as shown in Figure III-2-3 should be used for
all downspout dispersion applications except where splash blocks are allowed.
• Perforated stub-out connections shall not be used.
• For sites with septic systems, the discharge point of all dispersion systems must be
downgradient of the drainfield. This requirement may be waived if site topography
clearly prohibits flows from intersecting the drainfield.
• For sites with septic systems, the discharge point must be downslope of the primary
and reserve drainfield areas. This requirement may be waived if site topography
clearly prohibits flows from intersecting the drainfield or where site conditions (soil
permeability, distance between systems, etc) indicate that this is unnecessary.
• Place all dispersion systems at least 5 feet from any property line. If necessary,
setbacks shall be increased from the minimum 5 feet in order to maintain a 1:1 side
slope for future excavation and maintenance.
• Setback dispersion systems from sensitive areas, steep slopes, landslide hazard
areas, and erosion hazard areas as governed by Auburn City Code.
• All dispersions systems shall be at least 10 feet from any structure. If necessary,
setbacks shall be increased from the minimum 10 feet in order to maintain a 1H:1V
side slope for future excavation and maintenance.
• No erosion or flooding of downstream properties shall result.
1 Vegetative flow path is measured from the downspout or dispersion system discharge point perpendicular to the
topographic contours to the downstream property line, stream, wetland, or other impervious surface.
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• Runoff discharged towards landslide hazard areas must be evaluated by a
geotechnical engineer or a licensed geologist, hydrogeologist, or engineering
geologist. The discharge point shall not be placed on or above slopes greater than
20% (5H:1V) or above erosion hazard areas without evaluation by a geotechnical
engineer or qualified geologist and City approval.
Design Criteria for Dispersion Trenches
• A vegetated flowpath of at least 25 feet in length must be maintained between the
outlet of the trench and any property line, structure, stream, wetland, or impervious
surface. A vegetated flowpath of at least 50 feet in length must be maintained between
the outlet of the trench and any slope, 20% or greater. Sensitive area buffers may
count towards flowpath lengths if approved by the City of Auburn.
• Trenches serving up to 700 square feet of roof area may be simple 10-foot-long by
2-foot-wide gravel filled trenches as shown in Figure III-2-3. For roof areas larger than
700 square feet, a dispersion trench with notched grade board may be used as
approved by the City. The total length of this design must not exceed 50 feet and must
provide at least 10 feet of trench per 700 square feet of roof area.
• Dispersion systems shall be setback from sensitive areas, steep slopes, landslide
hazard areas, and erosion hazard areas as governed by Auburn City Code.
Design Criteria for Splashblocks
A typical downspout splashblock is shown in Volume VI section 2.2.1.1 BMP L610 Downspout
Dispersion. In general, if the ground is sloped away from the foundation and there is adequate
vegetation and area for effective dispersion, splashblocks will adequately disperse storm runoff. If the
ground is fairly level, if the structure includes a basement, or if foundation drains are proposed,
splashblocks with downspout extensions may be a better choice because the discharge point is
moved away from the foundation. Downspout extensions can include piping to a
splashblock/discharge point a considerable distance from the downspout, as long as the runoff can
travel through a well-vegetated area as described below.
• A vegetated flow path of at least 50 feet shall be maintained between the discharge
point and any property line, structure, steep slope, stream, wetland, lake, or other
impervious surface. Sensitive area buffers may count toward flow path lengths. The
minimum spacing between splash blocks shall be 10 feet on a contour line.
• Flows shall not be directed onto sidewalks.
• A maximum of 700 square feet of roof area may drain to each splashblock.
• A splashblock or a pad of crushed rock (2 feet wide by 3 feet long by 6 inches deep)
shall be placed at each downspout discharge point.
• No erosion or flooding of downstream properties may result.
• Runoff discharged towards landslide hazard areas must be evaluated by a
professional engineer with geotechnical expertise or a licensed geologist,
hydrogeologist, or engineering geologist. Splash blocks may not be placed on or
above slopes greater than 20% (5H:1V) or above erosion hazard areas without
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evaluation by a professional engineer with geotechnical expertise or qualified geologist
and City approval.
2.1.4 Bioinfiltration “Rain Gardens”
Purpose and Definition
Bioretention areas are shallow stormwater retention systems designed to mimic forested systems by
controlling stormwater through detention, infiltration, and evapotranspiration. Bioretention areas
provide water quality treatment through sedimentation, filtration, adsorption, and phytoremediation.
Bioretention facilities are integrated into the landscape to better mimic natural hydrologic conditions.
Bioretention facilities may be used as a water quality facility or a water quality and flow control
(retention) facility.
Use “Low Impact Development: Technical Guidance Manual for Puget Sound” and Washington State
University “Rain Garden Handbook for Western Washington Homeowners”, June 2007 as additional
guidance resources.
Applicability and Limitations
• Rain gardens can be used as on-lot retention facilities.
• Rain gardens may be used to receive roof runoff in areas where traditional infiltration is
not feasible.
• Three feet of clearance is necessary between the lowest elevation of the bioretention
soil or any underlying gravel layer and the seasonal high groundwater elevation or
other impermeable layer if the area tributary to the facility meets or exceeds any of the
following:
o 5000 square feet of pollution-generating impervious surface
o 10,000 square feet of impervious area
o ¾ acre of lawn and landscape
• For bioretention systems with a contributing area less than the above thresholds, a
minimum of 18 inches of clearance is required from the seasonal high groundwater or
other impermeable layer.
Setback and Site Constraints
• Assure that water movement through the surface soils and interflow will remain
unobstructed and soils will remain uncompacted.
• Locate bioretention facilities at least 10 feet from any structure or property line unless
approved in writing by the City.
• Locate bioretention facilities at least 50 feet back from slopes with a grade of 20% or
greater. A geotechnical analysis must be prepared addressing the potential impact of
the facility on the slope if closer than 50 feet or greater than 20%.
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Design Criteria
Flow Entrance/Presetting
• Flow velocity entering the facility should be less than 1 ft/sec.
• Use one of the four types of flow entrances:
o Dispersed, low velocity flow across a grade or landscape area.
o Pipe flow entrance. Include rock or other erosion protection material at the
facility entrance to dissipate energy and/or provide flow dispersion.
• Do not place woody plants directly in the entrance flow path as they can restrict or
concentrate flows.
• A minimum 1-inch grade change between the edge of a contributing impervious
surface and the vegetated flow entrance is required.
• Install flow diversion and erosion control measures to protect the bioretention area
from sedimentation until the upstream area is stabilized.
• If the catchment area exceeds 2,000 square feet, a presettling facility may be
required.
Cell Ponding Area
• The ponding area provides for surface storage and particulate settling,
• Ponding depth and drawdown rate provide variable conditions that allow for a range
of appropriate plant species. Soil must be allowed to dry out periodically in order to:
o Restore hydraulic capacity of system.
o Maintain infiltration rates.
o Maintain adequate soil oxygen levels for healthy soil biota and vegetation.
o Provide proper soil conditions for biodegradation and retention of pollutants.
o Prevent conditions supportive of mosquito breeding.
• The ponding depth shall be a maximum of 12 inches.
• The surface pool drawdown time shall be 24 hours.
• The minimum freeboard measured from the invert of the overflow pipe or earthen
channel to facility overtopping elevation shall be 2” for drainage areas less than
1,000 square feet and 6” for drainage areas 1,000 square feet or greater.
• If berming is used to achieve the minimum top elevation, maximum slope on berm
shall be 4H:1V, and minimum top width of design berm shall be 1 foot. Soil for
berming shall be imported bioretention soil or amended native soil compacted to a
minimum of 90% dry density.
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Overflow
• Unless designed for full infiltration of the entire runoff volume, bioretention systems
must include an overflow.
• A drain pipe installed at the designed maximum ponding elevation and connected to
a downstream BMP or an approved discharge point can be used as the overflow.
• Overflow shall be designed to convey the 100-year recurrence interval flow.
Soils
• For bioretention systems to meet the requirements for basic and enhanced treatment
the following requirements must be met:
• The bioretention soil mix (BSM) shall meet the following requirements:
o Have an infiltration rate between 1 and 2.4 inches per hour.
o The CEC (cation exchange capacity) must be at least 5 meq/100 grams of
dry soil.
o The soil mix should be about 40% by volume compost and about 60% by
volume aggregate component. The aggregate component shall meet the
specifications in Table III-2-3.
o The compost component shall be stable, mature, and derived from organic
waste materials including yard debris, wood wastes or other organic matter.
Compost must meet the Washington State compost regulations in WAC 173-
350-220, which is available at http://www.ecy.wa.gov/programs/swfa/compost
Table III-2-3. Bioretention Soil Mix Aggregate Component
Sieve Size Percent Passing
3/8” 100
#4 95-100
#10 75-90
#40 25-40
#100 4-10
#200 2-5
o Minimum depth of treatment soil must be 18 inches.
o Soil depths of 24” and greater should be considered to provide improved
removal of nutrients as needed, including phosphorus.
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• Provide a soils report, prepared by a soils professional, that addresses the following
for each bioretention area:
o A minimum of one soil log or test pit is required at each facility.
o The soil log shall extend a minimum of 4 feet below the bottom of the
subgrade (the lowest point of excavation).
o The soil log must describe the USDA textural class of the soil horizon through
the depth of the log and note any evidence of high groundwater level, such as
mottling.
Underdrain
Only install underdrains in bioretention areas if:
• Infiltration is not permitted and/or a liner is used, or
• Where infiltration rates are not adequate to meet the maximum pool drawdown
time.
• Where the facility is not utilized for infiltration.
Underdrain pipe diameter will depend on hydraulic capacity required, 6-inch minimum.
Use a geotextile fabric between the soil layer and underdrain.
Planting
• Plants must be tolerant of summer drought, ponding fluctuations, and saturated soil
conditions.
• Consider rooting depth when choosing plants. Roots must not damage underground
infrastructure.
• Locate slotted and perforated pipe at least 5 feet from tree roots and side sewer pipes.
• Consider adjacent plant communities and potential invasive species.
• Consider aesthetics, rain gardens should blend into surrounding landscapes.
• “Low Impact Development: Technical Guidance Manual for Puget Sound” is a good
tool for selecting proper bioretention vegetation.
Mulch Layer
• Bioretention areas should be designed with a mulch layer. Properly selected mulch
material reduces weed establishment, regulates soil temperatures and moisture, and
adds organic matter to soil. Mulch should be:
o Compost in the bottom of the facilities,
o Wood chip mulch composed of shredded or chipped hardwood or softwood
on cell slopes,
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o Free of weed seeds, soil, roots, and other material that is not trunk or branch
wood and bark,
o A maximum of 3 inches thick for compost or 4 inches thick for wood chips.
• Mulch shall not include grass clippings, mineral aggregate or pure bark.
• A dense groundcover can be used as an alternative to mulch though mulch most be
used until the dense groundcover is established.
Modeling and Sizing
For sites with contributing area less than 2,000 square feet:
Table III-2-4 gives the square footage of the bottom of the rain garden per 1000 square feet of roof
area.
Table III-2-4. Sizing Table for Rain Gardens
Soil Type Raingarden bottom
(square feet)
Coarse sands and cobbles 25
Medium sands 60
Fine sands, loamy sands 130
Sandy loam 160
Loam 225
For sites with contributing areas 2,000 square feet or more:
Use WWHM and model the facility as an infiltration facility with appropriate stage-storage and
overflow/outflow rates. Bioretention cells may be modeled as a layer of soil with infiltration to
underlying soil, ponding and overflow. The tributary area, cell bottom area, and ponding depth should
be iteratively sized until the duration curves and/or peak volumes meet the flow control requirements.
NOTE: WWHM Pro has the ability to model bioretention areas with or without underdrains so facility
will be sized differently than described above. Contact the Washington State Department of Ecology
for more information. Use the assumptions in Table III-2-5 when sizing the facilities.
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Table III-2-5. Modeling Assumptions for Rain Garden Sizing
Variable Assumption
Computational Time Step 15 minutes
Inflows to Facility Surface flow and interflow from drainage area routed to facility
Precipitation and Evaporation
Applied to Facility
Yes
Bioretention Soil Infiltration Rate For imported soils, for sites that have a contributing area of less
than 5,000 square feet of pollution generating surfaces, less
than 10,000 square feet of impervious area, and less than ¾
acre of landscaped area, reduce the infiltration rate of the BSM
by a factor of 2. For sites above these thresholds, a reductions
factor of 4 shall be applied.
For compost amended native soil, rate is equal to native soil
design infiltration rate.
Bioretention Soil Porosity 40%
Bioretention Soil Depth Minimum of 18 inches.
Native Soil Infiltration Rate Measured infiltration rate with applicable safety factors. See
Volume III for more information on infiltration rate determination.
Infiltration Across Wetted Surface
Area
Only if sides slopes are 3:1 or flatter
Overflow Overflow elevation set at maximum ponding elevation
(excluding freeboard). May be modeled as weir flow over rider
edge or riser notch.
Flow Credit
If roof runoff is infiltrated according to the requirements of this section, the roof area may be
discounted from the project area used for determining project thresholds and sizing stormwater
facilities.
2.1.5 Collect and Convey
Where it can be demonstrated that infiltration and dispersion are not feasible for roof downspout
controls, it may be allowable to collect and convey to the City stormwater system. This may be
either the curb, if present, or the actual pipe and structure conveyance system.
Conveyance to the curb will only be allowed if a catch basin is located within 350 feet
downstream of the discharge point. If a catch basin is not located within 350 feet of the
discharge location, a storm main extension shall be required.
Minimum pipe size for conveyance to the curb shall be 4 inches in diameter for single family
homes and a minimum of 6 inches in diameter for non-single family. Where capacity greater
than a 6 inch diameter pipe is required, the City shall review the proposal and may require a
storm main extension.
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For total roof areas 2,000 to 5,000 sf, roof runoff may be allowed to be collected and conveyed
to either the curb or directly connected to a structure. The runoff shall not be conveyed over
driveways, sidewalks or other areas reserved for pedestrian traffic. A detail for the discharge
shall be submitted to the City for review and approval.
For roof areas between 5,001 sf and 9,999 sf, roof runoff may be allowed to be collected and
conveyed to the curb or stormwater structure. Capacity analysis of the road gutter, conveyance
piping and catch basin leads will be required to ensure that adequate capacity exists. The City
may require more than one through curb outlet for discharge to the curb.
For roof areas 10,000 sf and greater, please refer to Minimum Requirement #7.
No flow credits will be allowed for the collect and convey option.
2.2 Infiltration Facilities for Stormwater Flow Control
2.2.1 Purpose
To provide infiltration capacity for stormwater runoff quantity and flow control. Infiltration facilities may
also be used for water quality treatment when designed appropriately.
2.2.2 Description
An infiltration BMP is typically an open basin (pond), trench, or buried perforated pipe used for
distributing the stormwater runoff into the underlying soil (See Figure III-2-4). (See Underground
Injection Control Program, Chapter 173-218 WAC).
The City prefers retention (infiltration) for storm drainage quantity control when soil conditions are
satisfactory for such application and water quality treatment can be provided.
Coarser more permeable soils can be used for quantity control provided that the stormwater
discharge does not cause a violation of groundwater quality criteria. Typically, treatment for removal
of TSS, oil, and/or soluble pollutants is necessary prior to conveyance to an infiltration BMP.
Use of the soil for treatment purposes is also an option as long as it is preceded by a pre-settling
basin or a basic treatment BMP. This section highlights design criteria that are applicable to infiltration
facilities serving a flow control function. See Volume V, Chapter 5 for design criteria for treatment.
2.2.3 Application
Infiltration facilities for flow control are used to convey stormwater runoff from new development or
redevelopment to the ground and groundwater after appropriate treatment. Infiltration facilities for
treatment purposes rely on the soil profile to provide treatment. In either case, runoff in excess of the
infiltration capacity of the facilities must be managed to comply with the flow control requirement in
Volume I, if flow control applies to the project.
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Figure III-2-4. Typical Infiltration Pond/Basin
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Infiltration trenches can be considered for residential lots, commercial areas, parking lots, and open
space areas. Infiltration facilities can help accomplish the following:
• Groundwater recharge
• Discharge of uncontaminated or properly treated stormwater in compliance with
Ecology’s UIC regulations (Chapter 173-218 WAC)
• Retrofits in limited land areas:
• Flood control
• Streambank erosion control
2.2.4 Design Methodology
Two methodologies outlining the steps for designing infiltration systems are presented in this manual.
The simplified approach is outlined in Section 2.2.5 and the detailed approach is outlined in
Section 2.2.6.
2.2.5 Simplified Approach
The simplified approach was derived from high groundwater and shallow pond sites in western
Washington, and in general will produce `conservative’ designs. The simplified approach can be
used when determining the trial geometry of the infiltration facility, for small or low impact facilities, or
for facilities where a more conservative design is acceptable. The simplified approach is applicable to
ponds and trenches and includes the following steps:
Step 1. Select a location:
This will be based on the ability to convey flow to the location and the expected soil conditions of the
location. Conduct a preliminary surface and sub-surface characterization study (Section 2.2.7). Do a
preliminary check of Site Suitability Criteria (Section 2.2.10) to estimate feasibility.
Step 2. Estimate volume of stormwater, Vdesign:
Use WWHM to estimate the design. The runoff volume developed for the project site serves as input
to the infiltration basin.
For infiltration basins sized simply to meet treatment requirements, the basin must successfully
infiltrate 91% of the influent runoff volume. The remaining 9% of the influent volume can bypass the
infiltration facility. However, if the bypass discharges to a surface water that is not exempt from flow
control, the bypass must meet the flow control standard.
For infiltration basins sized to meet the flow control standard, the basin must infiltrate either all of the
influent volume, or a sufficient amount of the influent volume such that any overflow/bypass meets
the flow duration standard.
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Step 3. Develop trial infiltration facility geometry:
To accomplish this, an infiltration rate will need to be assumed based on previously available data, or
a default infiltration rate of 0.5 inches/hour can be used. This trial facility geometry should be used to
help locate the facility and for planning purposes in developing the geotechnical subsurface
investigation plan.
Step 4. Complete more detailed site characterization study and consider site suitability
criteria:
Information gathered during initial geotechnical and surface investigations are necessary to know
whether infiltration is feasible. The geotechnical investigation evaluates the suitability of the site for
infiltration, establishes the infiltration rate for design, and evaluates slope stability, foundation
capacity, and other geotechnical design information needed to design and assess constructability of
the facility.
See Sections 2.2.7 and 2.2.10.
Step 5. Determine the infiltration rate as follows:
Three possible methods for estimating the long-term infiltration rate are provided in Section 2.2.9.
Step 6. Size the facility:
Ensure that the maximum pond depth stays below the minimum required freeboard. If sizing a
treatment facility, document that the 91st percentile, 24-hour runoff volume (indicated by an approved
continuous simulation model) can infiltrate through the infiltration basin surface within 48 hours. This
can be calculated by multiplying a horizontal projection of the infiltration basin mid-depth dimensions
by the estimated long-term infiltration rate; and multiplying the result by 48 hours.
Step 7. Construct the facility:
Maintain and monitor the facility for performance.
2.2.6 Detailed Approach
This detailed approach was obtained from Massmann (2003). Procedures for the detailed approach
are as follows:
Step 1: Select a location:
This will be based on the ability to convey flow to the location and the expected soil conditions. The
minimum setback distances must also be met. See Section 2.2.10 Site Suitability Criteria and
setback distances.
Step 2: Estimate volume of stormwater, Vdesign:
Use WWHM to estimate Vdesign.
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Step 3: Develop a trial infiltration facility geometry based on length, width, and depth:
To accomplish this, either assume an infiltration rate based on previously available data, or use a
default infiltration rate of 0.5 inches/hour. This trial geometry should be used to help locate the facility,
and for planning purposes in developing the geotechnical subsurface investigation plan.
Step 4: Conduct a geotechnical investigation and consider site suitability criteria:
See Sections 2.2.7 and 2.2.10.
Step 5: Determine the saturated hydraulic conductivity as follows:
For each defined layer below the pond to a depth below the pond bottom of 2.5 times the maximum
depth of water in the pond, but not less than 6 feet, estimate the saturated hydraulic conductivity in
cm/sec using the following relationship (see Massmann 2003, and Massmann et al., 2003)
log10(Ksat) = -1.57 + 1.90D10 + 0.015D60 – 0.013D90 – 2.08Ffines (equation 1)
Where, D10, D60 and D90 are the grain sizes in mm for which 10 percent, 60 percent and 90 percent of
the sample is more fine and ffines is the fraction of the soil (by weight) that passes the #200 sieve (Ksat
is in cm/s).
If the licensed professional conducting the investigation determines that deeper layers will influence
the rate of infiltration for the facility, soil layers at greater depths must be considered when assessing
the site’s hydraulic conductivity characteristics. Massmann (2003) indicates that where the water
table is deep, soil or rock strata up to 100 feet below an infiltration facility can influence the rate of
infiltration. Note that only the layers near and above the water table or low permeability zone (e.g., a
clay, dense glacial till, or rock layer) need to be considered, as the layers below the groundwater
table or low permeability zone do not significantly influence the rate of infiltration. Also note that this
equation for estimating hydraulic conductivity assumes minimal compaction consistent with the use of
tracked (i.e., low to moderate ground pressure) excavation equipment. If the soil layer being
characterized has been exposed to heavy compaction, or is heavily consolidated due to its geologic
history (e.g., overridden by continental glaciers), the hydraulic conductivity for the layer could be
approximately an order of magnitude less than what would be estimated based on grain size
characteristics alone (Pitt, 2003). In such cases, compaction effects must be taken into account when
estimating hydraulic conductivity. For clean, uniformly graded sands and gravels, the reduction in Ksat
due to compaction will be much less than an order of magnitude. For well-graded sands and gravels
with moderate to high silt content, the reduction in Ksat will be close to an order of magnitude. For
soils that contain clay, the reduction in Ksat could be greater than an order of magnitude.
For critical designs, the in-situ saturated conductivity of a specific layer can be obtained through field
tests such as the packer permeability test (above or below the water table), the piezocone (below the
water table), an air conductivity test (above the water table), or through the use of a pilot infiltration
test (PIT) as described in Volume III, Appendix B. Note that these field tests generally provide a
hydraulic conductivity combined with a hydraulic gradient (i.e., Equation 5). In some of these tests,
the hydraulic gradient may be close to 1.0; therefore, in effect, the magnitude of the test result is the
same as the hydraulic conductivity. In other cases, the hydraulic gradient may be close to the
gradient that is likely to occur in the full-scale infiltration facility. This issue will need to be evaluated
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on a case-by-case basis when interpreting the results of field tests. It is important to recognize that
the gradient in the test may not be the same as the gradient likely to occur in the full-scale infiltration
facility in the long-term (i.e., when groundwater mounding is fully developed).
Once the saturated hydraulic conductivity for each layer has been identified, determine the effective
average saturated hydraulic conductivity below the pond. Hydraulic conductivity estimates from
different layers can be combined using the harmonic mean:
(equation 2)
Where, d is the total depth of the soil column, di is the thickness of layer “i” in the soil column, and Ki
is the saturated hydraulic conductivity of layer “i” in the soil column. The depth of the soil column, d,
typically would include all layers between the pond bottom and the water table. However, for sites
with very deep water tables (>100 feet) where groundwater mounding to the base of the pond is not
likely to occur, it is recommended that the total depth of the soil column in Equation 2 be limited to
approximately 20 times the depth of pond. This is to ensure that the most important and relevant
layers are included in the hydraulic conductivity calculations. Deep layers that are not likely to affect
the infiltration rate near the pond bottom should not be included in Equation 2. Equation 2 may over-
estimate the effective hydraulic conductivity value at sites with low conductivity layers immediately
beneath the infiltration pond. For sites where the lowest conductivity layer is within five feet of the
base of the pond, it is suggested that this lowest hydraulic conductivity value be used as the
equivalent hydraulic conductivity rather than the value from Equation 2. The harmonic mean given by
Equation 2 is the appropriate effective hydraulic conductivity for flow that is perpendicular to
stratigraphic layers, and will produce conservative results when flow has a significant horizontal
component such as could occur due to groundwater mounding.
Step 6: Calculate the hydraulic gradient as follows:
The steady state hydraulic gradient is calculated as follows:
(equation 3)
Where, Dwt is the depth from the base of the infiltration facility to the water table in feet, K is the
saturated hydraulic conductivity in feet/day, Dpond is the depth of water in the facility in feet (see
Massmann et al., 2003, for the development of this equation), and CFsize, is the correction for pond
size. The correction factor was developed for ponds with bottom areas between 0.6 and 6 acres in
size. For small ponds (ponds with area equal to 2/3 acre), the correction factor is equal to 1.0. For
large ponds (ponds with area equal to 6 acres), the correction factor is 0.2, as shown in Equation 4.
(equation 4)
=
i
i
equiv
K
d
dK
size
pondwt CF
K
DD
)(62.138
igradient 1.0
+=
76.0)(73.0 -=pondsizeACF
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Where, Apond is the area of pond bottom in acres. This equation generally will result in a calculated
gradient of less than 1.0 for moderate to shallow groundwater depths (or to a low permeability layer)
below the facility, and conservatively accounts for the development of a groundwater mound. A more
detailed groundwater mounding analysis using a program such as MODFLOW will usually result in a
gradient that is equal to or greater than the gradient calculated using Equation 3. If the calculated
gradient is greater than 1.0, the water table is considered to be deep, and a maximum gradient of 1.0
must be used. Typically, a depth to groundwater of 100 feet or more is required to obtain a gradient
of 1.0 or more using this equation. Since the gradient is a function of depth of water in the facility, the
gradient will vary as the pond fills during the season. The gradient could be calculated as part of the
stage-discharge calculation used in the continuous runoff models. As of the date of this update,
neither the WWHM or MGSFlood have that capability. However, updates to those models may soon
incorporate the capability. Until that time, use a steady-state hydraulic gradient that corresponds with
a ponded depth of ¼ of the maximum ponded depth – as measured from the basin floor to the
overflow.
Step 7: Calculate the infiltration rate using Darcy’s law as follows:
(equation 5)
Where, f is the specific discharge or infiltration rate of water through a unit cross-section of the
infiltration facility (L/t), K is the hydraulic conductivity (L/t), dh/dz is the hydraulic gradient (L/L), and “i”
is the gradient.
Step 8: Adjust infiltration rate or infiltration stage-discharge relationship obtained in Steps 6
and 7:
This is done to account for reductions in the rate resulting from long-term siltation and biofouling,
taking into consideration the degree of long-term maintenance and performance monitoring
anticipated, the degree of influent control (e.g., pre-settling ponds biofiltration swales, etc.), and the
potential for siltation, litterfall, moss buildup, etc. based on the surrounding environment. It should be
assumed that an average to high degree of maintenance will be performed on these facilities. A low
degree of maintenance should be considered only when there is no other option (e.g., access
problems). The infiltration rate estimated in Step 8 and 9 is multiplied by the reduction factors
summarized in Table III-2-6.
Table III-2-6. Infiltration Rate Reduction Factors to Account for
Biofouling and Siltation Effects for Ponds
Potential for Biofouling Degree of Long-Term
Maintenance/Performance Monitoring
Infiltration Rate Reduction
Factor (CFsilt/bio)
Low Average to High 0.9
Low Low 0.6
High Average to High 0.5
High Low 0.2
(Massman, 2003)
Kidz
dhKf ==
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The values in this table assume that final excavation of the facility to the finished grade is deferred
until all disturbed areas in the upgradient drainage area have been stabilized or protected (e.g.,
construction runoff is not allowed into the facility after final excavation of the facility). Ponds located in
shady areas where moss and litterfall from adjacent vegetation can build up on the pond bottom and
sides, the upgradient drainage area will remain in a disturbed condition long-term, and no
pretreatment (e.g., pre-settling ponds, biofiltration swales, etc.) is provided, are one example of a
situation with a high potential for biofouling. A low degree of long-term maintenance includes, for
example, situations where access to the facility for maintenance is very difficult or limited, or where
there is minimal control of the party responsible for enforcing the required maintenance. A low degree
of maintenance should be considered only when there is no other option.
Also adjust this infiltration rate for the effect of pond aspect ratio by multiplying the infiltration rate
determined in Step 7 (Equation 5) by the aspect ratio correction factor Faspect as shown in the
following equation:
CFaspect = 0.02Ar + 0.98 (equation 6)
Where, Ar is the aspect ratio for the pond (length/width). In no case shall CFaspect be greater than
1.4.
The final infiltration rate will therefore be as follows:
f = Ki x CFaspect x CFsilt/bio (equation 7)
The rates calculated based on Equations 5 and 7 are long-term design rates. No additional reduction
factor or factor of safety is needed.
Step 9: Size the facility:
Size the facility to ensure that the desirable pond depth is three feet, with one-foot minimum required
freeboard. The maximum allowable pond depth is six feet.
Where the infiltration facility is being used to meet treatment requirements, check that the 91st
percentile, 24-hour runoff volume (indicated by WWHM or MGS Flood) can infiltrate through the
infiltration basin surface within 48 hours. This can be calculated by multiplying a horizontal projection
of the infiltration basin mid-depth dimensions by the estimated long-term infiltration rate; and
multiplying the result by 48 hours. Finally, check to make sure that the basin can drain its maximum
ponded water depth within 24 hours.
Step 10: Construct the facility:
Maintain and monitor the facility for performance in accordance with Section 2.2.11.
2.2.7 Site Characterization Criteria
One of the first steps in siting and designing infiltration facilities is to conduct a characterization study.
Information gathered during initial geotechnical investigations can be used for the site
characterization.
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2.2.7.1 Surface Features Characterization
• Topography within 500 feet of the proposed facility.
• Anticipated site use (street/highway, residential, commercial, high-use site).
• Location of water supply wells within 500 feet of proposed facility.
• Location of groundwater protection areas and/or 1, 5 and 10 year time of travel
zones for municipal well protection areas.
• A description of local site geology, including soil or rock units likely to be
encountered, the groundwater regime, and geologic history of the site.
2.2.7.2 Subsurface Characterization
• Conduct pit/hole explorations during the wet season (December 1st through April
30th) to provide accurate groundwater saturation and groundwater information.
• Subsurface explorations (test holes or test pits) to a depth below the base of the
infiltration facility of at least 5 times the maximum design depth of ponded water
proposed for the infiltration facility,
• Continuous sampling (representative samples from each soil type and/or unit within
the infiltration receptor) to a depth below the base of the infiltration facility of
2.5 times the maximum design ponded water depth, but not less than 6 feet.
• For basins, at least one test pit or test hole per 5,000 ft2 of basin infiltrating surface
(in no case less than two per basin).
• For trenches, at least one test pit or test hole per 50 feet of trench length (in no case
less than two per trench).
The depth and number of test holes or test pits, and samples should be increased, if in the judgment
of a licensed engineer with geotechnical expertise (P.E.), a licensed geologist, engineering geologist,
hydrogeologist, or other licensed professional acceptable to the City, the conditions are highly
variable and such increases are necessary to accurately estimate the performance of the infiltration
system. The exploration program may also be decreased if, in the opinion of the licensed engineer or
other professional, the conditions are relatively uniform and the borings/test pits omitted will not
influence the design or successful operation of the facility. In high water table sites, the subsurface
exploration sampling need not be conducted lower than two (2) feet below the groundwater table.
Prepare detailed logs for each test pit or test hole and a map showing the location of the test pits or
test holes. Logs must include at a minimum, depth of pit or hole, soil descriptions, depth to water,
presence of stratification (Logs must substantiate whether stratification does or does not exist. The
licensed professional may consider additional methods of analysis to substantiate the presence of
stratification that will significantly impact the design of the infiltration facility).
2.2.7.3 Infiltration Rate Determination
Determine the representative infiltration rate of the unsaturated vadose zone based on infiltration
tests and/or grain-size distribution/texture (see next section). Determine site infiltration rates using the
Pilot Infiltration Test (PIT) described in Volume III, Appendix B, if practicable. Such site testing should
be considered to verify infiltration rate estimates based on soil size distribution and textural analysis.
Infiltration rates may also be estimated based on soil grain-size distributions from test pits or test hole
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samples (particularly where a sufficient source of water does not exist to conduct a pilot infiltration
test). As a minimum, one soil grain-size analysis per soil stratum in each test hole shall be performed
within 2.5 times the maximum design water depth, but not less than 6 feet.
2.2.7.4 Soil Testing
Soil characterization for each soil unit (soils of the same texture, color, density, compaction,
consolidation and permeability) encountered should include:
• Grain-size distribution (ASTM D422 or equivalent AASHTO specification)
• Textural class (USDA) (See Figure III-2-5).
• Percent clay content (include type of clay, if known)
• Color/mottling
• Variations and nature of stratification
If the infiltration facility will be used to provide treatment as well as flow control, the soil
characterization should also include cation exchange capacity (CEC) and organic matter content for
each soil type and strata. Where distinct changes in soil properties occur, perform analysis to a depth
below the base of the facility of at least 2.5 times the maximum design water depth, but not less than
6 feet. Consider if soils are already contaminated, thus diminishing pollutant sorptive capacity.
For soils with low CEC and organic content, deeper characterization of soils may be warranted (refer
to Section 2.2.10 Site Suitability Criteria)
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Figure III-2-5. USDA Textural Triangle2
2.2.7.5 Infiltration Receptor
The requirements of this section will be applied as directed by the City. Infiltration receptor
(unsaturated and saturated soil receiving the stormwater) characterization should include:
• Installation of groundwater monitoring wells (at least three per infiltration facility, or
three hydraulically connected surface and groundwater features that will establish a
three-dimensional relationship for the groundwater table, unless the highest
groundwater level is known to be at least 50 feet below the proposed infiltration
facility) to:
o Monitor the seasonal groundwater levels at the site during at least one wet
season, and,
2 Shaded area is applicable for design of infiltration BMPs. Source, U.S. Department of Agriculture
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o Consider the potential for both unconfined and confined aquifers, or confining
units, at the site that may influence the proposed infiltration facility as well as
the groundwater gradient. Other approaches to determine groundwater levels
at the proposed site could be considered if pre-approved by the City, and
o Determine the ambient groundwater quality, if that is a concern.
• An estimate of the volumetric water holding capacity of the infiltration receptor soil.
This is the soil layer below the infiltration facility and above the seasonal high-water
mark, bedrock, hardpan, or other low permeability layer. This analysis should be
conducted at a conservatively high infiltration rate based on vadose zone porosity,
and the water quality runoff volume to be infiltrated. This, along with an analysis of
groundwater movement, will be useful in determining if there are volumetric
limitations that would adversely affect drawdown.
• Determination of:
o Depth to groundwater table and to bedrock/impermeable layers
o Seasonal variation of groundwater table based on well water levels and
observed mottling
o Existing groundwater flow direction and gradient
o Lateral extent of infiltration receptor
o Horizontal hydraulic conductivity of the saturated zone to assess the aquifer’s
ability to laterally transport the infiltrated water.
• Impact of the infiltration rate and volume at the project site on groundwater
mounding, flow direction, and water table; and the discharge point or area of the
infiltrating water. A groundwater mounding analysis should be conducted at all sites
where the depth to seasonal groundwater table or low permeability stratum is less
than 15 feet and the runoff to the infiltration facility is from more than one acre. (The
site professional may consider conducting an aquifer test, or slug test to aid in
determining the type of groundwater mounding analysis necessary at the site)
A detailed soils and hydrogeologic investigation should be conducted if potential pollutant
impacts to groundwater are a concern, or if the applicant is proposing to infiltrate in areas
underlain by till or other impermeable layers. (Suggested references: “Implementation
Guidance for the Groundwater Quality Standards”, Department of Ecology, publication 96-2,
2005).
2.2.8 Design Infiltration Rate Determination – Guidelines and Criteria
Infiltration rates can be determined using either a correlation to grain size distribution from soil
samples, textural analysis, or by in-situ field measurements. Short-term infiltration rates up to 2.4 in/hr
represent soils that typically have sufficient treatment properties. Long-term infiltration rates are used
for sizing the infiltration pond based on maximum pond level and drawdown time. Long-term
infiltration rates up to 2.0 inches per hour can also be considered for treatment if SSC-4 and SSC-6
are met, as defined in Section 2.2.10.
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Historically, infiltration rates have been estimated from soil grain size distribution (gradation) data
using the United States Department of Agriculture (USDA) textural analysis approach. To use the
USDA textural analysis approach, the grain size distribution test must be conducted in accordance
with the USDA test procedure (SOIL SURVEY MANUAL, U.S. Department of Agriculture, October
1993, page 136). This manual only considers soil passing the #10 sieve (2 mm) (U.S. Standard) to
determine percentages of sand, silt, and clay for use in Figure III-2-5 (USDA Textural Triangle).
However, many soil test laboratories use the ASTM soil size distribution test procedure (ASTM
D422), which considers the full range of soil particle sizes, to develop soil size distribution curves.
The ASTM soil gradation procedure must not be used with Figure III-2-5 to perform USDA soil
textural analyses.
2.2.9 Three Methods for Determining Long-term Infiltration Rates for Sizing
Infiltration Facilities
For designing the infiltration facility the site professional must select one of the three methods
described below that will best represent the long-term infiltration rate at the site. The long-term
infiltration rate should be used for routing and sizing the basin/trench for the maximum drawdown
time of 48 hours. If the pilot infiltration test (Table III-2-9) or ASTM gradation approach (Table III-2-8)
is selected corroboration with a textural based infiltration rate (Table III-2-7) is also desirable.
Appropriate correction factors must be applied as specified. Verification testing of the completed
facility is strongly encouraged and may be required by the City. (See Section 2.2.10.8 - Verification
Testing)
1. USDA Soil Textural Classification
Table III-2-7 provides the correlation between USDA soil texture and infiltration rates for estimating
infiltration rates for homogeneous soils based on gradations from soil samples and textural analysis.
The USDA soil texture – infiltration rate correlation in Table III-2-7 is based on the correlation
developed by Rawls, et al (1982), but with minor changes in the infiltration rates based on
WEF/ASCE (1998). The infiltration rates provided through this correlation represent short-term
conservative rates for homogeneous soils. These rates do not account for the effects of site
variability, long-term clogging due to siltation and biomass buildup in the infiltration facility, or other
processes that can decrease infiltration rates. Correction factors must be applied to these rates.
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Table III-2-7. Recommended Infiltration Rates based on USDA Soil Textural Classification
Short-Term
Infiltration Rate
(in/hr)1
Correction
Factor (CF)
Estimated Long-Term
(Design) Infiltration Rate
(in/hr)
Clean, sandy gravels and gravelly
sands (i.e., 90% of the total soil
sample is retained in the #10 sieve)
20 2 102
Sand 8 4 23
Loamy sand 2 4 0.5
Sandy loam 1 4 0.25
Loam 0.5 4 0.13
1 From WEF/ASCE, 1998 2 Not recommended for treatment
3 Refer to SSC-4 and SSC-6 for treatment acceptability criteria
Based on experience with long-term full-scale infiltration pond performance, Ecology’s Technical
Advisory Committee (TAC) recommends that the short-term infiltration rates be reduced as shown in
Table III-2-7, dividing by a correction factor of 2 to 4, depending on the soil textural classification. The
correction factors provided in Table III-2-7 represent an average degree of long-term facility
maintenance, TSS reduction through pretreatment, and site variability in the subsurface conditions.
These conditions might include deposits of ancient landslide debris, buried stream channels, lateral
grain size variability, and other factors that affect homogeneity).
These correction factors could be reduced, subject to the approval of the local jurisdiction, under the
following conditions:
For sites with little soil variability,
• Where there will be a high degree of long-term facility maintenance,
• Where specific, reliable pretreatment is employed to reduce TSS entering the
infiltration facility
In no case shall a correction factor less than 2.0 be used.
Correction factors higher than those provided in Table III-2-7 should be considered for situations
where long-term maintenance will be difficult to implement, where little or no pretreatment is
anticipated, or where site conditions are highly variable or uncertain. These situations require the use
of best professional judgment by the site engineer and the approval of the local jurisdiction. An
Operation and Maintenance plan and a financial bonding plan may be required by the local
jurisdiction.
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2. ASTM Gradation Testing at Full Scale Infiltration Facilities
As an alternative to Table III-2-7, recent studies by Massmann and Butchart (2000) were used to
develop the correlation provided in Table III-2-8. These studies compare infiltration measurements
from full-scale infiltration facilities to soil gradation data developed using the ASTM procedure (ASTM
D422). The primary source of the data used by Massmann and Butchart was from Wiltsie (1998),
who included limited infiltration studies only on Thurston County sites. However, Massmann and
Butchart also included limited data from King and Clark County sites in their analysis. This table
provides recommended long-term infiltration rates that have been correlated to soil gradation
parameters using the ASTM soil gradation procedure.
Table III-2-8 can be used to estimate long-term design infiltration rates directly from soil gradation
data. The City may require additional correction factors be applied to the values shown in Table III-2-
8 depending on the site conditions. As is true of Table III-2-7, the long-term rates provided in Table
III-2-8 represent average conditions regarding site variability, the degree of long-term maintenance
and pretreatment for TSS control, and represent a moderate depth to groundwater below the pond.
The long-term infiltration rates in Table III-2-8 may need to be decreased if the site is highly variable,
the groundwater table is shallow, there is fine layering present that would not be captured by the soil
gradation testing, or if maintenance and influent characteristics are not well controlled. The data that
forms the basis for Table III-2-8 was from soils that would be classified as sands or sandy gravels. No
data was available for finer soils at the time the table was developed. Therefore, Table III-2-8 should
not be used for soils with a D10 size (10% passing the size listed) less than 0.05 mm (U.S. Standard
Sieve).
Table III-2-8. Alternative Recommended Infiltration Rates
Based on ASTM Gradation Testing
D10 Size from ASTM D422 Soil Gradation Test
(mm)
Estimated Long-Term (Design) Infiltration Rate
(in/hr)
> 0.4 91
0.3 6.51
0.2 3.51
0.1 2.02
0.05 0.8
1 Not recommended for treatment
2 Refer to SSC-4 and SSC-6 for treatment acceptability criteria
The infiltration rates provided in Table III-2-7 and Table III-2-8 represent rates for homogeneous soil
conditions. If more than one soil unit is encountered within 6 feet of the base of the facility or
2.5 times the proposed maximum water design depth, use the lowest infiltration rate determined from
each of the soil units as the representative site infiltration rate.
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If soil mottling, fine silt or clay layers, which cannot be fully represented in the soil gradation tests, are
present below the bottom of the infiltration pond, the infiltration rates provided in the tables will be too
high and should be reduced. Based on limited full-scale infiltration data (Massmann and Butchart,
2000; Wiltsie, 1998), it appears that the presence of mottling indicates soil conditions that reduce the
infiltration rate for homogeneous conditions by a factor of 3 to 4.
3. In-situ Infiltration Measurements
Where feasible, Ecology encourages in-situ infiltration measurements, using a procedure such as the
Pilot Infiltration Test (PIT) described in Volume III, Appendix B. Small-scale infiltration tests such as
the EPA Falling Head or double ring infiltrometer test (ASTM D3385-88) are not recommended
unless modified versions are approved in writing by The City. These small-scale infiltration tests tend
to seriously overestimate infiltration rates and, based on recent Ecology experience, are considered
unreliable.
The infiltration rate obtained from the PIT test shall be considered to be a short-term rate. This short-
term rate must be reduced through correction factors to account for site variability and number of
tests conducted, degree of long-term maintenance and influent pretreatment/control, and potential for
long-term clogging due to siltation and bio-buildup.
The typical range of correction factors to account for these issues, based on Ecology experience, is
summarized in Table III-2-9. The range of correction factors is for general guidance only. The specific
correction factors used shall be determined based on the professional judgment of the licensed
engineer or other soils professional considering all issues which may affect the long-term infiltration
rate, subject to the approval of The City.
Table III-2-9. Correction Factors to be Used with In-Situ Infiltration
Measurements to Estimate Long-Term Design Infiltration Rates
Issue Partial Correction Factor
Site variability and number of locations tested CFy = 1.5 to 6
Degree of long-term maintenance to prevent siltation and bio-buildup CFm = 2 to 6
Degree of influent control to prevent siltation and bio-buildup CFi = 2 to 6
Total Correction Factor (CF) = CFy + CFm + CFi
The following discussions are to provide assistance in determining the partial correction factors to
apply in Table III-2-9.
Site variability and number of locations tested – The number of locations tested must be capable
of producing a picture of the subsurface conditions that fully represents the conditions throughout the
facility site. The partial correction factor used to compensate for site variability depends on the level of
uncertainty that adverse subsurface conditions may occur. If the range of uncertainty is low - for
example, conditions are known to be uniform through previous exploration and site geological factors
- one pilot infiltration test may be adequate to justify a partial correction factor at the low end of the
range. If the level of uncertainty is high, a partial correction factor near the high end of the range may
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be appropriate. This might be the case where the site conditions are highly variable due to a deposit
of ancient landslide debris, or buried stream channels. In these cases, even with many explorations
and several pilot infiltration tests, the level of uncertainty may still be high. A partial correction factor
near the high end of the range could be assigned where conditions have a more typical variability, but
few explorations and only one pilot infiltration test is conducted. That is, the number of explorations
and tests conducted do not match the degree of site variability anticipated.
Degree of long-term maintenance to prevent siltation and bio-buildup – The standard of
comparison here is the long-term maintenance requirements provided in Volume I, Appendix D, and
any additional requirements by local jurisdictional authorities. Full compliance with these
requirements would be justification to use a partial correction factor at the low end of the range. If
there is a high degree of uncertainty that long-term maintenance will be carried out consistently, or if
the maintenance plan is poorly defined, a partial correction factor near the high end of the range may
be justified.
Degree of influent control to prevent siltation and bio-buildup – A partial correction factor near
the high end of the range may be justified under the following circumstances:
• If the infiltration facility is located in a shady area where moss buildup or litter fall
buildup from the surrounding vegetation is likely and cannot be easily controlled
through long-term maintenance
• If there is minimal pre-treatment, and the influent is likely to contain moderately high
TSS levels.
• If influent into the facility can be well controlled such that the planned long-term
maintenance can easily control siltation and biomass buildup, then a partial
correction factor near the low end of the range may be justified.
The determination of long-term design infiltration rates from in-situ infiltration test data involves a
considerable amount of engineering judgment. Therefore, when reviewing or determining the final
long-term design infiltration rate the results of both textural analyses and in-situ infiltration tests
results will be considered when available and may be required by the City.
2.2.10 Site Suitability Criteria (SSC)
This section provides criteria that must be considered for siting infiltration systems. When a site
investigation reveals that any of the applicable criteria cannot be met appropriate mitigation
measures must be implemented so that the infiltration facility will not pose a threat to safety, health,
and the environment.
For site selection and design decisions a geotechnical and hydrogeologic report should be prepared
by a qualified engineer with geotechnical and hydrogeologic experience, or a licensed geologist,
hydrogeologist, or engineering geologist. The design engineer may utilize a team of certified or
registered professionals in soil science, hydrogeology, geology, and other related fields.
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2.2.10.1 SSC-1 Setback Criteria
Setback requirements contained within this manual and other applicable setbacks include those
contained within the uniform building code requirements, City of Auburn Zoning and Building Codes,
County Health District requirements, Washington State Department of Health On-Site Sewage
Systems Chapter 246-272A WAC, and other state regulations.
These additional setback requirements may be required as determined by the project engineer
and/or the City.
• Drinking water wells, septic tanks or drainfields, and springs used for public drinking
water supplies (DOH, Publication # 333-117, Chapter 246-272A WAC).
• Infiltration facilities upgradient of drinking water supplies and within 1, 5, and 10-year
time of travel zones must comply with Health Dept. requirements (Washington
Wellhead Protection Program, DOH, Publication #331-018).
• Additional setbacks must be considered if roadway deicers or herbicides are likely to
be present in the influent to the infiltration system.
• Building foundations within 20 feet downslope and within100 feet upslope
• Native Growth Protection Easement (NGPE) within 20 feet
• From the top of slopes >20% and within 50 feet.
• Evaluate on-site and off-site structural stability due to extended subgrade saturation
and/or head loading of the permeable layer, including the potential impacts to
downgradient properties, especially on hills with known side-hill seeps.
2.2.10.2 SSC-2 Groundwater Protection Areas
A site is not suitable if the infiltration facility will cause a violation of Ecology's Groundwater Quality
Standards (See SSC-9 for verification testing guidance).
2.2.10.3 SSC-3 High Vehicle Traffic Areas
An infiltration BMP may be considered for runoff from areas of industrial activity and the high vehicle
traffic areas described below. For such applications sufficient pollutant removal (including oil removal)
must be provided upstream of the infiltration facility to ensure that groundwater quality standards will
not be violated and that the infiltration facility is not adversely affected.
High Vehicle Traffic Areas are:
• Commercial or industrial sites subject to an expected average daily traffic count
(ADT) 100 vehicles/1,000 ft² gross building area (trip generation), and
• Road intersections with an ADT of 25,000 on the main roadway, or 15,000 on
any intersecting roadway.
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2.2.10.4 SSC-4 Drawdown Time
Infiltration facilities designed for flow control do not have a required drawdown time criteria.
2.2.10.5 SSC-5 Depth to Bedrock, Water Table, or Impermeable Layer
The base of all infiltration basins or trench systems shall be 5 feet above the seasonal high-water
mark, bedrock (or hardpan) or other low permeability layer. A separation down to 3 feet may be
considered if the groundwater mounding analysis, volumetric receptor capacity, and the design of the
overflow and/or bypass structures are judged by the site professional and the City to be adequate to
prevent overtopping and meet the site suitability criteria specified in this section.
2.2.10.6 SSC-6 Seepage Analysis and Control
Determine whether there would be any adverse effects caused by seepage zones on nearby building
foundations, basements, roads, parking lots or sloping sites.
2.2.10.7 SSC-7 Cold Climate and Impact of Roadway Deicers
For cold climate design criteria (snowmelt/ice impacts) refer to D. Caraco and R. Claytor Design
Supplement for Stormwater BMPs in Cold Climates, Center for Watershed Protection, 1997.
Potential impact of roadway deicers on potable water wells must be considered in the siting
determination. Mitigation measures must be implemented if infiltration of roadway deicers can cause
a violation of groundwater quality standards.
2.2.10.8 SSC-8 Verification Testing of the Completed Facility
Verification testing of the completed full-scale infiltration facility is recommended to confirm that the
design infiltration parameters are adequate. The site professional should determine the duration and
frequency of the verification testing program including the monitoring program for the potentially
impacted groundwater. The groundwater monitoring wells installed during site characterization (See
Section 2.2.7) may be used for this purpose. Long-term (more than two years) in-situ drawdown and
confirmatory monitoring of the infiltration facility would be preferable (See King County reference).
The City may require verification testing on a site-by-site basis.
2.2.11 Design Criteria for Infiltration Facilities
The design criteria for infiltration facilities shall be the same as for detention ponds described in
Section 2.3.1.2 as applicable. All retention ponds shall be appropriately and aesthetically located,
designed and planted. Pre-approval of the design concept, including landscaping, is required by the
City for all proposed public ponds. The size of the infiltration facility can be determined by routing the
influent runoff file generated by the continuous runoff model through it. The primary mode of
discharge from an infiltration facility is infiltration into the ground. However, when the infiltration
capacity of the facility is reached, additional runoff to the facility will cause the facility to overflow.
Overflows from an infiltration facility must comply with the Minimum Requirement #7 for flow control
in Volume I. Infiltration facilities used for runoff treatment must not overflow more than 9% of the
influent runoff volume.
In order to determine compliance with the flow control requirements, the Western Washington
Hydrology Model (WWHM) must be used.
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(A) For 100% infiltration
Ensure that the pond infiltrates 100% using the pond bottom area only.
(B) For 91% infiltration (water quality treatment volume)
Ensure that the pond infiltrates 91% using the pond bottom area only.
Infiltration facilities for treatment can be located upstream or downstream of detention and can be off-
line or on-line.
• On-line treatment facilities placed upstream or downstream of a detention facility
must be sized to infiltrate 91% of the runoff volume directed to it.
• Off-line treatment facilities placed upstream of a detention facility must have a flow
splitter designed to send all flows at or below the 15-minute water quality flow rate,
as predicted by WWHM to the treatment facility. The treatment facility must be sized
to infiltrate all the runoff sent to it (no overflows from the treatment facility are
allowed).
• Off-line treatment facilities placed downstream of a detention facility must have a
flow splitter designed to send all flows at or below the 2-year flow frequency from the
detention pond, as predicted by WWHM to the treatment facility. The treatment
facility must be sized to infiltrate all the runoff sent to it (no overflows from the
treatment facility are allowed).
See Volume V, Section 3.5.1 for flow splitter design details.
(C) To meet flow duration standard with infiltration ponds
This design will allow something less than 100% infiltration as long as any overflows will meet the
flow duration standard. You would need a discharge structure with orifices and risers similar to a
detention facility except that, in addition, you also have infiltration occurring from the pond.
Slope of the base of the infiltration facility must be <3 percent.
Spillways/overflow structures – A non-erodible outlet or spillway with a firmly established elevation
must be constructed to discharge overflow. Ponding depth, drawdown time, and storage volume are
calculated from that reference point.
For infiltration treatment facilities, side-wall seepage is not a concern if seepage occurs through the
same stratum as the bottom of the facility. However, for engineered soils or for soils with very low
permeability, the potential to bypass the treatment soil through the side-walls may be significant. In
those cases, the side-walls must be lined, either with an impervious liner or with at least 18 inches of
treatment soil, to prevent seepage of untreated flows through the side walls.
2.2.12 Construction Criteria
Initial basin excavation should be conducted to within 1-foot of the final elevation of the basin floor.
Excavate infiltration trenches and basins to final grade only after all disturbed areas in the upgradient
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project drainage area have been permanently stabilized. The final phase of excavation should
remove all accumulation of silt in the infiltration facility before putting it in service. After construction is
completed, prevent sediment from entering the infiltration facility by first conveying the runoff water
through an appropriate pretreatment system such as a pre-settling basin, wet pond, or sand filter.
Infiltration facilities should generally not be used as temporary sediment traps during construction. If
an infiltration facility is to be used as a sediment trap, it must not be excavated to final grade until after
the upgradient drainage area has been stabilized. Any accumulation of silt in the basin must be
removed before putting it in service.
Traffic Control – Relatively light-tracked equipment is recommended for this operation to avoid
compaction of the basin floor. The use of draglines and trackhoes should be considered for
constructing infiltration basins. The infiltration area should be flagged or marked to keep heavy
equipment away.
2.2.13 Maintenance Criteria
Provisions must be made for regular and perpetual maintenance of the infiltration basin/trench,
including replacement and/or reconstruction of the any media that are relied upon for treatment
purposes. Maintain when water remains in the basin or trench for more than 24 hours after the end of
a rainfall event, or when overflows occur more frequently than planned. Off-line infiltration facilities
should not overflow. Infiltration facilities designed to completely infiltrate all flows to meet flow control
standards should not overflow. An Operation and Maintenance Plan, approved by the local
jurisdiction, must ensure that the desired infiltration rate is maintained.
Adequate access for operation and maintenance must be included in the design of infiltration basins
and trenches.
Removal of accumulated debris/sediment in the basin/trench should be conducted every 6 months or
as needed to prevent clogging, or when water remains in the pond for greater than 24 hours after the
end of a rainfall event.
For more detailed information on maintenance, see Volume I, Appendix D – Maintenance Standards
for Drainage Facilities.
2.2.14 Verification of Performance
During the first 1-2 years of operation, verification testing (specified in SSC-9) is strongly
recommended, along with a maintenance program that results in achieving expected performance
levels. Operating and maintaining groundwater monitoring wells (specified in Section 2.2.10 - Site
Suitability Criteria) is also strongly encouraged.
2.2.15 Infiltration Basins
This section covers design and maintenance criteria specific for infiltration basins. See schematic in
Figure III-2-4.
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Description
Infiltration basins are earthen impoundments used for the collection, temporary storage and
infiltration of incoming stormwater runoff.
Design Criteria Specific for Basins
The finished floor elevation for buildings shall be a minimum of one foot (1’) above the maximum high
water elevation.
Access should be provided for vehicles to easily maintain the forebay (presettling basin) area and not
disturb vegetation, or resuspend sediment any more than is absolutely necessary.
The slope of the basin bottom should not exceed 3% in any direction.
A minimum of one foot of freeboard is recommended when establishing the design ponded water
depth. Freeboard is measured from the rim of the infiltration facility to the maximum ponding level or
from the rim down to the overflow point if overflow or a spillway is included.
Erosion protection of inflow points to the basin must also be provided (e.g., riprap, flow spreaders,
energy dissipators (See Volume III, Chapter 3). Select suitable vegetative materials for the basin floor
and side slopes to be stabilized. Refer to Volume V, Chapter 7 for recommended vegetation.
Lining material – Basins can be open or covered with a 6 to 12-inch layer of filter material such as
coarse sand, or a suitable filter fabric to help prevent the buildup of impervious deposits on the soil
surface. A nonwoven geotextile should be selected that will function sufficiently without plugging (see
geotextile specifications in Appendix C of Volume V). The filter layer can be replaced or cleaned
when/if it becomes clogged.
Vegetation – The embankment, emergency spillways, spoil and borrow areas, and other disturbed
areas should be stabilized and planted, preferably with grass, in accordance with Stormwater Site
Plan (See Minimum Requirement #1 of Volume I). Without healthy vegetation the surface soil pores
would quickly plug.
Refer to Section 2.3.1.2 for additional design criteria.
Maintenance Criteria for Basins
Maintain basin floor and side slopes to promote dense turf with extensive root growth. This enhances
infiltration, prevents erosion and consequent sedimentation of the basin floor, and prevents invasive
weed growth. Bare spots are to be immediately stabilized and revegetated.
Do not allow vegetation growth to exceed 18 inches in height. Mow the slopes periodically and check
for clogging, and erosion. Remove all clippings.
Seed mixtures should be the same as those recommended in Table II-3-3 (Volume II, Chapter 3).
The use of slow-growing, stoloniferous grasses will permit long intervals between mowing. Mowing
twice a year is generally satisfactory. Fertilizers shall not be applied.
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2.2.16 Infiltration Trenches
This section covers design, construction and maintenance criteria specific to infiltration trenches.
2.2.16.1 Description:
Infiltration trenches are generally at least 24 inches wide, and are backfilled with a coarse stone
aggregate, allowing for temporary storage of stormwater runoff in the voids of the aggregate material.
Stored runoff then gradually infiltrates into the surrounding soil. The surface of the trench can be
covered with grating and/or consist of stone, gabion, sand, or a grassed covered area with a surface
inlet. Perforated rigid pipe of at least 8-inch diameter can also be used to distribute the stormwater in
a stone trench. Perforated pipes used in conjunction with infiltration systems shall be installed with
the perforated holes facing downward toward the bottom of the trench.
2.2.16.2 Design Criteria
Due to accessibility and maintenance limitations, infiltration trenches must be carefully designed and
constructed.
Infiltration systems shall be located outside of parking and driving areas, unless otherwise approved
by the City.
Catch basins shall be provided on each end of the infiltration trench. Access to these catch basins is
required for maintenance and operation. Infiltration trenches and galleries shall be designed such that
no point in the facility is located more than fifty feet (50’) from an access structure.
Backfill Material - The aggregate material for the infiltration trench shall consist of a clean aggregate
with a maximum diameter of 3 inches and a minimum diameter of 1.5 inches. Void space for these
aggregates shall be in the range of 30 to 40 percent.
Geotextile fabric liner - The aggregate fill material shall be completely encased in an engineering
geotextile material. Geotextile should surround all of the aggregate fill material except for the top one-
foot, which is placed over the geotextile. Geotextile fabric with acceptable properties must be
carefully selected to avoid plugging (see Appendix C of Volume V).
The bottom sand or geotextile fabric is optional.
Refer to the Federal Highway Administration Manual “Geosynthetic Design and Construction
Guidelines,” Publication No. FHWA HI-95-038, May 1995 for design guidance on geotextiles in
drainage applications. Refer to the NCHRP Report 367, “Long-Term Performance of Geosynthetics
in Drainage Applications,” 1994, for long-term performance data and background on the potential for
geotextiles to clog, blind, or to allow piping to occur and how to design for these issues.
Overflow Channel - Because an infiltration trench is generally used for small drainage areas, an
emergency spillway is not necessary. However, a non-erosive overflow channel leading to a
stabilized watercourse should be provided.
Surface Cover - A stone filled trench can be placed under a porous or impervious surface cover to
conserve space.
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Observation Well - An observation well should be installed at the lower end of the infiltration trench
to check water levels, drawdown time, sediment accumulation, and conduct water quality monitoring.
Figure III-2-6 illustrates observation well details. It should consist of a perforated PVC pipe which is 4
to 6 inches in diameter and it should be constructed flush with the ground elevation. For larger
trenches a 12-36 inch diameter well can be installed to facilitate maintenance operations such as
pumping out the sediment. The top of the well should be capped to discourage vandalism and
tampering.
Figure III-2-6. Observation Well Details
2.2.16.3 Construction Criteria
Trench Preparation - Excavated materials must be placed away from the trench sides to enhance
trench wall stability. Care should also be taken to keep this material away from slopes, neighboring
property, sidewalks and streets. It is recommended that this material be covered with plastic. (see
Volume II, Chapter 3, BMP C123: Plastic Covering).
Stone Aggregate Placement and Compaction - The stone aggregate should be placed in lifts and
compacted using plate compactors. As a rule of thumb, a maximum loose lift thickness of 12 inches
is recommended. The compaction process ensures geotextile conformity to the excavation sides,
thereby reducing potential piping and geotextile clogging, and settlement problems.
Potential Contamination - Prevent natural or fill soils from intermixing with the stone aggregate. All
contaminated stone aggregate shall be removed and replaced with uncontaminated stone aggregate.
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Overlapping and Covering - Following the stone aggregate placement, the geotextile must be
folded over the stone aggregate to form a 12 inch minimum longitudinal overlap. When overlaps are
required between rolls, the upstream roll should overlap a minimum of 2 feet over the downstream
roll in order to provide a shingled effect.
Voids behind Geotextile - Voids between the geotextile and excavation sides must be avoided.
Removing boulders or other obstacles from the trench walls is one source of such voids. Natural soils
should be placed in these voids at the most convenient time during construction to ensure geotextile
conformity to the excavation sides. Soil piping, geotextile clogging, and possible surface subsidence
will be avoided by this remedial process.
Unstable Excavation Sites - Vertically excavated walls may be difficult to maintain in areas where
the soil moisture is high or where soft or cohesionless soils predominate. Trapezoidal, rather than
rectangular, cross-sections may be needed.
2.2.16.4 Maintenance Criteria
Sediment buildup in the top foot of stone aggregate or the surface inlet should be monitored on the
same schedule as the observation well.
2.3 Detention Facilities
This section presents the methods, criteria, and details for design and analysis of detention facilities.
These facilities provide for the temporary storage of increased surface water runoff resulting from
development pursuant to the performance standards set forth in Minimum Requirement #7 for flow
control (Volume I). Storm detention systems shall be designed such that the storm drainage from
public systems does not discharge into areas of private ownership or private maintenance
responsibility.
There are three primary types of detention facilities described in this section: detention ponds, tanks,
and vaults.
2.3.1 Detention Ponds
The design criteria in this section are for detention ponds. However, many of the criteria also apply to
infiltration ponds (Volume III, Section 2.2 and Volume V), and water quality wetponds and combined
detention/wetponds (Volume V). All detention ponds shall be appropriately and aesthetically located,
designed and planted. Pre-approval of the design concept, including landscaping is required by the
City for all proposed public ponds.
2.3.1.1 Dam Safety for Detention BMPs
Stormwater detention facilities that can impound 10 acre-feet (435,600 cubic feet; 3.26 million
gallons) or more above normal, surrounding grade with the water level at the embankment crest are
subject to Ecology’s dam safety requirements, even if water storage is intermittent and infrequent
(WAC 173-175-020). The principal safety concern is for the downstream population at risk if the dam
should breach and allow an uncontrolled release of the pond contents. Peak flows from dam failures
are typically much larger than the 100-year flows which these ponds are typically designed to
accommodate. The Applicant shall contact Ecology’s Dam Safety Engineers at Ecology
Headquarters if any of these conditions are met.
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2.3.1.2 Design Criteria
Standard details for detention ponds are provided in Figure III-2-7 through Figure III-2-10 and Table
III-2-10. Control structure discussion and details are provided in Section 2.3.4.
General
Ponds must be designed as flow-through systems (however, parking lot storage may
utilize a back-up system; see Section 2.3.5). Developed flows must enter through a
conveyance system separate from the control structure and outflow conveyance
system. Maximizing distance between the inlet and outlet is encouraged to promote
sedimentation.
Pond bottoms shall be level and be located a minimum of 0.5 feet below the inlet and
outlet to provide sediment storage.
Design criteria for outflow control structures are specified in Section 2.3.4.
A geotechnical analysis and report must be prepared for slopes 20% or greater, or if
located within 200 feet of the top of a slope 20% or greater or landslide hazard area.
The scope of the geotechnical report shall include the assessment of impoundment
seepage on the stability of the natural slope where the facility will be located within
the setback limits set forth in this section.
Detention ponds should be designed using rounded shapes and variations in slopes
to provide a more natural and aesthetically pleasing facility.
The total maximum depth of the detention pond from the bottom to the emergency
overflow water surface elevation shall be fifteen feet (15’).
Side Slopes
Interior side slopes above any wetpond surfaces, if present, shall not be steeper than
3H:1V unless an analysis is provided by a geotechnical engineer, demonstrating
that steeper slopes will be stable. The analysis shall include, at a minimum, an
assessment of the existing soil types, soil properties, groundwater conditions,
potential for seepage, and stability of proposed slopes. The geotechnical analysis
should also provide recommendations to ensure stability both during construction
and in perpetuity.
Exterior side slopes must not be steeper than 2H:1V unless analyzed for stability by
a geotechnical engineer.
A 10 foot level bench is required around the perimeter of the top of ponds to
separate the pond facility from adjacent slopes.
For maintenance and aesthetic reasons, pond designs should minimize structural
elements such as retaining walls. For ponds where retaining walls are required, they
should be limited to a maximum of three sides.
Pond walls may be vertical retaining walls, provided:
o They are constructed of minimum 3,000 psi structural reinforced concrete.
o A fence is provided along the top of the wall.
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o At least 25% of the pond perimeter shall be a vegetated soil slope not steeper
than 3H:1V.
o Access for maintenance per this section shall be provided.
o The design is stamped by a licensed civil engineer with structural expertise.
Other retaining walls such as rockeries, concrete, masonry unit walls, and keystone type walls may
be used if designed by a geotechnical engineer or civil engineer with structural expertise. If the entire
pond perimeter is to be retaining walls, ladders shall be provided on the walls for safety reasons.
Figure III-2-7. Typical Detention Pond
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Figure III-2-8. Typical Detention Pond Sections
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Figure III-2-9. Overflow Structure
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Embankments
• Pond berm embankments higher than 6 feet must be designed by a professional
engineer with geotechnical expertise.
• For berm embankments 6 feet or less in height, the minimum top width shall be 6
feet or as recommended by a geotechnical engineer.
• Pond berm embankments must be constructed on native consolidated soil (or
adequately compacted and stable fill soils analyzed by a geotechnical engineer) free
of loose surface soil materials, roots, and other organic debris.
• Pond berm embankments greater than 4 feet in height must be constructed by
excavating a key equal to 50 percent of the berm embankment cross-sectional height
and width unless specified otherwise by a geotechnical engineer. Embankment
compaction should be accomplished in such a manner as to produce a dense, low
permeability engineered fill that can tolerate post-construction settlements with a
minimum of cracking. The embankment fill shall be placed on a stable subgrade and
compacted to a minimum of 95% of the Standard Proctor Maximum Density, ASTM
Procedure D698. Placement moisture content should lie within 1% dry to 3% wet of
the optimum moisture content.
• The berm embankment shall be constructed of soils with the following minimum
characteristics per the United States Department of Agriculture’s Textural Triangle: a
minimum of 20% silt and clay, a maximum of 60% sand, a maximum of 60% silt, with
nominal gravel and cobble content.
• Anti-seepage filter-drain diaphragms must be placed on outflow pipes in berm
embankments impounding water with depths greater than 8 feet at the design water
surface. See Dam Safety Guidelines, Part IV, Section 3.3.B. An electronic version of
Dam Safety Guidelines is available in PDF format at
www.ecy.wa.gov/programs/wr/dams/dss.html
Overflow
• In all ponds, tanks, and vaults, a primary overflow (usually a riser pipe within the
control structure; see Section 2.3.4) shall be provided to bypass the 100-year
developed peak flow over or around the restrictor system. The design must provide
controlled discharge directly into the downstream conveyance system.
• A secondary inlet to the control structure shall be provided in ponds as additional
protection against overtopping should the inlet pipe to the control structure become
plugged. A grated opening (“jailhouse window”) in the control structure manhole
functions as a weir (see Figure III-2-8) when used as a secondary inlet.
The maximum circumferential length of this opening must not exceed one-half the control
structure circumference.
The “birdcage” overflow structure as shown in Figure III-2-9 may also be used as a
secondary inlet.
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Emergency Overflow Spillway
In addition to the above overflow provisions, ponds shall have an emergency
overflow spillway. For impoundments of 10 acre-feet or greater, the emergency
overflow spillway must meet the state’s dam safety requirements (see above). For
impoundments less than 10 acre-feet, ponds must have an emergency overflow
spillway that is sized to pass the 100-year developed peak flow. Emergency overflow
spillways shall control the location of pond overtopping such that flow is directed into
the downstream conveyance system or public right of way.
As an option for ponds with berms less than 2 feet in height and located at grades
less than 5 percent, emergency overflow may be provided by an emergency overflow
structure, such as a Type II manhole fitted with a birdcage as shown in Figure III-2-9.
The emergency overflow structure must be designed to pass the 100-year developed
peak flow, with a minimum of 6 inches of freeboard, directly to the downstream
conveyance system or another acceptable discharge point.
The emergency overflow spillway shall be armored with riprap in conformance with
the “Outlet Protection” BMP in Volume II (BMP C209). The spillway must be armored
full width, beginning at a point midway across the berm embankment and extending
downstream to where emergency overflows re-enter the conveyance system (See
Figure III-2-8).
Emergency overflow spillway designs must be analyzed as broad-crested trapezoidal
weirs as described in Methods of Analysis at the end of this section. Either one of the
weir sections shown in Figure III-2-8 may be used.
Access
The following access shall be provided.
Maintenance access road(s) shall be provided to the control structure and other
drainage structures associated with the pond (e.g., inlet or bypass structures).
An access ramp is required for pond cleaning and maintenance. The ramp must
extend to the pond bottom with a maximum slope of 7H:1V (see access road criteria
below).
The internal berm of a wetpond or combined detention and wetpond may be used for
access if it is designed to support a loaded 80,000 pound truck considering the berm is
normally submerged and saturated.
For combined detention and wetpond facilities, a 5 foot level bench area is required
around the perimeter a minimum of 1 foot, but no more than 3 feet, above the
wetpond surface elevation.
Where a portion of the pond is constructed within a fill slope, an access road shall be
provided adjacent to the detention pond along the entire length of the fill.
Access roads/ramps must meet the following requirements:
o Access roads may be constructed with an asphalt or gravel surface, or
modular grid pavement.
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o Maximum grade shall be 7H:1V percent.
o Outside turning radius shall be a minimum of 50 feet.
o Fence gates shall be located only on straight sections of road.
o Access roads shall be 15 feet in width.
o A driveway meeting City design standards must be provided where access
roads connect to paved public roadways.
If a fence is required, access shall be limited by a double-posted gate. If a fence is
not required, access shall be limited by two fixed bollards on each side of the access
road and two removable bollards equally located between the fixed bollards.
Additional easements or modification to proposed lot boundaries may be required to
provide adequate access to detention facilities. Right-of-way may be needed for
detention pond maintenance. Any tract not abutting public right-of-way shall have a
15-foot wide extension of the tract to an acceptable access location.
Fencing
A fence is required when a pond interior side slope is steeper than 3H:1V, or when
the wetpond depth is greater than 24 inches. Fencing is required for all vertical walls.
Fencing is required if more than 10 percent of slopes are steeper 3H:1V.
Also note that detention ponds on school sites shall comply with safety standards developed
by the Department of Health (DOH) and the Superintendent for Public Instruction (SPI).
These standards include what is called a ‘non-climbable fence.’
Fences shall be 42 inches in height (see WSDOT Standard Plan L-2, Type 1 chain
link fence).
Access gates shall be 16 feet in width consisting of two swinging sections 8 feet in
width.
Vertical metal balusters or 9 gauge galvanized steel fabric with bonded black vinyl
coating shall be used as fence material with the following aesthetic features:
o All posts, cross bars, and gates shall be painted or coated black.
o Fence posts and rails shall conform to WSDOT Standard Plan L-2 for Types
1, 3, or 4 chain link fence.
For metal baluster fences, Uniform Building Code standards apply.
Wood fences may be used in residential areas where the fence will be maintained by
homeowners associations or adjacent lot owners.
Wood fences shall have pressure treated posts (ground contact rated) either set in
24-inch deep concrete footings or attached to footings by galvanized brackets. Rails
and fence boards may be cedar, pressure-treated fir, or hemlock.
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Signage
Detention ponds, infiltration ponds, wetponds, and combined ponds in residential subdivisions shall
have a sign placed for maximum visibility from adjacent streets, sidewalks, and paths. An example
and specifications for a permanent surface water control pond are provided in Figure III-2-10 and
Table III-2-10.
Figure III-2-10. Examples of Permanent Surface Water Control Pond Sign
Table III-2-10. Permanent Surface Water Control Pond Sign Specifications
Size 48 inches by 24 inches
Material 0.125 gauge aluminum
Face Non-reflective vinyl or 3 coats outdoor enamel (sprayed)
Lettering Silk-screen enamel where possible, or vinyl letters
Colors Per City specifications where required
Type Face Helvetica condensed. Title: 3 inch; Sub-Title: 1-1/2 inch; Text: 1 inch;
Border Outer 1/8-inch border distance from edge: 1/4 inch
All text shall be at least 1-3/4 inches from border.
Installation Secure to chain link fence if available. Otherwise install on two posts as described below.
Top of sign no higher than 42 inches from ground surface.
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Posts Pressure-treated 4” x 4”; beveled tops 1-1/2 inches higher than the top of the sign; mounted
atop gravel bed, installed in 30-inch concrete-filled post holes (8-inch minimum diameter)
Placement Face sign in direction of primary visual or physical access. Do not block any access road.
Do not place within 6 feet of structural facilities (e.g. manholes, spillways, pipe inlets).
Special Notes This facility is lined to protect groundwater (if a liner restricting infiltration of stormwater is
used).
Setbacks
The City requires specific setbacks for sites with steep slopes, landslide areas, open water features,
springs, wells, and septic tank drain fields. Adequate room for maintenance access and equipment
shall also be considered. Project proponents should consult the Auburn City Codes to determine all
applicable setback requirements. Where a conflict occurs between setbacks, the most stringent of
the setback requirements applies.
Setbacks shall be as follows:
Stormwater ponds shall be set back at least 100 feet from drinking water wells, septic
tanks or drainfields, and springs used for public drinking water supplies.
Infiltration facilities upgradient of drinking water supplies and within 1, 5, and 10-year
time of travel zones must comply with Health Dept. requirements (Washington
Wellhead Protection Program, DOH Publication # 331-018). Additional setbacks for
infiltration facilities may be required per DOH publication #333-117, On-Site Sewage
Systems Chapter 246-272A WAC.
The 100-year water surface elevation shall be at least 10 feet from any structure or
property line. If necessary, setbacks shall be increased from the minimum 10 feet in
order to maintain a 1H:1V side slope for future excavation and maintenance. Vertical
pond walls may necessitate an increase in setbacks.
All pond systems shall be setback from sensitive areas, steep slopes, landslide hazard
areas, and erosion hazard areas as governed by the Auburn City Code. Facilities near
landslide hazard areas must be evaluated by a geotechnical engineer or qualified
geologist. The discharge point shall not be placed on or above slopes 20% (5H:1V) or
greater, or above erosion hazard areas without evaluation by a geotechnical engineer
or qualified geologist and City approval.
For sites with septic systems, ponds shall be downgradient of the drainfield unless the
site topography clearly prohibits subsurface flows from intersecting the drainfield.
Seeps and Springs
Intermittent seeps along cut slopes are typically fed by a shallow groundwater source (interflow)
flowing along a relatively impermeable soil stratum. These flows are storm driven. However, more
continuous seeps and springs, which extend through longer dry periods, are likely from a deeper
groundwater source. When continuous flows are intercepted and directed through flow control
facilities, adjustments to the facility design shall be made to account for the additional base flow. Flow
monitoring of intercepted flow may be required for design purposes.
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Planting Requirements
Exposed earth on the pond bottom and interior side slopes shall be sodded or seeded with an
appropriate seed mixture. All remaining areas of the tract shall be planted with grass or be
landscaped and mulched with a 4-inch cover of hog fuel or shredded wood mulch. Shredded wood
mulch is made from shredded tree trimmings, usually from trees cleared on site. The mulch should
be free of garbage and weeds and should not contain excessive resin, tannin, or other material
detrimental to plant growth. Multiple plantings and mulching may be required until vegetation has
established itself. A bond may be required to guarantee vegetation stabilization for detention facilities.
Landscaping
Public and private storm drainage facilities should enhance natural appearances, protect significant
cultural and natural resources, and be appropriate to the use of the site and the surrounding area.
Landscaping shall be designed to screen the storm drainage facilities and create a natural-appearing
setting while not adversely impacting the function and maintenance of the storm drainage facilities. A
Landscape Plan with the Stormwater Site Plan is required for City review and approval.
Landscaping is required for all stormwater tract areas (see below for areas not to be landscaped).
Landscaped stormwater tracts may, in some instances, provide a recreational space. In other
instances, “naturalistic” stormwater facilities may be placed in open space tracts.
The following criteria shall be incorporated when designing landscaping for storm drainage facilities.
Identify the type of landscaping and screening appropriate to the site taking into
account zoning and proposed use. Landscaping and screening requirements are
described in Auburn City Code (ACC) Title 18. The purpose of each type is to reflect
the level of landscaping and screening density needed to maintain compatibility with
the character of the neighborhood.
An effort should be made to retain all significant trees on site, evergreens six inches
(6”) or greater in diameter, or any deciduous tree four inches (4”) in diameter or greater
as defined in ACC Title 18. Diameter measurements are taken at four feet (4’) above
grade elevation. Authorization by the City is required for removal of any significant
trees.
Identify the soil type and hydrological regime or each portion of the storm drainage
facility to determine appropriate site criteria for plant selection.
Select tree and shrub species from the Plant Selection Guide contained herein. Plant
choices must reflect the functional and aesthetic needs of the site. Fall planting is
recommended for optimal acclimation and survivability. An irrigation system will be
required for public ponds to insure plant establishment. Irrigation systems may also be
needed for private ponds if plantings are done in the spring/summer or in times of
limited precipitation, unless other watering provisions are established.
Plant choices are not restricted to those listed in the Plant Selection Guide, but plant
selection must be based on ease of maintenance, appropriateness to the use of the
site (commercial, residential, or industrial), and survivability. Plant selection should
correspond with street tree requirements and neighborhood character as appropriate.
Selections are to be approved by the City during the review process. NOTE: Plants
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identified in the Guide are predominately native and reflect the soil conditions and
water regimes of the area.
Develop a Landscape Plan to scale identifying the location and species of existing
trees and the location and schedule of species, quantity and size of all proposed tree,
shrubs, and ground covers. Drawings should be scaled at 1”=10’ or 1”=20’ to optimally
relay information on the plant location and placement. Construction specifications
should indicate appropriate soil amendments where necessary and planting
specifications as recommended by the American Standards for Nursery Stock and the
American National Standards Institute (ANSI).
Excluding access points, a minimum of ten feet (10’) width of Type-III landscaping in
accordance with Auburn City Code 18.50 shall be provided around the exterior length
of the pond. This width may be reduced to five feet (5’) if the interior side slopes of the
pond are landscaped.
No tree and shrub planting is allowed with pipeline easements, traveled surfaces, or
over underground utilities.
No trees or shrubs shall be planted within 10 feet of inlet or outlet pipes or manmade
drainage structures such as spillways or flow spreaders. Species with roots that seek
water, such as willow or poplar, shall be avoided within 50 feet of pipes or manmade
structures.
Planting shall be restricted on berms that impound water either permanently or
temporarily during storms. This restriction does not apply to cut slopes that form pond
banks, only to berms.
o Trees or shrubs may not be planted on portions of water-impounding berms
taller than four feet high. Only grasses may be planted on berms taller than
four feet.
Grasses allow unobstructed visibility of berm slopes for detecting potential dam safety
problems, such as animal burrows, slumping, or fractures in the berm.
o Trees planted on portions of water-impounding berms less than 4 feet high
must be small, not higher than 20 feet mature height, and must have a fibrous
root system. Table III-2-11 gives some examples of trees with these
characteristics developed for the Central Puget Sound.
NOTE: The internal berm in a wetpond is not subject to this planting restriction since the
failure of an internal berm would be unlikely to create a safety problem.
All landscape material, including grass, shall be planted in topsoil. Native underlying
soils may be made suitable for planting if amended with 4 inches of compost tilled
into the subgrade. Compost used should meet specifications for Grade A compost
quality. See http://www.ecy.wa.gov/programs/swfa/compost/
For a naturalistic effect as well as ease of maintenance, trees of shrubs shall be
planted in clumps to form “landscape islands” rather than planting evenly spaced.
The landscaped islands shall be a minimum of six feet apart, and if set back from
fences or other barriers, the setback distance should also be a minimum of 6 feet.
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Where tree foliage extends low to the ground, the 6 feet setback should be counted
from the outer drip line of the trees (estimated at maturity).
This setback allows a 6-foot wide mower to pass around and between clumps.
Evergreen trees and trees which produce relatively little leaf-fall (such as Oregon ash,
mimosa, or locust) are preferred in areas draining to the pond.
Trees should be set back so that branches do not extend over the pond (to prevent
deposition of leaves into the pond).
Drought tolerant species are recommended.
The following lists contain the suggested trees, plants and grasses to be used in landscaping storm
drainage facilities. The trees and plants listed are native to the region and should be chosen over
non-native species. The lists shown are not all-inclusive, additional trees and plants may be
acceptable upon approval of the City.
Table III-2-11. Plant Selection Guide
Tree Selection Guide for Storm Drainage Detention/Retention Facilities
Suggested Trees Tolerates Wet to
Saturated Soils
Recommend Moderately
Wet to Dry Soils
Recommend
Dry Soils Botanical Name Common Name
Acer circinatum Vine Maple ♦
Alnus rubra Red Alder ♦
Betula papyrifera Paper Birch ♦
Corylus cornuta Hazelnut ♦
Crataegus douglasii Black Hawthorn ♦
Fraxinus latifolia Oregon Ash ♦
Picea sitchensis Sitka Spruce ♦
Pinus contorta Shore Pine ♦
Pinus monticula Western White Pine ♦
Populus tremuloides Quaking Aspen ♦
Prunus virginiana Choke Cherry ♦
Pseudotsuga menziesii Douglas Fir ♦
Salix lasiandra Pacific Willow ♦
Salix scouleriana Scouler Willow ♦
Salix sitchensis Sitka Willow ♦
Thuja pljcata Western Red Cedar ♦
Tsuga heterophylla Western Hemlock ♦
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Shrub Selection Guide for Storm Drainage Detention/Retention Facilities
Suggested Shrubs Tolerates Wet to
Saturated Soils
Recommend Moderately
Wet to Dry Soils
Recommend
Dry Soils Botanical Name Common Name
Amelanchier alnifolia Serviceberry ♦
Cornus sericea Red Osier Dogwood ♦
Gaultheria shallon Salal ♦
Holidiscus discolor Ocean Spray ♦
Lonicera involucrata Black Twinberry ♦
Mahonia aquifolium Tall Oregon Grape ♦
Mahonia repens Low Oregon Grape ♦
Oemleria cerasiformis Indian Plum ♦
Physocarpus capitatus Pacific Ninebark ♦
Ribes sanguineum Red Flowering Currant ♦
Rosa nutkana Nootka Rose ♦
Rosa rugosa Rugosa Rose ♦
Rubus spectabilis Salmonberry ♦
Rubus spectabilis Thimbleberry ♦
Sambucus racemosa Red Elderberry ♦
Symphoricarpos albus Snowberry ♦
Vaccinium ovatum Evergreen Huckleberry ♦
Vaccinium parviflorum Red Huckleberry ♦
Perennial Groundcover Selection Guide for Storm Drainage Detention/Retention Facilities
Suggested Perennial Groundcover Tolerates Wet
to Saturated
Soils
Recommend Moderately
Wet to Dry Soils
Recommend
Dry Soils Botanical Name Common Name
Athyrium filix-femina Lady Fern ♦
Dicentra formosa Bleeding Heart ♦
Polystichum munitum Sword Fern ♦
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Aquatic/Emergent Wetland Selection Guide for Storm Drainage Detention/Retention Facilities
Suggested Aquatics/Emergent Wetland Plants
Botanical Name Common Name
Tolerates Open Water (3’ + Depth) to
Shallow Standing Water (<1’ Depth)
Potamogeton natans Floating Pondweed
Lotus conicalitatus Birdsfoot Trefoil
Nymphaea odorata American Water Lily
Lemna minor Common Duckweed
Polygonum punctatum Dotted Smartweed
Polygonum amphibium Water Smartweed
Oenanthe sarmentosa Water Parsley
Alisma plantago-aquitica American Waterplantain
Sparganium spp. Bur-reed
Sagittaria spp. Arrowhead
Scirpus acutus Hardstem Bulrush
Scirpus microcarpus Small-fruited Bulrush
Carex obnupta Slough Sedge
Carex languinosa Wooly Sedge
Eleocharis spp. Spike Rush
Carex spp. Sedge
Tolmiea menziesii Piggy back plant
Hordcum brachyantherum Meadow Barley
Grass Seed Mixes for Detention/Retention Facilities
Moisture Condition By Weight Species Common Name Percent
Very Moist Agrosotis tenuis Colonial Bentgrass 50
Festuca ruba Red Fescue 10
Alopocuris pratensis Meadow Foxtail 40
Moist Festuca arundinacea Meadow Fescue 70
Agrosotis tenuis Colonial Bentgrass 15
Alopecurus pratensis Meadow Foxtail 10
Trifoluim hybridum White Clover 5
Moist-Dry Agrosotis tenuis Colonial Bentgrass 10
Festuca ruba Red Fescue 40
Lolium multiflorum Annual Ryegrass 40
Trifolium repens White Clover 10
Application rates: Hydroseed @ 60 lbs/acre Handseed @ 2 lbs/1000 square feet
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Maintenance
A maintenance plan shall be prepared for all surface water management facilities. See Volume I,
Appendix D for specific maintenance requirements.
All private drainage systems serving multiple lots shall require a signed Stormwater Maintenance
and Access agreement with the City. The agreement shall designate the systems to be maintained
and the parties responsible for maintenance. Contact the City to determine the applicability of this
requirement to a project.
Any standing water removed during the maintenance operation must be disposed of in a City
approved manner. See the dewatering requirements in Volume II of this manual. Pretreatment may
be necessary. Residuals must be disposed in accordance with state and local solid waste regulations
(See Minimum Functional Standards for Solid Waste Handling, Chapter 173-304 WAC).
2.3.1.3 Methods of Analysis
Detention Volume and Outflow
The volume and outflow design for detention ponds must be in accordance with Minimum
Requirements # 7 in Volume I and the hydrologic analysis and design methods in Chapter 1 of this
Volume. Design guidelines for restrictor orifice structures are given in Section 2.3.4.
The design water surface elevation is the highest elevation which occurs in order to meet the
required outflow performance for the pond.
Detention Ponds in Infiltrative Soils
Detention ponds may occasionally be sited on till soils that are sufficiently permeable for a properly
functioning infiltration system (see Section 2.2). These detention ponds have a surface discharge and
may also utilize infiltration as a second pond outflow. Detention ponds sized with infiltration as a
second outflow must meet all the requirements of Section 2.2 for infiltration ponds, including a soils
report, testing, groundwater protection, pre-settling, and construction techniques.
Emergency Overflow Spillway Capacity
For impoundments under 10-acre-feet, or ponds not subject to dam safety requirements, the
emergency overflow spillway weir section must be designed to pass the 100-year runoff event for
developed conditions assuming a broad-crested weir. The broad-crested weir equation for the
spillway section in Figure III-2-11, for example, would be:
Ql00 = C (2g) 1/2 [3
2 LH3/2 + 15
8 (Tan ) H5/2 ] (equation 1)
Where Ql00 = peak flow for the 100-year runoff event (cfs)
C = discharge coefficient (0.6)
g = gravity (32.2 ft/sec2)
L = length of weir (ft)
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H = height of water over weir (ft)
= angle of side slopes (degrees)
NOTE: Q100 is either the peak 10-minute flow computed from the 100-year, 24-hour storm and a
Type 1A distribution, or the 100-year, 1-hour flow, indicated by an approved continuous runoff model,
multiplied by a factor of 1.6
Assuming C = 0.6 and Tan = 3 (for 3:1 slopes), the equation becomes:
Ql00 = 3.21[LH3/2 + 2.4 H5/2 ] (equation 2)
To find width L for the weir section, the equation is rearranged to use the computed Ql00 and trial
values of H (0.2 feet minimum):
L = [Ql00/(3.21H3/2)] - 2.4 H or 6 feet minimum (equation 3)
Figure 3-19 Weir Section for Emergency Overflow Spillway
“Outlet Protection” in Vol. II
0.7 ft. min
0.5 ft. min
per “Outlet Protection” in Volume 2
overflow
water
surface
emergency overflow
water surface
0.2 ft. min
Figure III-2-11. Weir Section for Emergency Overflow Spillway
2.3.2 Detention Tanks
Detention tanks are underground storage facilities typically constructed with large diameter pipe.
Standard detention tank details are shown in Figure III-2-12 and Figure III-2-13. Control structure
details are shown in Section 2.3.4.
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Figure III-2-12. Typical Detention Tank
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Notes:
1. Use adjusting blocks as required to bring frame to grade.
2. All materials to be aluminum or galvanized and asphalt coated (Treatment 1 or
better).
3. Must be located for access by maintenance vehicles.
4. May substituteWSDOT special Type IV manhole (RCP only).
Figure III-2-13. Detention Tank Access Detail
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2.3.2.1 Design Criteria
General
• Tanks shall be designed as flow-through systems with manholes in line (see Figure
III-2-12) to promote sediment removal and facilitate maintenance. Tanks shall also
be designed to allow stormwater to back-up into the system if the tank is preceded
by water quality facilities.
• The detention tank bottom shall be located 6 inches below the inlet and outlet to
provide dead storage for sediment. If arch pipe is used, the minimum dead storage is
0.5 feet.
• The minimum pipe diameter for a detention tank is 36 inches.
• The minimum thickness for CMP shall be 12-gauge.
• Tanks larger than 36 inches may be connected to each adjoining structure with a
short section (2-foot maximum length) of 35-inch minimum diameter pipe. These
sections shall not be considered as access when determining required access
points.
• Details of outflow control structures are given in Section 2.3.4.
Materials
See City of Auburn Construction Standards Section 9.05.
Structural Stability
Tanks must meet structural requirements for overburden support and traffic loading if appropriate.
H-20 live loads shall be accommodated for tanks lying under parking areas and access roads. Metal
tank end plates shall be designed for structural stability at maximum hydrostatic loading conditions.
Tanks shall not be placed in fill slopes, unless analyzed in a geotechnical report for stability and
constructability.
Buoyancy
Buoyancy calculations shall be required where groundwater may induce flotation. Engineers are
required to address this issue in project design documentation.
Access
The following requirements for access shall be met along with those stipulated in Section 2.3.1.
• The maximum depth from finished grade to tank invert shall be 20 feet.
• Access openings shall be positioned a maximum of 50 feet from any location within
the tank. A minimum of one access opening per tank shall be provided.
• All tank access openings shall have round, solid locking lids (usually 1/2 to 5/8-inch
diameter Allen-head cap screws).
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• Thirty-six inch minimum diameter CMP riser-type manholes (see Figure III-2-13) of
the same gauge as the tank material may be used for access along the length of the
tank and at the upstream terminus of the tank in a backup system. The top slab is
separated (1-inch minimum gap) from the top of the riser to allow for deflections from
vehicle loadings without damaging the riser tank.
• All tank access openings must be readily accessible to maintenance vehicles.
• Tanks must comply with the OSHA confined space requirements, which include
clearly marking entrances to confined space areas. This may be accomplished by
hanging a removable sign in the access riser(s) just under the access lid.
Access Roads
Access roads are needed to all detention tanks, control structures, and risers. The access roads
must be designed and constructed as specified for detention ponds in Section 2.3.1.
Setbacks
For setback requirements see Section 2.3.1.
Maintenance
Provisions to facilitate maintenance operations must be built into the project when it is installed.
Maintenance must be a basic consideration in design and in determination of first cost. See Volume I,
Appendix D for specific maintenance requirements.
Methods of Analysis
Detention Volume and Outflow
The volume and outflow design for detention tanks must be in accordance with Minimum
Requirement # 7 in Volume I and the hydrologic analysis and design methods in Volume III,
Chapter 1. Restrictor and orifice design are given in Section 2.3.4.
2.3.3 Detention Vaults
Detention vaults are box-shaped underground storage facilities typically constructed with reinforced
concrete. A standard detention vault detail is shown in Figure III-2-14. Control structure details are
shown in Section 2.3.4. A detention vault may be used for commercial, industrial, or roadway projects
when there are space limitations precluding the use of aboveground storage options. Vaults are box-
shaped underground storage facilities typically constructed with reinforced concrete. The use of
public vaults for residential development is discouraged. A design of a detention vault may be
modified into a wetvault to provide stormwater quality (see Volume V 8.2.2)
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Figure III-2-14. Typical Detention Vault
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2.3.3.1 Design Criteria
General
• Detention vaults shall be designed as flow-through systems with bottoms level
(longitudinally) or sloped toward the inlet to facilitate sediment removal. Distance
between the inlet and outlet should be maximized (as feasible).
• The detention vault bottom shall slope at least 5 percent from each side towards the
center, forming a broad “v” to facilitate sediment removal. More than one “v” may be
used to minimize vault dept. The vault bottom may be flat with 0.5 – 1 foot of
sediment storage if removable panels are provided over the entire vault. It is
recommended that the removable panels be at grade, have stainless steel lifting
eyes, and weigh no more than 5 tons per panel.
• The invert elevation of the outlet shall be elevated above the bottom of the vault to
provide an average 6 inches of sediment storage over the entire bottom. The outlet
shall also be elevated a minimum of 2 feet above the orifice to retain oil within the
vault.
• Details of outflow control structures are given in Section 2.3.4.
Buoyancy
A buoyancy analysis is required to demonstrate that the vault will not be impacted by ground water.
Materials
Minimum 3,000 psi structural reinforced concrete may be used for detention vaults. All construction
joints must be provided with water stops.
Structural Stability
All vaults must meet structural requirements for overburden support and H-20 traffic loading (See
Standard Specifications for Highway Bridges, 1998 Interim Revisions, American Association of State
Highway and Transportation Officials). Vaults located under roadways must meet live load
requirements of the City. Cast-in-place wall sections must be designed as retaining walls. Structural
designs for cast-in-place vaults must be stamped by a licensed civil engineer with structural
expertise. Vaults must be placed on stable, well-consolidated native material with suitable bedding.
Vaults must not be placed in fill slopes, unless analyzed in a geotechnical report for stability and
constructability.
Access
Access must be provided over the inlet pipe and outlet structure. The following guidelines for access
shall be used.
• Access openings shall be positioned a maximum of 50 feet from any location within
the vault. Additional access points may be needed on large vaults.
• An access opening shall be provided directly above the lowest point of each “v” in
the vault floor.
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• An access opening shall be provided directly above each connection to the vault.
• For vaults with greater than 1,250 square feet of floor area, a 5’ x 10’ removable
panel should be provided over the inlet pipe (instead of a standard frame, grate and
solid cover). Alternatively, a separate access vault may be provided, as shown in
Figure III-2-14.
• For vaults under roadways, the removable panel must be located outside the travel
lanes. Alternatively, multiple standard locking manhole covers may be provided.
• Ladders and hand-holds shall be provided at all access openings, and as needed to
meet OSHA confined space requirements.
• All access openings, except those covered by removable panels, may have round,
solid locking lids, or 3-foot square, locking diamond plate covers.
• Vaults with widths 10 feet or less must have removable lids.
• The maximum depth from finished grade to the vault invert shall be 15 feet.
• Internal structural walls of large vaults should be provided with openings sufficient for
maintenance access between cells. The openings should be sized and situated to
allow access to the maintenance “v” in the fault floor.
• A minimum of two access openings shall be provided into each cell.
• The minimum internal height shall be 7 feet from the highest point of the vault floor
(not sump), and the minimum width shall be 4 feet. However, concrete vaults may be
a minimum 3 feet in height and width if used as a tank with access manholes at each
end, and if the width is no larger than the height. Also the minimum internal height
requirement may not be needed for any areas covered by removable panels.
• Vaults must comply with the OSHA confined space requirements, which include
clearly marking entrances to confined space areas. This may be accomplished by
hanging a removable sign in the access riser(s), just under the access lid.
• Ventilation pipes (minimum 12-inch diameter or equivalent) shall be provided in all
four corners of vaults to allow for artificial ventilation prior to entry of maintenance
personnel into the vault. Alternatively, removable panels over the entire vault, or
manhole access at 12-foot spacing, may be provided.
Access Roads
Access shall be designed and constructed as specified for detention ponds in Section 2.3.1.
Setbacks
For setback requirements see Section 2.3.1.
Maintenance
Provisions to facilitate maintenance operations must be built into the project when it is installed.
Maintenance must be a basic consideration in design and in determination of first cost. See Volume I,
Appendix D for specific maintenance requirements.
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2.3.3.2 Methods of Analysis
Detention Volume and Outflow
The volume and outflow design for detention vaults must be in accordance with Minimum
Requirement # 7 in Volume I and the hydrologic analysis and design methods in Chapter 1.
Restrictor and orifice design are given in Section 2.3.4.
2.3.4 Control Structures
Control structures are catch basins or manholes with a restrictor device for controlling outflow from a
facility to meet the desired performance.
The restrictor device usually consists of two or more orifices and/or a weir section sized to meet
performance requirements. Standard control structure details are shown in Figure III-2-15 through
Figure III-2-17.
2.3.4.1 Design Criteria
Multiple Orifice Restrictor
In most cases, control structures need only two orifices: one at the bottom and one near the top of
the riser, although additional orifices may best utilize detention storage volume. Several orifices may
be located at the same elevation if necessary to meet performance requirements.
• Minimum orifice diameter is 0.5 inches. In some instances, a 0.5-inch bottom orifice
will be too large to meet target release rates, even with minimal head. In these
cases, do not reduce the live storage depth to less than 3 feet in an attempt to meet
the performance standards. Under such circumstances, flow-throttling devices may
be a feasible option. These devices will throttle flows while maintaining a plug-
resistant opening.
• Orifices may be constructed on a tee section as shown in Figure III-2-15 or on a
baffle as shown in Figure III-2-16.
• In some cases, performance requirements may require the top orifice/elbow to be
located too high on the riser to be physically constructed (e.g. a 13-inch diameter
orifice positioned 0.5 feet from the top of the riser). In these cases, a notch weir in
the riser pipe may be used to meet performance requirements (see Figure III-2-17).
• Backwater effects from water surface elevations in the conveyance system shall be
evaluated. High tailwater elevations may affect performance of the restrictor system
and reduce live storage volumes. Backwater effects shall also be analyzed for areas
that are influenced by tides.
Riser and Weir Restrictor
• Properly designed weirs may be used as flow restrictors (see Figure III-2-17 and
Figure III-2-19 through Figure III-2-21). However, they must be designed to provide
for primary overflow of the developed 100-year peak flow discharging to the
detention facility.
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• The combined orifice and riser (or weir) overflow may be used to meet performance
requirements. However, the design must still provide for primary overflow of the
developed 100-year peak flow assuming all orifices are plugged. Figure III-2-22 can
be used to calculate the head in feet above a riser of given diameter and flow.
Access
The following guidelines for access shall be used.
• An access road to the control structure is needed for inspection and maintenance,
and must be designed and constructed as specified for detention ponds in
Section 2.3.1.
• Manhole and catch basin lids for control structures must be locking, and rim
elevations must match proposed finish grade.
• Manholes and catch basins must meet the OSHA confined space requirements,
which include clearly marking entrances to confined space areas. This may be
accomplished by hanging a removable sign in the access riser, just under the access
lid.
Information Plate
A brass or stainless steel plate shall be permanently attached inside each control structure with the
following information engraved on the plate:
• Name and file number of project
• Name and company of (1) developer, (2) engineer, and (3) contractor
• Date constructed
• Date of manual used for design
• Outflow performance criteria
• Release mechanism size, type, and invert elevation
• List of stage, discharge, and volume at one-foot increments
• Elevation of overflow
• Recommended frequency of maintenance.
2.3.4.2 Maintenance
Control structures require regular maintenance and cleaning. Maintenance frequency and
procedures shall be addressed in the facility maintenance manual.
Volume I, Appendix D provides maintenance recommendations for control structures and catch
basins.
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2.3.4.3 Methods of Analysis
This section presents the methods and equations for design of control structure restrictor devices.
Included are details for the design of orifices, rectangular sharp-crested weirs, v-notch weirs, sutro
weirs, and overflow risers.
Orifices
Flow-through orifice plates in the standard tee section or turn-down elbow may be approximated by
the general equation:
gh2A CQ= (equation 4)
where Q = flow (cfs)
C = coefficient of discharge (0.62 for plate orifice)
A = area of orifice (ft2)
h = hydraulic head (ft)
g = gravity (32.2 ft/sec2)
Figure III-2-18 illustrates this simplified application of the orifice equation.
The diameter of the orifice is calculated from the flow. The orifice equation is often useful when
expressed as the orifice diameter in inches:
h
Qd88.36= (equation 5)
where d = orifice diameter (inches)
Q = flow (cfs)
h = hydraulic head (ft)
Rectangular Sharp-Crested Weir
The rectangular sharp-crested weir design shown in Figure III-2-19 may be analyzed using standard
weir equations for the fully contracted condition.
Q = C (L - 0.2H)H 23 (equation 6)
where Q = flow (cfs)
C = 3.27 + 0.40 H/P (ft)
H, P = as shown in Figure III-2-19
L = length (ft) of the portion of the riser circumference
as necessary not to exceed 50 percent of the circumference
D = inside riser diameter (ft)
NOTE: This equation accounts for side contractions by subtracting 0.1H from L for each side of the
notch weir.
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V-Notch Sharp - Crested Weir
V-notch weirs as shown in Figure III-2-20 may be analyzed using standard equations for the fully
contracted condition.
Proportional or Sutro Weir
Sutro weirs are designed so that the discharge is proportional to the total head. This design may be
useful in some cases to meet performance requirements.
The sutro weir consists of a rectangular section joined to a curved portion that provides
proportionality for all heads above the line A-B (see Figure III-2-21). The weir may be symmetrical or
non-symmetrical.
For this type of weir, the curved portion is defined by the following equation (calculated in radians):
a
ZTanb
x 121--= (equation 7)
Where a, b, x and Z are as shown in Figure III-2-14.
The head-discharge relationship is:
)3h(2 b C 1d
agaQ-= (equation 8)
where Q = flow (cfs)
g = gravity
Values of Cd for both symmetrical and non-symmetrical sutro weirs are summarized in Table III-2-12.
When b > 1.50 or a > 0.30, use Cd=0.6.
Riser Overflow
The nomograph in Figure III-2-22 can be used to determine the head (in feet) above a riser of given
diameter and for a given flow (usually the 100-year peak flow for developed conditions).
NOTE: Q100 is either the peak 10-minute flow computed from the 100-year, 24-hour storm and a
Type 1A distribution, or the 100-year, 1-hour flow, indicated by an approved continuous runoff model,
multiplied by a factor of 1.6
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Table III-2-12. Values of Cd for Sutro Weirs
Cd Values, Symmetrical
b (ft)
a (ft)
0.50 0.75 1.0 1.25 1.50
0.02 0.608 0.613 0.617 0.6185 0.619
0.05 0.606 0.611 0.615 0.617 0.6175
0.10 0.603 0.608 0.612 0.6135 0.614
0.15 0.601 0.6055 0.610 0.6115 0.612
0.20 0.599 0.604 0.608 0.6095 0.610
0.25 0.598 0.6025 0.6065 0.608 0.6085
0.30 0.597 0.602 0.606 0.6075 0.608
Cd Values, Non-Symmetrical
b (ft) a (ft)
0.50 0.75 1.0 1.25 1.50
0.02 0.614 0.619 0.623 0.6245 0.625
0.05 0.612 0.617 0.621 0.623 0.6235
0.10 0.609 0.614 0.618 0.6195 0.620
0.15 0.607 0.6115 0.616 0.6175 0.618
0.20 0.605 0.610 0.614 0.6155 0.616
0.25 0.604 0.6085 0.6125 0.614 0.6145
0.30 0.603 0.608 0.612 0.6135 0.614
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Figure III-2-15. Flow Restrictor (TEE)
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NOTES:
Outlet capacity: 100 year developed peak flow
Metal parts: corrosion resistant steel parts
galvanized and asphalt coated
Catch basin: Type 2, minimum 72-inch diameter
Orifices: Sized and located as required with
lowest orifice a min. or 2" from base
Figure III-2-16. Flow Restrictor (Baffle)
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Spill containment must be provided to temporarily detain oil or floatable pollutants in runoff due to accidental spill
or illegal dumping.
Frames, grates and round solid
covers marked “DRAIN” with
locking bolts
shear gate with
control red for
drain
Figure III-2-17. Flow Restrictor (Weir)
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Figure III-2-18. Simple Orifice
Figure III-2-19. Rectangular, Sharp-Crested Weir
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Q = Cd(Tan /2)Y 5/2, in cfs
Y
H
Figure III-2-20. V-Notch, Sharp-Crested Weir
Figure III-2-21. Sutro Weir
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Figure III-2-22. Riser Inflow Curves
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2.3.5 Other Detention Options
This section presents other design options for detaining flows to meet flow control facility
requirements.
Use of Parking Lots for Additional Detention
Private parking lots may be used to provide additional detention volume for runoff events greater than
the 2-year runoff event provided all of the following are met:
• The depth of water detained does not exceed 0.5 feet (6 inches) at any location in
the parking lot for runoff events up to and including the 100-year event.
• The gradient of the parking lot area subject to ponding is 1 percent or greater.
• The emergency overflow path is identified and noted on the engineering plan. The
overflow must not create a significant adverse impact to downhill properties or
drainage system.
• Fire lanes be used for emergency equipment are free of ponding water for all runoff
events up to and including the 100-year event.
• The overflow elevation shall be a minimum of one foot (1’) below the finish floor
elevation of adjacent building, adjacent properties, landscaping and parking stalls.
• At no time shall parking lot ponding encroach on walking paths, sidewalks, or
American Disabilities Act (A.D.A) required parking stalls or adjacent A.D.A. access.
Use of Roofs for Detention
Detention ponding on roofs of structures may be used to meet flow control requirements provided all
of the following are met:
• The roof support structure is analyzed by a structural engineer to address the weight of
ponded water and meets the requirements of the applicable building code.
• The roof area subject to ponding is sufficiently waterproofed to achieve a minimum
service life of 30 years.
• The minimum pitch of the roof area subject to ponding is 1/4-inch per foot.
• An overflow system is included in the design to safely convey the 100-year peak flow
from the roof.
• A mechanism is included in the design to allow the ponding area to be drained for
maintenance purposes, or in the event the restrictor device is plugged.
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and Hydraulic Analysis Chapter 3 387
Chapter 3 Conveyance System Design
and Hydraulic Analysis
This chapter presents acceptable methods for the analysis and design of storm and surface water
conveyance systems. Conveyance systems can be separated into the following categories:
• Pipe systems
• Culverts
• Open Channels (ditches, swales)
• Outfalls
Pipe systems, culverts, and open channels are addressed in Section 3.4. Outfalls are addressed in
Section 3.5.
The purpose of a conveyance system is to drain surface water, up to a specific design flow, from
properties so as to provide protection to property and the environment. This chapter contains detailed
design criteria, methods of analysis and standard details for all components of a conveyance system.
A complete basic understanding of hydrology and hydraulics and the principles on which the
methodology of hydrologic analysis is based is essential for the proper and accurate application of
methods used in designing conveyance systems.
• A minimum of ten (10’) shall be provided between the centerline of the conveyance and any
property line or obstruction that would impede maintenance.
• Where storm drainage is directed against a curb, the curb shall be either a concrete curb and
gutter or concrete vertical curb. An extruded curb or asphalt wedge section in any form will
not be allowed.
3.1 Conveyance System Analysis Requirements
The project engineer shall provide calculations demonstrating the adequacy of all the project’s
existing and proposed surface water conveyance system components. The project engineer shall
provide calculations regarding all off-site flows as required by Volume I. All relevant work/calculations
shall be submitted for City review as part of a permit submittal. Small and/or isolated storm system
(detention and water quality treatment) designs shall address how they will be incorporated into
larger drainage systems likely to be built in the future. For example, site specific frontage and half
street improvement designs shall also use a corridor analysis approach to ensure that they can be
incorporated into larger future storm systems.
3.1.1 On-site Analysis
All proposed on-site surface water conveyance systems shall be sized to meet the required design
event per Section 3.2.
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3.1.2 Offsite Analysis (1/4 mile Downstream Analysis)
Refer to Minimum Requirement #10 (Offsite Analysis and Mitigation) in Volume I to determine
whether a downstream analysis is required for a specific project. All projects shall complete a
qualitative downstream analysis. A quantitative analysis shall be required as described in Minimum
Requirement #10.
The engineer must field survey all existing storm drainage systems downstream from the project for a
minimum of ¼ mile from the point of connection to the existing public drainage system, unless a City-
identified trunk-line is encountered. The goal of the inspection and analysis is to evaluate whether the
capacity of the drainage system(s) is adequate to handle the existing flows, flows generated by the
proposed project, and any overflow. Adequacy will be evaluated based on conveyance capacity,
flooding problems, erosion damage or potential, amount of freeboard in channels and pipes, and
storage potential within the system. All existing and proposed off-site surface water conveyance
systems shall be sized to convey flows from the required design storm event per Section 3.2.
The offsite analysis may be stopped shorter than the required ¼-mile downstream if the analysis
reaches a City identified trunk line. Storm drainage pipes greater than or equal to 36 inches in
diameter are generally considered trunk lines. However, where minimal grades (less than 0.5%)
necessitated the use of a larger pipe to maintain flows, the City may not consider a pipe greater than
or equal to 36 inches as a trunk line. Contact the City for final determination of whether a storm
drainage pipe is a trunk line.
If a capacity problem or streambank erosion problem is encountered, the flow durations from the
project will be restricted per Minimum Requirement #7 – Flow Control. The design shall meet the
requirements of Chapter 2 of this volume. For projects that do not meet the thresholds of Minimum
Requirement #7, and are therefore not required to provide flow control by the Department of Ecology,
the project proponent may be allowed to correct the downstream problem instead of providing on-site
flow control.
3.2 Design Event
The design events for all existing and new conveyance systems are as follows:
• All private pipe systems less than 24 inches in diameter shall be designed to convey at
minimum the 10-year, 24-hour peak flow rate without surcharging (the water depth in
the pipe must not exceed 90% of the pipe diameter).
• All private pipe systems greater than or equal to 24-inches in diameter and all public
pipe systems shall be designed to convey the 25-year, 24-hour peak flow rate without
surcharging (the water depth in the pipe must not exceed 90% of the pipe diameter).
• Culverts shall convey the 25-year, 24-hour peak flow rate without submerging the
culvert inlet (i.e. HW/D < 1).
• Constructed and natural channels shall contain the 100-year, 24-hour storm event.
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3.2.1 Additional Design Criteria
• For the 100-year event, overtopping of the pipe conveyance system may occur.
However, the additional flow shall not extend beyond half the lane width of the outside
lane of the traveled way and shall not exceed 4 inches in depth at its deepest point.
• All conveyance systems shall be designed for fully developed conditions. The fully
developed conditions for the project site shall be derived from the percentages of
proposed and existing impervious area. For off-site tributary areas, typical percentages
of impervious area for fully developed conditions are provided in Table III-3-13.
• Conveyance systems shall be modeled as if no on-site detention is provided
upstream.
Table III-3-13. Percentage Impervious for Fully Developed Conditions Offsite Tributary Areas
Land Use Description Percentage Impervious
Commercial/Industrial 85%
Residential 65%
3.3 Methods of Analysis
Proponent site surveys shall be used as the basis for determining the capacity of existing systems.
For preliminary analyses only, the proponent may use City of Auburn drainage maps and record
drawings. For naturally occurring drainage systems, drainage ditches, or undeveloped drainage
courses, the engineer must take into account the hydraulic capacity of the existing drainage course
and environmental considerations such as erosion, siltation, and increased water velocities or water
depths.
Describe capacities, design flows, and velocities in each reach. Describe required materials or
specifications for the design (e.g. rock lined for channels when velocity is exceeded; high density
polyethylene pipe needed for steep slope). Comprehensive maps showing the flow route and basins
for both the on-site and off-site surface water (for the minimum 1/4 mile downstream distance) must
be included in the storm drainage calculations.
If hydrologic modeling is required, the Project Engineer shall state methods, assumptions, model
parameters, data sources, and all other relevant information to the analysis. If model parameters are
used that are outside the standards of practice, or if parameters are different than those standards,
justify the parameters. Copies of all calculations for capacity of channels, culverts, drains, gutters and
other conveyance systems shall be included with the Stormwater Site Plan. If used, include all
standardized graphs and tables and indicate how they were used. Show headwater and tailwater
analysis for culverts when necessary. Provide details on references and sources of information used.
Single event modeling shall be used for designing conveyance systems, WWHM is not accepted.
For a full description of the information required for preparing a Stormwater Site Plan consult
Volume I, Chapter 4.
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3.3.1 Rational Method
This method shall only be used for preliminary pipe sizing and capacity analysis. For flow control
sizing derivations and water quality treatment sizing and flows see Chapter 2 of this volume and
Chapter 3 of Volume V.
The Rational Method is a simple, conservative method for analyzing and sizing conveyance elements
serving small drainage sub-basins, subject to the following specific limitations:
• Only for use in predicting peak flow rates for sizing conveyance elements (not for
use in sizing flow control or treatment facilities)
• Drainage sub-basin area, A, cannot exceed 10 acres for a single peak flow
calculation
• The time of concentration, Tc, must be computed using the method described below
and cannot exceed 100 minutes. A minimum Tc of 6.3 minutes shall be used.
• Unlike other methods of computing times of concentration, the 6.3 minutes is not an
initial collection time to be added to the total computed time of concentration.
3.3.1.1 Rational Method Equation
The following is the traditional Rational Method equation:
QR = CIRA (equation 1)
Where QR = peak flow (cfs) for a storm of return frequency R
C = estimated runoff coefficient (ratio of rainfall that becomes runoff)
IR = peak rainfall intensity (inches/hour) for a storm of return frequency R
A = drainage sub-basin area (acres)
When the composite runoff coefficient, Cc (see equation 2) of a drainage basin exceeds 0.60, the Tc
and peak flow rate from the impervious area should be computed separately. The computed peak
rate of flow for the impervious surface alone may exceed that for the entire drainage basin using the
value at Tc for the total drainage basin. The higher of the two peak flow rates shall then be used to
size the conveyance element.
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“C” Values
The allowable runoff coefficients to be used in this method are shown in Table III-3-14 by type of land
cover. These values were selected following a review of the values previously accepted by the City
for use in the Rational Method and as described in several engineering handbooks. The value for
single family residential areas were computed as composite values (as illustrated in the following
equation) based on the estimated percentage of coverage by roads, roofs, yards, and unimproved
areas for each density. For drainage basins containing several land cover types, the following formula
may be used to compute a composite runoff coefficient, Cc:
Cc = (C1A1+C2A2+…+CnAn)/At (equation 2)
Where At = total area (acres)
A1,2…n = areas of land cover types (acres)
C1,2…n = runoff coefficients for each area land cover type
Table III-3-14. Runoff Coefficients – “C” Values for the Rational Method
GENERAL LAND COVERS
LAND COVER C LAND COVER C
Dense forest 0.10 Playgrounds 0.30
Light forest 0.15 Gravel areas 0.80
Pasture 0.20 Pavement and roofs 0.90
Lawns 0.25 Open water (pond, lakes,
wetlands)
1.00
SINGLE FAMILY RESIDENTIAL AREAS*
[Density is in dwelling units per gross acreage (DU/GA)]
LAND COVER DENSITY C LAND COVER DENSITY C
0.20 DU/GA (1 unit per 5 ac.) 0.17 3.00 DU/GA 0.42
0.40 DU/GA (1 unit per 2.5 ac.) 0.20 3.50 DU/GA 0.45
0.80 DU/GA (1 unit per 1.25 ac.) 0.27 4.00 DU/GA 0.48
1.00 DU/GA 0.30 4.50 DU/GA 0.51
1.50 DU/GA 0.33 5.00 DU/GA 0.54
2.00 DU/GA 0.36 5.50 DU/GA 0.57
2.50 DU/GA 0.39 6.00 DU/GA 0.60
*Based on average 2,500 square feet per lot of impervious coverage.
For combinations of land covers listed above, an area-weighted “Cc x At” sum should be computed based on the equation
Cc x At = (C1 x A1)+(C2 x A2)…(Cn x An), where At = (A1+A2…An), the total drainage basin area
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“IR” Peak Rainfall Intensity
The peak rainfall intensity, IR, for the specified design storm of return frequency R is determined using
a unit peak rainfall intensity factor, iR, in the following equation:
IR = (PR)(iR) (equation 3)
Where PR = the total precipitation at the project site for the 24-hour duration storm event
for the given return frequency. Refer to Table III-3-15 for PR values. Total precipitation
can also be found in Chapter 1 of Volume III.
iR = the unit peak rainfall intensity factor
The unit peak rainfall intensity factor, iR, is determined by the following equation:
iR = (aR)(Tc)(-bR) (equation 4)
Where Tc = time of concentration (minutes), calculated using the method described below
and subject to equation limitations (6.3 < Tc < 100)
aR, bR = coefficients from Table III-3-15 used to adjust the equation for the design
storm
return frequency R
Table III-3-16 includes a table of rainfall intensity as a function of time of concentration, calculated
using the coefficients from Table III-3-15.
Table III-3-15. Coefficients for the Rational Method
Design Storm
Frequency
PR (inches) aR bR
2 years 2.0 1.58 0.58
5 years 2.5 2.33 0.63
10 years 3.0 2.44 0.64
25 years 3.5 2.66 0.65
50 years 3.5 2.75 0.65
100 years 4.0 2.61 0.63
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Table III-3-16. Rainfall Intensities for the City of Auburn
Rainfall Intensity (IR) (inches per hour)
Design storm recurrence interval (probability)
Time of
Concentration
(min)
2-year
(50%)
5-year
(20%)
10-year
(10%)
25-year
(4%)
50-year
(2%)
100-year
(1%)
6.3 1.09 1.83 2.25 2.81 2.91 3.27
7 1.02 1.71 2.11 2.63 2.72 3.06
8 0.95 1.57 1.93 2.41 2.49 2.82
9 0.88 1.46 1.79 2.23 2.31 2.62
10 0.83 1.37 1.68 2.08 2.15 2.45
11 0.79 1.29 1.58 1.96 2.03 2.30
12 0.75 1.22 1.49 1.85 1.91 2.18
13 0.71 1.16 1.42 1.76 1.82 2.07
14 0.68 1.10 1.35 1.67 1.73 1.98
15 0.66 1.06 1.29 1.60 1.66 1.90
16 0.63 1.02 1.24 1.54 1.59 1.82
17 0.61 0.98 1.19 1.48 1.53 1.75
18 0.59 0.94 1.15 1.42 1.47 1.69
19 0.57 0.91 1.11 1.37 1.42 1.63
20 0.56 0.88 1.08 1.33 1.37 1.58
25 0.49 0.77 0.93 1.15 1.19 1.37
30 0.44 0.68 0.83 1.02 1.06 1.22
35 0.40 0.62 0.75 0.92 0.95 1.11
40 0.37 0.57 0.69 0.85 0.88 1.02
45 0.35 0.53 0.64 0.78 0.81 0.95
50 0.33 0.50 0.60 0.73 0.76 0.89
55 0.31 0.47 0.56 0.69 0.71 0.84
60 0.29 0.44 0.53 0.65 0.67 0.79
70 0.27 0.40 0.48 0.59 0.61 0.72
80 0.25 0.37 0.44 0.54 0.56 0.66
90 0.23 0.34 0.41 0.50 0.52 0.61
100 0.22 0.32 0.38 0.47 0.48 0.57
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“Tc” Time of Concentration
The time of concentration is defined as the time it takes runoff to travel overland (from the onset of
precipitation) from the most hydraulically distant location in the drainage basin to the point of
discharge.
Due to the mathematical limits of the equation coefficients, values of Tc less than 6.3 minutes or
greater than 100 minutes cannot be used. Therefore, real values of Tc less than 6.3 minutes must be
assumed to be equal to 6.3 minutes, and values greater than 100 minutes must be assumed to be
equal to 100 minutes.
Tc is computed by summation of the travel times Tt of overland flow across separate flowpath
segments. The equation for time of concentration is:
Tc = T1 + T2 + … + Tn (equation 5)
Where T1,2…n = travel time for consecutive flowpath segments with different categories or
flowpath slope
Travel time for each segment, t, is computed using the following equation:
Tt = L/60V (equation 6)
where Tt = travel time (minutes)
Tt through an open water body (such as a pond) shall be assumed to be zero with this method.
Tt = Travel time for each segment (ft)
L = the distance of flow across a given segment (feet)
V = average velocity (ft/s) across the land cover = oRsk
Where kR = time of concentration velocity factor; see Table III-3-17.
s0 = slope of flowpath (feet/feet)
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Table III-3-17. “n” and “k” Values Used in Time Calculations for Hydrographs
“ns” Sheet Flow Equation Manning’s Values (for the initial 300 ft. of travel)
Manning values for sheet flow only, from Overton and Meadows 19761 ns
Smooth surfaces (concrete, asphalt, gravel, or bare hand packed soil) 0.011
Fallow fields or loose soil surface (no residue) 0.05
Cultivated soil with residue cover <20% 0.06
Cultivated soil with residue cover >20% 0.17
Short prairie grass and lawns 0.15
Dense grasses 0.24
Bermuda grass 0.41
Range (natural) 0.13
Woods or forest with light underbrush 0.40
Woods or forest with dense underbrush 0.80
“k” Values Used in Travel Time/Time of Concentration Calculations2
Sheet Flow kR
Forest with heavy ground litter and meadow 2.5
Fallow or minimum tillage cultivation 4.7
Short grass pasture and lawns 7.0
Nearly bare ground 10.1
Grasses waterway 15.0
Paved area (sheet flow) and shallow gutter flow 20.0
Shallow Concentrated Flow (After the initial 300 ft. of sheet flow, R = 0.1) ks
1. Forest with heavy ground litter and meadows (n = 0.10) 3
2. Brushy ground with some trees (n= 0.060) 5
3. Fallow or minimum tillage cultivation (n = 0.040) 8
4. High grass (n = 0.035) 9
5. Short grass, pasture and lawns (n = 0.030) 11
6. Nearly bare ground (n = 0.025) 13
7. Paved and gravel areas (n = 0.012) 27
Channel Flow (intermittent) (At the beginning of visible channels R = 0.2) kc
1. Forested swale with heavy ground litter (n = 0.10) 5
2. Forested drainage course/ravine with defined channel bed (n = 0.050) 10
3. Rock-lined waterway (n = 0.035) 15
4. Grassed waterway (n = 0.030) 17
5. Earth-lined waterway (n = 0.025) 20
6. CMP pipe, uniform flow (n = 0.024) 21
7. Concrete pipe, uniform flow (0.012) 42
8. Other waterways and pipe 0.508/n
Channel Flow (Continuous stream, R = 0.4) kc
9. Meandering stream with some pools (n = 0.040) 20
10. Rock-lined stream (n = 0.035) 23
11. Grass-lined stream (n = 0.030) 27
12. Other streams, man-made channels and pipe 0.807/n
1 See TR-55, 1986 2 210-VI-TR-55, Second Ed., June 1986
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3.4 Pipes, Culverts and Open Channels
This section presents the methods, criteria and details for analysis and design of pipe systems,
culverts, and open channel conveyance systems.
Storm drainage conveyance for public street requirements are as follows:
• Maximum surface run without considering curve super elevation (gutter flow) between
catch basins on paved roadway surfaces shall be as follows:
Pavement Slope, % Maximum Flow Length, ft
0.5 – 1 200
1 to 6 300
6 to 12 200
• The minimum longitudinal street gutter slope shall be one/half percent (0.5%).V
• Vaned catch basin grates and through-curb inlets may be required for roadway grades in
excess of six percent (6%).
• Storm manholes or catch basins shall not be designed within the vehicular wheel paths.
• The design of street drainage conveyance should seek to minimize the number of
structures and redundant pipes.
3.4.1 Pipe Systems
Pipe systems are networks of storm drain pipes, catch basins, manholes, inlets, and outfalls,
designed and constructed to convey surface water. The hydraulic analysis of flow in storm drainage
pipes typically is limited to gravity flow; however in analyzing existing systems it may be necessary to
address pressurized conditions. A properly designed pipe system will maximize hydraulic efficiency
by utilizing proper material, slope, and pipe size.
3.4.1.1 Design Flows
Design flows for sizing or assessing the capacity of pipe systems shall be determined using the
hydrologic analysis methods described in this chapter. Approved single event models described in
Chapter 1 of this volume may also be used to determine design flows. The design event is described
in Section 3.2. Pipe systems shall be designed to convey the design event without surcharging (water
depth in pipe shall not exceed 90% of the pipe diameter).
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3.4.1.2 Conveyance Capacity
Two methods of hydraulic analysis using Manning’s Equation are required by the City of Auburn for
the analysis of pipe systems. First, the Uniform Flow Analysis method is used for preliminary
design and analysis of pipe systems. Second, the Backwater Analysis method is used to analyze
both proposed and existing pipe systems to verify adequate capacity. See Section 3.2 for the
required design events for pipe systems.
Uniform Flow Analysis
This method is typically used for preliminary sizing of new pipe systems to convey the design flow as
calculated from the required design event from Section 3.2.
Assumptions:
• Flow is uniform in each pipe (i.e., depth and velocity remain constant throughout the
pipe for a given flow).
• Friction head loss in the pipe barrel alone controls capacity. Other head losses (e.g.,
entrance, exit, junction, etc.) and any backwater effects or inlet control conditions are
not specifically addressed.
• All pipes shall be designed for fully developed conditions. The fully developed
conditions shall be derived from the percentages of impervious area provided in
Table III-3-18.
Table III-3-18. Percentage Impervious for Modeling Fully Developed Conditions
Land Use Description1 % Impervious
Commercial/Industrial 85
Residential 65
1 For the land use descriptions, roads are included in the percentage impervious.
• All pipes shall be modeled as if no on-site detention is provided up-stream.
Each pipe within the system shall be sized and sloped such that its barrel capacity at normal full
flow is equal to or greater than the design flow calculated from the appropriate design storm as
identified in Section 3.2. The nomographs in Figure III-3-23 can be used for approximate sizing of the
pipes or Manning’s Equation can be solved for pipe size directly:
2/13/249.1 SRnV= (equation 7)
or use the continuity equation, Q = A•V, such that
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and Hydraulic Analysis Chapter 3 398
2/13/249.1 SARnQ= (equation 8)
Where Q = discharge (cfs)
V = velocity (fps)
A = area (sf)
n = Manning’s roughness coefficient; see Table III-3-19
R = hydraulic radius = area/wetted perimeter
S = slope of the energy grade line (ft/ft)
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Figure III-3-23. Nomograph for Sizing Circular Drains Flowing Full
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Table III-3-19. Manning’s “n” Values for Pipes
Analysis Method
Type of Pipe Material
Backwater Flow Manning’s
Equation Flow
A. Concrete pipe and CPEP-smooth interior pipe 0.012 0.014
B. Annular Corrugated Metal Pipe or Pipe Arch:
1. 2-2/3” x 1/2” corrugation (riveted)
a. plain or fully coated
b. paved invert (40% of circumference paved):
(1) flow full depth
(2) flow 0.8 depth
(3) flow 0.6 depth
c. treatment
2. 3” x 1” corrugation
3.6” x 2” corrugation (field bolted)
0.024
0.018
0.016
0.013
0.013
0.027
0.030
0.028
0.021
0.018
0.015
0.015
0.031
0.035
C. Helical 2-2/3” x 1/2” corrugation and CPEP-single
wall
0.024 0.028
D. Spiral rib metal pipe and PVC pipe 0.011 0.013
E. Ductile iron pipe cement lined 0.012 0.014
F. High density polyethylene pipe (butt fused only) 0.009 0.009
For pipes flowing partially full, the actual velocity may be estimated from the hydraulic properties
shown in Figure III-3-24 by calculating Qfull and Vfull and using the ratio of Qdesign/Qfull to find V and d
(depth of flow).
Table III-3-19 provides the recommended Manning’s “n” values for preliminary design for pipe
systems. The “n” values for this method are 15% higher in order to account for entrance, exit,
junction, and bend head losses.
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Figure III-3-24. Circular Channel Ratios
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3.4.1.3 Backwater Analysis
A backwater analysis shall be required when the design depth of flow is greater than 90% of the pipe
inside diameter or as directed by the City. The backwater analysis method described in this section is
used to analyze the capacity of both proposed and existing pipe systems to convey the required
design flow (i.e., either the 10-year or 25-year peak flow as required in Section 3.2). The backwater
analysis shall verify that the pipe system meets the following conditions:
• For the 25-year event, there shall be a minimum of 0.5 feet of freeboard between the
water surface and the top of any manhole or catch basin.
• For the 100-year event, overtopping of the pipe conveyance system may occur,
however, the additional flow shall not extend beyond half the lane width of the
outside lane of the traveled way and shall not exceed 4 inches in depth at its deepest
point. Refer to the Washington State Department of Transportation (WSDOT)
Hydraulics Manual for pavement drainage calculations. Off-channel storage on
private property is allowed with recording of the proper easements. When this
occurs, the additional flow over the ground surface is analyzed using the methods for
open channels described in Sections 3.2 and 3.4.3 and added to the flow capacity of
the pipe system.
This method is used to compute a simple backwater profile (hydraulic grade line) through a proposed
or existing pipe system for the purposes of verifying adequate capacity. It incorporates a re-arranged
form of Manning’s equation expressed in terms of friction slope (slope of the energy grade line in ft/ft).
The friction slope is used to determine the head loss in each pipe segment due to barrel friction,
which can then be combined with other head losses to obtain water surface elevation at all structures
along the pipe system.
The backwater analysis begins at the downstream end of the pipe system and is computed back
through each pipe segment and structure upstream. The friction, entrance, and exit head losses
computed for each pipe segment are added to that segment’s tailwater elevation (the water surface
elevation at the pipes’ outlet) to obtain its outlet control headwater elevation. This elevation is then
compared with the inlet control headwater elevation, computed assuming the pipe’s inlet alone is
controlling capacity using the methods for inlet control presented in Section 3.4.2. The condition that
creates the highest headwater elevation determines the pipe’s capacity. The approach velocity head
is then subtracted from controlling headwater elevation, and the junction and bend head losses are
added to compute the total headwater elevation, which is then used as the tailwater elevation for the
upstream pipe segment.
The Backwater Calculation Sheet in Figure III-3-25 can be used to compile the head losses and
headwater elevations for each pipe segment. The numbered columns on this sheet are described in
Table III-3-20. An example calculation is performed in Figure III-3-26.
This method should not be used to compute stage/discharge curves for level pool routing purposes.
See Volume III, Chapter 2 for level pool routing.
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Figure III-3-25. Backwater Calculation Sheet
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Table III-3-20. Backwater Calculation Sheet Notes
Column Description
(1) Design flow to be conveyed by pipe segment.
(2) Length of pipe segment.
(3) Pipe size: indicate pipe diameter or span % rise.
(4) Manning’s “n” value.
(5) Outlet Elevation of pipe segment.
(6) Inlet Elevation of pipe segment.
(7) Barrel Area: this is the full cross-sectional area of the pipe.
(8) Barrel Velocity: this is the full velocity in the pipe as determined by:
V = Q/A or Col. (8) = Col. (1)/Col. (7)
(9) Barrel Velocity Head = V3/2g or (Col. (8))2/2g;
Where g = 32.2 ft./sec.2 (acceleration due to gravity)
(10) Tailwater (TW) Elevation: this is the water surface elevation at the outlet of the pipe segment. If the pipe’s outlet is not
submerged by the TW and the TW depth is less than D+dc)/2, set TW equal to D+dc)/2 to keep the analysis simple and still
obtain reasonable results (D=pipe barrel height and dc=critical depth, both in feet. See Figure III-3-33 for determination of dc.
(11) Friction Loss = Sf x L (or Sf X Col (2));
Where Sf is the friction slope or head loss per linear foot of pipe as determined by Manning’s equation expressed in the form:
Sf = (nV)2/2.22R1.33
(12) Hydraulic Grade Line (HGL) Elevation just inside the entrance of the pipe barrel; this is determined by adding the friction loss
to the TW elevation: Col. (12) = Col. (11) + (Col. (10)
If this elevation falls below the pipe’s inlet crown, it no longer represents the true HGL when computed in this manner. The
true HGL will fall somewhere between the pipe’s crown and either normal flow depth or critical flow depth, whichever is
greater. To keep the analysis simple and still obtain reasonable results (i.e. erring on the conservative side), set the HGL
elevation equal to the crown elevation.
(13) Entrance Head Loss = Ke/2g (or Ke x Col (9))
Where Ke = Entrance Loss Coefficient from Table III-3-24. This is the head lost due to flow contractions at the pipe entrance.
(14) Exit Head Loss = 1.0 x V2/2g or 1.0 x Col. (9);
This is the velocity head lost or transferred downstream.
(15) Outdoor Control Elevation = Col. (12) + Col. (13) + Col. (14)
This is the maximum headwater elevation assuming the pipe’s barrel and inlet/outlet characteristics are controlling capacity. It
does not include structure losses or approach velocity considerations.
(16) Inlet Control Elevation (see Section 3.4.2.5 for computation of inlet control on culverts); this is the maximum headwater
elevation assuming the pipe’s inlet is controlling capacity. It does not include structure losses or approach velocity
considerations.
(17) Approach Velocity Head: This is the amount of head/energy being supplied by the discharge from an upstream pipe or
channel section, which serves to reduce the headwater elevation. If the discharge is from a pipe, the approach velocity head
is equal to the barrel velocity head computed for the upstream pipe. If the upstream pipe outlet is significantly higher in
elevation (as in a drop manhole) or lower in elevation such that its discharge energy would be dissipated, an approach
velocity head of zero should be assumed.
(18) Bend Head Loss = Kb x V2/2g (or Kb x Col. (17));
Where Kb = Bend Loss Coefficient (from Figure III-3-32). This is due to loss of head/energy required to change direction of
flow in an access structure.
(19) Junction Head Loss: This is the loss in head/energy which results from the turbulence created when two or more streams are
merged into one within the access structure. Figure III-3-30 can be used to determine this loss, or it an be computed using the
following equations derived from Figure III-3-30:
Junction Head Loss = Kj x V2/2g (or Kj x Col. (17)
where Kj is the Junction Loss Coefficient determined by:
Kj = (Q3/Q1)/(1.18 + 0.63(Q3/Q1))
(20) Headwater (HW) Elevation: This is determined by combining the energy heads in Columns 17, 18, and 19 with the highest
control elevation in either Column 15 or 16, as follows:
Col. (20) = Col. (15 or 16) – Col. (17) + Col. (18) + Col. (19)
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Figure III-3-26. Backwater Pipe Calculation Example
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3.4.1.4 Inlet Grate Capacity
The Washington State Department of Transportation (WSDOT) Hydraulics Manual can be used in
determining the capacity of inlet grates when capacity is of concern. When verifying capacity,
assume:
• Grate areas on slopes are 80 percent free of debris, and “vaned” grates are
95 percent free.
• Grate areas in sags or low spots are 50 percent free of debris, and “vaned” grates,
75 percent free.
3.4.1.5 Pipe Materials
See City of Auburn Construction Standards Division 7 for pipe specifications.
3.4.1.6 Pipe Sizes
• The following pipe sizes shall be used for pipe systems to be maintained by the City
of Auburn: 12-inch, 15-inch, 18-inch, 21-inch, 24-inch, 30-inch, 36-inch and 42-inch.
• Pipes smaller than 12-inch may only be used for privately maintained systems, or to
match the diameter of existing downstream mains, or as approved in writing by the
City.
• Catch basin leads shall be a minimum of 12-inch.
• Single-family home site roof, foundation and driveway drains may use pipe as small
as 4 inch.
• Non-single family roof, foundation and small driveway drains may use pipe as small
as 6-inch. Pipes under 10-inch may require capacity analysis if requested by the
City.
• For pipes larger than 30-inch increasing increments of 6-inch intervals shall be used
(36-inch, 42-inch, 48-inch, etc.).
3.4.1.7 Changes in Pipe Sizes
• Pipe direction changes or size increases or decreases are only allowed at manholes
and catch basins.
• Where a minimal fall is necessary between inlet and outlet pipes in a structure, pipes
must be aligned vertically by one of the following in order of preference:
a. Match pipe crowns
b. Match 80% diameters of pipes
c. Match pipe inverts or use City approved drop inlet connection
3.4.1.8 Pipe Alignment and Depth
• Pipes must be laid true to line and grade with no curves, bends, or deflections in any
direction.
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Exception: Vertical deflections in HDPE and ductile iron pipe with flanged restrained
mechanical joint bends (not greater than 30%) on steep slopes are allowed provided the pipe
adequately drains, with a minimum velocity of 2 feet per second (fps).
• A break in grade or alignment or changes in pipe material shall occur only at catch
basins or manholes.
• For the standard main alignment refer to the City’s Engineering Design and
Construction Standards.
• The standard depth for new mains measures six (6) feet from the center of the pipe
to the main street surface.
• The project engineer shall consult with the City for the potential of a future extension
of the storm system. In this case, the City may require modifications to the depth or
alignment.
• Connections to the main shall be at 90°. Slight variations may be allowed.
• Pipes shall be allowed to cross under retaining walls as specifically approved in
writing by the City when no other reasonable alternatives exist.
3.4.1.9 Pipe Slopes and Velocities
• The slope of the pipe shall be set so that a minimum velocity of 2 feet per second
can be maintained at full flow.
• A minimum slope for all pipes shall be 0.5% (under certain circumstances, a
minimum slope of 0.3% may be allowed with prior approval in writing from The City).
• Maximum slopes, velocities, and anchor spacings are shown in Table III-3-21. If
velocities exceed 15 feet per second for the conveyance system design event
described in Section 3.2, provide anchors and/or restrained joints at bends and
junctions.
3.4.1.10 Pipes on Steep Slopes
• Slopes 20% or greater shall require all drainage to be piped from the top to the
bottom in High Density Polyethylene (HDPE) pipe (butt-fused) or ductile iron pipe
welded or mechanically restrained. Additional anchoring design is required for these
pipes.
• Above-ground installation is required on slopes greater than 40% to minimize
disturbance to steep slopes, unless otherwise approved in writing by The City.
• HDPE pipe systems longer than 100 feet must be anchored at the upstream end if
the slope exceeds 20% or as required by The City.
• Above ground installations of HDPE shall address the high thermal
expansion/contraction coefficient of the pipe material. An analysis shall be completed
to demonstrate that the system as designed will tolerate the thermal expansion of the
pipe material.
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Table III-3-21. Maximum Pipe Slopes, Velocities and Anchor Requirements
Pipe Material Pipe Slope Above Which Pipe
Anchors Required and Minimum Anchor Spacing
Max. Slope
Allowed
Max. Velocity
@ Full Flow
Spiral Rib1, PVC1 20% (1 anchor per 100 L.F. of pipe) 30%(3) 30 fps
Concrete1 10% (1 anchor per 50 L.F. of pipe) 20%(3) 30 fps
Ductile Iron4 40% (1 anchor per pipe section) None None
HDPE2 50% (1 anchor per 100 L.F. of pipe – cross slope
installations may be allowed with additional
anchoring and analysis)
None None
1 Not allowed in landslide hazard areas.
2 Butt-fused pipe joints required. Above-ground installation is required on slopes greater than 40% to minimize disturbance
to steep slopes.
3 Maximum slope of 20% allowed for these pipe materials with no joints (one section) if structures are provided at each
end and the pipes are property grouted or otherwise restrained to the structures.
4 Restrained joints required on slopes greater than 25%. Above-ground installation is required on slopes greater than 40%
to minimize disturbance to steep slopes.
3.4.1.11 Structures
For the purposes of this Manual, all catch basins and manholes shall meet WSDOT standards such
as Type 1L, Type 1, and Type 2. Table III-3-22 presents the structures and pipe sizes allowed by size
of structure.
Table III-3-22. Allowable Structures and Pipe Sizes
Maximum Inside Pipe Diameter
Catch Basin Type1 CMP)(5), Spiral Rib)5, CPEP (single wall)5, HDPP,
Ductile Iron, PVC 2
(Inches)
Concrete, CPEP
(smooth interior),
(Inches)
Inlet 4
Type 1 3
Type IL 3
Type 2 - 48-inch dia.
Type 2 - 54-inch dia.
Type 2 – 60-inch dia.
Type 2 - 72-inch dia.
Type 2 - 96-inch dia.
12
15
21
30
36
42
54
72
12
12
18
24
30
36
42
60
1 Catch basins (including manhole steps, ladder, and handholds) shall conform to the W.S.D.O.T. Standard Plans or an
approved equal based upon submittal for approval.
2 Maintain the minimum sidewall thickness per this Section.
3 Maximum 5 vertical feet allowed between grate and invert elevation.
4 Normally allowed only for use in privately maintained drainage systems and must discharge to a catch basin immediately
downstream.
5 Allowed for private system installations only.
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The following criteria shall be used when designing a conveyance system that utilizes catch basins or
manholes:
• Catch basin (or manhole) diameter shall be determined by pipe diameter and
orientation at the junction structure. A plan view of the junction structure, drawn to
scale, will be required when more than four pipes enter the structure on the same
plane, or if angles of approach and clearance between pipes is of concern. The plan
view (and sections if necessary) must insure a minimum distance (of solid concrete
wall) between pipe openings of 8 inches for 48-inch and 54-inch diameter catch
basins and 12 inches for 72-inch and 96-inch diameter catch basins
• Type 1 catch basins should be used when overall catch basin height does not
exceed eight (8) feet or when the invert depth does not exceed five (5) feet below
rim.
• Type 1L catch basins should be used for the following situations:
o When overall catch basin height does not exceed eight (8) feet or when invert
depth does not exceed five (5) feet below rim.
o When any pipes tying into the structure exceed 21 inches connecting to the
long side, or 18 inches connecting to the short side at or very near to right
angles.
• Type 2 (48-inch minimum diameter) catch basins or manholes shall be used at the
following locations or for the following situations:
o When overall structure height exceed 8 feet.
o When all pipes tying into the structure exceed the limits set for Type 1
structures. Type 2 catch basins or manholes over 4 feet in height shall have
standard ladders.
o All Type 2 catch basins shall be specifically approved by the City. Type 2
catch basins shall not be substituted for manholes unless specifically
approved by The City.
• The maximum slope of ground surface for a radius of 5 feet around a catch basin
grate shall be 3:1. The preferred slope is 5:1 to facilitate maintenance access.
• Catch basin (or manhole) evaluation of structural integrity for H-20 loading will be
required for multiple junction catch basins and other structures that exceed the
recommendations of the manufacturers. The City may require further review for
determining structural integrity.
• Catch basins leads shall be no longer than 50 feet.
• Catch basins shall not be installed in graveled areas or sediment generating areas.
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• Catch basins shall be located:
o At the low point of any sag vertical curve or grade break where the grade of
roadway transitions from a negative to a positive grade.
o Prior to any intersection such that a minimal amount of water flows across the
intersection, through a curb ramp, or around a street return.
o Prior to transitions from a typical crown to a full warp through a down hill
grade.
• Catch basins shall not be placed in areas of expected pedestrian traffic. The
engineer shall avoid placing a catch basin in crosswalks, adjacent to curb ramps, or
in the gutter of a driveway. Care shall be taken on the part of the engineer to assure
that the catch basin will not be in conflict with any existing or proposed utilities.
• All catch basins, inlets, etc. shall be marked as directed by the City.
• Connections to structures and mains shall be at 90°. Slight variations may be
allowed.
• The maximum surface run between structures shall not exceed 400 linear feet.
• Changes in pipe direction, or increases or decreases in size, shall only be allowed at
structures.
• For pipe slope less than the required minimum, distance between structures shall be
decreased to 200 linear feet.
• For Type 1and 1L, catch basin to catch basin connections shall not be allowed.
• Bubble up systems shall not be allowed.
3.4.1.12 Pipe Clearances
Horizontal
A minimum of 5 feet horizontal separation shall be maintained between the storm main and all water
or sanitary sewer mains. This shall also apply to laterals.
Vertical
Where crossing an existing or proposed utility or sanitary sewer main, the alignment of the storm
system shall be such that the two systems cross as close to perpendicular as possible. Where
crossing a sanitary sewer main, provide a minimum 18 inches of vertical separation. For crossings of
water mains refer to the City Engineering Design and Construction Standards. The minimum vertical
separation for a storm main crossing any other utility shall be 6 inches. Note: Where the vertical
separation of two parallel systems exceeds the horizontal separation, additional horizontal separation
may be required to provide future access to the deeper system.
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3.4.1.13 Pipe Cover
• Suitable pipe cover over storm pipes in road rights-of-way shall be calculated for H-
20 loading by the Project Engineer. Pipe cover is measured from the finished grade
elevation down to the top of the outside surface of the pipe. Pipe manufacturer’s
recommendations are acceptable if verified by the Project Engineer.
• PVC (ASTM D3034 - SDR 35) minimum cover shall be three feet in areas subject to
vehicular traffic; maximum cover shall be 30 feet or per the manufacturer’s
recommendations and as verified with calculations from the Project Engineer.
• Cover for ductile iron pipe may be reduced to a 1-foot minimum. Use of reinforced
concrete pipe or AWWA C900 PVC pipe in this situation requires the engineer to
provide verifying calculations to confirm the adequacy of the selected pipe’s strength
for the burial condition.
• Pipe cover in areas not subject to vehicular loads, such as landscape planters and
yards, may be reduced to a 1-foot minimum.
• Catch basin evaluation of structural integrity for H-20 loading will be required for
multiple junction catch basins and other structures that exceed the recommendations
of the manufacturers.
3.4.1.14 System Connections
Connections to a pipe system shall be made only at catch basins or manholes. No wyes or tees are
allowed except on private roof/footing/yard drain systems on pipes 8 inches in diameter, or less.
Where wyes and tees are utilized, clean-outs shall be required upstream of each wye and tee.
Connections to structures and mains shall be at 90°. Slight variations may be allowed.
Minimum fall through manhole structures shall be 0. 1 foot. Pipes of different diameters shall be
aligned vertically in manholes by one of the following methods, listed in order of preference:
1. Match pipe crowns
2. Match 80% diameters of pipes.
3. Match pipe inverts or use City approved drop inlet connection.
Drop connections shall be considered on a case by case basis.
Private connections to the City storm system shall be at a drainage structure (i.e. catch basin or
manhole) and only if sufficient capacity exists. Tee connections into the side of a pipe shall not be
permitted.
Roof downspouts may be infiltrated or dispersed in accordance with the provisions of Chapter 2.
Infiltration and dispersion shall be evaluated first. If infiltration and dispersion are not feasible, roof
drains may be discharged through the curb per Section 2.1.5 into the roadway gutter or connected
into a drainage structure. Roof downspouts may not be connected directly into the side of a storm
drainage pipe.
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3.4.1.15 Debris Barriers
Access barriers are required on all pipes 12 inches and larger exiting a closed pipe system. Debris
barriers (trash racks) are required on all pipes entering a pipe system. See Figure III-3-27 for required
debris barriers on pipe ends outside of roadways and for requirements on pipe ends (culverts)
projecting from driveways or roadway side slopes.
Figure III-3-27. Debris Barrier
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3.4.2 Culverts
Culverts are relatively short segments of pipe of circular, elliptical, rectangular, or arch cross section
and typically convey flow under road embankments or driveways. Culverts installed in streams and
natural drainages shall meet the City’s Critical Areas Code and any fish passage requirements of the
Washington State Department of Fish and Wildlife.
3.4.2.1 Design Event
The design event for culverts is given in Section 3.2.
3.4.2.2 Design Flows
Design flows for sizing or assessing the capacity of culverts shall be determined using the hydrologic
analysis methods described in this chapter.
Other single event models as described in Chapter 2 of this volume may be used to determine
design flows. In addition, culverts shall not exceed the headwater requirements as established below:
3.4.2.3 Headwater
• For culverts 18-inch diameter or less, the maximum allowable headwater elevation
for the 100-year, 24-hour design storm (measured from the inlet invert) shall not
exceed 2 times the pipe diameter or arch-culvert-rise.
• For culverts larger than 18-inch diameter, the maximum allowable headwater
elevation for the 100-year, 24-hour design storm (measured from the inlet invert)
shall not exceed 1.5 times the pipe diameter or arch-culvert-rise.
• The maximum headwater elevation at the 100-year, 24-hour design flow shall be
below any road or parking lot subgrade.
3.4.2.4 Conveyance Capacity
Use the procedures presented in this section to analyze both inlet and outlet control conditions to
determine which governs. Culvert capacity is then determined using graphical methods.
3.4.2.5 Inlet Control Analysis
Nomographs such as those provided in Figure III-3-28 and Figure III-3-29 can be used to determine
the inlet control headwater depth at design flow for various types of culverts and inlet configurations.
These and other nomographs can be found in the FHWA publication Hydraulic Design of Highway
Culverts, HDS No. #5 (Report No. FHWA-NHI-01-020), September 2001; or the WSDOT Hydraulic
Manual.
Also available in the FHWA publication are the design equations used to develop the inlet control
nomographs. These equations are presented below.
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For unsubmerged inlet conditions (defined by Q/AD0.5 < 3.5);
Form 1*: HW/D = Hc /D + K(Q/AD0.5)M - 0.5S** (equation 9)
Form 2*: HW/D = K(Q/AD0.5)M (equation 10)
For submerged inlet conditions (defined by Q/AD0.5> 4.0);
HW/D = c(Q/AD0.5)2 + Y – 0.5S** (equation 11)
Where HW = headwater depth above inlet invert (ft)
D = interior height of culvert barrel (ft)
Hc = specific head (ft) at critical depth (dc + Vc
2/2g)
Q = flow (cfs)
A = full cross-sectional area of culvert barrel (sf)
S = culvert barrel slope (ft/ft)
K,M,c,Y = constants from Table III-3-23
The specified head Hc is determined by the following equation:
Hc = dc + Vc
2/2g (equation 12)
where dc = critical depth (ft); see Figure III-3-33
Vc = flow velocity at critical depth (fps)
g = acceleration due to gravity (32.2 ft/sec2)
* The appropriate equation form for various inlet types is specified in Table III-3-23
** For mitered inlets, use +0.7S instead of –0.5S.
NOTE: Between the unsubmerged and submerged conditions, there is a transition zone
(3.5 < Q/AD0.5<4.0) for which there is only limited hydraulic study information. The transition
zone is defined empirically by drawing a curve between and tangent to the curves defined by
the unsubmerged and submerged equations. In most cases, the transition zone is short and
the curve is easily constructed.
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Figure III-3-28. Headwater Depth for Smooth Interior Pipe Culverts with Inlet Control
DI
A
M
E
T
E
R
O
F
C
U
L
V
E
R
T
(
D
)
I
N
I
N
C
H
E
S
ENTRANCE
TYPE
HW
D SCALE
(1)
(2)
(3)
Square edge with
Groove end with
Groove end
headwall
headwall
projecting
EXAMPLE
D = 42 inches (3.0 feet).
Q = 120 cfs
HW*
*D in feet
HW
D (feet)
(1)
(2)
(3)
2.5
2.1
2.2
8.8
7.4
7.7
EXAMP
L
E
DI
S
C
H
A
R
G
E
(
Q
)
I
N
C
F
S
HE
A
D
W
A
T
E
R
D
E
P
T
H
I
N
D
I
A
M
E
T
E
R
S
(
H
W
/
D
)
To use scale (2) or (3) project
horizontally to scale (1), then
use straight inclined line through
D and Q scales, or reverse as
illustrated.
1.0
2
3
4
5
6
8
10
20
30
40
50
60
12
15
18
21
24
27
30
33
36
80
100
200
300
400
500
600
800
1,000
42
48
54
60
2,000
3,000
4,000
5,000
6,000
8,000
10,000
72
84
96
108
120
132
144
156
168
180
ENTRANCE TYPE
SQUARE EDGE WITH
HEADWALL
GROOVE END WITH
GROOVE END
PROJECTING
HEADWALL
PLAN
PLAN
(1)(2)(3)
.5
.5 .5
.6
.6 .6
.7
.7 .7
.8
.8 .8
.9
.9 .9
1.0
1.0 1.0
1.5
1.5 1.5
2.
2.
3.
3.3.
4.
4.
4.
5.
2.
(3)
(2)
(1)
5.
5.
6.
6.
6.
New Design Manual
Figure 4.3.1.B Headwater Depth for Smooth Interior Pipe Culverts with Inlet Control
Revised 12-2-97/Mdev
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Conveyance System Design Volume III
and Hydraulic Analysis Chapter 3 416
Figure III-3-29. Headwater Depth for Corrugated Pipe Culverts with Inlet Control
ST
A
N
D
A
R
D
C
.
M
.
DI
A
M
E
T
E
R
O
F
C
U
L
V
E
R
T
(
D
)
I
N
I
N
C
H
E
S
ST
R
U
C
T
U
R
A
L
P
L
A
T
E
C
.
M
.
ENTRANCE
TYPE
HW
D SCALE
(1)
(2)
(3)
Headwall
Mitered to conform
to slope
Projecting
EXAMPLE
D = 36 inches (3.0 feet).
Q = 66 cfs
HW*
*D in feet
HW
D (feet)
(1)
(2)
(3)
1.8
2.1
2.2
5.4
6.3
6.6
EXAM
P
L
E
DI
S
C
H
A
R
G
E
(
Q
)
I
N
C
F
S
HE
A
D
W
A
T
E
R
D
E
P
T
H
I
N
D
I
A
M
E
T
E
R
S
(
H
W
/
D
)
To use scale (2) or (3) project
horizontally to scale (1), then
use straight inclined line through
D and Q scales, or reverse as
illustrated
1.0
2
3
4
5
6
8
10
20
30
40
50
60
12
15
18
21
24
27
30
33
36
80
100
200
300
400
500
600
800
1,000
42
48
54
60
2,000
3,000
4,000
5,000
6,000
8,000
10,000
72
84
96
108
120
132
144
156
168
180
ENTRANCE TYPE
HEADWALL PLAN
MITERED TO
CONFORM
TO SLOPE
SECTION
PROJECTING
SECTION
(1)
(2)
(3)
.5 .5
.5
.6 .6
.6
.7 .7
.7
.8 .8
.8
.9 .9
.9
1.0 1.0
1.0
1.5
1.5 1.5
2.
2.
3.
3.
3.
4.
4.
4.
5.
2.
(3)
(2)
(1)
5.
5.
6.
6.
6.
New Design Manual
Figure 4.3.1.C Headwater Depth for Corrugated Pipe Culverts with Inlet Control
Revised 11-24-97/Mdev
SURFACE WATER MANAGEMENT MANUAL
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Conveyance System Design Volume III
and Hydraulic Analysis Chapter 3 417
Table III-3-23. Constants for Inlet Control Equations*
Unsubmerged Submerged Shape and
Material Inlet Edge Description Equation
Form K M c Y
Circular Concrete Square edge with headwall
Groove end with headwall
Groove end projecting
1 0.0098
0.0078
0.0045
2.0
2.0
2.0
0.0398
0.0292
0.0317
0.67
0.74
0.69
Circular CMP Headwall
Mitered to slope
Projecting
1 0.0078
0.0210
0.0340
2.0
1.33
1.50
0.0379
0.0463
0.0553
0.69
0.75
0.54
Rectangular Box 30o to 75o wingwall flares
90o and 15o wingwall flares
0o wingwall flares
1 0.026
0.061
0.061
1.0
0.75
0.75
0.0385
0.0400
0.0423
0.81
0.80
0.82
CM Boxes 90o headwall
Thick wall projecting
Thin wall projecting
1 0.0083
0.0145
0.0340
2.0
1.75
1.5
0.0379
0.0419
0.0496
0.69
0.64
0.57
Arch CMP 90o headwall
Mitered to slope
Projecting
1 0.0083
0.0300
0.0340
2.0
1.0
1.5
0.0496
0.0463
0.0496
0.57
0.75
0.53
Bottomless Arch
CMP
90o headwall
Mitered to slope
Thin wall projecting
1 0.0083
0.0300
0.0340
2.0
2.0
1.5
0.0379
0.0463
0.0496
0.69
0.75
0.57
*Source: FHWA HDS No. 5
SURFACE WATER MANAGEMENT MANUAL
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and Hydraulic Analysis Chapter 3 418
3.4.2.6 Outlet Control Analysis
Nomographs such as those provided in Figure III-3-31 and Figure III-3-32 can be used to determine
the outlet control headwater depth at design flow for various types of culverts and inlets. Outlet
control nomographs other than those provided can be found in FHWA HDS No. 5 or the WSDOT
Hydraulic Manual.
The outlet control headwater depth can also be determined using the simple Backwater Analysis
method presented in Section 3.4 for analyzing pipe system capacity. This procedure is summarized
as follows for culverts:
HW = H + TW – LS (equation 13)
where H = Hf + He + Hex
Hf = friction loss (ft) = (V2n2L)/(2.22R1.33)
NOTE: If (Hf+TW-LS) < D, adjust Hf such that (Hf+TW-LS) = D. This will keep the analysis
simple and still yield reasonable results (erring on the conservative side).
He = entrance head loss (ft) = Ke(V2/2g)
Hex = exit head loss (ft) = V2/2g
TW = tailwater depth above invert of culvert outlet (ft)
NOTE: If TW < (D+dc)/2, set TW = (D+dc)/2. This will keep the analysis simple and still yield
reasonable results.
L = length of culvert (ft)
S = slope of culvert barrel (ft/ft)
D = interior height of culvert barrel (ft)
V = barrel velocity (fps)
n = Manning’s roughness coefficient from Table III-3-19
R = hydraulic radius (ft)
Ke = entrance loss coefficient (from Table III-3-24)
G = acceleration due to gravity (32.2 ft/sec2)
dc = critical depth (ft); see Figure III-3-33
NOTE: The above procedure should not be used to develop stage/discharge curves for level
pool routing purposes because its results are not precise for flow conditions where the
hydraulic grade line falls significantly below the culvert crown (i.e., less than full flow
conditions).
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and Hydraulic Analysis Chapter 3 419
Figure III-3-30. Junction Head Loss in Structures
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Figure III-3-31. Head for Culverts (Pipe W/”N”=0.012) Flowing Full with Outlet Control
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Figure III-3-32. Head for Culverts (Pipe W/”N”=0.024) Flowing Full with Outlet Control
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and Hydraulic Analysis Chapter 3 422
Figure III-3-33. Critical Depth of Flow for Circular Culverts
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and Hydraulic Analysis Chapter 3 423
Table III-3-24. Entrance Loss Coefficients
Type of Structure and Design Entrance Coefficient, Ke
Pipe, Concrete, PVC, Spiral Rib, DI, and LCPE
Projecting from fill, socket (bell) end
Projecting from fill, square cut end
Headwall, headwall and wingwalls
Socket end of pipe (groove-end)
Square-edge
Rounded (radius = 1/12D)
Mitered to conform to fill slope
End section conforming to fill slope*
Beveled edges, 33.7o or 45o bevels
Side- or slope-tapered inlet
0.2
0.5
0.2
0.5
0.2
0.7
0.5
0.2
0.2
Pipe, Pipe-Arch, Corrugated Metal and Other Non-Concrete or D.I.
Projecting from fill (no headwall)
Headwall, or headwall and wingwalls (square-edge)
Mitered to conform to fill slope (paved or unpaved slope)
End section conforming to fill slope*
Beveled edges, 33.7o or 45o bevels
Side- or slope-tapered inlet
0.9
0.5
0.7
0.5
0.2
0.2
Box, Reinforced Concrete
Headwall parallel to embankment (no wingwalls)
Square-edged on 3 edges
Rounded on 3 edges to radius of 1/12 barrel dimension
or beveled edges on 3 sides
Wingwalls at 30o to 75o to barrel
Square-edged at crown
Crown edge rounded to radius of 1/12 barrel
dimension or beveled top edge
Wingwall at 10o to 25o to barrel
Square-edged at crown
Wingwalls parallel (extension of sides)
Square-edged at crown
Side- or slope-tapered inlet
0.5
0.2
0.4
0.2
0.5
0.7
0.2
NOTE: “End section conforming to fill slope” are the sections commonly available from
manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in
both inlet and outlet control. Some end sections incorporating a closed taper in their
design have a superior hydraulic performance.
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3.4.2.7 Inlets and Outlets
All inlets and outlets in or near roadway embankments must be flush with and conforming to the
slope of the embankments.
• For culverts 18-inch diameter and larger, the embankment around the culvert inlet
shall be protected from erosion by rock lining or riprap as specified in Table III-3-
27, except the length shall extend at least 5 feet upstream of the culvert, and the
height shall be at or above the design headwater elevation.
• Inlet structures, such as concrete headwalls, may provide a more economical
design by allowing the use of smaller entrance coefficients and, hence, smaller
diameter culverts. When properly designed, they will also protect the embankment
from erosion and eliminate the need for rock lining.
• In order to maintain the stability of roadway embankments, concrete headwalls,
wingwalls, or tapered inlets and outlets may be required if right-of-way or easement
constraints prohibit the culvert from extending to the toe of the embankment slopes.
All inlet structures or headwalls installed in or near roadway embankments must be
flush with and conforming to the slope of the embankment.
• Debris barriers (trash racks) are required on the inlets of all culverts that are over
60 feet in length and are 12 to 36 inches in diameter. This requirement also applies
to the inlets of pipe systems. See Figure III-3-27 for a debris barrier detail.
Exceptions are culverts on Type 1 or 2 streams.
• For culverts 18-inch diameter and larger, the receiving channel of the outlet shall be
protected from erosion by rock lining specified in Table III-3-27, except the height
shall be one foot above maximum tailwater elevation or one foot above the crown per
Figure III-3-41, whichever is higher.
3.4.3 Open Channels
This section presents the methods, criteria, and details for hydraulic analysis and design of open
channels.
3.4.3.1 Natural Channels
Natural channels are defined as those that have occurred naturally due to the flow of surface waters,
or those that, although originally constructed by human activity, have taken on the appearance of a
natural channel including a stable route and biological community. They may vary hydraulically along
each channel reach and should be left in their natural condition, wherever feasible or required, in
order to maintain natural hydrologic functions and wildlife habitat benefits from established
vegetation.
3.4.3.2 Constructed Channels
Constructed channels are those constructed or maintained by human activity and include bank
stabilization of natural channels. Constructed channels shall be either vegetation-lined, rock lined, or
lined with appropriately bioengineered vegetation.
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• Vegetation-lined channels are the most desirable of the constructed channels
when properly designed and constructed. The vegetation stabilizes the slopes of the
channel, controls erosion of the channel surface, and removes pollutants. The
channel storage, low velocities, water quality benefits, and greenbelt multiple-use
benefits create significant advantages over other constructed channels. The
presence of vegetation in channels creates turbulence, which results in loss of
energy and increased flow retardation; therefore, the design engineer must consider
sediment deposition and scour, as well as flow capacity, when designing the
channel.
• Rock-lined channels are necessary where a vegetative lining will not provide
adequate protection from erosive velocities they may be constructed with riprap,
gabions, or slope mattress linings. The rock lining increases the turbulence, resulting
in a loss of energy and increased flow retardation. Rock lining also permits a higher
design velocity and therefore a steeper design slope than in grass-lined channels.
Rock linings are also used for erosion control at culvert and storm drain outlets,
sharp channel bends, channel confluences, and locally steepened channel sections.
• Bioengineered vegetation lining is a desirable alternative to the conventional
methods of rock armoring. Soil bioengineering is a highly specialized science that
uses living plants and plant parts to stabilize eroded or damaged land. Properly
bioengineering systems are capable of providing a measure of immediate soil
protection and mechanical reinforcement. As the plants grow they produce
vegetative protective cover and a root reinforcing matrix in the soil mantle. This root
reinforcement serves several purposes:
a. The developed anchor roots provide both shear and tensile strength to the soil,
thereby providing protection from the frictional shear and tensile velocity
components to the soil mantle during the time when flows are receding and pore
pressure is high in the saturated bank.
b. The root mat provides a living filter in the soil mantle that allows for the natural
release of water after the high flows have receded.
c. The combined root system exhibits active friction transfer along the length of the
living roots. This consolidates soil particles in the bank and serves to protect the
soil structure from collapsing and the stabilization measures from failing.
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and Hydraulic Analysis Chapter 3 426
3.4.3.3 Design Flows
Design flows for sizing or assessing the capacity of open channels shall be determined using the
hydrologic analysis methods described in this chapter. Single event models as described in Chapter
2 of this volume may be used to determine design flows. In addition, open channel shall meet the
following:
• Open channels shall be designed to provide required conveyance capacity while
minimizing erosion and allowing for aesthetics, habitat preservation, and
enhancement.
• An access easement for maintenance is required along all constructed channels
located on private property. Required easement widths and building setback lines
vary with channel top width.
• The maximum distance from the edge of the adjacent access to the farthest point
shall be eighteen feet (18’).
• Channel cross-section geometry shall be trapezoidal, triangular, parabolic, or
segmental as shown in Figure III-3-34 through Figure III-3-36. Side slopes shall be
no steeper than 3:1 for vegetation-lined channels and 2:1 for rock-lined channels.
• Vegetation-lined channels shall have bottom slope gradients of 6% or less and a
maximum velocity at design flow of 5 fps (see Table III-3-26).
• Rock-lined channels or bank stabilization of natural channels shall be used
when design flow velocities exceed 5 feet per second. Rock stabilization shall be in
accordance with Table III-3-26 or stabilized with bioengineering methods as
described above in “Constructed Channels.”
SURFACE WATER MANAGEMENT MANUAL
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and Hydraulic Analysis Chapter 3 427
PROPERTIES OF DITCHES
DIMENSIONS HYDRAULICS
NO. Side Slopes B H W A WP R R(2/3)
D-1 -- -- 6.5" 5'-0" 1.84 5.16 0.356 0.502
D-1C -- -- 6" 25'-0" 6.25 25.50 0.245 0.392
D-2A 1.5:1 2'-0" 1'-0" 5'-0" 3.50 5.61 0.624 0.731
B 2:1 2'-0" 1'-0" 6'-0" 4.00 6.47 0.618 0.726
C 3:1 2'-0" 1'-0" 8'-0" 5.00 8.32 0.601 0.712
D-3A 1.5:1 3'-0" 1'-6" 7'-6" 7.88 8.41 0.937 0.957
B 2:1 3'-0" 1'-6" 9'-0" 9.00 9.71 0.927 0.951
C 3:1 3'-0" 1'-6" 12'-0" 11.25 12.49 0.901 0.933
D-4A 1.5:1 3'-0" 2'-0" 9'-0" 12.00 10.21 1.175 1.114
B 2:1 3'-0" 2'-0" 11'-0" 14.00 11.94 1.172 1.112
C 3:1 3'-0" 2'-0" 15'-0" 18.00 15.65 1.150 1.098
D-5A 1.5:1 4'-0" 3'-0" 13'-0" 25.50 13.82 1.846 1.505
B 2:1 4'-0" 3'-0" 16'-0" 30.00 16.42 1.827 1.495
C 3:1 4'-0" 3'-0" 22'-0" 39.00 21.97 1.775 1.466
D-6A 2:1 -- 1'-0" 4'-0" 2.00 4.47 0.447 0.585
B 3:1 -- 1'-0" 6'-0" 3.00 6.32 0.474 0.608
D-7A 2:1 -- 2'-0" 8'-0" 8.00 8.94 0.894 0.928
B 3:1 -- 2'-0" 12'-0" 12.00 12.65 0.949 0.965
D-8A 2:1 -- 3'-0" 12'-0" 18.00 13.42 1.342 1.216
B 3:1 -- 3'-0" 18'-0" 27.00 18.97 1.423 1.265
D-9 7:1 -- 1'-0" 14'-0" 7.00 14.14 0.495 0.626
D-10 7:1 -- 2'-0" 28'-0" 28.00 28.28 0.990 0.993
D-11 7:1 -- 3'-0" 42'-0" 63.00 42.43 1.485 1.302
Figure III-3-34. Ditches – Common Section
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Conveyance System Design Volume III
and Hydraulic Analysis Chapter 3 428
Figure III-3-35. Drainage Ditches – Common Sections
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Conveyance System Design Volume III
and Hydraulic Analysis Chapter 3 429
Figure III-3-36. Geometric Elements of Common Sections
Se
c
t
i
o
n
A
r
e
a
A
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t
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d
p
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r
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1.5
y
b
by
b
+
2y
by
b
+
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b
+
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2zy
or
zy
y
d
(b
+
z
y
)
y
b
+
2y
b
+
2y
1
+
z
2
zy
2
zy
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2
2
1 +
z
2
(b
+
z
y
)
y
1
+
z
2
b +
2zy
(b
+
z
y
)
y
1/2
1/8
1/2
[(b
+
z
y
)
y
]
b +
2zy
1.5
1.5
2.5
2.5
1.5
1.50.5
2
2
6Ty
32
2
3A 2y
2
0– si
n
si
n
0 0
˚
d
1/2
0
– s
i
n
si
n
0 0
˚
((
))
d
1/20
(sin
(
˚
˚)
)
y (
d
– y
)
y
2/3
P =
(
T/2)[
2/9
0
(1–
)
d
1/4
2/3
sin
˚
0
d0
˚
1/2
2
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–
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Ty
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(
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SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Conveyance System Design Volume III
and Hydraulic Analysis Chapter 3 430
Table III-3-25. Values of the Roughness Coefficient “n”
Type of Channel and Description
Manning’s
“n”*
(Normal)
Type of Channel and Description
Manning’s
“n”*
(Normal)
I. Constructed Channels II. Natural Streams
a. Earth, straight and uniform II-1 Minor Streams (top width at flood stage <100 ft)
1. Clean, recently completed 0.018 a. Streams on plain
2. Gravel, uniform section, clean 0.025 1. Clean, straight, full stage no rifts or deep pools 0.030
3. With short grass, few weeds 0.027 2. Same as #1, but more stones and weeds 0.035
b. Earth, winding and sluggish 3. Clean, winding, some pools and shoals 0.040
1. No vegetation 0.025 4. Same as #3, but some weeds 0.040
2. Grass, some weeds 0.030 5. Same as #4, but more stones 0.070
3. Dense weeds or aquatic plants in deep
channels
4. Earth bottom and rubble sides
5. Stony bottom and weedy banks
0.035
0.030
0.035
6. Sluggish reaches, weedy deep pools
7. Very weedy reaches, deep pools, or floodways
with heavy stand of timber and underbrush
0.100
0.050
6. Cobble bottom and clean sides 0.040 b. Mountain streams, no vegetation in channel,
banks usually steep, trees and brush along banks
submerged at high stages
c. Rock lined
1. Smooth and uniform
2. Jagged and irregular
0.035
0.040 1. Bottom: gravel, cobbles, and few boulders 0.040
2. Bottom: cobbles with large boulders 0.050
d. Channels not maintained, weeds and brush
uncut
1. Dense weeds, high as flow depth 0.080
2. Clean bottom, brush on sides
3. Same as #2, highest stage of flow
4. Dense brush, high stage
0.050
0.070
0.100
II-2 Floodplains
a. Pasture, no brush
1. Short grass
2. High grass
0.030
0.035
b. Cultivated areas
1. No crop 0.030
2. Mature row crops 0.035
3. Mature field crops 0.040
c. Brush
1. Scattered brush, heavy weeds 0.050
2. Light brush and trees 0.060
3. Medium to dense brush 0.070
4. Heavy, dense brush 0.100
d. Trees
1. Dense willows, straight 0.150
2. Cleared land with tree stumps, no sprouts 0.040
3. Same as #2, but with heavy growth of sprouts 0.060
4. Heavy stand of timber, a few down trees, little
undergrowth, flood stage below branches
0.100
5. Same as #4, but with flood stage reaching
branches
0.120
*Note: These “n“ values are “normal” values for use in analysis of channels. For conservative design for channel capacity, the maximum
values listed in other references should be considered. For channel bank stability, the minimum values should be considered.
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Table III-3-26. Channel Protection
Velocity at Design Flow (fps) REQUIRED PROTECTION
Greater than Less than or
equal to Type of Protection Thickness
Minimum Height
Above Design
Water Surface
0
5
Grass lining or bioengineered
lining N/A 0.5 foot
5 8 Rock lining(1) or
bioengineered lining 1 foot 1 foot
8 12 Riprap(2) 2 feet 2 feet
12 20 Slope mattress gabion, etc. Varies 2 feet
(1) Rock Lining shall be reasonable well graded as follows:
Maximum stone size: 12 inches
Median stone size: 8 inches
Minimum stone size: 2 inches
(2) Riprap shall be reasonably well graded as follows:
Maximum stone size: 24 inches
Median stone size: 16 inches
Minimum stone size: 4 inches
Note: Riprap sizing is governed by side slopes on channel, assumed to be approximately 3:1.
3.4.3.4 Conveyance Capacity
There are three acceptable methods of analysis for sizing and analyzing the capacity of open
channels:
• Manning’s equation for preliminary sizing
• Direct Step backwater method
• Standard Step backwater method
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3.4.3.5 Manning’s Equation for Preliminary Sizing
Manning’s equation is used for preliminary sizing of open channel reaches of uniform cross section
and slope (i.e., prismatic channels) and uniform roughness. This method assumes the flow depth (or
normal depth) and flow velocity remain constant throughout the channel reach for a given flow.
The charts in Figure III-3-34 and Figure III-3-35 can be used to obtain graphic solutions of Manning’s
equation for common ditch sections. For conditions outside the range of these charts or for more
precise results, Manning’s equation can be solved directly from its classic forms shown in
Equations 7 and 8 Section 3.4.1.2.
Table III-3-25 provides a reference for selecting the appropriate “n” values for open channels. A
number of engineering reference books, such as Open-Channel Hydraulics by V.T. Chow, may also
be used as guides to select “n” values. Figure III-3-36 contains the geometric elements of common
channel sections useful in determining area A, wetted perimeter WP, and hydraulic radius (R=A/WP).
If flow restrictions raise the water level above normal depth within a given channel reach, a backwater
condition (or non-uniform flow) is said to exist. This condition can result from flow restrictions created
by a downstream culvert, bridge, dam, pond, lake, etc., and even a downstream channel reach
having a higher normal flow depth. If backwater conditions are found to exist for the design flow, a
backwater profile must be computed to verify that the channel’s capacity is still adequate as
designed. The Direct Step or Standard Step backwater methods presented in this section can be
used for this purpose.
3.4.3.6 Direct Step Backwater Method
The Direct Step Backwater Method can be used to compute backwater profiles on prismatic channel
reaches (i.e. reaches having uniform cross section and slope) where a backwater condition or
restriction to normal flow is known to exist. The method can be applied to a series of prismatic
channel reaches in succession beginning at the downstream end of the channel and computing the
profile upstream.
Calculating the coordinates of the water surface profile using the method is an iterative process
achieved by choosing a range of flow depths, beginning at the downstream end, and proceeding
incrementally up to the point of interest or to the point of normal flow depth. This is best accomplished
by the use of a table (see Figure III-3-38) or computer programs.
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Figure III-3-37. Open Channel Flow Profile Computation
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Figure III-3-38. Direct Step Backwater Method – Example
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Equating the total head at cross section 1 and 2, the following equation may be written:
xSg
Vyg
VyxS f++=++22
2
2
22
2
1
110 (equation 14)
where, x = distance between cross sections (ft)
y1, y2 = depth of flow (ft at cross sections 1 and 2
V1, V2 = velocity (fps) at cross sections 1 and 2
1, 2 = energy coefficient at cross sections 1 and 2
S0 = bottom slope (ft/ft)
Sf = friction slope = (n2V2)/2.21R1.33)
g = acceleration due to gravity, (32.2 ft/sec2)
If the specific energy E at any one cross-section is defined as follows:
g
VyE2
2
+= (equation 15)
Assuming = 1 = 2 where is the energy coefficient which corrects for the non-uniform distribution
of velocity over the channel cross section, equations 14 and 15 can be combined and rearranged to
solve for x as follows:
)()(
)(
00
12
ffSS
E
SS
EEx -=-
-= (equation 16)
Typically values of the energy coefficient are as follows:
Channels, regular section 1.15
Natural streams 1.3
Shallow vegetated flood fringes (includes channel) 1.75
For a given flow, channel slope, Manning’s “n,” and energy coefficient , together with a beginning
water surface elevation y2, the values of x may be calculated for arbitrarily chosen values of y1. The
coordinates defining the water surface profile are obtained from the cumulative sum of x and
corresponding values of y.
The normal flow depth yn should first be calculated from Manning’s equation to establish the upper
limit of the backwater effect.
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3.4.3.7 Standard Step Backwater Method
The Standard Step Backwater Method is a variation of the Direct Step Backwater Method and can be
used to compute backwater profiles on both prismatic and non-prismatic channels. In this method,
stations are established along the channel where cross section data is known or has been
determined through field survey. The computation is carried out in steps from station to station rather
than throughout a given channel reach as is done in the Direct Step method. As a result, the analysis
involves significantly more trial-and-error calculation in order to determine the flow depth at each
station.
3.4.3.8 Computer Applications
There are several different computer programs capable of the iterative calculations involved for these
analyses. The project engineer is responsible for providing information describing how the program
was used, assumptions the program makes and descriptions of all variables, columns, rows,
summary tables, and graphs. The most current version of any software program shall be used for
analysis. Auburn may find specific programs not acceptable for use in design. Please check with
Public Works, to confirm the applicability of a particular program prior to starting design.
3.4.3.9 Riprap Design3
Proper riprap design requires the determination of the median size of stone, the thickness of the
riprap layer, the gradation of stone sizes, and the selection of angular stones, which will interlock
when placed. Research by the U.S. Army Corps of Engineers has provided criteria for selecting the
median stone weight, W50 (Figure III-3-39). If the riprap is to be used in a highly turbulent zone (such
as at a culvert outfall, downstream of a stilling basin, at sharp changes in channel geometry, etc.), the
median stone W50 should be increased from 200% to 600% depending on the severity of the locally
high turbulence. The thickness of the riprap layer should generally be twice the median stone
diameter (D50) or at least equivalent to the diameter of the maximum stone. The riprap should have a
reasonably well-graded assortment of stone sizes within the following gradation:
1.25 D max/D50 1.50
D15/D50 = 0.50
Dmin/D50 = 0.25
Riprap Filter Design
Riprap should be underlain by a sand and gravel filter (or filter fabric) to keep the fine materials in the
underlying channel bed from being washed through the voids in the riprap. Likewise, the filter
material must be selected so that it is not washed through the voids in the riprap. Adequate filters can
usually be provided by a reasonably well graded sand and gravel material where:
D15 < 5d85
3 From a paper prepared by M. Schaefer, Dam Safety Section, Washington State Department of Ecology.
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The variable d85 refers to the sieve opening through which 85% of the material being protected will
pass, and D15 has the same interpretation for the filter material. A filter material with a D50 of 0.5 mm
will protect any finer material including clay. Where very large riprap is used, it is sometimes
necessary to use two filter layers between the material being protected and the riprap.
Example:
What embedded riprap design should be used to protect a streambank at a level culvert outfall where
the outfall velocities in the vicinity of the downstream toe are expected to be about 8 fps.
From Figure III-3-39, W50 = 6.5 lbs, but since the downstream area below the outfall will be subjected
to severe turbulence, increase W50 by 400% so that:
W50 = 26 lbs, D50 = 8.0 inches
The gradation of the riprap is shown in Figure III-3-40, and the minimum thickness would be 1 foot
(from Table III-3-26); however, 16 inches to 24 inches of riprap thickness would provide some
additional insurance that the riprap will function properly in this highly turbulent area.
Figure III-3-40 shows that the gradation curve for ASTM C33, size number 57 coarse aggregate
(used in concrete mixes), would meet the filter criteria. Applying the filter criteria to the coarse
aggregate demonstrates that any underlying material whose gradation was coarser than that of
concrete sand would be protected.
For additional information and procedures for specifying filters for riprap, refer to the Army Corps of
Engineers Manual EM 1110-2-1601 (1970), Hydraulic Design of Flood Control Channels,
Paragraph 14, “Riprap Protection.”
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Figure III-3-39. Mean Channel Velocity vs Medium Stone Weight (W50)
and Equivalent Stone Diameter
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Figure III-3-40. Riprap Gradation Curve
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3.5 Outfalls Systems
This section presents the methods, criteria and details for analysis and design of outfall systems.
Properly designed outfalls are critical to reducing the chance of adverse impacts as the result of
concentrated discharges from pipe systems and culverts, both onsite and downstream. Outfall
systems include rock splash pads, flow dispersal trenches, gabion or other energy dissipaters, and
tightline systems. A tightline system is typically a continuous length of pipe used to convey flows
down a steep or sensitive slope with appropriate energy dissipation at the discharge end.
3.5.1 Outfall Design Criteria
All outfalls must be provided with an appropriate outlet / energy dissipation structure such as a
dispersal trench, gabion outfall, or rock splash pad (see Figure III-3-41) as specified below and in
Table III-3-27.
No erosion or flooding of downstream properties shall result from discharge from an outfall.
Table III-3-27. Rock Protection at Outfalls
Required Protection Discharge Velocity
at Design Flow in
feet per second
(fps) Minimum Dimensions
Type Thickness Width Length Height
0 – 5 Rock lining(1) 1 foot Diameter
+ 6 feet
8 feet or
4 x diameter,
whichever is
greater
Crown
+ 1 foot
>5 - 10 Riprap(2) 2 feet Diameter
+ 6 feet or
3 x diameter,
whichever is greater
12 feet or
4 x diameter,
whichever is
greater
Crown
+ 1 foot
>10 - 20 Gabion
outfall
As
required
As required As required Crown
+ 1 foot
>20 Engineered
energy
dissipater
required
1 Rock lining shall be quarry spalls with gradation as follows:
Passing 8-inch square sieve: 100%
Passing 3-inch square sieve: 40 to 60% maximum
Passing ¾-inch square sieve: 0 to 10% maximum
2 Riprap shall be reasonably well graded with gradation as follows:
Maximum stone size: 24 inches (nominal diameter)
Median stone size: 16 inches
Minimum stone size: 4 inches
Riprap sizing is based on outlet channel side slopes of approximately 3:1.
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3.5.1.1 Energy dissipation
• For freshwater outfalls with a design velocity greater than 10 fps, a gabion dissipater or
engineered energy dissipater may be required. The gabion outfall detail shown in
Figure III-3-44 is illustrative only. A design engineered to specific site conditions must
be developed.
• Engineered energy dissipaters, including stilling basins, drop pools, hydraulic jump
basins, baffled aprons, and bucket aprons, are required for outfalls with design velocity
greater than 20 fps. These should be designed using published or commonly known
techniques found in such references as Hydraulic Design of Energy Dissipaters for
Culverts and Channels, published by the Federal Highway Administration of the United
States Department of Transportation; Open Channel Flow, by V.T. Chow; Hydraulic
Design of Stilling Basins and Energy Dissipaters, EM 25, Bureau of Reclamation
(1978); and other publications, such as those prepared by the Soil Conservation
Service (now Natural Resource Conservation Service).
• Alternate mechanisms may be allowed with written approval of The City. Alternate
mechanisms shall be designed using sound hydraulic principles with consideration of
ease of construction and maintenance.
• Mechanisms that reduce velocity prior to discharge from an outfall are encouraged.
Some of these are drop manholes and rapid expansion into pipes of much larger size.
Other discharge end features may be used to dissipate the discharge energy. An
example of an end feature is the use of a Diffuser Tee with holes in the front half, as
shown in Figure III-3-45.
The in-stream sample gabion mattress energy dissipater may not be acceptable within the ordinary
high water mark of fish-bearing waters or where gabions will be subject to abrasion from upstream
channel sediments. A gabion basket located outside the ordinary high water mark should be
considered for these applications.
3.5.1.2 Flow dispersion
• The flow dispersal trenches shown in Figure III-3-42 and Figure III-3-43 shall not be
used unless both criteria below are met:
o An outfall is necessary to disperse concentrated flows across uplands where
no conveyance system exists and the natural (existing) discharge is
unconcentrated; and
o The 100-year peak discharge rate is less than or equal to 0.5 cfs.
• Flow dispersion may be allowed for discharges greater than 0.5 cfs, providing that
adequate design details and calculations for the dispersal trench to demonstrate that
discharge will be sheet flow are submitted and approved by The City.
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• For the dispersion trenches shown in Figure III-3-42 and Figure III-3-43, a vegetated
flowpath of at least 25 feet in length must be maintained between the outlet of the
trench and any property line, structure, stream, wetland, or impervious surface. A
vegetated flowpath of at least 50 feet in length must be maintained between the outlet
of the trench and any steep slope. Sensitive area buffers may count towards flowpath
lengths. For dispersion trenches discharging more than 0.5 cfs, additional vegetated
flow path may be required.
• All dispersions systems shall be at least 10 feet from any structure or property line. If
necessary, setbacks shall be increased from the minimum 10 feet in order to maintain
a 1H:1V side slope for future excavation and maintenance.
• Dispersion systems shall be setback from sensitive areas, steep slopes, slopes 20% or
greater, landslide hazard areas, and erosion hazard areas as governed by the Auburn
City Code or as outlined in this manual, whichever is more restrictive.
• For sites with multiple dispersion trenches, a minimum separation of 10 feet is required
between flowpaths. The City may require a larger separation based upon site
conditions such as slope, soil type and total contributing area.
• Runoff discharged towards landslide hazard areas must be evaluated by a
geotechnical engineer or a licensed geologist, hydrogeologist, or engineering
geologist. The discharge point shall not be placed on or above slopes 20% (5H:1V) or
greater or above erosion hazard areas without evaluation by a geotechnical engineer
or qualified geologist and City approval.
Please refer to the Auburn City Code for additional requirements. ACC 16.10 Critical Areas may
contain additional requirements depending upon the project proposal. A Hydraulic Project
Approval (Chapter 77.55 RCW) and an Army Corps of Engineers permit may be required for any
work within the ordinary high water mark.
Other provisions of that RCW or the Hydraulics Code - Chapter 220-110 WAC may also apply.
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Figure III-3-41. Pipe/Culvert Outfall Discharge Protection
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Figure III-3-42. Flow Dispersal Trench
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Figure III-3-43. Alternative Flow Dispersal Trench
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Figure III-3-44. Gabion Outfall Detail
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Figure III-3-45. Diffuser TEE (an example of energy dissipating end feature)
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3.5.2 Tightline Systems
• Outfall tightlines may be installed in trenches with standard bedding on slopes up to
20%. In order to minimize disturbance to slopes greater than 20%, it is
recommended that tightlines be placed at grade with proper pipe anchorage and
support.
• High density polyethylene pipe (HDPP) tightlines must be designed to address the
material limitations, particularly thermal expansion and contraction and pressure
design, as specified by the manufacturer.
• Due to the ability of HDPP tightlines to transmit flows of very high energy, special
consideration for energy dissipation must be made. Details of a sample gabion
mattress energy dissipater have been provided as Figure III-3-44. Flows of very high
energy will require a specifically engineered energy dissipater structure.
• Tightline systems may be needed to prevent aggravation or creation of a
downstream erosion problem.
• Tightline systems shall have appropriate anchoring designed, both along the slope
and to provide anchoring for the entire system.
3.5.3 Habitat Considerations
• New pipe outfalls can provide an opportunity for low-cost fish habitat improvements.
For example, an alcove of low-velocity water can be created by constructing the pipe
outfall and associated energy dissipater back from the stream edge and digging a
channel, over widened to the upstream side, from the outfall to the stream.
Overwintering juvenile and migrating adult salmonids may use the alcove as shelter
during high flows. Potential habitat improvements should be discussed with the
Washington Department of Fish and Wildlife biologist prior to inclusion in design.
• Bank stabilization, bioengineering and habitat features may be required for disturbed
areas.
• Outfall structures should be located where they minimize impacts to fish, shellfish, and
their habitats.
• The City’s Critical Area Code may regulate activities in these areas.
3.6 Pump Systems
Pump systems are only allowed if applied for through the City’s Exceptions process (see Volume I,
Section 3.5). Feasibility of all other methods of gravity conveyance, infiltration and dispersion shall
first be investigated and demonstrated to be infeasible in the following order of preference:
1. Infiltration of surface water on-site.
2. Dispersion of surface water on site.
3. Gravity connection to the City storm drainage system.
4. Pumping to a gravity system.
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3.6.1 Design Criteria
If approved by the City Exceptions process (see Volume I, Section 3.5), the pump system must
convey, at a minimum, the peak design flow for the 25-year 24-hour rainfall event. Pump capacity
plus system storage or overflow, must convey or store the 100-year, 24-hour storm event.
3.6.2 Pump Requirements
If approved by the City Exemptions/Variance process, proposed pump systems must meet the
following minimum requirements:
• The pump system shall be used to convey water from one location or elevation to
another within the project site.
• The gravity-flow components of the drainage system to and from the pump system
must be designed so that pump failure does not result in flooding of a building or
emergency access or overflow to a location other than the natural discharge point for
the project site.
• The pump system must have a dual pump (alternating) equipped with emergency
back-up power OR a single pump may be provided without back-up power if the
design provides the 100-year 24-hour storage volume.
• Pumps, wiring, and control systems shall be intrinsically safe per IBC requirements.
• All pump systems must be equipped with an external pump failure and high water
alarm system.
• The pump system will serve only one lot or business owner.
• The pump system must be privately owned and maintained.
• The pump system shall not be used to circumvent any other City drainage
requirements. Construction and operation of the pump system shall not violate any
City requirements.
3.6.3 Additional Requirements
Private pumped stormwater systems will require the following additional items:
• Operations and Maintenance Manual describing the system itself and all required
maintenance and operating instructions, including procedures to follow in the event of
a power outage. All the requirements of Volume I, Section 4.1 shall be included in the
O&M manual.
• Notice to Title on the property outlining that a private stormwater system is constructed
on the site and that the maintenance of that system is the responsibility of the property
owner. Wording of the Notice to Title shall be approved by the City prior to placing the
Notice.
• Operations and Maintenance Agreement signed by the property owner and the City.
After signature by the city, the agreement shall be recorded with the appropriate
County and listed in the Notice of Title with the recording number.
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All fees associated with preparing and recording documents and placing the Notice to Title shall be
the responsibility of the applicant.
3.6.4 Sump Pumps
The above pump requirements do not apply to internal sump pumps. However, internal sump pumps
do require a permit prior to connection to the City storm drainage system.
• Sump pumps shall be sized to properly remove water from basements and crawl
spaces.
• Sump pumps shall NOT be connected to the sanitary sewer system.
• Consult the pump manufacturer or an engineer for appropriate sizing of a sump
pump.
3.7 Easements and Access
All publicly owned, manmade drainage facilities and conveyances and all natural channels on the
project site used for conveyance of altered flows due to development (including swales, ditches,
stream channels, lake shores, wetlands, potholes, estuaries, gullies, ravines, etc.) shall be located
within easements as required by the City.
3.7.1 Public Easements
A stormwater easement is required for the placement, operation and maintenance of facilities upon
private property.
Public stormwater easements shall meet the following requirements:
• Public stormwater easements shall extend a minimum of seven and one-half feet (7 ½’) to
each side of the centerline of the storm pipe and seven and one-half (7 ½’) beyond the
outside extremity of a storm facility. Additional width may be required depending upon the
depth and site topography.
• Public stormwater easements shall be provided on the City’s standard easement form. Legal
description of the easement and the property that the easement encumbers, along with a
sketch showing both, shall be sealed by a licensed Land Surveyor and incorporated into the
easement form as exhibits. The legal descriptions and sketch shall be on plain bond paper
with margins acceptable to the County recording.
• Public stormwater easements shall be reviewed by the City and then recorded in the
appropriate County prior to acceptance of the public storm system.
All pipes and channels must be located within the easement so that each pipe face or top edge of
channel is no closer than 5 feet from its adjacent easement boundary. Pipes greater than 5 feet in
diameter and channels with top widths greater than 5 feet shall be placed in easements adjusted
accordingly, so as to meet the required dimensions from the easement boundaries.
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The depth or proximity of steep slopes to the public system may necessitate a larger easement
requirement for future excavation and maintenance purposes. See Table III-3-28 for appropriate
widths based on depth of pipe.
Table III-3-28. Additional Storm Drain Easement Widths
INVERT DEPTH WIDTH
< 10’ 20’
10’ - 15’ 25’
15’ - 20’ 30’
> 20’ 40’
Notes:
1. Greater width may be required for large diameter pipe or unfavorable site conditions.
2. Pipe shall be installed in center of easement.
3. If two pipes are to be installed in an easement, add 10 feet to the easement widths listed above. Use the deeper
of the two pipes in selecting the easement width from this table. Install pipes with 10 feet of horizontal clearance
between them.
3.7.2 Private Easements
Privately owned facilities shall be located in separate easements outside of dedicated public road
right-of-way areas. Private systems serving multiple lots require prior City approval.
• A separate storm drainage detention or retention system is required for each commercial or
industrial lot unless a combined storm drainage system is used for more than one lot. In such
cases, a private cross drainage easement and maintenance agreement is required for each
lot, unless cross drainage requirements are set up as a condition of the recorded final plat.
Copies of the recorded easements or plat condition, including the stormwater pollution
prevention plan must be provided to the City prior to civil plan approval.
• All projects shall execute with the City a standard Stormwater Easement and Maintenance
Agreement for the site’s private storm drainage facilities. The easement shall be approved by
the City and executed by the owner prior to issuance of occupancy permits for the
development.
3.7.3 Maintenance Access
A minimum 15-foot wide access easement shall be provided to drainage facilities from a public street
or right-of-way. Access easements shall be surfaced with a minimum 12-foot width of crushed rock,
or other approved surface to allow year-round equipment access to the facility.
Maintenance access must be provided for all manholes, catch basins, vaults, or other underground
drainage facilities operated by the City. Maintenance shall be through a public easement.
Maintenance access to privately maintained facilities may also be required.
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Appendix A 453
Appendix A Auburn Design Storm
Table III-A-29. Design Storm Precipitation Values
Return Frequency
24-Hour Storm Event (Years)
Precipitation
(Inches)
0.5 1.44
2 2.0
5 2.5
10 3.0
25 3.5
50 3.5
100 4.0
The depth of a 7-day, 100-year storm can be determined in one of three ways:
Use 12 inches for the lowland areas between sea level and 650 MSL.
Use the U.S. Department of Commerce Technical Paper No. 49, “Two- to Ten-Day
Precipitation for Return Periods of 2 to 100 Years in the Contiguous United States.”
Use the U.S. Department of Commerce NOAA Atlas 2, “Precipitation Frequency Atlas of
the Western United States,” Volume IX – Washington, 24-hour, 100-year Isopluvials and
add 6.0 inches to the appropriate isopluvial for the project area.
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Pilot Infiltration Test Appendix B 454
Appendix B Procedure for Conducting a Pilot
Infiltration Test
The Pilot Infiltration Test (PIT) consists of a relatively large-scale infiltration test to better approximate
infiltration rates for design of stormwater infiltration facilities. The PIT reduces some of the scale
errors associated with relatively small-scale double ring infiltrometer or “stove-pipe” infiltration tests. It
is not a standard test but rather a practical field procedure recommended by Ecology’s Technical
Advisory Committee.
Infiltration Test
• Excavate the test pit to the depth of the bottom of the proposed infiltration facility.
Lay back the slopes sufficiently to avoid caving and erosion during the test.
• The horizontal surface area of the bottom of the test pit should be approximately
100 square feet. For small drainages and where water availability is a problem
smaller areas may be considered as determined by the site professional.
• Accurately document the size and geometry of the test pit.
• Install a vertical measuring rod (minimum 5-ft. long) marked in half-inch
increments in the center of the pit bottom.
• Use a rigid 6-inch diameter pipe with a splash plate on the bottom to convey
water to the pit and reduce side-wall erosion or excessive disturbance of the
pond bottom. Excessive erosion and bottom disturbance will result in clogging of
the infiltration receptor and yield lower than actual infiltration rates.
• Add water to the pit at a rate that will maintain a water level between 3 and 4 feet
above the bottom of the pit. A rotometer can be used to measure the flow rate
into the pit.
A water level of 3 to 4 feet provides for easier measurement and flow stabilization control. However,
the depth should not exceed the proposed maximum depth of water expected in the completed
facility.
Every 15 – 30 min, record the cumulative volume and instantaneous flow rate in gallons per minute
necessary to maintain the water level at the same point (between 3 and 4 feet) on the measuring rod.
Add water to the pit until one hour after the flow rate into the pit has stabilized (constant flow rate)
while maintaining the same pond water level (usually 17 hours).
After the flow rate has stabilized, turn off the water and record the rate of infiltration in inches per hour
from the measuring rod data, until the pit is empty.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Procedure for Conducting a Volume III
Pilot Infiltration Test Appendix B 455
Data Analysis
Calculate and record the infiltration rate in inches per hour in 30 minutes or one-hour increments until
one hour after the flow has stabilized.
Use statistical/trend analysis to obtain the hourly flow rate when the flow stabilizes. This would be the
lowest hourly flow rate.
Apply appropriate correction factors for site heterogeneity, anticipated level of maintenance and
treatment to determine the site-specific design infiltration rate (see Table III-2-9).
Example
The area of the bottom of the test pit is 8.5-ft. by 11.5-ft.
Water flow rate was measured and recorded at intervals ranging from 15 to 30 minutes throughout
the test. Between 400 minutes and 1,000 minutes the flow rate stabilized between 10 and
12.5 gallons per minute or 600 to 750 gallons per hour, or an average of (9.8 + 12.3) / 2 =
11.1 inches per hour.
Applying a correction factor of 5.5 for gravelly sand in Table III-2-9 the design long-term infiltration
rate becomes 2 inches per hour, anticipating adequate maintenance and pre-treatment.
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