HomeMy WebLinkAboutSWMM Revision Pages.pdfREVISION DIRECTIONS
AUBURN SURFACE WATER MANAGEMENT MANUAL
EFFECTIVE DATE 06/06/2014
Revision No. 1
Preface, Volume I, Volume III, & Volume V
New, reprinted pages included with this revision are listed by page number in the right-
hand column below. The left-hand column lists obsolete material, which should be
removed from the book. Follow the instruction columns in sequence: remove and insert
pages as necessary.
When completed, insert these directions in a convenient place near the front of the
Surface Water Management Manual. For future reference, the person making these
changes may also date and initial this page.
Remove these pages Insert these pages
Preface page 3-4 Preface page 3-4
Volume I, Table of Contents Volume I, Table of Contents
Volume I, pages 25-32 Volume I, pages 25-32
Volume I, pages 47-48 Volume I, pages 47-48
Volume III, pages 307-310 Volume III, pages 307-310
Volume III, pages 351-354 Volume III, pages 351-354
Volume III, pages 357-364 Volume III, pages 357-364
Volume V, pages 701-706 Volume V, pages 701-706
Volume V, pages 715-716 Volume V, pages 715-716
Approved by City of Auburn, Public Works Committee April 21, 2014
SURFACE WATER MANAGE MENT MANUAL
NOVEMBER 2009
3 Preface
How to Use this Manual
This manual is designed for a variety of users.
Project proponents should start by reviewing the minimum requirements described in
Volume I. Volume I also describes a Stormwater Site Plan and provides guidance on
how to develop this plan.
City staff will use this manual to review Stormwater Site Plans, check BMP designs
and provide technical advice to project proponents. City staff will also use this
manual as a reference when designing public works projects. All development and
redevelopment projects within the City of Auburn shall meet the requirements of this
manual unless specifically exempted by this manual or the City Engineer.
The City Engineer shall have authority to modify requirements to protect the health,
safety or welfare of the public on the basis of information regarding threatened water
quality, erosion problems or potential habitat destruction, flooding, protection of
uninterruptible services, or endangerment to property. The City Engineer shall also
have the authority to modify requirements based upon increases in requirements
imposed by state or federal agencies, where existing requirements are not applicable
to the particular site, or other pertinent factors.
Permits may refer to this manual or the BMPs contained in this manual. In those
cases, affected permit-holders or applicants should use this manual for specific
guidance on how to comply with those permit conditions.
Where requirements in this manual are also mandated by any other law, ordinance, resolution, rule
or regulation, the more restrictive requirement shall apply.
Note: Drawing and detail figures included throughout these volumes are intended for
illustrative purposes only. Where any discrepancy exists between figure elements and
specific design criteria, the design criteria shall take precedence.
SURFACE WATER MANAGE MENT MANUAL
NOVEMBER 2009
4 Preface
Development of Best Management Practices (BMPs) for
Stormwater Management
The method by which this manual controls the adverse impacts of development and redevelopment
is through the application of Best Management Practices.
Best Management Practices (BMPs)
Best Management Practices 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 and other adverse impacts to waters of
Washington State. The types of BMPs are source control, treatment, and flow control. BMPs that
involve construction of engineered structures are often referred to as facilities in this manual. For
instance, the BMPs referenced in the menus of Chapter 2 in Volume 5 are called Treatment Facilities.
Source Control BMPs
Source control BMPs prevent or reduce pollution, or other adverse effects of stormwater, from
occurring. In this manual, source control BMPs are classified as operational or structural. Examples
of source control BMPs include methods as various as using mulches and covers on disturbed soil,
putting roofs over outside storage areas, and berming areas to prevent stormwater run-on and
pollutant runoff.
It is generally more cost-effective to use source controls to prevent pollutants from entering runoff
than to treat runoff to remove pollutants.
Treatment BMPs
Treatment BMPs include facilities that remove pollutants by simple gravity settling of particulate
pollutants, filtration, biological uptake, and soil adsorption. Treatment BMPs can accomplish
significant levels of pollutant load reductions if properly designed and maintained.
Flow Control BMPs
Flow control BMPs typically control the rate, frequency, and flow duration of stormwater surface
runoff. The need to provide flow control BMPs depends on whether a development site discharges to
a stream system or wetland, either directly or indirectly. Stream channel erosion control can be
accomplished by BMPs that detain runoff flows and also by those which physically stabilize eroding
stream banks. Both types of measures may be necessary. Only the former is covered in this manual.
The size of such a facility can be reduced by changing the extent to which a site is disturbed.
In regard to wetlands, it is necessary to not alter the natural hydroperiod. This means control of flows
from a development such that the wetland is within certain elevations at different times of the year
and short-term elevation changes are within the prescribed limits. If, however, the wetland was fed by
local groundwater elevations during the dry season, the impervious surface additions and the
bypassing practice may cause variations from the dry season elevations which might need mitigation.
The city has additional requirements that are related to surface water management, including
wetlands, critical areas, and flood protection. Refer to the City of Auburn Code.
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 MANAGE MENT MANUAL
NOVEMBER 2009
Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 25
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.
SURFACE WATER MANAGE MENT MANUAL
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New Development and Redevelopment Chapter 3 26
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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New Development and Redevelopment Chapter 3 27
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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New Development and Redevelopment Chapter 3 28
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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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 29
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.
SURFACE WATER MANAGE MENT MANUAL
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Minimum Requirements for Volume I
New Development and Redevelopment Chapter 3 30
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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New Development and Redevelopment Chapter 3 31
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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New Development and Redevelopment Chapter 3 32
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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Submittal Requirements Checklist Volume I
Appendix B 47
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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Submittal Requirements Checklist Volume I
Appendix B 48
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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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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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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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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8.2.1.4 Wetpool Geometry
Divide the wetpool into two cells separated by a baffle or berm. The first cell shall
contain between 25 to 35 percent of the total wetpool volume. The baffle or berm
volume shall not count as part of the total wetpool volume.
Provide sediment storage in the first cell. The sediment storage shall have a
minimum depth of 1-foot. Install a fixed sediment depth monitor in the first cell to
gauge sediment accumulation unless an alternative gauging method is proposed.
The minimum depth of the first cell shall be 4 feet, exclusive of sediment storage
requirements. The depth of the first cell may be greater than the depth of the second
cell.
The maximum depth of each cell shall not exceed 8 feet (exclusive of sediment
storage in the first cell). Plant pool depths of 3 feet or shallower (second cell) with
emergent wetland vegetation (see planting requirements).
Place inlets and outlets to maximize the flowpath through the facility. The ratio of
flowpath length to width from the inlet to the outlet shall be at least 3:1. The flowpath
length is defined as the distance from the inlet to the outlet, as measured at mid-
depth. The width at mid-depth can be found as follows: width = (average top width +
average bottom width)/2.
Wetponds with wetpool volumes less than or equal to 4,000 cubic feet may be single
celled (i.e., no baffle or berm is required). However, it is especially important in this
case that the flow path length be maximized. The ratio of flow path length to width
shall be at least 4:1 in single celled wetponds, but should preferably be 5:1.
All inlets shall enter the first cell. For multiple inlets, the length-to-width ratio shall be
based on the average flowpath length for all inlets.
Line the first cell in accordance with the liner requirements contained in Section 3.4.
8.2.1.5 Berms, Baffles, and Slopes
A berm or baffle shall extend across the full width of the wetpool, and tie into the
wetpond side slopes. If the berm embankments are greater than 4 feet in height, the
berm must be constructed by excavating a key with dimensions equal to 50 percent
of the embankment cross-sectional height and width. This requirement may be
waived if recommended by a geotechnical engineer for specific site conditions. The
geotechnical analysis shall address situations in which one of the two cells is empty
while the other remains full of water.
The top of the berm may extend to the WQ design water surface or be 1-foot below
the WQ design water surface. If at the WQ design water surface, berm side slopes
shall be 3H:1V. Berm side slopes may be steeper (up to 2H:1V) if the berm is
submerged 1-foot.
If good vegetation cover is not established on the berm, erosion control measures
shall be used to prevent erosion of the berm back-slope when the pond is initially
filled.
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The interior berm or baffle may be a retaining wall provided that the design is
prepared and stamped by a licensed civil engineer. If a baffle or retaining wall is
used, it shall be submerged one foot below the design water surface to discourage
access by pedestrians.
Criteria for wetpond side slopes are included in Section 3.3.
8.2.1.6 Embankments
Embankments that impound water must comply with the Washington State Dam Safety Regulations
(Chapter 173-175 WAC). If the impoundment has a storage capacity (including both water and
sediment storage volumes) greater than 10 acre-feet (435,600 cubic feet or 3.26 million gallons)
above natural ground level, then dam safety design and review are required by the Department of
Ecology. Contact Ecology for information about this regulation.
8.2.1.7 Inlet and Outlet
See Figure V-8-25 and Figure V-8-26 for details on the following requirements:
Submerge the inlet to the wetpond with the inlet pipe invert a minimum of two feet
from the pond bottom (not including sediment storage). The top of the inlet pipe shall
be submerged at least 1-foot, if possible.
Provide an outlet structure. Either a Type 2 catch basin with a grated opening (“jail
house window”) or a manhole with a cone grate (“birdcage”) may be used (see
Volume III, Figure III-2-9 for an illustration).
The pond outlet pipe (as opposed to the manhole or type 2 catch basin outlet pipe)
shall be back-sloped or have a down-turned elbow, and extend 1 foot below the WQ
design water surface.
Size the pond outlet pipe, at a minimum, to pass the on-line WQ design flow. The
highest invert of the outlet pipe sets the WQ design water surface elevation.
The overflow criteria for single-purpose (treatment only, not combined with flow
control) wetponds are as follows:
o The requirement for primary overflow is satisfied by either the grated inlet to
the outlet structure or by a birdcage above the pond outlet structure.
o The bottom of the grate opening in the outlet structure shall be set at or
above the height needed to pass the WQ design flow through the pond outlet
pipe. The grate invert elevation sets the overflow water surface elevation.
o The grated opening and downstream conveyance shall be sized to pass the
100-year design flow. The capacity of the outlet system shall be sized to pass
the peak flow for the conveyance requirements.
Provide an emergency spillway and design it according to the requirements for
detention ponds (see Volume III, Section 2.3.1).
The City may require a bypass/shutoff valve to enable the pond to be taken offline for
maintenance purposes.
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A gravity drain for maintenance is recommended if grade allows.
o The drain invert shall be at least 6 inches below the top elevation of the
dividing berm or baffle. Deeper drains are encouraged where feasible, but
must be no deeper than 18 inches above the pond bottom.
o The drain shall be at least 8 inches (minimum) diameter and shall be
controlled by a valve. Use of a shear gate is allowed only at the inlet end of a
pipe located within an approved structure.
o Provide operational access to the valve to the finished ground surface.
o The valve location shall be accessible and well marked with 1-foot of paving
placed around the box. It must also be protected from damage and
unauthorized operation.
o A valve box is allowed to a maximum depth of 5 feet without an access
manhole. If over 5 feet deep, an access manhole or vault is required.
All metal parts shall be corrosion-resistant. Do not use galvanized materials.
8.2.1.8 Access and Setbacks
All facilities shall be a minimum of 20 feet from any structure, property line, and any
vegetative buffer required by the local government, and 100 feet from any septic
tank/drainfield.
All facilities shall be a minimum of 50 feet from any slope greater than 20 percent. A
geotechnical report must address the potential impact of a wetpond on a slope steeper
than 20% or if closer than 50 feet.
Provide access and maintenance roads and designed them according to the
requirements for detention ponds. Access and maintenance roads shall extend to both
the wetpond inlet and outlet structures. An access ramp (7H minimum:1V) shall be
provided to the bottom of the first cell unless all portions of the cell can be reached and
sediment loaded from the top of the pond.
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.
8.2.1.9 Planting Requirements
Planting requirements for detention ponds also apply to wetponds.
Large wetponds intended for phosphorus control shall not be planted within the cells,
as the plants will release phosphorus in the winter when they die off.
If the second cell of a basic wetpond is 3 feet deep or shallower, the bottom area
shall be planted with emergent wetland vegetation. See Table V-8-15 for
recommended emergent wetland plant species for wetponds. The recommendations
in Table V-8-15 are for all of western Washington. Local knowledge should be used
to tailor this information to Auburn as appropriate.
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Cattails (Typha latifolia) shall not be used because they tend to crowd out other
species and will typically establish themselves anyway.
If the wetpond discharges to a phosphorus-sensitive lake or wetland, shrubs that
form a dense cover should be planted on slopes above the WQ design water surface
on at least three sides. Native vegetation species shall be used in all cases.
8.2.1.10 Recommended Design Features
The following features should be incorporated into the wetpond design where site conditions allow:
The method of construction of soil/landscape systems can cause natural selection of
specific plant species. Consult a soil restoration or wetland soil scientist for site-
specific recommendations. The soil formulation will impact the plant species that will
flourish or suffer on the site, and the formulation should be such that it encourages
desired species and discourages undesired species.
For private wetpond facilities, it is recommended that some form of recirculation be
provided, such as an aerator, to prevent stagnation and low dissolved oxygen
conditions. Recirculation is required for all public wetpond facilities.
A flow length-to-width ratio greater than the 3:1 minimum is desirable. If the ratio is
4:1 or greater, then the dividing berm is not required, and the pond may consist of
one cell rather than two. A one-cell pond must provide at least 6-inches of sediment
storage depth.
A tear-drop shape, with the inlet at the narrow end, rather than a rectangular pond is
preferred since it minimizes dead zones caused by corners.
A small amount of base flow is desirable to maintain circulation and reduce the
potential for low oxygen conditions during late summer.
Evergreen or columnar deciduous trees along the west and south sides of ponds are
recommended to reduce thermal heating, except that no trees or shrubs may be
planted on berms meeting the criteria of dams regulated for safety. In addition to
shade, trees and shrubs also discourage waterfowl use and the attendant
phosphorus enrichment problems they cause. Trees should be set back so that the
branches will not extend over the pond.
The number of inlets to the facility should be limited; ideally there should be only one
inlet. The flowpath length should be maximized from inlet to outlet for all inlets to the
facility.
The access and maintenance road could be extended along the full length of the
wetpond and could double as play courts or picnic areas. Placing finely ground bark
or other natural material over the road surface would render it more pedestrian
friendly.
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The following design features should be incorporated to enhance aesthetics where
possible:
o Provide pedestrian access to shallow pool areas enhanced with emergent
wetland vegetation. This allows the pond to be more accessible without
incurring safety risks.
o Provide side slopes that are sufficiently gentle (3H:1V or flatter).
o Create flat areas overlooking or adjoining the pond for picnic tables or seating
that can be used by residents. Walking or jogging trails around the pond are
easily integrated into site design.
o Include fountains or integrated waterfall features for privately maintained
facilities.
o Provide visual enhancement with clusters of trees and shrubs. On most pond
sites, it is important to amend the soil before planting since ponds are
typically placed well below the native soil horizon in very poor soils. Make
sure dam safety restrictions against planting do not apply.
o Orient the pond length along the direction of prevailing summer winds
(typically west or southwest) to enhance wind mixing.
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Table V-8-15. Emergent Wetland Species Recommended for Wetponds
Species Common Name Notes Maximum
Depth
INUNDATION TO 1-FOOT
Agrostis exarata1 Spike bent grass Prairie to coast 2 feet
Carex stipata Sawbeak sedge Wet ground
Eleocharis palustris Spike rush Margins of ponds, wet meadows 2 feet
Glyceria occidentalis Western mannagrass Marshes, pond margins 2 feet
Juncus tenuis Slender rush Wet soils, wetland margins
Oenanthe sarmentosa Water parsley Shallow water along stream and pond margins;
needs saturated soils all summer
Scirpus atrocinctus (formerly
S. cyperinus)
Woolgrass Tolerates shallow water; tall clumps
Scirpus microcarpus Small-fruited bulrush Wet ground to 18 inches depth 18 inches
Sagittaria latifolia Arrowhead
Inundation 1 to 2 feet
Agrostis exarata1 Spike bent grass Prairie to coast
Alisma plantago-aquatica Water plantain
Eleocharis palustris Spike rush Margins of ponds, wet meadows
Glyceria occidentalis Western mannagrass Marshes, pond margins
Juncus effuses Soft rush Wet meadows, pastures, wetland margins
Scirpus microcarpus Small-fruited bulrush Wet ground to 18 inches depth 18 inches
Sparganium emmersum Bur reed Shallow standing water, saturated soils
Inundation 1 to 3 feet
Carex obnupta Slough sedge Wet ground or standing water 1.5 to 3 feet
Beckmania syzigachne1 Western sloughgrass Wet prairie to pond margins
Scirpus acutus2 Hardstem bulrush Single tall stems, not clumping 3 feet
Scirpus validus2 Softstem bulrush
Inundation GREATER THAN 3 feet
Nuphar polysepalum Spatterdock Deep water 3 to 7.5 feet
Nymphaea odorata1 White waterlily Shallow to deep ponds 6 feet
1 Non-native species. Beckmania syzigachne is native to Oregon. Native species are preferred.
2 Scirpus tubers must be planted shallower for establishment, and protected from foraging waterfowl until established.
Emerging aerial stems should project above water surface to allow oxygen transport to the roots.
Primary sources: Municipality of Metropolitan Seattle, Water Pollution Control Aspects of Aquatic Plants, 1990.
Hortus Northwest, Wetland Plants for Western Oregon, Issue 2, 1991. Hitchcock and Cronquist, Flora of the Pacific
Northwest, 1973.
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8.2.3.5 Wetland Geometry
1. Stormwater wetlands shall consist of two cells, a presettling cell and a wetland cell.
2. The presettling cell shall contain approximately 33 percent of the wetpool volume
calculated in Step 1 above.
3. The depth of the presettling cell shall be between 4 feet (minimum) and 8 feet
(maximum), excluding sediment storage.
4. Provide one-foot of sediment storage in the presettling cell.
5. The wetland cell shall have an average water depth of about 1.5 feet (plus or minus
3 inches).
6. Shape the "berm" separating the two cells such that its downstream side gradually
slopes to form the second shallow wetland cell (see the section view in Figure V-8-
32). Alternatively, the second cell may be graded naturalistically from the top of the
dividing berm (see Step 8 below).
7. The top of the berm shall be either at the WQ design water surface or submerged
1 foot below the WQ design water surface. Correspondingly, the side slopes of the
berm must meet the following criteria:
a. If the top of berm is at the WQ design water surface, the berm side slopes
shall be no steeper than 3H:1V.
b. If the top of berm is submerged 1-foot, the upstream side slope may be up to
2H:1V.
8. Grade the bottom of the wetland cell in one of two ways:
a. Shallow evenly graded slope from the upstream to the downstream edge of the
wetland cell (see Figure V-8-32).
b. A "naturalistic" alternative, with the specified range of depths intermixed
throughout the second cell (see Figure V-8-33). A distribution of depths shall be
provided in the wetland cell depending on whether the dividing berm is at the
water surface or submerged (see Table V-8-16).
The maximum depth shall be 2.5 feet in either configuration.
8.2.3.6 Lining Requirements
In infiltrative soils, line both cells of the stormwater wetland. To determine whether a low-permeability
liner or a treatment liner is required, determine whether the following conditions will be met. If soil
permeability will allow sufficient water retention, lining may be waived.
1. The second cell must retain water for at least 10 months of the year.
2. The first cell must retain at least three feet of water year-round.
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3. Use a complete precipitation record when establishing these conditions. Take into
account evapo-transpiration losses as well as infiltration losses. Many wetland plants
can adapt to periods of summer drought, so a limited drought period is allowed in the
second cell. This may allow a treatment liner rather than a low permeability liner to be
used for the second cell. The first cell must retain water year-round in order for the
presettling function to be effective.
4. If a low permeability liner is used, place a minimum of 18 inches of native soil amended
with good topsoil or compost (one part compost mixed with 3 parts native soil) over the
liner. For geomembrane liners, a soil depth of 3 feet is recommended to prevent damage
to the liner during planting. Hydric soils are not required.
The criteria for liners given in Chapter 3 must be observed.
8.2.3.7 Inlet and Outlet
Same as for wetponds (see BMP T1010).
8.2.3.8 Access and Setbacks
Location of the stormwater wetland relative to site constraints (e.g., buildings, property
lines, etc.) shall be the same as for detention ponds (see Volume III). See Chapter 3 for
typical setback requirements for WQ facilities.
Provide access and maintenance roads and design them according to the requirements
for detention ponds (see Volume III). Extend access and maintenance roads shall to
both the wetland inlet and outlet structures. Provide an access ramp (7H minimum:1V) to
the bottom of the first cell unless all portions of the cell can be reached and sediment
loaded from the top of the wetland side slopes.
The internal berm of a stormwater treatment wetland 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.