HomeMy WebLinkAboutSupplemental Manual Combined VolumesCity of Auburn
Supplemental Manual to
2019 Stormwater
Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site?
Volume II – Construction Stormwater Pollution Prevention
Volume III – Choosing, Modeling, and Documenting Your BMPs
Volume IV – Source Control BMP Library
Volume V – Runoff Treatment, Flow Control, and LID BMP Library
City of Auburn Departments of Public Works and Community Development
Effective Date: 1/18/2022
i
Contents
Volume I...........................................................................................................................1
Executive Summary of Volume I – What Requirements Apply to My Site?...............................2
I-1 – Introduction to Stormwater Management in Western Washington ....................................2
I-1.1 About This Manual.........................................................................................................2
I-2 – Relationship of This Manual to Permits, Requirements, and Programs............................4
I-2.15 Other Requirements.....................................................................................................4
City of Auburn Codes and Ordinances........................................................................4
I-3 – Minimum Requirements for New Development and Redevelopment................................5
I-3.1 Introduction to the Minimum Requirements ...................................................................5
I-3.2 Exemptions ....................................................................................................................5
I-3.3 Applicability of the Minimum Requirements...................................................................6
I-3.4 Minimum Requirements (MRs) ......................................................................................7
I-3.4.1 MR1: Preparation of Stormwater Site Plans.....................................................7
I-3.4.2 MR2: Construction Stormwater Pollution Prevention (SWPPP).......................7
I-3.4.3 MR3: Source Control of Pollution.....................................................................8
I-3.4.4 MR4: Preservation of Natural Drainage Systems and Outfalls.........................8
I-3.4.5 MR5: On-Site Stormwater Management ..........................................................8
I-3.4.6 MR6: Runoff Treatment..................................................................................11
I-3.4.7 MR7: Flow Control..........................................................................................11
I-3.4.8 MR8: Wetlands Protection..............................................................................11
I-3.4.9 MR9: Operation and Maintenance..................................................................12
I-3.4.10 MR10: Off-Site Analysis and Mitigation........................................................12
I-3.5 Additional Protective Measures (APM) (Optional) .......................................................13
I-3.5.2 APM1: Financial Liability ................................................................................13
I-3.5.3 APM2: Off Site Analysis Report......................................................................13
I-3.6 Adjustments and Exceptions/Variances to the MRs ....................................................14
Appendix I-F Glossary and Notations.....................................................................................15
Appendix I-G City of Auburn Areas with Special Development Requirements.......................16
Appendix I-H On – Site Stormwater Management BMP List Option Infeasibility Criteria .......18
On-Site Stormwater Management BMP Infeasibility Summary............................................20
Appendix I-I Stormwater Site Plan (SSP) Report Submittal Requirements Checklist .............25
Appendix I-J Stormwater Facility Access Requirements .........................................................35
Volume II........................................................................................................................37
Executive Summary of Volume II – Construction Stormwater Pollution Prevention................38
ii
II-2 –Construction Stormwater Pollution Prevention Plans (Construction SWPPPs)...............40
II-2.2 When is a Construction SWPPP Required? ...............................................................40
II-2.4 Preparing Construction SWPPPs ...............................................................................40
II-3 – Construction Stormwater BMPs .....................................................................................41
BMP C163: Demolition of Buildings (Additional)........................................................41
BMP C164: Building, Repair, Remodeling and Construction (Additional) .................42
II-3.3 Construction Runoff BMPs..........................................................................................43
BMP C233: Silt Fence (Amended)............................................................................43
BMP C235: Wattles (Amended)................................................................................43
BMP C250: Construction Stormwater Chemical Treatment (Amended)...................44
Appendix II-B Construction Stormwater Pollution Prevention Plan Short Form ......................45
Volume III.......................................................................................................................46
Executive Summary of Volume III – Choosing, Modeling, and Documenting Your BMPs ......47
III-2 – Modeling Your BMPs.....................................................................................................48
III-2.2 Continuous Simulation Models ..................................................................................48
Western Washington Hydrology Model .....................................................................48
III-2.3 Single Event Hydrograph Method..............................................................................49
Water Quality Design Storm......................................................................................49
III-2.5 Closed Depression Analysis ......................................................................................49
III-3 - Stormwater Site Plans....................................................................................................49
III-3.2 Preparing a Stormwater Site Plan..............................................................................49
III-3.3 Changes to a Previously Approved Stormwater Site Plan.........................................49
Appendix III-D Conveyance System Design and Hydraulic Analysis ......................................50
III-D.1 Conveyance System Analysis Requirements............................................................50
III-D.2 Design Event.............................................................................................................51
III-D.3 Methods of Analysis ..................................................................................................52
III-D.4 Pipes, Culverts and Open Channels .........................................................................60
III-D.5 Outfalls Systems .....................................................................................................106
III-D.6 Pump Systems ........................................................................................................115
III-D.7 Easements and Access..........................................................................................117
Appendix III-E City of Auburn Design Storm..........................................................................118
Appendix III-F Procedure for Conducting a Pilot Infiltration Test...........................................119
Volume IV ....................................................................................................................121
Executive Summary of Volume IV - Source Control BMP Library .........................................122
IV-2 – Cleaning or Washing Source Control BMPs ...............................................................123
iii
S431: BMPs for Washing and Steam Cleaning Vehicles/Equipment/Building
Structures (Amended)...............................................................................123
S441: BMPs for Potable Water Line Flushing, Water Tank Maintenance, and Hydrant
Testing (Amended) ...................................................................................126
IV-3 – Roads, Ditches, and Parking Lot Source Control BMPs.............................................127
S630: BMPs for Disposing of Contaminated Stormwater and Waste Materials
Properly (Additional) .................................................................................127
S631: BMPs for Cleaning Catch Basins (Additional)...............................................128
S632: BMPs for Paving the Activity Area and Sloping to a Sump, Holding Tank, or
Oil/Water Separator (Additional)...............................................................128
S633: BMPs for Surrounding the Activity Area with a Curb, Dike, or Berm or
Elevating the Activity (Additional)..............................................................129
IV-4 – Soil Erosion, Sediment Control, and Landscaping Source Control BMPs ..................130
S435: BMPs for Pesticides and Integrated Pest Management Program (Amended)
..................................................................................................................130
S640: BMPs for Concrete and Asphalt Mixing and Production at Stationary Sites
(Additional)................................................................................................131
IV-5 – Storage and Stockpiling Source Control BMPs...........................................................132
S429: BMPs for Storage or Transfer (Outside) of Solid Raw Materials, Byproducts, or
Finished Products (Amended) ..................................................................132
S445: BMPs for Temporary Fruit Storage (Amended).............................................133
S650: BMPs for Storage of Solid Wastes and Food Wastes (Additional)................134
S651: BMPs for Storage and Treatment of Contaminated Soils (Additional)..........136
IV-7 – Other Source Control BMPs........................................................................................137
S414: BMPs for Maintenance and Repair of Vehicles and Equipment (Amended).137
S670: BMPs for Cleaning or Washing of Tools, Engines and Manufacturing
Equipment (Additional)..............................................................................138
S671: BMPs for Cleaning or Washing of Cooking Equipment (Additional)..............140
S672: BMPs for Collection and Disposal of Wastewater in Mobile Interior Washing
Operations (Additional) .............................................................................141
S673: BMPs for Manufacturing and Post-Processing of Metal Products (Additional)
..................................................................................................................142
S674: BMPs for Mining and Quarrying of Sand, Gravel, Rock, Peat, Clay and Other
Materials (Additional) ................................................................................144
S675: BMPs for Discharging Process Wastewater to a Sanitary Sewer, Holding
Tank, or Water Treatment System (Additional).........................................144
S676: BMPs for Covering the Activity with a Roof or Awning (Additional)...............145
Appendix IV – C Quick Reference Phone Numbers..............................................................146
Appendix IV – D Source Control BMPs for Homeowners......................................................147
Automobile Washing (for Single-Family Residences)..............................................147
Automobile Maintenance.........................................................................................148
Storage of Solid Wastes and Food Wastes.............................................................149
Composting..............................................................................................................149
Yard Maintenance and Gardening...........................................................................150
iv
Swimming Pool and Spa Cleaning and Maintenance..............................................151
Household Hazardous Material Use, Storage, and Disposal...................................152
General Home Maintenance....................................................................................153
Pet Waste................................................................................................................154
Volume V .....................................................................................................................155
Executive Summary of Volume V - Runoff Treatment, Flow Control, and LID BMP Library .156
V-1 – General BMP Design ...................................................................................................158
V-1.1 Sequence of Runoff Treatment and Detention BMPs ..............................................158
V-1.2 Setbacks, Slopes, and Embankments......................................................................158
V- 4 – Roof Downspout BMPs..............................................................................................159
V-4.1 Introduction to Roof Downspout BMPs.....................................................................159
V-5 – Infiltration BMPs...........................................................................................................160
V-5.6 Site Suitability Criteria (SSC) (Amended).................................................................160
SSC-1 Setback Criteria (Amended).........................................................................160
BMP T5.15 Permeable Pavements (Amended).......................................................160
BMP T7.30 Bioretention (Amended)........................................................................162
V-10 – Manufactured Treatment Devices as BMPs...............................................................164
V-10.3 Approval Process for Manufactured Treatment Devices........................................164
V-11 – Miscellaneous LID BMPs ...........................................................................................164
V-11.1 Introduction to Miscellaneous LID BMPs (Amended)............................................164
BMP T5.13 Post-Construction Soil Quality and Depth (Amended)..........................164
V-12 – Detention BMPs .........................................................................................................164
V-12.2 Control Structure Design (Amended) .....................................................................164
V-12.3 Other Detention Design Options (Amended)..........................................................164
BMP D.1 Detention Ponds .................................................................................................165
BMP D.3 Detention Vaults .................................................................................................174
v
List of Figures
Figure I-3.3A Flow Chart for Determining LID MR #5 Requirements in the City of Auburn ........10
Figure I-H-1 Map of Infiltration Restriction Areas ........................................................................19
Figure III-D.4-1 Pipe Sizing Nomograph .....................................................................................63
Figure III-D.4-2 Circular Channel Ratios .....................................................................................65
Figure III-D.4-3 Backwater Calculation Sheet .............................................................................67
Figure III-D.4-4 Critical Depth of Flow for Circular Culverts ........................................................70
Figure III-D.4-5 Junction Head Loss in Structures ......................................................................71
Figure III-D.4-6 Backwater Pipe Calculation Example ................................................................72
Figure III-D.4-7 Debris Barriers ...................................................................................................80
Figure III-D.4-8 Headwater Depth for Smooth Interior Pipe Culverts with Inlet Control ..............83
Figure III-D.4-9 Headwater Depth for Corrugated Pipe Culverts with Inlet Control.....................84
Figure III-D.4-10 Head for Culverts (Pipe W/”N”=0.012) Flowing Full with Outlet Control ..........87
Figure III-D.4-11 Head for Culverts (Pipe W/”N”=0.024) Flowing Full with Outlet Control ..........88
Figure III-D.4-12 Ditches – Common Section Properties ............................................................94
Figure III-D.4-13 Drainage Ditches – Slope/Discharge Chart .....................................................95
Figure III-D.4-14 Geometric Elements of Common Sections ......................................................96
Figure III_D.4-15 Direct Step Backwater Method Example.......................................................100
Figure III-D.4-16 Mean Channel Velocity vs Medium Stone Weight (W50) and Equivalent Stone
Diameter....................................................................................................................................104
Figure III-D.4-17 Riprap Gradation Curve .................................................................................105
Figure III-D.5-1 Pipe/Culvert Outfall Discharge Protection........................................................107
Figure III-D.5-2 Gabion Outfall Detail........................................................................................110
Figure III-D.5-3 Diffuser Tee – Energy Dissipating End Feature Example................................111
Figure III-D.5-4 Flow Dispersal Trench .....................................................................................113
Figure III-D.5-5 Alternative Flow Dispersal Trench ...................................................................114
Figure IV-3-1 Paved Area with Sump Drain ..............................................................................129
Figure IV-5.7A Stockpile Covering ............................................................................................133
vi
List of Tables
Table III-D.2-1 Percentage Impervious For Fully Developed Conditions Offsite Tributary Areas
....................................................................................................................................................52
Table III-D.3-1 Runoff Coefficients for the Rational Method........................................................55
Table III-D.3-2 Design Storm Frequency Coefficients for the Rational Method ..........................56
Table III-D.3-3 Rainfall Intensities for the City of Auburn ............................................................57
Table III-D.3-4 “n” and “k” Values for Hydrographs....................................................................60
Table III-D.4-1 Percentage Impervious for Modeling Fully Developed Conditions......................62
Table III-D.4-2 Manning’s “n” Values for Pipes ...........................................................................64
Table III-D.4-3 Backwater Calculation Sheet Notes....................................................................69
Table III-D.4-4 Maximum Pipe Slopes, Velocities, and Anchor Requirements ...........................75
Table III-D.4-5 Allowable Structures and Pipe Sizes...................................................................76
Table III-D.4-6 Constants for Inlet Control Equations..................................................................85
Table III-D.4-7 Entrance Loss Coefficients..................................................................................90
Table III-D.4-8 Channel Protection..............................................................................................97
Table III-D.4-9 Values of “n” for Channels...................................................................................99
Table III-D.5-1 Rock Protection at Outfalls................................................................................108
Table III-E-1 Design Storm Precipitation Values.......................................................................118
Table III-F-1 In-Situ Infiltration Measurement Correction Factors to Estimate Long-Term
Infiltration Rates ........................................................................................................................120
Table V-7.4-1 Grass Seed Mixes for Public Bioretention Facilities...........................................163
Table V-12.2-1 Plant Selection Guide.......................................................................................173
Table V-12.3-2 Grass Seed Mixes for Detention/Retention Facilities .......................................173
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
1
Volume I
What Requirements Apply to My Site?
City of Auburn Supplemental Manual
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
2
Executive Summary of Volume I – What Requirements Apply to My
Site?
I-1 – Introduction to Stormwater Management in Western Washington
I-1.1 About This Manual
The City of Auburn (COA) Supplemental Manual to the 2019 Department of Ecology’s (Ecology)
Stormwater Management Manual for Western Washington (SWMMWW) provides additional
requirements and guidance for applying the Minimum Requirements for stormwater
management to all new development and redevelopment projects. These two documents
together are referred to as the City of Auburn Surface Water Management Manual (SWMM), as
defined in Chapter 12.04.010 of the Auburn City Code (ACC). The SWMM is the manual of the
specific requirements related to storm drainage management in the City of Auburn.
Each volume of the COA Supplemental Manual is organized to correspond to the same volume
in the SWMMWW.
Each topic is identified with the volume number followed by the chapter and section (e.g., III-3.2
refers to Volume 3, Chapter 3, Section 2 of the SWMMWW or COA Supplemental Manual). Best
Management Practices (BMP) are grouped by activity in Volumes 2, 4, and 5 (e.g., IV-2 refers to
Volume 4 , Section 2 which addresses Cleaning or Washing Source Control BMPs).
The Ecology SWMMWW is available online at the link below:
2019 SWMMWW
Refer to the Glossary in the SWMMWW for definitions of terms used throughout this manual.
Additional definitions of terms not defined in the SWMMWW and acronyms are included in
Appendix I-F (Volume I) of the COA Supplemental Manual.
Important additions and changes contained in the COA Supplemental Manual for this Volume
include:
I-2.15: Other Requirements contains Auburn’s regulatory authority to require
stormwater management systems to prevent pollution of State waters.
I-3: Minimum Requirements for New Development and Redevelopment contains
clarifications, additional information regarding exceptions/exemptions to the Minimum
Requirements, and additions to the Minimum Requirements:
o I-3.1 provides additional requirements for multiple properties that share private
storm drainage facilities.
o I-3.2 provides additional definitions and clarifications for projects exempt from all
or part of the Minimum Requirements.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
3
o I-3.3 provides guidelines about when additional requirements to the Minimum
Requirements are required.
o I-3.4.1 – I-3.4.9 provide additional City requirements and guidance for meeting
Minimum Requirements #1 through #9.
o I-3.4.10 includes the additional Minimum Requirement #10 – Off-Site Analysis
and Mitigation, applicable to projects meeting the thresholds for Minimum
Requirements #1 - #5 and #1 - #9.
o I-3.5 clarifies the existing City of Auburn policies that are related to the Additional
Protective Measures (Optional) contained in that Section of the SWMMWW.
o I-3.6 contains information on Exceptions/Variances to the Minimum
Requirements within the City of Auburn.
Appendix I-F: Glossary and Notations - contains additional definitions of terms used
throughout the SWMMWW and the COA Supplemental Manual.
Additional Appendices contained in the COA Supplemental Manual include:
o Supplemental Appendix I-G: City of Auburn Areas with Special
Development Requirements identifies geographic areas within the City of
Auburn and the requirements specific to those areas.
o Supplemental Appendix I-H: On – Site Stormwater Management BMP List
Option Infeasibility Criteria provides infeasibility criteria for use in evaluating
BMPs to meet Minimum Requirement #5: On-Site Stormwater Management
using the List Option.
o Supplemental Appendix I-I: Stormwater Site Plan Report Submittal
Requirements Checklist is presented to assist with meeting the City of Auburn
submittal requirements for Stormwater Site Plan Reports.
o Supplemental Appendix I-J: Stormwater Facility Access Requirements
provides City of Auburn access and easement requirements for stormwater
facility design.
Sections with No Changes
The following portions of Volume I of the SWMMWW do not require any additional clarification in
the COA Supplemental Manual, and together with the COA Supplemental Manual, contain the
requirements and guidelines for stormwater management in the City of Auburn.
I-1: Introduction to Stormwater Management in Western Washington (all sections)
I-2: Relationship of This Manual to Permits, Requirements, and Programs
o I-2.1 – I-2.14
I-4: Introduction to UIC Wells (all sections)
Appendices I-A through I-E
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
4
I-2 – Relationship of This Manual to Permits, Requirements, and
Programs
I-2.15 Other Requirements
The COA Supplemental Manual to the SWMMWW is intended to provide additional technical
guidance for meeting the regulatory requirements outlined in the SWMMWW. The COA
Supplemental Manual assists developers, contractors, residents, and other project proponents
in development of stormwater management strategies that meet City of Auburn, State, and
Federal requirements.
The two main general stormwater permits relevant to the use of this volume of the COA
Supplemental Manual in conjunction with the SWMMWW are:
The Phase II Municipal Stormwater General Permit requires the City of Auburn to
adopt ordinances and other enforceable mechanisms that implement stormwater
controls for new development and redevelopment, including stormwater management of
construction sites. The City is responsible for the enforcement of the ordinances and
other enforceable mechanisms required by this permit.
The Construction Stormwater General Permit, the requirements of which are outlined
in I-2.7 of the SWMMWW.
Project proponents are required to obtain all applicable City of Auburn permits for construction
activities. Contact the Permit Center at 253-931-3020 or online at development@auburnwa.gov
for more information on required permits.
City of Auburn Codes and Ordinances
The following summarizes Auburn City Code that applies to surface water and pollution control.
The complete code may be found online at: http://www.auburnwa.gov/Home.asp
Chapter 8.08 ACC Solid Waste
Chapter 12.20 ACC Driveways
Title 13 ACC Water, Sewer and Public Utilities
Chapter 13.48 ACC Storm Drainage Utility
Section 13.48.330 ACC Off-site Improvements
Title 14 ACC Project Review
Chapter 15.74 ACC Land Clearing, Filling and Grading
Title 16 ACC Environment
Chapter 16.08 ACC Shoreline Regulation
Chapter 16.10 ACC Critical Areas
Title 17 ACC Subdivision
Title 18 ACC Zoning
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
5
I-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
ACC. New development and redevelopment projects may also be subject to other City code
requirements, depending on the nature and location of the project. These code requirements may
include, but are not limited to:
Chapter 12.20 ACC – Driveways
Title 14 ACC – Project Review, Land Adjustments and Divisions
Chapter 15.74 ACC – Land Clearing, Filling, and Grading
Chapter 16.08 ACC – Shoreline Management, Administrative, and Permitting Procedures
Chapter 16.10 ACC – Critical Areas
Title 17 ACC – Land Adjustments and Divisions
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.
I-3.1 Introduction to the Minimum Requirements
Private stormwater facilities shared by multiple properties require private cross drainage
easement and maintenance agreement(s) or other binding instruments such as plat conditions.
Copies of said instrument(s) must be provided to the City prior to civil plan approval.
I-3.2 Exemptions
Pavement Maintenance
When replacing or expanding public sidewalk in the City owned right of way, Minimum
Requirements #1 - #5 apply to the replaced sidewalk. The expanded sidewalk is subject to
Minimum Requirements #1 - #10.
Underground utility projects
Underground utility projects include, but are not limited to, installation of underground detention
or storm conveyance systems that are required due to other ground disturbing activities.
Additional Exemptions to the Minimum Requirements for the City of Auburn:
Oil and Gas Field Activities or Operations
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
6
Per Appendix I of the Ecology Western Washington Phase II Municipal Stormwater Permit,
construction of drilling sites, waste management pits, and access roads, as well as construction
of transportation and treatment infrastructure such as pipelines, natural gas treatment plants,
natural gas pipeline compressor stations, and crude oil pumping stations are exempt. Operators
are encouraged to implement and maintain BMPs to minimize erosion and control sediment
during and after construction activities to help ensure protection of surface water quality during
storm events.
Minor Clearing and Grading
The following minor clearing and grading activities are exempt from all the Minimum
Requirements except for Minimum Requirement #2 – Construction Stormwater Pollution
Prevention, unless located within a critical or sensitive area governed by the City’s Critical Areas
Ordinance, found in Chapter 16.10 of the ACC. Information on Critical Areas is available
through Community Development Services in the Community Development Department at (253)
931-3090.
Excavation for wells and exploratory excavations under the direction of soil engineers or
engineering geologists licensed in Washington State; fill made with the material from
these excavation activities remains subject to the Minimum Requirements and the City’s
Land Clearing, Filling and Grading requirements per Chapter 15.74 of the ACC;
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
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; and road reconstruction to restore damage from a flood,
earthquake, landslide, or volcanic activity. Permits authorizing the emergency work may be
required after completion of the emergency project.
I-3.3 Applicability of the Minimum Requirements
All New Development projects in the City of Auburn that meet the thresholds for Minimum
Requirements #1-#5 or #1-#9 are also required to meet Minimum Requirement #10 – Off-Site
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
7
Analysis and Mitigation. See I-3.4.10 of the COA Supplemental Manual for information
regarding Minimum Requirement #10.
All Redevelopment projects in the City of Auburn that meet the thresholds for Minimum
Requirements #1-#5 or #1-#9 are also required to meet Minimum Requirement #10 – Off-Site
Analysis and Mitigation. See I-3.4.10 of the COA Supplemental Manual for information
regarding Minimum Requirement #10.
I-3.4 Minimum Requirements (MRs)
Additional guidelines and requirements for applying the Minimum Requirements to projects
within the City of Auburn are provided in each section, where applicable.
I-3.4.1 MR1: Preparation of Stormwater Site Plans
To assist with meeting the submittal requirements of the SSP report, a SSP report outline and
checklist is provided in Appendix I-I of the COA Supplemental Manual.
I-3.4.2 MR2: Construction Stormwater Pollution Prevention (SWPPP)
A complete Construction Stormwater Pollution Prevention Plan (SWPPP) is required for projects
that:
Add or replace 5000 square feet or greater of hard surface, or
Disturb greater than 1 acre, or
Grade/Fill greater than 500 cubic yards of material, or
Disturb critical areas or buffers.
Projects that are required to submit a complete Construction SWPPP must use the most current
version of Ecology’s Construction SWPPP Template, available at:
http://www.ecy.wa.gov/programs/wq/stormwater/construction/
A Construction SWPPP Short Form is discussed in Appendix II-B of the COA Supplemental
Manual. The Construction SWPPP Short Form may be used for projects that meet the
thresholds found in that Appendix. A fillable version of the Construction SWPPP Short Form is
also available on the City’s website under “Civil” permit applications at the following link:
https://www.auburnwa.gov/forms
For more information on SWPPP submittal requirements refer to Chapter II-2 of the COA
Supplemental Manual.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
8
I-3.4.3 MR3: Source Control of Pollution
Source Control BMPs are used to control long-term pollutant sources from many practices in
commercial, industrial, and residential settings. Volume IV of both the SWMMWW and the COA
Supplemental Manual contain long-term Source Control BMPs for these practices. The BMP
resources available in Volume IV shall be used to develop a comprehensive SSP report by
including these BMPs in the Permanent Stormwater Control Plan section of that report (see
Chapter III-3 of the SWMMWW). Including and implementing the Source Control BMPs found in
Volume IV is a required component of the SSP report that will be submitted to the City for
review and approval. It is also required to include these Source Control BMPs in the Operations
and Maintenance Manual that is prepared in compliance with Minimum Requirement #9 –
Operation and Maintenance and included in the SSP report.
Additional Source Control BMPs are found in Volume II of both the SWMMWW and the COA
Supplemental Manual. These BMPs are specific to source control of pollution during the
construction process. Source Control BMPs for construction shall be included as part of the
Construction SWPPP that is prepared in compliance with Minimum Requirement #2 –
Construction Stormwater Pollution Prevention. Refer to Volume II of the SWMMWW and COA
Supplemental Manual for complete information on construction Source Control BMPs and the
Construction SWPPP.
I-3.4.4 MR4: Preservation of Natural Drainage Systems and Outfalls
No Additional Guidelines or Requirements
I-3.4.5 MR5: On-Site Stormwater Management
Flow Control Exemption
Flow Control Exemption in accordance with Section I-3.4.7 of the SWMMWW, as it relates to
Minimum Requirement #5, requires approval from the City Engineer per the Exemption process.
Refer to Section I-3.6 of the COA Supplemental Manual for information on the Exemption
application process. Projects that have received approval from the City Engineer for Flow
Control Exemption may meet Minimum Requirement #5 as described in Section I-3.4.5 of the
SWMMWW.
Additional Guidelines for the City of Auburn
All land within the City of Auburn is located within the Urban Growth Area (UGA); therefore, the
second and third requirements in Table I-3.1 of the SWMMWW do not apply.
Figure I-3.3A Flow Chart for Determining LID MR #5 Requirements in the City of Auburn below,
based on Ecology’s Figure I-3.3 of the SWMMWW, provides guidance in considering the
options available for complying with Minimum Requirement #5.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
9
Project proponents that must satisfy Minimum Requirement #5 can choose to employ the On-
Site Stormwater Management BMPs from Lists #1 or #2, as required by the project size, or
meet the Low Impact Development (LID) Performance Standard.
Applying the List Option
When choosing the List option, each On-Site Stormwater Management BMP must be
considered for each surface in the order they are given for the appropriate List. A BMP must be
determined to be infeasible prior to continuing to the next BMP for that surface on the List. Per
Minimum Requirement #1 and Chapter III-3 (Stormwater Site Plans) of the SWMMWW, any
BMPs from the list that have been determined to be infeasible for the project must include
documentation of the infeasibility within the SSP report. Where appropriate the infeasibility
documentation should include citations of site conditions identified in a geotechnical report.
Refer to Appendix I-H of the COA Supplemental Manual for more information on List Option LID
BMP infeasibility.
Once an On-Site Stormwater Management BMP from List #1 or #2 is considered feasible, this
BMP is applied to the project. On-Site Stormwater Management BMPs must be applied in
accordance with the design criteria presented in the SWMMWW and COA Supplemental
Manual. No further consideration of BMPs on the List is required for that surface, and the
remaining BMPs for that surface on the List that are below the applied BMP do not require
documentation of infeasibility. The BMPs for each surface type must be evaluated for feasibility.
If all BMPs from the List have been found to be infeasible, Minimum Requirement #5 has been
met and no further action is required. Projects that have satisfied Minimum Requirement #5
through the List option are not required to meet the LID Performance Standard, even if all LID
BMPs have been determined to be infeasible.
Applying the LID Performance Standard Option
Project proponents must demonstrate compliance with the LID Performance Standard using any
facility or combination of facilities that are approved by the City. Facilities are not limited to the
On-Site Stormwater Management BMPs from the List Option. Rain Gardens cannot be used
when selecting this option. Compliance with the LID Performance Standard must be shown
using the most current version of the Western Washington Hydraulic Model (WWHM). Model
results must be included in the appropriate section of the SSP report and submitted to the City.
Refer to Chapter III-2 of the SWMMWW and COA Supplemental Manual for more information
on hydrologic modeling requirements.
Projects that must apply only Minimum Requirements #1 - #5 and #10 are not required to
implement BMP T5.13 Post – Construction Soil Quality and Depth when the LID Performance
Standard option is applied. Projects that must apply Minimum Requirements #1 - #9 and #10
are required to implement BMP T5.13 if feasible when the LID Performance Standard option is
applied.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
10
Figure I-3.3A Flow Chart for Determining LID MR #5 Requirements in the City of Auburn
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
11
I-3.4.6 MR6: Runoff Treatment
Infiltration of any amount of runoff from a Pollution Generating Surface (PGS) within
Groundwater Protection Zones 1 and 2 requires the approval of the City Engineer, and at a
minimum requires that the site not be classified as a “high-use site” per the Glossary in the
SWMMWW and runoff from the PGS must undergo Enhanced Treatment prior to entering an
infiltration facility per III-1.2 of the SWMMWW.
I-3.4.7 MR7: Flow Control
Infrastructure Protection Requirement
The infrastructure protection requirement is intended to mitigate stormwater impacts from
projects that are not required to provide flow control, but that discharge to a system with
downstream capacity limitations.
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 pre-existing conditions.
Flow Control Exemption
Flow Control Exemption in accordance with I-3.4.7 of the SWMMWW requires approval from the
City Engineer per the Exemption process. Refer to I-3.6 of the COA Supplemental Manual for
information on the Exemption application process.
Please Note: The Green River identified in Appendix I-A of the SWMMWW is identified as WRIA
26 in Cowlitz County, Washington. This river is not the Green River located in the City of
Auburn, and flow control exemption does not apply to projects that discharge directly to the
Green River within the City of Auburn.
The City of Auburn does not contain any areas that are recognized by the Department of
Ecology as a basin with 40% or more of total impervious area as of 1985. Pre-existing site
conditions shall be modeled using Forested conditions unless reasonable, historic evidence
exists showing that the area should be modeled as Prairie.
I-3.4.8 MR8: Wetlands Protection
Stormwater discharges to wetlands are regulated under the City’s Critical Areas Ordinance. See
Chapter 16.10 of the ACC.
The requirements in I-3.4.8 of the SWMMWW are in addition to requirements given in Chapter
16.10 of the ACC 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
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
12
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.
Additional Guidelines for the City of Auburn
For more information about wetlands, wetland permits and development close to wetlands,
please contact the Community Development Department at (253) 931-3090.
I-3.4.9 MR9: Operation and Maintenance
The Operations and Maintenance Manual is a component of the SSP report, but it should be
prepared as a stand-alone document. The Operations and Maintenance Manual for privately
maintained facilities shall be retained on-site and transferred with the ownership of the property
with the Stormwater Easement and Maintenance Agreement per Section 13.48.435 of the ACC.
The Operations and Maintenance Manual for public facilities shall be provided to the City and
retained within the appropriate department. Guidance for developing the Operations and
Maintenance Manual is available in multiple locations within the SWMMWW. Important sections
of the SWMMWW include:
Appendix V-A contains maintenance standards for drainage facilities
Volumes II, IV, and V contain sections on maintenance in the description of each BMP
Volume IV contains source control BMPs that can be included in the operations and
maintenance manual where applicable
III-3.2 contains an overview of operations and maintenance manual requirements
Additionally, the Stormwater Site Plan Submittal Requirements Checklist in Appendix I-I of the
COA Supplemental Manual provides key components on completing the Operations and
Maintenance Manual for submittal.
I-3.4.10 MR10: Off-Site Analysis and Mitigation
The City of Auburn requires Off-Site Analysis and Mitigation in addition to Minimum
Requirements #1-5 and #1-9. Minimum Requirement #10 is required for all projects meeting the
project thresholds and criteria for Minimum Requirements #1-5 and #1-#9.
Development projects that discharge stormwater off-site shall submit as part of their SSP 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.
The City may require a quantitative analysis for any project deemed to need additional downstream
information. A quantitative analysis may be required where there are known downstream flow
constraints or there are known or suspected drainage issues downstream from the proposed
development.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
13
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 on-site and off-site 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 on-site drainage facilities. Details on how to perform this analysis
and submittal requirements are located in Appendix I-I.3 of the COA Supplemental Manual.
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 Appendix III-D of the COA
Supplemental Manual.
Objective
To identify and evaluate off-site 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. Aggravating 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. Off-site
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.
I-3.5 Additional Protective Measures (APM) (Optional)
I-3.5.2 APM1: Financial Liability
The following information regarding Financial Liability and bonding shall supersede the APM 1
in I-3.5.2 of the SWMMWW:
Per Auburn City Code, performance and maintenance bonding is required for public stormwater
facilities within the City of Auburn.
I-3.5.3 APM2: Off Site Analysis Report
The off-site analysis and mitigation requirements described in I-3.4.10 of the COA Supplemental
Manual shall supersede the APM 2 in I-3.5.3 of the SWMMWW. Minimum Requirement #10 –
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
14
Off Site Analysis and Mitigation is required in the City of Auburn for projects that meet the
thresholds for Minimum Requirements #1-#5 and #1-#9.
I-3.6 Adjustments and Exceptions/Variances to the MRs
Exceptions from the Minimum Requirements
Deviations from the Minimum Requirements may be requested in writing, in accordance with
Chapter 13.48 of the ACC.
Exceptions from Engineering Design Standards
Deviations from any City of Auburn engineering design standards requires approval by the City
Engineer through the General Deviation request process outlined in Chapter 1 of the
Engineering Design Standards.
All Deviation Requests
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 report or analysis by a civil
engineer licensed in Washington State, 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.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
15
Appendix I-F Glossary and Notations
The following terms and acronyms are provided as reference for use in the SWMMWW and
COA Supplemental Manual in addition to those available in the SWMMWW.
AC
Asphalt Concrete – pavement consisting of a composite mixture of aggregate and asphalt.
ACC
Auburn City Code – the codification of ordinances for the City of Auburn, Washington.
ADT
Average Daily Traffic (ADT) means the volume of traffic passing a point or segment of a
highway, in both directions, during a period of time, divided by the number of days in the period
and factored to represent an estimate of traffic volume for an average day of the year.
COA
City of Auburn, Washington.
City of Auburn Design Standards
The engineering design standards and details developed and adopted by the City of Auburn,
Washington Public Works Department.
Groundwater Protection Zone
Land areas designated by the City of Auburn, Washington beneath which groundwater occurs
that is a current or potential future source of drinking water for the City.
SSP
Stormwater Site Plan – a comprehensive report containing all of the technical information and
analysis necessary for the City of Auburn to evaluate a proposed new development or
redevelopment for compliance with the stormwater requirements in the SWMMWW and COA
Supplemental Manual.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
16
Appendix I-G City of Auburn Areas with Special Development
Requirements
This Appendix identifies geographic areas within the City of Auburn and the requirements
specific to those areas. These requirements shall be in addition to the Minimum Requirements
found in Chapter 3 of this volume of the SWMMWW and the COA Supplemental Manual unless
the text in this chapter specifically indicates that the area-specific requirement supersedes or
replaces a Minimum Requirement.
Groundwater Protection Areas 1 and 2
In 2005, the City of Auburn adopted the Critical Areas Ordinance, Chapter 16.10 of the ACC,
which formally designates Groundwater Protection Areas (Zones) within the City of Auburn.
Groundwater Protection Zones 1 and 2 represent 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 Zones 1 and 2 that receive
stormwater from any pollution-generating surfaces (PGS) including streets, parking areas, or
galvanized roofs are prohibited unless approved by the City Engineer or his/her designee per
the General Deviation requirements in Chapter 1 of the Engineering Design Standards. To
receive approval, a PGS must not be classified as a “high-use site” per the Glossary of the
SWMMWW and runoff from the PGS must undergo Enhanced Treatment prior to discharge into
the infiltration facility per Volume V of the SWMMWW. In such case a private disposal system
may be approved.
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 are 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 water bodies, additional treatment or flow
control requirements may apply.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
17
Floodplains
Floodplains are not regulated through the SWMMWW and COA Supplemental Manual.
However, stormwater facilities proposed within floodplains will be reviewed on a case-by-case
basis to determine if the facilities are acceptable. Additional analysis and requirements may be
needed for stormwater facilities located within floodplains. Refer to ACC 15.68 for additional
guidance.
Airport Operations
Open water in stormwater facilities may attract birds and other wildlife, which can pose a hazard
for aircraft.
The following requirements apply to any parcel located within Airport Zones 1, 2, and 5 for the
City of Auburn Municipal Airport, as defined in Chapter 18.38 of the Auburn City Code. See
www.auburnwa.gov/maps for Zoning Map and for Airport Overlay Map. Any stormwater facilities
that are situated within the boundaries of these zones must use underground vaults,
underground detention/infiltration facilities, or LID facilities that drain completely within 48 hours.
Open stormwater ponds are not allowed.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
18
Appendix I-H On – Site Stormwater Management BMP List Option
Infeasibility Criteria
This Appendix summarizes infeasibility criteria provided in the SWMMWW for evaluating BMPs
when applying the List Option to meet Minimum Requirement #5 – On-Site Stormwater
Management. Projects that chose to meet the LID Performance Standard may use any BMP or
facility in the SWMMWW to achieve that standard except Rain Gardens. Projects that chose the
List #1 or #2 option to comply with Minimum Requirement #5 must select On-Site Stormwater
Management BMPs from the applicable list and consider them in the order given. If a BMP is
found to be infeasible based on the criteria presented below or in the SWMMWW, consider the
next BMP on the applicable list. Refer to Section I-3.4.5 of the SWMMWW and COA
Supplemental Manual to determine which On-Site Stormwater Management BMPs require
evaluation for a project. Evaluation is based on project type, discharge location, and other
criteria.
Prior to evaluating On-Site Stormwater Management BMPs, review the site design
considerations in III-3 of the SWMMWW to conserve natural areas, retain native vegetation,
reduce impervious surfaces, and integrate stormwater controls into the existing site layout to the
extent feasible. Also consider the design criteria for individual BMPs as given in the SWMMWW
and amended in the COA Supplemental Manual.
The Infeasibility Criteria summarized below apply to BMPs when selecting the List Option if the
area proposed for the BMP is the only available area for the BMP, after all reasonable efforts to
regrade the site and allow for alternative placement of the BMP have been made.
When using the On-Site Stormwater Management List Option, an On-Site BMP is considered
infeasible if any one infeasibility criteria for that BMP is met. Citation of infeasibility when using
the List Option is a component of the SSP report. Refer to III-3 of the SWMMWW and Appendix
I-H of the COA Supplemental Manual for more information on the SSP report and documenting
infeasibility criteria.
Through historical data and existing topography, the City of Auburn has defined specific areas
where On – Site Stormwater Management BMPs that require infiltration are restricted or limited,
illustrated in Figure I-H-1 Map of Infiltration Restriction Areas below. Projects located in
Groundwater Protection Zones 1 and 2 require Enhanced Treatment and approval from the City
Engineer to use infiltrative BMPs for managing stormwater from pollution generating surfaces.
Other areas are identified as infeasible for infiltrative List Option On-Site Stormwater
Management BMPs based on slope proximity, typical groundwater elevations, proximity to
critical areas, and other site-specific criteria. Contact the Public Works Department at (253)-931-
3010 for more information and assistance in identifying areas where List Option On-Site
Stormwater Management Infiltration BMPs are restricted.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
19
Figure I-H-1 Map of Infiltration Restriction Areas
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
20
On-Site Stormwater Management BMP Infeasibility Summary
BMP T5.13 Post-Construction Soil Quality and Depth
When the site is located on till soil slopes greater than 50%.
All Dispersion BMPs (includes T5.30, T5.10B, T5.11, T5.12)
In areas where BMP installation creates a reasonable risk of erosion, slope
failure, or flooding, as determined by a licensed engineer, geologist, or
hydrogeologist registered in the State of Washington.
On project sites where the dispersion flowpath area is within 10 feet of a
proposed or existing septic system or drainfield.
On project sites where the dispersion flowpath area is within a Critical Area
Buffer. For more information on identifying Critical Area Buffers within the City of
Auburn, contact the Community Development Department at (253) 931-3090.
On project sites where the dispersion flowpath area is on a slope greater than
20%.
The flowpath does not meet minimum horizontal setback requirements to property
lines, structures, and other flow paths.
All Infiltration BMPs (includes T5.10A, T7.30, T5.14, T5.15, T5.10C)
A licensed engineer, geologist, or hydrogeologist registered in the State of
Washington has determined that the area available for installing the infiltration
BMP:
o Would threaten the safety or reliability of pre-existing underground utilities,
pre-existing underground storage facilities, pre-existing structures, or pre-
existing road or parking lot surfaces or subgrades.
o Would threaten flood control structures such as levees.
o Is within a steep slope area or a landslide prone area or setback.
o Would create a risk of erosion, slope failure, or flooding.
The area available for installing the infiltration BMP does not meet the minimum
horizontal and/or vertical setback requirements.
Infiltration is restricted due to known contaminated soil or groundwater.
Infiltration is restricted due to Seasonal High Groundwater limitations as
determined through analysis by a licensed engineer, geologist, hydrogeologist, or
engineering geologist registered in the State of Washington (see individual BMPs
for Seasonal High Groundwater depth requirements).
T5.30 Full Dispersion
Where any of the Infeasibility Criteria listed for All Dispersion BMPs above apply.
Where the site has less than a 65 to 10 ratio of native vegetation area to
impervious area.
The minimum native vegetation flowpath is less than 100 feet.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
21
The flowpath is steeper than 15% for any 20 ft. reach of flowpath.
A copy of the conservation easement agreement that would be required by the
City in connection with this BMP can be provided upon request.
T5.10A Downspout Full Infiltration
Where any of the Infeasibility Criteria listed for All Infiltration BMPs above apply.
The Seasonal High Groundwater elevation has been determined by a licensed
professional engineer, geologist, hydrogeologist, or engineering geologist
registered in the State of Washington, and:
o There is less than 3 feet of permeable soil from the proposed final grade
to the Seasonal High Groundwater elevation.
o There is less than 1 foot of clearance from the expected bottom elevation
of the infiltration trench or dry well to the Seasonal High Groundwater
elevation.
The area available for installation of dry wells and infiltration trenches is on or
discharges to a slope greater than 25%.
T7.30 Bioretention and T5.14 Rain Gardens
Where any of the Infeasibility Criteria listed for All Infiltration BMPs above apply.
Where the City Engineer has determined that the only area available for installing
the bioretention or rain garden facility does not allow for a safe overflow pathway
to the municipal separate storm water system or private storm water system.
Where there is a lack of usable space for bioretention or rain garden facilities at
re-development sites, or where there is insufficient space within the existing
public right-of-way on public road projects.
Where bioretention or rain garden facilities are not compatible with surrounding
drainage system as determined by the City Engineer (e.g., project drains to an
existing stormwater collection system whose elevation or location precludes
connection to a properly functioning bioretention facility).
Where the site cannot be reasonably designed to locate bioretention or rain
garden facilities on slopes less than 8%.
Within 50 feet from the top of slopes that are greater than 20% and over 10 feet
of vertical relief.
Within 100 ft of a closed or active landfill.
Within 100 ft of a drinking water well, aquifer, or a spring used for drinking water
supply.
Within 10 ft of any known underground storage tank and connecting underground
pipes, when tank and pipe system capacity is 1100 gallons or less.
Within 100 ft of any known underground storage tank and connecting
underground pipes, when tank and pipe system capacity is 1100 gallons or
greater.
Where the minimum vertical separation of 1 foot to the seasonal high water table,
bedrock, or other impervious layer would not be achieved below bioretention or
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
22
rain garden facilities that would serve a drainage area that is: 1) less than 5,000
sq. ft. of pollution- generating impervious surface, and 2) less than 10,000 sq. ft.
of impervious surface; and 3) less than ¾ acres of pervious surface.
Where the minimum vertical separation of 3 feet to the seasonal high water table,
bedrock or other impervious layer would not be achieved below bioretention
facilities that: 1) would serve a drainage area that meets or exceeds: a) 5,000 sq.
ft. of pollution generating impervious surface, or b) 10,000 sq. ft. of impervious
surface, or c) 3/4 acres of pervious surfaces; and 2) cannot reasonably be broken
down into amounts smaller than indicated in (1).
Where field testing indicates soils have a measured native soil saturated
hydraulic conductivity less than 0.3 in/hr. Minimum requirements for field testing
are found in the Small-Scale Pilot Infiltration Test (PIT Test) section in Appendix
III-F of the SWMMWW.
Within geographic areas that the City of Auburn has determined that bioretention
or rain gardens are designated as infeasible due to year-round, seasonal or
periodic high groundwater conditions, or due to inadequate infiltration rates.
Contact the Public Works Department at (253) 931-3010 for more information and
assistance in identifying areas where bioretention and raingardens are
designated as infeasible. Refer to Figure I-H-1 Map of Infiltration Restriction
Areas for an indication of where infiltration and LID may be feasible.
The minimum bottom width of the infiltrating bioretention facility cannot be met
due to, but not limited to: encroachment within the critical root zone of an existing
tree(s) or minimum setback to structures, utilities, or property lines.
T5.15 Permeable Pavement
Where any of the Infeasibility Criteria listed for All Infiltration BMPs above apply.
Roads that receive more than very low traffic volumes and more than very low
truck traffic. Roads with a projected average daily traffic volume of 400 vehicles or
less are very low volume roads; very low truck traffic roads are not subject to
through truck traffic but may receive up to weekly use by utility trucks, buses, etc.
Where infiltrating and ponded water below new permeable pavement area would
compromise adjacent impervious pavements.
Down slope of steep, erosion prone areas that are likely to deliver sediment.
Where fill soils area used that can become unstable when saturated.
Excessively steep slopes (>15%) where water within aggregate base layer or at
sub-grade surface cannot be controlled by detention structures and may cause
erosion and structural failure, or where surface runoff velocities may preclude
adequate infiltration at the pavement surface.
Where permeable pavements cannot provide sufficient strength to support heavy
loads at industrial facilities.
Within 50 ft from the top of slopes that are greater than 20%.
Within 100 ft of an area known to have deep soil contamination.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
23
Within 10 ft of any known underground storage tank and connecting underground
pipes, regardless of tank size.
Within 100 ft of a closed or active landfill.
Within 100 ft of a drinking water well, aquifer, or a spring used for drinking water
supply.
The pavement is over a structure, such as, but not limited to: parking garages and
over culverts and bridges.
In areas likely to have long-term excessive sediment deposition after
construction.
Where site cannot be designed to have porous AC at less than a 5 % slope,
pervious concrete at less than a 10% slope, or permeable pavers at less than
10% slope. Use of permeable pavers is limited to walkways, private parking lots,
and private driveways.
Where seasonal high ground water or an underlying impermeable/low
permeability layer would create saturated conditions within one foot of the bottom
of the lowest gravel base course.
Where underlying soils are unsuitable for supporting traffic loads when saturated.
Soils with a California Bearing Ratio of less than 5% are considered unsuitable for
residential access roads.
Where field testing indicates soils have a measured native soil saturated
hydraulic conductivity less than 0.3 in/hr. Minimum requirements for field testing
are found in the Small-Scale Pilot Infiltration Test (PIT Test) section in Section V-
5.4 of the SWMMWW. May be built with an underdrain if flow control benefits to
meet Minimum Requirement #7 are desired, but this will not meet Minimum
Requirement #5.
Where replacing existing impervious surfaces unless the existing surface is a
non-pollution generating surface over an outwash soil with a saturated hydraulic
conductivity of 4 in/hr or more.
Where the risk of concentrated pollutant spills is more likely (gas stations, truck
stops, industrial chemical storage sites, etc.).
Where routine, heavy applications of sand occur in frequent snow zones.
Within geographic areas that the City of Auburn has determined that permeable
pavements are designated as infeasible due to year-round, seasonal or periodic
high groundwater conditions, or due to inadequate infiltration rates
Where the site is considered “high use” due to one of the following criteria being
met:
o The site is a commercial or industrial site subject to an expected ADT
count equal to or greater than 100 vehicles per 1,000 SF of gross building
area.
o The site is a commercial or industrial site subject to petroleum storage and
transfer more than 1,500 gal/yr, not including routinely delivered heating
oil.
o The site is a commercial or industrial site subject to parking, storage, or
maintenance of 25 or more vehicles that are over 10 tons gross weight.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
24
o The site is a road intersection with a measured ADT count of 25,000
vehicles or more on the main roadway and 15,000 vehicles or more on
any intersecting roadway, excluding projects proposing primarily
pedestrian or bicycle use improvements.
T5.10B Downspout Dispersion Systems
Where any of the Infeasibility Criteria listed for All Dispersion BMPs above apply.
The vegetated flow path is less than 25 feet between the trench outlet and any
property line, structure, stream, wetland, or impervious surface.
The vegetated flow path is less than 50 feet between the trench outlet and any
slope steeper than 15%.
There is insufficient space to accommodate the dispersion trench or trenches
required for the surface area draining to it. The minimum trench length is 10 feet
per 700 SF of surface area.
The dispersion area is flat and would result in ground saturation.
T5.11 Concentrated Flow Dispersion
Where any of the Infeasibility Criteria listed for All Dispersion BMPs above apply.
There are no concentrated flows to disperse.
The vegetated flow path is less than 50 feet between the trench outlet and any
property line, structure, stream, wetland, or impervious surface.
The surface area draining to each concentrated flow area is greater than 700 SF.
The vegetated flow path is less than 50 feet between the trench outlet and any
slope steeper than 15%.
T5.12 Sheet Flow Dispersion
Where any of the Infeasibility Criteria listed for All Dispersion BMPs above apply.
The area to be dispersed has a slope greater than 15%.
The minimum vegetated flow path for sheet flow dispersion cannot be met. Note:
A 10-foot flow is required to disperse runoff from a contributing flow length of up
to 20 feet. An additional 10 feet of flow path is required for each additional 20 feet
of contributing flow path or fraction thereof.
T5.10C Perforated Stub-Out Connections
The location for the perforated pipe portion of the system is under impervious or
heavily compacted surfaces.
The minimum perforated stub-out length of 10 feet per 5,000 SF of contributing
roof area cannot be met.
Where the site cannot be reasonably designed to locate a catch basin between
the perforated stub-out and point of connection to the public storm drainage
system.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
25
Appendix I-I Stormwater Site Plan (SSP) Report Submittal
Requirements Checklist
The Submittal Requirements Checklist is intended to aid the design engineer in preparing a SSP
report. All items included in the following checklist must be addressed as part of any SSP report. The
City requires the design engineer follow the order and structure of the checklist to facilitate review,
which in turn will expedite permit issuance.
Title Page and Formatting
Provide a title page with applicant, permit number, engineer, and property owner information including
name, address, telephone number and email address for all parties.
Provide date of document preparation on the title page.
Include a Table of Contents.
Include List of Figures.
Number all pages, figures, maps and attachments.
Chapter 1 – Project Overview
The project overview is intended to be a summary of detailed information describing the project.
Information that is provided in detail in following chapters should be summarized here.
Identify type of City of Auburn permit requested and permit number
Identify all permits and approvals required by other regulatory agencies.
Identify the project location, including address, legal description, and parcel number.
Provide a brief narrative description of the project, including a summary of site constraints.
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. Analyze this data to determine site
constraints, including but not limited to:
Areas with high potential for erosion and sediment deposition based on soil properties, slope,
etc.; and
Locations of sensitive and critical areas; and
Areas of existing Native Vegetation; and
Points where existing surface water enters and exits the project site.
Delineate these areas on the vicinity map and/or site map and provide a narrative summary outlining
key details. Include all reports and studies used to develop this summary in Attachment D of the SSP
report.
Describe, discuss and identify the following for the project site:
Topography.
Land use and ground cover.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
26
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. Contact the City of Auburn
Storm Utility at 253-931-3010 for more information on historical drainage problems.
Existing on-site and adjacent utilities, including stormwater facilities, water, and sewer.
Areas with high potential for erosion and sediment deposition.
Locations of sensitive and critical areas (e.g., vegetative buffers, wetlands, steep slopes,
floodplains, geologic hazard areas, streams, creeks, ponds, ravines, springs).
Existing fuel tanks.
Groundwater wells on-site and within 100 feet of site.
Septic systems on-site and/or within 100 feet of the site.
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. For
information on the City’s wellhead protection areas (Groundwater Protection Zones 1-4), contact the
City of Auburn Water Utility at 253-931-3010.
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, or water quality reports. Where such reports impose additional conditions on the
Proponent, state these conditions, and describe any proposed mitigation measures.
A summary of a geotechnical report prepared and stamped by a geotechnical engineer licensed in
Washington State.
Describe the 100-year flood hazard zone, if applicable.
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. If a quantitative analysis is required, detailed calculations shall be contained
in Attachment C of the SSP report. The COA Supplemental Manual I-3.4.10 describes the Off-site
Analysis. In addition, a list of elements to be included is provided below.
Qualitative Analysis
Investigate the drainage system ¼ mile downstream from the project by site visit and plan review, and
discuss the following items:
Problems reported or observed during the plan review and site visit.
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 downstream of the project site and their corresponding
widths.
Qualitative data on features such as land use, impervious surface, topography, soils, presence of
streams, and wetlands.
Information on pipe sizes, channel characteristics and drainage structures.
Verification of tributary drainage areas.
Date and weather at the time of the site visit.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
27
Describe the drainage system and its existing and predicted problems through observations and
reports.
A map of each basin showing the drainage system and all existing and predicted problems.
Site visit photographs illustrating the existing system and conditions.
Complete the following table for each basin/subbasin:
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
28
Off Site Analysis Drainage System Table
Basin:Subbasin
Name:
Subbasin
Number:
Date:
Symbol Drainage
Component Type,
Size, and Name
Description Slope Distance
from Site
Discharge
Existing
Problems
Potential
Problems
Observations
See Map Type: sheet flow, swale,
channel, pipe, pond,
flow control/water
quality BMP
Size: diameter, surface
area
Drainage basin,
vegetation, cover,
depth, type of sensitive
area, volume
%¼ mile = 1,320
ft.
Constrictions, under capacity,
ponding, overtopping, flooding,
habitat or organism destruction,
scouring, bank sloughing,
sedimentation, incision, other
erosion
Tributary area,
likelihood of
problem, overflow
pathways, potential
impacts
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
29
Quantitative Analysis (see Appendix III-D of the COA Supplemental Manual)
Clearly describe tail water assumptions.
Summarize the hydraulic modeling of the City-specified design storm event (Appendix III-E) as
described in Chapter III-2 of the COA Supplemental Manual.
Include downstream system capacity calculations in Attachment E of the SSP report.
Discuss potential fixes for capacity problems.
Include off-site areas in drainage calculations.
Provide storm system pipe 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 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 the Permanent Stormwater Control Plan.
Refer to Volume V of the SWMMWW for on-site stormwater management BMP selection and
feasibility analysis related to Minimum Requirement #5 – On-Site Stormwater Management.
Refer to Volume V of the SWMMWW for treatment facility selection related to Minimum Requirement
#6 – Runoff Treatment.
Refer to Volume III of the SWMMWW and COA Supplemental Manual for flow control facility design
related to Minimum Requirement #7 – Flow Control.
The City requires that model files to be provided electronically. Hydrologic modeling must conform to
the City of Auburn requirements presented in III-2 of the COA Supplemental Manual.
The Permanent Stormwater Control Plan shall contain the following sections:
Threshold Discharge Areas
Complete all applicable portions of the following table for each threshold discharge area:
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
30
Threshold Discharge Area (TDA) Table
Basin:TDA
Name:
TDA
Number:
Date:
Descriptiona On-site Off-site Total
Existing Conditions
Total Project Areab (ft2)
Existing hard surface (ft2)
Existing Native Vegetation area (ft2)
Existing lawn/landscaped area (ft2)
Proposed Conditions
Total Project Areab (ft2)
Amount of new hard surface (ft2)
Amount of new pollution generating hard surface (PGHS)c
(ft2)
Amount of replaced hard surface (ft2)
Amount of replaced PGHSd (ft2)
Amount of new + replaced hard surface (ft2)
Amount of new + replaced PGHS (ft2)
Amount of existing hard surfaces converted to vegetation
(ft2)
Amount of Land Disturbed (ft2)
Amount of native vegetation converted to lawn/landscaped
areas (ft2)
Amount of native vegetation converted to pasture (ft2)
Existing native vegetation area to remain (ft2)
Existing hard surface to remain unaltered (ft2)
Value of proposed improvements ($)
Assessed value of existing site improvements ($)
Amount to be graded/filled (Cubic Feet)
a. All terms are defined in the Glossary found in the SWMMWW
b. The total project area in the existing conditions should typically match the total project area
in the proposed condition
c. The “amount of new PGHS” should be part of or all of “amount of new hard surfaces”
d. The “amount of replaced PGHS” should be part of or all of “amount of replaced hard
surfaces”
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
31
Pre-Developed Site Hydrology
This section describes the pre-development characteristics of the site. This includes the
acreage, soil types, and land covers used to determine the pre-development flow
characteristics.
When developing the hydrologic model in Western Washing Hydrology Model (WWHM), the
pre-developed condition shall be a forested land cover. This applies unless reasonable, historic
information is provided that indicates the site was prairie prior to settlement.
Provide justification for land uses other than forest.
Summarize Pre-Developed Site Hydrology model input. Include complete WWHM reports in
Attachment C.
Developed Site Hydrology
Project Summary
Provide a list of assumptions and site parameters for the developed condition.
Describe Low Impact Development (LID) principles used in developing the site, and steps taken to
preserve native vegetation, minimize impervious areas, and reduce stormwater runoff.
Identify all developed sub-basins within, or flowing through, the site. Use consistent labeling for all sub-
basins throughout figures, calculations, and text.
Summarize WWHM input/output data from the developed condition.
Include the WWHM report “Formatted Report With Charts” in Attachment C. Include the WWHM reports
“LID Duration Standard” and “Wetland Fluctuation” in Attachment C, where applicable.
State conclusions from decision and flow charts (Figure I-3.1, Figure I-3.2 from Volume I of the
SWMMWW, and Figure I-3.3A from Volume I of the COA Supplemental Manual). Include the flow
charts in Chapter 5 of the SSP report.
On-Site Stormwater Management System – Minimum Requirement #5
Include total area of Native Vegetation retained.
Indicate if using the LID Performance Standard or List Option to comply with Minimum
Requirement #5 – On-Site Stormwater Management.
If using the LID Performance Standard to meet Minimum Requirement #5, summarize model
results showing that the Performance Standard has been met. Provide complete WWHM reports
in Attachment C.
If using the LID List Option, describe the On-Site Stormwater Management BMPs that are
infeasible and provide documentation of the infeasibility criteria that apply. Summarize associated
geotechnical reports and other documentation of feasibility/infeasibility. Refer to Appendix I-H of
the COA Supplemental Manual.
Provide drawings of the on-site stormwater management facilities and appurtenances, including:
Dimensions.
Location of inflow, outflow, and overflow, where applicable.
Location of each facility on the project site.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
32
Pipe material and diameter.
Other design details, including infiltration rates, Bioretention Soil Media and/or permeable
pavement specifications, plant lists, subgrade compaction, and base materials.
Include a summary of proposed public or private ownership and maintenance responsibilities of
On-Site Stormwater Management BMPs and any areas serving a stormwater function.
Provided areas of disturbed soils to be amended per BMP T5.13 Post-Construction Soil Quality
and Depth.
Water Quality System – Minimum Requirement #6 (where required)
Identify the sizing method used.
Where appropriate summarize model results related to Minimum Requirement #6 – Runoff Treatment.
Provide complete WWHM reports in Attachment C.
Calculations for the water quality design storm and facility sizing calculations must be included in
Attachment C of the SSP report.
Identify treatment methods used, including size, type, and characteristics of treatment facility and
appurtenances.
Identify proposed distributed bioretention facilities and/or infiltration below pollution-generating hard
surfaces used to meet water quality requirements.
Provide drawings of the treatment facilities, including:
Dimensions.
Inlet/outlet sizes and elevations.
Location of the facilities on the project site.
Appurtenances/fittings.
Where appropriate, include manufacturer’s specifications as an additional Attachment of the SSP
report.
Proposed proprietary treatment devices that have not received GULD approval through Ecology’s
TAPE process must be accompanied with documentation showing how the device meets the runoff
treatment requirements of Minimum Requirement #6.
Flow Control System – Minimum Requirement #7 (where required)
Identify sizing system used.
Describe proposed flow control system and appurtenances, including size, type, and characteristics of
storage facility and control structure.
Identify proposed distributed bioretention facilities, storage below permeable pavement, and any other on-
site stormwater management facilities used to meet flow control requirements.
Provide drawings of the flow control facilities, including:
Dimensions.
Inlet/outlet sizes and elevations.
Location of the facilities on the project site.
Appurtenances/fittings.
Summarize model results showing that Minimum Requirement #7 – Flow Control has been met. Provide
complete WWHM reports in Attachment C of the SSP report.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
33
Conventional Conveyance System Analysis and Design
Provide complete pipe system plan and profile drawings. See Required Drawings section below.
Identify pipe sizes, types, and slopes.
Describe capacities, design flows, and velocities for each reach.
Include conveyance calculations in Attachment C of the SSP 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 and documented in the SSP report.
Include applicable flowcharts for determining Minimum Requirements (Figure I-3.1, Figure I-3.2 from the
SWMMWW, and Figure I-3.3A from the COA Supplemental Manual) with decision paths clearly
marked.
List the Minimum Requirements that apply to the project.
Briefly discuss how the project satisfies each Minimum Requirement.
Indicate where in the SSP report documentation can be found that shows how the Minimum
Requirements are satisfied.
SSP Report Attachments
Attachments A and B are components of the SSP report that are also intended as stand-alone
documents for the project during and after construction. This section also contains all
background data, calculations, reports, and other documents that support the information
summarized in the SSP report. Additional Attachments should be labeled clearly and included in
the SSP report Table of Contents.
SSP Attachment A – Operations and Maintenance Manual, when required to satisfy Minimum
Requirement #9. Note: The Operations and Maintenance Manual shall be written as a stand-alone
document for the project owner when the project is complete.
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/on-site
stormwater management/source control/etc. facilities prominently noted. This is needed to
enable the Operations 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 Operations and Maintenance Manual is to be kept. Note that it must be made available to
the City for inspection.
A description of each stormwater 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 stormwater facility.
Include all applicable Maintenance Standards for Drainage Facilities per Appendix V-A of the
SWMMWW, and any manufacturer’s recommendations.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
34
A list and description of all source control BMPs per Volume IV of the SWMMWW that are applied to
the site.
A sample maintenance activity log indicating emergency and routine actions to be taken.
SSP Attachment B – Construction Stormwater Pollution Prevention Plan (SWPPP)
SWPPP Long Form – Use the most current version of the Department of Ecology’s SWPPP
Template, available at:
Construction SWPPP
SWPPP Short Form – See COA Supplemental Manual Vol II, Appendix II-B.
SSP Attachment C – Hydraulic /Hydrologic Analysis and Modeling Results.
WWHM “Formatted Report With Charts” report shall be included.
WWHM “LID Duration Standard” report shall be included when applicable.
WWHM “Wetland Fluctuation” report shall be included when applicable.
Include all other water quality, oil/water separator, GULD approval, conveyance, and hydraulic/
hydrologic calculations and information related to the project.
Basin Maps used for all hydraulic/hydrologic analysis, modeling results, and Off-Site Analyses (See
Chapter 3 above).
SSP Attachment D – Other Special Reports – See Engineering Design Standards Chapter 4 for report
preparation requirements.
Geotechnical Soils Report.
Critical Area Report.
Other reports related to historical site conditions, proposed project impacts, Low Impact
Development feasibility analysis, and stormwater management.
SSP Attachment E – Declaration of Covenant for Privately Maintained Facilities (Final SSP report
following completion and acceptance of construction only).
Required Drawings
Include the following project drawings:
Vicinity Map
Existing Conditions Summary Site Map
Site Map and Grading Plan
Basin Map
Site Plan showing all stormwater facilities, including on-site stormwater management facilities, treatment
facilities, flow control facilities, conveyance systems, and mitigated impervious surfaces
Storm System Plan and Profile
Erosion Control Plan
Detail Sheets
An 8.5”x11” exhibit with minimum 0.1” height text showing the site plan, storm system and labels for
major storm system elements (detention facility, control structure, water quality treatment facility, etc.).
This exhibit is required for Stormwater Easement and Maintenance Agreements.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
35
Appendix I-J Stormwater Facility Access Requirements
All publicly owned, manmade stormwater 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.
Maintenance Access Easements
A minimum 15-foot-wide maintenance access easement shall be provided to drainage facilities
from a public street or right-of-way.
Maintenance access must be provided for all manholes, catch basins, vaults, detention ponds,
or underground drainage facilities operated by the City. Maintenance access shall be through a
public easement. Driving access to all parts of detention ponds for maintenance of structures
and for mowing shall be provided, including locking vehicle gates if access is restricted by
fencing. Such access shall not be blocked by vegetation, fencing or standing water. Access to
the site shall be limited by a double-posted gate with a minimum width of 15 feet, or other
approved equal.
Access roads shall be constructed in accordance with City of Auburn Design Standards,
Chapter 6.
Public Stormwater Facility Easements
A stormwater easement is required for the placement, operation and maintenance of facilities
upon private property.
Public stormwater easements shall meet the requirements of City of Auburn Design Standards,
Chapter 6.
Detention Ponds
The following access shall be provided for public detention ponds:
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). See
access road criteria above.
An access ramp meeting the gravel access road standards 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 above.
The internal berm of a wetpond, or combined detention and wetpond, may NOT be
used for access.
City of Auburn Supplemental Manual to the 2019 Ecology Stormwater Management Manual for
Western Washington
Volume I – What Requirements Apply to My Site? 2022 Version 1
36
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. See access
road criteria above.
Additional easements or modification to proposed lot boundaries, fencing, and/or
gates 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 easement or extension of the tract to an
acceptable access location.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
37
Volume II
Construction Stormwater Pollution
Prevention
City of Auburn Supplemental Manual
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
38
Executive Summary of Volume II – Construction Stormwater Pollution
Prevention
Volume II of the City of Auburn (COA) Supplemental Manual to the Department of Ecology’s
(Ecology) Stormwater Management Manual for Western Washington (SWMMWW) provides
additional guidance for implementing Minimum Requirement #2 – Construction Stormwater
Pollution Prevention within the City of Auburn. The COA Supplemental Manual Volume II is
organized to correspond to the SWMMWW Volume II. This Volume should be used in
conjunction with the SWMMWW to prepare and implement Construction Stormwater Pollution
Prevention Plans (SWPPP). The Best Management Practices (BMPs) presented in both
documents are required for construction projects located within the City.
The Ecology SWMMWW is available online at the link below:
2019 SWMMWW
Refer to the Glossary in the SWMMWW for definitions of terms used throughout this manual.
Additional definitions of terms not defined in the SWMMWW and acronyms are included in
Appendix I-F of the COA Supplemental Manual.
Important new BMPs (indicated as “Additional”) and changes to the SWMMWW BMPs
(indicated as “Amended”) contained in the COA Supplemental Manual for this Volume include:
II-2: Construction Stormwater Pollution Prevention Plans (Construction SWPPPs)
o II-2.2 provides the thresholds for projects requiring a Construction SWPPP for
the City of Auburn.
o II-2.4 provides the Construction SWPPP submittal requirements for the City of
Auburn.
II-3: Construction Stormwater BMPs
o BMP C163: Demolition of Buildings (Additional)
o BMP C164: Building, Repair, Remodeling and Construction (Additional)
o BMP C233, C235, C250 (Amended)
Supplemental Appendix II-B: Construction Stormwater Pollution Prevention Plan
Short Form provides a short form Construction SWPPP for use on projects that meet
the criteria described in that appendix.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
40
Sections with No Changes
The following portions of Volume II of the SWMMWW do not require any additional clarification
in the COA Supplemental Manual, and together with the COA Supplemental Manual, contain
the requirements and guidelines for stormwater management in the City of Auburn.
II-1: Soil Erosion and Sedimentation from Construction
o II-1.1 – II-1.3 (all subsections)
II-2: Construction Stormwater Pollution Prevention Plans (Construction
SWPPPs)
o II-2.1 and II-2.3 (all subsections)
II-3: Construction Stormwater BMPs
o II-3.1 (all subsections)
o II-3.2 Construction Source Control BMPs (all subsections)
o II-3.3 Construction Runoff BMPs
BMP C200-209, C220, C231-C232, C234, C236, C240-C241, C251-
C252 (all subsections)
Appendix II-A
II-2 –Construction Stormwater Pollution Prevention Plans
(Construction SWPPPs)
II-2.2 When is a Construction SWPPP Required?
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-#13. The Construction SWPPP must include
an explanation for any elements that are not used.
Unless located in a critical area, a SWPPP is not required for projects that:
Add or replace less than 2000 square feet of hard surface, or
Disturb less than 7000 square feet of land, or
Grade/fill less than 500 cubic yards of material.
II-2.4 Preparing Construction SWPPPs
The Construction SWPPP Short Form may be used for projects that meet the thresholds found
in Appendix II-B of the COA Supplemental Manual.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
41
Unless use of the SWPPP Short Form is approved by the City Engineer or his/her designee, a
complete SWPPP is required for projects that:
Add or replace 5000 square feet or greater of hard surface, or,
Disturb greater than 1 acre, or
Grade/Fill greater than 500 cubic yards of material, or
Disturb critical areas or buffers.
Projects that are required to submit a complete Construction SWPPP must use the most current
version of the Department of Ecology’s Construction SWPPP Template, available at:
Construction SWPPP
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.
II-3 – Construction Stormwater BMPs
Changes to BMPs in the SWMMWW and additional BMPs required in the City are contained in
this section.
BMP C163: Demolition of Buildings (Additional)
Description of Pollutant Sources
This applies to removal of existing buildings by controlled explosions, wrecking balls, or manual
methods, and subsequent clearing of the rubble. The loose debris can contaminate stormwater.
Pollutants of concern include toxic organic compounds, heavy metals, asbestos, and suspended
solids.
Pollutant Control Approach
Regularly clean up debris that can contaminate stormwater. Protect the storm drainage system
from dirty runoff and loose particles. Sweep paved surfaces daily. Vacuum sweeping is
preferred.
The following BMPs are required of all businesses and public agencies engaged in building
demolition:
Identify, obtain required City permits, and properly abandon all utility connections such
as sanitary sewer, water, gas, fuel lines and tanks.
If directed to keep water out of the storm system during demolition activity, storm drain
covers or a similarly effective containment device must be placed on all nearby drains to
prevent dirty runoff and loose particles from entering the storm drainage system. If storm
drains are not present, dikes, berms, or other methods must be used to protect overland
discharge paths from runoff.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
42
Utilize storm drain inlet protection BMP C220: Inlet Protection in Volume II of the
SWMMWW.
Street gutters, sidewalks, driveways, and other paved surfaces in the immediate area of
the demolition must be swept at the end of each work day to collect and properly
dispose of loose debris and garbage.
Use dust control methods BMP C140: Dust Control in Volume II of the SWMMWW
and/or BMP S407: BMPs for Dust Control at Disturbed Land Areas and Unpaved
Roads and Parking Lots in Volume IV of the SWMMWW.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but can provide additional pollution protection:
If possible, a wall should be constructed to prevent stray building materials and dust
from escaping the area during demolition.
Schedule demolition to take place at a dry time of the year.
BMP C164: Building, Repair, Remodeling and Construction (Additional)
Description of Pollutant Sources
This activity refers to activities associated with construction of buildings and other structures,
remodeling of existing buildings and houses, and general exterior building repair work. Washing
of buildings is covered under S431: BMPs for Washing and Steam Cleaning
Vehicles/Equipment/Building Structures in Volume IV of the SWMMWW. Painting of
buildings is covered under S420: BMPs for Painting, Finishing, and Coating of Vehicles,
Boats, Buildings, and Equipment in Volume IV of the SWMMWW.
Pollutants of concern include toxic organics, suspended solids, heavy metals, asbestos, pH,
oils, and greases.
Pollutant Control Approach
Employees must be educated about the need to control site activities. Control leaks, spills, and
loose material. Utilize good housekeeping practices.
The following BMPs are required of all businesses engaged in building repair, remodeling, and
construction:
Employees must be educated about the need to control site activities to prevent
stormwater pollution, and also must be trained in spill cleanup procedures.
Spill cleanup materials, appropriate to the chemicals being used on site, must be
available at the work site at all times.
The work site must be cleaned up at the end of each work day, with materials
such as solvents put away indoors or covered and secured so that vandals will
not have access to them.
The area must be swept daily to collect loose litter, paint chips, grit, and dirt.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
43
Absolutely no substance can be dumped on pavement, the ground, or in or
toward storm drains, regardless of its content, unless it is only uncontaminated
water.
Bermed ground or drop cloths must be used underneath scraping and
sandblasting work. Ground cloths, buckets, or tubs must also be used anywhere
that work materials are laid down.
BMP C154: Concrete Washout Area in Volume II of the SWMMWW shall be
applied when performing interior or exterior concrete work.
Tools covered with non-water-based finishes or other materials must be cleaned
in a manner that enables collection of used solvents for recycling or proper
disposal. See BMP S445: BMPs for Discharging Process Wastewater to a
Sanitary Sewer, Holding Tank, or Water Treatment System in Volume IV of
the COA Supplemental Manual for disposal options.
Inlet protection as described in BMP C220: Inlet Protection in Volume II of the
SWMMWW must be used if dust, grit, wash water, or other pollutants may
escape the work area. This is particularly necessary on rainy days. Provide inlet
protection over the storm drain at the beginning of the work day. Do not perform
outdoor work during wet weather if contaminants could be washed off-site by
rainfall.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but can provide additional pollution protection:
Recycle materials whenever possible.
Activities such as tool cleaning should occur over a ground cloth or within a
containment device such as a tub.
II-3.3 Construction Runoff BMPs
BMP C233: Silt Fence (Amended)
Additional Requirements for the City of Auburn
All silt fence applications shall conform to the City of Auburn Standard Detail E-02.
BMP C235: Wattles (Amended)
Additional Requirements for the City of Auburn
The City requires removal of non-biodegradable wattles and wattle netting at project completion.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
44
BMP C250: Construction Stormwater Chemical Treatment (Amended)
Additional Requirements for the City of Auburn
Conditions of Use
Formal written approval from the Department of Ecology and the City of Auburn is required for
the use of chemical treatment regardless of size. When approved, include the chemical
treatment system in the Construction SWPPP. Discharges to the wastewater system must also
comply with King County wastewater discharge requirements and permits. Refer to the King
County Wastewater Services webpage for more information.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume II - Construction Stormwater Pollution Prevention 2022 Version 1
45
Appendix II-B Construction Stormwater Pollution Prevention Plan
Short Form
Projects falling within the thresholds listed below may use this short form instead of preparing a
professionally-designed Construction 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 Construction SWPPP Short Form may be used for projects meeting one of the following
thresholds:
Add or replace between 2,000 and 5,000 square feet of hard 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 any of these thresholds, prepare a formal Construction SWPPP as
described in Chapter 3 of this volume.
The SWPPP Short Form may also be used, with approval by the City Engineer or his/her
designee, for projects that:
Add or replace between 5000 square feet and 10,000 square feet of hard surface, or,
Disturb critical areas or buffers.
The Construction SWPPP Short form is available as a separate download at
https://www.auburnwa.gov/Forms
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
46
Volume III
Choosing, Modeling, and Documenting
Your BMPs
City of Auburn Supplemental Manual
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
47
Executive Summary of Volume III – Choosing, Modeling, and
Documenting Your BMPs
Volume III of the City of Auburn (COA) Supplemental Manual to the Department of Ecology’s
(Ecology) Stormwater Management Manual for Western Washington (SWMMWW) provides
additional guidance for performing hydrologic and hydraulic analyses and designing flow control
facilities to meet Minimum Requirement #7 – Flow Control.
The Ecology SWMMWW is available online at the link below:
2019 SWMMWW
COA Supplemental Manual Volume III is organized to correspond to the SWMMWW Volume III.
This Volume should be used in conjunction with the SWMMWW to meet the hydrologic
modeling requirements of the City and design flow control facilities for installation within the City
of Auburn.
Refer to the Glossary in the SWMMWW for definitions of terms used throughout this manual.
Additional definitions of terms not defined in the SWMMWW and acronyms are included in
Appendix I-F, Volume I of the COA Supplemental Manual.
Important additions and changes contained in the COA Supplemental Manual for this Volume
include:
III-2: Modeling Your BMPs contains several sections to assist with meeting the
hydrologic analysis requirements for project submittal to the City, including:
o III-2.2 Continuous Simulation Models provides specific hydrologic modeling
requirements for project submittal.
o III-2.3 Single Event Hydrograph Method contains the Water Quality Design
Storm for Single Event Hydrograph Method modeling to be used for the design of
piped conveyance systems within the City.
o II-2.5 Closed Depression Analysis contains important information on Closed
Depression Analysis in the City.
III-3: Stormwater Site Plans (SSPs)
o III-3.2 Preparing a Stormwater Site Plan describes the submittal requirements
for SSPs submitted to the City.
Appendix III-D contains detailed information on hydraulic analysis and the design of
traditional storm conveyance systems.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
48
Sections with No Changes
The following portions of Volume III of the SWMMWW do not require any additional clarification
in the COA Supplemental Manual, and together with the COA Supplemental Manual, contain
the requirements and guidelines for stormwater management in the City of Auburn.
III-1: Introduction
o III-1.1 – III-1.4
III-2: Hydrologic Analysis
o III-2.1.1
o III-2.2.1 – III-2.2.3
o III-2.3.2
o III-2.4
III-3: Stormwater Site Plans
o III-3.1, III-3.3, and III-3.4
Appendix III-A through III-C
III-2 – Modeling Your BMPs
III-2.2 Continuous Simulation Models
Western Washington Hydrology Model
For flow control, treatment, and on-site stormwater management design submittal to the City, the
most current version of the Western Washington Hydrology Model (WWHM) shall be used.
Information on the WWHM is provided in the SWMMWW. 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
Hydrologic modeling submittal requirements are outlined the Stormwater Site Plan (SSP) report
checklist found in Appendix I-I of the COA Supplemental Manual.
The City does not accept MGS Flood or KCRTS (King County Runoff Time Series) modeling results
for flow control, treatment, or on-site stormwater management design submittal.
Synthetic turf field/grass shall be modeled as grass over underlying soil type (till or outwash) without
underlying perforated drain pipes, and as impervious surface with underlying perforated drain pipes.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
49
III-2.3 Single Event Hydrograph Method
Single event hydrologic modeling shall be used for the design of conveyance systems only. See
Appendix III-D of the COA Supplemental Manual for more information on the design and modeling of
conveyance systems.
Water Quality Design Storm
For sizing wetpool treatment facilities, the following design storm shall be used for the City:
6-month, 24-hour design storm: 1.44 inches
III-2.5 Closed Depression Analysis
The requirements of Chapter 16.10 of the Auburn City Code (ACC) apply to closed depression
analyses.
III-3 - Stormwater Site Plans
III-3.2 Preparing a Stormwater Site Plan
The SSP is the comprehensive report containing all the technical information and analysis
needed for review. All SSP reports must conform to the format outlined in the SSP Submittal
Requirements Checklist found in Appendix I-I of the COA Supplemental Manual. The SSP
report outline is provided in a checklist format to assist project proponents in meeting the
submittal requirements of the City. All items in the checklist must be addressed in the SSP
report.
III-3.3 Changes to a Previously Approved Stormwater Site Plan
In lieu of the process in the SWMMWW, changes or revisions to the originally-approved
Stormwater Site Plan or stormwater plans shall use one or more of the following requirements in
the submittal for City review:
1. Submit revised pages of the SSP with a footnote indicating the revision date.
2. Submit a dated memorandum or letter explaining the proposed changes to the SSP.
3. Submit revised drawings with the revision date in the revision block.
4. Submit redlined drawings indicating the proposed changes.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
50
Appendix III-D Conveyance System Design and Hydraulic Analysis
This Appendix 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 III-D.4. Outfalls are
addressed in Section III-D.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 Appendix
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.
Refer to Appendix I-J of the COA Supplemental Manual for access easement
requirements for storm conveyance systems.
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.
III-D.1 Conveyance System Analysis Requirements
Additional Requirements for the City of Auburn
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 of the SWMMWW
and the COA Supplemental Manual. All relevant work/calculations shall be submitted for City
review in the Stormwater Site Plan (SSP) report 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
51
D.1.1 On-site Analysis
All proposed on-site surface water conveyance systems shall be sized to meet the required
design event per Section D.2 of this Appendix.
D.1.2 Offsite Analysis (1/4 mile Downstream Analysis)
Refer to Minimum Requirement #10 – Offsite Analysis and Mitigation in Volume I of the COA
Supplemental Manual for more information on downstream analysis. All projects required to
meet Minimum Requirements #1-#5 or #1-#9 shall complete a qualitative downstream analysis.
A quantitative analysis shall be required as described in Appendix I-I.3 of the COA
Supplemental Manual.
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 trunkline 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 D.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 of Auburn Storm Utility at 253-931-
3010 for final determination of whether a storm drainage pipe is a trunk line.
If a capacity problem or stream bank 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 III-3 of the SWMMWW and the COA Supplemental Manual. 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.
III-D.2 Design Event
Additional Requirements for the City of Auburn
The design events for all existing and new conveyance systems are as follows:
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
52
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.
D.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-D.2-1
Percentage Impervious For Fully Developed Conditions Offsite Tributary Areas below.
Conveyance systems shall be modeled as if no on-site detention is provided upstream.
Land Use Description Percentage Impervious
Commercial/Industrial 85%
Residential 65%
Table III-D.2-1 Percentage Impervious For Fully Developed Conditions Offsite Tributary
Areas
III-D.3 Methods of Analysis
Additional Requirements for the City of Auburn
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 drainage maps and record
drawings. For naturally occurring drainage systems, drainage ditches, or undeveloped drainage
courses, the engineer must consider the hydraulic capacity of the existing drainage course and
environmental considerations such as erosion, siltation, and increased water velocities or water
depths.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
53
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 SSP report. 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 SSP report consult III-3 of the
SWMMWW and the Stormwater Site Plan Submittal Requirements Checklist found in Appendix
I-I of the COA Supplemental Manual.
D.3.1 Rational Method
This method shall only be used for preliminary pipe sizing and capacity analysis.
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.
D.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)
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
54
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.
“C” Values
The allowable runoff coefficients to be used in this method are shown by type of land cover in
Table III-D.3-1 Runoff Coefficients for the Rational Method below. 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
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*
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
55
[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
Table III-D.3-1 Runoff Coefficients for the Rational Method
“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-D.3-2 Design Storm Frequency Coefficients for the
Rational Method below for PR values.
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-D.3-2 used to adjust the equation for the design storm
return frequency R
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
56
Table III-D.3-3 Rainfall Intensities for the City of Auburn below includes values of rainfall
intensity as a function of time of concentration, calculated using the coefficients from Table III-
D.3-2.
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
Table III-D.3-2 Design Storm Frequency Coefficients for the Rational Method
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
57
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
Table III-D.3-3 Rainfall Intensities for the City of Auburn
“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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
58
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-D.3-4 “n” and “k” Values for
Hydrographs.
s0 = slope of flowpath (feet/feet)
“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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
59
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
60
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
Table III-D.3-4 “n” and “k” Values for Hydrographs
III-D.4 Pipes, Culverts and Open Channels
Additional Requirements for the City of Auburn
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:
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
61
Pavement Slope, %Maximum Flow Length, ft
0.5 – 1 200
1 to 6 300
6 to 10 200
Refer to Chapter 8 - Sewer and Storm Drainage Facilities and Chapter 9 - Facilities
in the Right of Way of the City of Auburn Design Standards for more requirements
related to drainage conveyance located in public street right-of-way.
D.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.
D.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 appendix. Approved single event models
described in III-2 of the SWMMWW may also be used to determine design flows for pipe
systems. The design event is described above in Section D.2, Appendix D of the COA
Supplemental Manual. Pipe systems shall be designed to convey the design event without
surcharging (water depth in pipe shall not exceed 90% of the pipe diameter).
D.4.1.2 Conveyance Capacity
Two methods of hydraulic analysis using Manning’s Equation are required by the City 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 D.2,
Appendix D of the COA Supplemental Manual 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
62
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 modeled as if no on-site detention is provided up-stream.
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-D.4-1
Percentage Impervious for Modeling Fully Developed Conditions below.
Land Use Description1 % Impervious
Commercial/Industrial 85
Residential 65
1 For the land use descriptions, roads are included in the percentage impervious.
Table III-D.4-1 Percentage Impervious for Modeling Fully Developed Conditions
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 D.2. The nomographs in Figure III-D.4-1 Pipe Sizing Nomograph below
can be used for approximate sizing of the pipes or Manning’s Equation can be solved for pipe
size directly:
(equation 7)
2/13/249.1 SRnV
or use the continuity equation, Q = A•V, such that
(equation 8)
2/13/249.1 SARnQ
Where Q = discharge (cfs)
V = velocity (fps)
A = area (sf)
n = Manning’s roughness coefficient; see Table III-D.4-2 Manning’s “n” Values for Pipes
R = hydraulic radius = area/wetted perimeter
S = slope of the energy grade line (ft/ft)
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
63
Figure III-D.4-1 Pipe Sizing Nomograph
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
64
Analysis Method
Type of Pipe Material Backwater
Flow
Manning’s
Equation Flow
A.Polypropylene CPEP-smooth interior pipe 0.012 0.014
B.Solid Wall PVC pipe 0.011 0.013
C.Ductile iron pipe cement lined 0.012 0.014
D.High density polyethylene pipe (butt fused only)0.009 0.009
Table III-D.4-2 Manning’s “n” Values for Pipes
Table III-D.4-2 Manning’s “n” Values for Pipes above 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.
For pipes flowing partially full, the actual velocity may be estimated from the hydraulic
properties shown below in Figure III-D.4-2 Circular Channel Ratios by calculating Qfull and Vfull
and using the ratio of Qdesign/Qfull to find V and d (depth of flow).
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
65
Figure III-D.4-2 Circular Channel Ratios
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
66
D.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
D.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 D.2 and D.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
D.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-D.4-3 Backwater Calculation Sheet 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-D.4-3 Backwater Calculation Sheet Notes. An
example calculation is performed in Figure III-D.4-6 Backwater Pipe Calculation Example. This
method should not be used to compute stage/discharge curves for level pool routing purposes.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
67
Figure III-D.4-3 Backwater Calculation Sheet
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
68
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-D.4-4 Critical Depth of Flow for Circular
Culverts 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))
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
69
Where Ke = Entrance Loss Coefficient from Table III-D.4-7 Entrance Loss Coefficients 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 D.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-D.4-11 Head for Culverts (Pipe
W/”N”=0.024) Flowing Full with Outlet Control). 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-D.4-5
Junction Head Loss in Structures can be used to determine this loss, or it can be computed
using the following equations derived from Figure III-D.4-5:
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)
Table III-D.4-3 Backwater Calculation Sheet Notes
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
70
Figure III-D.4-4 Critical Depth of Flow for Circular Culverts
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
71
Figure III-D.4-5 Junction Head Loss in Structures
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
72
Figure III-D.4-6 Backwater Pipe Calculation Example
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
73
D.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.
D.4.1.5 Pipe Materials
See City of Auburn Engineering Construction Standards, Division 8, for pipe specifications.
D.4.1.6 Pipe Sizes
The following pipe sizes shall be used for pipe systems to be maintained by the City: 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 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
inches.
Non-single family roof, foundation and small driveway drains may use pipe as small as 6
inches. 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.).
D.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:
o Match pipe crowns
o Match 80% diameters of pipes
o Match pipe inverts or use City approved drop inlet connection
D.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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
74
o 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.
D.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-D.4-4 Maximum
Pipe Slopes, Velocities, and Anchor Requirements below. If velocities exceed 15 feet
per second for the conveyance system design event described in Section D.2, provide
anchors and/or restrained joints at bends and junctions.
D.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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
75
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
1Not allowed in landslide hazard areas.
2Butt-fused pipe joints required. Above-ground installation is required on slopes greater than
40% to minimize disturbance to steep slopes.
3Maximum 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.
4Restrained joints required on slopes greater than 25%. Above-ground installation is required
on slopes greater than 40% to minimize disturbance to steep slopes.
Table III-D.4-4 Maximum Pipe Slopes, Velocities, and Anchor Requirements
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
76
D.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-D.4-5 Allowable Structures and Pipe Sizes
below presents the structures and pipe sizes allowed by size of structure.
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
1Catch 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.
2Maintain the minimum sidewall thickness per this Section.
3Maximum 5 vertical feet allowed between grate and invert elevation.
4Normally allowed only for use in privately maintained drainage systems and must discharge to a catch
basin immediately downstream.
5Allowed for private system installations only.
Table III-D.4-5 Allowable Structures and Pipe Sizes
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
77
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 ensure 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 exceeds 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.
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.
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 downhill 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.
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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
78
For pipe slope less than the required minimum, distance between structures shall be
decreased to 200 linear feet.
For Type 1 and 1L, catch basin to catch basin connections shall not be allowed.
Bubble up systems shall not be allowed.
D.4.1.12 Pipe Clearances
Horizontal and Vertical
For horizontal and vertical separation of storm drainage pipes from other utilities, refer to the
City of Auburn Design Standards, Chapter 6.
D.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 as long as it is not within
the structural pavement of the roadway surfacing. 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.
D.4.1.14 System Connections
Connections to a pipe system shall be made only at catch basins or manholes.
Connections to structures and mains shall be at 90. Slight variations may be allowed.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
79
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 V-4 of the
SWMMWW. Infiltration and dispersion shall be evaluated first. If infiltration and dispersion are
not feasible, roof drains may be discharged through the curb for residential projects per V-4 of
the COA Supplemental Manual 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.
D.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-D.4-
7 Debris Barriers for required debris barriers on pipe ends outside of roadways and for
requirements on pipe ends (culverts) projecting from driveways or roadway side slopes.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
80
Figure III-D.4-7 Debris Barriers
D.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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
81
D.4.2.1 Design Event
The design event for culverts is given in Section D.2.
D.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 appendix.
Other single event models as described in III-2 of the SWMMWW may be used to determine
design flows. In addition, culverts shall not exceed the headwater requirements as established
below:
D.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.
D.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.
D.4.2.5 Inlet Control Analysis
Nomographs such as those provided in Figure III-D.4-8 Headwater Depth for Smooth Interior
Pipe Culverts with Inlet Control and Figure III-D.4-9 Headwater Depth for Corrugated Pipe
Culverts with Inlet Control below 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
82
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 + Vc2/2g)
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-D.4-6 Constants for Inlet Control
Equations
The specified head Hc is determined by the following equation:
Hc = dc + Vc2/2g (equation 12)
where dc = critical depth (ft); see Figure III-D.4-4 Critical Depth of Flow for Circular
Culverts
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-D.4-6 Constants
for Inlet Control Equations
**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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
83
Figure III-D.4-8 Headwater Depth for Smooth Interior Pipe Culverts with Inlet Control
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
84
Figure III-D.4-9 Headwater Depth for Corrugated Pipe Culverts with Inlet Control
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
85
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
Table III-D.4-6 Constants for Inlet Control Equations
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
86
D.4.2.6 Outlet Control Analysis
Nomographs such as those provided in Figure III-D.4-10 Head for Culverts (Pipe W/”N”=0.012)
Flowing Full with Outlet Control and Figure III-D.4-11 Head for Culverts (Pipe W/”N”=0.024)
Flowing Full with Outlet Control 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 D.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-D.4-2 Manning’s “n”
Values for Pipes
R = hydraulic radius (ft)
Ke = entrance loss coefficient from Table III-D.4-7 Entrance Loss Coefficients
G = acceleration due to gravity (32.2 ft/sec2)
dc = critical depth (ft); see Figure III-D.4-4 Critical Depth of Flow for Circular
Culverts
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).
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
87
Figure III-D.4-10 Head for Culverts (Pipe W/”N”=0.012) Flowing Full with Outlet Control
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
88
Figure III-D.4-11 Head for Culverts (Pipe W/”N”=0.024) Flowing Full with Outlet Control
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
89
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
0.5
0.2
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
90
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
0.4
0.2
0.5
0.7
0.2
Table III-D.4-7 Entrance Loss Coefficients
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.
D.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 Section D.5.1, 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-D.4-7 Debris Barriers for a debris barrier detail.
Exceptions are culverts on Type 1 or 2 streams.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
91
For culverts 18-inch diameter and larger, the receiving channel of the outlet shall be
protected from erosion by rock lining specified in Section D.5.1, except the height shall
be one foot above maximum tailwater elevation or one foot above the crown per Error!
Reference source not found. in Section D.5., whichever is higher.
D.4.3 Open Channels
This section presents the methods, criteria, and details for hydraulic analysis and design of
open channels.
D.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.
D.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.
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
92
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:
o 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.
o 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.
o 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.
D.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
III-2 of the SWMMWW 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-D.4-12 through Figure III-D.4-14. 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-D.4-8 Channel Protection).
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-D.4-8 Channel Protection or stabilized with bioengineering
methods as described above in “Constructed Channels.”
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
93
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
94
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-D.4-12 Ditches – Common Section Properties
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
95
Figure III-D.4-13 Drainage Ditches – Slope/Discharge Chart
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
96
Figure III-D.4-14 Geometric Elements of Common Sections
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
97
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.
Table III-D.4-8 Channel Protection
D.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
D.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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
98
flow depth (or normal depth) and flow velocity remain constant throughout the channel reach for
a given flow.
The charts in Figure III-D.4-12 Ditches – Common Section Properties and Figure III-D.4-13
Drainage Ditches – Slope/Discharge Chart 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 D.4.1.2.
Table III-D.4-9 Values of “n” for Channels below 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-D.4-14 Geometric Elements of Common Sections 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.
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
99
submerged at high stagesc. 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 0.050d. 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.
Table III-D.4-9 Values of “n” for Channels
D.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_D.4-15 Direct Step Backwater Method
Example) or computer programs.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
100
Figure III_D.4-15 Direct Step Backwater Method Example
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
101
Equating the total head at cross section 1 and 2, the following equation may be written:
(equation 14)
xSg
Vyg
VyxS f 22
2
2
22
2
1
110
where,Δx = 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
α1, α2 = 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:
(equation 15)g
VyE2
2
Assuming α = α1 = α2 where α 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 Δx as follows:
(equation 16))()(
)(
00
12
ffSS
E
SS
EEx
Typically values of the energy coefficient α are as follows:
Channels, regular section 1.15
Natural streams 1.3
Shallow vegetated flood fringes (includes channel) 1.75
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
102
For a given flow, channel slope, Manning’s “n,” and energy coefficient α, together with a
beginning water surface elevation y2, the values of Δx may be calculated for arbitrarily chosen
values of y1. The coordinates defining the water surface profile are obtained from the cumulative
sum of Δx 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.
D.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.
D.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 City of Auburn Development Services at 253-931-3020, to confirm the applicability of
a particular program prior to starting design.
D.4.3.9 Riprap Design1
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-D.4-16 Mean Channel Velocity vs Medium
Stone Weight (W50) and Equivalent Stone Diameter). 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
1 From a paper prepared by M. Schaefer, Dam Safety Section, Washington State Department of Ecology.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
103
maximum stone. The riprap should have a reasonably well-graded assortment of stone sizes
within the following gradation:
1.25 ≤ Dmax/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
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-D.4-16 Mean Channel Velocity vs Medium Stone Weight (W50) and Equivalent
Stone Diameter, 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-D.4-17 Riprap Gradation Curve, and the
minimum thickness would be 1 foot (from Table III-D.4-8 Channel Protection); 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-D.4-17 Riprap Gradation Curve 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.”
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
104
Figure III-D.4-16 Mean Channel Velocity vs Medium Stone Weight (W50) and Equivalent
Stone Diameter
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
105
Figure III-D.4-17 Riprap Gradation Curve
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
106
III-D.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.
D.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 Error! Reference source not found.)
as specified below and in Table III-D.5-1 Rock Protection at Outfalls.
No erosion or flooding of downstream properties shall result from discharge from an outfall.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
107
Figure III-D.5-1 Pipe/Culvert Outfall Discharge Protection
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
108
Required ProtectionDischarge
Velocity at
Design Flow in
feet per second
(fps)Minimum Dimensions
Type
Thicknes
s 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.
Table III-D.5-1 Rock Protection at Outfalls
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
109
D.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-D.5-2 Gabion Outfall Detail 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-D.5-3 Diffuser Tee – Energy Dissipating End Feature Example.
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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
110
Figure III-D.5-2 Gabion Outfall Detail
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
111
Figure III-D.5-3 Diffuser Tee – Energy Dissipating End Feature Example
D.5.1.2 Flow dispersion
The flow dispersal trenches shown in Figure III-D.5-4 Flow Dispersal Trench and Figure
III-D.5-5 Alternative Flow Dispersal Trench 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
112
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.
For the dispersion trenches shown in Figure III-D.5-4 Flow Dispersal Trench and Figure
III-D.5-5 Alternative Flow Dispersal Trench, 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 flow paths. 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. Chapter 16.10 Critical Areas
of the ACC 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
113
Figure III-D.5-4 Flow Dispersal Trench
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
114
D.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
Figure III-D.5-5 Alternative Flow Dispersal Trench
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
115
energy dissipater have been provided as Figure III-D.5-2 Gabion Outfall Detail. 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.
D.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. See Chapter 16.10
of the ACC.
III-D.6 Pump Systems
Additional Requirements for the City of Auburn
Pump systems are only allowed if applied for through the City’s deviation process (see I-3.6 of
the COA Supplemental Manual). Feasibility of all other methods of gravity conveyance,
infiltration, dispersion, and other on-site stormwater management strategies 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
116
D.6.1 Design Criteria
If approved by the City’s deviation process (see Chapter 1 of the Engineering Design
Standards), 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.
D.6.2 Pump Requirements
If approved through the City‘s deviation 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.
A gravity-flow overflow shall be provided from the control structure back to the storage
system.
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.
Pumping systems that are downstream of detention will require a detailed exhibit
demonstrating that the pump flow discharges will meet the required pre-developed flow
durations and peaks up to the 50 year storm flow.
D.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 Section Appendix V-A of the SWMMWW shall be
included in the O&M manual.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
117
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.
All fees associated with preparing and recording documents and placing the Notice to Title shall
be the responsibility of the applicant.
D.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.
III-D.7 Easements and Access
Refer to Appendix I-J of the COA Supplemental Manual for storm facility access criteria.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
118
Appendix III-E City of Auburn Design Storm
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
Table III-E-1 Design Storm Precipitation Values
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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
119
Appendix III-F 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).
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume III - Choosing, Modeling, and Documenting Your BMPs 2022 Version 1
120
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.
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-F-1 In-Situ
Infiltration Measurement Correction Factors to Estimate Long-Term Infiltration Rates).
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-F-1 the design long-term
infiltration rate becomes 2 inches per hour, anticipating adequate maintenance and pre-
treatment.
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
Table III-F-1 In-Situ Infiltration Measurement Correction Factors to Estimate Long-Term
Infiltration Rates
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
121
Volume IV
Source Control BMP Library
City of Auburn Supplemental Manual
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
122
Executive Summary of Volume IV - Source Control BMP Library
Volume IV of the City of Auburn (COA) Supplemental Manual to the Department of Ecology’s
(Ecology) Stormwater Management Manual for Western Washington (SWMMWW) provides
additional guidance for selection of source control Best Management Practices (BMPs) to meet
Minimum Requirement #3 – Source Control of Pollution.
The Ecology SWMMWW is available online at the link below:
2019 SWMMWW
The COA Supplemental Manual Volume IV is organized to correspond to the SWMMWW
Volume IV. This Volume should be used in conjunction with the SWMMWW to select and
implement source control BMPs to prevent pollutants from contaminating stormwater runoff and
entering our rivers, streams and groundwater.
Source Control BMPs in this Volume of the SWMMWW and COA Supplemental Manual that are
required must always be applied to the associated activity. Recommended BMPs are to be
considered when and where the required BMPs are not achieving or expected to achieve the
outcome of the required BMPs. The outcome of all required Source Control BMPs is to eliminate
the discharge of pollutants into stormwater runoff to the maximum extent practicable.
Refer to the Glossary in the SWMMWW for definitions of terms used throughout this manual.
Additional definitions of terms not defined in the SWMMWW and acronyms are included in
Appendix I-F, Volume I of the COA Supplemental Manual.
Important new BMPs (indicated as “Additional”) and changes to the SWMMWW BMPs
(indicated as “Amended”) in the SWMMWW Volume IV are summarized below:
IV-2: Cleaning or Washing Source Control BMPs
o BMP S431 (amended)
o BMP S441 (amended)
IV-3: Roads, Ditches, and Parking Lot Source Control BMPs
o BMP S630 (additional)
o BMP S631 (additional)
o BMP S632 (additional)
o BMP S633 (additional)
IV-4: Soil Erosion, Sediment Control, and Landscaping Source Control BMPs
o BMP S435 (amended)
o BMP S640 (additional)
IV-5: Storage and Stockpiling Source Control BMPs
o BMP S445 (amended)
o BMP S650 (additional)
o BMP S651 (additional)
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
123
IV-7: Other Source Control BMPs
o BMP S414 (amended)
o BMP S670 (additional)
o BMP S671 (additional)
o BMP S672 (additional)
o BMP S673 (additional)
o BMP S674 (additional)
o BMP S675 (additional)
o BMP S676 (additional)
Supplemental Appendix IV-C contains a quick reference phone number guide for local,
county, and state agencies.
Supplemental Appendix IV-D provides required Source Control BMPs for homeowners.
Sections with No Changes
The following portions of Volume IV of the SWMMWW do not require any additional clarification
in the COA Supplemental Manual, and together with the COA Supplemental Manual, contain
the requirements and guidelines for stormwater management in the City of Auburn.
IV-1: Introduction (all BMPs)
IV-2: Cleaning or Washing Source Control BMPs
o S434
IV-3: Roads, Ditches, and Parking Lot Source Control BMPs
o BMP S405, S406, S415 - S417, S424, and S430
IV-4: Soil Erosion, Sediment Control, and Landscaping Source Control BMPs
o BMP S407 – S408, S411, S425, S444, S449, and S450
IV-5: Storage and Stockpiling Source Control BMPs
o BMP S427 – S429
IV-6: Transfer of Liquid or Solid Materials Source Control BMPs
o BMP S409, S412, S419, S426,and S439
IV-7: Other Source Control BMPs
o BMP S401 – S404, S413 – S414, S418, S420, S422 – S424, S432 – S433,
S436. S438, S440, S442 – S443, S446 – S447, S451, and S452
Appendices IV-A – IV-B
IV-2 – Cleaning or Washing Source Control BMPs
S431: BMPs for Washing and Steam Cleaning Vehicles/Equipment/Building Structures
(Amended)
General
Wash water shall be discharged to the sanitary sewer with appropriate pretreatment to
remove solids.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
124
If heavy accumulations of solids are to be removed during washing, then appropriate
pretreatment to capture those solids must be provided. Volume II of the SWMMWW
contains temporary sediment removal BMPs that can be utilized to capture solids before
discharging wastewater to the sanitary sewer system
Two-step washing may be allowed at all facilities discharging to the sanitary sewer.
Provisions must be in place to neutralize the wash water prior to introduction into the
sanitary sewer system.
Facilities with dedicated wash pads with a catch basin having a tee discharge and/or
pretreatment equipment, sampling and monitoring ports will be required. Additionally, the
installation of a valve may be required to prevent discharge from the system in the event
of a spill. All ports must be accessible for inspection and sampling at all times. Any
equipment needed for access must be available at all times.
Contact the City of Auburn Sanitary Sewer Utility for more information 253-931-3010.
New and Used Car Dealer Lots
If washing previously clean vehicles is accomplished only with cold water and consists of
washing only the outside of the vehicles, (no soaps or detergents used) discharge to the
private storm drainage system will be allow provided the site has a flow control structure
that acts as the sediment and oil trap. See Ecology guidance WQ-R-95-56, “Vehicle and
Equipment Washwater Discharges/Best Management Practices Manual”, November
2012 or most recent update, for more information.
If soaps or detergents are to be used, washing must occur on a dedicated wash pad.
Only the washing of the outside of the vehicles is permitted. The wash pad must be
equipped with a catch basin/sediment trap that discharges through a tee outlet to the
sanitary sewer. The tee outlet will allow containment of minor amounts of free-floating
oil. Wastewater must meet local limitations on wastewater strength and quality. If
engines and/or undercarriages are to be washed additional pretreatment will be
required. Contact the City of Auburn Sewer Utility at 253-931-3010 for further
information.
Other Washing Events
Other types of washing events and their locations (such as regional used car sales, RV
shows, etc.) will be evaluated on a case-by-case basis according to the discharge
criteria stated above.
Fundraising car washes may wash only the exterior of vehicles. If soap is used, the
wash water must be captured and directed to the sanitary sewer. Contact City of Auburn
Public Works Department at 253-931-3010 for alternative fundraising activities that do
not pose a risk of pollution.
Pressure washing of building facades, rooftops, pavement, and other large surfaces
must be conducted in such a way that all of the runoff is collected for proper disposal.
Sediment removal BMPs include those as described in Volume II of the SWMMWW and
include measures such as check dams, sorbent booms, and catch basin inserts.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
125
Temporary curbs, dikes, or berms may be used to direct the water to a collection point or
catch basins may be covered to help contain the water. The collected water, provided it
meets limits established by King County Wastewater Treatment Division or defined in
Chapter 13.20 of the Auburn City Code, should be disposed of to the sanitary sewer.
Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010 for more information
on allowable discharges to the sanitary sewer.
On a case by case basis, if runoff does not contain pollutants, following appropriate
pretreatment, such as filtration or sedimentation, then this water may be allowed to be
discharged to the storm drainage system. Contact the City of Auburn Storm Drainage
Utility at 253-931-3010 for more information.
Automatic and Manual Car Wash
See Chapter 13.20-Sewers of the Auburn City Code and the Auburn Engineering Design
Standards.
Truck Washing Facilities
Wash on a concrete or asphalt paved dedicated wash pad connected to a designed
pretreatment device, which discharges to the sanitary sewer See Chapter 13.20-Sewers
of the Auburn City Code and the Auburn Engineering Design Standards.
Mobile Vehicle Washers
The following summarizes the requirements for mobile vehicle and grocery cart washers doing
work in the City of Auburn. Contact the City of Auburn Storm Drainage Utility at 253-931-3010
for more information.
Mobile vehicle washers must possess a current City of Auburn Business License.
Discharge all wash water to the sanitary sewer.
Mobile vehicle washers shall capture all the wash water generated and discharge it to
the sanitary sewer system through an on-site cleanout if available. The use of temporary
wash pads, catch basin inserts, and vacuum systems are some possible means to
capture the wash water.
Other requirements may apply depending upon the items to be washed.
Only wastewater generated within the City of Auburn may be discharged into its
municipal sewer system.
Mobile vehicle washers may wash only the exteriors of the vehicles. Engine washing,
cleaning the undercarriage or fifth wheel is strictly prohibited unless prior approval is
received from the City Engineer or his/her designee. Additional pretreatment may be
required. Discharges from mobile washing must meet the requirements in Chapter 13.20
of the ACC regarding discharge limitations for pH.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
126
S441: BMPs for Potable Water Line Flushing, Water Tank Maintenance, and Hydrant
Testing (Amended)
Description of Pollutant Sources
Construction and operation of drinking water infrastructure, including water mains, water
reservoirs, water transmission mains, hydrants, and water wells, as well as emergency
response activities, can generate sediments, rust, turbidity and suspended solids, bacteria, and
chlorinated water. Flushing of the water delivery system is necessary to maintain drinking water
quality and ensure public health. Flushing activities can result in increased flows in downstream
conveyances. These high flows may cause flooding and create erosion in downstream
channels.
Pollutant Control Approach
Establish operational controls for flow rate and volume of discharges, removal of sediments,
neutralization of chlorine, and maximizing the beneficial use of the resource.
Applicable Operational BMPs
Discharges of untreated hyperchlorinated water must go to the sanitary sewer. Prior City
approval is required. Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010
for more information.
Alternatively, non-emergency discharges of de-chlorinated potable water may go to the
storm drainage system at prior approved flow rates provided the following limits are met:
o Chlorine residual ≤0.1 ppm
o pH 6.5 – 8.5
o Turbidity ≤10 NTU
Coordinate with the City of Auburn Storm Drainage Utility at 253-931-3010. The
receiving stormwater conveyance shall be monitored for the duration of the discharge.
Evaluation of the receiving conveyance system for capacity and/or obstructions may be
required.
City approval may be required for draining and flushing reservoirs, standpipes,
wellheads, and transmission lines. Notification, monitoring, reporting, flow control
measures, and other special conditions may apply. Contact the City of Auburn Public
Works Department at 253-931-3010 for the requirements.
For routine hydrant and water main flushing, coordinate with the City of Auburn Storm
Maintenance and Operations section at 253-931-3048. In all cases, the receiving storm
pipe shall be monitored for the duration of the discharge to maintain no more than a half
full-pipe flow rate.
Significant releases of water can have a detrimental effect on the storm and sanitary
transmission system as well as receiving waters. Notification of these releases must be
promptly made to the City by calling 253-931-3048.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
127
Applicable Operational BMPs
Additional Guidelines for the City of Auburn
During emergency repairs and activities, such as mainline breaks, erosion control
measures shall be taken as practicable. Use of sandbags, check dams, plastic sheeting,
pumps, and other erosion control measures should be employed to minimize erosion as
much as possible.
Excavation de-watering should be managed to minimize downstream environmental
impacts. Use of vactor trucks, diverting flow to grassy areas, filter bags, and retention
ponds should be employed.
IV-3 – Roads, Ditches, and Parking Lot Source Control BMPs
S630: BMPs for Disposing of Contaminated Stormwater and Waste Materials Properly
(Additional)
Every business and residence in Auburn must dispose of solid and liquid wastes and contaminated
stormwater properly. There are generally four options for disposal depending on the type of materials.
These options include:
Sanitary sewer and septic systems
Recycling facilities
Public or private, permitted solid waste disposal facilities
Permitted hazardous waste treatment, storage, and disposal facilities
Many liquid wastes and contaminated stormwater (depending on the pollutants and associated
concentrations present) can be put into the sanitary sewer. Animal wastes can also be disposed of in
a sanitary sewer. A permit may be required for discharges to the sanitary sewer system. Please
contact the City of Auburn Utility Billing Section at 253-931-3038 for design and permit requirements.
See Appendix IV-B of the SWMMWW for Management of Street Wastes.
If wastes cannot be legally discharged to a sanitary sewer or septic system, one of the other three
disposal options must be used. Recycling facilities are a recommended option for many commercial
and household items, including used oils, used batteries, old equipment, glass, some plastics, metal
scrap materials, solvents, paints, wood and land clearing wastes, and various other solid wastes.
Solid wastes that cannot be recycled and that are not hazardous must be disposed of at a licensed
solid waste disposal facility. Dangerous and hazardous wastes must be properly transported to an
appropriate hazardous waste treatment, storage, and disposal facility. The City of Auburn Solid
Waste Utility at 253-931-3038 can provide information on waste disposal options.
Maintain records for all materials that are recycled or disposed.
Appendix IV-C of the COA Supplemental Manual has a list of telephone numbers to contact for
assistance.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
128
S631: BMPs for Cleaning Catch Basins (Additional)
Cleaning catch basins regularly is one of the most important stormwater source control
measures that a business can take. Catch basins are typically located at low spots in parking
lots, along curbs and road edges, and where storm drain pipes combine flows. Catch basins
collect surface runoff for storm drains that are typically located directly underneath them. Most
catch basins have some storage in the bottom that never drains to an outflow pipe. This
permanent storage area is intended to trap sediments, debris, and other particles that can settle
out of stormwater, thus preventing clogging of downstream pipes and washing of these solids
into receiving waters. All of the solids and stagnant water collected from catch basin sumps
must be disposed of properly. The sump contents shall not be flushed into the catch basin
outflow pipe.
For additional information on the maintenance of catch basins, refer to Appendix V-A of the
SWMMWW. Perform regular inspections of the basins and their grates. Remove trash and
collected sediment when 60% of the sump depth has been filled or sediments are within 6
inches of the bottom of the outlet pipe.
It should be apparent that the use of other BMPs, such as frequent sweeping of activity areas,
covering activity areas, reducing activity occurrence, and containing run-off from activity areas
will help reduce catch basin cleaning frequency, thus saving time and money. All businesses
and agencies should set up maintenance schedules for all of their BMPs so coordinated BMP
maintenance efforts results in reduced catch basin cleaning frequencies.
Use of catch basin inserts such as filter socks, absorbent pillows, and filter baskets require an
increased inspection frequency to prevent plugging and flooding.
Dispose of street waste in accordance with Appendix IV-B of the SWMMWW for Management of
Street Wastes.
S632: BMPs for Paving the Activity Area and Sloping to a Sump, Holding Tank, or
Oil/Water Separator (Additional)
This BMP applies to several activities that cannot be covered effectively. It is particularly suited
to activities with the potential for leaks and spills, but that otherwise do not generate excessive
amounts of polluted runoff. The activity area shall be paved and sloped to a central collection
point. A sump, holding tank, or oil/water separator (Figure IV-3-1 Paved Area with Sump Drain)
serves to provide spill containment until the liquids can be pumped out and properly disposed.
The minimum volume for the sump shall be equivalent to the volume generated by the
anticipated activity plus rain water. Sizing justification shall be included in design submittals.
To prevent run-on, the area should be enclosed with a berm, curb, or dike. Frequent inspections
of the sump, holding tank, or oil/water separator are necessary. Inspections and maintenance
shall be recorded in a log in the Operations and Maintenance Manual for the site. For
assistance, contact commercial services that pump sumps and holding tanks.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
129
Figure IV-3-1 Paved Area with Sump Drain
S633: BMPs for Surrounding the Activity Area with a Curb, Dike, or Berm or Elevating the
Activity (Additional)
This set of BMP options can be an effective means for prevention of stormwater run-on to an
activity area. In addition, a curb, berm, or dike can be used for containment of spills in the
activity area, or for containment of contaminated activity runoff. Generally, a containment BMP,
is most applicable to spill control situations; that is, sites where runoff is relatively clean, but
occasional spills may occur.
If a curb, berm, or dike is used for runoff containment, and other containment sizing regulations
(such as fire codes, Environmental Protection Agency, or Department of Ecology restrictions) do
not apply, the containment volume shall be 100% of the volume of the largest tank plus the
volume of stormwater runoff from rain events up to the 25-year, 24-hour storm within the
containment area is contained or 110% of the volume of the largest tank, whichever is greater.
Impervious containment may consist of membrane lined soil enclosures, containment pallets,
plastic pools, mortar mixing tubs, and water troughs.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
130
Regular inspections of the containment area and proper management of any collected
stormwater is required.
Development of a spill plan may be necessary for storage of liquids. See BMP S426 in Volume
IV of the SWMMWW. For permanent storage facilities see BMPs S409, S427, S428, and S429
in Volume IV of the SWMMWW.
IV-4 – Soil Erosion, Sediment Control, and Landscaping Source
Control BMPs
S435: BMPs for Pesticides and Integrated Pest Management Program (Amended)
Description of Pollutant Sources
Use of herbicides, fungicides, and rodenticides should always be done with extreme caution, not
only because of the potential harm to humans and pets, but also because of the potential harm
to fish, wildlife, and our water resources. In light of the toxic nature of these compounds, special
attention should be given to pesticide usage in all applications. The discussion below applies
more to large-scale licensed pesticide users but should be considered for backyard applications
as well.
Pollutant Control Approach
Commercial, agricultural, and other large scale pesticide users, such as golf courses and parks,
shall adhere to the principles of integrated pest management (IPM), a decision-making process
for pest management that strives for intelligent, environmentally sound control of pests. It is a
systems approach to pest management that combines agronomic, biological, chemical, and
genetic information for educated decisions on the type of control to use, the timing and extent of
chemical application, and whether non-chemical means can attain an acceptable level of pest
control.
IPM is a preventive measure aimed at knowing the exact pests being targeted for control, the
locations and times when pests will pose problems, the level of pest-induced damage that can
be tolerated without taking action, the most vulnerable life stage, and control actions that are
least damaging to the environment. The major components of IPM are as follows:
Monitoring and inventory of pest populations
Determination of pest-induced injury and action levels
Identification of priority pest problems
Selection and timing of least toxic management tools
Site-specific treatment with minimized chemical use
Evaluation and adjustment of pesticide applications
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
131
Monitoring of pest populations is key to successful IPM implementation. Pest problems are
universally easier to control if the problem can be discovered early. With IPM, pesticides are
used only as a last resort. Maximization of natural controls, including biological controls and
removal of pests by hand, is always the first choice.
Additional concerns are storage, equipment clean-up, spill protocols, and waste disposal.
More information on IPM is available from the Washington State Department of Agriculture and
from the Washington State University Extension Service.
S640: BMPs for Concrete and Asphalt Mixing and Production at Stationary Sites
(Additional)
Description of Pollutant Sources
This applies to businesses and agencies that mix raw materials onsite to produce concrete or
asphalt. It also applies to subsequent uses such as pouring concrete structures and making
other concrete or asphalt products. Mobile concrete pouring and asphalt application are covered
in Volume II of the SWMMWW. Requirements for stockpiling of raw materials are covered under
BMP S429 in the Volume IV of the SWMMWW.
Pollutants of concern include toxic hydrocarbons, toxic organic compounds, oils and greases,
heavy metals, and pH.
Pollutant Control Approach
Cover and contain processes where possible and prevent stormwater run-on and
contamination, where feasible.
Any facility categorized under SIC Code 2951 or SIC Code 3273 may need to comply with
Ecology’s Sand and Gravel General Permit. Contact the Department of Ecology at 360-407-
6979 for additional information. These facilities may also be subject to City of Auburn
requirements. Contact the City of Auburn Public Works Department at 253-931-3010 for further
information.
The following BMPs are required of all businesses and public agencies active in concrete and
asphalt mixing and production:
Eliminate all illicit connections to the storm drainage system. See BMP S410 in Volume
IV of the SWMMWW for a detailed discussion on identifying and eliminating these
connections.
All process water from production, pouring, and equipment cleaning must be discharged
to a dead-end sump, a process water treatment system, connected to the sanitary
sewer, or recycled. Never wash fresh concrete or concrete mixer washout into streets,
storm drainage systems, streams, or other water bodies.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
132
A BMP maintenance schedule must be established, maintenance documented, and
employees educated about the need to prevent stormwater contamination through the
use and proper maintenance of BMPs.
Production and pouring areas must be protected from stormwater run-on. See BMP
S633 in Volume IV of the COA Supplemental Manual for methods of run-on protection.
Cover the production area for prevention of stormwater run-on. See BMP S676 and
BMP S633 in Volume IV of the COA Supplemental Manual for information on covers and
run-on prevention.
Use absorbent materials or catch basin filters in and around storm drains and catch
basins to filter out contaminants.
Sweep areas that show accumulation of materials. Vacuum sweeping is also an
acceptable method of removing accumulated material.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but can provide additional pollution protection:
The production and pouring area should be swept at the end of each work day to collect
loose chunks of aggregate and raw materials for recycling or proper disposal. See BMP
S630 in Volume IV of the COA Supplemental Manual for disposal options.
Sweep all driveways and gutters that show accumulation of materials to minimize the
amount that could be carried offsite by rain and enter the storm drainage system. Use of
vacuum sweepers is most efficient.
Asphalt plants should use an oil/water separator to treat stormwater runoff. See Volume
V of the SWMMWW, Water Quality Treatment BMPs, for more information.
Pave the mixing, production, and pouring areas. A sump drain in these areas is probably
not advisable due to potential clogging problems but could be used in a curing area.
Sweep these areas to remove loose aggregate and recycle or dispose of properly.
Use storm drain covers or similarly effective containment devices to prevent runoff from
entering the storm drainage system. Accumulations of dirty runoff must be disposed of
properly.
IV-5 – Storage and Stockpiling Source Control BMPs
S429: BMPs for Storage or Transfer (Outside) of Solid Raw Materials, Byproducts, or
Finished Products (Amended)
Description of Pollutant Sources
Some activities, such as stockpiling of raw materials, can be effectively covered with a sturdy
tarp or heavy plastic sheet made of impermeable material.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
133
Pollutant Control Approach
Weights such as bricks, tires, or sandbags should be used to anchor the cover in place. Run-on
shall be prevented from reaching the activity or material. Stormwater run-off from the cover shall
be directed away from the stockpile and work zone, and if uncontaminated, directed to the
stormwater collection system. The tarp must be inspected daily to ensure that no holes or gaps
are present in the tarp coverage. An example of this type of cover is shown in Figure IV-5.7A
Stockpile Covering, which supersedes SWMMWW Figure IV-5.7.
Figure IV-5.7A Stockpile Covering
S445: BMPs for Temporary Fruit Storage (Amended)
Businesses that store or process fruits and vegetables are required to be covered under an
Individual NPDES Permit and may require an Industrial Wastewater Discharge Permit from King
County Metro. Contact the Washington State Department of Ecology for information on BMPs
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
134
related to fruit and vegetable processing and storing. Additional permitting may be required from
the County Health Department.
S650: BMPs for Storage of Solid Wastes and Food Wastes (Additional)
Description of Pollutant Sources
This applies to facilities such as hospitals, restaurants, meat and seafood markets, veterinarian
clinics, schools, grocery stores, assisted living centers, multifamily residential properties and
group assembly halls that store solid wastes and food wastes outdoors. This includes ordinary
garbage. If improperly stored, these wastes can contribute a variety of different pollutants to
stormwater, for more information call the Auburn Solid Waste and Recycling Utility at 253-931-
3038.
NOTE: Dangerous solid wastes must be stored and handled under special guidelines.
Businesses and agencies that store dangerous wastes must follow specific regulations outlined
by the Department of Ecology. Ecology regulations are outlined in Section I-2.15 of the
SWMMWW. Please contact the Department of Ecology’s Hazardous Waste and Toxics
Reduction Program for the specific requirements and permitting information.
Pollutants of concern include toxic organic compounds, oils and greases, heavy metals,
nutrients, suspended solids, chemical oxygen demand (COD), and biochemical oxygen demand
(BOD).
Pollutant Control Approach
Store wastes in suitable containers with leak-proof lids. Sweep or shovel loose solids. Educate
employees about the need to check for and replace leaking containers.
Additional Requirements for the City of Auburn
The following BMPs are required of all businesses and public agencies engaged in storage of
non-dangerous solid wastes or food wastes:
All solid and food wastes must be stored in suitable containers. Piling of wastes without
any cover is prohibited.
Waste storage areas and trash enclosures for food or liquid bearing wastes must be
connected to the sanitary sewer and bermed or sloped to prevent stormwater run-on.
Waste storage areas and trash enclosures over 200 square feet in area require a
permanent cover.
Trash compactors or dumpsters for food or liquid-bearing wastes shall drain to the
sanitary sewer system using a design approved by the City of Auburn.
Storage containers must be checked for leaks and replaced if they are leaking, corroded,
or otherwise deteriorating. If storage containers contain liquid wastes of any kind, then
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
135
the container shall be located on a pad equipped with a drainage system connected to
the City sanitary sewer.
Storage containers must have leak-proof lids or be covered by some other means. Lids
must be kept closed at all times. This is especially important for dumpsters, as birds can
pick out garbage and drop it, promoting rodent, health, and stormwater problems.
Do not completely fill containers of waste grease and oil. Leave a minimum of four
inches of freeboard to prevent spills when the containers are moved or handled for
recycling.
Employees must be trained to check storage containers frequently for leaks and to
ensure that the lids are on tightly.
The waste storage area must be swept or otherwise cleaned frequently to collect all
loose solids for proper disposal in a storage container. Do not hose the area to collect or
clean solids.
If containers are cleaned, all rinse water from cleaning must be disposed of in a sanitary
sewer or septic system.
Inspect regularly and clean out catch basins on the property that receive drainage from
waste storage area. See BMP S631 in Volume IV of the COA Supplemental Manual for
details on catch basin cleaning.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but can provide additional pollution protection:
If the amount of waste accumulated appears to frequently exceed the capacity of the
storage container, then another storage container should be obtained and utilized.
Provide a backup storage container if the amount of waste accumulated appears to
frequently exceed the capacity of the storage container.
Locate drain to sanitary sewer at one end or side of the enclosure to ease cleaning the
drain.
In enclosures with drains to the sanitary sewer, provide an area for washing floor mats.
Store containers such that wind will not be able to knock them over.
Designate a storage area, pave the area, and slope the drainage to a holding tank or
sanitary sewer drain. If a holding tank is used, the contents must be pumped out before
the tank is full and properly disposed. See BMP S630 in Volume IV of the COA
Supplemental Manual for more information on disposal options.
Compost appropriate wastes. Contact Auburn Solid Waste and Recycling Utility
Recycling at 253-931-3038 for more information on composting.
Recycle solid wastes. The Industrial Materials Exchange (IMEX) program facilitates the
transfer of excess materials and wastes to those who can use them. IMEX can be
reached at 206-263-8465.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
136
S651: BMPs for Storage and Treatment of Contaminated Soils (Additional)
Description of Pollutant Sources
This applies to businesses and agencies that store and treat soils contaminated with toxic
organic compounds, petroleum products, or heavy metals. Stormwater runoff that comes in
contact with contaminated soil can carry those contaminants along with loose dirt into receiving
waters.
The State Department of Ecology regulates businesses disposing and treating contaminated
soil. A permit from the Puget Sound Clean Air Agency is required if the treatment method for
removing soil contaminants involves forcing air through, or sucking air from, the soil. The Puget
Sound Clean Air Agency can be reached at 1-800-552-3565. In addition, approval from the King
County Wastewater Services Department may be required if potentially contaminated water is to
be discharged from the site. Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010
for information on proper disposal of contaminated water.
The BMPs included here are intended as a supplement to other regulations. The following
BMPs are required of all businesses engaged in storage and treatment of contaminated soils:
The storage area for contaminated soils must be enclosed indoors, covered, or
contained by a curb, dike, or berm constructed around the material storage area. If the
contaminated soils are covered, stormwater run-on protection must also be provided.
BMP S633 in Volume IV of the COA Supplemental Manual provides further details on
containment and run-on prevention.
Employees must be educated on methods to prevent contamination from leaving the
site.
Cleanup materials must be stocked near the storage area.
Gutters, storm drains, catch basins, and other drainage system features on the site must
be cleaned following the completion of site work, or at least once per year, whichever
comes first. Sediments from such cleaning must be disposed of properly. See BMPs
S630 and S631 in Volume IV of the COA Supplemental Manual for disposal options and
details on catch basin cleaning.
Additional Guidelines for the City of Auburn
The following BMPs are not required but can provide additional pollution protection:
If feasible, the storage area should be swept weekly for collection of stray soil, which can
be added back to the piles or properly disposed. See BMP S630 in Volume IV of the
COA Supplemental Manual for information on disposal options.
Implement a vegetated biofilter in conjunction with a runoff containment plan.
Implement treatment BMP for the targeted pollutant (see Volume V of the SWMMWW).
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
137
The use of any treatment BMP must not result in the violation of groundwater, surface water,
wastewater, or drinking water quality standards.
IV-7 – Other Source Control BMPs
S414: BMPs for Maintenance and Repair of Vehicles and Equipment (Amended)
Description of Pollutant Sources
This applies to businesses and public agencies where fuel filters, engine oil, and other fluids
such as battery acid, coolants, and transmission and brake fluids are removed and replaced in
vehicles and equipment. It also applies to mobile vehicle maintenance operations, such as at
construction sites. Related vehicle maintenance activities are covered under the following
activity headings in the SWMMWW.
S419 BMPs for Mobile Fueling of Vehicles and Heavy Equipment
S420 BMPs for Painting/Finishing/Coating of Vehicles/Boats/Buildings/Equipment
S421 BMPs for Parking and Storage for Vehicles and Equipment
Pollutants of concern include toxic hydrocarbons, toxic organic compounds, oils and greases,
pH, and heavy metals.
Applicable Operational BMPs:
The following BMPs or equivalent measures are required of all businesses and agencies
engaged in engine and vehicle repair:
Employees must be educated about the need for careful handling of automotive fluids.
Employees at businesses or agencies who routinely change or handle these fluids must
be trained in spill response and cleanup procedures. Spill cleanup materials, such as
rags and absorbent materials, must always be kept close at hand when changing oil and
other fluids. Soiled rags and other cleanup material must be properly disposed of or
cleaned and reused.
Floor drains inside buildings shall connect to sanitary sewer, be routed through an
appropriately sized oil/water separator and shall be approved by the City.
Do not hose down the maintenance/repair area. Instead, sweep the area weekly to
collect dirt, and wipe up spills with rags and other absorbent materials.
A bermed tarp, ground cloth, or drip pans must be used beneath the vehicle or
equipment to capture all spills and drips. The collected drips and spills must be recycled
or disposed of properly. See BMP S630 in Volume IV of the COA Supplemental Manual
for disposal options.
If this activity occurs at a stationary business location, the activity area must be moved
indoors. An exception to this requirement would be equipment that is too large to fit
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
138
under a roofed area. In this case, the outdoor area must be paved, provided with a sump
drain, and provision made for stormwater run-on prevention. See BMP S632 and S633
in in Volume IV of the COA Supplemental Manual for more on paving, sump drains and
holding tanks, and run-on prevention. Contact the City of Auburn Sanitary Sewer Utility
at 253-931-3010 for information on requirements for disposal to sewer. If the site utilizes
a septic tank, sump contents will need to be pumped and disposed of by an oil recycler
or hazardous waste company.
Recycle oil, antifreeze, batteries, and air conditioning coolant.
If engine washing is to be performed, then appropriate pretreatment will be required.
Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010 for the requirements.
Contaminated stormwater runoff from vehicle staging and maintenance areas must be
conveyed to an American Petroleum Institute (API) or Coalescing Plate (CP) oil and
water separator followed by a basic treatment BMP (see Volume V of the SWMMWW),
applicable filter, or other equivalent oil treatment system.
Recommended Additional Operational BMPs:
Additional Guidelines for the City of Auburn
The following BMPs are not required but can provide additional pollution prevention.
Drain all fluids from wrecked vehicles and car parts upon arrival. Recover air
conditioning gases.
Use reusable cloth rags to clean up drips and small spills instead of disposables: these
can be professionally laundered and reused. Do not attempt to launder these at home or
at a coin-op laundry.
Use absorbent pillows or booms in or around storm drains and catch basins to absorb oil
and fuel.
Maintain vehicles under cover where possible.
S670: BMPs for Cleaning or Washing of Tools, Engines and Manufacturing Equipment
(Additional)
Description of Pollutant Sources
This applies to businesses and public agencies that clean manufacturing equipment such as saws,
grinders, screens, and other processing devices outside of buildings, and to businesses engaged in
pressure washing of engines, equipment, and portable objects.
Pollutants sources include toxic hydrocarbons, organic compounds, oils and greases, nutrients,
heavy metals, pH, suspended solids, biochemical oxygen demand (BOD), and chemical oxygen
demand (COD).
Pollutant Control Approach
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
139
The preferred approach is to cover and/or contain the cleaning activity or conduct the activity inside a
building, to separate the uncontaminated stormwater from the pollutant sources. Wash water must be
conveyed to a sanitary sewer after approval by the City of Auburn, temporarily stored before proper
disposal, or recycled, with no discharge to the ground, a storm drain, or surface water.
The quality of any discharge to the ground after proper treatment must comply with Ecology’s
Groundwater Quality Standards, Chapter 173-200 WAC and may require a State Waste Discharge
Permit. Contact the Department of Ecology for an NPDES Permit application for discharge of
washwater to surface water, storm drain or the ground.
The following BMPs are required of all businesses and public agencies engaged in cleaning or
washing of tools, engines, equipment, and portable objects:
Illicit connections to the storm drainage system must be eliminated. See BMP S410
in Volume IV of the SWMMWW for detailed information.
Employees shall be educated to control washing operations to prevent stormwater
contamination.
Wash water from cleaning roof-top equipment, such as exhaust fans, shall be
captured and disposed to the sanitary sewer. The use of wet/dry vacuums,
temporary berms or containers, such as plastic pools, are possible ways to capture
this water. Wash water shall not be discharged to the storm drainage system.
Pressure washing must be done in a designated area (such as a wash pad) provided
with a sump drain and stormwater run-on prevention. See BMPs S632 and S633 in
Volume IV of the COA Supplemental Manual for information on sumps (or holding
tanks) and run-on prevention. Contact the City of Auburn Storm Drainage Utility at
253-931-3010 for washing operation policy.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but they can provide additional pollution control:
If soaps or detergents are used, use the least toxic cleaner capable of doing the job.
Use non-phosphate detergent, if possible, to reduce loadings at your local
wastewater treatment plant.
Limit the amount of water used in washing activities to reduce the potential of runoff
carrying pollutants beyond the designated wash pad or capture system.
Recycle wash water for subsequent washings.
Implement one or more of the following stormwater treatment BMPs in addition to the
Required BMPs:
o Oil/water separator (do not use an oil/water separator for wash water
containing soaps or detergents).
o Wet vault for settling.
o Infiltration basin.
o Filtration with media designed for the pollutants present.
o Catch basin with a filter insert for pressure washing to collect suspended
solids.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
140
Catch basin filters and/or sorbent inserts should be selected based on the type of
contaminants in the stormwater.
For discharging wash water containing soaps and detergents, the use of infiltration, biofiltration, wet
ponds, and wetlands must not result in the violation of groundwater quality standards.
S671: BMPs for Cleaning or Washing of Cooking Equipment (Additional)
Description of Pollutant Sources
This applies to businesses that clean cooking equipment such as vent filters, grills, hoods, and
grease traps outside of buildings and clean paved areas and floor mats around cooking equipment.
Pollutants of concern consist of oil and grease, nutrients, suspended solids, biochemical oxygen
demand (BOD) and chemical oxygen demand (COD)
Pollutant Control Approach
Businesses engaged in this activity that cannot connect discharges to a sanitary sewer, holding tank,
or process water treatment system must contact the Department of Ecology and obtain a National
Pollutant Discharge Elimination System (NPDES) wastewater permit.
The following BMPs are required of all businesses engaged in cleaning or washing of cooking
equipment:
Illicit connections to the storm drainage system must be eliminated. See BMP S410
in Volume IV of the SWMMWW for detailed requirements.
Employees must be educated about the need to prevent stormwater contamination
from washing operations.
Wash water cannot be discharged to the storm drainage system.
Paved washing areas must be swept daily to collect loose solid materials for proper
disposal.
Cleaning and washing of cooking equipment shall take place indoors with drainage
to the sanitary sewer system, holding tank, or process treatment system or captured
using a tub or similar device to contain all the wash water. The wash water shall be
recycled or disposed into the sanitary sewer system, holding tank, or process
treatment system. Provisions must be in place to neutralize the wash water prior to
discharge to the sanitary sewer system.
Greasy buildup on cooking equipment must be removed and properly disposed of
prior to washing to reduce the amount of material that can potentially contaminate
the wash water. Washing must either take place on a wash pad connected to the
sanitary sewer, or the wastewater must be collected and disposed in the sanitary
sewer.
Move the activity indoors, into either an existing building or a newly constructed
building or shed, with drainage to a sanitary sewer, holding tank, or process
treatment system. See BMP S675 in Volume IV of the COA Supplemental Manual
for further information on drainage alternatives. Any connection to the sanitary sewer
requires the approval of the City of Auburn.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
141
If the washing activity cannot be moved indoors or contained in a tub, then the
washing area must drain to a sanitary sewer, holding tank, or process treatment
system, and provisions must be made to prevent stormwater run-on onto the
washing area. See BMP S675 in Volume IV of the COA Supplemental Manual for
detailed drainage requirements and BMP S633 in Volume IV of the COA
Supplemental Manual for methods of run-on prevention. If discharging to a sanitary
sewer, permits must be obtained from the City of Auburn Permit Center. Call 253-
931-3020 for more information.
If a holding tank is used for storage of wash water, the contents must be pumped out
before it is full and disposed of appropriately to a sanitary sewer or wastewater
treatment system.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but can provide additional pollution protection:
A cover should be placed over a designated wash area to keep rain from falling on
dirty equipment and producing contaminated runoff.
Implement one or more of the following treatment BMPs in addition to the required
BMPs:
o Oil/water separator.
o Wet vault for settling.
o Infiltration basin with pretreatment.
o Filtration with media designed for the pollutants present.
S672: BMPs for Collection and Disposal of Wastewater in Mobile Interior Washing
Operations (Additional)
Description of Pollutant Sources
This applies to businesses that wash floors, carpets and other interior items on a mobile site-to-
site basis. The typical washing process includes removing wax and use of machines that spray
the wash solution onto the carpet or upholstery and then suck the dirty solution up into a
portable tank with limited capacity.
Pollutants of concern consist of nutrients, suspended solids, organic compounds (such as
pesticides and chemicals used for flea and odor control), biochemical oxygen demand (BOD),
and chemical oxygen demand (COD).
Pollutant Control Approach
Wastewater must be poured into a sanitary sewer drain at the site of collection, the business
office, or at another proper location. If sanitary sewer disposal is not available or not allowed,
the collected wastewater must be returned to the business site for process treatment or transfer
to a holding tank.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
142
Absolutely no wastewater from mobile interior wash activities shall be disposed of
outdoors, or to a drain connected to the storm drainage system.
Wastewater from mobile washing operations shall be discharged to a sanitary sewer if it
does not contain high concentrations of toxic materials. Some of the chemicals used for
flea and odor control are listed by EPA as toxics. Contact the City of Auburn Sanitary
Sewer Utility at 253-931-3010 if you intend to use and discharge these types of
chemicals. All wastewater must be poured into a sanitary sewer drain at the site of
collection, the business office, or at another proper location.
If sanitary sewer disposal is not available or not allowed at the site of collection, the
collected wastewater must be returned to the mobile business site for process treatment
or transfer to a holding tank. See BMP S675 in Volume IV of the COA Supplemental
Manual for details on these drainage/disposal alternatives.
Carpet cleaning wash water must be disposed of to the sanitary sewer. It is preferred
that the dirty wash water be discharged into a toilet or sink at the place where it was
generated. Alternatively, the carpet cleaner may discharge the water into the sanitary
sewer back at their place of business if located in Auburn. Otherwise, they must contact
the sewerage agency providing their service for that agency’s approval.
Additional Guidelines for the City of Auburn
The following BMPs are not required, but can provide additional pollution protection:
Use the least toxic detergents and cleaners that will get the job done. Select non-
phosphate detergents when possible.
Limit the amount of water used in interior washing operations. This will save you time,
money, and effort when it comes to proper disposal.
Recycle wash water for more than one use.
S673: BMPs for Manufacturing and Post-Processing of Metal Products (Additional)
Description of Pollutant Sources
This applies to businesses such as mills, foundries, and fabricators that manufacture or post-
process metal products. A variety of activities such as machining, grinding, soldering, cutting,
welding, quenching, cooling, and rinsing may take place. These businesses may be required to
obtain a National Pollutant Discharge Elimination System (NPDES) permit from the Department
of Ecology and/or an Industrial Wastewater Discharge Permit from King County-Metro. See
Section I-2.4 of the SWMMWW and COA Supplemental Manual for a discussion of NPDES
requirements. Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010 to determine if
an industrial wastewater discharge permit is necessary.
NOTE: Painting, finishing and coating of metal products is covered under BMP S420 in Volume
IV of the SWMMWW.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
143
Pollutants of concern include toxic organic compounds, heavy metals, oils and greases, pH,
suspended solids, and biological oxygen demand (BOD).
Pollutant Control Approach
Cover and contain operations and apply good housekeeping and preventive maintenance
practices to prevent the contamination of stormwater.
The following BMPs are required of all businesses engaged in metals manufacturing or post-
processing:
Eliminate illicit connections to the storm drainage system. See BMP S410 in Volume IV
of the SWMMWW for detailed information on identifying and eliminating illicit
connections.
Process wastewater (including contact cooling water, filter backwash, cooling tower
blowdown, etc.) and stormwater runoff from activity areas, must discharge to a sanitary
sewer, holding tank, or process treatment system before discharge to surface water or
storm drain. Contact the City of Auburn Permit Center at 253-931-3020 to obtain permits
for discharge to the sewer. See BMP S675 in Volume IV of the COA Supplemental
Manual for detailed requirements.
Employees must be educated to control their work with metal products to minimize
pollution.
The activity area must be swept at the end of each work day to collect and dispose of
metal fragments and product residues properly. See BMP S630 in Volume IV of the
COA Supplemental Manual for disposal alternatives.
Additional Guidelines for the City of Auburn
The following BMPs are not required but can provide additional pollution protection:
Limit the amount of water used in quenching and rinsing. Recycle used water where
possible.
Cover the activity area to prevent rain from contacting the process and reduce the
amount of runoff that has to be detained or treated.
Use a catch basin filter or screen basket insert to capture stray metal particles.
Implement a program to track purchase and consumption of lubricants, solvents, and
additives. Check with operating managers for an explanation if consumption increases.
Recommend actions if significant equipment leaks or spills are identified.
Utilize any additional BMPs which are applicable for materials storage and maintenance
activities in your shop.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
144
S674: BMPs for Mining and Quarrying of Sand, Gravel, Rock, Peat, Clay and Other
Materials (Additional)
Description of Pollutant Sources
This applies to surface excavation and on-site storage of sand, gravel, and other materials that
are mined. All mining operations that have stormwater runoff from the site are required to apply
for a National Pollutant Discharge Elimination System (NPDES) permit with the Department of
Ecology. Ecology has specific BMPs required by the permit. Some additional BMPs to help
meet Ecology’s discharge performance standards are listed below. Other permits from the
Washington Department of Natural Resources and the City of Auburn may be required.
Pollutants of concern are suspended solids, nutrients, pH, oils, and metals.
Pollutant Control Approach
Provide containment and or cover for any on-site storage areas to prevent run-on and discharge
of suspended solids and other pollutants.
Measures to control track-out and dust shall be implemented. Wheel washes, sweeping and
paving high traffic areas are some common practices.
Additional Guidelines for the City of Auburn
If the material is appropriate, use excavated spoil material to form compacted berms
along downslope sides of the site to contain runoff. Berms should be seeded to promote
growth of grass or other vegetation to limit erosion from the berms. Safety measures to
prevent flooding due to berm failure shall be considered.
Semi-permanent stockpiles should be seeded to promote vegetation growth which can
help to limit erosion from the stockpiles.
Use sediment ponds to promote settling of suspended solids. Refer to Volume V of the
SWMMWW for more information.
Use anchored tarps to cover stockpiles at small-scale mining operations if there is a
potential for contaminated stormwater to leave the site.
Provide containment and or cover for any on-site storage areas to prevent run-on and
discharge of suspended solids and other pollutants.
S675: BMPs for Discharging Process Wastewater to a Sanitary Sewer, Holding Tank, or
Water Treatment System (Additional)
This BMP is a minimum requirement for all industrial and commercial activities that generate
contaminated process wastewater, such as washing activities, composting activities, and
production and processing activities. The water used in these activities shall not drain to surface
waters or groundwater untreated. Process water must drain to a sanitary sewer, holding tank,
on-site treatment system, wastewater treatment system, or be recycled.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
145
In order to connect to the sanitary sewer, contact the City of Auburn Permit Center at 253-931-
3020 for information on sanitary sewer connection permits. Call the City of Auburn Sanitary
Sewer Utility at 253-931-3010 for pretreatment and permit information.
If a sanitary sewer is not available, the only remaining options are holding tanks or an on-site
wastewater treatment facility.
The contents of the holding tank must be pumped out or drained before the tank is full and
disposed of properly (see BMP S630 in Volume IV of the COA Supplemental Manual).
If the on-site wastewater treatment facility option is taken, then it must be designed to receive
and effectively treat all discharges of process water from the business. The Washington State
Department of Ecology must be contacted for approval of such a facility.
If the activity is to remain uncovered, then define a designated area for the activity and provide a
mechanism for prevention of stormwater run-on into the activity area. (e.g., a curb, dike, or
berm).The designated area shall be paved and sloped to a central collection drain and be
connected to the sanitary sewer, (with pretreatment if required), the on-site holding tank, or the
on-site treatment facility, whichever method is selected.
Monitoring and maintaining all collection systems and keeping records of inspections and
maintenance may be required.
S676: BMPs for Covering the Activity with a Roof or Awning (Additional)
In many cases, a simple roof or awning will protect the activity from coming into contact with
stormwater, and usually at a lower cost than a complete building. These structures require
building permits to construct. Contact the City of Auburn Permit Center at 253-931-3020 to
obtain permits.
The area of the roof cover shall be sufficient to prevent any precipitation from reaching the
covered materials. Provisions shall be made to prevent stormwater run-on into the covered
area. The installation of sumps or sanitary sewer drains may also be necessary. Roof drains
shall discharge outside and be directed away from the covered area.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
146
Appendix IV – C Quick Reference Phone Numbers
City of Auburn
Storm Drainage and Sewer Utilities (Public Works Department) 253-931-3010
Permit Center 253-931-3020
Community Development 253-931-3090
Utility Billing 253-931-3038
Waste Management (Garbage and Recycle) 253-939-9792
Household Hazardous Waste (King County) 206-296-4692
Valley Regional Fire Authority 253-288-5800
Environmental Protection Agency (EPA) - Region X 800-424-4372
IMEX (Industrial Materials Exchange) 206-263-8465
King County
Wastewater Program (septic) 206-477-8050
Household Hazardous Waste 206-296-4692
Tacoma-Pierce County Health Department
On-Site Sewage 253-798-6470
Hazardous Waste Line 800-287-6429
University of Washington Center for Urban Water Resources 253-254-7030
Washington State Department of Agriculture 360-902-1800
Washington State Department of Ecology 360-407-6000
Northwest Regional Office 206-594-0000
Dangerous/Hazardous Waste 425-649-7000
NPDES Stormwater or Wastewater Permits 360-407-6000
Spill Reporting (spill to water) 800-424-8802
and 800-258-5990
Spill Reporting (spill to land King County) 206-594-0000
Spill Reporting (spill to land Pierce County) 360-407-6300
Recycling 800-732-9253
Groundwater Quality and Protection 360-407-6000
Underground Ground Storage Tanks 800-826-7716
Washington State University/Pierce County Cooperative Extension 253-798-7180
Puget Sound Clean Air Agency 206-343-8800
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
147
Appendix IV – D Source Control BMPs for Homeowners
Actions taken each day in and around homes have a profound effect on surface water quality and
fish habitat in this region. Stormwater goes directly to rivers, streams and to Puget Sound.
Stormwater does not go to the wastewater treatment plant. Any pollutants that get into the
stormwater go directly to surface water. Small amounts of pollution from many different sources can
significantly affect our waterways. Yard maintenance, waste storage, car washing and maintenance,
and pool cleaning are some of the activities that can adversely impact water quality. The best
management practices (BMPs) discussed in this section are practical ways to keep stormwater from
becoming polluted in the first place. It is recommended that all residents in Auburn use these BMPs.
Please note that some of these procedures are required by various state, or city laws, and are noted
as required BMPs.
A general list of BMPs for homeowners is described in this chapter. Some of the BMPs described in
Chapter 4 may also be applicable to homeowners.
Automobile Washing
Automobile Maintenance
Storage of Solid Wastes and Food Wastes
Composting
Yard Maintenance and Gardening
Swimming Pool and Spa Cleaning and Maintenance
Household Hazardous Material Use, Storage, and Disposal
General Home Maintenance
Pet Waste
Automobile Washing (for Single-Family Residences)
Car washing at home will cause wash water to enter the storm system and flow untreated into
surface waters. Soaps and detergents, even the biodegradable ones, can have immediate and long-
term effects. Wash water from car washing could be considered an illicit discharge and is prohibited
from entering the stormwater system per Auburn City Code and state requirements.
Additional Guidelines for the City of Auburn
The best option is to take cars to a commercial car wash that has a recycle system and
discharges wastewater to the sanitary system for treatment. If this option is not feasible, use the
suggested BMPs below.
At Home
Wash cars directly over lawn areas or make sure the wash water drains to a
vegetated area.
Ideally, no soaps or detergents should be used, but if one is used, select one
without phosphates.
Consider using commercial products that allow cleaning a vehicle without water.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
148
Use a hose nozzle with a shut-off valve to save water.
Do not wash cars if rain is expected.
Pour the bucket of soapy, dirty wash water down your sink.
Away from Home
Take cars to a commercial car wash that has a recycle system and discharges
wastewater to the sanitary sewer for treatment.
Support fundraising activities that sell tickets for a wash at a commercial car
wash.
Automobile Maintenance
Recycle all oils, antifreeze, solvents, and batteries. Many local car parts dealers
and gas stations accept used oil. The King County Household Hazardous
Wastemobile makes regular, scheduled visits to Auburn and accepts oil, oil
filters, antifreeze, and solvents, (website:
http://www.hazwastehelp.org/HHW/proper-disposal.aspx or call 206-296-4692).
Old batteries can actually be worth money. Recycle old batteries at automotive or
battery shops.
Never dump new or used automotive fluids or solvents on the ground, in a storm
drain or street gutter, or in a waterbody.
Do not mix wastes. Always keep your wastes in separate containers which are
properly labeled and store them out of the weather.
Additional Guidelines for the City of Auburn
Fix all leaks, to keep the leaky material off the streets and out of the stormwater
system and local waterways.
To dispose of oil filters, punch a hole in the top and let drain for 24 hours. After
draining, wrap in 2 layers of plastic and dispose of in your regular garbage or
recycle by taking it to the King County Household Hazardous Waste
Wastemobile. Note: used oil must be recycled at an approved facility and cannot
be disposed of in your regular garbage. Use care in draining and collecting
antifreeze to prevent accidental spills. Spilled antifreeze can be deadly to cats
and dogs that ingest it.
A tarp, ground cloth, sheet of cardboard, drip pans, or other materials to contain
drips must be used beneath the vehicle or equipment to capture all spills and
drips. Keep a bag of kitty litter on hand to absorb spills. Sprinkle a good layer on
the spill, let it absorb and then sweep it up. Place the contaminated litter in a
double plastic bag (bag in a bag), tie it up, and dispose of it in your regular
garbage. Do not leave kitty litter out in the rain.
If body work is performed outside, be sure to use a tarp to catch material
resulting from grinding, sanding, and painting. Dispose of this waste by double
bagging in plastic and placing in garbage.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
149
Storage of Solid Wastes and Food Wastes
Improper storage of food and solid waste at residences can lead not only to water pollution problems,
but problems with neighborhood pets and vermin as well. Following the BMPs listed below can help
keep property a clean and healthy place to live.
All waste containers kept outside should have lids and be free of leaks. If the hauler
container lid is damaged or the container is leaking, please call the City of Auburn Utility
Billing at (253) 931-3038 for information on lid repair and container replacement.
Additional Guidelines for the City of Auburn
Store waste containers under cover if possible, or on grassy areas.
Inspect the storage area regularly to pick up loose scraps of material and dispose
of them properly.
Recycle as much as you can. The City of Auburn offers curbside recycling. Also,
look under "Recycling" in the phone book for firms which take other recyclables
or call the City of Auburn Solid Waste Division at (253) 931-3038.
Purchase products which have the least amount of packaging materials.
Recycle biodegradable materials such as grass clippings and vegetable scraps in
your yard waste cart instead of throwing them away. Call the City of Auburn
Utility Billing at (253) 931-3038 for more information on yard and food scrap
recycling.
Composting
Composting is an earth-friendly activity as long as the rules outlined below are followed. The
following BMPs are applicable to composting. For more information go to the City of Auburn’s
website at www.auburnwa.gov or call the City of Auburn Solid Waste Division at (253) 931-3038.
Locate compost piles on an unpaved area that is not subject to stormwater run-on or runoff;
that is not prone to water ponding during storms, and well away from wetlands, streams,
lakes and other drainage ways.
Additional Guidelines for the City of Auburn
Compost piles must be maintained and turned over regularly to work properly.
Large piles of unattended compost may create odor and vermin problems and
are not allowed within City limits.
Do not put hazardous or non-decomposable waste in the pile.
Cover the pile for two reasons:
o To keep excess water from cooling down the pile, which will slow down
the rate of decomposition.
o To keep stormwater from washing nutrients into waterways.
Building a small earthen dike around a compost pile is an effective means of
preventing nutrient-rich compost drainage from reaching stormwater paths.
An alternative to traditional backyard composting is worm composting. For more
information on getting started with worm composting, call the City of Auburn Solid
Waste Division at (253) 931-3038.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
150
Yard Maintenance and Gardening
This section deals with the normal yard maintenance activities typically performed at residences.
Overwatering, overfertilizing, improper herbicide application, and improper disposal of trimmings and
clippings can all contribute to serious water pollution problems. Following the BMPs listed below will
help alleviate pollutant runoff.
Follow the manufacturer's directions exactly for mixing and applying herbicides,
fungicides, and pesticides, and use them sparingly. Never apply when it is windy or
when rain is expected. Never apply over water, within 100 feet of a well-head, or
adjacent to streams, wetlands, or other waterbodies. Triple-rinse empty containers,
using the rinse water for mixing your next batch of spray, and then double-bag and
dispose of the empty container in your regular garbage.
Never dispose of grass clippings or other vegetation in or near storm drains,
streams, lakes, or Puget Sound.
Make sure all fertilizers and pesticides are stored in a covered location.
Additional Guidelines for the City of Auburn
Use natural, organic soil amendments. Visit
http://your.kingcounty.gov/solidwaste/naturalyardcare/ Click on Build healthy
soil.
Use an integrated pest management program (IPM), which is a natural, long-
term, ecologically based approach to controlling pest populations. See BMP
S411 and BMP S435 in Volume IV of the SWMMWW.
Follow manufacturer's directions when applying fertilizers. More is not better,
either for your lawn or for local waterbodies. Never apply fertilizers over water or
adjacent to ditches, streams, or other water bodies. Remember that organic
fertilizers have a slow release of nitrogen, and less potential to pollute then
synthetic fertilizers.
Save water and prevent pollution problems by watering lawns sensibly. Lawns
and gardens typically need the equivalent of 1-inch of rainfall per week. Put a
wide mouth jar out where watering is occurring and measure the water with a
small plastic ruler. Overwatering to the point of runoff can carry polluting nutrients
to the nearest waterbody.
Consider using native plants as a vegetated buffer zone adjacent to streams or
other water bodies. Call the Garden hotline at 206-633-0224 for advice and
assistance in developing a planting plan or visit
http://your.kingcounty.gov/solidwaste/naturalyardcare/ Click on Plant right for
your site.
Reduce the need for pesticides and fertilizers on lawns by improving the health of
the soil. Aerating, thatching, and topdressing with compost will improve soil
health and help wanted grasses compete with weeds and moss.
Use a mulching mower and mow higher to improve soil/grass health and reduce
or eliminate pesticide use.
Compost all yard clippings or use them as mulch to save water and keep down
weeds in your garden. See Composting BMP for more information.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
151
Practice organic gardening and virtually eliminate the need to use pesticides and
fertilizers. Contact King County Master Gardener Phone Clinic at 206-296-3440
for information and classes on earth-friendly gardening.
Pull weeds instead of spraying and get some healthy exercise, too. If you must
spray, use the least toxic formulations that will get the job done. The Master
Gardener program listed above can help advise you on which spray to use.
Work fertilizers into the soil instead of letting them lie on the ground surface
exposed to the next rain storm.
Plant vegetation suited to Northwest conditions because they require less water
and fewer to no fertilizers and pesticides.
The City of Auburn has a curbside yard waste recycling program. Call 253-931-
3038 for more information.
Swimming Pool and Spa Cleaning and Maintenance
Despite the fact that we immerse ourselves in it, the water from pools and spas is far from chemically
clean. Nutrients, pH, and chlorine can adversely affect fish and wildlife in waterbodies. Following the
requirements listed below and in BMP S433 in Volume IV of the SWMMWW will ensure the
cleanliness of your pool and the environment.
Pool and spa water must be dechlorinated if it is to be emptied into a ditch, on
the ground or a lawn, or to the storm drainage system. Contact a pool chemical
supplier to obtain the neutralizing chemicals needed. The rate of flow into the
ditch or drainage system must be regulated so that it does not cause problems
such as erosion, surcharging, or flooding. Contact the City of Auburn Strom
Drainage Utility at 253-931-3010 for the conditions for discharge approval. Water
discharged to the ground or a lawn must not cross property lines and must not
produce runoff.
Swimming pool cleaning wastewater and filter backwash shall not be discharged
to the municipal storm drainage system.
If pool and spa water cannot be dechlorinated, it must be discharged to the
sanitary sewer. Prior to draining a pool or spa, contact the City of Auburn
Sanitary Sewer Utility at 253-931-3010 for the conditions for discharge approval.
A pool service company can help determine the frequency of cleaning and
backwash of filters.
Diatomaceous earth used in pool filters cannot be disposed of in surface waters,
on the ground, or into storm drainage systems or septic systems. Dry it out as
much as possible, bag it in plastic, and dispose of at the landfill.
Additional Guidelines for the City of Auburn
Hire a professional pool service company to collect all pool water for proper
disposal. Make sure to ask where the water will be disposed of and ensure the
proper permits have been obtained.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
152
Household Hazardous Material Use, Storage, and Disposal
Oil-based paints and stains, paint thinner, gasoline, charcoal starter fluid, cleaners, waxes,
pesticides, fingernail polish remover, and wood preservatives are just a few hazardous materials
typically used in a residential setting.
When hazardous materials are dumped on the ground or in a storm drain, they can be washed
directly to receiving waters where fish and wildlife can be harmed. Hazardous materials can also
infiltrate into the ground and contaminate drinking water supplies. If disposed of with regular garbage,
hazardous chemical containers can leak at the landfill and contaminate groundwater. Groundwater
contamination can also occur if hazardous products are poured down a sink or toilet into a septic
system. Do not pour hazardous chemicals down the drain if household plumbing is connected to
municipal sewers, either. Many compounds will "pass through" the wastewater treatment plant
without treatment and contaminate receiving waters, or they can harm the biological process used at
the treatment plant, reducing overall treatment efficiency.
With such a diversity of hazardous products present in all homes in Auburn, a large potential for
serious environmental harm exists if improper methods of storage, usage, and disposal are
employed. Using the following BMPs will help keep these materials out of soils, sediments, and
waters.
Dispose of hazardous materials and their containers properly. Never dump
products labeled as poisonous, corrosive, caustic, flammable, inflammable,
volatile, explosive danger, warning, caution, or dangerous outdoors, in a storm
drain, or into sinks, toilets or drains. Call the King County Hazardous Waste Line
at 206-296-4692 or 1-888-TOXIC ED for information on disposal methods,
collection events, and alternative products. Household hazardous wastes from
City of Auburn residents are accepted at King County Household Hazardous
Waste Facilities.
Store hazardous materials containers under cover and off the ground. Keep
them out of the weather to avoid rusting, freezing, cracking, labels being washed
off, etc.
Additional Guidelines for the City of Auburn
Store hazardous materials out of the reach of children.
Check hazardous material containers frequently for signs of leakage. If a
container is rusty and has the potential of leaking soon, place it in a secondary
container before the leak occurs and prevent a clean-up problem.
Store hazardous materials containers under cover and off the ground. Keep them
out of the weather to avoid rusting, freezing, cracking, labels being washed off,
etc.
Keep appropriate spill cleanup materials on hand. Kitty litter is good for many oil-
based spills.
Ground cloths and drip pans must be used under any work outdoors which
involves hazardous materials such as oil-based paints, stains, rust removers,
masonry cleaners, and others bearing label warnings as outlined above.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
153
Latex paints are not a hazardous waste but are not accepted in liquid form at the
landfill. To dispose, leave uncovered in a protected place until dry, then place in
the garbage. If you wish to dry waste paint quickly, pour kitty litter in the can to
absorb the paint. Once paint is dry, leave the lid off when you place it in the
garbage so the garbage collector can see that it is no longer liquid.
Use less toxic products whenever possible. The King County Household Hazards
Line at 206-296-4692 or 1-888-TOXIC ED and the Washington Toxics Coalition
at 206-632-1545 have information detailing alternatives to toxic products.
If an activity involving the use of a hazardous material can be moved indoors out
of the weather, then do so. Make sure proper ventilation is provided.
Follow manufacturers' directions in the use of all materials. Over-application of
yard chemicals, for instance, can result in the washing of these compounds into
receiving waterbodies. Never apply pesticides when rain is expected.
When hazardous materials are in use, place the container inside a tub or bucket
to minimize spills.
Purchase only the amount of product that is needed.
General Home Maintenance
This section deals with the normal maintenance activities typically performed in residential settings.
Following the BMPs listed below will help alleviate pollutant runoff.
Pressure washing of building facades, rooftops, pavement, and other large objects must
be conducted in such a way that all of the runoff is collected for proper disposal. No
runoff shall leave the site. Temporary curbs, dikes, or berms may be used to direct the
water away from storm drains. Sweep up and collect debris for disposal as solid waste
as an alternative to pressure washing.
Additional Guidelines for the City of Auburn
Carpet cleaning wash water must be disposed of to the sanitary sewer. It is preferred that the
dirty wash water be discharged into a toilet or mop sink at the place where it was generated.
Clean brushes and tools coated with non-water-based paints, finishes, or other materials in a
manner that allow collection of used solvents (e.g., paint thinner, turpentine, xylol, etc.) for
proper disposal at a Household Hazardous Waste Facility. Call the King County Household
Hazards Line at 206-296-4692 or 1-888-TOXIC ED for information on disposal methods,
collection events, and alternative products. Household hazardous wastes from City of Auburn
residents are accepted at King County Household Hazardous Waste Facilities.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume IV Source Control BMP Library 2022 Version 1
154
Pet Waste
Pets can generate pollutants from fecal deposits, animal washing, and cage or kennel cleaning.
Pollutants include bacteria which can pollute water ways and make people sick. To prevent pet waste
pollutants from entering the storm drains, follow the requirements listed below. For more information,
see BMP S440 in Volume IV of the SWMMWW.
Regularly scoop, sweep and clean up pet waste deposited on walks and at home.
Dispose of pet waste in the garbage or flush it down the toilet.
When cleaning out cages and kennels, wash directly over lawn areas or make sure
the wash water drains to a vegetated area. Alternately, dispose of the wash water
down the toilet or a mop sink.
Do not dispose of unused pet pharmaceuticals in a storm drain, or flush down a
toilet, or wash down a sink. Visit www.takebackyourmeds.org for a list of locations
that take back unused pharmaceuticals.
Kennels, boarding facilities, veterinarians: Refer to S402 BMPs for Commercial
Animal Handling Areas in Volume IV of the SWMMWW.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
155
Volume V
Runoff Treatment, Flow Control,
and LID BMP Library
City of Auburn Supplemental Manual
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
156
Executive Summary of Volume V - Runoff Treatment, Flow Control,
and LID BMP Library
Volume V of the City of Auburn (COA) Supplemental Manual to the Department of Ecology’s
(Ecology) Stormwater Management Manual for Western Washington (SWMMWW) provides
additional guidance to comply with Minimum Requirement #5 – On-Site Stormwater
Management, Minimum Requirement #6 – Runoff Treatment, and Minimum Requirement #7 –
Flow Control.
The Ecology SWMMWW is available online at the link below:
2019 SWMMWW
The COA Supplemental Manual Volume V is organized to correspond to the SWMMWW
Volume V. This Volume should be used in conjunction with the SWMMWW to select, design,
and maintain BMPs for on-site stormwater management, treatment, and flow control facilities
for installation within the City of Auburn. Additional requirements for SWMMWW BMPs for use
within the City of Auburn are indicated as “Amended”, as noted below.
Important additions and changes contained in the COA Supplemental Manual for this Volume
include:
V-1: General BMP Design gives additional general requirements for facilities in the City
of Auburn.
o V-1.1 provides requirements for runoff from the right of way.
o V-1.2 provides requirements for setbacks from slopes and embankments.
V-4: Roof Downspout BMPs gives additional general requirements for installation of
downspouts including setbacks and collection and conveyance requirements.
V-5: Infiltration BMPs
o V-5.6 Site Suitability Criteria (Amended) provides additional requirements to
protect Auburn’s groundwater drinking water supply.
o SSC-1 Setback Criteria (Amended)
o BMP T5.15 (Amended) provides requirements on when Permeable Pavements
can be used.
o BMP T7.30 (Amended) provides additional design criteria for Bioretention
Facilities.
V-10: Manufactured Treatment Devices as BMPs
o V-10.3 gives requirements on the use of proprietary products for Private and
Public treatment facilities for runoff from pollution-generating hard surfaces.
V-11: Miscellaneous LID BMPs
o BMP T5.13 (Amended) requires that projects in the City of Auburn follow the
guidelines and procedures found in Guidelines and Resources For Implementing
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
157
Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual
for Western Washington.
V-12: Detention BMPs
o V-12.2 Control Structure Design (Amended)
o V-12.2 Other Detention Design Options (Amended)
o BMP D.1 Detention Ponds provides specific requirements for the design and
construction of detention ponds in Auburn.
o BMP D.2 Detention Vaults provides specific requirements for the design and
construction of detention vaults in Auburn.
Sections with No Changes
The following portions of Volume V of the SWMMWW do not require any additional clarification
in the COA Supplemental Manual, and together with the COA Supplemental Manual, contain
the requirements and guidelines for stormwater management in the City of Auburn.
V-1: General BMP Design
o V-1.3 and 1.4 (all subsections)
V-2: Site Design BMPs (all sections)
V-3: Dispersion BMPs (all sections)
V-4: Roof Downspout BMPs
o BMPs T5.10A, T5.10B, and T5.10C (all subsections)
V-5: Infiltration BMPs
o V-5.1, 5.2, 5.3, 5.4, and 5.5 (all subsections)
o BMPs T7.10, T7.20, T7.30, and T7.50 (all subsections)
V-6: Filtration BMPs (all sections)
V-7: Biofiltration BMPs (all sections)
V-8: Wetpool BMPs (all sections)
V-9: Pretreatment BMPs (all sections)
V-10: Manufactured Treatment Devices as BMPs
o V-10.1, 10.2, and 10.4 (all subsections)
V-11: Miscellaneous LID BMPs
o BMPs T5.14, T5.16 through T5.20 (all subsections)
V-12: Detention BMPs (all sections)
o V-12.1
V-13: Oil and Water Separator BMPs (all sections)
Appendix V-A
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
158
V-1 – General BMP Design
V-1.1 Sequence of Runoff Treatment and Detention BMPs
The following requirements apply to treatment of runoff from the public right of way.
Stormwater runoff treatment systems utilized in the public right of way shall comply with
all other relevant design standards including requirements for street tree clearance and
height. Systems that, in the opinion of the City Engineer, do not have adequate
settling/debris separation chambers/basins, may conflict with existing or future utilities,
or do not have acceptable maintenance requirements shall not be allowed.
All storm facility landscape planting and seeding plans shall be prepared and sealed by
a Washington State licensed professional Landscape Architect or similar specialist
approved by the City of Auburn.
With approval from the City Engineer, stormwater runoff from required improvements in
the public right-of-way may be designed to discharge onto the private property and be
mitigated within the on-site storm drainage system with pretreatment of the runoff from
the public right of way. In this case, the applicant would be required to enter into a
Stormwater Easement and Maintenance Agreement (SWEMA) holding the City harmless
for the connection, and the applicant shall agree to maintain the system. The SWEMA
would be per the City’s template document without revisions that would place additional
liability or responsibility upon the City.
V-1.2 Setbacks, Slopes, and Embankments
Project proponents should consult the Auburn City Codes 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. Auburn City
Code titles and chapters that are relevant to setback requirements are as follows:
Chapter 13.16 Fire Hydrants – Requirements related to fire hydrant visibility
Title 15 Buildings and Construction – Requirements meeting building and fire
regulations
Chapter 16.10 Critical Areas – Requirements for wetland, stream, wildlife habitat area,
and geologic hazard area buffer setbacks
Title 18 Zoning – Requirements for lot line setbacks in all zoning classifications
Required setbacks for storm facilities are as follows:
Minimum spacing between trenches shall be 4 feet measured from the edge of trench.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
159
All 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 1:1 side slope for future
excavation and maintenance.
All 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 1:1 side slope for
future excavation and maintenance.
All facilities shall be setback from sensitive areas, steep slopes, landslide hazard areas,
and erosion hazard areas as governed by the Auburn City Code. Runoff discharged 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 greater than 15%
or above erosion hazard areas without evaluation by a geotechnical engineer or qualified
geologist and City approval.
For sites with septic systems, all infiltration systems, unlined wetponds and detention
ponds shall be downgradient of the drainfield unless the site topography clearly prohibits
subsurface flows from intersecting the drainfield.
Additional setbacks for specific stormwater facilities will be noted in the appropriate section.
V- 4 – Roof Downspout BMPs
V-4.1 Introduction to Roof Downspout BMPs
The roof downspout control Best Management Practices (BMPs) listed in V-4 of the SWMMWW
are subject to the setback requirements defined in the Auburn City Code.
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.
Collect and Convey
Conveyance of roof runoff to the City stormwater system is allowable when all roof downspout control
BMPs listed in V-4 of the SWMMWW have been determined to be infeasible by the City Engineer or
his/her designee. Conveyance from roof runoff shall be connected to the City stormwater system at a
catch basin or manhole. If a catch basin or manhole is not located at the discharge location, a storm
main extension shall be required.
The runoff shall not be conveyed over driveways, sidewalks or other areas reserved for pedestrian
traffic. A detail for the connection shall be submitted to the City for review and approval. Capacity
analysis of the conveyance piping and catch basin leads will be required to ensure that adequate
capacity exists.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
160
For roof areas 10,000 sf and greater, please refer to Minimum Requirement #7, Flow Control.
Conveyance and discharge to the curb is allowable for single family homes when all roof downspout
control BMPs listed in V-4 of the SWMMWW and a direct connection to the City stormwater system
has been determined to be infeasible by the City Engineer or his/her designee. 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 3 inches in diameter for single
family homes. A detail for the curb discharge shall be submitted to the City for review and approval.
No flow credits will be allowed for the collect and convey option.
V-5 – Infiltration BMPs
V-5.6 Site Suitability Criteria (SSC) (Amended)
Perform a site suitability analysis per V-5 of the SWMMWW for all infiltration and bioretention
facilities. Due to Seasonal High Groundwater, Groundwater Protection Zones, and other site
conditions, the City of Auburn restricts the use of infiltration in certain areas. Refer to Appendix
I-H of the COA Supplemental Manual for more information on infiltration infeasibility when
selecting the List Option to meet Minimum Requirement #5 On-Site Stormwater Management.
SSC-1 Setback Criteria (Amended)
Refer to V-1.2 of the COA Supplemental Manual for general stormwater facility setback requirements
and Auburn City Code titles and chapters relevant to setback requirements. 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.
BMP T5.15 Permeable Pavements (Amended)
Permeable pavements installed within the City of Auburn, including porous asphalt, pervious
concrete, and permeable pavers, shall comply with the City of Auburn Engineering Design and
Construction Standards, including the following additional Design Criteria:
All permeable pavements, including subgrade, base materials, and surface treatments,
shall conform to the City of Auburn Engineering Design and Construction Standards.
Permeable pavements shall have a minimum separation of 1 foot from the bottom of the
lowest gravel base layer and/or treatment layer to the Seasonal High Groundwater
Elevation, bedrock, or other low permeability layer as determined by a geologist or
engineer licensed in the State of Washington.
Geosynthetic fabrics shall be installed between the subgrade and base layer for soil
separation and stabilization. Geosynthetic fabrics used with permeable pavements shall
meet the specifications defined in the City of Auburn Engineering Construction
Standards.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
161
Permeable pavement road designs shall include conventional stormwater conveyance
design with sufficient capacity per Appendix III-D of the COA Supplemental Manual, and
the City’s Engineering Design and Construction Standards. The purpose of including
conventional stormwater conveyance systems with permeable pavement designs is to
provide adequate drainage capacity for storm events that exceed the capacity of the
permeable pavement.
Underdrain systems for permeable pavement surfaces are subject to approval of the City
Engineer or his/her designee.
Acceptance and/or infiltration testing for all permeable pavement surfaces shall be
performed per the City of Auburn Engineering Construction Standards.
Permeable pavements shall not be allowed for roadways with over 400 ADT and greater
than very low truck traffic. Very low truck traffic shall be 2% or less of the total ADT for
the roadway and includes vehicles with a FHWA Vehicle Classification of 4 or 5.
Permeable pavements shall not be allowed on any roadway with vehicle(s) with a FHWA
Vehicle Classification of greater than 5.
The adjusted native soil infiltration rate beneath all proposed permeable pavements shall
be a minimum of 0.1 inches/hour for public facilities. The adjusted infiltration rate is
determined by applying appropriate correction factors the measured saturated hydraulic
conductivity per V-5.4 of the SWMMWW.
The entire surface of all permeable pavements shall be accessible by maintenance
equipment.
To avoid clogging permeable pavements, pervious areas such as lawn and landscape
areas shall not drain to permeable pavements.
The drainage of additional impervious area to permeable pavements located on private
property shall not exceed a maximum ratio of 1:1. Downspout outlets or ground level
impervious surfaces shall not drain more than 1,000 sq. ft. to a single point on the
permeable pavement.
Permeable pavements located within the public right-of-way shall not receive drainage
from surfaces located on private property.
All permeable pavement located in the public right of way shall be constructed of porous
asphalt or pervious concrete. Permeable pavers and plastic or concrete grid systems
shall not be used in the public right of way.
Permeable pavement designs shall demonstrate compliance with the Site Suitability
Criteria for Permeable Pavement found in V-5.4 of the SWMMWW.
The following maximum longitudinal slopes shall apply to permeable pavements:
o Porous Asphalt – 5% maximum slope
o Pervious Concrete – 10% maximum slope
o Permeable pavers – 12% maximum slope
Permeable pavements with a longitudinal slope greater than 3% shall incorporate check
dams into the subgrade design to reduce subsurface flow rates. Permeable pavements
with a longitudinal slope greater than 5% shall incorporate terraced subgrades and
baffles to reduce uneven ponding.
Permeable pavements shall not be allowed in the following areas:
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
162
o At sites defined as “high use” in the Glossary of the SWMMWW.
o In areas with “industrial activity” as identified in 40 CFR 122.26(b)(14).
o At sites where the risk of concentrated pollutant spills is higher than average,
including but not limited to gas stations, truck stops, and industrial chemical
storage sites.
o Where routine, heavy applications of sand occur in frequent snow zones to
maintain traction during weeks of snow and ice accumulation.
o In areas of required pavement design elements where the material properties of
permeable pavements present significant constructability issues as determined
by the City Engineer, including but not limited to ADA ramps, driveway aprons,
and curb transitions.
o Within 50 ft of or on slopes greater than 20%.
o Within 10 ft of any known underground storage tank and connecting underground
pipes, regardless of tank size.
Permeable pavement located on sites within the City of Auburn Groundwater Protection
Zones 1 & 2 requires approval by the City Engineer. Permeable pavements within these
Zones shall incorporate advanced treatment.
If the proposed permeable pavement is considered a pollution-generating surface and is
located within the public right-of-way, the subgrade shall meet all of the following criteria
to a depth of one foot below the lowest gravel base layer:
o Cation Exchange Capacity is greater than 5%
o Organic Content is greater than 1%
o Measured (initial) saturated hydraulic conductivity is less than 9 in./hr.
o Depth of infiltration treatment layer is equal to or greater than 18 inches.
BMP T7.30 Bioretention (Amended)
Design Criteria:
All bioretention facilities receiving stormwater exclusively from private sources shall be
located on private property and privately maintained per the approved Operations and
Maintenance manual for the facility.
Bioretention facility designs shall be prepared by a professional engineer licensed in the
State of Washington. Bioretention facility designs shall be clearly labeled and include the
following components:
o Ponding Zone: From the top of the Bioretention Soil Media to the overflow
elevation provide a minimum 6 inch to maximum 12 inch ponding zone.
o Overflow: A piped or overland overflow structure is required. The overflow shall
discharge to the public system per the City of Auburn Engineering Standards. A
piped overflow is required when the underlying native soil has a measured
infiltration rate below 0.25 inches per hour. Overland overflow paths shall be
stabilized with streambed cobble per WSDOT 9-03.11(2) and direct to an
approve downstream drainage area. The inlet elevation shall allow for a minimum
of 6 inches freeboard.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
163
o Inlet: The inlet shall be designed and constructed with the appropriate slope and
elevation to allow for the free flow of stormwater into the facility The facility inlet
shall include a 6 inch thick layer washed drain rock or streambed cobble per
WSDOT 9-03.11(2) (2 to 4 inch minimum) pad to transition from inlet or splash
pad to Bioretention Soil Media to dissipate energy and/or disperse flow. The
energy dispersion pad shall have a minimum width equal to the inlet width.
o Dimensions: Provide a flat bottom with a minimum width of 1 foot. Side slopes of
bioretention facilities shall be a maximum 2.5:1 adjacent to sidewalks, maximum
4:1 at sheet flow inlets, and maximum 3:1 in all other locations.
Public bioretention planter boxes require prior approval from the City Engineer.
Public bioretention cells and swales shall be sodded or seeded using the seed mixtures
in Error! Reference source not found. below.
Grass Seed Mixes for Public Bioretention 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
Trifolium 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 Hand seed @ 2 lbs/1000 square feet
Table V-7.4-1 Grass Seed Mixes for Public Bioretention Facilities
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
164
V-10 – Manufactured Treatment Devices as BMPs
V-10.3 Approval Process for Manufactured Treatment Devices
All emerging technologies that have received General Use Level Designation (GULD) may be
considered for use on privately owned and maintained stormwater treatment facilities.
Currently, the only proprietary technologies approved for City maintained and owned stormwater
treatment facilities are the Modular Wetland without vegetation, BioPod without vegetation, and
the Boxless BioPod for treatment flows equal to or greater than 3 cfs. All Modular Wetland units
shall have either manhole access lids or spring assisted hatches.
V-11 – Miscellaneous LID BMPs
V-11.1 Introduction to Miscellaneous LID BMPs (Amended)
BMP T5.13 Post-Construction Soil Quality and Depth (Amended)
The City of Auburn requires using the guidelines and procedures found in Guidelines and
Resources For Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater
Management Manual for Western Washington when implementing BMP T5.13. This document
is available at:
http://www.soilsforsalmon.org/pdf/Soil_BMP_Manual.pdf
V-12 – Detention BMPs
V-12.2 Control Structure Design (Amended)
Design Criteria
Control structures shall meet the following requirements:
Access opening shall be oriented in a manner to facilitate inspection and maintenance.
The City of Auburn Standard Detail S-09 shall be used as a design guideline and be
updated to be site specific, and shall supersede the control structure details found in the
SWMMWW
V-12.3 Other Detention Design Options (Amended)
Use of Parking Lots for Additional Detention
The depth of water detained shall 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
165
The emergency overflow elevation shall be a minimum of 1 foot 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 (ADA) required parking stalls or adjacent ADA access.
Use of Alternate Storm Detention Facilities (Additional)
Alternate Storm Detention Facilities are defined as detention facilities that are not
classified as ponds, tanks, or vaults.
Fabric-wrapped rock Storm Detention Facilities are not allowed.
Use of Alternate Storm Detention Facilities are subject to deviation approval except if the
facility is one of the following allowed proprietary Alternate Storm Detention systems or
their equivalents: RTanks, StormTech, ChamberMaxx.
Deviations require documentation that addresses ground water levels, buoyancy, owner
inspection, cleaning and other maintenance activities, and City inspection requirements.
BMP D.1 Detention Ponds
Design Criteria
The design criteria in this section are specific to detention ponds located within the City. Many of
the criteria also apply to infiltration ponds, water quality wetponds, and combined
detention/wetponds in Volume V of the SWMMWW. All detention ponds shall be appropriately
and aesthetically located, designed, and planted.
Private ponds must adhere to the design criteria for detention ponds presented in V-12 and
BMP D.1 of the SWMMWW.
Pre-approval of the design concept, including landscaping, is required by the City for all
proposed public ponds. All proposed public ponds are subject to the following minimum design
criteria in addition to the criteria presented in the SWMMWW.
General
Plans for projects that propose to construct temporary stormwater management
facilities that will ultimately become permanent facilities are required to show how
those temporary facilities will be improved to become the permanent facilities
including, but not limited to, sequencing/phasing.
Detention ponds shall be designed using rounded shapes and variations in slopes to
provide a more visually aesthetic appearance and avoid a uniform shape on all
sides.
The total maximum depth of the detention pond from the bottom to the emergency
overflow water surface elevation shall be 15 feet.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
166
A 3-foot wide bench shall be provided at the 10-year storm storage elevation except
where there are walls forming the sides of the pond. In this case, the bench shall
terminate at the wall.
No rock, ecology block, gabion or mesh basket pond cell dividing walls are allowed.
The design of the cell dividing wall shall meet the following criteria:
o The minimum width of the pond adjacent to the wall shall be 10 feet.
o The wall shall be no higher than 3’ if the width of the adjacent pond is 10-15
feet wide and a maximum of 5 feet high if the pond width is 15-30 feet. Wall
heights for areas exceeding this width will be reviewed on a case-by-case
basis.
o Constructed of minimum 3,000 psi structural reinforced concrete.
o Walls must be watertight cast-in-place concrete.
o The walls are designed and stamped by a structural engineer licensed in the
State of Washington and structural calculations are provided.
Side Slopes
For maintenance and aesthetic reasons, pond designs should minimize structural
elements such as retaining walls. For ponds where retaining walls are required, they
shall be limited to a maximum of three sides.
Pond walls may be vertical retaining walls, provided:
o The maximum height of pond walls shall be 15 feet.
o All pond walls shall be retaining walls with soil backfill. Free-standing pond
walls with exposed exterior sides are considered impoundments and shall not
be allowed.
o They are constructed of minimum 3,000 psi structural reinforced concrete.
o Walls must be watertight cast-in-place concrete.
o At least 25% of the pond perimeter shall be a vegetated soil slope not steeper
than 3H:1V as measured at the 100-year water surface elevation.
o Access for maintenance per Appendix I-J of the COA Supplemental Manual
shall be provided.
o The walls are designed and stamped by a structural engineer licensed in the
State of Washington and structural calculations are provided.
o Pond wall drains are required to be routed around the walls and connected
directly into the pond.
o Ladders or other safety measures may be required.
Emergency Overflow Spillway
An emergency overflow spillway shall be provided and designed according to the
criteria given in the SWMMWW.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
167
Access
Refer to the COA Supplemental Manual Appendix I-J – Stormwater Facility Access
Requirements and to COA Design Standards Chapter 6 for detention pond access
criteria.
Per COA Design Standards Chapter 6, all pond access ramps shall be constructed with
a minimum of 8” compacted base course topped with a minimum of 2” compacted top
course. The pond ramp subgrade shall be compacted to a minimum of 95% modified
Proctor.
Fencing
The following fencing shall be provided for all detention ponds:
Fencing is required at the 10-year storage elevation and shall be installed on the
outside edge of a 3-foot wide bench. Fencing is required at the top of all vertical
walls.
Fences shall be 48 inches in height (see WSDOT Standard Plan L-20.10-03, Type 4
chain link fence).
Access gates to the interior of the pond shall be a minimum 15 feet in width
consisting of two swinging sections of equal width.
Access gates will be set back a minimum of 20 feet from the point of entry to the
public right-of-way.
Additional access gates may be required where access for maintenance (such as to
a control structure(s) or for mowing) is limited by the configuration of the pond and
fencing. Additional access roads shall be provided to the additional access gates.
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-20.10-03 for
Types 3 or 4 chain link fence.
Setbacks
Setbacks for detention ponds shall include the following:
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
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
168
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 set back 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 licensed in Washington State. The discharge point shall not be
placed on or above slopes 15% or greater, or above erosion hazard areas without
evaluation by a geotechnical engineer or qualified geologist licensed in Washington
State and approval by the City Engineer or his/her designee.
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
Seeps and springs that produce continuous intercepted flows on the project site shall be considered
during the design process and included in the Stormwater Site Plan report. Flow monitoring of
intercepted flow may be required for design purposes.
Planting Requirements
The following planting requirements shall be provided for all detention ponds and shall be noted on
the plans:
The pond side slopes shall be scarified, and all vegetative and construction debris and rocks
larger than 2 inches shall be removed prior to hydroseeding.
Hydroseeding shall be applied over the entire detention pond bottom (or on the slopes above
the water quality elevation for wetponds) up to the fence line or to the maximum design water
level (overflow elevation), whichever is higher.
The pond bottom and all interior side slopes per the requirement above shall be hydroseeded
with an appropriate grass seed mixture per Table V-12.3-2 Grass Seed Mixes for
Detention/Retention Facilities below, which shall be listed on the landscaping plans.
The need for watering to establish the hydroseeding will depend on what time of year the
hydroseeding is applied and how long it takes for the seed to be established. If a temporary
irrigation system is used, it shall be attached to the fence or placed at locations that do not
interfere with mowing.
All pond construction, landscaping, and temporary irrigation, if used, shall be completed prior
to scheduling an initial punchlist inspection. The grass shall have sprouted over at least 80%
of the area to be seeded.
The contractor is responsible for weed control and removal of litter prior to planting and
throughout the plant establishment period.
The contractor is responsible for removing the temporary irrigation system prior to expiration
of the 1-year warranty period.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
169
Landscaping
The following landscaping requirements shall be provided stormwater tracts and shall be noted on
the plans:
All remaining areas of the tract shall be planted with grass or be landscaped (except as noted
below) and mulched with a 4-inch cover of shredded wood mulch. Multiple plantings and
mulching may be required until grass has established itself. A bond may be required to
guarantee vegetation stabilization for detention facilities.
Public and private storm drainage facilities should enhance natural appearances and be
appropriate to the use of the site and the surrounding area. Landscaping shall be designed to
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). No tree or shrubs shall be planted at or below the 10-year storage elevation,
within the fenced area of the pond, or in public easement areas.
The following criteria shall be incorporated when designing landscaping for storm drainage tracts
above the 10-year storage elevation, excluding the fenced area for the pond and public easement
areas.
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 Title 18 of the ACC. 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 Title 18 of the ACC. Diameter measurements are taken at four feet (4’)
above grade elevation. Authorization by the City is required for removal of any
significant trees.
Select tree and shrub species from Table V-12.2-1 Plant Selection Guide below. Plant
choices must reflect the functional and aesthetic needs of the site. Fall planting is
recommended for optimal acclimation and survivability.
Permanent irrigation is only allowed above the 10-year storage elevation within those
portions of the Tract to be maintained by the HOA.
Appropriate grass seed mixes for detention ponds are given in Table V-12.3-2 Grass
Seed Mixes for Detention/Retention Facilities below.
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
identified in the Guide are predominately native and reflect the soil conditions and
water regimes of the area.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
170
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).
No tree or shrub planting is allowed within pipeline easements, traveled surfaces, or
over underground utilities.
Trees shall be placed no closer than 10 feet and shrubs no closer than 5 feet from all
pond fences.
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.
The following tables 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
171
Tree Selection for Storm Drainage Facilities
Suggested Trees
1.Botanical
Name
2.Common
Name
Tolerates Wet
to Saturated
Soils
Recommend
Moderately Wet to
Dry Soils
Recommen
d Dry Soils
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 3.♦
Salix scouleriana Scouler Willow ♦
Salix sitchensis Sitka Willow ♦
Thuja plicata Western Red
Cedar ♦
Tsuga heterophylla Western
Hemlock ♦
Shrub Selection for Storm Drainage Facilities
Suggested Shrubs
4.Botanical
Name
5.Common
Name
Tolerates Wet
to Saturated
Soils
Recommend
Moderately Wet to
Dry Soils
Recommen
d Dry Soils
Amelanchier alnifolia Serviceberry ♦
Cornus sericea Red Osier
Dogwood ♦
Gaultheria shallon Salal ♦
Holidiscus discolor Ocean Spray ♦
Lonicera involucrata Black Twinberry ♦
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
172
Mahonia aquifolium Tall Oregon
Grape ♦
Mahonia repens Low Oregon
Grape 6.♦
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 for Storm Drainage Facilities
Suggested Perennial Groundcover
7.Botanical
Name
8.Common
Name
Tolerates
Wet to
Saturated
Soils
Recommend
Moderately Wet to
Dry Soils
Recommen
d Dry Soils
Athyrium filix-femina Lady Fern ♦
Dicentra formosa Bleeding Heart ♦
Polystichum munitum Sword Fern ♦
Aquatic/Emergent Wetland Selection for Storm Drainage Facilities
Suggested Aquatics/Emergent Wetland Plants
9.Botanical
Name 10. 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 ♦
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
173
Polygonum
amphibium Water Smartweed ♦
Oenanthe sarmentosa Water Parsley 11.♦
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 ♦
Table V-12.2-1 Plant Selection Guide
Grass Seed Mixes for Storm Drainage Facilities
Botanical Name Common Name Percent
Festuca elatior arundinacea Turf-Type Tall Fescue 40
Lolium perenne Turf-Type Perennial Rye 30
Festuca ruba Red Fescue 25
Agrosotis tenuis Colonial Bentgrass 5
Application rates: Hydroseed @ 60 lbs/acre; Hand seed @ 2 lbs/1000 square feet
Table V-12.3-2 Grass Seed Mixes for Detention/Retention Facilities
Maintenance
All private drainage systems shall require a signed Stormwater Easement and Maintenance
Agreement (SWEMA) 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.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
174
Any standing water removed during the maintenance operation must be disposed of in a City-
approved manner. See the dewatering requirements in Volume II and Appendix IV-B of the
SWMMWW. 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).
BMP D.3 Detention Vaults
All publicly owned and maintained detention vaults are required to meet the minimum design
criteria below, in addition to the criteria provided in the SWMMWW:
A separate building permit is required for detention vaults.
A buoyancy analysis is required to demonstrate that the vault will not be impacted by
ground water.
The maximum allowable depth of the deepest sump shall be 20 feet from finished grade.
Vault floors shall slope to a central location in addition to the 5% “v” slope outlined in the
SWMMWW.
All primary access points shall meet the following requirements:
o A 5-foot x 10-foot 2-door solid hatch shall be provided.
o Hatch cover shall be made from ¼-inch aluminum with diamond-pattern tread plate
reinforced for 150 psf live load.
o Frame shall be made from extruded aluminum angle frame with strap anchors bolted
around the perimeter.
o A Type 316 stainless steel slam lock with fixed interior handle and removable
exterior turn/lift handle with steel cam-action hinges that are concealed from the
exterior of the door shall be used.
o A spring-assisted lift with hold-open feature torsion bars shall be installed. Automatic
hold-open arm with grip handle release shall be added.
o Polypropylene rungs or an aluminum ladder shall be installed extending from the
hatch or manhole opening to the bottom of the vault.
o A ladder-up mechanism shall be installed centered over the rungs or ladder below
the hatch.
o All hardware shall be zinc-coated.
The 5-foot x 10-foot hatch opening shall be located over the sump or low point or points
in the vault.
Vault outlet control structures shall be located in a separate manhole.
The vault design shall include internal galvanized hooks capable of supporting 250
pounds each and located within 2 feet of each opening for the temporary attachment of
lights and other maintenance equipment.
An access opening shall be provided directly above each inlet connection to the vault,
and at each opening between cells.
A minimum of two access openings shall be provided into each cell.
COA Supplemental Manual to the 2019 Ecology Stormwater Management Manual for Western
Washington
Volume V - Runoff Treatment, Flow Control, and LID BMP Library 2022 Version 1
175
In unpaved locations, access hatches and openings shall have a 3-foot-wide paved
collar.
Vaults shall be located on a separate tract. Vaults shall not be located in the public right
of way or access tracts.
Vault locations shall include a drivable surface to all access openings.
Vault access must accommodate large vehicle access with turning radius of 48 feet.
Additional site access shall be provided per Appendix I-J of the COA Supplemental
Manual.
Setbacks
Vaults 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, access, or other site conditions.