HomeMy WebLinkAboutSWMM Volume 4-6.pdf Volume IV
i Table of Contents
Volume IV –
Source Control
Best Management Practices
Table of Contents
Purpose of this Volume...................................................................................................................457
Content and Organization of this Volume.......................................................................................457
Chapter 1 Frequently Asked Questions...........................................................................458
1.1 Applicability............................................................................................................................458
1.2 Pollutants of Concern............................................................................................................458
1.2.1 pH....................................................................................................................................458
1.2.2 Total Suspended Solids..................................................................................................458
1.2.3 Oils and Greases............................................................................................................458
1.2.4 Oxygen-demanding Substances.....................................................................................458
1.2.5 Metals..............................................................................................................................459
1.2.6 Bacteria and Viruses.......................................................................................................459
1.2.7 Nutrients..........................................................................................................................459
1.2.8 Toxic Organic Compounds.............................................................................................459
1.2.9 Other Chemicals and Substances..................................................................................459
1.3 Types of Source Control BMPs.............................................................................................460
1.3.1 Operational BMPs...........................................................................................................460
1.3.2 Structural BMPs..............................................................................................................460
Chapter 2 Worksheet for Commercial and Industrial Activities.....................................461
Chapter 3 BMPs for Homeowners....................................................................................465
3.1 Automobile Washing (for Single-Family Residences)...........................................................465
3.1.1 Suggested BMPs............................................................................................................465
3.1.1.1 At Home................................................................................................................465
3.1.1.2 Away from Home...................................................................................................466
3.2 Automobile Maintenance.......................................................................................................466
3.2.1 Required BMPs...............................................................................................................466
3.2.2 Suggested BMPs............................................................................................................466
3.3 Storage of Solid Wastes and Food Wastes...........................................................................467
3.3.1 Suggested BMPs............................................................................................................467
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3.4 Composting............................................................................................................................467
3.4.1 Suggested BMPs............................................................................................................467
3.5 Yard Maintenance and Gardening........................................................................................468
3.5.1 Required BMPs...............................................................................................................468
3.5.2 Suggested BMPs............................................................................................................468
3.6 Swimming Pool and Spa Cleaning and Maintenance...........................................................469
3.6.1 Required BMPs...............................................................................................................469
3.6.2 Suggested BMPs............................................................................................................469
3.7 Household Hazardous Material Use, Storage, and Disposal................................................469
3.7.1 Required BMPs...............................................................................................................470
3.7.2 Suggested BMPs............................................................................................................470
3.8 General Home Maintenance..................................................................................................471
3.8.1 Suggested BMPs............................................................................................................471
3.9 Pet Waste..............................................................................................................................471
3.9.1 Suggested BMPs............................................................................................................471
Chapter 4 BMPs for Commercial and Industrial Activities..............................................472
4.1 BMPs to Consider for all Activities........................................................................................472
4.2 Cleaning and Washing Activities...........................................................................................473
4.2.1 BMP A101: Cleaning or Washing of Tools, Engines and Manufacturing Equipment....473
4.2.1.1 Description of Pollutant Sources...........................................................................473
4.2.1.2 Pollutant Control Approach...................................................................................473
4.2.1.3 Required BMPs.....................................................................................................473
4.2.1.4 Recommended BMPs...........................................................................................474
4.2.2 BMP A102: Cleaning or Washing of Cooking Equipment...............................................476
4.2.2.1 Description of Pollutant Sources...........................................................................476
4.2.2.2 Pollutant Control Approach...................................................................................476
4.2.2.3 Required BMPs.....................................................................................................476
4.2.2.4 Recommended BMPs...........................................................................................477
4.2.3 BMP A103: Washing, Pressure Washing and Steam Cleaning of
Vehicles/Equipment/Building Structures......................................................................................478
4.2.3.1 Description of Pollutant Sources...........................................................................478
4.2.3.2 Pollutant Control Approach...................................................................................478
4.2.3.3 Required BMPs:....................................................................................................478
4.2.3.4 General..................................................................................................................480
4.2.4 BMP A104: Collection and Disposal of Wastewater in Mobile Interior Washing
Operations.......................................................................................................................481
4.2.4.1 Description of Pollutant Sources...........................................................................481
4.2.4.2 Pollutant Control Approach...................................................................................481
4.2.4.3 Required BMPs.....................................................................................................481
4.2.4.4 Recommended BMPs...........................................................................................482
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4.3 Transfer of Liquid or Solid Materials......................................................................................483
4.3.1 BMP A201: Loading and Unloading Areas for Liquid or Solid Material..........................483
4.3.1.1 Description of Pollutant Sources...........................................................................483
4.3.1.2 Pollutant Control Approach...................................................................................483
4.3.1.3 Required BMPs.....................................................................................................483
4.3.1.4 Recommended BMPs:..........................................................................................486
4.3.2 BMP A202: Fueling at Dedicated Stations......................................................................488
4.3.2.1 Description of Pollutant Sources...........................................................................488
4.3.2.2 Pollutant Control Approach...................................................................................488
4.3.2.3 Required BMPs:....................................................................................................488
4.3.3 BMP A203: Vehicle Maintenance Activities....................................................................494
4.3.3.1 Description of Pollutant Sources...........................................................................494
4.3.3.2 Pollutant Control Approach...................................................................................494
4.3.3.3 Required BMPs.....................................................................................................494
4.3.3.4 Recommended BMPs...........................................................................................495
4.3.4 BMP A204: Mobile Fueling of Vehicles and Heavy Equipment......................................496
4.3.4.1 Description of Pollutant Sources...........................................................................496
4.3.4.2 Pollutant Control Approach...................................................................................496
4.3.4.3 Required BMPs.....................................................................................................496
4.4 Production and Application Activities....................................................................................499
4.4.1 BMP A301: Concrete and Asphalt Mixing and Production at Stationary Sites...............499
4.4.1.1 Description of Pollutant Sources...........................................................................499
4.4.1.2 Pollutant Control Approach...................................................................................499
4.4.1.3 Required BMPs.....................................................................................................499
4.4.1.4 Recommended BMPs...........................................................................................500
4.4.2 BMP A302: Concrete Pouring, Concrete Cutting, and Asphalt Application at
Temporary Sites..............................................................................................................501
4.4.2.1 Description of Pollutant Sources...........................................................................501
4.4.2.2 Pollutant Control Approach...................................................................................501
4.4.2.3 Required BMPs.....................................................................................................501
4.4.2.4 Recommended BMPs...........................................................................................502
4.4.3 BMP A303: Manufacturing and Post-Processing of Metal Products..............................503
4.4.3.1 Description of Pollutant Sources...........................................................................503
4.4.3.2 Pollutant Control Approach...................................................................................503
4.4.3.3 Required BMPs.....................................................................................................503
4.4.3.4 Recommended BMPs...........................................................................................504
4.4.4 BMP A304: Wood Treatment Areas................................................................................505
4.4.4.1 Description of Pollutant Sources...........................................................................505
4.4.4.2 Pollutant Control Approach...................................................................................505
4.4.4.3 Required BMPs.....................................................................................................505
4.4.4.4 Recommended BMP.............................................................................................506
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4.4.5 BMP A305: Commercial Composting.............................................................................507
4.4.5.1 Description of Pollutant Sources...........................................................................507
4.4.5.2 NPDES Permit Requirements...............................................................................507
4.4.5.3 Pollutant Control Approach...................................................................................507
4.4.5.4 Required BMPs.....................................................................................................507
4.4.5.5 Recommended BMPs...........................................................................................508
4.4.6 BMP A306: Landscaping and Lawn/Vegetation Management.......................................509
4.4.6.1 Description of Pollutant Sources...........................................................................509
4.4.6.2 Pollutant Control Approach...................................................................................509
4.4.6.3 Required BMPs for Landscaping..........................................................................509
4.4.6.4 Recommended BMPs for Landscaping.................................................................509
4.4.6.5 Required BMPs for the Use of Pesticides.............................................................510
4.4.6.6 Recommended BMPs for the use of Pesticides....................................................511
4.4.6.7 Required BMPs for Vegetation Management.......................................................512
4.4.6.8 Fertilizer Management:..........................................................................................512
4.4.6.9 Integrated Pest Management................................................................................513
4.4.7 BMP A307: Painting, Finishing and Coating of Vehicles, Boats, Buildings and
Equipment....................................................................................................................................514
4.4.7.1 Description of Pollutant Sources...........................................................................514
4.4.7.2 Pollutant Control Approach...................................................................................514
4.4.7.3 Required BMPs.....................................................................................................514
4.4.7.4 Recommended BMPs...........................................................................................515
4.4.8 BMP A308: Commercial Printing Operations..................................................................516
4.4.8.1 Description of Pollutant Sources...........................................................................516
4.4.8.2 Pollutant Control Approach...................................................................................516
4.4.8.3 Required BMPs.....................................................................................................516
4.4.8.4 Recommended BMPs...........................................................................................516
4.4.9 BMP A309: Manufacturing Operations – Outside...........................................................517
4.4.9.1 Description of Pollutant Sources...........................................................................517
4.4.9.2 Pollution Control Approach...................................................................................517
4.4.9.3 Required BMPs.....................................................................................................517
4.5 Storage and Stockpiling Activities.........................................................................................519
4.5.1 BMP A401: Storage or Transfer (Outside) of Solid Raw Materials, By-Products or
Finished Products........................................................................................................................519
4.5.1.1 Description of Pollutant Sources...........................................................................519
4.5.1.2 Pollutant Control Approach...................................................................................519
4.5.1.3 Required BMPs.....................................................................................................519
4.5.1.4 Recommended BMPs...........................................................................................520
4.5.2 BMP A402: Storage and Treatment of Contaminated Soils...........................................522
4.5.2.1 Description of Pollutant Sources...........................................................................522
4.5.2.2 Pollutant Control Approach...................................................................................522
4.5.2.3 Required BMPs.....................................................................................................522
4.5.2.4 Recommended BMPs...........................................................................................522
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4.5.3 BMP A403: Temporary Storage or Processing of Fruits or Vegetables.........................523
4.5.3.1 Description of Pollutant Sources...........................................................................523
4.5.3.2 Pollutant Control Approach...................................................................................523
4.5.3.3 Required BMPs.....................................................................................................523
4.5.3.4 Recommended BMPs...........................................................................................524
4.5.4 BMP A404: Storage of Solid Wastes and Food Wastes.................................................525
4.5.4.1 Description of Pollutant Sources...........................................................................525
4.5.4.2 Pollutant Control Approach...................................................................................525
4.5.4.3 Required BMPs.....................................................................................................525
4.5.4.4 Recommended BMPs...........................................................................................526
4.5.5 BMP A405: Recyclers and Scrap Yards.........................................................................527
4.5.5.1 Description of Pollutant Sources...........................................................................527
4.5.5.2 Required BMPs.....................................................................................................527
4.5.6 BMP A406: Treatment, Storage or Disposal of Dangerous Wastes...............................528
4.5.7 BMP A407: Storage of Liquid, Food Waste or Dangerous Waste Containers...............529
4.5.7.1 Description of Pollutant Sources...........................................................................529
4.5.7.2 Pollutant Control Approach...................................................................................529
4.5.7.3 Required BMPs.....................................................................................................529
4.5.8 BMP A408: Storage of Liquids in Above-Ground Tanks.................................................532
4.5.8.1 Description of Pollutant Sources...........................................................................532
4.5.8.2 Pollutant Control Approach...................................................................................532
4.5.8.3 Required BMPs for All Tanks................................................................................532
4.5.8.4 Required BMPs for Single-walled Tanks...............................................................533
4.5.8.5 Recommended BMPs for Double-walled Tanks...................................................533
4.5.9 BMP A409: Parking and Storage for Vehicles and Equipment.......................................535
4.5.9.1 Description of Pollutant Sources...........................................................................535
4.5.9.2 Required BMPs.....................................................................................................535
4.6 Construction and Demolition Activities..................................................................................536
4.6.1 BMP A501: Clearing, Grading and Preparation of Construction Sites...........................536
4.6.2 BMP A502: Demolition of Buildings................................................................................537
4.6.2.1 Description of Pollutant Sources...........................................................................537
4.6.2.2 Pollutant Control Approach...................................................................................537
4.6.2.3 Required BMPs.....................................................................................................537
4.6.2.4 Recommended BMPs...........................................................................................537
4.6.3 BMP A503: Building, Repair, Remodeling and Construction..........................................538
4.6.3.1 Description of Pollutant Sources...........................................................................538
4.6.3.2 Pollutant Control Approach...................................................................................538
4.6.3.3 Required BMPs.....................................................................................................538
4.6.3.4 Recommended BMPs...........................................................................................539
4.7 Dust Control, and Soil and Sediment Control........................................................................540
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4.7.1 BMP A601: Dust Control at Disturbed Land Areas and Unpaved Roadways and
Parking Lots....................................................................................................................540
4.7.1.1 Description of Pollutant Sources...........................................................................540
4.7.1.2 Pollutant Control Approach...................................................................................540
4.7.1.3 Required BMPs.....................................................................................................540
4.7.1.4 Recommended BMPs for Roadways and Other Trafficked Areas:.......................540
4.7.1.5 Recommended BMPs for Dust Generating Areas:...............................................541
4.7.2 BMP A602: Dust Control at Manufacturing Sites............................................................542
4.7.2.1 Description of Pollutant Sources...........................................................................542
4.7.2.2 Pollutant Control Approach...................................................................................542
4.7.2.3 Required BMPs.....................................................................................................542
4.7.2.4 Recommended BMPs...........................................................................................542
4.7.3 BMP A603: Soil Erosion and Sediment Control at Industrial Sites.................................543
4.7.3.1 Description of Pollutant Sources...........................................................................543
4.7.3.2 Pollutant Control Approach...................................................................................543
4.7.3.3 Required BMPs.....................................................................................................543
4.8 Other Activities.......................................................................................................................544
4.8.1 BMP A701: Commercial Animal Handling Areas............................................................544
4.8.1.1 Description of Pollutant Sources...........................................................................544
4.8.1.2 Pollutant Control Approach...................................................................................544
4.8.1.3 Required BMPs.....................................................................................................544
4.8.2 BMP A702: Log Sorting and Handling............................................................................545
4.8.2.1 Description of Pollutant Sources...........................................................................545
4.8.2.2 Ecology’s Baseline General Permit Requirements...............................................545
4.8.3 BMP A703: Boat Building, Maintenance and Repair......................................................546
4.8.3.1 Description of Pollutant Sources...........................................................................546
4.8.3.2 Pollutant Control Approach...................................................................................546
4.8.3.3 Required BMPs.....................................................................................................546
4.8.3.4 Recommended BMPs...........................................................................................547
4.8.4 BMP A704: Logging........................................................................................................549
4.8.4.1 Description of Pollutant Sources...........................................................................549
4.8.4.2 Pollutant Control Approach...................................................................................549
4.8.4.3 Required BMPs.....................................................................................................549
4.8.4.4 Recommended BMPs...........................................................................................550
4.8.5 BMP A705: Mining and Quarrying of Sand, Gravel, Rock, Peat, Clay and
Other Materials................................................................................................................551
4.8.5.1 Description of Pollutant Sources...........................................................................551
4.8.5.2 Recommended BMPs...........................................................................................551
4.8.6 BMP A706: Swimming Pool and Spa Cleaning and Maintenance.................................552
4.8.6.1 Description of Pollutant Sources...........................................................................552
4.8.6.2 Pollutant Control Approach...................................................................................552
4.8.6.3 Required BMPs.....................................................................................................552
4.8.6.4 Recommended BMP.............................................................................................552
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4.8.7 BMP A707: De-Icing and Anti-Icing Operations for Streets & Highways........................553
4.8.7.1 Description of Pollutant Sources...........................................................................553
4.8.7.2 Required BMPs.....................................................................................................553
4.8.7.3 Recommended BMPs...........................................................................................553
4.8.8 BMP A708: Roof and Building Drains at Manufacturing and Commercial Buildings......554
4.8.8.1 Description of Pollutant Sources...........................................................................554
4.8.8.2 Pollutant Control Approach...................................................................................554
4.8.8.3 Required BMPs.....................................................................................................554
4.8.9 BMP A709: Urban Streets...............................................................................................555
4.8.9.1 Description of Pollutant Sources...........................................................................555
4.8.9.2 Pollutant Control Approach...................................................................................555
4.8.9.3 Recommended BMPs...........................................................................................555
4.8.10 BMP A710: Railroad Yards.............................................................................................557
4.8.10.1 Description of Pollutant Sources...........................................................................557
4.8.10.2 Pollutant Control Approach...................................................................................557
4.8.10.3 Required BMPs.....................................................................................................557
4.8.11 BMP A711: Maintenance of Public and Utility Corridors and Facilities..........................558
4.8.11.1 Description of Pollutant Sources...........................................................................558
4.8.11.2 Pollutant Control Approach...................................................................................558
4.8.11.3 Required BMPs.....................................................................................................558
4.8.11.4 Recommended BMPs...........................................................................................559
4.8.12 BMP A712: Maintenance of Roadside Ditches...............................................................560
4.8.12.1 Description of Pollutant Sources...........................................................................560
4.8.12.2 Pollutant Control Approach...................................................................................560
4.8.12.3 Required BMPs.....................................................................................................560
4.8.12.4 Recommended BMPs...........................................................................................561
4.8.13 BMP A713: Maintenance of Stormwater Drainage and Treatment Facilities................562
4.8.13.1 Description of Pollutant Sources...........................................................................562
4.8.13.2 Pollutant Control Approach...................................................................................562
4.8.13.3 Required BMPs.....................................................................................................562
4.8.14 BMP A714: Spills of Oil and Hazardous Substances.....................................................564
4.8.14.1 Description of Pollutant Sources...........................................................................564
4.8.14.2 Pollutant Control Approach...................................................................................564
4.8.14.3 Required BMPs.....................................................................................................564
4.8.14.4 Recommended BMPs...........................................................................................565
4.8.15 BMP A715: Water Reservoir, Transmission Mainline, Wellhead, and Hydrant
Flushing Activities...........................................................................................................566
4.8.15.1 Description of Pollutant Sources...........................................................................566
4.8.15.2 Pollutant Control Approach...................................................................................566
4.8.15.3 Required BMPs.....................................................................................................566
4.8.15.4 Recommended BMPs...........................................................................................566
4.8.16 BMP S101: Eliminate Illicit Storm Drainage System Connections.................................568
4.8.17 BMP S102: Dispose of Contaminated Stormwater and Waste Materials Properly........569
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4.8.18 BMP S103: Discharge Process Wastewater to a Sanitary Sewer, Holding Tank,
or Water Treatment System............................................................................................570
4.8.19 BMP S108: Implement Integrated Pest Management Measures...................................571
4.8.20 BMP S109: Cleaning Catch Basins................................................................................572
4.9 Cover and Surround Activities...............................................................................................573
4.9.1 BMP S104: Cover the Activity with a Roof or Awning.....................................................573
4.9.2 BMP S105: Cover the Activity with an Anchored Tarp or Plastic Sheet.........................574
4.9.3 BMP S106: Pave the Activity Area and Slope to a Sump, Holding Tank, or
Oil/Water Separator........................................................................................................575
4.9.4 BMP S107: Surround the Activity Area with a Curb, Dike, or Berm or Elevate the
Activity.............................................................................................................................576
Chapter 5 Regulations and Requirements.......................................................................580
5.1 City of Auburn Codes and Ordinances..................................................................................580
5.2 State, Federal, and Other Regulations and Requirements...................................................580
5.2.1 Washington State Department of Ecology Requirements for the Discharge of
Process Wastewaters Directly to Surface Waters..........................................................580
5.2.2 Washington State Department of Ecology Requirements for Dangerous
Waste Generators...........................................................................................................581
5.2.3 Washington State Department of Ecology Stormwater NPDES Permit
Requirements..................................................................................................................581
5.2.4 Washington State Department of Ecology Requirements for Underground and
Above Ground Storage Tanks........................................................................................582
5.2.5 U.S. Environmental Protection Agency and Ecology Emergency Spill Cleanup
Requirements..................................................................................................................582
5.2.6 Washington State Department of Agriculture Pesticide Regulations..............................582
5.2.7 Puget Sound Clean Air Agency Air Quality Regulations.................................................582
Appendix A Quick Reference Phone Numbers...................................................................583
Appendix B Recycling/Disposal of Vehicle Fluids and Other Wastes...............................584
Appendix C Example of an Integrated Pest Management Program (IPM).........................585
Appendix D Recommendations for Management of Street Wastes..................................588
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Purpose Volume IV
Content and Organization Introduction 457
Volume IV:
Source Control Best
Management Practices
Purpose of this Volume
This volume was designed to help businesses, homeowners and public agencies in Auburn
implement source control best management practices (BMPs) to prevent pollutants from
contaminating stormwater runoff and entering our rivers, streams and groundwater.
Content and Organization of this Volume
Volume IV contains five chapters and four appendices.
• Chapter 1 provides an overview of who should use this volume and the type of
pollutants being targeted.
• Chapter 2 provides a worksheet for commercial and industrial activity.
• Chapter 3 provides BMPs for single-family residences.
• Chapter 4 provides BMPs for commercial and industrial activities.
• Chapter 5 lists related regulations and requirements.
• Appendix A provides a list of phone numbers for related agencies.
• Appendix B lists recommended management procedures for the handling of
hazardous wastes.
• Appendix C provides an example of an Integrated Pest Management program.
• Appendix D provides recommendations for the management of street wastes.
Volume
IV
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Frequently Asked Questions Volume IV
Chapter 1 458
Chapter 1 Frequently Asked Questions
1.1 Applicability
The implementation of BMPs applies to all businesses, residences, and public agencies in Auburn.
1.2 Pollutants of Concern
The City is required to show progress in eliminating virtually all non-stormwater discharges to the
storm drainage system. Only uncontaminated stormwater may be discharged to the City of Auburn
storm drainage system. Illicit discharges, intentional or unintentional, are not allowed and polluters
may be subject to state and federal penalties. It is the property owner’s responsibility to keep
pollutants from leaving a property and entering the City storm drainage system.
Pollutants can be placed into several broad categories, as listed below.
1.2.1 pH
The pH value of a substance is a relative measure of whether it is acidic or basic. Most aquatic
species can only survive in neutral conditions. Sources that can contribute to a change in pH of
stormwater and waterbodies include cement in concrete pouring, paving, and recycling operations;
solutions from metal plating; chemicals from printing businesses and other industrial processes; and
household cleaners such as bleaches and deck washes.
1.2.2 Total Suspended Solids
This represents particulate solids such as eroded soil, heavy metal precipitates, and biological solids
which can cause sedimentation in streams and turbidity in receiving surface waters. Sediments can
destroy the desired habitat for fish and can impact drinking water supplies. Sediment may be carried
to streams, rivers and eventually to Puget Sound where they may be toxic to aquatic life and destroy
habitat.
1.2.3 Oils and Greases
Oils and greases can be either petroleum-based or food-related sources. Petroleum-based
compounds can be immediately toxic to fish and wildlife, and can destroy our drinking water aquifers.
Food-based oils and greases can coat fish gills and insects, suffocating them. Oils and greases can
clog conveyance systems, which may cause flooding.
1.2.4 Oxygen-demanding Substances
Degradable organic matter, such as yard, food, and pet wastes, and some chemical wastes, can
have a drastic effect on water quality. These substances, when broken down by bacteria, consume
the oxygen in the water. This stresses and can eventually kill fish and other creatures in the water.
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NOVEMBER 2009
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Chapter 1 459
1.2.5 Metals
Metals are utilized in many products important to our daily lives. Certain metals, known as heavy
metals, wear off of our car brakes and tires, and come from the paint and moss-killing roof strips and
herbicides we use at our homes. These metals can cause severe health and reproductive problems
in fish and animals that live in water. Metals can be transported on sediments to waterbodies.
1.2.6 Bacteria and Viruses
Bacteria and viruses from pet wastes, failing septic systems and agricultural areas can contaminate
drinking water and close down swimming and shellfish areas. A group of bacteria called fecal
coliform bacteria are typically used as the indicators for all bacteria and viruses, so large amounts of
fecal coliform may indicate serious problems.
1.2.7 Nutrients
In the context of water quality, nutrients are mainly compounds of nitrogen and phosphorus. When
nutrients are allowed to enter waterbodies, undesirable effects such as algae overgrowth, oxygen
depletion, channel clogging due to overgrowth of vegetation, and fish and animal death can occur.
Sources of nutrients can include fertilizers, failing septic systems, and yard and animal wastes.
1.2.8 Toxic Organic Compounds
A number of organic chemicals are just plain toxic when they get into the aquatic environment.
Pesticides, herbicides, rodenticides, and fungicides are deadly to aquatic life. Compounds such as
antifreeze, wood preservatives, cleansers, and a host of other, more exotic organics derived from
industries or past practices (such as polychlorinated biphenyls (PCBs), DDT, and chlordane) can also
have detrimental effects on the environment.
1.2.9 Other Chemicals and Substances
There are a host of other chemicals that can cause problems if allowed to enter the aquatic
environment. Common household bleach can be deadly to fish and other critters if drained directly to
waterbodies. Diatomaceous earth backwash from swimming pool filters can clog gills and suffocate
fish. Arsenic has been used in rat and mole killing compounds. Even those compounds classified as
biodegradable or environmentally friendly can have immediate devastating effects on aquatic life.
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Frequently Asked Questions Volume IV
Chapter 1 460
1.3 Types of Source Control BMPs
As the name implies, source control BMPs prevent contamination from entering stormwater runoff by
controlling them at the source. There are two categories of source control BMPs:
• Operational BMPs
• Structural BMPs
1.3.1 Operational BMPs
Operational source control BMPs are considered to be the most cost effective pollutant minimization
practices. Operational source control BMPs are non-structural practices that prevent or reduce
pollutants form entering stormwater. They can also include process changes such as raw
material/product changes and recycling wastes.
Examples include:
• Formation of a pollution prevention team
• Good housekeeping practices
• Preventive maintenance procedures
• Spill prevention and cleanup
• Employee training
• Inspections of pollutant sources
• Record keeping
1.3.2 Structural BMPs
Structural source control BMPs are physical, structural or mechanical devices or facilities that are
intended to prevent pollutants from entering stormwater.
Examples of structural source control BMPs typically include:
• Enclosing and/or covering the pollutant source, i.e., within a building or other
enclosure, a roof over storage and working areas, a temporary tarp
• Physically segregating the pollutant source to prevent run-on of uncontaminated
stormwater
• Devices that direct only contaminated stormwater to appropriate treatment BMPs, i.e.
discharge to a sanitary sewer if allowed by the local sewer utility.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
1 If any of these activities occur indoors, then BMPs are not required, provided no indoor drains or processes can ultimately
contact stormwater or be transported to surface waters such as rivers and streams. Ensure that liquids, powders, dusts
and fine granular materials stay confined indoors. Otherwise BMPs will be required.
2 If any of these activities occur outdoors, then use the activity code to find the appropriate BMPs described in Chapter 4.
Worksheet for Commercial Volume IV
and Industrial Activities 461 Chapter 2
Chapter 2 Worksheet for Commercial and Industrial
Activities
This worksheet is designed for use by business and industry operators. Complete the entire
worksheet by checking the appropriate boxes for all activities that take place at the work site. If any of
the activities as being performed outdoors, use the activity code on the worksheet to find the
recommended BMPs contained in Chapter 4.
Are you involved in this?
If so, check if it occurs:
Activity
Code
(BMP)
TYPE OF ACTIVITY
Indoors1 Outdoors2
SECTION A1 – CLEANING AND WASHING ACTIVITIES
A101 Cleaning or Washing of Tools, Engines, and Manufacturing
Equipment – includes parts washers and all types of manufactured
equipment components.
A102 Cleaning or Washing of Cooking Equipment – includes vents, filters,
pots and pans, grills, and related items.
A103 Washing, Pressure Washing, and Steam Cleaning of
Vehicles/Equipment/Building Structures – covers cleaning and
washing at all types of establishments, including fleet vehicle yards,
car dealerships, car washes, and maintenance facilities.
A104 Collection and Disposal of Wastewater from Mobile Interior Washing
Operations – includes carpet cleaners, upholstery cleaners, and
drapery cleaners.
SECTION A2 – TRANSFERS OF LIQUID OR SOLID MATERIALS
A201 Loading and Unloading Areas for Liquid or Solid Material – for
loading and unloading of materials at industrial and commercial
facilities
A202 Fueling at Dedicated Stations – includes gas stations, pumps at fleet
vehicle yards or shops, and other privately owned pumps.
A203 Vehicle Maintenance Activities – covers oil changes and other
engine fluids.
A204 Mobile Fueling of Vehicles and Heavy Equipment – includes fleet
fueling, wet fueling, and wet hosing
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1 If any of these activities occur indoors, then BMPs are not required, provided no indoor drains or processes can ultimately
contact stormwater or be transported to surface waters such as rivers and streams. Ensure that liquids, powders, dusts
and fine granular materials stay confined indoors. Otherwise BMPs will be required.
2 If any of these activities occur outdoors, then use the activity code to find the appropriate BMPs described in Chapter 4.
Worksheet for Commercial Volume IV
and Industrial Activities 462 Chapter 2
Are you involved in this?
If so, check if it occurs:
Activity
Code
(BMP)
TYPE OF ACTIVITY
Indoors1 Outdoors2
SECTION A3 – PRODUCTION AND APPLICATION ACTIVITIES
A301 Concrete and Asphalt Mixing and Production at Stationary Sites –
applies to mixing of raw materials on-site to produce concrete or
asphalt.
A302 Concrete Pouring, Concrete Cutting, and Asphalt Application at
Temporary Sites – includes construction sites, and driveway and
parking lot resurfacing.
A303 Manufacturing and Post-processing of Metal Products – includes
machining, grinding, soldering, cutting, welding, quenching, rinsing,
etc.
A304 Wood Treatment Areas – includes wood treatment using pressure
processes or by dipping or spraying.
A305 Commercial Composting – includes commercial composting facilities
operating outside.
A306 Landscaping and Lawn/Vegetation Maintenance, Including
Vegetation Removal, Herbicide and Insecticide Application, Fertilizer
Application, Irrigation, Watering, Gardening, and Lawn Care –
includes businesses involved in landscaping, applying pesticides
and managing vegetation.
A307 Painting, Finishing, and Coating of Vehicles, Boats, Buildings, and
Equipment – includes surface preparation and the applications of
paints, finishes, and/or coatings.
A308 Commercial Printing Operations – includes materials used in the
printing process.
A309 Manufacturing Activities (Outside) - includes outdoor manufacturing
areas.
SECTION A4 – STORAGE ACTIVITIES
A401 Storage or Transfer (Outside) of Solid Raw Materials, By-products,
or Finished Products
A402 Storage and Treatment of Contaminated Soils – applies to
contaminated soils that are excavated and left on-site.
A403 Temporary Storage or Processing of Fruits or Vegetables – includes
processing activities at wineries, fresh and frozen juice makers, and
other food and beverage processing operations.
A404 Storage of Solid Wastes and Food Wastes – includes regular
garbage and all other discarded non-liquid items.
A405 Recyclers and Scrap Yards – includes scrapped equipment,
vehicles, empty metal drums, and assorted recyclables.
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1 If any of these activities occur indoors, then BMPs are not required, provided no indoor drains or processes can ultimately
contact stormwater or be transported to surface waters such as rivers and streams. Ensure that liquids, powders, dusts
and fine granular materials stay confined indoors. Otherwise BMPs will be required.
2 If any of these activities occur outdoors, then use the activity code to find the appropriate BMPs described in Chapter 4.
Worksheet for Commercial Volume IV
and Industrial Activities 463 Chapter 2
Are you involved in this?
If so, check if it occurs:
Activity
Code
(BMP)
TYPE OF ACTIVITY
Indoors1 Outdoors2
A406 Treatment, Storage, or Disposal of Dangerous Wastes – Refer to
Ecology and the appropriate County Health Department for more
information, see Chapter 6.
A407 Storage of Liquid, Food Waste, or Dangerous Waste Containers –
includes containers located outside a building and used for
temporary storage
A408 Storage of Liquids in Permanent Above-ground Tanks – includes all
liquids in above-ground tanks
A409 Parking and Storage for Vehicles and Equipment – includes public
and commercial parking lots
SECTION A5 – CONSTRUCTION ACTIVITIES
A501 Clearing, Grading, and Preparation of Construction Sites – applies to
land developing activities and to residential yard clearing and
grading projects.
A502 Demolition of Buildings – applies to removal of existing buildings and
subsequent clearing of the rubble.
A503 Building Repair, Remodeling, and Construction – applies to
construction of buildings, general exterior building repair work and
remodeling of buildings.
SECTION A6 – DUST CONTROL AND SOIL AND SEDIMENT CONTROL
A601 Dust Control at Disturbed Land Areas and Unpaved Roadways and
Parking Lots
A602 Dust Control at Manufacturing Sites – includes grain dust, sawdust,
coal, gravel, crushed rock, cement, and boiler fly ash.
A603 Soil Erosion and Sediment Control at Industrial Sites – includes
industrial activities that take place on soil.
SECTION A7 – OTHER ACTIVITIES
A701 Commercial Animal Handling Areas – includes kennels, fenced
pens, veterinarians, and businesses that board animals
A702 Log Sorting and Handling – applies to log yards typically located at
sawmills, ports, and pulp mills.
A703 Boat building, Mooring, Maintenance, and Repair – includes all types
of maintenance, repair, and building operations.
A704 Logging – applies to logging activities that fall under Class IV
general forest practices.
SURFACE WATER MANAGEMENT MANUAL
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1 If any of these activities occur indoors, then BMPs are not required, provided no indoor drains or processes can ultimately
contact stormwater or be transported to surface waters such as rivers and streams. Ensure that liquids, powders, dusts
and fine granular materials stay confined indoors. Otherwise BMPs will be required.
2 If any of these activities occur outdoors, then use the activity code to find the appropriate BMPs described in Chapter 4.
Worksheet for Commercial Volume IV
and Industrial Activities 464 Chapter 2
Are you involved in this?
If so, check if it occurs:
Activity
Code
(BMP)
TYPE OF ACTIVITY
Indoors1 Outdoors2
A705 Mining and Quarrying of Sand, Gravel, Minerals, Peat, Clay, Rock,
and Other Materials – does not include excavation at construction
sites.
A706 Swimming Pool and Spa Cleaning and Maintenance – includes
every swimming pool and spa not at a single family residence.
Commercial pool cleaners are included here for all pools.
A707 Deicing and Anti-icing Operations for Airports and Streets -includes
aircraft, runways/taxiways, streets and highways.
A708 Roof and Building Drains at Manufacturing and Commercial
Buildings – These sites will be referred to the Puget Sound Clean Air
Agency.
A709 Urban Streets – includes recommended BMPs.
A710 Railroad Yards
A711 Maintenance of Public and Private Utility Corridors and Facilities –
includes public and private utility maintenance activities.
A712 Maintenance of Roadside Ditches
A713 Maintenance of Stormwater Drainage and Treatment Facilities
A714 Spills of Oil and Hazardous Substances
A715 Water Reservoir, Transmission Mainline, Wellhead, and Hydrant
Flushing Activities
SECTION S1 – SOURCE CONTROL BMPs
S101 Eliminate Illicit Sewer to Storm Drainage System Connections
S102 Dispose of Contaminated Stormwater and Waste Materials Properly
S103 Discharge Process Wastewater to a Sanitary Sewer, Holding Tank,
or Water Treatment System
S104 Cover the Activity with a Roof of Awning
S105 Cover the Activity with an Anchored Tarp or Plastic Sheet
S106 Pave the Activity Area and Slope to a Sump or Holding Tank, or
Oil/Water Generator
S107 Surround the Activity Area with a Curb, Dike, or Berm or Elevate the
Activity
S108 Implement Integrated Pest Management Measures
S109 Cleaning Catch Basins
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Chapter 3 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.
• Section 3.1 – Automobile Washing
• Section 3.2 – Automobile Maintenance
• Section 3.3 – Storage of Solid Wastes and Food Wastes
• Section 3.4 – Composting
• Section 3.5 – Yard Maintenance and Gardening
• Section 3.6 – Swimming Pool and Spa Cleaning and Maintenance
• Section 3.7 – Household Hazardous Material Use, Storage, and Disposal
• Section 3.8 – General Home Maintenance
3.1 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.
3.1.1 Suggested BMPs
3.1.1.1 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.
• Use a hose nozzle with a shut-off valve to save water.
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• Do not wash cars if rain is expected.
• Pour the bucket of soapy, dirty wash water down your sink.
3.1.1.2 Away from Home
• Take cars to a commercial car wash that has a recycle system and discharges
wastewater to the sanitary sewer for treatment.
• Go to fundraising car washes where sponsors use Auburn’s Car Wash loaner kits.
Use a Car Wash loaner kit. If your group is planning a car wash in Auburn, call
(253) 931-3010 to get information about using a Car Wash loaner kit.
3.2 Automobile Maintenance
3.2.1 Required BMPs
• 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.govlink.org/hazwaste/house/disposal/AnyProduct.cfm?catID=974 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.
3.2.2 Suggested BMPs
• Fix all leaks, to keep the leaky material off the streets and out of the surface water.
• 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. Pending
State law may make disposal in your home garbage illegal, so please call the King
County Household Hazards Line at (206) 296-4692 for up-to-date information.
• Use care in draining and collecting antifreeze to prevent accidental spills. Spilled
antifreeze can be deadly to cats and dogs that ingest it.
• Perform service activities on concrete or asphalt or over a plastic tarp to make spill
clean-up easier. 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.
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3.3 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.
3.3.1 Suggested BMPs
• All waste containers kept outside should have lids. If the hauler container lid is
damaged, please call the City of Auburn Utilities at (253) 931-3038 for information on
lid repair and replacement.
• Leaking waste containers should be replaced. If the container is damaged, please
call City of Auburn Utilities at (253) 931-3038.
• 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) 931-3047.
• 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 Utilities
at (253) 931-3038 for more information on yard and food scrap recycling.
3.4 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) 931-3047.
3.4.1 Suggested BMPs
• Locate compost bins on an unpaved area that is not prone to water ponding during
storms, and well away from wetlands, streams, lakes and other drainage paths.
• Compost bins 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 bin.
• Cover the bin to keep excess water from cooling down the pile, which will slow down
the rate of decomposition.
• 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) 931-3047.
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3.5 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.
3.5.1 Required BMPs
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 rinsate 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.
3.5.2 Suggested BMPs
• Use natural, organic soil amendments. Visit www.kingcounty.gov/environment. Click
on Composting, Natural Lawn Care for natural yard care information.
• Use an integrated pest management program (IPM), which is a natural, long-term,
ecologically based approach to controlling pest populations. See Section 4.4.6 and
Appendix C – Example of an Integrated Best Management Program.
• 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 www.kingcounty.gov/Environment, Click on
Compost, 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.
• Make sure all fertilizers and pesticides are stored in a covered location.
• 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 section for more information.
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• 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.
3.6 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
these BMPs will ensure the cleanliness of your pool and the environment.
3.6.1 Required BMPs
• 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 any conditions for discharge approval. Water discharged to the
ground or a lawn must not cross property lines and must not produce runoff.
• 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 any 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.
3.6.2 Suggested BMPs
• 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.
3.7 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.
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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.
3.7.1 Required BMPs
• Hazardous materials must be stored out of the reach of children.
• 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.
3.7.2 Suggested BMPs
• 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.
• 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, just 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.
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• 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.
3.8 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.
3.8.1 Suggested BMPs
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.
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.
3.9 Pet Waste
Pets can generate pollutants from manure 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 BMPs listed below.
3.9.1 Suggested BMPs
• 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.
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Chapter 4 BMPs for Commercial and Industrial Activities
This chapter coordinates with the worksheet completed in Chapter 2. That worksheet and the BMPs
are organized by the different activities that businesses perform. If the listed activity is performed
indoors and all discharges from the activity are controlled (e.g., process water, wash water,
lubricants, solvents, fugitive dust, granular material, blowdown waste, etc.) such that no exposure to
stormwater occurs, then no new BMPs for that activity are required. However, if the column for
activities performed outdoors was checked, match the number from the worksheet to the activities
listed in this section to find the BMPs suggested.
Contact the City’s Storm Drainage Utility at 253-931-3010 for more information or technical
assistance. Assistance can be provided over the phone or at on-site consultations.
Every person/business in Auburn is required to use BMPs as outlined in this manual. Utilizing
additional BMPs to protect water further quality is encouraged.
Some businesses are or will be required to obtain a National Pollutant Discharge Elimination System
(NPDES) permit for stormwater discharges. These permits are issued and regulated by the
Washington State Department of Ecology.
There are several BMPs contained in this chapter that may also apply to residences and other non-
commercial or non-industrial sites.
4.1 BMPs to Consider for all Activities
Some common best management practices that should be considered for all activities include:
• Avoid the activity or reduce its occurrence.
• Move the activity indoors if possible.
• Clean up spills quickly.
• Use less material.
• Use the least toxic materials available.
• Create and maintain vegetated areas near activity locations.
• Locate activities as far as possible from surface drainage paths.
• Keep storm drain systems clean.
• Reduce, reuse, and recycle as much as possible.
• Be an advocate for stormwater pollution prevention.
• Report violators to Storm Drainage Utility 253-931-3010.
• Provide oversight and training.
• Sweep or vacuum to control dust and debris.
• Regularly inspect, clean, and repair all facilities and BMPs.
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4.2 Cleaning and Washing Activities
4.2.1 BMP A101: Cleaning or Washing of Tools, Engines and
Manufacturing Equipment
4.2.1.1 Description of Pollutant Sources
This activity 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).
4.2.1.2 Pollutant Control Approach
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. Washwater
may be discharged to the ground after proper treatment in accordance with Ecology Guidance WQ-
R-95-56, “Vehicle and Equipment Washwater Discharges,” revised September 2007. The quality of
any discharge to the ground after proper treatment must comply with Ecology’s Groundwater Quality
Standards, Chapter 173-200 WAC. Contact the Ecology Southwest Regional Office for an NPDES
Permit application for discharge of washwater to surface water or to a storm drain after on-site
treatment.
4.2.1.3 Required BMPs
The following BMPs, or equivalent measures, 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 S101
for detailed information.
• Employees shall be educated to control washing operations to prevent stormwater
contamination.
• All washwater must discharge to a holding tank, process treatment system, or
sanitary sewer, never to the storm drain system. See BMP S103 for detailed
information on how this must be accomplished.
• Pressure washing must be done in a designated area (such as a wash pad) provided
with a sump drain and stormwater run-on prevention (Figure IV-4-1). See BMPs
S106 and S107 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.
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Figure IV-4-1. Pressure Wash Water Recovery System
4.2.1.4 Recommended BMPs
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 washwater 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.
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• Catch basin filters and/or sorbent inserts should be selected based on the type of
contaminants in the stormwater.
For discharging washwater containing soaps and detergents, the use of infiltration, biofiltration, wet
ponds, and wetlands must not result in the violation of groundwater quality standards.
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4.2.2 BMP A102: Cleaning or Washing of Cooking Equipment
4.2.2.1 Description of Pollutant Sources
This activity 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)
4.2.2.2 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.
4.2.2.3 Required BMPs
The following BMPs or equivalent measures 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 S101
for detailed requirements.
• Employees must be educated about the need to prevent stormwater contamination
from washing operations.
• Washwater cannot be discharged to the storm drainage system.
• Paved washing areas must be swept daily to collect loose solid materials for proper
disposal.
• 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
runoff. 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 S103 for further information on drainage alternatives.
Any connection to the sanitary sewer requires the approval of the City of Auburn. 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 S103 for detailed drainage requirements and BMP S107 for methods of run-on
prevention. If discharging to a sanitary sewer, permits must be obtained from the City
of Auburn Permit Center 253-931-3090.
OR
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Use a tub or similar device to contain washwater. This water must be recycled for subsequent
washing, or disposed into a holding tank or sanitary sewer.
• If a holding tank is used for storage of washwater, the contents must be pumped out
before it is full and disposed of appropriately to a sanitary sewer or wastewater
treatment system.
4.2.2.4 Recommended BMPs
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.
For discharging washwater containing soaps and detergents, the use of infiltration, biofiltration, wet
ponds, and wetlands must not result in the violation of groundwater quality standards.
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4.2.3 BMP A103: Washing, Pressure Washing and Steam Cleaning of
Vehicles/Equipment/Building Structures
4.2.3.1 Description of Pollutant Sources
Vehicles, aircraft, vessels/boats, grocery carts, carpets, industrial equipment, and large buildings or
structures may be commercially cleaned with low or high pressure water or steam. This also includes
removing graffiti and “charity” car washes at gas stations and commercial parking lots. The cleaning
can include hand washing, scrubbing, sanding, etc. Washwater from cleaning activities can contain
oil and grease, suspended solids, heavy metals, soluble organics, soaps, and detergents that can
contaminate stormwater.
4.2.3.2 Pollutant Control Approach
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. Washwater must be
conveyed to a sanitary sewer after approval by the City of Auburn. See Ecology guidance
WQ-R-95-56, “Vehicle and Equipment Washwater Discharges,” June 1995 for more information.
4.2.3.3 Required BMPs:
New and Used Car Dealer Lots
If washing is accomplished only with cold water and consists of washing only the outside of the motor
vehicles, (no soaps or detergents used) there should be no discharge to the sanitary sewer. See
Ecology guidance WQ-R-95-56, “Vehicle and Equipment Washwater Discharge”, June 1995, 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 Storm Drainage Utility at 253-931-3010 for
further information. The facility will be inspected periodically by City staff.
Other Washing Events
Identify types of washing events and their locations (such as regional used car sales, RV shows,
etc.) and evaluate options on a case-by-case basis according to the discharge criteria stated above.
Charity car washes should wash only the exterior of vehicles. If soap is used the wash water must be
captured and directed to the sanitary sewer. For information concerning the use of charity car wash
kits, contact City of Auburn Public Works Department at 253-931-3010.
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 to a collection point or catch
basins may be covered to help contain the water. The collected water, provided it meets local limits,
should be disposed of to the sanitary sewer.
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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 Storm Drainage Utility at 253-931-3010 for more information.
Automatic Car Wash
At a minimum, a catch basin/sediment trap that discharges through a tee outlet to the sanitary sewer
is required. Other requirements may be necessary on a case-by-case basis.
Manual (Wand) Car Wash
There must be covered and bermed bays with a catch basin/sediment trap connected to a designed
pretreatment device discharging to the sanitary sewer. At a minimum a coalescing plate oil water
separator will be required. The facility will be inspected periodically by City staff.
Truck Washing Facilities
Wash on a concrete or asphalt paved dedicated wash pad with a catch basin/sediment trap
connected to a designed pretreatment device, which discharges to the sanitary sewer. At a minimum
a coalescing plate oil water separator will be required. The facility will be inspected periodically by
City staff.
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 and they must
possess a Letter of Authorization issued by the City’s Storm Drainage Utility.
Mobile Vehicle Washers must also:
• Discharge all wash water to the sanitary sewer;
• Submit MSDS sheets for all chemicals used;
• Obtain Source Control approval for any chemicals used; and
• Provide adequate means to prevent contaminated wash water from entering the
storm drainage system or discharging onto unpaved ground.
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 Storm
Drainage Utility. Additional pretreatment may be required. Discharges from mobile washing must
meet City of Auburn code requirement, Chapter 13.20 discharge limitations for pH.
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Those washers employing the two-step (acid-alkaline) process must obtain approval from City of
Auburn Sanitary Sewer Utility on a case-by-case basis. At a minimum, two-step washers must:
• Provide some means of holding and mixing the wash water generated;
• Provide an accepted means of testing the pH of the mixed wash water;
• Provide a means of adjusting the pH, if necessary, to within City of Auburn limits;
• Discharge the wastewater to the sanitary sewer; or
• Capture all of the water and dispose of it at a licensed treatment, storage, and
disposal facility.
Mobile vehicle washers must discharge wash water to the sanitary sewer system through an on-site
cleanout if available. Only wastewater generated within the City of Auburn may be discharged into
the municipal sewer system.
4.2.3.4 General
Two-step washing may be allowed at all facilities discharging to the sanitary sewer. Provisions must
be in place to neutralize the wash water rinsate prior to introduction into the sanitary sewer system.
For facilities with dedicated wash pads with either a catch basin with 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.
Any sampling and testing required of industry to verify pretreatment equipment performance shall be
accomplished by using 40 CFR Part 136 approved methods. For Total Petroleum Hydrocarbons
(TPH) EPA Method 1664 may be used.
Applicants requesting approval of designed oil/water separator systems must submit three (3) sets of
plans.
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4.2.4 BMP A104: Collection and Disposal of Wastewater in Mobile Interior
Washing Operations
4.2.4.1 Description of Pollutant Sources
This activity applies to businesses that wash carpets and other interior items on a mobile site-to-site
basis. The typical washing process includes use of machines that spray the wash water 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).
4.2.4.2 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.
4.2.4.3 Required BMPs
This BMP is required of all businesses doing mobile interior wash activities:
• Absolutely no wastewater from mobile interior wash activities shall be disposed of
outdoors, or to a drain connected to the storm drainage system. This point must be
made clear to all employees. Wastewater from mobile washing operations may be
permitted for sanitary sewer disposal 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, the collected wastewater
must be returned to the business site for process treatment or transfer to a holding
tank. See BMP S103 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.
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4.2.4.4 Recommended BMPs
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 washwater for more than one use.
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4.3 Transfer of Liquid or Solid Materials
4.3.1 BMP A201: Loading and Unloading Areas for Liquid or Solid Material
4.3.1.1 Description of Pollutant Sources
Loading/unloading of liquid and solid materials at industrial and commercial facilities is typically
conducted at shipping and receiving, outside storage, fueling areas, etc. Materials transferred can
include products, raw materials, intermediate products, waste materials, fuels, scrap metals, etc.
Leaks and spills of fuels, oils, powders, organics, heavy metals, salts, acids, alkalis, etc. during
transfer are potential causes of stormwater contamination. Spills from hydraulic line breaks are a
common problem at loading docks.
4.3.1.2 Pollutant Control Approach
Cover and contain the loading/ unloading area where necessary to prevent run-on of stormwater and
runoff of contaminated stormwater.
4.3.1.3 Required BMPs
At All Loading/ Unloading Areas:
• A significant amount of debris can accumulate at outside, uncovered
loading/unloading areas. Sweep these surfaces frequently to remove material that
could otherwise be washed off by stormwater. Sweep outside areas that are covered
for a period of time by containers, logs, or other material after the areas are cleared.
• Place drip pans or other appropriate temporary containment devices at locations
where leaks or spills may occur such as hose connections, hose reels, and filler
nozzles. Drip pans shall always be used when making and breaking connections
(see Figure IV-4-2). Check loading/unloading equipment such as valves, pumps,
flanges, and connections regularly for leaks and repair as needed. Frequent
monitoring of drip pans is required to ensure captured materials are not displaced by
wind or rainwater.
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Figure 4.2 – Drip Pan
Figure IV-4-2. Drip Pan
At Tanker Truck and Rail Transfer Areas to Above/Below-ground Storage Tanks:
• To minimize the risk of accidental spillage, prepare an "Operations Plan" that
describes procedures for loading/unloading. Train the employees, especially fork lift
operators, in its execution and post it or otherwise have it readily available to
employees.
• Report spills of reportable quantities to Ecology’s Northwest Regional Office
425-649-7000.
• Prepare and implement an Emergency Spill Cleanup Plan for the facility (BMP A714
Spills of Oil and Hazardous Substances) which includes the following BMPs:
o Ensure the clean up of liquid/solid spills in the loading/unloading area
immediately if a significant spill occurs, upon completion of the
loading/unloading activity, or at the end of the working day.
o Retain and maintain an appropriate oil spill cleanup kit on-site for rapid
cleanup of material spills (see BMP A714 Spills of Oil and Hazardous
Substances).
o Ensure that an employee trained in spill containment and cleanup is present
during loading/unloading.
At Rail Transfer Areas to Above/Below-ground Storage Tanks:
Install a drip pan system as illustrated (see Figure IV-4-3) within the rails to collect
spills/leaks from tank cars and hose connections, hose reels, and filler nozzles.
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Figure IV-4-3. Drip Pan Within Rails
Loading/Unloading from/to Marine Vessels:
Facilities and procedures for the loading or unloading of petroleum products must comply
with Coast Guard requirements.
Transfer of Small Quantities from Tanks and Containers:
Refer to BMPs A408 Storage of Liquids in Permanent Above-Ground Tanks and A407
Storage of Liquid, Food Waste, or Dangerous Waste Containers for requirements on the
transfer of small quantities from tanks and containers, respectively.
At All Loading/ Unloading Areas:
• Consistent with Uniform Fire Code requirements and to the extent practicable,
conduct unloading or loading of solids and liquids in a manufacturing building or
under a roof, lean-to, or other appropriate cover.
• Berm, dike, and/or slope the loading/unloading area to prevent run-on of stormwater
and to prevent the runoff or loss of any spilled material from the area.
• Large loading areas frequently are not curbed along the shoreline. As a result,
stormwater passes directly off the paved surface into surface water. Place curbs
along the edge, or slope the edge such that the stormwater can flow to an internal
storm drain system that leads to an approved treatment BMP.
• Pave and slope loading/unloading areas to prevent the pooling of water. The use of
catch basins and drain lines within the interior of the paved area must be minimized
as they will frequently be covered by material, or they shall be placed in designated
“alleyways” that are not covered by material, containers, or equipment.
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4.3.1.4 Recommended BMPs:
• For the transfer of pollutant liquids in areas that cannot contain a catastrophic spill,
install an automatic shutoff system in case of unanticipated off-loading interruption
(e.g. coupling break, hose rupture, overfill, etc.).
At Loading and Unloading Docks:
• Install/maintain overhangs or door skirts that enclose the trailer end
(see Figure IV-4-4 and Figure IV-4-5) to prevent contact with rainwater.
• Design the loading/unloading area with berms, sloping, etc. to prevent the run-on of
stormwater.
• Retain on-site the necessary materials for rapid cleanup of spills.
At Tanker Truck Transfer Areas to Above/Below-Ground Storage Tanks:
• Pave the area on which the transfer takes place. If any transferred liquid, such as
gasoline, is reactive with asphalt, pave the area with Portland cement concrete.
• Slope, berm, or dike the transfer area to a dead-end sump, spill containment sump,
spill control (SC) oil/water separator, or other spill control device. The minimum spill
retention time should be 15 minutes at the highest fuel dispenser nozzle through-put
rate or the peak flow rate of the 6-month, 24-hour storm event over the surface of the
containment pad, whichever is greater. The volume of the spill containment sump
should be a minimum of 50 gallons with an adequate grit sedimentation volume.
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Figure IV-4-4. Loading Dock with Door Skirt
Figure 4.5 – Loading Dock with Overhang
Figure IV-4-5. Loading Dock with Overhang
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4.3.2 BMP A202: Fueling at Dedicated Stations
4.3.2.1 Description of Pollutant Sources
A fueling station is a facility dedicated to the transfer of fuels from a stationary pumping station to
mobile vehicles or equipment. It includes above or under-ground fuel storage facilities. In addition to
general service gas stations, fueling may also occur at 24-hour convenience stores, construction
sites, warehouses, car washes, manufacturing establishments, port facilities, and businesses with
fleet vehicles. Typically, stormwater contamination at fueling stations is caused by leaks/spills of
fuels, lube oils, radiator coolants, and vehicle wash water.
4.3.2.2 Pollutant Control Approach
New or substantially remodeled fueling stations must be constructed on an impervious concrete pad
under a roof to keep out rainfall and stormwater run-on. Substantial remodeling includes replacing the
canopy or relocating or adding one or more fuel dispensers in such a way that the Portland cement
concrete (or equivalent) paving in the fueling area is modified. A treatment BMP must be used for
contaminated stormwater and wastewaters in the fueling containment area.
4.3.2.3 Required BMPs:
General Requirements
• Fuel islands shall not drain into the storm drainage system.
• Fuel islands shall be paved and provide a means to protect the storm drainage and
sanitary sewers from spills.
• Fuel islands may provide blind sumps for spill containment, or they may drain into
the sanitary sewer through a properly sized oil/water separator protected by an
emergency shut-off valve. Contact the City of Auburn Sanitary Sewer Utility at 253-
931-3010 for assistance.
• Fueling areas must encompass the reach of the longest fueling hose.
Oil/Water Separator Requirements (see Figure IV-4-6)
• Separators shall have as a minimum a 4” diameter tee installed in the discharge line.
• Separators shall have an emergency shut-off valve installed on the discharge line. A
valve key shall be provided and be prominently displayed near the shut-off valve.
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Figure IV-4-6. Oil/Water Separator Layout
Accidental Spill Prevention Plan
An Accidental Spill Prevention Plan developed by the operator and approved by Storm
Drainage Utility shall be available for inspection. See BMP A714 Spills of Oil and Hazardous
Materials for the elements of a spill plan.
Covered Fuel Islands
• Areas outside the canopy cover shall be sloped or bermed to divert precipitation
away from the fuel island and into the storm drainage system.
• For a covered fuel island with incidental stormwater run-on and no more than four
(4) hose bibs, a minimum 18 gpm rated, 530 gallon capacity oil/water separator
(Hanson Pipe Vault 466-S or equivalent) shall be used when connecting to a sanitary
sewer.
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• Covered fueling islands must have a roof or canopy to prevent the direct entry of
precipitation onto the spill containment pad (see Figure IV-4-7 below). The roof or
canopy shall, at a minimum, cover the spill containment pad (within the grade break
or fuel dispensing area) and preferably extend several additional feet to reduce the
introduction of windblown rain. Convey all roof drains to storm drains outside the
fueling containment area.
Figure IV-4-7. Covered Fuel Island
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Uncovered Fuel Islands
a. Fuel islands shall be sized to minimize the area which drains to the sanitary
sewer. Areas outside the pad shall be sloped to divert precipitation into the storm
drainage system.
b. Separators used for spill protection with uncovered fuel islands must be sized
using standard engineering practices and shall be based on a 25-year storm
event (see Table IV-4-1 below).
Table IV-4-1. Uncovered Fuel Island Separator Sizing Requirements
Calculated Flow
(gpm)
Separator Size Requirement
18 530 gallon capacity PIPE Vault 466-S, or equivalent
30 900 gallon capacity PIPE Vault 577-S, or equivalent
72 2,160 gallon capacity PIPE Vault 5106-S, or equivalent
108 3,230 gallon capacity PIPE Vault 612-S, or equivalent
126 3,770 gallon capacity PIPE Vault 712-S, or equivalent
182 5,450 gallon capacity PIPE Vault 814-8-S, or equivalent
215 6,460 gallon capacity PIPE Vault 818-8-S, or equivalent
NOTE: This sizing is not appropriate for a stormwater quality treatment device. Refer to
Volume V, Chapter 9.
BMPs Applicable to Both Covered and Uncovered Fuel Islands
• Prepare an emergency spill response and cleanup plan (per BMP A714 Spills of Oil
and Hazardous Substances) and have designated trained person(s) available either
on site or on call at all times to promptly and properly implement that plan and
immediately cleanup all spills. Keep suitable cleanup materials, such as dry
adsorbent materials, on site to allow prompt cleanup of a spill.
• Train employees on the proper use of fuel dispensers. Post signs in accordance with
the International Fire Code (IFC). Post “No Topping Off” signs (topping off gas tanks
causes spillage and vents gas fumes to the air). Make sure that the automatic shutoff
on the fuel nozzle is functioning properly.
• The person conducting the fuel transfer must be present at the fueling pump during
fuel transfer, particularly at unattended or self-serve stations.
• Provide suitable containers for waste materials such as oil filters, oil cans, and
garbage.
• Design the fueling island to control spills (a spill control oil/water separator in
compliance with City of Auburn requirements) and to treat collected stormwater
and/or wastewater to required levels. Slope the concrete containment pad around
the fueling island toward drains: trench drains or, catch basins. The slope of the
drains shall not be less than 1 percent (Section 3405.3.8.1 of the IFC). The outlet
from the spill control oil/water separator shall have a shutoff valve, which must be
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closed in the event of a spill. The spill control sump must be sized using standard
engineering practices and shall be based a 25-year storm event when the fueling
area is uncovered; or
• Design the fueling island as a spill containment pad with a sill or berm raised to a
minimum of four inches to prevent the runoff of spilled liquids and to prevent run-on
of stormwater from the surrounding area. Raised sills are not required at the open-
grate trenches that connect to an approved drainage-control system.
• The fueling pad must be paved with Portland cement concrete, or equivalent. Asphalt
is not considered an equivalent material.
• Stormwater collected on the fuel island containment pad must be conveyed to a
sanitary sewer system, as approved by the City of Auburn. In rare cases, stormwater
may be conveyed to the storm drainage system through an approved treatment
system such as an oil/water separator and a basic treatment BMP (basic treatment
BMPs are listed in Volume V and include media filters and biofilters). Discharges
from treatment systems to storm drains or surface water or to the ground must not
display ongoing or recurring visible sheen and must not contain levels of oil and
grease above Water Quality or Model Toxics Control Act criteria.
• Alternatively, stormwater collected on the fuel island containment pad may be
collected and held for proper off site disposal.
• Conveyance of any fuel-contaminated stormwater to a sanitary sewer must be
approved by the City of Auburn. All discharge must meet the criteria specified in
Auburn City Code Chapter 13.20.
• Transfer fuel from the delivery tank truck to the fuel storage tank on an impervious
surface and ensure that appropriate overflow protection is used. Use drip pans under
all hose connections.
Additional BMP for Vehicles 10 feet in height or greater:
A roof or canopy may not be practicable at fueling stations that regularly fuel vehicles that are 10 feet
in height or greater, particularly at industrial sites. At those types of fueling facilities, the following
BMPs apply, as well as all of the other required BMPs and fire code requirements.
• The concrete fueling pad must be equipped with emergency spill control, which may
include an oil/water separator, with a shutoff valve for the drainage from the fueling
area. The valve must be closed in the event of a spill. Spills must be cleaned up and
disposed off-site in accordance with BMP A714 Spills of Oil and Hazardous
Substances.
• Stormwater collected on the fuel island containment pad must be conveyed to a
sanitary sewer system, as approved by the City of Auburn. In rare cases, and only
when approved by the City, stormwater may be conveyed to the storm drainage
system through an approved treatment system such as an oil/water separator and a
basic treatment BMP (basic treatment BMPs are listed in Volume V and include
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media filters and biofilters). Discharges from treatment systems to storm drains or
surface water or to the ground must not display ongoing or recurring visible sheen
and must not contain levels of oil and grease above Water Quality or Model Toxics
Control Act criteria.
An explosive or flammable mixture is defined under state and federal pretreatment regulations, based
on a flash point determination of the mixture. If contaminated stormwater is determined not to be
explosive or flammable, then it could be conveyed to a sanitary sewer system, if approved by the City
of Auburn.
Figure IV-4-8. Surrounding Activities Containing Oversized Equipment
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4.3.3 BMP A203: Vehicle Maintenance Activities
4.3.3.1 Description of Pollutant Sources
This activity 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 this manual, and other BMPs provided in this manual:
A103 Washing, Pressure Washing, and Steam Cleaning of
Vehicles/Equipment/Building Structures
A201 Loading and Unloading Areas for Liquid or Solid Material
A202 Fueling at Dedicated Stations
A204 Mobile Fueling
A307 Painting, Finishing and Coating of Vehicles, Boats, Buildings, and Equipment
A401 Storage or Transfer (Outside) of Solid Raw Materials, By-Products,
or Finished Products
A407 Storage of Liquid, Food Waste, or Dangerous Waste Containers
A408 Storage of Liquids in Permanent Above-ground Tanks
A409 Parking and Storage for Vehicles and Equipment
A714 Spills of Oil and Hazardous Substances
Pollutants of concern include toxic hydrocarbons, toxic organic compounds, oils and greases, pH,
and heavy metals.
4.3.3.2 Pollutant Control Approach
Control of leaks and spills of fluids using good housekeeping and cover and containment BMPs.
4.3.3.3 Required 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. Inspect all incoming
vehicles, parts, and equipment stored temporarily outside for leaks.
• Remove batteries and liquids from vehicles and equipment in designated areas
designed to prevent stormwater contamination. Store cracked batteries in a covered
non-leaking secondary containment system.
• Empty fuel and fuel filters before disposal.
• 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.
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• 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 S102 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 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 S106 and S107 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.
• Implement the following treatment BMP in addition to the Required BMPs:
• Contaminated stormwater runoff from vehicle staging and maintenance areas must
be conveyed to an API or CP oil and water separator followed by a basic treatment
BMP (see Volume V), applicable filter, or other equivalent oil treatment system.
4.3.3.4 Recommended BMPs
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.
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4.3.4 BMP A204: Mobile Fueling of Vehicles and Heavy Equipment
4.3.4.1 Description of Pollutant Sources
Mobile fueling, also known as fleet fueling, wet fueling, or wet hosing, is the practice of filling fuel
tanks of vehicles by tank trucks that are driven to the yards or sites where the vehicles to be fueled
are located. Mobile fueling is only conducted using diesel fuel, as mobile fueling of gasoline is
prohibited. Diesel fuel is considered a Class II Combustible Liquid, whereas gasoline is considered a
Flammable Liquid.
Historically mobile fueling has been conducted for off-road vehicles that are operated for extended
periods of time in remote areas. This includes construction sites, logging operations, and farms.
Mobile fueling of on-road vehicles is also conducted commercially in the State of Washington.
4.3.4.2 Pollutant Control Approach
Proper training of the fueling operator, and the use of spill/drip control and reliable fuel transfer
equipment with backup shutoff valving are typically needed.
4.3.4.3 Required BMPs
Organizations and individuals conducting mobile fueling operations must implement the following
BMPs. The operating procedures for the driver/operator shall be simple, clear, effective and their
implementation verified by the organization that will potentially be liable for environmental and third
party damage.
• Ensure that all mobile fueling operations are approved and permitted by the Valley
Regional Fire Authority and comply with local and Washington State fire codes.
Contact the Valley Regional Fire Authority at 253-931-3060.
• In fueling locations that are in close proximity to sensitive aquifers, designated
wetlands, wetland buffers, or other waters of the State, approval by the City of
Auburn is necessary to ensure compliance with additional local requirements. It shall
be the responsibility of the site owner to obtain approval under this exception. Any
permit holder permitted under this exception shall cover all catch basins prior to
commencing any fueling operations. Sites which are adjacent to designated
wetlands, wetland buffers, streams, or bodies of water shall have on site, in a
marked conspicuous location a minimum of 50 feet of 4-inch diameter, non-water
absorbing containment boom.
• Ensure compliance with all 49 CFR 178 requirements for DOT 406 cargo tanker.
Documentation from a Department of Transportation (DOT) Registered Inspector
shall be proof of compliance.
• Ensure the presence and the constant observation/monitoring by the driver/operator
at the fuel transfer location at all times during fuel transfer and ensure that the
following procedures are implemented at the fuel transfer locations:
o Locate the point of fueling at least 25 feet from the nearest storm drain or
inside an impervious containment area with a volumetric holding capacity
equal to or greater than 110 percent of the receiving tank volume, or place an
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impervious covering over the storm drain to ensure no inflow of spilled or
leaked fuel. Storm drains that convey the inflow to a spill control separator
approved by the City of Auburn and the Valley Regional Fire Authority need
not be covered. Potential spill/leak conveyance surfaces must be impervious
and in good repair.
o Place a drip pan or an absorbent pad under each fueling location prior to and
during all dispensing operations. The pan (must be liquid tight) and the
absorbent pad must have a capacity of 3 gallons. Spills retained in the drip
pan or the pad need not be reported.
o Handle and operate fuel transfer hoses and nozzle, drip pan(s), and
absorbent pads to prevent spills/leaks of fuel from reaching the ground, storm
drains, and receiving waters.
o Do not extend the fueling hoses across a traffic lane without fluorescent traffic
cones, or equivalent devices.
o Remove the fill nozzle and cease filling when the automatic shut-off valve
engages. Do not allow automatic shutoff fueling nozzles to be locked in the
open position.
o Do not “top off” the equipment receiving fuel.
• Provide the driver/operator of the fueling vehicle with:
o Adequate headlamps, flashlights or other mobile lighting to view fill openings
with poor accessibility. Consult with the Valley Regional Fire Authority for
additional lighting requirements.
o Two-way communication with home base.
• Train the driver/operator annually in spill prevention, reporting and cleanup measures
and emergency procedures. Make all employees aware of the significant liability
associated with fuel spills.
• The fueling operation procedures shall be properly signed and dated by the
responsible manager, distributed to the operators, retained in the organization files,
and made available in the event an authorized government agency requests a
review.
• Ensure that the Valley Regional Fire Authority (911) and the Ecology Northwest
Regional Office (425) 649-7000 are immediately notified in the event of any spill
entering surface or groundwaters, including catch basins. Establish a 24-hour “call
down list” to ensure the rapid and proper notification of management and
government officials should any amount of product be spilled on-site. Keep the list in
a protected but readily accessible location in the mobile fueling truck. The “call down
list” shall also pre-identify spill response contractors available in the area to ensure
the rapid removal of significant product spillage into the environment.
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• Maintain a minimum of the following spill clean-up materials in all fueling vehicles,
that are readily available for use:
o Non-water absorbents (pads, pillows, sump skimmers) capable of absorbing
15 gallons of diesel fuel;
o A storm drain plug or cover kit;
o A non-water absorbent containment boom of a minimum 10 feet in length with
a 12-gallon absorbent capacity;
o A non-metallic shovel; and
o Two, five-gallon buckets with lids.
o Loose granular absorbent material capable of absorbing a minimum of
5 gallons of diesel. Use only non-water absorbing materials, such as peat
moss, during wet weather conditions.
• Use automatic shutoff nozzles for dispensing the fuel. Replace automatic shut-off
nozzles as recommended by the manufacturer.
• Maintain and replace equipment on fueling vehicles, particularly hoses and nozzles,
at established intervals to prevent failures.
• Do not overfill tanks. Allow room for heat expansion of fuel during warm weather.
• Include the following fuel transfer site components:
o Automatic fuel transfer shut-off nozzles; and
o An adequate lighting system at the filling point.
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4.4 Production and Application Activities
4.4.1 BMP A301: Concrete and Asphalt Mixing and Production at Stationary
Sites
4.4.1.1 Description of Pollutant Sources
This activity 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 under
BMP A302. Requirements for stockpiling of raw materials are covered under BMP A401 Storage or
Transfer (Outside) of Solid Raw Materials, By-products or Finished Products.
Pollutants of concern include toxic hydrocarbons, toxic organic compounds, oils and greases, heavy
metals, and pH.
4.4.1.2 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 Ecology at 360-407-6400 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.
4.4.1.3 Required BMPs
The following BMPs or equivalent measures 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 S101 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.
• 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
S107 for methods of run-on protection.
• Cover the production area for prevention of stormwater run-on. See BMP S104 and
S107 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.
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4.4.1.4 Recommended BMPs
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 S102 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, 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.
Contact the City of Auburn Storm Drainage Utility at 253-931-3010 for information about water quality
treatment BMPs for these types of operations. Contact the Department of Ecology’s web page for
accepted water quality treatment at http://www.ecy.wa.gov/programs/wq/stormwater/index.html.
The use of any treatment BMP must not result in the violation of groundwater, surface water, or
wastewater standards.
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4.4.2 BMP A302: Concrete Pouring, Concrete Cutting, and Asphalt
Application at Temporary Sites
4.4.2.1 Description of Pollutant Sources
This activity applies to businesses and public agencies that apply asphalt or pour or cut concrete for
building construction and remodeling, road construction, utility projects, sidewalk, curb and gutter
repairs and construction, sealing of driveways and roofs, and other applications. These activities are
typically done on a temporary site-to-site basis where permanent BMP measures do not apply.
Concrete pouring activities can not only severely alter the pH of receiving waters, but slurry from
aggregate washing can harden in storm pipes, thus reducing capacity and creating flooding
problems.
Pollutants of concern include toxic hydrocarbons, toxic organic compounds, oils and greases, heavy
metals, suspended solids, and pH.
4.4.2.2 Pollutant Control Approach
Train employees on proper procedures, sweep or shovel aggregate chunks, collect accumulated
runoff and solids, and wash equipment in designated areas.
4.4.2.3 Required BMPs
The following BMPs or equivalent measures are required of all businesses and agencies doing
concrete pouring and asphalt application at temporary sites:
• Employees must be educated on the pollution hazards of concrete and asphalt
application and cutting.
• Loose aggregate chunks and dust must be swept or shoveled and collected (not
hosed down a storm drain) for recycling or proper disposal at the end of each work
day, especially at work sites such as streets, driveways, parking lots, sidewalks,
curbs, and gutters where rain can readily pick up the loose material and carry it to
the nearest stormwater conveyance. Small amounts of excess concrete, grout, and
mortar can be disposed of in the trash.
• Storm drain covers or similarly effective containment devices must be placed over all
nearby drains at the beginning of each day. Shovel or vacuum slurry and remove
from the site. All accumulated runoff and solids must be collected and properly
disposed (see BMP S102 for disposal options) at the end of each work day, or more
often if necessary.
• Exposed aggregate washing, where the top layer of unhardened concrete is hosed
or scraped off to leave a rough finish, must be done with a mechanism for
containment and collection of the discarded concrete slurry (such as the storm drain
covers mentioned above). The easiest way to contain the washwater is to direct the
washings to a trench in the ground where the water can percolate into the ground
and the solids later covered with soil.
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• Cleaning of concrete application and mixing equipment or concrete vehicles on the
work site must be done in a designated area where the rinse water is controlled. The
rinse water must either be collected for proper disposal or put into a trench in the
ground where the water can percolate away and the solids later covered with soil or
recovered and disposed or recycled.
The use of any treatment BMP must not result in the violation of groundwater, surface water, or
drinking water quality standards.
4.4.2.4 Recommended BMPs
The following BMPs are not required but can provide additional pollution prevention:
• Avoid the activity when rain is occurring or expected.
• If possible, portable asphalt mixing equipment should be covered by an awning, a
lean-to, or another simple structure to avoid contact with rain. See BMP S104 for
further details on cover structures.
• Recycle broken concrete and asphalt. Look under Recycling Services in the Yellow
pages of the phone book to find the nearest recycler.
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4.4.3 BMP A303: Manufacturing and Post-Processing of Metal Products
4.4.3.1 Description of Pollutant Sources
This activity 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 or an Industrial Wastewater Discharge Permit from King County-Metro. See Chapter 5 for a
discussion of NPDES requirements and contact the City of Auburn Sanitary Sewer Utility at
253-931-3010 to determine if a wastewater discharge permit is necessary.
NOTE: Painting, finishing and coating of metal products is covered under BMP A307 Painting,
Finishing, and Coating of Vehicles, Boats, Buildings, and Equipment.
Pollutants of concern include toxic organic compounds, heavy metals, oils and greases, pH,
suspended solids, and biological oxygen demand (BOD).
4.4.3.2 Pollutant Control Approach
Cover and contain operations and apply good housekeeping and preventive maintenance practices
to prevent the contamination of stormwater.
4.4.3.3 Required BMPs
The following BMPs or equivalent measures are required of all businesses engaged in metals
manufacturing or post-processing:
• Eliminate illicit connections to the storm drainage system. See BMP S101 for
detailed information on identifying and eliminating illicit connections.
• Process wastewater (including contact cooling water, filter backwash, cooling tower
blowdown, etc.) from this activity, and stormwater runoff from activity areas, must
discharge to a sanitary sewer, holding tank, or process treatment system that would
need an Ecology NPDES Permit for discharge to surface water or storm drain.
Contact the City of Auburn Permit Center at 253-931-3090 to obtain permits for
discharge to the sewer. See BMP S103 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 S102 for disposal
alternatives.
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4.4.3.4 Recommended BMPs
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.
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4.4.4 BMP A304: Wood Treatment Areas
4.4.4.1 Description of Pollutant Sources
Wood treatment includes both anti-staining and wood preserving using pressure processes or by
dipping or spraying. Wood preservatives include creosote, creosote/coal tar, pentachlorophenol,
copper naphthenate, arsenic trioxide, malathion, or inorganic arsenicals such as chromated copper
arsenate, acid copper chromate, chromate zinc chloride, and fluor-chrome-arsenate-phenol. Anti-
staining chemical additives include iodo-prophenyl-butyl carbamate, dimethyl sulfoxide, didecyl
dimethyl ammonium chloride, sodium azide, 8-quinolinol, copper (II) chelate, sodium ortho-
phenylphenate, 2-(thiocyanomethylthio)-benzothiazole (TCMTB) and methylene bis- (thiocyanate),
and zinc naphthenate.
Pollutant sources include drips of condensate or preservative after pressurized treatment, product
washwater (in the treatment or storage areas), spills and leaks from process equipment and
preservative tanks, fugitive emissions from vapors in the process, blowouts and emergency pressure
releases, and kick-back from lumber (phenomenon where preservative leaks as it returns to normal
pressure). Potential pollutants typically include the wood treating chemicals, BOD, suspended solids,
oil and grease, benzene, toluene, ethylbenzene, phenol, chlorophenols, nitrophenols, heavy metals,
and PAH, depending on the chemical additive used.
4.4.4.2 Pollutant Control Approach
Cover and contain all wood treating areas and prevent all leaching of and stormwater contamination
by wood treating chemicals. All wood treating facilities in Washington State are required to be
covered under an individual NPDES Permit and may require an Industrial Wastewater Discharge
Permit from King County-Metro.
4.4.4.3 Required BMPs
The individual NPDES Permit will require the following BMPs at a minimum:
• Dedicate equipment that is used for treatment activities to prevent the tracking of
treatment chemicals to other areas on the site.
• Eliminate non-process traffic on the drip pad. Scrub down non-dedicated lift trucks on
the drip pad.
• Immediately remove and properly dispose of soils with visible surface contamination
(green soil) to prevent the spread of chemicals to groundwater and/or surface water
via stormwater runoff.
• If any wood is observed to be contributing chemicals to the environment in the
treated wood storage area, relocate it on a concrete chemical containment structure
until the surface is clean and until it is drip free and surface dry.
• Cover and/or enclose, and contain with impervious surfaces, all wood treatment
areas. Slope and drain areas around dip tanks, spray booths, retorts, and any other
process equipment in a manner that allows return of treatment chemicals to the
wood treatment process.
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• Cover storage areas for freshly treated wood to prevent contact of treated wood
products with stormwater. Segregate clean stormwater from process water. Ensure
that all process water is conveyed to an approved treatment system.
• Seal any holes or cracks in the asphalt areas that are subject to wood treatment
chemical contamination.
• Elevate stored, treated wood products to prevent contact with stormwater run-on and
runoff.
• Place dipped lumber over the dip tank or on an inclined ramp for a minimum of
30 minutes to allow excess chemical to drip back to the dip tank.
• Place treated lumber either from dip tanks or retorts in a covered paved storage area
for at least 24 hours before placement in outside storage. Use a longer storage
period during cold weather unless the temporary storage building is heated. The
wood shall be drip free and surface dry before it is moved outside.
4.4.4.4 Recommended BMP
Consider using preservative chemicals that do not adversely impact receiving surface water and
groundwater.
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4.4.5 BMP A305: Commercial Composting
4.4.5.1 Description of Pollutant Sources
Commercial compost facilities operating outside without cover require large areas to decompose
wastes and other feedstocks. These facilities should be designed to separate stormwater from
leachate (i.e., industrial wastewater) to the greatest extent possible. When stormwater is allowed to
contact any active composting areas, including waste receiving and processing areas, it becomes
leachate. Pollutants in leachate include nutrients, biochemical oxygen demand (BOD), organics,
coliform bacteria, acidic pH, color, and suspended solids. Stormwater at a compost facility consists of
runoff from areas at the facility that are not associated with active processing and curing, such as
product storage areas, vehicle maintenance areas, and access roads.
4.4.5.2 NPDES Permit Requirements
Discharge of leachate from a compost facility will require a State or NPDES permit from Ecology,
depending on the disposal method chosen for managing leachate at the facility (see the Ecology
website www.ecy.wa.gov/programs/swfa/compost ). An additional alternative, zero discharge, is
possible by containing all leachate from the facility (in tanks or ponds) or preventing production of
leachate (by composting under a roof or in an enclosed building).
4.4.5.3 Pollutant Control Approach
Consider the leachate control specified on the Ecology website:
www.ecy.wa.gov/programs/swfa/compost or zero discharge of leachate.
4.4.5.4 Required BMPs
• Ensure that the compost feedstocks do not contain dangerous wastes regulated
under Chapter 173-303 WAC or hazardous products of a similar nature or solid
wastes that are not beneficial to the composting process. Employees must be trained
to screen these materials in incoming wastes.
• Contact other federal, state, and City of Auburn departments with environmental or
zoning authority for applicable permit and regulatory information. County Health
Departments are responsible for issuing solid waste handling permits for commercial
compost facilities.
• Apply for coverage under the General Permit to Discharge Stormwater Associated
with Industrial Activities if the facility discharges stormwater to surface water or a
municipal stormwater system. If all stormwater from the facility infiltrates into the
surrounding area, the General Permit is not required.
• Develop a plan of operations as outlined in the Composting Facility Standards
(WAC 173-350-220).
• Store finished compost in a manner to prevent contamination of stormwater.
• Compost pads are required for all uncovered facilities in areas of the state with wet
climates (per water quality regulations).
• Provide curbing for all compost pads to prevent stormwater run-on and leachate
run-off.
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• Slope all compost pads sufficiently to direct leachate to the collection device.
• Provide one or more sumps or catch basins capable of collecting all leachate
generated by the design storm and conveying it to the leachate holding structure for
all compost pads.
• Convey all leachate from composting operations to a sanitary sewer, holding tank, or
on-site treatment system designed to treat the leachate. Discharge of leachate to the
sanitary sewer may require an Industrial Wastewater Discharge Permit. Contact the
City of Auburn Sanitary Sewer Utility at 253-931-3010 for a determination.
• Ponds used to collect, store, or treat leachate and other contaminated waters
associated with the composting process must be lined to prevent groundwater
contamination. Apply “AKART” or All Known Available and Reasonable Methods of
Prevention and Treatment to all pond liners, regardless of the construction materials.
Refer to “Compost Facility Resource Handbook, Guidance for Washington State,” November 1998,
Publication # 97-502, for additional design criteria and information.
4.4.5.5 Recommended BMPs
• Clean up debris from yard areas regularly.
• Locate stored residues in areas designed to collect leachate.
• Limit storage times of residues to prevent degradation and generation of leachate.
• Consider using leachate as make-up water in early stages of the composting
process. Since leachate can contain pathogenic bacteria, care should be taken to
avoid contaminating finished product or nearly finished product with leachate.
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4.4.6 BMP A306: Landscaping and Lawn/Vegetation Management
4.4.6.1 Description of Pollutant Sources
Landscaping can include grading, soil transfer, vegetation removal, pesticide and fertilizer application,
and watering. Stormwater contaminants include toxic organic compounds, heavy metals, oils, total
suspended solids, coliform bacteria, fertilizers, and pesticides.
Lawn and vegetation management can include control of objectionable weeds, insects, mold,
bacteria, and other pests with chemical pesticides and is conducted commercially at commercial,
industrial, and residential sites. Examples include weed control on golf course lawns, access roads,
and utility corridors and during landscaping; sap stain and insect control on lumber and logs; rooftop
moss removal; killing nuisance rodents; fungicide application to patio decks; and residential
lawn/plant care. Toxic pesticides such as pentachlorophenol, carbamates, and organometallics can
be released to the environment by leaching and dripping from treated parts, container leaks, product
misuse, and outside storage of pesticide contaminated materials and equipment. Poor management
of the vegetation, poor application of pesticides or fertilizers, and non-targeted irrigation water or over
watering can cause appreciable stormwater contamination.
4.4.6.2 Pollutant Control Approach
Control of fertilizer and pesticide applications, soil erosion, and site debris to prevent contamination of
stormwater.
Develop and implement an Integrated Pest Management Plan (IPM) and use pesticides only as a
last resort. Refer to Volume IV, Appendix C – Example of an Integrated Pest Management Program
for more information. If pesticides/herbicides are used they must be carefully applied in accordance
with label instructions on U.S. Environmental Protection Agency (EPA) registered materials. Maintain
appropriate vegetation, with proper fertilizer application where practicable, to control erosion and the
discharge of stormwater pollutants. Where practicable, grow plant species appropriate for the site, or
adjust the soil properties of the subject site to grow desired plant species.
4.4.6.3 Required BMPs for Landscaping
• Install engineered soil/landscape systems to improve the infiltration and regulation of
stormwater in landscaped areas.
• Do not dispose of collected vegetation into wetlands, waterways or storm drainage
systems.
4.4.6.4 Recommended BMPs for Landscaping
• Conduct mulch-mowing whenever practicable.
• Dispose of grass clippings, leaves, sticks, or other collected vegetation by
composting, if feasible.
• Collect all clippings, leaves, bark, and trimmings blown onto the sidewalk or street.
Do not leave this material in the gutter or where it can be washed into the storm
drainage system.
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• Use mulch or other erosion control measures when soils are exposed for more than
one week during the dry season or two days during the rainy season.
• If oil or other chemicals are handled, store and maintain appropriate oil and chemical
spill cleanup materials in readily accessible locations. Ensure that employees are
familiar with proper spill cleanup procedures.
• Till fertilizers into the soil rather than dumping or broadcasting onto the surface.
Determine the proper fertilizer application for the types of soil and vegetation
encountered.
• Till a topsoil mix or composted organic material into the soil to create a well-mixed
transition layer that encourages deeper root systems and drought-resistant plants.
• Use manual and/or mechanical methods of vegetation removal rather than applying
herbicides, where practical.
• Target irrigation water on vegetated areas and limit irrigation time to reduce the
potential of carrying fertilizers and pesticides off-site.
4.4.6.5 Required BMPs for the Use of Pesticides
• Develop and implement an integrated pest management system (IPM) (See section
on IPM at end of BMP) and use pesticides only as a last resort.
• Implement a pesticide-use plan and include at a minimum: a list of selected
pesticides and their specific uses; brands, formulations, application methods, and
quantities to be used; equipment use and maintenance procedures; safety, storage,
and disposal methods; and monitoring, record keeping, and public notice
procedures. All procedures shall conform to the requirements of Chapter 17.21 RCW
and Chapter 16-228 WAC (Appendix 4 – D.R.7).
• Choose the least toxic pesticide available that is capable of reducing the infestation
to acceptable levels. The pesticide should readily degrade in the environment and/or
have properties that strongly bind it to the soil. Any pest control used should be
conducted at the life stage when the pest is most vulnerable. Any method used
should be site-specific and not used wholesale over a wide area.
• Apply the pesticide according to label directions. Under no conditions shall pesticides
be applied in quantities that exceed manufacturer’s instructions.
• Mix the pesticides and clean the application equipment in an area where accidental
spills will not enter surface or groundwaters, and will not contaminate the soil.
• Store pesticides in enclosed areas or in covered impervious containment. Ensure
that pesticide contaminated stormwater or spills/leaks of pesticides are not
discharged to storm drains. Do not hose down paved areas to a storm drain or
conveyance ditch. Store and maintain appropriate spill cleanup materials in a
location known to all near the storage area.
• Clean up any spilled pesticides and ensure that the pesticide contaminated waste
materials are kept in designated covered and contained areas.
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• The pesticide application equipment must be capable of immediate shutoff in the
event of an emergency.
• Do not spray pesticides within 100 feet of open waters including wetlands; ponds;
and streams, sloughs, and any drainage ditch or channel that leads to open water,
except when approved by Ecology or by the City of Auburn. All sensitive areas
including wells, creeks, and wetlands must be flagged prior to spraying.
• As required by the City of Auburn or by Ecology, complete public posting of the area
to be sprayed prior to the application.
• Spray applications should only be conducted during weather conditions as specified
in the label direction and applicable local and state regulations. Do not apply during
rain or immediately before expected rain.
4.4.6.6 Recommended BMPs for the use of Pesticides
• Consider alternatives to the use of pesticides such as covering or harvesting weeds,
substitute vegetative growth, and manual weed control/moss removal.
• Consider the use of soil amendments, such as compost, that are known to control
some common diseases in plants, such as Pythium root rot, ashy stem blight, and
parasitic nematodes. The following are three possible mechanisms for disease
control by compost addition (USEPA Publication 530-F-9-044):
o Successful competition for nutrients by antibiotic production;
o Successful predation against pathogens by beneficial microorganism; and
o Activation of disease-resistant genes in plants by composts.
Installing an amended soil/landscape system can preserve both the plant system and the soil
system more effectively. This type of approach provides a soil/landscape system with
adequate depth, permeability, and organic matter to sustain itself and continue working as an
effective stormwater infiltration system and a sustainable nutrient cycle.
• Once a pesticide is applied, its effectiveness should be evaluated for possible
improvement. Records should be kept showing the applicability and inapplicability of
the pesticides considered.
• An annual evaluation procedure should be developed including a review of the
effectiveness of pesticide applications, impact on buffers and sensitive areas
(including potable wells), public concerns, and recent toxicological information on
pesticides used/proposed for use. If individual or public potable wells are located in
the proximity of commercial pesticide applications, contact the regional Ecology
hydrologist to determine if additional pesticide application control measures are
necessary.
• Rinsate from equipment cleaning and/or triple-rinsing of pesticide containers should
be used as product or recycled into product.
For more information, contact the WSU Extension Home-Assist Program at 253-445-4556; Bio-
Integral Resource Center (BIRC), P.O. Box 7414, Berkeley, CA 94707; or the Washington
Department of Ecology to obtain “Hazardous Waste Pesticides” (Publication #89-41); contact EPA to
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obtain a publication entitled “Suspended, Canceled and Restricted Pesticides” which lists all
restricted pesticides and the specific uses that are allowed. Valuable information from these sources
may also be available on the internet.
4.4.6.7 Required BMPs for Vegetation Management
• Use at least an eight-inch topsoil layer with at least 8 percent organic matter to
provide a sufficient vegetation–growing medium.
• Select the appropriate turf grass mixture for climate and soil type based on
recommendations from a licensed landscape architect.
• Selection of desired plant species can be made by adjusting the soil properties of the
subject site. For example, a constructed wetland can be designed to resist the
invasion of reed canary grass by layering specific strata of organic matters (e.g.,
compost forest product residuals) and creating a mildly acidic pH and carbon-rich soil
medium. Consult a soil restoration specialist for site-specific conditions.
• Aerate lawns regularly in areas of heavy use, where the soil tends to become
compacted. Aeration shall be conducted while the grasses in the lawn are growing
most vigorously. Remove layers of thatch greater than ¾-inch deep.
• Set the mowing height at the highest acceptable level and mow at times and
intervals designed to minimize stress on the turf. Generally mowing only 1/3 of the
grass blade height will prevent stressing the turf.
4.4.6.8 Fertilizer Management:
• Fertilization needs vary by site depending on plant, soil, and climatic conditions.
Evaluation of soil nutrient levels through regular testing ensures the best possible
efficiency and economy of fertilization. For details on soils testing, contact the Pierce
Conservation District or Cooperative Extension Service.
• Fertilizers shall be applied in amounts appropriate for the target vegetation and at the
time of year that minimizes losses to surface and groundwaters. Do not fertilize
during a drought or when the soil is dry. Alternatively, do not apply fertilizers within
three days prior to predicted rainfall. The longer the period between fertilizer
application and either rainfall or irrigation, the less fertilizer runoff occurs.
• Use slow release fertilizers such as methylene urea, IDBU, or resin coated fertilizers
when appropriate, generally in the spring. Use of slow release fertilizers is especially
important in areas with sandy or gravelly soils.
• Time the fertilizer application to periods of maximum plant uptake. Generally fall and
spring applications are recommended.
• Properly trained persons shall apply all fertilizers. Fertilizers shall not be applied to
grass swales, filter strips, or buffer areas that drain to surface water bodies.
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4.4.6.9 Integrated Pest Management
An IPM program might consist of the following steps:
1. Correctly identify problem pests and understand their life cycle.
2. Establish tolerance thresholds for pests.
3. Monitor to detect and prevent pest problems.
4. Modify the maintenance program to promote healthy plants and discourage pests.
5. Use cultural, physical, mechanical, or biological controls first if pests exceed the
tolerance thresholds.
6. Evaluate and record the effectiveness of the control and modify maintenance practices
to support lawn or landscape recovery and prevent recurrence.
For an elaboration of these steps refer to Volume IV, Appendix C – Example of an Integrated Pest
Management Program.
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4.4.7 BMP A307: Painting, Finishing and Coating of Vehicles, Boats, Buildings
and Equipment
4.4.7.1 Description of Pollutant Sources
Surface preparation and the application of paints, finishes, and/or coatings to vehicles, boats,
buildings, and/or equipment outdoors can be sources of pollutants. Potential pollutants include
organic compounds, oils and greases, heavy metals, and suspended solids.
4.4.7.2 Pollutant Control Approach
Cover and contain painting and sanding operations and apply good housekeeping and preventive
maintenance practices to prevent the contamination of stormwater with painting overspray and grit
from sanding.
4.4.7.3 Required BMPs
• Train employees in the careful application of paints, finishes, and coatings to reduce
misuse and over spray. Use ground or drop cloths or temporary berms underneath
outdoor painting, scraping, sandblasting work, and properly clean and temporarily
store collected debris daily.
• Do not conduct spraying, blasting, or sanding activities over open water or where
wind may blow paint into water.
• Wipe up spills with rags and other absorbent materials immediately. Do not hose
down the area to a storm drain, receiving water, or conveyance ditch to receiving
water.
• On dock areas, sweep or vacuum rather than hose down debris. Collect any hose
water generated and convey to appropriate treatment and disposal.
• Use a storm drain cover, filter fabric, or similarly effective runoff control device if dust,
grit, washwater, or other pollutants may escape the work area and enter a catch
basin. The containment device(s) must be in place at the beginning of the workday.
Collect contaminated runoff and solids and properly dispose of such wastes before
removing the containment device(s) at the end of the workday.
• Use a ground cloth, pail, drum, drip pan, tarpaulin, or other protective device (e.g.
plastic wading pool) for activities such as paint mixing and tool cleaning outside or
where spills can contaminate stormwater.
• Properly dispose of all wastes and prevent all uncontrolled releases to the air,
ground, or water.
• Clean brushes and tools covered with non-water-based paints, finishes, or other
materials in a manner that allows collection of used solvents (e.g., paint thinner,
turpentine, xylol, etc.) for recycling or proper disposal.
• Store toxic materials under cover (tarp, etc.) during precipitation events and when
not in use to prevent contact with stormwater.
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• Enclose and/or contain all work while using a spray gun or conducting sand blasting
and in compliance with applicable air pollution control, OSHA, and WISHA
requirements. Do not conduct outside spraying, grit blasting, or sanding activities
during windy conditions which render containment ineffective.
4.4.7.4 Recommended BMPs
• Clean paintbrushes and tools covered with water-based paints in sinks connected to
sanitary sewers or in portable containers that can be dumped into a sanitary sewer
drain.
• Recycle paint, paint thinner, solvents, pressure wash water, and any other recyclable
materials.
• Use efficient spray equipment such as electrostatic, air-atomized, high volume/low
pressure, or gravity feed spray equipment.
• Purchase recycled paints, paint thinner, solvents, and other products if feasible.
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4.4.8 BMP A308: Commercial Printing Operations
4.4.8.1 Description of Pollutant Sources
Materials used in the printing process include inorganic and organic acids, resins, solvents, polyester
film, developers, alcohol, vinyl lacquer, dyes, acetates, and polymers. Waste products may include
waste inks and ink sludge, resins, photographic chemicals, solvents, acid and alkaline solutions,
chlorides, chromium, zinc, lead, spent formaldehyde, silver, plasticizers, and used lubricating oils. As
the printing operations are conducted indoors, the only likely points of potential contact with
stormwater are the outside temporary waste material storage area and area where chemicals are
offloaded at external unloading bays. Pollutants can include TSS, pH, heavy metals, oil and grease,
and COD.
4.4.8.2 Pollutant Control Approach
Ensure appropriate disposal and NPDES permitting of process wastes. Cover and contain stored raw
and waste materials.
4.4.8.3 Required BMPs
• Discharge process wastewaters to a sanitary sewer (if approved by the City of
Auburn) or to an approved process wastewater treatment system. Contact the City of
Auburn Sanitary Sewer Utility at 253-931-3010 for discharge requirements.
• Do not discharge process wastes or wastewaters into storm drains or surface water.
• Determine whether any of these wastes qualify for regulation as dangerous wastes
and dispose of them accordingly.
• Store raw materials or waste materials that could contaminate stormwater in covered
and contained areas.
4.4.8.4 Recommended BMPs
• Train all employees in pollution prevention, spill response, spill reporting, and
environmentally acceptable materials handling procedures.
• Store materials in proper, appropriately labeled containers. Identify and label all
chemical substances.
• All stormwater management devices should be inspected regularly and maintained
as necessary.
• Try to use press washes without listed solvents, and with the lowest VOC content
possible. Do not evaporate ink cleanup trays to the outside atmosphere.
• Place cleanup sludges into a properly labeled container with a tight lid and dispose of
as hazardous waste. Do not dispose of cleanup sludges in the garbage or in
containers of soiled towels.
For additional information on pollution prevention the following Washington Department of Ecology
publications are recommended: A Guide for Screen Printers, Publication #94-137 and A Guide for
Lithographic Printers, Publication #94-139.
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4.4.9 BMP A309: Manufacturing Operations – Outside
4.4.9.1 Description of Pollutant Sources
Manufacturing pollutant sources include outside process areas, stack emissions, and areas where
manufacturing activity has taken place in the past and significant pollutant materials remain and are
exposed to stormwater.
4.4.9.2 Pollution Control Approach
Cover and contain outside manufacturing and prevent stormwater run-on and contamination, where
feasible.
4.4.9.3 Required BMPs
• Sweep paved areas regularly, as needed, to prevent contamination of stormwater.
Vacuum sweeping is preferred.
• Alter the activity by eliminating or minimizing the contamination of stormwater.
• Enclose the activity (see Figure IV-4-9). If possible, enclose the manufacturing
activity in a building.
• Cover the activity and connect floor drains to a sanitary sewer, if approved by the
City of Auburn. Berm or slope the floor as needed to prevent drainage of pollutants to
areas outside (see Figure IV-4-10).
• Isolate and segregate pollutants, as feasible. Convey the segregated pollutants to a
sanitary sewer, process treatment, or dead-end sump, depending on available
methods and applicable permit requirements.
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Figure 4.7 – Enclose the Activity
Figure IV-4-9. Enclose the Activity
Figure 4.8 – Cover the Activity
Figure IV-4-10. Cover the Activity
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4.5 Storage and Stockpiling Activities
4.5.1 BMP A401: Storage or Transfer (Outside) of Solid Raw Materials, By-
Products or Finished Products
4.5.1.1 Description of Pollutant Sources
Solid raw materials, by-products, or products such as gravel, sand, salts, topsoil, compost, logs,
sawdust, wood chips, lumber and other building materials, concrete, and metal products are typically
stored outside in large piles, stacks, etc. at commercial or industrial establishments. Contact of
outside bulk materials with stormwater can cause leachate and/or erosion of the stored materials.
Contaminants may include TSS, BOD, organics, and dissolved salts (sodium, calcium, magnesium
chloride, etc).
4.5.1.2 Pollutant Control Approach
Provide impervious containment with berms, dikes, etc. and/or cover to prevent run-on and discharge
of leachate, pollutant(s) and TSS.
4.5.1.3 Required BMPs
• Do not hose down the contained stockpile area to a storm drain or other conveyance
leading to a storm drain or receiving water.
• Choose one or more of the source control BMP options listed below for stockpiles
greater than 5 cubic yards of erodible or water soluble materials such as soil, road
deicing salts, compost, unwashed sand and gravel, sawdust, etc. or for outside
storage areas for solid materials such as logs, bark, lumber, metal products, etc.:
o Store in a building or paved and bermed covered area as shown in Figure IV-
4-11.
o Place temporary plastic sheeting (polyethylene, polypropylene, hypalon, or
equivalent) over the material as shown in Figure IV-4-12.
OR
o Pave the area and install a stormwater drainage system. Place curbs or
berms along the perimeter of the area to prevent the run-on of unconta-
minated stormwater and to collect and convey runoff to treatment. Slope the
paved area in a manner that minimizes contact between stormwater (e.g.,
pooling) and leachable materials in compost, logs, bark, wood chips, etc.
o For large stockpiles that cannot be covered, implement containment practices
at the perimeter of the site and at any catch basins as needed to prevent
erosion and discharge of the stockpiled material offsite or to a storm drain.
Ensure that contaminated stormwater is not discharged directly to catch
basins without being conveyed through a treatment BMP. For log yards see
Ecology publication “Industrial Stormwater General Permit Implementation
Manual for Log Yards:, publication # 04-10-031.
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o Convey contaminated stormwater from the stockpile area to a wet pond, wet
vault, settling basin, media filter, or other appropriate treatment system,
depending on the contaminate.
Figure IV-4-11. Covered Storage Area for Bulk Solids (including berm if needed)
4.5.1.4 Recommended BMPs
• Maintain drainage areas in and around storage of solid materials with a minimum
slope of 1.5 percent to prevent pooling and minimize leachate formation. Areas
should be sloped to drain stormwater to the perimeter where it can be collected, or to
internal drainage “alleyways” where material is not stockpiled.
• Sweep paved storage areas regularly for collection and disposal of loose solid
materials.
• If and when feasible, collect and recycle water-soluble materials (leachates) to the
stockpile.
• Stock cleanup materials such as brooms, dustpans, and vacuum sweepers near the
storage area.
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Figure IV-4-12. Material Covered with Plastic Sheeting
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4.5.2 BMP A402: Storage and Treatment of Contaminated Soils
4.5.2.1 Description of Pollutant Sources
This activity 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.
4.5.2.2 Pollutant Control Approach
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. In addition, a Special Approved Discharge
Authorization from the Metro-King County may be required if potentially contaminated water is to be
discharged from the site. The Puget Sound Clean Air Agency can be reached at 1-800-552-3565.
Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010.
4.5.2.3 Required BMPs
The BMPs included here are intended as a supplement to other regulations. The following BMPs or
equivalent measures 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 S107 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 BMP S109 and S102 for details on catch basin cleaning and disposal options.
4.5.2.4 Recommended BMPs
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 S102 for information on
disposal options.
• Implement one of the following treatment BMPs in conjunction with a runoff
containment plan:
o Vegetated biofilter.
o Equivalent BMP for the targeted pollutant (see Volume V).
The use of any treatment BMP must not result in the violation of groundwater, surface water,
wastewater, or drinking water quality standards.
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4.5.3 BMP A403: Temporary Storage or Processing of Fruits or Vegetables
4.5.3.1 Description of Pollutant Sources
This activity applies to businesses that temporarily store fruits and vegetables outdoors prior to
processing or sale, or that crush, cut, or shred fruits or vegetables for wines, frozen juices, and other
food and beverage products. These businesses may include farmers markets, fruit and vegetable
stands, and fruit and vegetable processors. Nutrients and soil washing off of fruit and vegetables can
have a detrimental effect on receiving waters.
Pollutants of concern include nutrients, suspended solids, biochemical oxygen demand (BOD), and
color.
4.5.3.2 Pollutant Control Approach
Store and process fruits and vegetables indoors or under cover whenever possible. Educate
employees about proper procedures. Eliminate illicit connections to the storm drainage system.
Cover and contain operations and apply good housekeeping and preventive maintenance practices
to prevent the contamination of stormwater.
4.5.3.3 Required BMPs
The following BMPs or equivalent measures are required of all businesses engaged in storage of
fruits or vegetables:
• Employees must be educated on benefits of keeping a clean storage area.
• Eliminate illicit connections to the storm drainage system. See BMP S101 for details
on detecting and eliminating these connections.
• Water used to clean produce or other liquid wastes cannot enter the storm drainage
system unless treated. Minimize the use of water when cleaning produce to avoid
excess runoff.
• Cleanup materials, such as brooms and dustpans, must be kept near the storage area.
• Gutters, storm drains, and catch basins on the property must be cleaned as needed.
See BMP S109 for details on catch basin cleaning requirements.
The following BMPs or equivalent measures are required of all businesses that process fruits or
vegetables:
• Eliminate illicit connections to the storm drainage system. See BMP S101 for details
on detecting and eliminating these connections.
• Employees must be educated on benefits of keeping a clean processing area.
• Cleanup materials, such as brooms, dustpans, and shovels, must be kept near the
storage area.
• The processing area must be swept or shoveled daily to collect dirt and fruit and
vegetable fragments for proper disposal.
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• The processing area must be enclosed in a building or shed, or covered with
provisions for stormwater run-on prevention. See BMP S104, S105, and S107 for more
on covering and run-on prevention.
OR
The processing area must be paved and sloped to a sanitary sewer drain, holding tank,
or process treatment system collection drain, and stormwater run-on prevention must be
provided for the processing area. Call the Auburn Permit Center at 253-931-3090 for
information on discharging to the sanitary sewer. See BMP S106 and S103 for details on
paving and drainage.
4.5.3.4 Recommended BMPs
The following BMPs are not required but can provide additional pollution protection:
• Cover storage areas for fruits and vegetables. See BMP S104 and S105 for more
details on coverings.
• A containment curb, dike, or berm can be used to prevent offsite runoff from storage or
processing areas and also to prevent stormwater run-on. See BMP S107 for more
information. Note that run-on prevention is required for processing areas, but not for
storage areas.
• The storage area should be swept or shoveled daily to collect dirt and fruit and
vegetable fragments for proper disposal. Keep hosing to a minimum.
• Use one or a combination of the following treatment BMPs:
o Wet pond or wet vault to treat storage area runoff.
o Vegetated biofilter to treat storage area runoff.
o Catch basin with appropriate insert for the targeted pollutants to treat storage
area runoff. See S109 for information on catch basin cleaning.
o Equivalent BMP (see Volume V).
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4.5.4 BMP A404: Storage of Solid Wastes and Food Wastes
4.5.4.1 Description of Pollutant Sources
This activity applies to facilities such as hospitals, restaurants, meat and seafood markets,
veterinarian clinics, schools, grocery stores, assisted living centers, 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. Requirements for handling
and storing solid waste may include a permit from the local health departments; Tacoma-Pierce
County Health Department or Seattle-King County Health Department
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 Volume IV, Chapter 5. Please contact the
Department of Ecology at 360-407-6300 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).
4.5.4.2 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.
4.5.4.3 Required BMPs
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.
• Trash compactors or dumpsters for food or liquid-bearing wastes shall drain to the
sanitary sewer system and be designed as required 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 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.
OR
• If lids cannot be provided for the waste containers, or they cannot otherwise be
covered, there is another option: a designated waste storage area must be provided
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with a containment berm, dike, or curb, and the designated area must drain to a
sanitary sewer or holding tank for further treatment. See BMP S107 and S103 for more
information.
• 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 S109 for details on catch basin cleaning.
4.5.4.4 Recommended BMPs
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.
• 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 S102 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 206-296-4899, or use the IMEX computer bulletin board modem access
number at 1-800-858-6625.
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4.5.5 BMP A405: Recyclers and Scrap Yards
4.5.5.1 Description of Pollutant Sources
Includes businesses that reclaim various materials for resale or for scrap, such as vehicles and
vehicle/ equipment parts, construction materials, metals, computers, appliances, beverage
containers, and papers.
Potential sources of pollutants include paper, plastic, metal scrap debris, engines, transmissions,
radiators, batteries, and other materials that contain fluids or are contaminated with fluids. Other
pollutant sources include leachate from metal components, contaminated soil, and the erosion of soil.
Activities that can generate pollutants include the transfer, dismantling, and crushing of vehicles and
scrap metal; the transfer and removal of fluids; maintenance and cleaning of vehicles, parts, and
equipment; and storage of fluids, parts for resale, solid wastes, scrap parts, and materials, equipment
and vehicles that contain fluids, generally in uncovered areas.
Potential pollutants typically found at vehicle recycle and scrap yards include oil and grease, ethylene
and propylene glycol, total suspended solids, BOD, heavy metals, and acidic pH.
4.5.5.2 Required BMPs
For facilities subject to Ecology’s Industrial Stormwater General Permit refer to BMP Guidance
Document #94-146, “Vehicle Recyclers: A Guide to Implementing the Industrial Stormwater General
National Pollutant Discharge Elimination System (NPDES) Permit Requirements”, Washington
Department of Ecology, January 2006 for selection of BMPs. The BMPs in that guidance document
can also be applied to scrap material recycling facilities (depending on the pollutant sources existing
at those facilities) and to non-permitted facilities.
Contact the City of Auburn Public Works Department at 253-931-3010 if contact stormwater or
process wastewater is to be discharged from your site.
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4.5.6 BMP A406: Treatment, Storage or Disposal of Dangerous Wastes
This activity applies to businesses and public agencies that are permitted by the Washington State
Department of Ecology (DOE) to treat, store, or dispose of dangerous wastes. DOE regulates these
facilities with specific requirements, which include the need for a National Pollutant Discharge
Elimination System (NPDES) permit. Detailed BMPs are not included in this manual since site
requirements for these facilities are well beyond the level of typical BMP applications. See Chapter 5
of this volume for reference information.
Contact the City of Auburn Sanitary Sewer Utility at 253-931-3010 for their requirements. An
Industrial Wastewater Discharge Permit is required before discharging contact stormwater or process
wastewater to the municipal sewer system. The Department of Ecology regulates the treatment,
storage and disposal of Dangerous Wastes.
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4.5.7 BMP A407: Storage of Liquid, Food Waste or Dangerous Waste
Containers
4.5.7.1 Description of Pollutant Sources
Steel and plastic drums with volumetric capacities of 55 gallons or less are typically used at industrial
facilities for storage of liquids and powders. The BMPs specified below apply to container(s) located
outside a building used for temporary storage of accumulated food wastes, vegetable or animal
grease, used oil, liquid feedstock cleaning chemicals, or Dangerous Wastes (liquid or solid), unless
the business is permitted by Ecology to store the wastes. Leaks and spills of pollutant materials
during handling and storage are the primary sources of pollutants. Oil and grease, acid/alkali pH,
BOD, and metals are potential pollutant constituents.
4.5.7.2 Pollutant Control Approach
Store containers in impervious containment under a roof or other appropriate cover, or in a building.
For roll-containers (for example, dumpsters) that are picked up directly by the collection truck, a filet
can be placed on both sides of the curb to facilitate moving the dumpster. If a storage area is to be
used on-site for less than 30 days, a portable temporary secondary system like that shown in Figure
IV-4-13 can be used in lieu of a permanent system as described above.
4.5.7.3 Required BMPs
• Place tight-fitting lids on all containers. Provide adequate freeboard/headspace.
• Place drip pans beneath all mounted container taps and at all potential drip and spill
locations during filling and unloading of containers.
• Inspect container storage areas regularly for corrosion, structural failure, spills, leaks,
overfills, and failure of piping systems. Check containers daily for leaks/spills. Replace
containers, secure lids, and replace and tighten bungs in drums, as needed.
• Drums stored in an area where unauthorized persons may gain access must be
secured in a manner that prevents accidental spillage, pilferage, or any unauthorized
use (see Figure IV-4-14).
• If the material is a Dangerous Waste, the business owner must comply with any
additional Ecology requirements as specified in Chapter 5.
• Storage of reactive, ignitable, or flammable liquids must comply with the Uniform Fire
Code.
• Cover dumpsters or keep them under cover, such as a lean-to, to prevent the entry of
stormwater. Replace or repair leaking garbage dumpsters.
• Drain dumpsters and/or dumpster pads to sanitary sewer. Keep dumpster lids closed.
Install waterproof liners.
• Keep containers with Dangerous Waste, food waste or other potential pollutant liquids
inside a building unless this is impracticable due to site constraints or Uniform Fire
Code requirements.
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• Store containers in a designated area that is covered, bermed, or diked; paved; and
impervious in order to contain leaks and spills (see Figure IV-4-15). The secondary
containment shall be sloped to drain into a dead-end sump for the collection of leaks
and small spills.
• For liquid wastes, surround the containers with a dike as illustrated in Figure IV-4-15.
The dike must be of sufficient height to provide a volume of 10 percent of the total
enclosed container volume or 110 percent of the volume contained in the largest
container, whichever is greater, or, if a single container, 110 percent of the volume of
that container.
• Where material is temporarily stored in drums, a containment system can be used,
as illustrated, in lieu of the above system (see Figure IV-4-13).
• Place containers mounted for direct removal of a liquid chemical for use by
employees inside a containment area as described above. Use a drip pan during
liquid transfer (see Figure IV-4-16).
• For contaminated stormwater in the containment area, connect the sump outlet to a
sanitary sewer, if approved by the City of Auburn, or to appropriate treatment such
as an API or CP oil/water separator, catch basin filter, or other appropriate system
(see Volume V). Equip the sump outlet with a normally closed and locked steel line
and valve to prevent the release of spilled or leaked liquids, especially flammables
(in compliance with Fire Codes), and dangerous liquids. This valve may be opened
only for the conveyance of contaminated stormwater to treatment. Maintain a log to
record inspections and when the containment area is drained to treatment.
• Another option for discharge of contaminated stormwater is to provide a dead-end
sump or catchment. Stormwater can then be pumped to a tank truck or other
appropriate vehicle for off-site treatment and/or disposal.
Figure 4.11 – Secondary Containment System
Figure IV-4-13. Secondary Containment Vessel
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Figure 4.12 – Locking System for Drum Lid
Figure IV-4-14. Locking System for Drum Lid
Figure IV-4-15. Covered and Bermed Containment Area
Figure IV-4-16. Mounted Container
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4.5.8 BMP A408: Storage of Liquids in Above-Ground Tanks
4.5.8.1 Description of Pollutant Sources
Above-ground tanks containing liquids (excluding uncontaminated water) may be equipped with a
valved drain, vent, pump, and bottom hose connection. These tanks may be heated with steam heat
exchangers equipped with steam traps. Leaks and spills can occur at connections and during liquid
transfer. Oil and grease, organics, acids, alkalis, and heavy metals in tank water and condensate
drainage can also cause stormwater contamination at storage tanks.
4.5.8.2 Pollutant Control Approach
Install secondary containment or a double-walled tank. Slope the containment area to a drain with a
sump. Stormwater collected in the containment area may need to be discharged to treatment such as
an API or CP oil/water separator, or equivalent BMP. Add safeguards against accidental releases
including protective guards around tanks to protect against vehicle or forklift damage, and tag valves
to reduce human error. Tank water and condensate discharges are process wastewater that may
need an NPDES Permit or approval to discharge to the sanitary system.
4.5.8.3 Required BMPs for All Tanks
• Inspect the tank containment areas regularly to identify leaks/spills, cracks,
corrosion, etc. in problem components such as fittings, pipe connections, and valves.
• Develop a spill plan as per the requirements of BMP A714: Spills of Oil and
Hazardous Substances.
• Place adequately sized drip pans beneath all mounted taps and drip/spill locations
during filling/unloading of tanks. Valved drain tubing may be needed in mounted drip
pans.
• Sweep and clean the tank storage area regularly, if paved.
• Replace or repair tanks or other components that are leaking, corroded, or otherwise
deteriorating.
• All installations shall comply with the International Fire Code and the National
Electric Code.
• Locate permanent tanks in impervious (Portland cement concrete or equivalent)
secondary containment surrounded by dikes as illustrated in Figure IV-4-17, or use
UL approved double-walled tanks.
• Double-walled tanks with exposure to traffic must be protected with bollards, jersey
barriers, or walls. Bollards should be at least 4 feet high, at least 3 feet from the tank,
and no more than 4 feet apart.
• Include a tank overfill protection system to minimize the risk of spillage during
loading.
• There must be at least 5 feet of space between the tanks and any enclosures.
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4.5.8.4 Required BMPs for Single-walled Tanks
• The containment volume shall be 100% of the volume of the largest tank plus the
volume of stormwater from rain events up to a 25-year, 24-hour storm within the
containment area or 110% of the volume of the largest tank, whichever is greater.
• There must be at least 5 feet of space between the tanks and any enclosures.
• Slope the secondary containment to drain to a dead-end sump (optional), or
equivalent, for the collection of small spills.
• If the tank containment area is uncovered, equip the outlet from the spill-containment
sump with a shutoff valve, which is normally closed and locked. Valves for
flammables containment shall be made of steel. Valves for corrosives containment
shall be compatible with the material being stored.
• The external valve may be opened manually only to convey contaminated
stormwater to an approved treatment or disposal facility, or to convey
uncontaminated stormwater to a storm drain. Evidence of contamination can include
the presence of visible sheen, color, or turbidity in the runoff, or existing or historical
operational problems at the facility. Simple pH measurements with litmus or pH
paper can be used for areas subject to acid or alkaline contamination.
• At petroleum tank farms, convey stormwater contaminated with floating oil or debris
in the contained area through an API or CP-type oil/water separator (Volume V,
Treatment BMPs) or other approved treatment facility prior to discharge to storm
drain or surface water. Direct discharges require an NPDES permit from the
Department of Ecology.
• Loading racks and transfer areas associated with tank farms shall provide spill
containment and treatment sized to encompass the largest vessel (trailer, railcar,
intermodal tank) using the area.
4.5.8.5 Recommended BMPs for Double-walled Tanks
• Tank pads and the fuel delivery area should be protected from stormwater run-on but sized
and sloped to capture any leaks or spills from the tank or fueling process.
• Feed and return lines from the tanks shall be doubled walled or entirely contained within the
utility corridor.
• Catch basins receiving drainage from the tank pad and fueling area should be oversized and
have down-turned elbows in their outlets or flow to an appropriately sized oil/water separator.
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Figure IV-4-17. Above-Ground Tank Storage
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4.5.9 BMP A409: Parking and Storage for Vehicles and Equipment
4.5.9.1 Description of Pollutant Sources
Parked vehicles at public and commercial parking lots and garages, such as retail store, fleet vehicle
(including rent-a-car lots and car dealerships), equipment sale and rental parking lots, and parking lot
driveways, can be sources of toxic hydrocarbons and other organic compounds, oils and greases,
metals, and suspended solids.
4.5.9.2 Required BMPs
• If washing of a parking lot is conducted, discharge the wash water to a sanitary
sewer (if allowed by the City of Auburn) or other approved wastewater treatment
system, or collect it for off-site disposal.
• Do not hose down the area to a storm drain or receiving water. Sweep (vacuum
sweeping is preferred) parking lots, storage areas, and driveways regularly to collect
dirt, waste, and debris.
• An oil removal system such as an API or CP oil and water separator, catch basin
filter, or equivalent BMP (see VV), approved by the City of Auburn, is applicable for
parking lots meeting the threshold vehicle traffic intensity level of a high-use site. For
more information on high-use sites, refer to Volume V, Chapter 1.
• Covered floors of parking garages must drain to the sanitary sewer through an
approved oil/water separator. Uncovered floors must be routed to the storm drainage
system through an approved treatment device.
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4.6 Construction and Demolition Activities
4.6.1 BMP A501: Clearing, Grading and Preparation of Construction Sites
This activity applies to businesses and municipal agencies that develop lands for construction. It also
applies to residences that undertake large yard clearing and grading projects. Stormwater runoff from
bare ground can be loaded with dirt and other pollutants. This material can clog ditches and stream
channels, thus reducing carrying capacity and increasing flooding, as well as smothering spawning
beds for fish. Simply controlling runoff and not allowing it to leave the site will prevent these harmful
effects. Clearing, grading, and preparation activities are covered in detail in Volume II of this manual,
Construction Stormwater Pollution Prevention. Grading activities are also regulated in the City of
Auburn by the Grading and Excavation Code, Chapter 15.74 ACC.
Control of stormwater run-on and soil stabilization is critical. Limiting the area to be cleared and
graded during wet weather seasons will make site stabilization and sediment control easier.
Coverage under Ecology’s Construction Stormwater General Permit is required for construction sites
that result in the disturbance of one acre or more of land. Compliance with the Construction
Stormwater Pollution Prevention requirements in Ecology’s manual is required, as applicable.
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4.6.2 BMP A502: Demolition of Buildings
4.6.2.1 Description of Pollutant Sources
This activity 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.
4.6.2.2 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.
4.6.2.3 Required BMPs
The following BMPs or equivalent measures are required of all businesses and public agencies
engaged in building demolition:
• Identify and properly abandon all utility connections such as sanitary sewer, 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. See BMP S102 and S107 for
more information on runoff control and disposal options.
• Utilize storm drain inlet protection per BMP C220.
• 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.
4.6.2.4 Recommended BMPs
The following BMPs are not required, but can provide additional pollution protection:
• Water should be sprayed throughout the site to help control wind blowing of fine
materials such as soil, concrete dust, and paint chips. The amount of water must be
controlled so that runoff from the site does not occur, yet dust control is achieved.
Oils must never be used for dust control.
• If possible, a wall should be constructed to prevent stray building materials and dust
from escaping the area during demolition.
• Install catch basin filter inserts to treat site runoff. Additional information about catch
basin filter inserts can be found in Volume II, BMP C220.
• Schedule demolition to take place at a dry time of the year.
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4.6.3 BMP A503: Building, Repair, Remodeling and Construction
4.6.3.1 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 A103 Washing, Pressure Washing, and Steam Cleaning. Painting of
buildings is covered under A307 Painting, Finishing, and Coating of Vehicles, Boats, Buildings, and
Equipment. Concrete pouring is covered under A302 Concrete Pouring and Asphalt Application at
Temporary Sites.
Pollutants of concern include toxic organics, suspended solids, heavy metals, asbestos, pH, oils, and
greases.
4.6.3.2 Pollutant Control Approach
Employees must be educated about the need to control site activities. Control leaks, spills, and loose
material. Utilize good housekeeping practices.
4.6.3.3 Required BMPs
The following BMPs or equivalent measures 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.
• 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.
• 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 S102 for disposal options.
• Inlet protection as described in Volume II, BMP C220 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.
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4.6.3.4 Recommended BMPs
The following BMPs are not required, but can provide additional pollution protection:
• Recycle materials whenever possible.
• Light spraying of water on the work site can control some of the dust and grit that can
blow away. Oils must never be used for dust control. Never spray to the point of
runoff from the site.
• Activities such as tool cleaning should occur over a ground cloth or within a
containment device such as a tub.
• Catch basin filter inserts may be considered.
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4.7 Dust Control, and Soil and Sediment Control
4.7.1 BMP A601: Dust Control at Disturbed Land Areas and Unpaved
Roadways and Parking Lots
4.7.1.1 Description of Pollutant Sources
Dust can cause air and water pollution problems particularly at demolition sites, disturbed land areas,
and unpaved roadways and parking lots.
4.7.1.2 Pollutant Control Approach
Minimize dust generation and apply environmentally friendly and government approved dust
suppressant chemicals, if necessary.
4.7.1.3 Required BMPs
• Sprinkle or wet down soil or dust with water as long as it does not result in runoff or a
wastewater discharge. Minimize the amount of water to avoid washing pollutants into
the storm drainage system. At active construction sites, street sweeping shall be
performed prior to washing the street.
• Use only local and/or state government approved dust suppressant chemicals such
as those listed in Ecology Publication #96-433, “Techniques for Dust Prevention and
Suppression.” See BMP C126, Polyacrylamide for Soil Erosion Protection, in
Volume II, Chapter 3 of this manual.
• Avoid excessive and repeated applications of dust suppressant chemicals. Time the
application of dust suppressants to avoid or minimize their wash-off by rainfall or
human activity such as irrigation.
• Use stormwater containment to prevent the conveyance of solids by stormwater into
storm drains or receiving waters.
• The use of motor oil or other oils for dust control is prohibited. Care shall be taken
when using lignin derivatives and other high BOD chemicals in excavations or areas
easily accessible to surface water or groundwater.
• Consult with the Ecology Northwest Regional Office at 425-649-7000 on discharge
permit requirements if the dust suppression process results in a wastewater
discharge to the ground, groundwater, storm drain, or surface water.
4.7.1.4 Recommended BMPs for Roadways and Other Trafficked Areas:
• Consider limiting use of off-road recreational vehicles on dust generating land.
• Consider paving unpaved permanent roads, approaches, exits, access lanes, and
other trafficked areas at municipal, commercial, and industrial areas.
• Consider paving or stabilizing shoulders of paved roads with gravel, vegetation, or
City of Auburn approved chemicals.
• Encourage use of alternate paved routes, if available.
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• Vacuum or wet sweep fine dirt and skid control materials from paved roads soon
after winter weather ends or as needed.
• Consider using traction sand that is pre-washed to reduce dust emissions.
4.7.1.5 Recommended BMPs for Dust Generating Areas:
• Prepare a dust control plan. Helpful references include: Control of Open Fugitive
Dust Sources (EPA-450/3-88-088) and Fugitive Dust Background Document and
Technical Information Document for Best Available Control Measures
(EPA-450/2-92-004).
• Limit exposure of soil (dust source) as much as feasible.
• Stabilize dust-generating soil by growing and maintaining vegetation, mulching,
topsoiling, and/or applying stone, sand, or gravel.
• Apply windbreaks in the soil such as trees, board fences, tarp curtains, bales of hay,
etc.
• Cover dust-generating piles with wind-impervious fabric, or equivalent material.
Additional information on dust control can be found in Volume II of this manual.
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4.7.2 BMP A602: Dust Control at Manufacturing Sites
4.7.2.1 Description of Pollutant Sources
Industrial material handling activities can generate considerable amounts of dust that is typically
removed using exhaust systems. This can generate air emissions that can contaminate stormwater.
Dusts can be generated at cement and concrete product mixing facilities, foundries, and wherever
powdered materials are handled. Particulate materials that are of concern to air pollution control
agencies include grain dust, sawdust, coal, gravel, crushed rock, cement, and boiler fly ash. The
objective of this BMP is to reduce the stormwater pollutants caused by dust generation and control.
4.7.2.2 Pollutant Control Approach
Prevent dust generation and emissions where feasible, regularly clean-up dust that can contaminate
stormwater, and convey dust contaminated stormwater to proper treatment.
4.7.2.3 Required BMPs
• Clean building roofs, powder material handling equipment, and vehicles that can be
sources of stormwater pollutants as needed to remove accumulated dust and residue.
• Regularly sweep dust accumulation areas that can contaminate stormwater.
Sweeping shall be conducted using vacuum filter equipment to minimize dust
generation and to ensure optimal dust removal.
• Minimize the amount of water used for dust control to avoid washing pollutants into
the storm drainage system.
4.7.2.4 Recommended BMPs
• In manufacturing operations, train employees to handle powders carefully to prevent
generation of dust.
• Use dust filtration/collection systems such as bag house filters, cyclone separators,
etc. to control vented dust emissions that could contaminate stormwater. It may be
necessary to monitor rooftops for possible accumulations of materials and take
appropriate measures to prevent this material from entering the storm drainage
system. Control of dust at foundries, metal shredders, and material transfer and
handling facilities are some examples.
• Use water spray to flush dust accumulations to sanitary sewers where allowed by the
City of Auburn or to other appropriate treatment system.
• Use approved dust suppressants such as those listed in Ecology Publication
“Techniques for Dust Prevention and Suppression,” #96-433. (Ecology, 2003).
Application of some products may not be appropriate in close proximity to receiving
waters or conveyances close to receiving waters. For more information check with
the Ecology Northwest Regional Office or the City of Auburn.
• For removal of TSS in stormwater, use sedimentation basins, wet ponds, wet vaults,
catch basin filters, vegetated filter strips, or equivalent sediment removal BMPs.
Refer to Volume V for more information about these BMPs.
• Additional information on dust control can be found in Volume II of this manual.
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4.7.3 BMP A603: Soil Erosion and Sediment Control at Industrial Sites
4.7.3.1 Description of Pollutant Sources
Industrial activities on soil areas, exposed and disturbed soils, steep grades, etc. can be sources of
sediments that can contaminate stormwater runoff.
4.7.3.2 Pollutant Control Approach
Limit the exposure of erodible soil, stabilize or cover erodible soil where necessary to prevent
erosion, and/or provide treatment for stormwater contaminated with TSS caused by eroded soil.
4.7.3.3 Required BMPs
Apply one or more of the following cover practices:
• Vegetative cover such as grass, trees, large bark, bonded fiber matrix, or shrubs on
erodible soil areas;
• Covering with mats such as clear plastic, jute, synthetic fiber; or gunite; and/or
• Preservation of natural vegetation including grass, trees, shrubs, and vines.
Control sediment through installing a vegetated swale, dike, silt fence, check dam, gravel filter berm,
and/or sedimentation basin and properly grading. For design information refer to Volume II.
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4.8 Other Activities
4.8.1 BMP A701: Commercial Animal Handling Areas
4.8.1.1 Description of Pollutant Sources
Racetracks, kennels, fenced pens, veterinarians, and businesses that provide boarding services for
horses, dogs, cats, etc. can generate pollutants from activities such as manure deposits, animal
washing, grazing, and any other animal handling activity that could contaminate stormwater.
Pollutants can include coliform bacteria, nutrients, and total suspended solids.
4.8.1.2 Pollutant Control Approach
To prevent, to the maximum extent practicable, the discharge of contaminated stormwater from
animal handling and keeping areas.
4.8.1.3 Required BMPs
• Regularly scoop, sweep and clean animal keeping areas to collect and properly
dispose of droppings, uneaten food, and other potential stormwater contaminants.
• Do not hose down to storm drains or receiving waters those areas that contain
potential stormwater contaminants.
• Contaminated water must go to the sanitary sewer. A hair and sediment trap may be
required.
• Do not allow any wash water to be discharged to storm drains or to receiving water
without proper treatment.
• If animals are kept in unpaved and uncovered areas, the ground must either have
vegetative cover or some other type of ground cover, such as mulch.
• If animals are not leashed or in cages, the area where animals are kept must be
surrounded by a fence or other devices to prevent animals from moving away from
the controlled area where BMPs are used.
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4.8.2 BMP A702: Log Sorting and Handling
4.8.2.1 Description of Pollutant Sources
Log yards are paved or unpaved areas where logs are transferred, sorted, debarked, cut, and stored
to prepare them for shipment or for the production of dimensional lumber, plywood, chips, poles, or
other products. Log yards are generally maintained at sawmills, shipping ports, and pulp mills.
Typical pollutants include oil and grease, BOD, settleable solids, total suspended solids (including
soil), high and low pH, heavy metals, pesticides, wood-based debris, and leachate.
The following are pollutant sources:
• Log storage, rollout, sorting, scaling, and cutting areas
• Log and liquid loading areas
• Log sprinkling
• Debarking, bark bin, and conveyor areas
• Bark, ash, sawdust and wood debris piles, and other solid wastes
• Metal salvage areas
• Truck, rail, ship, stacker, and loader access areas
• Log trucks, stackers, loaders, forklifts, and other heavy equipment
• Maintenance shops and parking areas
• Cleaning areas for vehicles, parts, and equipment
• Storage and handling areas for hydraulic oils, lubricants, fuels, paints, liquid wastes,
and other liquid materials
• Pesticide usage for log preservation and surface protection
• Application of herbicides for weed control
• Contaminated soil resulting from leaks or spills of fluids
4.8.2.2 Ecology’s Baseline General Permit Requirements
Industries with log yards are required to obtain coverage under the baseline General Permit for
Discharges of Stormwater Associated with Industrial Activities to Surface Water. The permit requires
preparation and on-site retention of Stormwater Pollution Prevention Plans (SWPPP). The SWPPP
must identify operational, source control, erosion and sediment control, and, if necessary, treatment
BMPs. Required and recommended operational, source control, and treatment BMPs are presented
in detail in Ecology’s Guidance Document: Industrial Stormwater General Permit Implementation
Manual for Log Yards, Publication # 04-10-031. It is recommended that all log yard facilities obtain a
copy of this document.
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4.8.3 BMP A703: Boat Building, Maintenance and Repair
4.8.3.1 Description of Pollutant Sources
Sources of pollutants at boat and ship building, repair, and maintenance facilities include pressure
washing, surface preparation, paint removal, sanding, painting, engine maintenance and repairs, and
material handling and storage, if conducted outdoors. Potential pollutants include spent abrasive
grits, solvents, oils, ethylene glycol, wash water, paint over-spray, cleaners/detergents, anti-corrosive
compounds, paint chips, scrap metal, welding rods, resins, glass fibers, dust, and miscellaneous
trash. Pollutant constituents include TSS, oil and grease, organics, copper, lead, tin, and zinc.
Related activities are covered under the following activity headings in this manual, and other BMPs
provided in this manual:
• A103 Washing, Pressure Washing, and Steam Cleaning of
Vehicles/Equipment/Building Structures
• A202 Fueling at Dedicated Stations
• A714 Spills of Oil and Hazardous Substances
4.8.3.2 Pollutant Control Approach
Apply good housekeeping, preventive maintenance, and cover and containment BMPs in and around
work areas.
4.8.3.3 Required BMPs
The following BMPs or equivalent measures are required of all businesses, public agencies, and
private boat owners engaged in boat building, mooring, maintenance and repair that are not covered
by the NPDES permit for boatyards:
• Maintenance and repair activities that can be moved on-shore must be moved
accordingly. This action reduces some of the potential for direct pollution impact on
waterbodies.
• Blasting and spray painting activities must be sheltered by hanging tarps to block the
wind and prevent dust and overspray from escaping. Move the activity indoors if
possible. See Chapter 5 for details on Puget Sound Clean Air Agency limitations.
• Bermed ground cloths must be used for collection of drips and spills in painting and
finishing operations, and paint chips and used blasting sand from sand blasting.
• Collect spent abrasives regularly and store under cover to await proper disposal.
• Dispose of greasy rags, oil filters, air filters, batteries, spent coolant, and degreasers
properly.
• Drain oil filters before disposal or recycling.
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• Bilge water must be collected for proper disposal rather than discharged on land or
water. See BMP S102 for detail on disposal options. Discharge of contaminated bilge
water may be avoided if oil-absorbent pads are used to capture the oil in the bilge
water before or during pumping. If pads are used, they must be recycled or properly
disposed.
• Convey sanitary sewage to pump-out stations, portable on-site pump-outs or other
appropriate onshore facilities.
• Maintenance yard areas must be swept and cleaned, without hosing down the area,
at least once per week or as needed. This prevents sandblasting materials,
scrapings, paint chips, oils, and other loose debris from being carried away with
stormwater. The collected materials must be disposed of properly. See BMP S102
for disposal options.
• Docks and boat ramps must be swept at least once per week or as needed and the
collected materials must be disposed of properly.
• Paint and solvent mixing, fuel mixing and similar handling of liquids shall be
performed on shore, or such that no spillage can occur directly into surface
waterbodies.
• Routine cleanup materials such as oil-absorbent pads, brooms, dustpans, mops,
buckets, and sponges must be stocked near docks.
• Comply with BMP A203 and A101 if engine repair and maintenance are conducted.
• In the event of an accidental discharge of oil or hazardous material into waters of the
state or onto land with a potential for entry into state waters, immediately notify the
Department of Ecology, and the National Response Center at 1-800-424-8802
(24 hour).
4.8.3.4 Recommended BMPs
The following BMPs are not required but can provide additional pollution protection:
• Boat construction and structural repair activities should be covered.
• Materials such as paints, tools, and ground cloths should be stored indoors or in a
covered area when not in use.
• Select the least toxic anti-fouling paint available.
• Use sanders that have dust containment bags and avoid sanding in windy
conditions.
• All used oil should be recycled if feasible. To dispose of filters, let drain 24 hours,
then double wrap in plastic and dispose in the regular garbage. Pending state
legislation may make disposal in the garbage illegal, so call the Hazardous Waste
Line at 1-800-287-6429 for current information.
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• Use one of the following treatment BMPs when paint chips or blasting grit are
prevalent in the work area
o Catch basin filter insert
o Infiltration basin
o Wet pond or vault
o Constructed wetland
o Vegetated biofilter
o Filtration with media designed for the pollutants that are present
o Equivalent BMP (see Volume V)
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4.8.4 BMP A704: Logging
4.8.4.1 Description of Pollutant Sources
This activity covers logging activities that fall under the Washington State Forest Practices Act
category of Class IV general forest practices. These are situations where timber harvesting is done in
the process of converting forest lands into other land uses, such as home and business construction.
Stormwater runoff from bare ground can be loaded with dirt and other pollutants. This material can
clog ditches and stream channels, thus reducing carrying capacity and increasing flooding, as well as
smothering spawning beds for fish. Simply controlling runoff and not allowing it to leave the site will
prevent these harmful effects. Clearing and grading activities are covered in detail in Volume II of this
manual, Construction Stormwater Pollution Prevention. Grading activities are also regulated in the
City of Auburn by the Land Clearing, Filling and Grading Code, Chapter 15.74 ACC.
Control of stormwater run-on and soil stabilization is critical. Limiting the area to be cleared and
graded during wet weather seasons will make site stabilization and sediment control easier.
Coverage under Ecology’s Construction Stormwater General Permit is required for construction sites
that result in the disturbance of one acre or more of land. Compliance with the Construction
Stormwater Pollution Prevention requirements in Ecology’s manual is required, as applicable.
Virtually all logging operations will require a permit from the Washington State Department of Natural
Resources. Sensitive/critical areas and wetlands ordinances for Auburn also contain requirements for
logging activities in the vicinity of water bodies.
Pollutants of concern include suspended solids, oils and greases, biochemical oxygen demand
(BOD), nutrients, toxic organic compounds, and heavy metals.
4.8.4.2 Pollutant Control Approach
Maintain required buffers adjacent to critical areas, including streams and wetlands. Keep sediments
out of waterbodies and off paved areas.
4.8.4.3 Required BMPs
• Vegetation along stream corridors, and adjacent to other waterbodies and wetlands,
must be preserved. Maintenance of a vegetated buffer enables filtration of most of
the pollutants of concern for this activity. The above-mentioned ordinances contain
specific requirements for buffer setbacks.
• Logging access roads must have a crushed rock or spall apron construction entrance
where they join the pavement to prevent sediments from being tracked onto the
pavement.
• On-site fueling and maintenance operations must follow the required BMPs as
outlined in A204 Mobile Fueling; A203 Vehicle Maintenance; and A407 Storage of
Liquid, Food Waste, or Dangerous Waste Containers.
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4.8.4.4 Recommended BMPs
The following BMPs are not required, but can provide additional pollution protection:
• Erosion potential can be reduced by avoiding logging on steep slopes.
• If access roads are constructed for logging, they should be provided with drainage
ditches that divert runoff into vegetated areas or stormwater treatment systems.
• Plant vegetated buffers in areas where they are already downslope of proposed
logging areas, with sufficient lead time to allow for effective growth.
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4.8.5 BMP A705: Mining and Quarrying of Sand, Gravel, Rock, Peat, Clay and
Other Materials
4.8.5.1 Description of Pollutant Sources
This activity 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.
4.8.5.2 Recommended BMPs
• 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.
• Measures to control track-out and dust should be implemented. Wheel washes,
sweeping, and paving high traffic areas are some common practices.
• 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
this manual 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.
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4.8.6 BMP A706: Swimming Pool and Spa Cleaning and Maintenance
4.8.6.1 Description of Pollutant Sources
This activity applies to all municipal and commercial swimming pools and spas. Pools and spas at
hotels, motels, and apartment and condominium complexes are covered here. Pools at single-family
residences are covered in Chapter 3 of this volume. Commercial pool and spa cleaning services
must follow the required BMPs for all pools serviced.
Pollutants of concern include nutrients, suspended solids, chlorine, pH, and chemical oxygen
demand (COD).
4.8.6.2 Pollutant Control Approach
Dispose of pool or spa water to the sanitary sewer.
4.8.6.3 Required BMPs
• The preferred method of pool or spa water disposal is to the sanitary sewer. If a
sanitary sewer is available, all Health Department regulated facilities are required to
connect for draining and backwash. Contact the City of Auburn Sanitary Sewer Utility
at 253-931-3010 prior to discharge for instructions on allowable flow rates and timing
before starting to drain the pool. Never discharge pool water to a septic system, as it
may cause the system to fail.
• If discharge to the sanitary sewer is not possible, pool and spa water may be
discharged to a ditch or storm drainage system, provided that the water has been
dechlorinated first. The proponent is required to contact the City of Auburn Storm
Drainage Utility at 253-931-3010 prior to discharge for instructions on allowable flow
rates for the system or ditch that is being discharged to. All discharges shall be de-
chlorinated to a concentration of 0.1 parts per million (ppm or mg/L) or less, and pH
adjusted 6.5 to 8.5 standard units, if necessary. Neutralizing chemicals are available
for dechlorinating water and adjusting the pH. Turbidity shall not exceed 10 NTU.
Letting the pool or spa "sit" may also reduce chlorine levels. Use a test kit to
determine if the chlorine concentration has reached zero and the pH is within
acceptable limits.
• State law allows discharges of pool water to the ground, not to a water body or storm
drainage system, with a chlorine level of up to 3 parts per million. However, the water
must not cross property lines or impact neighboring properties, and a satisfactory
means for distributing the water to the ground must be used so there is no runoff.
• Backwash from pool filters cannot be discharged to surface waters, storm drainage
systems, septic systems, or on the ground.
• Diatomaceous earth used in pool filters cannot be discharged to surface waters,
storm drainage systems, septic systems, or on the ground.
4.8.6.4 Recommended BMP
• Hire a professional pool-draining service to collect all pool water for offsite disposal.
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4.8.7 BMP A707: De-Icing and Anti-Icing Operations for Streets & Highways
4.8.7.1 Description of Pollutant Sources
Deicing and/or anti-icing compounds are used on highways, streets, and sidewalks to control ice and
snow. Typically ethylene glycol and propylene glycol are deicers used on aircraft. Deicers commonly
used on highways, streets and sidewalks include calcium magnesium acetate (CMA), calcium
chloride, magnesium chloride, sodium chloride, urea, and potassium acetate. The deicing and anti-
icing compounds become pollutants when they are conveyed to storm drains or to surface water after
application. Leaks and spills of these chemicals can also occur during their handling and storage.
4.8.7.2 Required BMPs
• Select de-icers and anti-icers that cause the least adverse environmental impact.
Apply only as needed using minimum quantities.
• Where feasible and practicable, use roadway deicers, such as calcium magnesium
acetate, potassium acetate, or similar materials that cause less adverse
environmental impact than urea and sodium chloride.
• Store and transfer de/anti-icing materials on an impervious containment pad in
accordance with BMP A401 Storage or Transfer (Outside) of Solid Raw Materials,
By-Products, or Finished Products and A408 Storage of Liquids in Above-Ground
Tanks.
• Sweep/clean up accumulated de/anti-icing materials and grit from roads as soon as
possible after the road surface clears.
4.8.7.3 Recommended BMPs
• Intensify roadway cleaning in early spring to help remove particulates from road
surfaces.
• Include limits on toxic metals in the specifications for de/anti-icers.
• Additional guidance can be found in the Regional Road Maintenance - Endangered
Species Act (ESA) program guidelines.
• State guidelines contain additional information for de-icing activities at airports.
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4.8.8 BMP A708: Roof and Building Drains at Manufacturing and Commercial
Buildings
4.8.8.1 Description of Pollutant Sources
Stormwater runoff from roofs and sides of manufacturing and commercial buildings can be sources of
pollutants caused by leaching of roofing materials, building vents, and other air emission sources.
Vapors and entrained liquid and solid droplets/particles have been identified as potential pollutants in
roof/building runoff. Metals, solvents, acidic/alkaline pH, BOD, and organics are some of the pollutant
constituents identified.
4.8.8.2 Pollutant Control Approach
Evaluate the potential sources of stormwater pollutants and apply source control BMPs where
feasible. Use dust filtration/collection systems such as bag house filters, cyclone separators, etc. to
control vented dust emissions that could contaminate stormwater. It may be necessary to monitor
roof tops for possible accumulations of materials and take appropriate measures to prevent this
material from entering the storm drainage system. Control of dusts at foundries, metal shredders, and
material transfer and handling facilities are some examples.
4.8.8.3 Required BMPs
• Bare galvanized metal shall not be used for materials that convey stormwater, such
as roofs, canopies, siding, gutters, downspouts, roof drains, and pipes. Any
galvanized materials shall have an inert, non-leachable finish, such as baked
enamel, fluorocarbon paint (such as Kynar or Hylar ), factory-applied epoxy, pure
aluminum, or asphalt coating. Acrylic paint, polyester paint, field-applied, and part
zinc (such as Galvalume ) coatings are not acceptable.
• If leachates and/or emissions from buildings are suspected sources of stormwater
pollutants, then sample and analyze the stormwater draining from the building.
• If a roof/building stormwater pollutant source is identified, implement appropriate
source control measures such as air pollution control equipment, selection of
materials, operational changes, material recycle, process changes, etc.
• Water quality treatment BMPs are found in Volume V of this manual.
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4.8.9 BMP A709: Urban Streets
4.8.9.1 Description of Pollutant Sources
Streets can be sources of vegetative debris, paper, fine dust, vehicle liquids, tire wear residues,
heavy metals (lead and zinc), phthalates, soil particles, ice control salts, domestic wastes, animal
wastes, lawn chemicals, and vehicle combustion by-products. Street surface contaminants have
been found to contain significant concentrations of particle sizes less than 250 microns (Sartor and
Boyd, 1972).
4.8.9.2 Pollutant Control Approach
Conduct efficient street sweeping where and when appropriate to minimize the contamination of
stormwater. Do not wash street debris into storm drains.
4.8.9.3 Recommended BMPs
• For maximum stormwater pollutant reductions on curbed streets and high volume
parking lots, use efficient vacuum sweepers.
• High-efficiency street sweepers utilize strong vacuums and the mechanical action of
main and gutter brooms combined with an air filtration system that only returns clean
air to the atmosphere (i.e., filters very fine particulates). They sweep dry and use no
water since they do not emit any dust.
• For moderate stormwater pollutant reductions on curbed streets, use regenerative air
sweepers or tandem sweeping operations.
• A tandem sweeping operation involves a single pass of a mechanical sweeper
followed immediately by a single pass of a vacuum sweeper or regenerative air
sweeper.
o A regenerative air sweeper blows air down on the pavement to entrain
particles and uses a return vacuum to transport the material to the hopper.
o These operations usually use water to control dust. This reduces their ability
to pick up fine particulates.
• For minimal stormwater pollutant reductions on curbed streets, use mechanical
sweepers.
NOTE: Mechanical sweepers are referred to as broom sweepers and use the mechanical
action of main and gutter brooms to throw material on a conveyor belt that transports it to the
hopper. These sweepers usually use water to control dust, reducing their ability to pick up
fine particulates.
• Conduct sweeping at optimal frequencies. Optimal frequencies are those scheduled
sweeping intervals that produce the most cost-effective annual reduction of
pollutants normally found in stormwater and can vary depending on land use, traffic
volume, and rainfall patterns.
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• Train operators in those factors that result in optimal pollutant removal. These factors
include sweeper speed, brush adjustment and rotation rate, sweeping pattern,
maneuvering around parked vehicles, and interim storage and disposal methods.
• Minimize the amount of water applied for dust control to avoid washing pollutants into
the storm drainage system.
• At active construction sites, street sweeping shall be performed prior to washing the
street.
• Consider the use of periodic parking restrictions and public notification in residential
areas to ensure the sweeper’s ability to sweep along the curb.
• Establish procedures for prompt sweeping, removal, and disposal of spill clean-up
materials and debris from special events that will generate higher than normal
loadings.
• Disposal of street sweeping solids must comply with “Recommendations for
Management of Street Wastes” described in Appendix D of this volume. Additional
guidance can be found in the Regional Road Maintenance – Endangered Species
Act (ESA) program guidelines.
• Inform citizens about the importance of eliminating yard debris, oil, and other wastes
in street gutters in order to reduce street pollutant sources.
• When encountering questionable sweeping waste contact the City of Auburn Storm
Drainage Maintenance and Operations at 253-931-3048.
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4.8.10 BMP A710: Railroad Yards
4.8.10.1 Description of Pollutant Sources
Pollutant sources can include drips/leaks of vehicle fluids and cargo onto the railroad bed; human
waste disposal; litter; locomotive/railcar/equipment cleaning; fueling; outside material storage; the
erosion and loss of soil particles from the railroad bed; maintenance and repair activities at railroad
terminals, switching yards, and maintenance yards; and herbicides used for vegetation management.
Waste materials can include waste oil, solvents, degreasers, antifreeze solutions, radiator flush,
acids, brake fluids, dust, soiled rags, oil filters, sulfuric acid and battery sludge, machine chips with
residual machining oil, and toxic fluids/solids lost during transit. Potential pollutants include oil and
grease, TSS, BOD, organics, pesticides, and metals.
4.8.10.2 Pollutant Control Approach
Apply good housekeeping and preventive maintenance practices to control leaks and spills of liquids
in railroad yard areas.
4.8.10.3 Required BMPs
• Implement the applicable BMPs in this chapter depending on the pollutant generating
activities/sources at a railroad yard facility.
• Do not allow toilets to discharge to outside areas while a train is in transit or at the
station. Pump-out facilities shall be used to service train toilets.
• Use drip and track pans at hose/pipe connections during liquid transfer and other
leak-prone areas.
• During maintenance do not discard debris or waste liquids along the tracks or in
railroad yards.
• Promptly clean up all spilled materials.
In areas subjected to leaks/spills of oils or other chemicals, convey the contaminated stormwater to
appropriate treatment such as a sanitary sewer (if approved by the City of Auburn), to a CP or API
oil/water separator for floating oils, or other appropriate treatment BMP (as approved by the City of
Auburn). See Volume V.
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4.8.11 BMP A711: Maintenance of Public and Utility Corridors and Facilities
4.8.11.1 Description of Pollutant Sources
Passageways and equipment at petroleum product, natural gas, and water pipelines and electrical
power transmission corridors and rights-of-way can be sources of pollutants, such as herbicides used
for vegetation management and eroded soil particles from unpaved access roads. At pump stations,
waste materials generated during maintenance activities may be temporarily stored outside.
Additional potential pollutant sources include the leaching of preservatives from wood utility poles,
PCBs in older transformers, water that is removed from underground transformer vaults, and
leaks/spills from petroleum pipelines. The following are potential pollutants: oil and grease, TSS,
BOD, organics, PCB, pesticides, and heavy metals.
4.8.11.2 Pollutant Control Approach
Control fertilizer and pesticide applications, soil erosion, and site debris that can contaminate
stormwater.
4.8.11.3 Required BMPs
• Implement BMPs included in Chapter 4, BMP A306: Landscaping and
Lawn/Vegetation Management.
• When water or sediments are removed from electric transformer vaults, determine
whether contaminants might be present before disposing of the water and
sediments. This includes inspecting for the presence of oil or sheen, and determining
from records or testing if the transformers contain PCBs. If records or tests indicate
that the sediment or water are contaminated above applicable levels, manage these
media in accordance with applicable federal and state regulations, including the
federal PCB rules (40 CFR 761) and the state MTCA cleanup regulations (Chapter
173-340 WAC). Water removed from the vaults can be discharged in accordance
with the federal 40 CFR 761.79, and state regulations (Chapter 173-201A WAC and
Chapter 173-200 WAC), or via the sanitary sewer if the requirements, including
applicable permits, for such a discharge are met. (See also Chapter 5).
• Within utility corridors, consider preparing maintenance procedures and an
implementation schedule that provides for vegetative, gravel, or equivalent cover that
minimizes bare or thinly vegetated ground surfaces within the corridor to prevent the
erosion of soil. Pave high traffic corridors.
• Provide maintenance practices to prevent stormwater from accumulating and
draining across and/or onto roadways. Stormwater shall be conveyed through
roadside ditches and culverts. The road shall be crowned, out-sloped, water barred,
or otherwise left in a condition not conducive to erosion. Appropriately maintaining
grassy roadside ditches discharging to surface waters is an effective way of
removing some pollutants associated with sediments carried by stormwater.
• Maintain ditches and culverts at an appropriate frequency to ensure that plugging
and flooding across the roadbed, with resulting overflow erosion, does not occur.
• Apply the appropriate BMPs from Volume IV for the storage of waste materials that
can contaminate stormwater.
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4.8.11.4 Recommended BMPs
• When selecting utility poles for a specific location, consideration should be given to
the potential environmental effects of the pole or poles during storage, handling, and
end-use, as well as its cost, safety, efficacy, and expected life. If a wood product
treated with chemical preservatives is used, it should be made in accordance with
generally accepted industry standards such as the American Wood Preservers
Association Standards. If the pole or poles will be placed in or near an
environmentally sensitive area, such as a wetland or a drinking water well,
alternative materials or technologies should be considered. These include poles
constructed with material(s) other than wood, such as fiberglass composites, metal,
or concrete. Other technologies and materials, such as sleeves or caissons for wood
poles, may also be considered when they are determined to be practicable and
available.
• Ring the base of treated poles with adsorbent material if leaching of preservative
may occur. Monitor the adsorbents as needed.
• As soon as practicable, remove all litter from wire cutting/replacing operations, etc.
• Implement temporary erosion and sediment control in areas where clear-cuts are
conducted and new roads are constructed.
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4.8.12 BMP A712: Maintenance of Roadside Ditches
4.8.12.1 Description of Pollutant Sources
Common road debris including litter, eroded soil, oils, vegetative particles, and heavy metals can be
sources of stormwater pollutants.
4.8.12.2 Pollutant Control Approach
Roadside ditches should be maintained to preserve the condition and capacity for which they were
originally constructed, and to minimize bare or thinly vegetated ground surfaces. Maintenance
practices should provide for erosion and sediment control (Refer to BMP A306 Landscaping and
Lawn/Vegetation Management).
4.8.12.3 Required BMPs
• Inspect roadside ditches regularly, as needed, to identify sediment accumulations
and localized erosion.
• Clean ditches on a regular basis, as needed. Ditches shall be kept free of rubbish
and debris.
• Vegetation in ditches often prevents erosion and cleanses runoff waters. Remove
vegetation only when flow is blocked or excess sediments have accumulated.
Conduct ditch maintenance (seeding, fertilizer application, harvesting) in late spring
and/or early fall, where possible. Consider leaving segments of undisturbed
vegetation to provide natural filtration.
• In the area between the edge of the pavement and the bottom of the ditch,
commonly known as the “bare earth zone,” use grass vegetation, wherever possible.
Vegetation shall be established from the edge of the pavement if possible or at least
from the top of the slope of the ditch.
• Diversion ditches on top of cut slopes that are constructed to prevent slope erosion
by intercepting surface drainage must be maintained to retain their diversion shape
and capability.
• Ditch cleanings are not to be left on the roadway surfaces. Sweep dirt and debris
remaining on the pavement at the completion of ditch cleaning operations.
• Roadside ditch cleanings not contaminated by spills or other releases and not
associated with a stormwater treatment system such as a bioswale may be screened
to remove litter and separated into soil and vegetative matter (leaves, grass,
needles, branches, etc.). The soil fraction may be handled as ‘clean soils’ and the
vegetative matter can be composted or disposed of in a municipal waste landfill. For
more information, please see “Recommendations for Management of Street
Wastes,” in Appendix D of this volume.
• Roadside ditch cleanings contaminated by spills or other releases known or
suspected to contain dangerous waste must be handled following the Dangerous
Waste Regulations (Chapter 173-303 WAC) unless testing determines it is not
dangerous waste. Specific cleanup standards are set forth in the Model Toxics
Control Act (Chapter 70.105D RCW) and Regulations (Chapter 173-340 WAC).
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• Inspect culverts on a regular basis for scour or sedimentation at the inlet and outlet,
and repair as necessary. Give priority to those culverts conveying perennial and/or
salmon-bearing streams and culverts near streams in areas of high sediment load,
such as those near subdivisions during construction.
4.8.12.4 Recommended BMPs
• Install biofiltration swales and filter strips to treat roadside runoff wherever
practicable and use engineered topsoils wherever necessary to maintain adequate
vegetation. These systems can improve infiltration and stormwater pollutant control
upstream of roadside ditches. Refer to Volume V of this manual for additional
information about biofiltration swales and filter strips.
• Additional guidance can be found in the Regional Road Maintenance - Endangered
Species Act (ESA) program guidelines.
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4.8.13 BMP A713: Maintenance of Stormwater Drainage and
Treatment Facilities
4.8.13.1 Description of Pollutant Sources
Facilities include roadside catch basins on arterials and within residential areas, conveyance
systems, detention facilities such as ponds and vaults, oil and water separators, biofilters, settling
basins, infiltration systems, and all other types of stormwater treatment systems presented in
Volume V. Roadside catch basins can remove from 5 to 15 percent of the pollutants present in
stormwater. When catch basins are about 60 percent full of sediment, they cease removing
sediments. Oil and grease, hydrocarbons, debris, heavy metals, sediments, and contaminated water
are found in catch basins, oil and water separators, settling basins, etc.
4.8.13.2 Pollutant Control Approach
Provide maintenance and cleaning of debris, sediments, and oil from stormwater collection,
conveyance, and treatment systems to obtain proper operation.
4.8.13.3 Required BMPs
Maintain stormwater treatment facilities according to the O&M procedures presented in
Volume I, Appendix D, in addition to the following BMPs:
• Inspect and clean treatment BMPs, conveyance systems, and catch basins as
needed, and determine whether improvements in O&M are needed.
• Promptly repair any deterioration threatening the structural integrity of the facilities.
These include replacement of clean-out gates, catch basin lids, and rock in
emergency spillways.
• Ensure that storm sewer capacities are not exceeded and that heavy sediment
discharges to the sewer system are prevented.
• Regularly remove debris and solids from BMPs used for peak-rate control, treatment,
etc. and discharge to a sanitary sewer if approved by the City of Auburn, or truck to a
local or state government approved disposal site.
• Clean catch basins in accordance with the information provided in Volume I,
Appendix D. Additional information is also included in BMP S109: Cleaning Catch
Basins.
• Clean debris in a catch basin as frequently as needed to ensure proper operation of
the catch basin.
• Post warning signs or curb markers; “Dump No Waste - Drains to Groundwater,”
“Streams,” “Lakes,” or emboss on or adjacent to all storm drain inlets where
practical.
• Disposal of sediments and liquids from the catch basins must comply with
“Recommendations for Management of Street Wastes” described in Appendix D of
this volume.
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• Select additional applicable BMPs from Chapter 4 of this volume depending on the
pollutant sources and activities conducted at the facility. Those BMPs include:
o A407 Storage of Liquid, Food Waste, or Dangerous Waste Containers.
o A603 Soil Erosion and Sediment Control at Industrial Sites
o A709 Urban Streets
o A714 Spills of Oil and Hazardous Substances
o S101 Eliminate Illicit Storm Drainage System Connections
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4.8.14 BMP A714: Spills of Oil and Hazardous Substances
4.8.14.1 Description of Pollutant Sources
Owners or operators of facilities engaged in drilling, producing, handling, gathering, storing,
processing, transferring, distributing, refining or consuming oil and/or oil products are required by
Federal Law to have a Spill Prevention and Control Plan. The federal definition of oil is oil of any kind
or any form, including, but not limited to, petroleum, fuel oil, sludge, oil refuse, and oil mixed with
wastes other than dredged spoil. Specific regulations can be found in 40 CFR Part 112, as amended
December, 2006. These regulations are administered by the Environmental Protection Agency and
the United States Coast Guard. Large petroleum handling facilities and vessels are also subject to
regulations contained in Chapter 90.56 RCW and Chapter 173-180A WAC.
Owners of businesses that produce Dangerous Wastes are also required by State Law, Chapter
70.105 RCW and Chapter 173-303 WAC, to have a spill control plan. These businesses should refer
to Chapter 5 of this volume. The City of Auburn may also require a spill plan to protect the municipal
sewer system and groundwater resources. Plans required by the above listed regulations may
suffice.
4.8.14.2 Pollutant Control Approach
Maintain, update, and implement an oil spill prevention/cleanup plan.
4.8.14.3 Required BMPs
• Prepare an Emergency Spill Control Plan (SCP), which includes:
o A description of the facility including the owner's name and address.
o The nature of the activity at the facility.
o The general types of chemicals used or stored at the facility.
o A site plan showing the location of storage areas for chemicals, the locations
of storm and sanitary drains, the areas draining to them, the ultimate point of
discharge, and the location and description of any devices to stop spills from
leaving the site such as positive control valves;
o Cleanup procedures and supplies.
o Notification procedures to be used in the event of a spill, such as notifying
key personnel. Agencies such as Ecology, Valley Regional Fire Authority,
Washington State Patrol, City of Auburn, and the U.S. Environmental
Protection Agency shall be notified.
o The name and 24-hour contact telephone number of the designated person,
and their alternate with overall spill cleanup and notification responsibility.
o Identify contractors that can be contacted to provide spill clean-up and
disposal services. A service agreement is encouraged.
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• Train key personnel in the implementation of the Emergency SCP. Prepare a
summary of the plan and post it at appropriate points in the building, identifying the
spill cleanup coordinators, location of cleanup kits, and 24-hour phone numbers of
regulatory agencies to be contacted in the event of a spill.
• Update the SCP regularly.
• Immediately notify Ecology and the City of Auburn if a spill may reach sanitary or
storm sewers, groundwater, or surface water, in accordance with federal and
Ecology spill reporting requirements.
• Immediately clean up spills using appropriate personal protection equipment and
following the facility safety standards. Do not use emulsifiers for cleanup unless an
appropriate disposal method for the resulting oily wastewater is implemented.
Absorbent material shall not be washed down a floor drain or storm sewer.
• Locate emergency spill containment and cleanup kit(s) in high potential spill areas.
The contents of the kit shall be appropriate for the type and quantities of chemical
liquids stored at the facility.
4.8.14.4 Recommended BMPs
• Spill kits should include appropriately lined drums, absorbent pads, and granular or
powdered materials for neutralizing acids or alkaline liquids where applicable. In
fueling areas: absorbent should be packaged in small bags for easy use and small
drums should be available for storage of absorbent and/or used absorbent. Spill kits
should be deployed in a manner that allows rapid access and use by employees.
• Example spill plans may be obtained from the Washington State Department of
Transportation and the Environmental Protection Agency.
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4.8.15 BMP A715: Water Reservoir, Transmission Mainline, Wellhead, and
Hydrant Flushing Activities
4.8.15.1 Description of Pollutant Sources
Construction and operation of drinking water infrastructure, 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.
4.8.15.2 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.
4.8.15.3 Required BMPs
• Discharges of untreated hyperchlorinate water must go to the sanitary sewer. Prior
City approval is required.
• 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
• 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
half the full-pipe flow rate.
4.8.15.4 Recommended BMPs
• 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.
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• 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.
Figure IV-4-18. Hydrant Flushing
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4.8.16 BMP S101: Eliminate Illicit Storm Drainage System Connections
A common problem found in storm drainage systems is illegal hook-ups to the system. Conversely,
discharging clean, uncontaminated water to the sanitary sewer system is also prohibited.
All businesses and residences in Auburn shall examine their plumbing systems to determine if illicit
connections exist. Any time it is found that toilets, sinks, appliances, showers and bathtubs, floor
drains, industrial process waters, cooling towers, or other indoor activities are connected to the storm
drainage system, the connections must be immediately rerouted to the sanitary or septic system,
holding tanks, or process treatment system. Exceptions to this requirement would be those industries
and businesses that have been issued an NPDES Permit by Ecology, and are allowed specific
discharges under that permit. Please refer to Chapter 5 to determine if a specific type of business is
required to have a NPDES permit.
Dye testing with a non-toxic dye, smoke testing, electronic locators, and television inspection
equipment can help to determine where a pipe or structure drains if it is not obvious by observations
or on plans. Contact the City of Auburn Public Works Department at 253-931-3010 for assistance in
locating City structures adjacent to a property.
Drains which are found to be connected to the wrong system must either be permanently plugged or
disconnected and rerouted as soon as possible. Permits must be obtained from The City of Auburn
(253-931-3090) to reroute drains. If the discharge is contaminated and sanitary service is not
available, alternate measures will be necessary. If the discharge is simply domestic waste, a septic
system may be feasible. Contact the appropriate County Health Department for a septic system
permit. If the discharge is anything other than domestic, then a holding tank or on-site treatment may
be necessary. Contact the City of Auburn Storm Drainage Utility at 253-931-3010 for assistance.
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4.8.17 BMP S102: Dispose of Contaminated Stormwater and Waste Materials
Properly
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
• Municipal 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 Auburn Utilities Section at 253-931-3010 for design and permit requirements.
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
municipal 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 253-931-3047 can provide information on waste disposal options.
Maintain records for all materials that are recycled or disposed.
Appendix A of this volume has a list of telephone numbers to contact for assistance.
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4.8.18 BMP S103: Discharge Process Wastewater to a Sanitary Sewer,
Holding Tank, or Water Treatment System
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.
In order to connect to the sanitary sewer, contact Auburn Permit Center at 253-931-3090 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 S102).
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.
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4.8.19 BMP S108: Implement Integrated Pest Management Measures
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.
Commercial, agricultural, municipal, and other large scale pesticide users, such as golf courses and
parks, should 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
Monitoring of pest populations is a 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, or in Appendix C of this volume.
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4.8.20 BMP S109: Cleaning Catch Basins
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 Volume I, Appendix D.
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.
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4.9 Cover and Surround Activities
4.9.1 BMP S104: Cover the Activity with a Roof or Awning
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 253-931-3090 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. Examples of these types of structures are
shown in
Figure IV-4-19.
Figure IV-4-19. Examples of Covered Activities
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4.9.2 BMP S105: Cover the Activity with an Anchored Tarp or Plastic Sheet
Some activities, such as stockpiling of raw materials, can be effectively covered with a sturdy tarp or
heavy plastic sheet made of impermeable material. 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-4-20.
Figure IV-4-20. Tarp Covering
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4.9.3 BMP S106: Pave the Activity Area and Slope to a Sump, Holding Tank,
or Oil/Water Separator
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-4-21) 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 as shown in Figure IV-4-
23. Frequent inspections of the sump, holding tank, or oil/water separator are necessary. Inspections
and maintenance shall be recorded in a log. Commercial services that pump sumps and holding
tanks are listed in the Yellow Pages of the phone directory under Environmental and Ecological
Services.
Figure IV-4-21. Paved Area with Sump Drain
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4.9.4 BMP S107: Surround the Activity Area with a Curb, Dike, or Berm or
Elevate the Activity
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.
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 A714. For permanent
storage facilities see BMP A202, A401, A407, and A408.
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Figure IV-4-22. Above-Ground Tank with Impervious Walls and Floor
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Figure IV-4-23. Containment Types
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Figure IV-4-24. Standby Generator
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Chapter 5 Regulations and Requirements
The information in this chapter is provided to aid in compliance with other Auburn and Washington
State regulations, which may apply to a project, industry, or business in terms of protecting water
quality. A listing of relevant regulations is provided but should be verified because of the continuing
modification of statutes, regulations, and City ordinances. It is the applicant’s responsibility to obtain
the current version of any ordinances, statutes, or regulations that apply to a specific project or
activity. Copies of City ordinances are available at the City Clerk’s office in City Hall located at
25 West Main Street, 253-931-3039.
5.1 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
8.08 Solid Waste
12.20 Driveways
13 Water, Sewer and Public Utilities
13.48 Storm Drainage Utility
13.48.330 Off-site Improvements
14 Project Review
15.74 Land Clearing, Filling and Grading
16 Environment
16.08 Shoreline Regulation
16.10 Critical Areas
17 Subdivision
18 Zoning
5.2 State, Federal, and Other Regulations and Requirements
5.2.1 Washington State Department of Ecology Requirements for the Discharge
of Process Wastewaters Directly to Surface Waters
If a public sanitary sewer is not available, process wastewater may be discharged, after suitable
treatment, to a surface water body like a lake or stream, or to a drainage field. If the discharge is to a
surface water body, Ecology must approve the type and design of the treatment system, as well as
the outfall design. If a septic tank and drain field are used for treatment, requirements of the Seattle-
King County Health Department (SKCHD) or the Tacoma-Pierce County Health Department
(TPCHD) will also apply. Contact SKCHD at 206-296-4932 or TPCHD at 253-798-6470 for more
information.
Ecology’s requirements can be found at WAC Chapter 173.240.
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5.2.2 Washington State Department of Ecology Requirements for Dangerous
Waste Generators
The state dangerous waste regulations (WAC Chapter 173-303) cover accumulation, storage,
transportation, treatment, and disposal. Of interest to this manual is the temporary accumulation of
waste until taken from the site to a permitted disposal site. Only portions of those regulations that
apply to temporary storage are summarized here.
Permitted Generators
Businesses that generate 220 pounds or more of waste, either per batch or in the aggregate, over
one month must comply with the storage specifications outlined below:
Small-Quantity Waste Generators
These are businesses that generate less than 220 pounds of dangerous waste per month or per
batch (or 2.2 pounds of extremely hazardous waste). Small-quantity generators still fall under
Ecology regulations to the extent that the materials must be properly stored on site until shipment.
The wastes must be moved from the property whenever the accumulated quantity equals or exceeds
220 pounds or whenever the material has resided on site for 180 days. The waste must be disposed
of at an approved facility. If the business is in compliance with these requirements, they are also
considered solid waste generators, and are regulated by the local health departments. For technical
assistance and site visits, contact the Tacoma-Pierce County Health Department at 253-798-6047 or
the King County Hazardous Waste Management program at 206-296-4692. Regulations governing
small-quantity generators are currently being reviewed to possibly raise the accumulation limit. Call
the Hazardous Waste Line at 800-287-6429 for the most up-to-date information.
Dangerous Waste Pollution Prevention Plans
A recent state law established the requirement that generators of dangerous wastes in excess of
220 lbs/month (2,640 lbs/year) prepare a waste reduction plan, called a pollution prevention plan, not
to be confused with the stormwater pollution prevention plan (see R.4). The required content of the
plan is set forth in Pollution Prevention Planning - Guidance Manual, January 1992,
Publication #91-2, for WAC Chapter 173-307.
5.2.3 Washington State Department of Ecology Stormwater NPDES Permit
Requirements
The Federal National Pollutant Discharge Elimination System (NPDES) program requires industries
or industrial-type activities to obtain permits for stormwater discharge.
Coverage under Ecology’s general permit for Stormwater Discharges Associated with Industrial
Activities is required for each regulated facility. A business must obtain permit coverage if its primary
activity falls under one of the categories listed in the permit or its fact sheet. The permit and fact sheet
may be viewed on Ecology’s website at www.ecy.wa.gov/programs/wq/permits/index.html.
The program requires the preparation of a stormwater pollution prevention plan (SWPPP). A NPDES
permit is required for certain categories of industries and municipalities for discharge to surface
water, or a storm drain that discharges to surface water or to surface water and groundwater.
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5.2.4 Washington State Department of Ecology Requirements for Underground
and Above Ground Storage Tanks
Underground storage tanks (UST) that contain fuel and other petroleum products are regulated by
the Department of Ecology under WAC Chapter 173-360 Underground Storage Tank
Regulations. Above-ground storage tanks (ASTs) may also be regulated. Inquiries about business-
specific requirements and permitting for USTs and ASTs should be directed to the Department of
Ecology, Northwest Regional Office at 425-649-7000.
5.2.5 U.S. Environmental Protection Agency and Ecology Emergency Spill
Cleanup Requirements
USEPA - Spill Prevention Control and Cleanup (SPCC) Plans (40 CFR 112)
This federal regulation requires that owners or operators of facilities engaged in drilling, producing,
gathering, storing, processing, refining, transferring, or consuming oil and oil products are required to
have a spill prevention and control plan (SPCC), provided that the facility is not transportation related;
and, that the aboveground storage of a single container is in excess of 660 gallons, or an aggregate
capacity greater than 1,320 gallons, or a total below ground capacity in excess of 42,000 gallons.
Department of Ecology Dangerous Wastes (WAC 173-303-350)
The regulations state that generators must have a contingency plan that must include:
1. Actions taken in the event of a spill.
2. Descriptions of arrangements with local agencies.
3. Identification of the owner's emergency coordinator.
4. List of emergency equipment.
5. Evaluation plan for business personnel.
See Federal Regulation 40 CFR 112 and WAC 173-303-350 for further information.
5.2.6 Washington State Department of Agriculture Pesticide Regulations
Washington State pesticide laws are administered by the State's Department of Agriculture, under
the Washington Pesticide Control Act (RCW Chapter 15.58), Washington Pesticide Application
Action (RCW Chapter 17.21), and regulations in WAC Chapter 16.228. In Auburn, all pest control
operators and fumigators are required to obtain certification from the appropriate County Health
Department. Contact the Seattle-King County Health Department at 206-205-4394 or Tacoma-Pierce
County Health Department’s Compliance Program at 253-798-6440 for more information.
5.2.7 Puget Sound Clean Air Agency Air Quality Regulations
The Puget Sound region is under the jurisdiction of regional air quality authorities who in turn must
function under Washington State and federal air quality regulations. The Puget Sound Clear Air
Agency (PSCAA) is the regulatory agency for air quality in Auburn.
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Quick Reference Phone Numbers Volume IV
Appendix A 583
Appendix A Quick Reference Phone Numbers
City of Auburn
Storm Drainage Utility 253-931-3010
Permit Center 253-931-3090
Valley Regional Fire Authority 253-591-5740
Solid Waste Management Utility 253-931-3038
Household Hazardous Waste 253-931-3038
Recycling Services 253-931-3038
Environmental Protection Agency (EPA) - Region X 800-424-4372
IMEX (Industrial Materials Exchange) 206-296-4899
King County
Wastewater Program (septic) 206-296-4932
Household Hazardous Waste 206-296-4692
Pest Control Operators 206-205-4394
Tacoma-Pierce County Health Department
On-Site Sewage and Underground Storage Tanks 253-798-6470
Hazardous Waste Section 253-798-6047
Solid Waste 253-798-6047
Hazardous Waste Line 800-287-6429
Pest Control Operators and Fumigators 253-798-6470
University of Washington Center for Urban Water Resources 206-543-6272
Washington State Department of Agriculture 360-902-2010
877-301-4555
Washington State Department of Ecology 360-407-6000
Northwest Regional Office 425-649-7000
Dangerous/Hazardous Waste 360-407-6300
NPDES Stormwater or Wastewater Permits 360-407-6400
Spill Reporting 800-424-8802
Recycling 800-732-9253
Groundwater Quality and Protection 360-407-6400
Underground and Above Ground Storage Tanks 360-407-7170
Washington State University/Pierce County Cooperative Extension 253-798-7180
Puget Sound Clean Air Agency 800-552-3565
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Recycling/Disposal of Vehicle Fluids Volume IV
and Other Wastes Appendix B 584
Appendix B Recycling/Disposal of Vehicle Fluids and
Other Wastes
The information in this appendix was obtained from Ecology’s Hazardous Waste Program. For a
copy of “Hazardous Waste Services Directory”, Publication #91-12S, Revised December 1994, call
Ecology’s Hazardous Waste and Toxic Reduction Program at 360-407-6721.
RECOMMENDED MANAGEMENT
Antifreeze Store separately for resale. Separate ethylene glycol from propylene glycol for off-site
recycling.
If not recyclable, send to Treatment, Storage, and Disposal Facility (TSDF) for disposal.
Batteries INTACT: Accumulate under cover prior to sale, deliver to recycler or, return to
manufacturer.
BROKEN: Accumulate acid from broken batteries in resistant containers with secondary
containment. Send to TSDF for disposal.
Brake fluid Accumulate in separate, marked, closed container. Do not mix with waste oil. Recycle.
Fuel Store gasoline, and diesel separately for use or resale.
Mixtures of diesel, gasoline, oil, and other fluids may not be recyclable and may require
expensive disposal.
Fuel filters Drain fluids for use as product. With approval of local landfill operator, dispose to dumpster,
if needed.
Oil filters Puncture the filter dome and drain it for 24 hours. Put oil drained from filters into a "USED
OIL ONLY" container. Keep drained filters in a separate container marked "USED OIL
FILTERS ONLY." Locate a scrap metal dealer who will pick up and recycle filters. With
approval of local landfill operator, dispose of drained filters to dumpster.
Paint Accumulate oil-based and water-based paints separately for use or resale.
If not recyclable, send accumulations to TSDF for disposal.
Power steering fluid Same as for used oils
Shop towels/oily rags Use cloth towels that can be laundered and reused. Accumulate used shop towels in a
closed container.
Sign up with an industrial laundry service that can recycle your towels.
Solvents Consider using less hazardous solvents or switching to a spray cabinet that does not use
solvent.
Accumulate solvents separately. Consider purchasing a solvent still and recycling solvent
on site.
Do not mix with used oil. Do not evaporate as a means of disposal.
Transmission oil,
differential and rear end
fluids
Accumulate in your "USED OIL ONLY" container. Arrange for pickup for off-site recycling.
Used oils; including,
crankcase oil,
transmission oil, power
steering fluid and
differential/rear end oil
Keep used oil in a separate container marked "USED OIL ONLY." Do not mix with brake
fluid, or used antifreeze. Do not mix with any other waste if material will be burned for
heating. Arrange for pickup for off-site recycling.
Windshield washer
fluid
Accumulate separately for use or resale. Discharge to on-site sewage disposal, or, if
acceptable by the local sewer authority, discharge to sanitary sewer.
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NOVEMBER 2009
Example of an Integrated Pest Volume IV
Management Program (IPM) Appendix C 585
Appendix C Example of an Integrated Pest
Management Program (IPM)
Integrated Pest Management (IPM) is a natural, long-term, ecologically based systems approach to
controlling pest populations. This system uses techniques either to reduce pest populations or
maintain them at levels below those causing economic injury, or to manipulate the populations so
that they are prevented from causing injury. The goals of IPM are to encourage optimal selective
pesticide use (away from prophylactic, broad spectrum use), and to maximize natural controls to
minimize the environmental side effects.
A step-by-step comprehensive Integrated Pest Management (IPM) Program is provided below as a
guide.
Introduction
This section provides a sound cultural approach to managing lawns and landscapes and minimizing
runoff. Many homeowners or property managers will be able to implement most or all of this
approach. Others will wish to hire these services out. For the do-it yourselfer, an array of resources
are available to assist in the effort. Landscaping businesses, agricultural extensions, local agencies,
master gardener programs, local nurseries, and the library can all provide assistance. Landscaping
professionals (businesses) are particularly encouraged to practice IPM.
Definition
“Integrated pest management, or IPM, is an approach to pest control that uses regular monitoring to
determine if and when treatments are needed, and employs physical, mechanical, cultural, and
biological tactics to keep pest numbers low enough to prevent intolerable damage or annoyance.
Least-toxic chemical controls are used as a last resort.”
True IPM is a powerful approach that anticipates and prevents most problems through proper cultural
practices and careful observation. Knowledge of the life cycles of the host plants and both beneficial
and pest organisms is also important. The IPM section of this example is adapted from “Least Toxic
Pest Management for Lawns” by Sheila Daar. Following the IPM process yields the information
needed to minimize damage by weeds, diseases, and pests and to treat those problems with the
least toxic approaches.
The IPM Process
Step 1. Correctly identify problem pests and understand their life cycle.
Learn more about the pest. Observe it and pay attention to any damage that may be occurring. Learn
about the life cycle. Many pests are only a problem during certain seasons, or can only be treated
effectively in certain phases of the life cycle.
Step 2. Establish tolerance thresholds for pests.
Every landscape has a population of some pest insects, weeds, and diseases. This is good because
it supports a population of beneficial species that keep pest numbers in check. Beneficial organisms
may compete with, eat, or parasitize disease or pest organisms. Decide on the level of infestation
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Example of an Integrated Pest Volume IV
Management Program (IPM) Appendix C 586
that must be exceeded before treatment needs to be considered. Pest populations under this
threshold should be monitored but do not need treatment. For instance, European crane flies usually
do not do serious damage to a lawn unless there are between 25 to 40 larvae per square foot
feeding on the turf in February (in normal weather years). Also, most people consider a lawn healthy
and well maintained even with up to 20% weed cover, so treatment, other than continuing good
maintenance practices, is generally unnecessary.
Step 3. Monitor to detect and prevent pest problems.
Regular monitoring is a key practice to anticipate and prevent major pest outbreaks. It begins with a
visual evaluation of the lawn or landscape's condition. Take a few minutes before mowing to walk
around and look for problems. Keep a notebook, record when and where a problem occurs, then
monitor for it at about the same time in future years. Specific monitoring techniques can be used in
the appropriate season for some potential problem pests, such as European crane fly.
Step 4. Modify the maintenance program to promote healthy plants and discourage pests.
A healthy landscape is resistant to most pest problems. Lawn aeration and overseeding along with
proper mowing height, fertilization, and irrigation will help the grass out-compete weeds. Correcting
drainage problems and letting soil dry out between waterings in the summer may reduce the number
of crane-fly larvae that survive.
Step 5. If pests exceed the tolerance thresholds
Use cultural, physical, mechanical, or biological controls first. If those prove insufficient, use the
chemical controls described below that have the least non-target impact. When a pest outbreak
strikes (or monitoring shows one is imminent), implement IPM then consider control options that are
the least toxic, or have the least non-target impact. Here are two examples of an IPM approach:
• Red thread disease is most likely under low nitrogen fertility conditions and most
severe during slow growth conditions. Mow and bag the clippings to remove
diseased blades. Fertilize lightly to help the grass recover, then begin
grasscycling and change to fall fertilization with a slow-release or natural-organic
fertilizer to provide an even supply of nutrients. Chemical fungicides are not
recommended because red thread cannot kill the lawn.
• Crane fly damage is most prevalent on lawns that stay wet in the winter and are
irrigated in the summer. Correct the winter drainage and/or allow the soil to dry
between irrigation cycles; larvae are susceptible to drying out, so these changes
can reduce their numbers. It may also be possible to reduce crane fly larvae
numbers by using a power de-thatcher on a cool, cloudy day when feeding is
occurring close to the surface. Studies are being conducted using beneficial
nematodes that parasitize the crane fly larvae; this type of treatment may
eventually be a reasonable alternative.
Only after trying suitable non-chemical control methods, or determining that the pest outbreak is
causing too much serious damage, should chemical controls be considered. Study to determine what
products are available and choose a product that is the least toxic and has the least non-target
impact. Refer to the Operational BMPs for the use of Pesticides below for guidelines on choosing,
storing, and using lawn and garden chemicals.
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Example of an Integrated Pest Volume IV
Management Program (IPM) Appendix C 587
Step 6. Evaluate and record the effectiveness of the control, and modify maintenance
practices to support lawn or landscape recovery and prevent recurrence.
Keep records! Note when, where, and what symptoms occurred, or when monitoring revealed a
potential pest problem. Note what controls were applied and when, and the effectiveness of the
control. Monitor next year for the same problems. Review your landscape maintenance and cultural
practices to see if they can be modified to prevent or reduce the problem.
A comprehensive IPM Program should also include the proper use of pesticides as a last resort, and
vegetation/fertilizer management to eliminate or minimize the contamination of stormwater.
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Recommendations for Management Volume IV
of Street Wastes Appendix D 588
Appendix D Recommendations for Management
of Street Wastes
Introduction
This appendix is a summary, taken from the June 1999 draft Ecology publication titled
Recommendations for Management of Street Waste (Publication WQ 99-09). The guidance
document addresses waste generated from stormwater maintenance activities such as street
sweeping and the cleaning of catch basins and, to a limited extent, other stormwater conveyance and
treatment facilities. Limited information is available on the characteristics of wastes from
detention/retention ponds, bioswales, and similar stormwater treatment facilities. The
recommendations provided here may be generally applicable to these facilities, with extra diligence
given to waste characterization.
These recommendations do not constitute rules or regulations, but are suggestions for street
waste handling, reuse, and disposal supported by current regulations and the present state of
knowledge of street waste constituents. The recommendations are intended to address the liquid
and solid wastes collected during routine maintenance of stormwater catch basins,
detention/retention ponds and ditches and similar stormwater treatment and conveyance
structures, and street and parking lot sweeping. In addition to these recommendations, end users
and other authorities may have their own requirements for street waste reuse and handling.
"Street Wastes" include liquid and solid wastes collected during maintenance of stormwater catch
basins and detention/retention ponds and ditches and similar stormwater treatment and conveyance
structures, and solid wastes collected during street and parking lot sweeping.
"Street Wastes," as defined here, does not include solids and liquids from street washing using
detergents, cleaning of electrical vaults, vehicle wash sediment traps, restaurant grease traps,
industrial process waste, sanitary sewage, mixed process, or combined sewage/stormwater wastes.
Wastes from oil/water separators at sites that load fuel are not included as street waste. Street waste
also does not include flood debris, land slide debris, and chip seal gravel.
Street waste does not ordinarily classify as dangerous waste. The owner of the stormwater
facility and/or collector of street waste is considered the waste generator and is responsible for
determining whether or not the waste designates as dangerous waste. Sampling to date has shown
that material from routine maintenance of streets and stormwater facilities does not classify as
dangerous waste (See Table IV-D-7). However, it is possible that street waste from spill sites could
classify as dangerous waste. Street waste from areas with exceptionally high average daily traffic
counts may contain contaminants - such as heavy metals, total petroleum hydrocarbons (TPH), and
carcinogenic polycyclic aromatic hydrocarbons (c-PAH) - at levels that limit reuse options.
Street Waste Solids
Street waste is solid waste. While street waste from normal street and highway maintenance is not
dangerous waste, it is solid waste, as defined under The Solid Waste Management Act (Chapter
70.95 RCW) and under Solid Waste Handling Standards (Chapter 173-350 WAC). Under the Solid
Waste Management Act, local health departments have primary jurisdiction over solid waste
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of Street Wastes Appendix D 589
management. Street waste solids may contain contaminants at levels too high to allow unrestricted
reuse. There are currently no specific references in the Solid Waste Handling Standards to facilities
managing street waste solids. These facilities will typically fit under the section dealing with Piles
Used for Storage and Treatment (Section 320 of the regulation). There are no specific references for
reuse and disposal options for street wastes in the Solid Waste Handling Standards, although the
Solid Waste Handling Standards do not apply to clean soils. In the rule, clean soils are defined as
‘soils that do not contain contaminants at concentrations which could degrade the quality of air,
waters of the state, soils, or sediments; or pose a thereat to the health of humans or other living
organisms’ (WAC 173-350-100). Whether or not a soil is a “clean soil” depends primarily upon the
level of contaminants and, to a lesser degree, on the background level of contaminants at a particular
location and the exposure potential to humans or other living organisms. Therefore, both the soil and
potential land application sites must be evaluated to determine if a soil is a clean soil. Local health
departments should be contacted to determine if a street waste meets the definition of “clean soil”
when it will be reused as a soil.
There is no simple regulatory mechanism available to classify street waste solids as "clean" for
uncontrolled reuse or disposal. Local health districts have historically used the Model Toxics Control
Act Cleanup Regulation (MTCA) Method A residential soil cleanup levels to approximate "clean" and
to make decisions on land application proposals. These regulations were amended in February 2001.
The MTCA regulation is not intended to be directly applied to setting contaminant concentration
levels for land application proposals. However, they may provide human health and environmental
threat information and a useful framework for such decisions, when used in conjunction with other
health and environmental considerations. The local health department should be contacted to
determine local requirements for making this determination.
Using the old MTCA regulations, many local health departments have set a criteria of 200 mg/kg
Total Petroleum Hydrocarbons (TPH) for diesel and heavy fuel oils as a threshold level for clean soil.
Using the new MTCA terrestrial ecological evaluation procedures, allowable TPH levels for land
application could range from 200 – 460 mg/kg, depending on site characteristics and intended land
use. Street waste sampling has historically yielded TPH values greater than 200 mg/kg for
hydrocarbons in the diesel and heavy oil range. These values typically reflect interference from
natural organic material and, to a lesser extent, relatively immobile petroleum hydrocarbons. The
mobile hydrocarbons that are of concern for groundwater protection are generally not retained with
street waste solids. Ecology's Manchester Lab has developed an analytical method to reduce the
problem of natural organic material being included in the TPH analysis for diesel and heavier range
hydrocarbons. This new method, called NWTPH-Dx, reduces the background interference
associated with vegetative matter by as much as 85% to 95%. However, even with the new
methodology, TPH test results for street waste may still be biased by the presence of natural
vegetative material and may still exceed 200 mg/kg. Where the laboratory results report no
‘fingerprint’ or chromatographic match to known petroleum hydrocarbons, the soils should not be
considered to be petroleum contaminated soils.
Street waste solids frequently contain levels of carcinogenic PAHs (c-PAH) that make unrestricted
use inappropriate. This is further complicated by analytical interference caused by organic matter that
raises practical quantitation or reporting limits. To reduce the level of interference the use of US EPA
Test Method 8270, incorporating the silica gel cleanup step, is recommended. The calculated c-PAH
value can vary greatly depending upon how non-detect values are handled. The new MTCA Method
A criterion for c-PAH is 0.1 mg/kg (the sum of all seven c-PAH parameters multiplied by the
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appropriate toxicity equivalency factor) for unrestricted land uses. The MTCA criteria for soil cleanup
levels for industrial properties is 2.0 mg/kg. Following this guidance, most sites where street wastes
could be reused as soil will be commercial or industrial sites, or sites where public exposure will be
limited or prevented.
Street waste treatment and storage facilities shall be permitted by the local health department
as applicable. Under the Solid Waste Management Act, local health departments have primary
jurisdiction over solid waste management
Street waste handling facilities are subject to the requirements of the Solid Waste Handling
Standards. The specific requirements will depend upon the manner in which the waste is managed.
Most facilities will probably be permitted under the section dealing with Piles Used for Storage and
Treatment (Section 320 of the regulation)
For most facilities, permit requirements include a plan of operations, sampling, record keeping and
reporting, inspections, and compliance with other state and local requirements. The plan of operation
should include a procedure for characterization of the waste and appropriate reuse and disposal
options, consistent with the recommendations in this document and applicable federal, state, and
local requirements.
A street waste site evaluation (see sample at end of this appendix) is suggested for all street
waste as a method to identify spill sites or locations that are more polluted than normal. The
disposal and reuse options listed below are based on characteristics of routine street waste and are
not appropriate for more polluted wastes. The collector of street waste should evaluate it both for its
potential to be classified as dangerous waste and to not meet end users requirements.
Street waste that is suspected to be dangerous waste should not be collected with other
street waste. Material in catch basins with obvious contamination (unusual color, staining, corrosion,
unusual odors, fumes, and oily sheen) should be left in place or segregated until tested. Testing
should be based on probable contaminants. Street waste that is suspected to be dangerous waste
should be collected and handled by someone experienced in handling dangerous waste. If potential
dangerous waste must be collected because of emergency conditions, or if the waste becomes
suspect after it is collected, it should be handled and stored separately until a determination as to
proper disposal is made. Street waste treatment and storage facilities should have separate "hot
load" storage areas for such waste. Dangerous Waste includes street waste known and suspected
to be dangerous waste. This waste must be handled following the Dangerous Waste Regulations
(Chapter 173-303 WAC) unless testing determines it is not dangerous waste.
Spills should be handled by trained specialists. Public works maintenance crews and private
operators conducting street sweeping or cleaning catch basins should have written policies and
procedures for dealing with spills or suspected spill materials. Emergency Spill Response telephone
numbers should be immediately available as part of these operating policies and procedures.
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The end recipient of street waste must be informed of its source and may have additional
requirements for its use or testing that are not listed here. This document is based primarily on
average street waste's chemical constituents and their potential effect on human health and the
environment. There are physical constituents (for example, broken glass or hypodermic needles) or
characteristics (for example, fine grain size) that could also limit reuse options. Additional treatment
such as drying, sorting, or screening may also be required, depending on the needs and
requirements of the end user.
Street waste treatment and storage facilities owned or operated by governmental agencies
should be made available to private waste collectors and other governmental agencies on a
cost recovery basis. Proper street waste collection and disposal reduces the amount of waste
released to the environment. The operators of street waste facilities should restrict the use of their
facilities to certified and/or licensed waste collectors who meet their training and liability requirements.
The use of street waste solids under this guidance should not lead to designation as a
hazardous waste site, requiring cleanup under MTCA. Exceeding MTCA Method A unrestricted
land use cleanup levels in street waste and products made from street waste, does not automatically
make the site where street waste is reused a cleanup site. A site is reportable only if "-a release
poses a threat to human health or the environment-" (Model Toxic Control Act). The reuse options
proposed below are designed to meet the condition of not posing a threat to human health or the
environment.
Testing of street waste solids will generally be required as part of a plan of operation that
includes procedures for characterization of the waste. Testing frequency, numbers of samples,
parameters to be analyzed, and contaminant limit criteria should all be provided as part of an
approved plan of operation. Table IV-D-5 and Table IV-D-6 provide some recommended parameters
and sampling frequencies for piles of street waste solids from routine street maintenance. These are
provided as guidance only and are intended to assist the utility and the local health department in
determining appropriate requirements. Sampling requirements may be modified, over time, based on
accumulated data. When the material is from a street waste facility or an area that has never been
characterized by testing, the test should be conducted on a representative sample before co-mingling
with other material. Testing in these instances would be to demonstrate that the waste does not
designate as dangerous waste and to characterize the waste for reuse. At a minimum, the
parameters in Table IV-D-5 are recommended for these cases. Note that it will generally not be
necessary to conduct Toxic Characteristic Leaching Procedure (TCLP) analyses when the observed
values do not exceed the recommended values in Table IV-D-5. Table IV-D-7 illustrates some
observed relationships between total metals and TCLP metals values.
For further information on testing methods and sampling plans, refer to:
• SW 846 (US EPA, Office of Solid Waste, Test Methods for Evaluating Solid
Wastes, 3rd Edition) and
• Standard Methods for the Examination of Water and Wastewater (American
Public Health Association, et al., 18th Edition, 1992)
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For street waste not exceeding the suggested maximum values in Table IV-D-5 the following
street waste solids reuse and disposal options are recommended:
• Street sweepings that consist primarily of leaves, pine needles and branches,
and grass cuttings from mowing grassy swales can be composted. Litter and
other foreign material must be removed prior to composting or the composting
facility must provide for such removal as part of the process. The screened trash
is solid waste and must be disposed of at an appropriate solid waste handling
facility.
• Coarse sand screened from street sweeping after recent road sanding may be
reused for street sanding, provided there is no obvious contamination from spills.
The screened trash is solid waste and must be disposed of at an appropriate
solid waste handling facility.
• Roadside ditch cleanings, not contaminated by a spill or other release and not
associated with a stormwater treatment system such as a bioswale, may be
screened to remove litter and separated into soil and vegetative matter (leaves,
grass, needles, branches, etc.). The soils from these activities are not generally
regulated as solid waste. Ditching material that may be contaminated must be
stored, tested, and handled in the same manner as other street waste solids. It is
the generator’s responsibility to visually inspect and otherwise determine whether
the materials may be contaminated.
• Construction street waste - solids collected from sweeping or in stormwater
treatment systems at active construction sites - may be placed back onto the site
that generated it, or managed by one of the methods listed below, provided that it
has not been contaminated as a result of a spill. For concrete handling at
construction site, refer to BMP C151 in Volume II.
• Screened street waste soils may be used as feedstock materials for topsoil
operations. This option should be reserved for street waste soils with very low
levels of contaminants. Diluting street waste soils with clean soils or composted
material must not be used as a substitute for treatment or disposal. There may
be physical contaminants (for example, glass, metal, nails, etc.) in street waste
that cannot be entirely screened from the waste. Where present, these
contaminants in street waste could preclude its use as feedstock material for
topsoil operations.
• Fill in parks, play fields, golf courses, and other recreational settings where direct
exposure by the public is limited or prevented. One way to prevent or limit direct
exposure is to cover the fill with sod, grass, or other capping material to reduce
the risk of soil being ingested. The level of contaminants in the street waste must
be evaluated to ensure that the soils meet the definition of clean soils when used
in this manner.
• Fill in commercial and industrial areas, including soil or top dressing for use at
industrial sites, roadway medians, airport infields, and similar sites where there is
limited direct human contact with the soil and the soils will be stabilized with
vegetation or other means. The level of contaminants in the street waste must be
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of Street Wastes Appendix D 593
evaluated to ensure that the soils meet the definition of clean soils when used in
this manner.
• Top dressing on roadway slopes, road or parking lot construction material, road
or parking lot subgrade, or other road fill. The level of contaminants in the street
waste must be evaluated to ensure that the soils meet the definition of clean soils
when used in this manner.
• Daily cover or fill in a permitted municipal solid waste landfill, provided the street
waste solids have been dewatered. Street waste solids may be acceptable as
final cover during a landfill closure. The local health department and landfill
operator should be consulted to determine conditions of acceptance.
• Treatment at a permitted contaminated soil treatment facility.
• Recycling through incorporation into a manufactured product, such as Portland
cement, prefab concrete, or asphalt. The facility operator should be consulted to
determine conditions of acceptance.
• Other end uses as approved by the local health department.
• Disposal at an appropriate solid waste handling facility.
For street waste that exceeds the suggested maximum values in Table IV-D-5, the following
street waste solids reuse and disposal options are recommended:
• Treatment at a permitted contaminated soil treatment facility.
• Recycling through incorporation into a manufactured product, such as Portland
cement, prefab concrete, or asphalt. The facility operator should be consulted to
determine conditions of acceptance.
• Other end uses as approved by the local health department.
• Disposal at an appropriate solid waste handling facility.
Street Waste Liquids
Street waste collection should emphasize solids in preference to liquids. Street waste solids
are the principal objective in street waste collection and are substantially easier to store and treat
than liquids.
Street waste liquids require treatment and/or must follow location limitations before their
discharge. Street waste liquids usually contain high amounts of suspended and total solids and
adsorbed metals. Treatment requirements depend on the discharge location.
Discharges to sanitary sewer and storm sewer systems must be approved by the entity
responsible for operation and maintenance of the system. Ecology will not generally require
waste discharge permits for discharge of stormwater decant to sanitary sewers or to stormwater
treatment BMPs constructed and maintained in accordance with Ecology’s Stormwater Management
Manual for Western Washington. (See Volume V for further detail).
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The following disposal options are recommended, in order of preference, for catch basin
decant liquid and for water removed from stormwater treatment facilities.
Under the Municipal General Permit, municipalities are required to use this guidance in determining
appropriate means of dealing with street wastes from stormwater maintenance activities. Ecology
Northwest Regional Office water quality staff can help you with treatment standards and permit
requirements for your particular situation.
Discharge of catch basin decant liquids to a municipal sanitary sewer connected to a Public
Owned Treatment Works (POTW) is the preferred disposal option. Discharge to a municipal
sanitary sewer requires the approval of the sewer authority. Street waste liquids discharged to a
POTW may be treated at a combined street waste liquid and solid facility (decant facility) or at
separate liquids only facilities. These liquid only facilities may consist of modified type 2 catch basins
(with a flow restrictor or oil/water separator) or water quality vaults, strategically located through the
sanitary collection system. These should provide 24 hour detention for the expected volumes and
should be constructed and operated to ensure that the decant discharge does not re-suspend
sediments. Sewer authorities should require periodic sampling and decant facility operators should
test their waste effluent on a regular basis, but street waste decant liquid should meet the most
restrictive local limits with 24 hours of undisturbed gravity settling. Overnight settling is more practical
and will likely meet most local pretreatment requirements. (See Table IV-D-10 for typical catch basin
decant values from King County’s decant facility at Renton).
Discharge of stormwater runoff into sanitary sewers requires a Metro-King County Discharge permit
to avoid hydraulic overloads and treatment performance problems.
Stormwater removed from catch basins and stormwater treatment wetvaults may be
discharged into a Basic or Enhanced Stormwater Treatment BMP.
Decant liquid collected from cleaning catch basins and stormwater treatment wetvaults may be
discharged back into the storm sewer system under the following conditions:
• The preferred disposal option of discharge to sanitary sewer is not reasonably
available, and
• The discharge is to a Basic or Enhanced Stormwater Treatment Facility (See
Volume V), and
• The storm sewer system owner/operator has granted approval and has
determined that the treatment facility will accommodate the increased loading.
Pretreatment may be required to protect the treatment BMP.
Reasonable availability will be determined by the stormwater utility and by the circumstances
including such factors as distance, time of travel, load restrictions, and capacity of the stormwater
treatment facility. Auburn may choose not to allow discharge back to the storm sewer system.
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of Street Wastes Appendix D 595
Discharging back into the storm sewer is an acceptable option, under certain conditions:
• Other practical means are not reasonably available, and
• Pretreatment is provided by discharging to a modified type 2 catch basin (with a
flow restrictor or oil/water separator) or water quality vault, and
• The discharge is upstream of a basic or enhanced stormwater treatment BMP,
and
• The storm sewer system owner/operator has granted approval.
Other practical means include the use of decanting facilities and field decant sites that discharge to
sanitary sewers or discharge to an approved stormwater treatment BMP.
Limited field testing of flocculent aids has been conducted. While the use of flocculent aids is
promising, sufficient testing has not been conducted to allow approval of any specific product or
process. In general, the following conditions must be met for flocculent use to be approved:
• The flocculent must be non-toxic under circumstances of use and approved for
use by the Department of Ecology.
• The decant must be discharged to an approved basic or enhanced stormwater
treatment BMP, with sufficient capacity and appropriate design to handle the
anticipated volume and pollutant loading.
• The discharge must be approved by the storm sewer system owner/operator.
Water removed from stormwater ponds, vaults, and oversized catch basins may be returned
to storm sewer system. Stormwater ponds, vaults, and oversized catch basins contain substantial
amounts of liquid, which hampers the collection of solids and poses problems if the removed waste
must be hauled away from the site. Water removed from these facilities may be discharged back into
the pond, vault, or catch basin provided:
• Clear water removed from a stormwater treatment structure may be discharged
directly to a downgradient cell of a treatment pond or into the storm sewer
system.
• Turbid water may be discharged back into the structure it was removed from if:
o The removed water has been stored in a clean container (eductor truck,
Baker tank, or other appropriate container used specifically for handling
stormwater or clean water) and
o There will be no discharge from the treatment structure for at least
24 hours.
• The discharge must be approved by the storm sewer system owner/operator.
Vegetation management and structural integrity concerns sometimes require that the ponds be
refilled as soon after solids removal as possible. For ponds and other systems relying on biological
processes for waste treatment, it is often preferable to reuse at least some portion of the removed
water.
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of Street Wastes Appendix D 596
Site Evaluation
A site evaluation is suggested as method to identify spill sites or locations that are more polluted than
normal.
The site evaluation will aid in determining if waste should be handled as dangerous waste and in
determining what to test for if dangerous waste is suspected. The site evaluation will also help to
determine if the waste does not meet the requirements of the end users.
There are three steps to a site evaluation:
1. A historical review of the site for spills, previous contamination, and nearby toxic cleanup
sites and dangerous waste and materials.
The historical review will be easier if done on an area wide basis prior to scheduling any waste
collection. The historical review should be more thorough for operators who never collected
waste at a site before. At a minimum, the historical review should include operator knowledge of
the area's collection history or records kept from previous waste collections.
Private operators should ask the owner of the site for records of previous contamination and the
timing of the most recent cleaning. Ecology’s Hazardous Substance Information Office maintains
a Toxic Release Inventory and a “Facility Site” web page, tracking more than 15,000 sites. This
information is available through the Internet at http://www.wa.gov/ecology/iss/fsweb/fshome.html
or by calling a toll-free telephone number (800-633-7585). The web page allows anyone with
web-access to search for facility information by address, facility name, town, zip code, and SIC
code, etc. It lists why the Department of Ecology is tracking each one (NPDES, TSCA, RCRA,
Clean Air Act, etc.), as well as who to call within Ecology to find out more about the given facility.
2. An area visual inspection for potential contaminant sources such as a past fire, leaking
tanks and electrical transformers, and surface stains.
The area around the site should be evaluated for contaminant sources prior to collection of the
waste. The area visual inspection may be done either as part of multiple or as single site
inspections. If a potential contaminant source is found, the waste collection should be delayed
until the potential contaminant is assessed.
A second portion of the area visual inspection is a subjective good housekeeping evaluation of
the area. Locations with poor housekeeping commonly cut corners in less obvious places and
should be inspected in greater detail for illegal dumping and other contamination spreading
practices.
3. A waste and container inspection before and during collection.
The inspection of the waste and catch basin or vault is the last and perhaps most critical step in
the site evaluation.
For example, if the stormwater facility has an unusual color in or around it, then there is a strong
possibility that something could have been dumped into it. Some colors to be particularly wary of
are yellow-green from antifreeze dumping and black and/or rainbow sheen from oil and/or grease
dumping. In addition, if any staining or corrosion is observed, then a solvent may have been
dumped.
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of Street Wastes Appendix D 597
Fumes are also good indicators of potential dangerous conditions or dangerous waste.
Deliberate smelling of catch basins should be avoided for worker safety, but suspicious odors
may be encountered from catch basins thought to be safe. Some suspicious odors are rotten
eggs (hydrogen sulfide is present), gasoline or diesel fumes, or solvent odors. If unusual odors
are noted, contact a dangerous waste inspector before cleaning the basin.
Finally, operator experience is the best guide to avoid collection of contaminated waste.
Table IV-D-2. Typical TPH Levels in Street Sweeping and Catch Basin Solids
Reference Street Sweeping
(mg/kg)
Catch Basin Solid
(mg/kg)
Snohomish County 1
(Landau 1995)
390 – 4300
King County (1)
(Herrera 1995)
123 – 11049
(Median 1036)
Snohomish County & Selected Cities 1
(W & H Pacific, 1993)
163 - 1500
(Median 760)
163 – 1562
(Median 760)
City of Portland 2
(Bresch)
MDL – 1830
(Median – 208)
Oregon1
(Collins; ODOT 1998)
Oregon 3
(Collins; ODOT 1998)
1600 – 2380
98 - 125
1 Method WTPH 418.1; does not incorporate new methods to reduce background interference due to vegetative material
2 Method NWTPH-Dx
3 Method WTPH – HCID
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of Street Wastes Appendix D 598
Table IV-D-3. Typical c-PAH Values in Street Waste Solids and Related Materials
Sample Source City of Everett WSDOT
Analyte Street
Sweepings
Soil 3-Way
Topsoil
Vactor
Solids
Leaf &
Sand
Sweepings
– Fresh
Sweepings
Weathered
Benzo(a)anthracene 0.1U 0.076U 0.074U 0.21 0.45 0.56 0.40
Chrysene 0.14 0.09 0.074U 0.32 0.53 0.35 0.35
Benzo(b)fluoranthene 0.11 0.076U 0.074U 0.27 0.52 0.43 0.51
Benzo(k)fluoranthene 0.13 0.076U 0.074U 0.25 0.38 0.39 0.40
Benzo(a)pyrene 0.13 0.076U 0.074U 0.26 0.5 0.41 0.33U
Indeno(1,2,3-cd)pyrene 0.1U 0.076U 0.074U 0.19 0.39 NR NR
Dibenzo(a,h)anthracene 0.1U 0.076U 0.074U 0.081 0.12 0.39 0.33U
Revised MTCA
Benzo(a)pyrene
[ND=PQL]
0.215 0.134 0.134 0.388 0.727 0.708 0.597
Benzo(a)pyrene
[ND=1/2 PQL]
0.185 0.069 0.067 0.388 0.727 0.708 0.366
Benzo(a)pyrene [See *
below]
0.185 0.069 0 0.388 0.727 0.708 0.366
Benzo(a)pyrene [ND=0] 0.155 0.001 0 0.388 0.727 0.708 0.135
*If the analyte was not detected for any PAH, then ND=0; If analyte was detected in at least 1 PAH, then ND=1/2PQL;
If the average concentration (using ND=1/2 PQL) is greater than the maximum detected value, then ND=Maximum
value.
The new Method A soil cleanup level for unrestricted land use is 0.1 mg/Kg for BAP. (WAC 173-340-900, Table 740-1)
The new Method A soil cleanup level for industrial properties is 2 mg/Kg for BAP. (WAC 173-340-900, Table 745-1)
Table IV-D-4. Typical Metals Concentrations in Catch Basin Sediments
PARAMETER Ecology 1993 Thurston 1993 King County
1995
King County 1995
METALS; TOTAL
(mg/kg)
(Min – Max) (Min – Max) (Min - Max) Mean
As <3 -- 24 .39 -- 5.4 4 – 56 0.250
Cd 0.5 -- 2.0 < 0.22 -- 4.9 0.2 – 5.0 0.5
Cr 19 -- 241 5.9 -- 71 13 - 100 25.8
Cu 18 -- 560 25 -- 110 12 - 730 29
Pb 24 -- 194 42 -- 640 4 – 850 80
Ni 33 -- 86 23 -- 51 14 – 41 23
Zn 90 -- 558 97 -- 580 50 – 2000 130
Hg .04 -- .16 .024 -- .193
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of Street Wastes Appendix D 599
Table IV-D-5. Recommended Parameters & Suggested Values for Determining
Reuse & Disposal Options
Parameter Suggested Maximum Value
Arsenic, Total 20.0 mg/kg (a)
Cadmium, Total 2.0 mg/kg (b)
Chromium, Total 42 mg/kg (c)
Lead, total 250 mg/kg (d)
Nickel 100 mg/kg (e)
Zinc 270 mg/kg (e)
Mercury (Inorganic) 2.0 mg/kg (f)
PAHs (Carcinogenic) 0.1 – 2.0 mg/kg (see Note at (g) below)
TPH (Heavy Fuel Oil) 200 - 460 mg/kg (see Note at (h) below)
TPH (Diesel) 200 – 460 mg/kg (see Note at (h) below)
TPH (Gasoline) 100 mg/kg (i)
Benzene 0.03 mg/kg (i)
Ethylbenzene 6 mg/kg (i)
Toluene 7 mg/kg (i)
Xylenes (Total) 9 mg/kg (i)
a Arsenic: from MTCA Method A - Table 740-1: Soil cleanup levels for unrestricted land uses
b Cadmium: from MTCA Method A – Table 740-1: Soil cleanup levels for unrestricted land uses.
c Chromium; from MTCA Method A - Table 740-1: Soil cleanup levels for unrestricted land uses
d Lead; from MTCA Method A – Table 740-1: Soil cleanup levels for unrestricted land uses
e Nickel and Zinc; from MTCA Table 749-2: Protection of Terrestrial Plants and Animals
f Mercury; from MTCA Method A – Table 740-1: Soil cleanup levels for unrestricted land uses
g PAH-Carcinogenic; from MTCA Method A – Table 740-1: Soil cleanup levels for unrestricted land uses and Table
745-1, industrial properties, based on cancer risk via direct contact with contaminated soil (ingestion of soil) in
residential land use situations and commercial/industrial land uses. Note: The local health department may permit
higher levels as part of a Plan of Operation, where they determine that the proposed end use poses little risk of
direct human contact or ingestion of soil.
h TPH: from MTCA Tables 749-2 & 749-3: Protection of Terrestrial Plants and Animals. Values up to 460 mg/kg may
be acceptable where the soils are capped or covered to reduce or prevent exposure to terrestrial plants and
animals. Where the laboratory results report no ‘fingerprint’ or chromatographic match to known petroleum
hydrocarbons, the soils will not be considered to be petroleum contaminated soils.
i BETX; from MTCA Method A - Table 740-1: Soil cleanup levels for unrestricted land uses.
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of Street Wastes Appendix D 600
Table IV-D-6. Recommended Sampling Frequency for Street Waste Solids
Cubic Yards of Solids Minimum Number of Samples
0 – 100 3
101 – 500 5
501 – 1000 7
1001 – 2000 10
>2000 10 + 1 for each additional 500 cubic yards
*Modified from Ecology’s Interim Compost Guidelines
Table IV-D-7. Pollutants in Catch Basin Solids – Comparison to Dangerous Waste Criteria
PARAMETER Range of Values in Catch
Basin Waste
Range of Values in
Catch Basin Waste Dangerous Waste Criteria
METALS Total Metals (mg/kg) TCLP Metals
(mg/kg) TCLP values (mg/l)
Arsenic <3 – 56 < .02 - 0 .5 5.0
Cadmium <.22 – 5 .0002 - .03 1.0
Chromium 5.9 - 241 .0025 - .1 5.0
Copper 12 - 730 .002 -- .88 none
Lead 4 - 850 .015 -- 3.8 5.0
Nickel 23 - 86 < .01 -- .36 none
Zinc 50 - 2000 .04 -- 6.7 none
Mercury .02 - .19 .0001 -- .0002 0.2
* Data from Thurston County (Thurston County 1993), King County (Herrera 1995) and Ecology (Serdar; Ecology
1993).
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Table IV-D-8. Typical Catch Basin Decant Values Compared to Surface Water Quality Criteria
PARAMETER State Surface Water Quality Criteria Range of Values
Reported
Range of Values
Reported
METALS
Freshwater Acute
(ug/l – dissolved
metals)
Freshwater Chronic
(ug/l – dissolved
metals)
Total Metals
(ug/l)
Dissolved Metals
(ug/l)
Arsenic 360 190 100 – 43000 60 - 100
Cadmium* 2.73 0.84 64 - 2400 2 - 5
Chromium (total) 13 -- 90000 3 - 6
Chromium (III)* 435 141
Chromium (VI) 0.5 10
Copper* 13.04 8.92 81 -- 200000 3 - 66
Lead* 47.3 1.85 255 -- 230000 1 - 50
Nickel* 1114 124 40 -- 330 20 - 80
Zinc* 90.1 82.3 401 -- 440000 1900 - 61000
Mercury 2.10 .012 0.5 -- 21.9
*Hardness dependent; hardness assumed to be 75 mg/l
Table IV-D-9. Typical Values for Conventional Pollutants in Catch Basin Decant
PARAMETER Ecology 1993 (Min - Max) King County 1995 (Min - Max)
Values as mg/l; except where
stated Mean Mean
PH 6.94 6.18 - 7.98 8 6.18 - 11.25
Conductivity (umhos/cm) 364 184 - 1110 480 129 - 10,100
Hardness (mg/l CaCO3) 234 73 - 762
Fecal Coliform (MPN/100 ml) 3000
BOD 151 28 - 1250
COD 900 120 - 26,900
Oil & Grease 11 7.0 - 40 471 15 - 6242
TOC 136 49 - 7880 3670 203 - 30,185
Total Solids 1930 586 - 70,400
Total Dissolved Solids 212 95 - 550
Total Suspended Solids 2960 265 - 111,000
Settleable Solids (ml/l/hr) 27 2 - 234 57 1 - 740
Turbidity (ntu) 1000 55 - 52,000 4673 43 - 78,000
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Table IV-D-10. Catch Basin Decant Values Following Settling*
Parameter; Total Metals in
mg/l
Portland – Inverness
Site
Min - Max
King County -
Renton
Min - Max
METRO
Pretreatment
Discharge Limits
Arsenic .0027 .015 < MDL – 0.12 4
Cadmium .0009 - .0150 < MDL – 0.11 0.6
Chromium .0046 - .0980 .017 – .189 5
Copper .015 - .8600 .0501 – .408 8
Lead .050 – 6.60 .152 – 2.83 4
Nickel .0052 - .10 .056 - .187 5
Silver .0003 - .010 < MDL 3
Zinc .130 – 1.90 .152 – 3.10 10
Settleable Solids; ml/L No Data .02 - 2 7
Nonpolar FOG 5.7 - 25 5 - 22 100
Ph (std) 6.1 – 7.2 6.74 – 8.26 5.0 - 12.0
TSS 2.8 - 1310
Recorded Total Monthly Flow;
Gallons
Data not available 31,850 - 111,050
Recorded Max. Daily Flow;
Gallons
Data not available 4,500 - 18,600 25,000 GPD
Calculated Average Daily
Flow; GPD
Data not available 1517 - 5428
* Data from King County’s Renton Facility (data from 1998 – 1999) and the City of Portland’s Inverness Site (data
from 1999 – 2001); detention times not provided
Volume V
i Table of Contents
Volume V – Water Quality
Treatment BMPs
Table of Contents
Purpose of this Volume...................................................................................................................603
Content and Organization of this Volume.......................................................................................603
Chapter 1 Treatment Facility Selection Process.............................................................604
1.1 Step-by-Step Selection Process for Treatment Facilities......................................................604
Chapter 2 Treatment Facility Menus.................................................................................610
2.1 Oil Control Menu....................................................................................................................610
2.1.1 Performance Goal............................................................................................................610
2.1.2 Options.............................................................................................................................610
2.1.3 Application on the Project Site.........................................................................................611
2.2 Phosphorus Treatment Menu................................................................................................611
2.2.1 Performance Goal............................................................................................................611
2.2.2 Options.............................................................................................................................611
2.3 Enhanced Treatment Menu...................................................................................................612
2.3.1 Performance Goal............................................................................................................612
2.3.2 Options.............................................................................................................................612
2.4 Basic Treatment Menu..........................................................................................................613
2.4.1 Performance Goal............................................................................................................613
2.4.2 Options.............................................................................................................................614
Chapter 3 General Requirements for Stormwater Facilities...........................................615
3.1 Design Flow Volume and Flow Rate.....................................................................................615
3.1.1 Water Quality Design Flow Volume.................................................................................615
3.1.2 Water Quality Design Flow Rate.....................................................................................616
3.1.3 Flows Requiring Treatment..............................................................................................616
3.2 Sequence of Facilities...........................................................................................................617
3.3 Setbacks, Slopes, and Embankments...................................................................................618
3.3.1 Setbacks..........................................................................................................................618
3.3.2 Side Slopes and Embankments......................................................................................619
3.4 Facility Liners.........................................................................................................................620
3.4.1 General Design Criteria...................................................................................................620
3.4.2 Design Criteria for Treatment Liners...............................................................................622
3.4.3 Design Criteria for Low Permeability Liner Options.........................................................622
Volume V
ii Table of Contents
3.4.3.1 Compacted Till Liners............................................................................................622
3.4.3.2 Clay Liners............................................................................................................623
3.4.3.3 Geomembrane Liners............................................................................................623
3.4.3.4 Concrete Liners.....................................................................................................624
3.5 Hydraulic Structures..............................................................................................................624
3.5.1 Flow Splitter Designs.......................................................................................................624
3.5.1.1 General Design Criteria.........................................................................................624
3.5.1.2 Materials................................................................................................................624
3.5.2 Flow Spreading Options..................................................................................................627
3.5.2.1 General Design Criteria.........................................................................................627
Chapter 4 Pretreatment.....................................................................................................632
4.1 Purpose.................................................................................................................................632
4.2 Application.............................................................................................................................632
4.3 BMPs for Pretreatment..........................................................................................................632
4.3.1 BMP T610 Presettling Basin............................................................................................632
4.3.1.1 Purpose and Definition..........................................................................................632
4.3.1.2 Application and Limitations...................................................................................632
4.3.1.3 Design Criteria.......................................................................................................632
4.3.1.4 Site Constraints and Setback Requirements........................................................633
Chapter 5 Infiltration and Bio-infiltration Treatment Facilities.......................................634
5.1 Purpose.................................................................................................................................634
5.2 Application.............................................................................................................................634
5.3 Site Suitability........................................................................................................................634
5.3.1 Setback Criteria (SSC-1).................................................................................................635
5.3.2 Groundwater Protection Areas (SSC-2)..........................................................................635
5.3.3 High Vehicle Traffic Areas (SSC-3).................................................................................635
5.3.4 Soil Infiltration Rate/Drawdown Time for Treatment (SSC-4)..........................................636
5.3.4.1 Infiltration Rates: Short-term and Long-term.........................................................636
5.3.4.2 Drawdown Time....................................................................................................636
5.3.5 Depth to Bedrock, Water Table, or Impermeable Layer (SSC-5)....................................636
5.3.6 Soil Physical and Chemical Suitability for Treatment (SSC-6)........................................637
5.3.7 Seepage Analysis and Control (SSC-7)..........................................................................637
5.3.8 Cold Climate and Impact of Roadway Deicers (SSC-8)..................................................637
5.3.9 Verification Testing of the Completed Facility.................................................................637
5.4 Site Characterization.............................................................................................................638
5.4.1 Field Methods used to Determine Subsurface Characterization.....................................638
5.4.1.1 Test Holes or Pits..................................................................................................638
5.4.1.2 Infiltration Rate Determination...............................................................................639
5.4.1.3 Infiltration Receptor...............................................................................................639
Volume V
iii Table of Contents
5.4.2 Design Infiltration Rate Determination.............................................................................640
5.4.2.1 Three Methods for Determining Long-term Infiltration Rate for Sizing the Infiltration
Basin, Trench, or Swale........................................................................................640
5.4.2.2 General Sizing Criteria..........................................................................................642
5.4.2.3 General Design Criteria.........................................................................................643
5.4.2.4 General Construction Criteria................................................................................643
5.4.2.5 Maintenance Criteria.............................................................................................644
5.4.2.6 Verification of Performance...................................................................................644
5.5 BMPs for Infiltration and Bio-infiltration Treatment................................................................644
5.5.1 BMP T710 Infiltration Basins...........................................................................................645
5.5.1.1 Description............................................................................................................645
5.5.1.2 Design Criteria Specific for Basins........................................................................645
5.5.1.3 Maintenance Criteria for Basins............................................................................645
5.5.2 BMP T720 Infiltration Trenches.......................................................................................646
5.5.2.1 Description............................................................................................................646
5.5.2.2 Design Criteria.......................................................................................................646
5.5.2.3 Construction Criteria..............................................................................................646
5.5.2.4 Maintenance Criteria.............................................................................................647
5.5.3 BMP T730 Bio-infiltration Swale......................................................................................648
5.5.3.1 Description............................................................................................................648
5.5.3.2 Additional Design Criteria Specific for Bio-infiltration Swales...............................648
Chapter 6 Sand Filtration Treatment Facilities................................................................650
6.1 Purpose.................................................................................................................................650
6.2 Description.............................................................................................................................650
6.3 Applications and Limitations..................................................................................................656
6.4 Site Suitability........................................................................................................................656
6.5 Design Criteria.......................................................................................................................656
6.5.1 Objective..........................................................................................................................656
6.5.2 Sand Filter Sizing.............................................................................................................656
6.6 Construction Criteria..............................................................................................................659
6.7 Maintenance Criteria.............................................................................................................659
6.7.1 BMP T810 Sand Filter Vault............................................................................................661
6.7.1.1 Description: (Figure V-6-13 and Figure V-6-14)....................................................661
6.7.1.2 Applications and Limitations..................................................................................661
6.7.1.3 Additional Design Criteria for Vaults.....................................................................661
6.7.2 BMP T820 Linear Sand Filter.........................................................................................664
6.7.2.1 Description............................................................................................................664
6.7.2.2 Application and Limitations...................................................................................664
6.7.2.3 Additional Design Criteria for Linear Sand Filters.................................................664
Chapter 7 Biofiltration Treatment Facilities.....................................................................666
7.1 Purpose.................................................................................................................................666
7.2 Applications...........................................................................................................................666
Volume V
iv Table of Contents
7.3 Site Suitability........................................................................................................................666
7.4 Best Management Practices..................................................................................................666
7.4.1 BMP T910 Basic Biofiltration Swale................................................................................667
7.4.1.1 Description:...........................................................................................................667
7.4.1.2 Design Criteria:......................................................................................................667
7.4.1.3 Bypass Guidance..................................................................................................671
7.4.1.4 Sizing Procedure for Biofiltration Swales..............................................................671
7.4.1.5 Soil Criteria............................................................................................................678
7.4.1.6 Vegetation Criteria.................................................................................................679
7.4.1.7 Construction Criteria..............................................................................................679
7.4.1.8 Maintenance Criteria.............................................................................................679
7.4.2 BMP T920 Wet Biofiltration Swale...................................................................................683
7.4.2.1 Description............................................................................................................683
7.4.2.2 Performance Objectives........................................................................................683
7.4.2.3 Applications/Limitations.........................................................................................683
7.4.2.4 Criteria...................................................................................................................683
7.4.3 BMP T930 Continuous Inflow Biofiltration Swale............................................................685
7.4.3.1 Description:...........................................................................................................685
7.4.3.2 Applications...........................................................................................................685
7.4.3.3 Design Criteria.......................................................................................................685
7.4.4 BMP T940 Basic Filter Strip.............................................................................................686
7.4.4.1 Description............................................................................................................686
7.4.4.2 Applications/Limitations.........................................................................................686
7.4.4.3 Design Criteria for Filter strips:..............................................................................688
7.4.4.4 Sizing Procedure...................................................................................................688
7.4.5 BMP T950 Narrow Area Filter Strip.................................................................................689
7.4.5.1 Description:...........................................................................................................689
7.4.5.2 Applications/Limitations:........................................................................................689
7.4.5.3 Design Criteria:......................................................................................................689
Chapter 8 Wetpool Facilities.............................................................................................691
8.1 Purpose.................................................................................................................................691
8.2 Best Management Practices..................................................................................................691
8.2.1 BMP T1010 Wetponds - Basic and Large.......................................................................692
8.2.1.1 Description:...........................................................................................................692
8.2.1.2 Design Criteria:......................................................................................................692
8.2.1.3 Sizing Procedure...................................................................................................695
8.2.1.4 Wetpool Geometry................................................................................................701
8.2.1.5 Berms, Baffles, and Slopes...................................................................................701
8.2.1.6 Embankments.......................................................................................................702
8.2.1.7 Inlet and Outlet......................................................................................................702
8.2.1.8 Access and Setbacks............................................................................................703
8.2.1.9 Planting Requirements..........................................................................................703
8.2.1.10 Recommended Design Features..........................................................................704
Volume V
v Table of Contents
8.2.1.11 Construction Criteria:.............................................................................................707
8.2.1.12 Operation and Maintenance:.................................................................................707
8.2.2 BMP T1020 Wetvaults.....................................................................................................708
8.2.2.1 Description:...........................................................................................................708
8.2.2.2 Applications and Limitations:.................................................................................708
8.2.2.3 Design Criteria:......................................................................................................708
8.2.2.4 Wetpool Geometry................................................................................................710
8.2.2.5 Vault Structure.......................................................................................................710
8.2.2.6 Inlet and Outlet......................................................................................................711
8.2.2.7 Access Requirements...........................................................................................711
8.2.2.8 Access Roads, Right of Way, and Setbacks.........................................................711
8.2.2.9 Recommended Design Features..........................................................................712
8.2.2.10 Construction Criteria..............................................................................................712
8.2.2.11 Operation and Maintenance..................................................................................712
8.2.2.12 Modifications for Combining with a Baffle Oil/Water Separator............................713
8.2.3 BMP T1030 Stormwater Treatment Wetlands.................................................................714
8.2.3.1 Description............................................................................................................714
8.2.3.2 Applications and Limitations..................................................................................714
8.2.3.3 Design Criteria.......................................................................................................714
8.2.3.4 Sizing Procedure...................................................................................................714
8.2.3.5 Wetland Geometry................................................................................................715
8.2.3.6 Lining Requirements.............................................................................................715
8.2.3.7 Inlet and Outlet......................................................................................................716
8.2.3.8 Access and Setbacks............................................................................................716
8.2.3.9 Planting Requirements..........................................................................................719
8.2.3.10 Construction Criteria..............................................................................................719
8.2.3.11 Operation and Maintenance..................................................................................719
8.2.4 BMP T1040 Combined Detention and Wetpool Facilities ...............................................720
8.2.4.1 Description:...........................................................................................................720
8.2.4.2 Applications and Limitations:.................................................................................720
8.2.4.3 Design Criteria:......................................................................................................720
8.2.4.4 Sizing.....................................................................................................................723
8.2.4.5 Detention and Wetpool Geometry.........................................................................723
8.2.4.6 Berms, Baffles and Slopes....................................................................................723
8.2.4.7 Inlet and Outlet......................................................................................................723
8.2.4.8 Access and Setbacks............................................................................................723
8.2.4.9 Planting Requirements..........................................................................................723
8.3 Combined Detention and Wetvault........................................................................................724
8.4 Combined Detention and Stormwater Wetland.....................................................................726
8.4.1 Sizing Criteria..................................................................................................................726
8.4.2 Design Criteria.................................................................................................................726
8.4.3 Inlet and Outlet Criteria....................................................................................................726
8.4.4 Planting Requirements....................................................................................................726
Volume V
vi Table of Contents
Chapter 9 Oil and Water Separators.................................................................................727
9.1 Purpose.................................................................................................................................727
9.2 Description.............................................................................................................................727
9.3 Applications/Limitations.........................................................................................................727
9.4 Site Suitability........................................................................................................................730
9.5 Design Criteria.......................................................................................................................731
9.5.1 General Considerations...................................................................................................731
9.5.2 Criteria for Separator Bays..............................................................................................731
9.5.3 Criteria for Baffles............................................................................................................732
9.6 Oil and Water Separator BMPs.............................................................................................732
9.6.1 BMP T1110 API (Baffle type) Separator Bay..................................................................733
9.6.1.1 Design Criteria.......................................................................................................733
9.6.1.2 Sizing Criteria........................................................................................................733
9.6.2 BMP T1111 Coalescing Plate (CP) Separator Bay.........................................................735
9.6.2.1 Design Criteria.......................................................................................................735
9.6.2.2 Operation and Maintenance..................................................................................735
Chapter 10 Emerging Technologies...................................................................................737
10.1 Background............................................................................................................................737
10.2 Emerging Technology and the City of Auburn.......................................................................737
10.3 Ecology Role in Evaluating Emerging Technologies.............................................................737
10.4 Evaluation of Emerging Technologies...................................................................................737
10.5 Assessing Levels of Development of Emerging Technologies.............................................738
10.6 Examples of Emerging Technologies for Stormwater Treatment and Control......................739
Appendix A Basic Treatment Receiving Waters.................................................................741
Appendix B Procedure for Conducting a Pilot Infiltration Test.........................................743
Appendix C Geotextile Specifications.................................................................................745
Appendix D Turbulence and Short-Circuiting Factor.........................................................747
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Purpose Volume V
Content and Organization Introduction 603
Volume V:
Water Quality Treatment BMPs
Purpose of this Volume
This volume focuses on treatment of runoff to remove pollutants at developed sites. Typical pollutants
of concern include sand, silt, and other suspended solids; metals such as copper, lead, and zinc;
nutrients (e.g., nitrogen and phosphorous); certain bacteria and viruses; and organics such as
petroleum hydrocarbons and pesticides. Methods of pollutant removal include sedimentation/settling,
filtration, infiltration, plant uptake, ion exchange, adsorption, and bacterial decomposition. Floatable
pollutants such as oil, debris, and scum can be removed with separator structures. The purpose of
this volume is to provide criteria for selection and design of permanent runoff treatment facilities.
Use this volume to select permanent water quality treatment BMPs. Include the BMPs and the design
criteria used in your Stormwater Site Plan (see Volume I). This volume should be used as an aid in
designing and constructing water quality treatment BMPs.
Content and Organization of this Volume
Volume V contains ten chapters and four appendices:
• Chapter 1 outlines a step-by-step process for selecting treatment facilities for new
development and redevelopment projects.
• Chapter 2 presents treatment facility “menus” that are used in applying the step-by-
step process presented in Chapter 1. These menus cover different treatment needs
that are associated with different sites.
• Chapter 3 discusses general requirements for treatment facilities.
• Chapter 4 describes using a pretreatment facility to remove suspended solids prior to
runoff treatment facilities.
• Chapter 5 through Chapter 9 provide detailed information regarding specific types of
treatment facilities identified in the menus in Chapter 2.
• Chapter 10 discusses special considerations for emerging technologies for
stormwater treatment.
• The appendices provide more detailed information on selected topics referenced in
the preceding chapters.
Volume
V
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Chapter 1 Treatment Facility Selection Process
This chapter describes a step-by-step process for selecting the type of treatment facilities that will
apply to individual projects.
1.1 Step-by-Step Selection Process for Treatment Facilities
Use the step-by-step process outlined below to determine the type of treatment facilities applicable to
the project.
Step 1. Determine the receiving waters and pollutants of concern
Step 2. Determine if oil treatment is required
Step 3. Determine if infiltration for pollutant removal is possible
Step 4. Determine if control of phosphorus is required
Step 5. Determine if enhanced treatment is required
Step 6. Determine if basic treatment is required
Step 7. Consider other factors that may influence the selection of a treatment facility
Step 8. Select an appropriate treatment facility or treatment train for each type of
treatment required
Step 1: Determine the Receiving Waters and Pollutants of Concern Based on Offsite Analysis
The project proponent must determine the natural receiving water for the stormwater drainage from
the project site (groundwater, wetland, lake, stream, or salt water). This is necessary to determine the
applicable treatment menu from which to select treatment facilities. If the discharge is to the City of
Auburn’s municipal storm drainage system, the applicant must determine the final discharge point.
Watershed specific requirements and/or specific protection areas are outlined in Chapter 2 of
Volume V.
Step 2: Determine if an Oil Control Facility/Device is Required
Oil control is required for projects that have high-use sites. High-use sites are those that typically
generate high concentrations of oil due to high traffic turnover or the frequent transfer of oil. High-use
sites include:
• An area of a commercial or industrial site subject to an expected average daily traffic
(ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross
building area;
• An area of a commercial or industrial site subject to petroleum storage and transfer
in excess of 1,500 gallons per year, not including routinely delivered heating oil;
• An area of a commercial or industrial site subject to parking, storage or maintenance
of 25 or more motorized vehicles that are over 10 tons gross weight (trucks, buses,
trains, heavy equipment, etc.);
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• 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.
The traffic count can be estimated using information from “Trip Generation” published by the Institute
of Transportation Engineers, or from a traffic study prepared by a professional engineer or
transportation specialist with experience in traffic estimation.
The following urban land uses are likely to have areas that fall within the definition of “high-use sites”
or have sufficient quantities of free oil present that can be treated by an API or CP-type oil/water
separator:
• Industrial Machinery and Equipment, and Railroad Equipment Maintenance
• Log Storage and Sorting Yards
• Aircraft Maintenance Areas
• Railroad Yards
• Fueling Stations
• Vehicle Maintenance and Repair
• Construction Businesses (paving, heavy equipment storage and maintenance,
storage of petroleum products)
• Any other sites that generate high concentrations of oil.
In general, all-day parking areas are not intended to be defined as high-use sites, and should not
require oil control treatment outlined in Section 2.1. Gasoline stations, with or without small food
stores, will likely exceed the high-use site threshold. The petroleum storage and transfer criterion is
intended to address regular transfer operations such as gasoline service stations, not occasional
filling of heating oil tanks.
Note: Some land use types require the use of a spill control (SC-type) oil/water separator. Those
situations are described in IV and are separate from this oil treatment requirement.
Step 3: Determine if Infiltration for Pollutant Removal is Possible
Use Volume V, Chapter 5 as a guide to determine if the site contains soils that are conducive to
infiltration. For sites located in Groundwater Protection Area 2 (see Volume I, Chapter 2) this
option is not allowed.
Infiltration treatment facilities must be preceded by a pretreatment facility, such as a presettling basin
or vault, to reduce the occurrence of plugging. Any of the basic treatment facilities, and detention
ponds designed to meet flow control requirements, can also be used for pretreatment. If an oil/water
separator is necessary for oil control, it can also function as the pre-settling basin as long as the
influent suspended solids concentrations are not high. However, frequent inspections are necessary
to determine when accumulated solids exceed the 6-inch depth at which clean-out is recommended.
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Step 4: Determine if Phosphorus Treatment is Required
Step 5: Determine if Enhanced Treatment is Required
Enhanced treatment is required for the following project sites that discharge to fish-bearing streams,
lakes, or to waters or conveyance systems tributary to fish-bearing streams or lakes:
• Industrial sites,
• Commercial sites,
• Multi-family sites, and
• High AADT roads as follows:
o Fully controlled and partially controlled limited access highways with Annual
Average Daily Traffic (AADT) counts of 15,000 or more
o All other roads with an AADT of 7,500 or greater
Sites listed above that discharge directly (or, indirectly through a municipal storm sewer system) to
Basic Treatment Receiving Waters, and portions of the above-listed project sites that are only subject
to Basic Treatment requirements (see Step 6) are not required to provide enhanced treatment. For
developments with a mix of land use types, the Enhanced Treatment requirement shall apply when
the runoff from the areas subject to the Enhanced Treatment requirement comprises 50% or more of
the total runoff within a threshold discharge area. Basic Treatment Receiving Waters within the City
of Auburn include the Green River and the White River. Basic Treatment Receiving Waters identified
by Ecology are listed in Appendix A.
If the project must apply Enhanced Treatment, select and apply an appropriate Enhanced
Treatment facility. Please refer to the Enhanced Treatment Menu in Section 2.3. Select an option
from the menu after reviewing the applicability and limitations, site suitability, and design criteria of
each for compatibility with the site. You may also use Table V-1-1 for an initial screening of options.
Step 6: Select a Basic Treatment Facility
Basic treatment is required for all sites that meet the treatment thresholds of minimum
requirement #6, see Volume I, Chapter 3.
Typical sites that require basic treatment include:
• Project sites that discharge to the ground, UNLESS:
o The soil suitability criteria for infiltration treatment are met (see Volume III,
Chapter 3), or
o The project uses infiltration strictly for flow control – not treatment - and the
discharge is within ¼ mile of a phosphorus sensitive lake (use the
Phosphorus Treatment Menu) or within ¼ mile of a fish-bearing stream or
lake (use the Enhanced Treatment Menu).
• Residential projects not otherwise needing phosphorus control as determined in
Step 4.
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• Project sites discharging directly to salt waters, river segments and lakes listed in
Volume V, Appendix A.
• Project sites that drain to streams that are not fish-bearing, or to waters not tributary
to fish-bearing streams.
• Landscaped areas of industrial, commercial, and multi-family project sites, and
parking lots of industrial and commercial project sites, dedicated solely to parking of
employees’ private vehicles that do not involve any other pollution-generating
sources (e.g., industrial activities, customer parking, and storage of erodible or
leachable material, wastes, or chemicals). For developments with a mix of land use
types, the Basic Treatment requirement shall apply when the runoff from the areas
subject to the Basic Treatment requirement comprises 50% or more of the total
runoff within a threshold discharge area.
Please refer to the Basic Treatment Menu in Section 2.4. Select an option from the menu
after reviewing the applicability and limitations, site suitability, and design criteria of each for
compatibility with the site.
Step 7: Consider Other Factors that May Influence the Selection of a Treatment Device
Base the selection of a treatment facility on site physical factors and pollutants of concern. Try to
choose a facility that is more likely to do a better job removing the types of pollutants generated on
the site regardless of treatment requirements. The types of site physical factors that influence facility
selection are summarized below.
1. Pollutants of Concern
Consider the land uses and potential pollutants associated with that land use.
2. Soil Type (Table V-1-1)
The permeability of the soil underlying an infiltration treatment facility can influence effectiveness.
This is particularly true for infiltration treatment facilities that are best sited in sandy to loamy sand
soils. They are not generally appropriate for sites that have final infiltration rates (f) of less than
0.5 inches per hour. Wet pond facilities situated on coarser soils will need a synthetic liner or the soils
will need to be amended to reduce the infiltration rate and provide treatment. Maintaining a
permanent pool in the first cell is necessary to avoid resuspension of settled solids. Biofiltration
swales in coarse soils can also be amended to reduce the infiltration rate.
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3. High Sediment Input
High total suspended solids (TSS) loads can clog infiltration soil, sand filters, and coalescing plate
oil/water separators. Pretreatment with a presettling basin, wet vault, or another basic treatment
facility would typically be necessary.
Other Physical Factors
• Slope: Steep slopes restrict the use of several BMPs. For example, biofiltration
swales are usually situated on sites with slopes of less than 6%, although greater
slopes can be considered. Infiltration BMPs are not suitable when the slope exceeds
20%.
• High Water Table: Unless there is sufficient horizontal hydraulic receptor capacity
the water table acts as an effective barrier to exfiltration and can sharply reduce the
efficiency of an infiltration system. If the high water table extends to within five
(5) feet of the bottom of an infiltration BMP, the site is seldom suitable.
• Depth to Bedrock/ Hardpan/Till: The downward exfiltration of stormwater is also
impeded if a bedrock or till layer lies too close to the surface. If the impervious layer
lies within five feet below the bottom of the infiltration BMP the site is not suitable.
Similarly, pond BMPs are often not feasible if bedrock lies within the area that must
be excavated.
• Proximity to Foundations and Wells: Since infiltration BMPs convey runoff back
into the soil, some sites may experience problems with local seepage. This can be a
real problem if the BMP is located too close to a building foundation. Another risk is
groundwater pollution; hence the requirement to site infiltration systems more than
100 feet away from drinking water wells.
• Maximum Depth: Wet ponds are also subject to a maximum depth limit for the
"permanent pool" volume. Deep ponds (greater than 8 feet) may stratify during
summer and create low oxygen conditions near the bottom resulting in re-release of
phosphorus and other pollutants back into the water.
Step 8: Select an Appropriate Treatment Device
Refer to the treatment facility menus in Chapter 2 for treatment facility options. Use Table V-1-1 as an
aid in determining which treatment device is most appropriate for the site.
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Table V-1-1. Screening Treatment Facilities Based on Soil Type
Soil Type Infiltration
Wet
Pond*
Biofiltration*
(Swale or Filter Strip)
Coarse Sand or Cobbles N N N
Sand Y N N
Loamy Sand Y N Y
Sandy Loam Y N Y
Loam N N Y
Silt Loam N N Y
Sandy Clay Loam N Y Y
Silty Clay Loam N Y Y
Sandy Clay N Y Y
Silty Clay N Y N
Clay N Y N
Y Indicates that use of the technology is generally appropriate for this soil type.
N Indicates that use of the technology is generally not appropriate for this soil type
* Coarser soils may be used for these facilities if a liner is installed to prevent infiltration,
or if the soils are amended to reduce the infiltration rate.
Sand filtration is not listed because its feasibility is not dependent on soil type.
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Chapter 2 Treatment Facility Menus
This chapter identifies the treatment facility menus. The menus in this chapter are as follows:
• Oil Control Menu, Section 2.1
• Phosphorus Treatment Menu, Section 2.2
• Enhanced Treatment Menu, Section 2.3
• Basic Treatment Menu, Section 2.4
Performance goals apply to the water quality design storm volume or flowrate, whichever is
applicable, and apply on an average annual basis to the entire annual discharge volume (treated plus
bypassed). The incremental portion of runoff in excess of water quality design flowrate can be routed
around the facility (off-line treatment facilities) or can be passed through the facility untreated (on-line
treatment facilities) provided a net pollutant reduction is maintained. All performance goals apply to
stormwater typically found in Pacific Northwest maritime climates, where long duration, low intensity
storms predominate and stormwater contains mostly silt-sized particles.
2.1 Oil Control Menu
This menu is applicable in addition to facilities required by other treatment menus.
2.1.1 Performance Goal
The oil control menu facility options should achieve the goals of no ongoing or recurring visible
sheen, and to have a 24-hour average Total Petroleum Hydrocarbon (TPH) concentration no greater
than 10 mg/l, and a maximum of 15 mg/l for a discrete sample (grab sample).
2.1.2 Options
Oil control options include facilities that are small, treat runoff from a limited area, and require
frequent maintenance as well as facilities that treat runoff from larger areas and generally require less
frequent maintenance.
• API-Type Oil/Water Separator – See Chapter 9
• Coalescing Plate Oil/Water Separator – See Chapter 9
• Linear Sand Filter – See Chapter 6
The linear sand filter is used in the Basic, Enhanced, and Phosphorus Treatment menus
also. If used to satisfy one of those treatment requirements, the same facility shall not also be
used to satisfy the oil control requirement unless enhanced maintenance is assured. This is
to prevent clogging of the filter by oil so that it will function for suspended solids and other
pollutant removal as well. Quarterly cleaning is required when used as a combination facility.
• Emerging Stormwater Treatment Technologies for Oil Treatment – See Chapter
10
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2.1.3 Application on the Project Site
Place oil control facilities upstream of other facilities, as close to the source of oil generation as
practical. For high-use sites located within a larger commercial center, only the impervious surface
associated with the high-use portion of the site is subject to oil treatment requirements. If common
parking for multiple businesses is provided, treatment shall be applied to the number of parking stalls
required for the high-use business only. If the treatment collection area also receives runoff from
other areas, the treatment facility must be sized to treat all water passing through it.
High-use roadway intersections shall treat lanes where vehicles accumulate during the traffic signal
cycle, including left and right turn lanes and through lanes, from the beginning of the left turn pocket.
If no left turn pocket exists, the treatable area shall begin at a distance equal to three car lengths from
the stop line. If runoff from the intersection drains to more than two collection areas that do not
combine within the intersection, treatment may be limited to any two of the collection areas.
2.2 Phosphorus Treatment Menu
2.2.1 Performance Goal
The phosphorus menu facility choices should achieve a goal of 50% total phosphorus removal for a
range of influent concentrations between 0.1 – 0.5 mg/l. The phosphorus menu facility choices must
achieve basic treatment goals in addition to phosphorus.
2.2.2 Options
Any one of the following options may be chosen to satisfy the phosphorus treatment requirement.
• Infiltration with appropriate pretreatment – See Chapter 4 and Chapter 5
o Infiltration Treatment - If infiltration is through soils meeting the minimum
site suitability criteria for infiltration treatment (see Chapter 5), a presettling
basin or a basic treatment facility can serve for pretreatment.
o Infiltration Proceeded by Basic Treatment - If infiltration is through soils
that do not meet the soil suitability criteria for infiltration treatment, treatment
must be provided by a basic treatment facility unless the soil and site fit the
description in the next option below.
o Infiltration Preceded by Phosphorus Treatment - If the soils do not meet
the soil suitability criteria and the infiltration site is within ¼ mile of a
phosphorus-sensitive receiving water, or a tributary to that water, treatment
must be provided by one of the other treatment facility options listed below.
• Large Sand Filter – See Chapter 6
• Large Wetpond – See Chapter 8
• Emerging Stormwater Treatment Technologies for Phosphorus Treatment –
See Chapter 10
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• Two-Facility Treatment Trains – See Table V-2-2.
Table V-2-2. Treatment Trains for Phosphorus Removal
First Basic Treatment Facility Second Treatment Facility
Biofiltration Swale Basic Sand Filter or Sand Filter Vault
Filter Strip Linear Sand Filter (no presettling needed)
Linear Sand Filter Filter Strip
Basic Wetpond Basic Sand Filter or Sand Filter Vault
Wetvault Basic Sand Filter or Sand Filter Vault
Stormwater Treatment Wetland Basic Sand Filter or Sand Filter Vault
Basic Combined Detention and
Wetpool
Basic Sand Filter or Sand Filter Vault
2.3 Enhanced Treatment Menu
2.3.1 Performance Goal
The enhanced treatment facility choices should provide a higher rate of removal of dissolved metals
than basic treatment facilities. The performance goal assumes that the facility is treating stormwater
with influent dissolved copper ranging from 0.003 to 0.02 mg/l, and dissolved zinc ranging from 0.02
to 0.3 mg/l. Enhanced treatment facilities must achieve basic treatment goals in addition to enhanced
treatment goals.
2.3.2 Options
Any one of the following options may be chosen to satisfy the enhanced treatment requirement:
• Infiltration with appropriate pretreatment – See Chapter 4 and Chapter 5
o Infiltration Treatment - If infiltration is through soils meeting the minimum
site suitability criteria for infiltration treatment (see Chapter 5), a presettling
basin or a basic treatment facility can serve for pretreatment.
o Infiltration Proceeded by Basic Treatment - If infiltration is through soils
that do not meet the soil suitability criteria for infiltration treatment, treatment
must be provided by a basic treatment facility unless the soil and site fit the
description in the next option below.
o Infiltration Preceded by Enhanced Treatment - If the soils do not meet the
soil suitability criteria and the infiltration site is within ¼ mile of a fish-bearing
stream or a tributary to a fish-bearing stream treatment must be provided by
one of the other treatment facility options listed below.
• Large Sand Filter – See Chapter 6
• Stormwater Treatment Wetland – See Chapter 8
• Two Facility Treatment Trains – See Table V-2-3
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• Compost-Amended Filter – See Chapter 7
• Bioretention/rain garden – See Volume VI
NOTE: Where bioretention/rain gardens are intended to fully meet treatment requirements,
they must be designed, using an approved continuous runoff model, to infiltrate 91% of the
influent runoff volume.
• Emerging Stormwater Treatment Technologies for Enhanced Treatment – See
Chapter 10
Table V-2-3. Treatment Trains for Enhanced Treatment
First Basic Treatment Facility Second Treatment Facility
Biofiltration Swale Basic Sand Filter or Sand Filter Vault or Media
Filter1
Filter Strip Linear Sand Filter with no pre-settling cell
needed
Linear Sand Filter Filter Strip
Basic Wetpond Basic Sand Filter or Sand Filter Vault or Media
Filter1
Wetvault Basic Sand Filter or Sand Filter Vault or Media
Filter1
Basic Combined Detention/Wetpool Basic Sand Filter or Sand Filter Vault or Media
Filter1
Basic Sand Filter or Sand Filter Vault
with a presettling cell if the filter isn’t
preceded by a detention facility
Media Filter1
1 The media must be of a type approved for use by Ecology. Refer to Ecology’s website.
2.4 Basic Treatment Menu
2.4.1 Performance Goal
The basic treatment menu facility options should achieve 80% removal of total suspended solids
(TSS) for influent concentrations ranging from 100 to 200 mg/L. For influent concentrations greater
than 200 mg/l, a higher treatment goal is appropriate. For influent concentrations less than 100 mg/l,
the facilities should achieve an effluent goal of 20 mg/l total suspended solids.
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2.4.2 Options
Any one of the following options may be chosen to satisfy the basic treatment requirement:
• Bio-infiltration Swale – See Chapter 5
• Infiltration – See Chapter 5
• Sand Filters – See Chapter 6
• Biofiltration Swales – See Chapter 7
• Filter Strips – See Chapter 7
• Basic Wetpond – See Chapter 8
• Wetvault – See Chapter 8
A wetvault may be used for commercial, industrial, or road projects if there are space
limitations. The use of wetvaults is discouraged for residential projects. Combined
detention/wetvaults are allowed (see Section 8.3).
• Stormwater Treatment Wetland – See Chapter 8
• Combined Detention and Wetpool Facilities – See Chapter 8
• Bioretention/Rain Garden – See Volume VI
• Emerging Stormwater Treatment Technologies for Basic Treatment – See
Chapter 10
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Chapter 3 General Requirements for Stormwater Facilities
This chapter addresses general requirements for treatment facilities. Requirements discussed in this
chapter include design volumes and flows, sequencing of facilities, liners, and hydraulic structures for
splitting or dispersing flows.
• All open detention and retention ponds shall be appropriately and aesthetically located,
designed, and planted. Pre-approval of the design concept, including landscaping, is required
for all proposed public ponds. Joint use facilities incorporating recreation or open space
opportunities are encouraged.
• 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.
• Water quality systems shall be designed for simplicity and ease of maintenance.
• Treatment systems shall be designed such that storm drainage from public streets does not
discharge into areas of private ownership or private maintenance responsibility unless a hold-
harmless and cross drainage agreement has been secured.
• The City prefers retention (infiltration) for storm drainage quantity control when soil conditions
are satisfactory for such application and water quality treatment can be provided.
3.1 Design Flow Volume and Flow Rate
3.1.1 Water Quality Design Flow Volume
The volume of runoff predicted from a 24-hour storm with a 6-month return frequency (a.k.a.,
6-month, 24-hour storm) obtained from an approved single event model. The 6-month, 24-hour
design storm of 1.44 inches shall be used in the City of Auburn.
Alternatively, the 91st percentile, 24-hour runoff volume indicated by the Western Washington
Hydrology Model (WWHM) will be used to determine the water quality design flow volume.
Treatment facility sizes are the same whether they precede, follow or are incorporated (i.e., combined
detention and wetpool facilities) into the detention facility.
Treatment options that can be sized by this method include:
• Wetponds
• Wetvaults
• Stormwater Wetlands
• Combined Detention and Wetpool Facilities
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3.1.1 Water Quality Design Flow Rate
A. Preceding Detention Facilities or when Detention Facilities are not required: The
flow rate at or below which 91% of the runoff volume, as estimated by WWHM, will
be treated
• All BMPs except wetpool type facilities (Chapter 8) shall use the 15-minute time
series from WWHM. Design criteria for treatment facilities are assigned to achieve the
applicable performance goal (e.g., 80 percent TSS removal) at the water quality
design flow rate.
• Off-line facilities: For treatment facilities not preceded by an equalization or storage
basin, and when runoff flow rates exceed the water quality design flow rate, the
treatment facility shall continue to receive and treat the water quality design flow rate
to the applicable treatment performance goal. The incremental portion of runoff in
excess of the water quality design flowrate can be routed around the facility.
Treatment facilities preceded by an equalization or storage basin may identify a lower
water quality design flow rate provided that at least 91 percent of the estimated runoff
volume in the time series of an approved continuous runoff model is treated to the
applicable performance goals (e.g., 80 percent TSS removal at the water quality
design flow rate and 80 percent TSS removal on an annual average basis).
• On-line facilities: Runoff flow rates in excess of the water quality design flow rate
can be routed through the facility provided a net pollutant reduction is maintained, and
the applicable annual average performance goal is met.
B. Downstream of Detention Facilities: The full 2-year release rate from the detention
facility
• An approved continuous runoff model shall identify the 2-year return frequency flow
rate discharged by a detention facility that is designed to meet the flow duration
standard.
• Treatment facilities downstream of detention can be designed on-line or off-line. For
off-line facilities, the entire water quality design flow volume/rate must be treated.
Only flows in excess of the design flow may be bypassed.
3.1.2 Flows Requiring Treatment
Runoff from pollution-generating impervious or pervious surfaces exceeding the thresholds outlined
in Minimum Requirement #6 (Volume I, Chapter 3) must be treated using the water quality facilities in
this volume. If runoff from non-pollution generating surfaces reaches a runoff treatment BMP, flows
from those areas must be included in the sizing calculations for the facility. Once runoff from non-
pollution generating areas is mixed with runoff from pollution-generating areas, it cannot be
separated before treatment.
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The following are some examples of surfaces that are required to provide treatment if threshold limits
for treatment are met:
• Runoff from pollution-generating impervious surfaces (e.g. roads (paved or unpaved),
driveways, parking lots, storage yards, bike lanes within the roadway, unvegetated road
shoulders, etc.)
• Runoff from any pervious or non-impervious surface subject to the use of pesticides and
fertilizers or soil loss.
• Runoff from pervious parking areas.
• Runoff from metal roofs unless they are coated with an inert non-leachable material.
3.2 Sequence of Facilities
Treatment facilities can be placed in a variety of configurations. Some are required to be upstream
from detention facilities while others may perform better if located downstream. Detention facilities
can act as settling basins and therefore can reduce the load going to a treatment facility. Additionally,
treatment facilities can be sequenced together to provide a higher level of treatment that could be
achieved by a single facility. For instance, the enhanced treatment and phosphorus removal menus,
described in Chapter 2, include treatment options in which more than one type of treatment facility is
used, for these treatment trains the sequencing is prescribed.
Table V-3-4 summarizes placement considerations of treatment facilities in relation to detention
facilities.
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Table V-3-4. Treatment Facility Placement in Relation to Detention Facilities
Water Quality Facility
Preceding
Detention Following Detention
Basic biofiltration swale (Chapter 7) OK OK. Prolonged flows may reduce grass
survival. Consider wet biofiltration swale
Wet biofiltration swale (Chapter 7) OK OK
Filter strip (Chapter 7) OK No—must be installed before flows
concentrate.
Basic or large wetpond (Chapter 8) OK OK—less water level fluctuation in
ponds downstream of detention may
improve aesthetic qualities and
performance.
Wetvault (Chapter 8) OK OK
Basic or large sand filter or sand
filter vault (Chapter 6)
OK, but
presettling and
control of
floatables
needed
OK—sand filters downstream of
detention facilities may require field
adjustments if prolonged flows cause
sand saturation and interfere with
phosphorus removal.
Stormwater treatment wetland/pond
(Chapter 8)
OK OK—less water level fluctuation and
better plant diversity are possible if the
stormwater wetland is located
downstream of the detention facility.
3.3 Setbacks, Slopes, and Embankments
The following guidelines for setbacks, slopes, and embankments are intended to provide for
adequate maintenance accessibility to runoff treatment facilities. Setback requirements may also be
specified by Auburn City Code, Uniform Building Code requirements, or other state regulations.
3.3.1 Setbacks
The City requires specific setbacks for sites with steep slopes, landslide areas, open water features,
springs, wells, and septic tank drain fields. Setbacks from tract lines are necessary for maintenance
access and equipment maneuverability. Adequate room for maintenance equipment shall also be
considered during site design. 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. Required setbacks are as follows or as determined by a qualified geotechnical
engineer:
• Minimum spacing between trenches shall be 4 feet measured from the edge of trench.
• Stormwater infiltration facilities, unlined wetponds and detention ponds shall be set back at
least 100 feet from open water features, drinking water wells, septic tanks or drainfields, and
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springs used for public drinking water supplies. Infiltration facilities, unlined wetponds and
detention ponds upgradient of drinking water supplies and within 1, 5, and 10-year time of
travel zones must comply with Health Department requirements (Washington Wellhead
Protection Program, DOH, 12/93).
• 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 20% or above
erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and
City approval.
• For sites with septic systems, 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.
• Infiltration system shall be set back 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 by a geotechnical engineer
or qualified geologist. The discharge point shall not be placed on or above slopes greater
than 20% (5H:1V) or above erosion hazard areas without evaluation by a geotechnical
engineer or qualified geologist and City approval.
Additional setbacks for specific stormwater facilities will be noted in the appropriate section.
3.3.2 Side Slopes and Embankments
• Side slopes shall not exceed a slope of 3H:1V. Moderately undulating slopes are acceptable
and can provide a more natural setting for the facility. In general, gentle side slopes improve
the aesthetic attributes of the facility and enhance safety.
• Interior side slopes may be retaining walls if the design is prepared and stamped by a
licensed civil engineer. A fence shall be provided along the top of the wall.
• Maintenance access shall be provided through an access ramp or other adequate means
(see Volume III, Chapter 2, Section 2.3.1).
• Embankments that impound water must comply with the Washington State Dam Safety
Regulations (Chapter 173-175 WAC). See Volume III, Chapter 3 for more detail concerning
Detention Ponds.
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3.4 Facility Liners
Liners are intended to reduce the likelihood that pollutants in stormwater will reach groundwater.
Where necessary, a liner is incorporated into the base of the treatment facility as the facility is
constructed. In addition to groundwater protection considerations, some facility types require
permanent water for proper functioning. An example is the first cell of a wetpond.
3.4.1 General Design Criteria
• Table V-3-5 shows recommendations for the type of liner generally best suited for use with
various runoff treatment facilities.
• Liners shall be evenly placed over the bottom and/or sides of the treatment area of the facility
as indicated in Table V-3-5. Areas above the treatment volume that are required to pass
flows greater than the water quality treatment flow (or volume) need not be lined. However,
the lining must be extended to the top of the interior side slope and anchored if it cannot be
permanently secured by other means.
• For low permeability liners, the following criteria apply:
o Where the seasonal high groundwater elevation is likely to contact a low
permeability liner, liner buoyancy may be a concern. A low permeability liner
shall not be used in this situation unless evaluated and recommended by a
geotechnical engineer.
o Where grass must be planted over a low permeability liner per the facility
design, a minimum of 6 inches of good topsoil or compost-amended native
soil (2 inches compost tilled into 6 inches of native till soil) must be placed
over the liner in the area to be planted. Twelve inches of cover is preferred.
• Check all liners for buoyancy stability and include calculations in project documentation.
Provide anchors as needed.
• If a treatment liner will be below the seasonal high water level, the pollutant removal
performance of the liner must be evaluated by a geotechnical or groundwater specialist and
found to be as protective as if the liner were above the level of the groundwater.
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Table V-3-5. Lining Types Recommended for Runoff Treatment Facilities
WQ Facility Area to be Lined Type of Liner Recommended
Presettling basin Bottom and sides
Low permeability liner or
Treatment liner (If the basin
will intercept the seasonal high
groundwater table, a treatment
liner is recommended.)
Wetpond First cell: bottom and sides to
WQ design water surface
----------------------------------
Second cell: bottom and sides
to WQ design water surface
Low permeability liner or
Treatment liner (If the wet
pond will intercept the
seasonal high groundwater
table, a treatment liner is
recommended.)
--------------------------------
Treatment liner
Combined detention/WQ
facility
First cell: bottom and sides to
WQ design water surface
----------------------------------
Second cell: bottom and sides
to WQ design water surface
Low permeability liner or
treatment liner (If the facility
will intercept the seasonal high
groundwater table a treatment
liner is recommended.)
--------------------------------
Treatment liner
Stormwater wetland Bottom and sides, both cells Low permeability liner (If the
facility will intercept the
seasonal high groundwater
table, a treatment liner is
recommended.)
Sand filtration basin Basin sides only Treatment liner
Sand filter vault Not applicable No liner needed
Linear sand filter Not applicable if in vault
Bottom and sides of presettling
cell if not in vault
No liner needed
Low permeability or treatment
liner
Media filter (in vault) Not applicable No liner needed
Wet vault Not applicable No liner needed
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3.4.2 Design Criteria for Treatment Liners
• A two-foot thick layer of soil with a minimum organic content of 5% and a minimum cation
exchange capacity (CEC) of 5 milliequivalents/100 grams can be used as a treatment
layer beneath a water quality or detention facility.
• To demonstrate that in-place soils meet the above criteria, one sample per 1,000 square
feet of facility area shall be tested. Each sample shall be a composite of subsamples
taken throughout the depth of the treatment layer (usually two to six feet below the
expected facility invert).
• Typically, side wall seepage is not a concern if the seepage flows through the same
stratum as the bottom of the treatment BMP. However, if the treatment soil is an
engineered soil or has very low permeability, the potential to bypass the treatment soil
through the side walls may be significant. In those cases, the treatment BMP side walls
may be lined with at least 18 inches of treatment soil, as described above, to prevent
untreated seepage. This lesser soil thickness is based on unsaturated flow as a result of
alternating wet-dry periods.
• Organic content shall be measured on a dry weight basis using ASTM D2974.
• Cation exchange capacity (CEC) shall be tested using EPA laboratory method 9081.
• Certification by a soils testing laboratory that imported soil meets the organic content and
CEC criteria above shall be provided to the City of Auburn.
• Animal manures used in treatment soil layers must be sterilized because of potential for
bacterial contamination of the groundwater.
3.4.3 Design Criteria for Low Permeability Liner Options
This section presents the design criteria for each of the following four low permeability liner options:
compacted till liners, clay liners, geomembrane liners, and concrete liners.
3.4.3.1 Compacted Till Liners
• Liner thickness shall be 18 inches after compaction.
• Soil shall be compacted to 95% minimum dry density, modified proctor method (ASTM
D-1557).
• A different depth and density sufficient to retard the infiltration rate to 2.4 x 10-5 inches per
minute (1 x 10-6 cm/s) may also be used instead of the above criteria.
• Soil shall be placed in 6-inch lifts.
• Reference Table V-3-6 for Acceptable Gradation for Compacted Till Liners.
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Table V-3-6. Acceptable Gradation for Compacted Till Liners
Sieve Size Percent Passing
6-inch 100
4-inch 90
#4 70 - 100
#200 20
3.4.3.2 Clay Liners
• Liner thickness shall be 12 inches.
• Clay shall be compacted to 95% minimum dry density, modified proctor method
(ASTM D-1557).
• A different depth and density sufficient to retard the infiltration rate to 2.4 x
10-5 inches per minute (1 x 10-6 cm/s) may also be used instead of the above criteria.
• The slope of clay liners must be restricted to 3H: IV for all areas requiring soil cover;
otherwise, the soil layer must be stabilized by another method so that soil slippage
into the facility does not occur. Any alternative soil stabilization method must take
maintenance access into consideration.
• Where clay liners form the sides of ponds, the interior side slope shall not be steeper
than 3H:1V, irrespective of fencing.
3.4.3.3 Geomembrane Liners
• Geomembrane liners shall be ultraviolet (UV) light resistant and have a minimum
thickness of 30 mils. A thickness of 40 mils shall be used in areas of maintenance
access or where heavy machinery must be operated over the membrane.
• Geomembranes shall be bedded according to the manufacturer's recommendations.
• Liners shall be installed so that they can be covered with 12 inches of top dressing
forming the bottom and sides of the water quality facility, except for liner sand filters.
Top dressing shall consist of 6 inches of crushed rock covered with 6 inches of
native soil. The rock layer is to mark the location of the liner for future maintenance
operations. As an alternative to crushed rock, 12 inches of native soil may be used if
orange plastic “safety fencing” or another highly-visible, continuous marker is
embedded 6 inches above the membrane.
• If possible, liners should be of a contrasting color so that maintenance workers are
aware of any areas where a liner may have become exposed when maintaining the
facility.
• Geomembrane liners shall not be used on slopes steeper than 5H:1V to prevent the
top dressing material from slipping. Textured liners may be used on slopes up to
3H:1V upon recommendation by a geotechnical engineer that the top dressing will be
stable for all site conditions, including maintenance.
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3.4.3.4 Concrete Liners
• Portland cement liners are allowed irrespective of facility size, and shotcrete may be
used on slopes. However, specifications must be developed by a professional
engineer who certifies the liner against cracking or losing water retention ability
under expected conditions of operation, including facility maintenance operations.
Weight of maintenance equipment can be up to 80,000 pounds when fully loaded.
• Asphalt concrete may not be used for liners due to its permeability to many organic
pollutants.
• If grass is to be grown over a concrete liner, slopes must be no steeper than 5H:1V
to prevent the top dressing material from slipping.
3.5 Hydraulic Structures
3.5.1 Flow Splitter Designs
3.5.1.1 General Design Criteria
• A flow splitter must be designed to deliver the WQ design flow rate specified in this
volume to the WQ treatment facility.
• The top of the weir must be located at the water surface for the design flow.
Remaining flows enter the bypass line. Flows modeled using a WWHM shall use
15-minute time steps, if available. Otherwise use 1-hour time steps.
• The maximum head must be minimized for flow in excess of the WQ design flow.
Specifically, flow to the WQ facility at the 100-year water surface must not increase
the design WQ flow by more than 10%.
• Design as shown in Figure V-3-1 or Figure V-3-2 or provide an equivalent design.
• As an alternative to using a solid top plate in Figure V-3-2, a full tee section may be
used with the top of the tee at the 100-year water surface. This alternative would
route emergency overflows (if the overflow pipe were plugged) through the WQ
facility rather than back up from the manhole.
• Special applications, such as roads, may require the use of a modified flow splitter.
The baffle wall may be fitted with a notch and adjustable weir plate to proportion
runoff volumes other than high flows.
• For ponding facilities, backwater effects must be considered in determining the
height of the standpipe in the manhole.
• Ladder or step and handhold access must be provided. If the weir wall is higher than
36 inches, two ladders, one to either side of the wall, must be used.
3.5.1.2 Materials
• The splitter baffle may be installed in a Type 2 manhole or vault.
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• The baffle wall must be made of reinforced concrete or another suitable material
resistant to corrosion, and have a minimum 4-inch thickness. The minimum
clearance between the top of the baffle wall and the bottom of the manhole cover
must be 4 feet; otherwise, dual access points shall be provided.
• All metal parts must be corrosion resistant. Examples of preferred materials include
aluminum, stainless steel, and plastic. Zinc and galvanized materials are prohibited
unless coated as approved by the City. Painted metal parts shall not be used
because of poor longevity.
Figure V-3-1. Flow Splitter, Option A
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Figure V-3-2. Flow Splitter, Option B
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3.5.2 Flow Spreading Options
Flow spreaders function to uniformly spread flows across the inflow portion of water quality facilities
(e.g., sand filter, biofiltration swale, or filter strip). There are five flow spreader options presented in
this section:
• Option A – Anchored plate (Figure V-3-3)
• Option B – Concrete sump box (Figure V-3-4)
• Option C – Notched curb spreader (Figure V-3-5)
• Option D – Through-curb ports (Figure V-3-6)
• Option E – Interrupted curb
Options A through C can be used for spreading flows that are concentrated. Any one of these options
can be used when spreading is required by the facility design criteria. Options A through C can also
be used for unconcentrated flows, and in some cases must be used, such as to correct for moderate
grade changes along a filter strip.
Options D and E are only for flows that are already unconcentrated and enter a filter strip or
continuous inflow biofiltration swale. Other flow spreader options may be allowed with written
approval from The City.
3.5.2.1 General Design Criteria
• Where flow enters the flow spreader through a pipe, it is recommended that the pipe
be submerged to the extent practical to dissipate as much energy as possible.
• For higher inflows (greater than 5 cfs for the 100-yr storm), a Type 1 catch basin
shall be positioned in the spreader and the inflow pipe shall enter the catch basin
with flows exiting through the top grate. The top of the grate shall be lower than the
level spreader plate, or if a notched spreader is used, lower than the bottom of the
v-notches.
Option A -- Anchored Plate
• An anchored plate flow spreader must be preceded by a sump having a minimum
depth of 8 inches and minimum width of 24 inches. If not otherwise stabilized, the
sump area must be lined to reduce erosion and to provide energy dissipation.
• The top surface of the flow spreader plate must be level, projecting a minimum of
2 inches above the ground surface of the water quality facility, or V-notched with
notches 6 to 10 inches on center and 1 to 6 inches deep (use shallower notches with
closer spacing). Alternative designs may also be used if approved by the City.
• A flow spreader plate must extend horizontally beyond the bottom width of the facility
to prevent water from eroding the side slope. The horizontal extent shall be such that
the bank is protected for all flows up to the 100-year flow or the maximum flow that
will enter the Water Quality (WQ) facility.
• Flow spreader plates must be securely fixed in place.
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• Flow spreader plates may be made of either wood, metal, fiberglass reinforced
plastic, or other durable material. If wood, pressure treated 4 by 10-inch lumber or
landscape timbers are acceptable.
• Anchor posts must be 4-inch square concrete, tubular stainless steel, or other
material resistant to decay.
Figure V-3-3. Flow Spreader Option A – Anchored Plate
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Option B -- Concrete Sump Box
• The wall of the downstream side of a rectangular concrete sump box must extend a
minimum of 2 inches above the treatment bed. This serves as a weir to spread the
flows uniformly across the bed.
• The downstream wall of a sump box must have “wing walls” at both ends. Side walls
and returns must be slightly higher than the weir so that erosion of the side slope is
minimized.
• Concrete for a sump box can be either cast-in-place or precast, but the bottom of the
sump must be reinforced with wire mesh for cast-in-place sumps.
• Sump boxes must be placed over bases that consists of 4 inches of crushed rock,
5/8-inch minus to help assure the sump remains level.
Figure V-3-4. Flow Spreader Option B – Concrete Sump Box
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Option C -- Notched Curb Spreader
Notched curb spreader sections must be made of extruded concrete laid side-by-side and level.
Typically five “teeth” per four-foot section provide good spacing. The space between adjacent “teeth”
forms a v-notch.
Figure V-3-5. Flow Spreader Option C – Notched Curb Spreader
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Option D -- Through-Curb Ports (Figure V-3-6)
Unconcentrated flows from paved areas entering filter strips or continuous inflow biofiltration swales
can use curb ports or interrupted curbs (Option E) to allow flows to enter the strip or swale. Curb
ports use fabricated openings that allow concrete curbing to be poured or extruded while still
providing an opening through the curb to admit water to the water quality facility.
Openings in the curb must be at regular intervals but at least every 6 feet (minimum). The width of
each curb port opening must be a minimum of 11 inches. Approximately 15 percent or more of the
curb section length shall be in open ports, and no port shall discharge more than about 10 percent of
the flow.
Figure V-3-6. Flow Spreader Option D – Through-Curb Ports
Option E -- Interrupted Curb
Interrupted curbs are sections of curb placed to have gaps spaced at regular intervals along the total
width (or length, depending on facility) of the treatment area. At a minimum, gaps must be every
6 feet to allow distribution of flows into the treatment facility before they become too concentrated.
The opening must be a minimum of 11 inches. As a general rule, no opening shall discharge more
than 10 percent of the overall flow entering the facility.
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Chapter 4 Pretreatment
4.1 Purpose
This chapter presents the methods that may be used to provide pretreatment prior to runoff treatment
facilities. Pretreatment must be provided in the following applications:
• For sand filtration and infiltration Best Management Practices (BMPs) to protect them
from excessive siltation and debris;
• Where the basic treatment facility or the receiving water may be adversely affected
by non-targeted pollutants (e.g., oil), or may be overwhelmed by a heavy load of
targeted pollutants (e.g., suspended solids).
4.2 Application
Presettling basins are a typical pretreatment BMP used to remove suspended solids. All basic,
enhanced, and phosphorus treatment options may be used for pretreatment to reduce suspended
solids.
A detention pond sized to meet the flow control standard in Volume I may also be used to provide
pretreatment for suspended solids removal.
4.3 BMPs for Pretreatment
4.3.1 BMP T610 Presettling Basin
4.3.1.1 Purpose and Definition
A presettling basin provides pretreatment of runoff in order to remove suspended solids, which can
impact other runoff treatment BMPs.
4.3.1.2 Application and Limitations
Runoff treated by a presettling basin may not be discharged directly to a receiving water; it must be
further treated by a basic, enhanced, or phosphorus runoff treatment BMP.
4.3.1.3 Design Criteria
1. A presettling basin shall be designed using analysis techniques for a wetpool or using
WWHM. The treatment volume shall be at least 30 percent of the total volume of runoff
from the 6-month, 24-hour storm event.
2. If the runoff in the presettling basin will be in direct contact with the soil, it must be lined
per the liner requirement in Section 3.4.
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3. The presettling basin shall conform to the following:
o The length-to-width ratio shall be at least 3:1. Berms or baffles may be used
to lengthen the flowpath.
o The minimum depth shall be 4 feet; the maximum depth shall be 6 feet.
4. Inlets and outlets shall be designed to minimize velocity and reduce turbulence.
Inlet and outlet structures should be located at extreme ends of the basin in order
to maximize particle-settling opportunities.
4.3.1.4 Site Constraints and Setback Requirements
Site constraints are any manmade restrictions such as property lines, easements, structures, etc. that
impose constraints on development. Constraints may also be imposed from natural features such as
requirements in the Auburn City Code. These should also be reviewed for specific application to the
proposed development. For Setback Requirements see Section 3.3.1.
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Chapter 5 Infiltration and Bio-infiltration Treatment Facilities
5.1 Purpose
This chapter provides site suitability, design, and maintenance criteria for infiltration treatment
systems. Infiltration treatment Best Management Practices (BMPs) serve the dual purpose of
removing pollutants (TSS, heavy metals, phosphates, and organics) from stormwater and recharging
aquifers.
The infiltration BMPs described in this chapter include:
BMP T710 Infiltration basins
BMP T720 Infiltration trenches
BMP T730 Bio-infiltration swales
5.2 Application
These infiltration and bio-infiltration treatment measures are capable of achieving the performance
objectives cited in Chapter 3 for specific treatment menus. In general, these treatment techniques
can capture and remove or reduce the target pollutants to levels that will not adversely affect public
health or beneficial uses of surface and groundwater resources.
Infiltration treatment systems are typically installed:
• As off-line systems, or on-line for small drainages.
• As a polishing treatment for street/highway runoff after pretreatment for TSS and oil.
• As part of a treatment train.
• As retrofits at sites with limited land areas, such as residential lots, commercial
areas, parking lots, and open space areas.
• With appropriate pretreatment for oil and silt control to prevent clogging.
An infiltration basin is preferred, where applicable, and where a trench or bio-infiltration swale cannot
be sufficiently maintained.
5.3 Site Suitability
The following site suitability criteria (SSC) must be considered. When a site investigation reveals that
any of the eight applicable criteria cannot be met, appropriate mitigation measures must be
implemented so that the infiltration facility will not pose a threat to safety, health, and the
environment.
For site selection and design decisions a geotechnical and hydrogeologic report must be prepared by
a qualified engineer with geotechnical and hydrogeologic experience, or an equivalent professional
acceptable to the City, under the seal of a registered Professional Engineer. The design engineer
may utilize a team of certified or registered professionals in soil science, hydrogeology, geology, and
other related fields.
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5.3.1 Setback Criteria (SSC-1)
Setback requirements are generally required by the City, uniform building code requirements, or
other state regulations. Where a conflict between setbacks occurs, the City shall require compliance
with the most stringent of the setback requirements from the various codes/regulations.
Setback criteria for the various infiltration and dispersion facilities can be found in the design criteria
for each BMP in this chapter. Below are conditions that the soils professional must evaluate to
determine the need for additional or more stringent setbacks than outlined in this manual.
The professional must evaluate:
• Potential impacts to drinking water wells, septic tanks or drainfields, and springs
used for public drinking water supplies. Minimum setback will be 100 feet. Infiltration
facilities upgradient of drinking water supplies and within 1, 5, and 10-year time of
travel zones must comply with Health Dept. requirements (Publication # 331-018).
• Potential impacts from roadways subject to deicers or herbicides which are likely to
be present in the influent to the infiltration system.
• Potential impacts to all building foundations in the vicinity of the proposed infiltration
facility. Recommend investigating all building foundations: within 100 feet upslope
and 20 feet downslope from the facility.
• Potential impacts to all property lines within 20 feet of the facility.
• Potential impacts to a Native Growth Protection Easement (NGPE); 20 feet.
• Potential impacts to the top of slopes >20% and within 50 feet.
• On-site and off-site structural stability due to extended subgrade saturation and/or
head loading of the permeable layer, including the potential impacts to downgradient
properties, especially on hills with known side-hill seeps.
5.3.2 Groundwater Protection Areas (SSC-2)
A site is not suitable if the infiltration facility will cause a violation of Ecology's Groundwater Quality
Standards (See Section 5.3.7 for verification testing guidance).
5.3.3 High Vehicle Traffic Areas (SSC-3)
An infiltration BMP may be considered for runoff from areas of industrial activity and the high vehicle
traffic areas described below. For such applications sufficient pollutant removal (including oil removal)
shall be provided upstream of the infiltration facility to ensure that groundwater quality standards will
not be violated and that the infiltration facility is not adversely affected.
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High Vehicle Traffic Areas are:
• Commercial or industrial sites subject to an expected average daily traffic count
(ADT) 100 vehicles/1,000 ft² gross building area (trip generation), and
• Road intersections with an ADT of 25,000 on the main roadway, or 15,000 on
any intersecting roadway.
5.3.4 Soil Infiltration Rate/Drawdown Time for Treatment (SSC-4)
5.3.4.1 Infiltration Rates: Short-term and Long-term
For treatment purposes, the short-term soil infiltration rate should be 2.4 in/hour, or less, to a depth of
2.5 times the maximum design pond water depth, or a minimum of 6 ft. below the base of the
infiltration facility. This infiltration rate is also typical for soil textures that possess sufficient physical
and chemical properties for adequate treatment, particularly for soluble pollutant removal (see
Section 5.3.6). It is comparable to the textures represented by Hydrologic Groups B and C. Long-
term infiltration rates up to 2.0 inches/hour can also be considered, if the infiltration receptor is not a
sole-source aquifer, and in the judgment of the site professional, the treatment soil has characteristics
comparable to those specified in SSC-6 to adequately control the target pollutants.
The long-term infiltration rate should be used for drawdown time and routing calculations.
5.3.4.2 Drawdown Time
Refer to Section 5.4 for infiltration rate determination. Document that the 91st percentile, 24-hour
runoff volume (as indicated by the Western Washington Hydrology Model [WWHM]) can infiltrate
through the infiltration basin surface within 48 hours. This can be calculated using a horizontal
projection of the infiltration basin mid-depth dimensions and the estimated long-term infiltration rate.
This drawdown restriction is intended to meet the following objectives:
• Restore hydraulic capacity to receive runoff from a new storm,
• Maintain infiltration rates,
• Aerate vegetation and soil to keep the vegetation healthy, and
• Enhance the biodegradation of pollutants and organics in the soil.
5.3.5 Depth to Bedrock, Water Table, or Impermeable Layer (SSC-5)
The base of all infiltration basins or trench systems shall be 5 feet above the seasonal high-water
mark, bedrock (or hardpan) or other low permeability layer. A separation down to 3 feet may be
considered if the groundwater mounding analysis, volumetric receptor capacity, and the design of the
overflow and/or bypass structures are judged by the site professional to be adequate to prevent
overtopping and meet the site suitability criteria specified in this section.
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5.3.6 Soil Physical and Chemical Suitability for Treatment (SSC-6)
The soil texture and design infiltration rates should be considered along with the physical and
chemical characteristics specified below to determine if the soil is adequate for removing the target
pollutants. The following soil properties must be carefully considered in making such a determination:
• Cation exchange capacity (CEC) of the treatment soil must be 5 milliequivalents
CEC/100 g dry soil (USEPA Method 9081). Consider empirical testing of soil sorption
capacity, if practicable. Ensure that soil CEC is sufficient for expected pollutant
loadings, particularly heavy metals. CEC values of >5 meq/100g are expected in
loamy sands, according to Rawls, et al. Lower CEC content may be considered if it is
based on a soil loading capacity determination for the target pollutants that is
accepted by the City.
• Depth of soil used for infiltration treatment must be a minimum of 18 inches.
• Organic Content of the treatment soil (ASTM D 2974): Organic matter can increase
the sorptive capacity of the soil for some pollutants. The site professional should
evaluate whether the organic matter content is sufficient for control of the target
pollutant(s).
• Waste fill materials should not be used as infiltration soil media nor should such
media be placed over uncontrolled or non-engineered fill soils.
Engineered soils may be used to meet the design criteria in this chapter and the performance goals
in Chapter 2 and Chapter 3. Field performance evaluation(s), using acceptable protocols, would be
needed to determine feasibility and acceptability by the City of Auburn.
5.3.7 Seepage Analysis and Control (SSC-7)
Determine whether there would be any adverse effects caused by seepage zones on nearby building
foundations, basements, roads, parking lots or sloping sites.
5.3.8 Cold Climate and Impact of Roadway Deicers (SSC-8)
For cold climate design criteria (snowmelt/ice impacts) refer to D. Caraco and R. Claytor, “Design
Supplement for Stormwater BMPs in Cold Climates”, Center for Watershed Protection, 1997.
Potential impact of roadway deicers on potable water wells must be considered in the siting
determination. Mitigation measures must be implemented if infiltration of roadway deicers can cause
a violation of groundwater quality standards.
5.3.9 Verification Testing of the Completed Facility
Verification testing of the completed full-scale infiltration facility is recommended to confirm that the
design infiltration parameters are adequate. The site professional should determine the duration and
frequency of the verification testing program including the monitoring program for the potentially
impacted groundwater. The groundwater monitoring wells installed during site characterization
(see 5.4) may be used for this purpose. Long-term (more than two years) in-situ drawdown and
confirmatory monitoring of the infiltration facility would be preferable. The City may require verification
testing on a case-by-case basis.
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5.4 Site Characterization
Use the following guidelines to determine if the site suitability criteria have been met.
5.4.1 Field Methods used to Determine Subsurface Characterization
5.4.1.1 Test Holes or Pits
• Dig test holes or pits to a depth below the base of the infiltration facility of at least
5 times the maximum design depth of ponded water proposed for the infiltration
facility,
• Conduct pit/hole exploration during the wet season (December 1st through April 30th)
to provide accurate groundwater saturation and groundwater information.
• Take soil samples (representative samples from each soil type and/or unit within the
infiltration receptor) to a depth below the base of the infiltration facility of 2.5 times
the maximum design ponded water depth, but not less than 6 feet.
o For basins, at least one test pit or test hole per 5,000 ft2 of basin infiltrating
surface (in no case less than two per basin) is required.
o For trenches, at least one test pit or test hole per 50 feet of trench length (in
no case less than two per trench) is required.
The depth and number of test holes or test pits and samples should be increased if, in the judgment
of a licensed engineer with geotechnical expertise (P.E.) or other licensed professional acceptable to
the City, the conditions are highly variable and such increases are necessary to accurately estimate
the performance of the infiltration system. The exploration program may also be decreased if, in the
opinion of the licensed engineer or other professional, the conditions are relatively uniform and the
borings/test pits omitted will not influence the design or successful operation of the facility. In high
water table sites the subsurface exploration sampling need not be conducted lower than two (2) feet
below the groundwater table.
• Prepare detailed logs for each test pit or test hole and a map showing the location of
the test pits or test holes. Logs must include at a minimum, depth of pit or hole, soil
descriptions, depth to water, presence of stratification.
At a minimum, soil characterization for each soil unit (soils of the same texture, color, density,
compaction, consolidation and permeability) encountered shall include:
o Grain-size distribution (ASTM D422 or equivalent AASHTO specification).
o Textural class (USDA) (see Volume III, Figure III-2-5).
o Percent clay content (include type of clay, if known).
o Cation exchange capacity (CEC) and organic matter content for each soil
type and strata. Where distinct changes in soil properties occur, to a depth
below the base of the facility of at least 2.5 times the maximum design water
depth, but not less than 6 feet. Consider if soils are already contaminated,
thus diminishing pollutant sorptive capacity (for water quality design only).
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o For soils with low CEC and organic content, deeper characterization of soils
may be warranted.
o Color/mottling.
o Variations and nature of stratification.
5.4.1.2 Infiltration Rate Determination
Determine the representative infiltration rate of the unsaturated vadose zone based on field infiltration
tests and/or grain size/texture determinations. Field infiltration rates can be determined using the Pilot
Infiltration Test (see PIT-Appendix B). Such site testing should be considered to verify infiltration rate
estimates based on soil size distribution and/or texture. Infiltration rates may also be estimated based
on soil grain-size distributions from test pits or test hole samples. This may be particularly useful
where a sufficient source of water does not exist to conduct a pilot infiltration test. As a minimum, one
soil grain-size analysis per soil stratum in each test hole shall be performed within 2.5 times the
maximum design water depth, but not less than 6 feet.
The infiltration rate is needed for routing and sizing purposes and for classifying the soil for treatment
adequacy.
5.4.1.3 Infiltration Receptor
Infiltration receptor (unsaturated and saturated soil receiving the storm water) characterization should
include:
• Installation of groundwater monitoring wells. Use at least three per infiltration facility,
or three hydraulically connected surface and groundwater features. This will
establish a three-dimensional relationship for the groundwater table, unless the
highest groundwater level is known to be at least 50 feet below the proposed
infiltration facility. The monitoring wells will:
o Monitor the seasonal groundwater levels at the site during at least one wet
season, and,
o Consider the potential for both unconfined and confined aquifers, or confining
units, at the site that may influence the proposed infiltration facility as well as
the groundwater gradient. Other approaches to determine groundwater levels
at the proposed site could be considered if pre-approved by the City, and,
o Determine the ambient groundwater quality, if that is a concern.
• An estimate of the volumetric water holding capacity of the infiltration receptor soil.
This is the soil layer below the infiltration facility and above the seasonal high-water
mark, bedrock, hardpan, or other low permeability layer. This analysis should be
conducted at a conservatively high infiltration rate based on vadose zone porosity,
and the water quality runoff volume to be infiltrated. This, along with an analysis of
groundwater movement, will be useful in determining if there are volumetric
limitations that would adversely affect drawdown.
• Depth to groundwater table and to bedrock/impermeable layers.
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• Seasonal variation of groundwater table based on well water levels and observed
mottling.
• Existing groundwater flow direction and gradient.
• Lateral extent of infiltration receptor.
• Horizontal hydraulic conductivity of the saturated zone to assess the aquifer’s ability
to laterally transport the infiltrated water.
• Impact of the infiltration rate and volume at the project site on groundwater
mounding, flow direction, and water table; and the discharge point or area of the
infiltrating water. A groundwater mounding analysis shall be conducted at all sites
where the depth to seasonal groundwater table or low permeability stratum is less
than 15 feet and the runoff to the infiltration facility is from more than one acre. The
site professional can consider conducting an aquifer test, or slug test and the type of
groundwater mounding analysis necessary at the site.
5.4.2 Design Infiltration Rate Determination
Infiltration rates for treatment can be determined using either a correlation to grain size distribution
from soil samples, textural analysis, or by in-situ field measurements.
Infiltration rate design information shall be prepared and sealed by a licensed engineer or licensed
geologist and submitted to the City along with the Stormwater Site Plan.
5.4.2.1 Three Methods for Determining Long-term Infiltration Rate for Sizing the
Infiltration Basin, Trench, or Swale
For designing the infiltration facility, the site professional should select one of the three methods
described below that will best represent the long-term infiltration rate at the site. The long-term
infiltration rate should be used for routing and sizing the basin/trench for the maximum drawdown
time of 24 hours. Verification testing of the completed facility is strongly encouraged using the criteria
described in Section 5.3.9.
Method 1 — USDA Soil Textural Classification
The infiltration rates provided in Volume III, Table III-2-7 represents rates for homogeneous soil
conditions. If more than one soil unit is encountered within 6 feet of the base of the facility, or
2.5 times the proposed maximum water design depth, use the lowest infiltration rate determined from
each of the soil units as the representative site infiltration rate.
• Use Volume III, Figure III-2-5 (USDA Textural Triangle) to determine the soil textural
classification. Only soils passing the #10 sieve may be used to determine the
percentages of sand, silt, and clay.
• More information on the USDA textural classification can be found in the Soils
Survey Manual (U.S. Department of Agriculture, October 1993).
• Use the textural classification obtained from Volume III, Figure III-2-5 and Volume III,
Table III-2-7 to determine the short and long-term infiltration rates. Treatment site
suitability criteria (Section 5.3.4) must be met.
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• Consider alternate correction factors if site conditions warrant. Alternate correction
factors require City approval.
In no case shall a correction factor less than 2.0 be used.
Correction factors higher than those provided in Volume III, Table III-2-7 should be
considered for situations where: long-term maintenance will be difficult to implement; where
little or no pretreatment is anticipated, where site conditions are highly variable or uncertain,
or where soil mottling exists that cannot be fully represented in the soil gradation tests. These
situations require the use of best professional judgment by the site engineer and the approval
of the City. An operation and maintenance plan and a financial bonding plan may also be
required by the City.
Use Method 2 if the soil gradation was determined using ASTM D422.
Method 2 — ASTM Gradation Testing at Full Scale Infiltration Facilities
This method may not be used for soils with a d10 size less than 0.05 mm (U.S. Standard Sieve).
The infiltration rates provided in Volume III, Table III-2-8 represent rates for homogeneous soil
conditions. If more than one soil unit is encountered within 6 feet of the base of the facility, or
2.5 times the proposed maximum water design depth, use the lowest infiltration rate determined from
each of the soil units as the representative site infiltration rate.
1. Determine the long-term infiltration rate using the d10 value obtained from ASTM D422
and Volume III, Table III-2-8.
2. Consider alternate correction factors if site conditions warrant. Alternate correction
factors require City approval.
3. Correction factors higher than those provided in Volume III, Table III-2-7 should be
considered for situations where: long-term maintenance will be difficult to implement;
where little or no pretreatment is anticipated, where site conditions are highly variable or
uncertain, or where soil mottling exists that cannot be fully represented in the soil
gradation tests. These situations require the use of best professional judgment by the
site engineer and the approval of the City. An operation and maintenance plan and a
financial bonding plan may also be required by the City.
Method 3 - In-situ Infiltration Measurements or Pilot Infiltration Tests (PIT)
Where feasible, use in-situ infiltration measurements. Use a procedure such as the Pilot Infiltration
Test (PIT) described in Appendix B.
As with the previous methods, the infiltration rate obtained from the PIT shall be considered to be a
short-term rate. To obtain long-term infiltration rates the short-term rates must be reduced by applying
a total correction factor. The total correction factor is the sum of the partial correction factors,
presented in Table V-B-17 (Appendix B), which account for site variability, number of tests
conducted, degree of long-term maintenance, influent pretreatment/control, and potential for long-
term clogging due to siltation and bio-buildup.
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The range of partial correction factors is for general guidance only. The specific partial correction
factors used shall be determined based on the professional judgment of the licensed engineer or
other site professional considering all issues which may affect the long-term infiltration rate, subject to
the approval of the City.
Use the following guidance to determine the partial correction factors to apply from Table V-B-17
(Appendix B).
• Site variability and number of locations tested. The number of locations tested must
represent the subsurface conditions throughout the facility site. If the range of
uncertainty is low (for example, conditions are known to be uniform through previous
exploration and site geological factors), one pilot infiltration test may be adequate to
justify a partial correction factor at the low end of the range. If the level of uncertainty
is high, due to highly variable site conditions or limited local testing data, a partial
correction factor near the high end of the range may be appropriate. This might be
the case where the site conditions are highly variable due to a deposit of ancient
landslide debris, or buried stream channels. In these cases, even with many
explorations and several pilot infiltration tests, the level of uncertainty may still be
high.
• Degree of long-term maintenance to prevent siltation and bio-buildup. The standard
of comparison here is the long-term maintenance requirements provided in Volume I,
Appendix D and any additional requirements by the City. Full compliance with these
requirements would be justification to use a partial correction factor at the low end of
the range. If there is a high degree of uncertainty that long-term maintenance will be
carried out consistently, or if the maintenance plan is poorly defined, a partial
correction factor near the high end of the range may be justified.
• Degree of influent control to prevent siltation and bio-buildup. A partial correction
factor near the high end of the range may be justified under the following
circumstances:
o If the infiltration facility is located in a shady area where moss or litter fall,
buildup from the surrounding vegetation is likely and cannot be easily
controlled through long-term maintenance.
o If there is minimal pretreatment, and the influent is likely to contain
moderately-high TSS levels.
The determination of long-term design infiltration rates from in-situ infiltration test data involves a
considerable amount of engineering judgment. Therefore, when reviewing or determining the final
long-term design infiltration rate, the City may consider the results of both textural analyses and in-
situ infiltration tests results when available.
5.4.2.2 General Sizing Criteria
This information is applicable to infiltration basins, trenches, and bio-infiltration facilities.
Size the device by routing 91% of the runoff volume, as predicted by the WWHM through the facility.
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Infiltration facilities for treatment can be located upstream or downstream of detention and can be off-
line or on-line.
• On-line treatment facilities placed upstream or downstream of a detention facility
must be sized to infiltrate 91% of the runoff file volume directed to it.
• Off-line treatment facilities placed upstream of a detention facility must have a flow
splitter designed to send all flows at or below the 15-minute water quality flow rate,
as predicted by WWHM, to the treatment facility. The treatment facility must be sized
to infiltrate all the runoff sent to it (no overflows from the treatment facility are
allowed).
• Off-line treatment facilities placed downstream of a detention facility must have a
flow splitter designed to send all flows at or below the 2-year flow frequency from the
detention pond, as predicted by WWHM, to the treatment facility. The treatment
facility must be sized to infiltrate all the runoff sent to it (no overflows from the
treatment facility are allowed).
5.4.2.3 General Design Criteria
• Slope of the base of the infiltration facility should be < 3 percent.
• Spillways/overflow structures – A non-erodible outlet or spillway with a firmly
established elevation must be constructed to discharge overflow. Ponding depth,
drawdown time, and storage volume are calculated from that reference point. Refer to
Volume III, Section 2.3 for overflow structure design details.
• For infiltration treatment facilities, side-wall seepage is not a concern if seepage occurs
through the same stratum as the bottom of the facility. However, for engineered soils
or for soils with very low permeability, the potential to bypass the treatment soil
through the side-walls may be significant. In those cases, the side-walls must be lined,
either with an impervious liner or with at least 18 inches of treatment soil, to prevent
seepage of untreated flows through the side walls.
5.4.2.4 General Construction Criteria
• Initially excavate to within 1-foot of the final floor elevation of the infiltration facility. Do
not excavate to the final grade until all disturbed areas in the upgradient watershed
have been stabilized or protected. Remove all accumulated sediment in the final phase
of excavation.
• Post-construction, all water must be conveyed through a pretreatment device to
prevent sedimentation.
• Infiltration facilities should not be used as temporary sediment traps during
construction.
• Use light-tracked equipment for excavation to avoid compaction of the floor of the
infiltration facility. The use of draglines and trackhoes should be considered. The
infiltration area should be flagged or marked to keep equipment away.
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5.4.2.5 Maintenance Criteria
• Make provision for regular and perpetual maintenance of the infiltration basin/trench,
including replacement and/or reconstruction of the treatment infiltration medium.
• Include access for operation and maintenance in system design.
• Include an operation and maintenance plan. The plan must be approved by the City.
• Remove accumulated debris/sediment every 6 months or as needed to prevent
clogging, or when water remains in the pond for greater than 24 hours.
• Replace or amend the treatment soil as needed to ensure maintaining adequate
treatment capacity.
5.4.2.6 Verification of Performance
During the first 1 to 2 years of operation, verification testing as specified in Section 5.3.9 is strongly
recommended, along with a maintenance program that achieves expected performance levels.
Operating and maintaining groundwater monitoring wells is also strongly encouraged.
In order to determine compliance with the flow control requirements, the WWHM must be used.
5.5 BMPs for Infiltration and Bio-infiltration Treatment
The three BMPs discussed below are recognized currently as effective treatment techniques using
infiltration and bio-infiltration. Selection of a specific BMP should be based on the Treatment Facility
Menus provided in Chapter 2.
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5.5.1 BMP T710 Infiltration Basins
5.5.1.1 Description
Infiltration basins are earthen impoundments used for the collection, temporary storage and
infiltration of incoming stormwater runoff.
5.5.1.2 Design Criteria Specific for Basins
• Complete a site suitability analysis per Section 5.3.
• Provide access for vehicles to easily maintain the forebay (presettling basin) area
and not disturb vegetation, or resuspend sediment any more than is absolutely
necessary.
• The slope of the basin bottom should not exceed 3% in any direction.
• Provide a minimum of one foot of freeboard when establishing the design ponded
water depth. Freeboard is measured from the rim of the infiltration facility to the
maximum ponding level or from the rim down to the overflow point if overflow or a
spillway is included.
• Establish vegetation on the basin floor and side slopes to prevent erosion and
sloughing and to provide additional pollutant removal. Provide erosion protection
of inflow points to the basin (e.g., riprap, flow spreaders, energy dissipators (See
Chapter 3). Refer to Chapter 7 for recommended vegetation.
• Lining material – Basins can be open or covered with a 6 to 12-inch layer of filter
material such as coarse sand, or a suitable filter fabric to help prevent the buildup
of impervious deposits on the soil surface. A nonwoven geotextile should be
selected that will function sufficiently without plugging (see geotextile
specifications in Appendix C). The filter layer shall be replaced or cleaned when/if
it becomes clogged.
• Stabilize and plant embankment, emergency spillways, spoil and borrow areas,
and other disturbed areas. Without healthy vegetation the surface soil pores
would quickly plug.
5.5.1.3 Maintenance Criteria for Basins
• Maintain basin floor and side slopes to promote dense turf with extensive root
growth. This enhances infiltration, prevents erosion and consequent sedimentation of
the basin floor, and prevents invasive weed growth. Immediately stabilize and
revegetate any bare spots.
• Vegetation growth should not be allowed to exceed 18 inches in height. Mow the
slopes periodically and check for clogging, and erosion. Remove clippings from
mowing, weeding and pruning operations.
• Seed mixtures should be the same as those recommended in Table V-7-12. The use
of slow-growing, stoloniferous grasses will permit long intervals between mowing.
Mowing twice a year is generally satisfactory. Fertilizers are not allowed.
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5.5.2 BMP T720 Infiltration Trenches
5.5.2.1 Description
Infiltration trenches are generally at least 24 inches wide, and are backfilled with a coarse stone
aggregate, allowing for temporary storage of stormwater runoff in the voids of the aggregate material.
Stored runoff gradually infiltrates into the surrounding soil. The surface of the trench can be covered
with grating and/or consist of stone, gabion, sand, or a grassed covered area with a surface inlet.
Perforated rigid pipe of at least 8-inch diameter can also be used to distribute the stormwater in a
stone trench.
5.5.2.2 Design Criteria
• Complete a site suitability analysis per Section 5.3.
• Include an access port or open or grated top for accessibility to conduct inspections
and maintenance.
• Use clean aggregate with a maximum diameter of 3 inches and a minimum diameter
of 1.5 inches to fill trench. Void space for these aggregates should be in the range of
30 to 40 percent.
• Line sides of trench with an engineered geotextile material. Geotextile should
surround all of the aggregate fill material except for the top one-foot, which is placed
over the geotextile. Geotextile fabric with acceptable properties must be carefully
selected to avoid plugging (see Appendix C).
• The bottom sand or geotextile fabric is optional.
• Overflow Channel - Because an infiltration trench is generally used for small
drainage areas, an emergency spillway is not necessary. However, a non-erosive
overflow channel leading to a stabilized watercourse should be provided.
• Trench can be placed under a porous or impervious surface cover to conserve
space.
• Install an observation well at the lower end of the infiltration trench to check water
levels, drawdown time, sediment accumulation, and conduct water quality
monitoring. Figure V-5-7 illustrates observation well details. It should consist of a
perforated PVC pipe which is 4 to 6 inches in diameter and it should be constructed
flush with the ground elevation. For larger trenches a 12-36 inch diameter well can
be installed to facilitate maintenance operations such as pumping out the sediment.
Cap the top of the well to discourage vandalism and tampering.
5.5.2.3 Construction Criteria
• Trench Preparation –Place excavated materials away from the trench sides to
enhance trench wall stability. Keep excavated material away from slopes,
neighboring property, sidewalks and streets.
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• Stone Aggregate Placement and Compaction – Place the stone aggregate in lifts
and compact using plate compactors. As a rule of thumb, a maximum loose lift
thickness of 12 inches is recommended. The compaction process ensures geotextile
conformity to the excavation sides, thereby reducing potential piping and geotextile
clogging, and settlement problems.
• Potential Contamination - Prevent natural or fill soils from intermixing with the
stone aggregate. Remove all contaminated stone aggregate and replace with
uncontaminated stone aggregate.
• Overlapping and Covering - Following the stone aggregate placement, fold the
geotextile over the stone aggregate to form a 12-inch-minimum longitudinal overlap.
When overlaps are required between rolls, overlap the upstream roll a minimum of
2 feet over the downstream roll in order to provide a shingled effect.
• Voids behind Geotextile – Avoid voids between the geotextile and excavation
sides. Remove boulders or other obstacles from the trench walls. Place natural soils
in voids at the most convenient time during construction to ensure geotextile
conformity to the excavation sides. Soil piping, geotextile clogging, and possible
surface subsidence will be avoided by this remedial process.
• Unstable Excavation Sites - Vertically excavated walls may be difficult to maintain
in areas where the soil moisture is high or where soft or cohesionless soils
predominate. Trapezoidal, rather than rectangular, cross-sections may be needed.
5.5.2.4 Maintenance Criteria
• Monitor sediment buildup in the top foot of stone aggregate or the surface inlet.
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5.5.3 BMP T730 Bio-infiltration Swale
5.5.3.1 Description
Bio-infiltration swales, also known as Grass Percolation Areas, combine grassy vegetation and soils
to remove stormwater pollutants by percolation into the ground. Their pollutant removal mechanisms
include filtration, soil sorption, and uptake by vegetative root zones.
In general, bio-infiltration swales are used for treating stormwater runoff from roofs, roads and parking
lots. Runoff volumes greater than water quality design volume are typically overflowed to the
subsurface through an appropriate conveyance facility such as a dry well, or an overflow channel to
surface water. Overflows that are directed to a surface water must meet Minimum Requirement #7 or
#8 (whichever is applicable). If applicable, see Volume I, Chapter 2.
5.5.3.2 Additional Design Criteria Specific for Bio-infiltration Swales
• Complete a site suitability analysis per Section 5.3.
• Swale bottom: flat with a longitudinal slope less than 1%.
• Maximum ponded level: 6 inches.
• The design soil thickness may be reduced to as low as 6 inches if appropriate
performance data demonstrates that the vegetated root zone and the natural soil can
be expected to provide adequate removal and loading capacities for the target
pollutants. The design professional should calculate the pollutant loading capacity of
the treatment soil to estimate if there is sufficient treatment soil volume for an
acceptable design period.
• Other combinations of treatment soil thickness, CEC, and organic content design
factors can be considered if it is demonstrated that the soil and vegetation will
provide a target pollutant loading capacity and performance level acceptable.
• The treatment zone depth must contain sufficient organics and texture to ensure
good growth of the vegetation.
• If demonstrated that 6 inches of treatment depth is sufficient, the maximum infiltration
rate is 1 in/hr. In these cases, all other portions of the Site Suitability Criteria apply.
• Use native or adapted grass.
• Pretreat to prevent the clogging of the treatment soil and/or growth of the vegetation.
• Identify pollutants, particularly in industrial and commercial area runoff, that could
cause a violation of Ecology's Groundwater Quality Standards (Chapter 173-200
WAC). Include appropriate mitigation measures (pretreatment, source control, etc.)
for those pollutants.
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Figure V-5-7. Observation Well Details
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Chapter 6 Sand Filtration Treatment Facilities
6.1 Purpose
This chapter presents criteria for the design, construction and maintenance of runoff treatment sand
filters. Treatment sand filters are used to collect, treat and remove TSS, phosphorous, and insoluble
organics (including oils) from stormwater.
Sand filtration options discussed in this Chapter are:
Basic Sand Filter
Large Sand Filter
BMP T810 Sand Filter Vault
BMP T820 Linear Sand Filter
6.2 Description
A typical sand filtration system consists of a pretreatment facility, flow spreader(s), a sand bed, and
the underdrain piping. The sand filter bed includes a geotextile fabric between the sand bed and
the bottom underdrain system.
An impermeable liner under the facility may also be needed if the filtered runoff requires additional
treatment to remove soluble groundwater pollutants, or in cases where additional groundwater
protection is mandated. The variations of a sand filter include a basic or large sand filter, sand filter
with level spreader, sand filter vault, and linear sand filter. Figures V-6-8 through V-6-15 provide
examples of various sand filter configurations.
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Source: City of Austin, TX
Filtered Outflow (Route through detention basin)
Figure V-6-8. Sand Filtration Basin Preceded by Presettling Basin
(Variation of a Basic Sand Filter)
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Figure V-6-9. Sand Filter with a Pretreatment Cell (top view)
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Figure V-6-10. Sand Filter with a Pretreatment Cell (side view)
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Figure V-6-11. Sand Filter with Level Spreader (top view)
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Figure V-6-12. Sand Filter with Level Spreader (side view)
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6.3 Applications and Limitations
Sand filtration can be used in most residential, commercial, and industrial developments where
debris, heavy sediment loads, and oils and greases will not clog or prematurely overload the sand, or
where adequate pretreatment is provided for these pollutants.
Pretreatment is necessary to reduce velocities to the sand filter and remove debris, floatables, large
particulate matter, and oils. In high water table areas, adequate drainage of the sand filter may
require additional engineering analysis and design considerations. An underground filter should be
considered in areas subject to freezing conditions. (Urbonas, 1997)
6.4 Site Suitability
The following site characteristics should be considered in siting a sand filtration system:
• Space availability, including the space needed for a presettling basin.
• Sufficient hydraulic head, at least 4 feet from inlet to outlet.
• Adequate Operation and Maintenance capability including accessibility for O&M.
• Sufficient pretreatment of oil, debris and solids in the tributary runoff.
6.5 Design Criteria
6.5.1 Objective
The objective is to capture and treat 91% of the total runoff volume (95% for large sand filters) as
predicted by the Western Washington Hydrology Model (WWHM).
6.5.2 Sand Filter Sizing
Sand filter design criteria are as follows:
1. The design hydraulic conductivity shall be 1 in/hr.
2. On-line sand filters must NOT be placed upstream of a detention facility. This is to prevent
exposure of the sand filter surface to high flow rates that could cause loss of media and
previously removed pollutants.
3. On-line sand filters placed downstream of a detention facility must be sized using WWHM to
filter 91% of the runoff volume.
4. Off-line sand filters placed upstream of a detention facility must have a flow splitter designed
to send all flows at or below the 15-minute water quality flow rate, as predicted by WWHM to
the sand filter. The sand filter must be sized to filter all the runoff sent to it (no overflows from
the treatment facility should occur).
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5. Off-line sand filters placed downstream of a detention facility must have a flow splitter
designed to send all flows at or below the 2-year frequency peak flow as predicted by
WWHM, from the detention facility to the treatment facility. The treatment facility must be
sized to filter all the runoff sent to it (no overflows from the treatment facility should occur).
6. Include an overflow in the design. The overflow height shall be at the maximum hydraulic
head of the water above the sand bed.
7. Pretreat runoff to be treated by the sand filter (e.g., presettling basin, etc. depending on
pollutants) to remove debris and other solids, and oil from high use sites.
8. Design inlet bypass and flow spreading structures (e.g., flow spreaders, weirs or multiple
orifice openings) to capture the applicable design flow rate, minimize turbulence and to
spread the flow uniformly across the surface of the sand filter. Install stone riprap or other
energy dissipation devices to prevent gouging of the sand medium and to promote uniform
flow. Include emergency spillway or overflow structures (see Volume III).
9. Include underdrain piping in sand filter design. Types of underdrains include a central
collector pipe with lateral feeder pipes; or a geotextile drain strip in an 8-inch gravel backfill or
drain rock bed; or longitudinal pipes in an 8-inch gravel backfill or drain rock with a collector
pipe at the outlet end.
• Upstream of detention, underdrain piping shall be sized to handle double the
two-year return frequency flow indicated by the WWHM (the doubling factor is
a factor of safety). Downstream of detention the underdrain piping shall be
sized for the two-year return frequency flow indicated by the WWHM. In both
instances there shall be at least one foot of hydraulic head above the invert of
the upstream end of the collector pipe.
• Internal diameters of underdrain pipes shall be a minimum of six inches
having two rows of ½-inch holes spaced 6 inches apart longitudinally
(maximum), with rows 120 degrees apart (laid with holes downward).
Maximum perpendicular distance between two feeder pipes must be 15 feet.
All piping is to be schedule 40 PVC or greater wall thickness. Drain piping
could be installed in basin and trench configurations. Other equivalent
underdrains can be used.
• Main collector underdrain pipe shall be at a slope of 0.5 percent minimum.
• A geotextile fabric (specifications in Appendix C) must be used between the
sand layer and drain rock or gravel and placed so that 1-inch of drain
rock/gravel is above the fabric. Drain rock shall be 0.75-1.5 inch rock or
gravel backfill, washed free of clay and organic material.
10. Provide cleanout wyes with caps or junction boxes at both ends of the collector pipes. Extend
cleanouts to the surface of the filter. Provide a valve box for access to the cleanouts. Provide
access for cleaning all underdrain piping. This may consist of installing cleanout ports that tee
into the underdrain system and surface above the top of the sand bed. To facilitate
maintenance of the sand filter an inlet shutoff/bypass valve is recommended.
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11. Sand specification: The sand in a filter must meet the size gradation (by weight) given in
Table V-6-7. The contractor must obtain a grain size analysis from the supplier to certify that
the No. 100 and No. 200 sieve requirements are met.
Table V-6-7. Sand Specifications
U.S. Sieve Number Percent Passing
4 95-100
8 70-100
16 40-90
30 25-75
50 2-25
100 <4
200 <2
Source: King County Surface Water Design Manual, September 1998
12. Impermeable Liners for Sand Bed Bottom: Impermeable liners are generally required for
soluble pollutants such as metals and toxic organics and where the underflow could cause
problems with structures. Impermeable liners may be made of clay, concrete or
geomembrane. Clay liners shall have a minimum thickness of 12 inches and meet the
specifications given in Table V-6-8.
Table V-6-8. Clay Liner Specifications
Property Test Method Unit Specification
Permeability ASTM D-2434 cm/sec 1 x 10-6 max.
Plasticity Index of Clay ASTM D-423 & D-
424
percent Not less than
15
Liquid Limit of Clay ASTM D-2216 percent Not less than
30
Clay Particles Passing ASTM D-422 percent Not less than
30
Clay Compaction ASTM D-2216 percent 95% of
Standard
Proctor Density
Source: City of Austin, 1988
• If a geomembrane liner is used it shall have a minimum thickness of 30 mils
and be ultraviolet light resistant. The geomembrane liner shall be protected
from puncture, tearing, and abrasion by installing geotextile fabric on the top
and bottom of the geomembrane.
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• Concrete liners may also be used for sedimentation chambers and for
sedimentation and sand filtration basins less than 1,000 square feet in area.
Concrete shall be 5 inches thick Class A or better and shall be reinforced by
steel wire mesh. The steel wire mesh shall be 6 gauge wire or larger and 6-inch
by 6-inch mesh or smaller. An "Ordinary Surface Finish" is required. When the
underlying soil is clay or has an unconfined compressive strength of 0.25 ton per
square foot or less, the concrete shall have a minimum 6-inch compacted
aggregate base. This base must consist of coarse sand and river stone, crushed
stone or equivalent with diameter of 0.75- to 1-inch.
• If an impermeable liner is not required then a geotextile fabric liner shall be
installed that retains the sand and meets the specifications listed in Appendix C
unless the basin has been excavated to bedrock.
• If an impermeable liner is not provided, then an analysis shall be made of
possible adverse effects of seepage zones on groundwater, and near building
foundations, basements, roads, parking lots and sloping sites. Sand filters
without impermeable liners shall not be built on fill sites and shall be located at
least 20-feet downslope and 100-feet upslope from building foundations.
13. Include an access ramp with a slope not to exceed 7H:1V, or equivalent, for maintenance
purposes at the inlet and the outlet of a surface filter.
14. Side slopes for earthen/grass embankments shall not exceed 3H:1V to facilitate mowing.
15. There shall be at least 2 feet clearance between the seasonal high groundwater level and the
bottom of the sand filter.
6.6 Construction Criteria
• The sand filter shall not be placed into service until site construction is complete and
the site is stabilized.
• Place sand in a uniform thickness and compact using a water settling method.
Settling shall be accomplished by flooding the sand with 10-15 gallons of water per
cubic foot of sand. After flooding, the sand shall be smoothed and leveled.
6.7 Maintenance Criteria
Inspections of sand filters and pretreatment systems shall be conducted every 6 months and after
storm events as needed during the first year of operation, and annually thereafter. Maintenance
suggestions include:
• Remove accumulated silt and debris on top of the sand filter when depth exceeds
1/2-inch. Scrape the silt off during dry periods with steel rakes or other devices.
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• Frequent overflow or slow drawdown are indicators of plugging problems. A sand
filter shall empty in 24 hours following a storm event (24 hours for the pre-settling
chamber), depending on the ponding depth over the filter bed surface. If the
hydraulic conductivity drops to 1-inch per hour, corrective action is needed.
Examples include:
o Scraping the top layer of fine-grain sediment accumulation (mid-winter
scraping is suggested)
o Removal of thatch
o Aerating the filter surface
o Tilling the filter surface (late-summer rototilling is suggested)
o Replacing the top 4 inches of sand.
o Inspecting geotextiles for clogging.
• Rapid drawdown in the sand bed (greater than 12 inches per hour) indicates short-
circuiting of the filter. Inspect the periphery of the filter bed and cleanouts on the
underdrain pipes for leakage.
• Drawdown tests for the sand bed shall be conducted every two years. These tests
can be conducted by allowing the filter to fill (or partially fill) during a storm event,
then measuring the decline in water level over a 4-8 hour period. An inlet and an
underdrain outlet valve would be necessary to conduct such a test.
• Formation of rills and gullies on the surface of the filter indicates improper function of
the inlet flow spreader, or poor sand compaction. Check for accumulation of debris
on or in the flow spreader and refill rills and gullies with sand.
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6.7.1 BMP T810 Sand Filter Vault
6.7.1.1 Description: (Figure V-6-13 and Figure V-6-14)
A sand filter vault is similar to an open sand filter except that the sand layer and underdrains are
installed below grade in a vault. It consists of presettling and sand filtration cells.
6.7.1.2 Applications and Limitations
• Use where space limitations preclude above ground facilities
• Not suitable where high water table and heavy sediment loads are expected
• An elevation difference of 4 feet between inlet and outlet is needed
6.7.1.3 Additional Design Criteria for Vaults
• Optimize sand inlet flow distribution with minimal sand bed disturbance. A maximum
8-inch distance between the top of the spreader and the top of the sand bed is
required. Flows may enter the sand bed by spilling over the top of the wall into a flow
spreader pad or alternatively a pipe and manifold system may be used. Any pipe and
manifold system must retain the required dead storage volume in the first cell,
minimize turbulence, and be readily maintainable.
• If an inlet pipe and manifold system is used, the minimum pipe size shall be 8 inches.
Multiple inlets are required to minimize turbulence and reduce local flow velocities.
• Provide erosion protection along the first foot of the sand bed adjacent to the
spreader. Geotextile fabric secured on the surface of the sand bed, or equivalent
method, may be used.
• Design the presettling cell for sediment collection and removal. Use a V-shaped
bottom, removable bottom panels, or equivalent sludge handling system. Provide
one-foot of sediment storage in the presettling cell.
• Seal the pre-settling chamber to trap oil and trash. This chamber is usually
connected to the sand filtration chamber using a pipe with an inverted elbow to
protect the filter surface from oil and trash.
• If a retaining baffle is necessary for oil/floatables in the presettling cell, it must extend
at least one foot above to one foot below the design flow water level. Provide
provision for the passage of flows in the event of plugging. Provide access opening
and ladder on both sides of the baffle.
• To prevent anoxic conditions, provide a minimum of 24 square feet of ventilation
grate for each 250 square feet of sand bed surface area. For sufficient distribution of
airflow across the sand bed, grates may be located in one area if the sand filter is
small, but placement at each end is preferred. Small grates may also be dispersed
over the entire sand bed area.
• Sand filter vaults must conform to the materials and structural suitability criteria
specified for wet vaults in Chapter 8.
• Provide a sand filter inlet shutoff/bypass valve for maintenance
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• A geotextile fabric over the entire sand bed may be installed that is flexible, highly
permeable, three-dimensional matrix, and adequately secured. This is useful in
trapping trash and litter.
Figure V-6-13. Sand Filter Vault (top view)
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Figure V-6-14. Sand Filter Vault (side view)
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6.7.2 BMP T820 Linear Sand Filter
6.7.2.1 Description
Linear sand filters are typically long, shallow, two-celled, rectangular vaults. The first cell is designed
for settling coarse particles, and the second cell contains the sand bed. Stormwater enters the
second cell via a weir section that also functions as a flow spreader.
Figure V-6-15 illustrates a linear sand filter.
6.7.2.2 Application and Limitations
• Applicable in long narrow spaces such as the perimeter of a paved surface.
• As a part of a treatment train such as downstream of a filter strip, upstream of an
infiltration system, or upstream of a wet pond or a biofilter for oil control.
• To treat small drainages (less than 2 acres of impervious area).
• To treat runoff from high-use sites for TSS and oil/grease removal, if applicable.
6.7.2.3 Additional Design Criteria for Linear Sand Filters
• Divide the two cells by a divider wall that is level and extends a minimum of
12 inches above the sand bed.
• Stormwater may enter the sediment cell by sheet flow or a piped inlet.
• The width of the sand cell must be 1-foot minimum to 15 feet maximum.
• The sand filter bed must be a minimum of 12 inches deep and have an 8-inch layer
of drain rock with perforated drainpipe beneath the sand layer.
• The drainpipe must be 6-inch diameter minimum and be wrapped in geotextile and
sloped a minimum of 0.5 percent to promote positive drainage.
• Maximum sand bed ponding depth: 12 inches.
• Must be vented as described above for sand filter vaults.
• Linear sand filters must conform to the materials and structural suitability criteria
specified for wet vaults described in Chapter 8.
• Sediment cell width shall be selected based on sand filter width as follows:
Sand filter width, (w) inches 12-24 24-48 48-72 72+
Sediment cell width, inches 12 18 24 w/3
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Figure V-6-15. Linear Sand Filter
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Chapter 7 Biofiltration Treatment Facilities
7.1 Purpose
This chapter addresses five Best Management Practices (BMPs) that are classified as biofiltration
treatment facilities.
Biofilters are vegetated treatment systems (typically grass) that remove pollutants by means of
sedimentation, filtration, soil sorption, and/or plant uptake. They are typically configured as swales or
flat filter strips.
The BMPs discussed in this chapter are designed to remove low concentrations and quantities of
total suspended solids (TSS), heavy metals, petroleum hydrocarbons, and/or nutrients from
stormwater.
7.2 Applications
A biofilter can be used as a basic treatment BMP for stormwater runoff from roadways, driveways,
parking lots, and highly impervious ultra-urban areas or as the first stage of a treatment train. In
cases where hydrocarbons, high TSS concentrations, or debris would be present in the runoff, such
as high-use sites, a pretreatment system for those components is necessary. Placement of the
biofilter in an off-line location is preferred to avoid flattening of the vegetation and the erosive effects
of high flows.
7.3 Site Suitability
The following factors must be considered for determining site suitability:
• Accessibility for operation and maintenance.
• Suitable growth environment (soil, exposure to sunlight, etc.) for the vegetation.
• Adequate siting for a pre-treatment facility if high petroleum hydrocarbon levels
(oil/grease) or high TSS loads could impair treatment capacity or efficiency.
7.4 Best Management Practices
The following five Biofiltration Treatment Facilities BMPs are discussed in this chapter:
BMP T910 – Basic Biofiltration Swale
BMP T920 – Wet Biofiltration Swale
BMP T930 – Continuous Inflow Biofiltration Swale
BMP T940 – Basic Filter Strip & Compost-Amended Filter Strip
BMP T950 – Narrow Area Filter Strip
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7.4.1 BMP T910 Basic Biofiltration Swale
7.4.1.1 Description:
Biofiltration swales are typically shaped as a trapezoid or a parabola in cross section as shown in
Figure V-7-16 and Figure V-7-17.
Figure V-7-16. Typical Swale Section
7.4.1.2 Design Criteria:
• Size the swale using sizing criteria specified in Table V-7-9. Minimum length shall be
100 feet.
• Check the hydraulic capacity/stability using Qmax.
• Select a vegetation cover suitable for the site. Refer to Table V-7-12 through Table
V-7-14.
• Install level spreaders (minimum 1 inch gravel) at the head of all swales, and every
50 feet in swales of 4 feet width. Include sediment cleanouts (weir, settling basin, or
equivalent) at the head of the biofilter as needed.
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• Use energy dissipaters (such as quarry spalls or riprap) for increased downslopes.
• A minimum of ten feet (10’) shall be provided between the outside edge of the
bioswale and any property line or obstruction that would impede maintenance.
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Source: Livingston, et al, 1984
Figure V-7-17. Geometric Formulas for Common Swale Shapes
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Table V-7-9. Sizing Criteria
Design parameter BMP T 910-Biofiltration swale BMP T 940-Filter strip
Longitudinal Slope 0.015 - 0.0251 0.01 - 0.15
Maximum velocity 1 ft / sec @ K2 multiplied by
the WQ design flow rate
0.5 ft / sec @ K multiplied by
the WQ design flow rate
Maximum velocity for channel
stability3
3 ft/sec ---
Maximum water depth4 2”- if mowed frequently; 4” if
mowed infrequently
1-inch max.
Manning coefficient (0.2 – 0.3)5 (0.24 if mowed
infrequently)
0.35 (0.45 if compost
amended, or mowed to
maintain grass height 4”)
Bed width (bottom) (2 - 10 ft)6 ---
Freeboard height 0.5 ft ---
Minimum hydraulic residence
time at K multiplied by Water
Quality Design Flow Rate
9 minutes (18 minutes for
continuous inflow)
9 minutes
Minimum length 100 ft Sufficient to achieve hydraulic
residence time in the filter strip
Maximum sideslope 3 H:1 V
4H:1V preferred
Inlet edge 1” lower than
contributing paved area
Max. tributary drainage flowpath --- 150 feet
Max. longitudinal slope of
contributing area
--- 0.05 (steeper than 0.05 need
upslope flow spreading and
energy dissipation)
Max. lateral slope of contributing
area
--- 0.02 (at the edge of the strip
inlet)
1 For swales, if the slope is less than 1.5% install an underdrain using a perforated pipe, or equivalent. Amend the soil
if necessary to allow effective percolation of water to the underdrain. Install the low-flow drain 6-inches deep in the
soil. Slopes greater than 2.5% need check dams (riprap) at vertical drops of 12-15 inches. Underdrains can be made
of 6-inch Schedule 40 PVC perforated pipe or equivalent with 6” of drain gravel on the pipe. The gravel and pipe
must be enclosed by geotextile fabric (see Figure V-7-19 and Figure V-7-20).
2 K=A ratio of the peak 10-minunte flow predicted by SBUH to the water quality design flow rate estimated using the
WWHM. The value of K for off-line systems is 3.5, and for on-line systems is 2.0 in the City of Auburn.
3 Maximum flowrate for channel stability shall be the 100-year, 24-hour discharge (Q100) calculated with WWHM using
a 15-minute time step. If an hourly time step is used, multiply the Q100 by 1.6.
4 Below the design water depth install an erosion control blanket, at least 4” of topsoil, and the selected biofiltration
mix. Above the water line use a straw mulch or sod.
5 This range of Manning’s n can be used in the equation; b = Qn/1.49y(1.67) s(0.5) – Zy with wider bottom width b, and
lower depth, y, at the same flow. This provides the designer with the option of varying the bottom width of the swale
depending on space limitations. Designing at the higher n within this range at the same flow decreases the hydraulic
design depth, thus placing the pollutants in closer contact with the vegetation and the soil.
6 For swale widths up to 16 feet the cross-section can be divided with a berm (concrete, plastic, compacted earthfill)
using a flow spreader at the inlet (Figure V-7-21).
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7.4.1.3 Bypass Guidance
Most biofiltration swales are currently designed to be on-line facilities. However, an off-line design is
possible. Swales designed in an off-line mode should not engage a bypass until the flow rate
exceeds a value determined by multiplying Q, the off-line water quality design flow rate predicted by
the WWHM, by 3.5 for off-line systems and 2.0 for on-line systems. This modified design flow rate is
an estimate of the design flow rate determined by using SBUH procedures. Ecology’s intent is to
maintain recent biofiltration sizing recommendations until more definitive information is collected
concerning bioswale performance. The only advantage of designing a swale to be off-line is that the
stability check, which may make the swale larger, is not necessary.
7.4.1.4 Sizing Procedure for Biofiltration Swales
Preliminary Steps
P.1 Determine the water quality design flow rate (Q) in 15-minute time steps using WWHM.
P.2 Establish the longitudinal slope of the proposed biofilter.
P.3 Select an appropriate vegetated cover for the site. Refer to Table V-7-12 through Table V-7-14.
Design Steps
D.1 Select the type of vegetation and depth of flow (based on frequency of mowing and type of
vegetation).
D.2 Select a value of Manning’s n.
D.3 Select swale shape.
D.4 Use a variation on Manning’s equation to solve for bottom width, b.
b 2.5Qn
1.49ys Zy1.670.5 -
Where:
Q = Water Quality Design flow rate in 15-minute time steps based on WWHM, (ft³/s, cfs)
n = Manning's n (dimensionless)
s = Longitudinal slope as a ratio of vertical rise/horizontal run (dimensionless)
y = depth of flow (ft)
b = bottom width of trapezoid (ft)
For a trapezoid, select a side slope Z of at least 3. Compute b and then top width T, where
T = b + 2yZ.
NOTE: Adjustment factor of 2.5 accounts for the differential between Water Quality design
flow rate and the SBUH design flow. This equation is used to estimate an initial cross-
sectional area. It does not affect the overall biofiltration swale size.
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If b for a swale is greater than 10 ft, either investigate how Q can be reduced, divide the flow
by installing a low berm, or arbitrarily set b = 10 ft and continue with the analysis. For other
swale shapes refer to Figure V-7-17.
D.5 Compute A
ATyrectangle= or Atrapezoid = by + Zy2
Afilter strip = Ty
Where:
A = cross-sectional area (ft²)
T = top width of trapezoid or width of a rectangle (ft)
y = depth of flow (ft)
b = bottom width of trapezoid (ft)
Z = side slope
D.6 Compute the flow velocity at design flow rate:
V = K (Q/A)
A = cross-sectional area (ft2)
K = A ratio of the peak 10-minute flow predicted by SBUH to the water quality
design flow rate estimated using the WWHM. The value of K for off-line
systems is 3.5 and for on-line systems is 2.0
Q = water quality design flow rate in 15-minute time steps based on WWHM.
If V >1.0 ft/sec (or V>0.5 ft/sec for a filter strip), repeat steps D-1 to D-6 until the condition is
met. A velocity greater than 1.0 ft/sec was found to flatten grasses, thus reducing filtration. A
velocity lower than this maximum value will allow a 9-minute hydraulic residence time
criterion in a shorter biofilter. If the value of V suggests that a longer biofilter will be needed
than space permits, investigate how Q can be reduced (e.g., use of low impact development
BMPs), or increase y and/or T (up to the allowable maximum values) and repeat the analysis.
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D.7 Compute the swale length (L, ft)
L = Vt (60 sec/min)
Where: t = hydraulic residence time (min)
V = flow velocity
Use t = 9 minutes for this calculation (use t = 18 minutes for a continuous inflow biofiltration
swale). If a biofilter length is greater than the space permits, follow the advice in step 6.
If a length less than 100 feet results from this analysis, increase it to 100 feet, the minimum
allowed. In this case, it may be possible to save some space in width and still meet all criteria.
This possibility can be checked by computing V in the 100 ft biofilter for t = 9 minutes,
recalculating A (if V < 1.0 ft/sec) and recalculating T.
D.8 If there are space constraints, the local government and the project proponent should consider
the following solutions (listed in order of preference):
a. Divide the site drainage to flow to multiple biofilters.
b. Use infiltration to provide lower discharge rates to the biofilter (only if the Site
Suitability Criteria in Section 5.3 of this volume are met).
c. Increase vegetation height and design depth of flow (note: the design must ensure
that vegetation remains standing during design flow).
d. Reduce the developed surface area to gain space for biofiltration.
e. Increase the longitudinal slope.
f. Increase the side slopes.
g. Nest the biofilter within or around another BMP.
Stability Check Steps
The stability check must be performed for the combination of highest expected flow and least
vegetation coverage and height. A check is not required for biofiltration swales that are located "off-
line" from the primary conveyance/detention system. Maintain the same units as in the biofiltration
capacity analysis.
The maximum permissible velocity for erosion prevention (Vmax) is 3 feet per second.
S.1 Perform the stability check for the 100-year, return frequency flow using 15-minute time steps
using WWHM. The designer can use the WWHM 100-yr. hourly peak flows times an adjustment
factor of 1.6 to approximate peak flows in 15-minute time steps.
S.2 Estimate the vegetation coverage ("good" or "fair") and height on the first occasion that the
biofilter will receive flow, or whenever the coverage and height will be least. Avoid flow
introduction during the vegetation establishment period by timing planting.
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S.3 Estimate the degree of retardance from Table V-7-10. When uncertain, be conservative by
selecting a relatively low degree of retardance.
Table V-7-10. Stability Check Steps (SC)
Guide for Selecting Degree of Retardance
Coverage Average Grass Height
(inches)
Degree of Retardance
Good 2 E. Very Low
2-6 D. Low
6-10 C. Moderate
11-24 B. High
30 A. Very High
Fair 2 E. Very Low
2-6 D. Low
6-10 D. Low
11-24 C. Moderate
30 B. High
* See Chow (1959), In addition, Chow recommended selection of retardance C for a
grass-legume mixture 6-8 inches high and D for a mixture 4-5 inches high. No
retardance recommendations have appeared for emergent wetland species.
Therefore, judgment must be used. Since these species generally grow less densely
than grasses, using a “fair” coverage would be a reasonable approach.
S.4 Select a trial Manning's n for the high flow condition. The minimum value for poor vegetation
cover and low height (possibly, knocked from the vertical by high flow) is 0.033. A good initial
choice under these conditions is 0.04.
S.5 Refer to Figure V-7-18 to obtain a first approximation for VR.
S.6 Compute hydraulic radius, R, from VR in Figure V-7-18 and a Vmax in Table V-7-11.
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Source: Livingston, et al, 1984
VR (feet 2 /second)
Figure V-7-18. The Relationship of Manning’s n with VR
for Various Degrees of Flow Retardance (A-E)
Table V-7-11. Guide to Selecting Maximum Permissible Swale Velocities for Stability*
Max Velocity – ft/sec (m/sec)
Cover
Slope
(percent) Erosion-Resistant Soils Easily Eroded Soils
Kentucky bluegrass
Tall fescue 0-5 6 (1.8) 5 (1.5)
Kentucky bluegrass
Ryegrasses
Western wheatgrass
5-10 5 (1.5) 4 (1.2)
0-5 5 (1.5) 4 (1.2) Grass-legume mixture 5-10 4 (1.2) 3 (0.9)
Red fescue 0-5 3 (0.9) 2.5 (0.8)
Redtop 5-10 Not recommended Not recommended
* Adapted from Chow (1959), Livingston et al (1984), and Goldman et al (1986)
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S.7 Use Manning’s equation to solve for the actual VR.
S.8 Compare the actual VR from step S.7 and first approximation from step S.5. If they do not
agree within 5 percent, repeat steps S.4 to S.8 until acceptable agreement is reached. If
n<0.033 is needed to get agreement, set n = 0.033, repeat step S.7, and then proceed to
step S.9.
S.9 Compute the actual V for the final design conditions:
S.10 Check to be sure V < Vmax of 3 feet/second.
S.11 Compute the required swale cross-sectional area, A, for stability:
S.12 Compare the A, computed in step S.11 of the stability analysis, with the A from the
biofiltration capacity analysis (step D.5).
If less area is required for stability than is provided for capacity, the capacity design is
acceptable. If not, use A from step S.11 of the stability analysis and recalculate channel
dimensions.
S.13 Calculate the depth of flow at the stability check design flow rate condition for the final
dimensions and use A from step S.11.
S.14 Compare the depth from step S.13 to the depth used in the biofiltration capacity design (Step
D.1). Use the larger of the two and add 0.5 ft. of freeboard to obtain the total depth (yt) of the
swale. Calculate the top width for the full depth using the appropriate equation.
SC.15 Recalculate the hydraulic radius: (use b from Step D.4 calculated previously for biofiltration
capacity, or Step S.12, as appropriate, and yt = total depth from Step S.14)
SC.16 Make a final check for capacity based on the stability check design storm (this check will
ensure that capacity is adequate if the largest expected event coincides with the greatest
retardance). Use Equation 1, a Manning's n selected in step D.2, and the calculated channel
dimensions, including freeboard, to compute the flow capacity of the channel under these
conditions. Use R from step SC-14, above, and A = b(yt) + Z(yt)² using b from Step D.4,
D.15, or S.12 as appropriate.
If the flow capacity is less than the stability check design storm flow rate, increase the
channel cross-sectional area as needed for this conveyance. Specify the new channel
dimensions.
Completion Step
Review all of the criteria and guidelines for biofilter planning, design, installation, and operation above
and specify all of the appropriate features for the application.
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Figure V-7-19. Biofiltration Swale Underdrain Detail
Figure V-7-20. Biofiltration Swale Low-Flow Drain Detail
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Dividing Berm Height = Design Flow
Depth (y) + 2” (min.)
Figure V-7-21. Swale Dividing Berm
7.4.1.5 Soil Criteria
• Use the following list as a guide for choosing appropriate soils for the biofiltration
swale. Use at least 8-inches of the following top soil mix:
o Sandy loam 60-90%
o Clay 0-10%
o Composted organic matter, (excluding animal waste, toxics) 10-30%
• Use compost amended soil where practicable.
• Till to at least 8-inch depth.
• For longitudinal slopes of < 2 percent use more sand to obtain more infiltration.
• If groundwater contamination is a concern, seal the bed with clay or a geomembrane
liner.
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7.4.1.6 Vegetation Criteria
• See Table V-7-12 through Table V-7-14 for recommended grasses, wetland plants,
and groundcovers.
• Select fine, turf-forming, water-resistant grasses where vegetative growth and
moisture will be adequate for growth.
• Irrigate if moisture is insufficient during dry weather season.
• Use sod with low clay content and where needed to initiate adequate vegetative
growth. Preferably sod should be laid to a minimum of one-foot vertical depth above
the swale bottom.
• Consider sun/shade conditions for adequate vegetative growth and avoid prolonged
shading of any portion not planted with shade tolerant vegetation.
• Stabilize soil areas upslope of the biofilter to prevent erosion.
• Fertilizing a biofilter shall not be allowed.
7.4.1.7 Construction Criteria
• Do not put swale into operation until exposed soil in contributing drainage area is stabilized.
• Keep erosion and sediment control measures in place until swale vegetation is established.
• Avoid compaction during construction.
• Grade biofilters to attain uniform longitudinal and lateral slopes.
7.4.1.8 Maintenance Criteria
• Inspect biofilters at least once every 6 months, preferably during storm events, and
also after storm events of > 0.5 inch rainfall/ 24 hours. Maintain adequate grass
growth and eliminate bare spots.
• Mow grasses, if needed for good growth. Typically maintain at 4 – 9 inches but not
below design flow level.
• Remove sediment as needed at head of the swale if grass growth is inhibited in
greater than 10 percent of the swale, or if the sediment is blocking the distribution
and entry of the water.
• Remove leaves, litter, and oily materials, and re-seed or resod, and regrade, as
needed. Clean curb cuts and level spreaders as needed.
Prevent scouring and soil erosion in the biofilter. If flow channeling occurs, regrade and
reseed the biofilter, as necessary.
• Maintain access to biofilter inlet, outlet, and to mowing (Figure V-7-22).
• If a swale is equipped with underdrains, vehicular traffic on the swale bottom (other
than grass mowing equipment) shall be avoided to prevent damage to the
drainpipes.
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Table V-7-12. Grass Seed Mixes Suitable for Biofiltration Swale Treatment Areas
Mix 1 Mix 2
75-80 percent tall or meadow fescue 60-70 percent tall fescue
10-15 percent seaside/colonial
bentgrass
10-15 percent seaside/colonial
bentgrass
5-10 percent redtop 10-15 percent meadow foxtail
6-10 percent alsike clover
1-5 percent marshfield big trefoil
1-6 percent redtop
Note: All percentages are by weight, based on Briargreen, Inc.
Table V-7-13. Groundcovers & Grasses Suitable for the Upper Side Slopes
of a Biofiltration Swale in Western Washington
Groundcovers
kinnikinnick* Arctostaphylos uva-ursi
Epimedium Epimedium grandiflorum
creeping forget-me-not Omphalodes verna
-- Euonymus lanceolata
yellow-root Xanthorhiza simplissima
-- Genista
white lawn clover Trifolium repens
white sweet clover* Melilotus alba
------- Rubus calycinoides
strawberry* Fragaria chiloensis
broadleaf lupine* Lupinus latifolius
Grasses (drought-tolerant, minimum mowing)
dwarf tall fescues Festuca spp. (e.g., Many Mustang, Silverado)
hard fescue Festuca ovina duriuscula (e.g., Reliant, Aurora)
tufted fescue Festuca amethystina
buffalo grass Buchloe dactyloides
red fescue* Festuca rubra
tall fescue grass* Festuca arundinacea
blue oatgrass Helictotrichon sempervirens
* Good choices for swales with significant periods of flow, such as those downstream of a detention facility.
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Table V-7-14. Recommended Plants for Wet Biofiltration Swale
Common Name Scientific Name Spacing
(on center)
Shortawn foxtail Alopecurus aequalis seed
Water foxtail Alopecurus geniculatus seed
Spike rush Eleocharis spp. 4 inches
Slough sedge* Carex obnupta 6 inches or seed
Sawbeak sedge Carex stipata 6 inches
Sedge Carex spp. 6 inches
Western mannagrass Glyceria occidentalis seed
Velvetgrass Holcus mollis seed
Slender rush Juncus tenuis 6 inches
Watercress* Rorippa nasturtium-aquaticum 12 inches
Water parsley* Oenanthe sarmentosa 6 inches
Hardstem bulrush Scirpus acutus 6 inches
Small-fruited bulrush Scirpus microcarpus 12 inches
* Good choices for swales with significant periods of flow, such as those downstream of
a detention facility.
Cattail (Typha latifolia) is not appropriate for most wet swales because of its very dense and clumping growth habit
which prevents water from filtering through the clump.
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Figure V-7-22. Biofiltration Swale Access Features
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7.4.2 BMP T920 Wet Biofiltration Swale
7.4.2.1 Description
A wet biofiltration swale is a variation of a basic biofiltration swale for use where the longitudinal slope
is slight, water tables are high, or continuous low base flow is likely to result in saturated soil
conditions. Where saturation exceeds about 2 weeks, typical grasses will die. Thus, vegetation
specifically adapted to saturated soil conditions is needed. Different vegetation in turn requires
modification of several of the design parameters for the basic biofiltration swale.
7.4.2.2 Performance Objectives
To remove low concentrations of pollutants such as TSS, heavy metals, nutrients, and petroleum
hydrocarbons.
7.4.2.3 Applications/Limitations
Wet biofiltration swales are applied where a basic biofiltration swale is desired but not allowed or
advisable because one or more of the following conditions exist:
• The swale is located on glacial till soils and is downstream of a detention pond
providing flow control.
• Saturated soil conditions are likely because of seeps or base flows on the site.
• Longitudinal slopes shall be less than 2 percent.
7.4.2.4 Criteria
Use the same sizing and criteria as for basic biofiltration swales except for the following:
1. Adjust for extended wet season flow.
• If the swale will be downstream of a detention pond or vault providing flow control,
multiply the treatment area (bottom width times length) of the swale by 2, and
readjust the swale length, if desired. Maintain a 5:1 length to width ratio.
2. Swale geometry.
• The bottom width may be increased to 25 feet maximum, but a length-to-width ratio
of 5:1 must be provided. No longitudinal dividing berm is needed.
The minimum swale length is 100 feet.
• If longitudinal slopes are greater than 2 percent, the wet swale must be stepped so
that the slope within the stepped sections averages 2 percent. Steps may be made
of retaining walls, log check dams, or short riprap sections. No underdrain or low-
flow drain is required.
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3. High-flow bypass
• A high-flow bypass (i.e., an off-line design) is required for flows greater than the off-
line water quality design flow that has been increased by 3.5. The bypass may be an
open channel parallel to the wet biofiltration swale.
4. Water Depth and Base Flow
• Design water depth shall be 4 inches for all wetland vegetation selections.
• No underdrains or low-flow drains are required.
5. Flow Velocity, Energy Dissipation, and Flow Spreading
• No flow spreader is required.
6. Access
• Access is only required to the inflow and outflow of the swale. Access along the
swale is not required.
• Wheel strips may not be used for access.
7. Planting Requirements
• A list of acceptable plants and recommended spacing is shown in Table V-7-14.
• A wetland seed mix may be applied by hydroseeding, but if coverage is poor,
planting of rootstock or nursery stock is required. Poor coverage is considered to be
more than 30 percent bare area through the upper 2/3 of the swale after four weeks.
8. Maintenance Considerations
• Mowing of wetland vegetation is not required. However, harvesting of very dense
vegetation may be desirable in the fall after plant die-back to prevent the sloughing of
excess organic material into receiving waters. Fall harvesting of Juncus species is
not recommended.
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7.4.3 BMP T930 Continuous Inflow Biofiltration Swale
7.4.3.1 Description:
In situations where water enters a biofiltration swale continuously along the side slope rather than
discretely at the head, a different design approach–the continuous inflow biofiltration swale–is
needed. The basic swale design is modified by increasing swale length to achieve an equivalent
average residence time.
7.4.3.2 Applications
A continuous inflow biofiltration swale is to be used when inflows are not concentrated, such as
locations along the shoulder of a road without curbs. This design may also be used where frequent,
small point flows enter a swale, such as through curb inlet ports spaced at intervals along a road, or
from a parking lot with frequent curb cuts. In general, no inlet port shall carry more than about
10 percent of the flow.
A continuous inflow swale is not appropriate for a situation in which significant lateral flows enter a
swale at some point downstream from the head of the swale. In this situation, the swale width and
length must be recalculated from the point of confluence to the discharge point in order to provide
adequate treatment for the increased flows.
7.4.3.3 Design Criteria
Same as specified for basic biofiltration swale except for the following:
• The design flow for continuous inflow swales must include runoff from the pervious
side slopes draining to the swale along the entire swale length. Therefore, they must
be on-line facilities.
• If only a single design flow is used, the flow rate at the outlet should be used. The
goal is to achieve an average residence time through the swale of 9 minutes as
calculated using the on-line water quality design flow rate multiplied by the ratio, K
(see footnotes in Table V-7-9). Assuming an even distribution of inflow into the side
of the swale double the hydraulic residence time to a minimum of 18 minutes.
• Interior side slopes above the water quality design treatment elevation shall be
planted in grass. A typical lawn seed mix or the biofiltration seed mixes are
acceptable. Landscape plants or groundcovers other than grass may not be used
anywhere between the runoff inflow elevation and the bottom of the swale.
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7.4.4 BMP T940 Basic Filter Strip
7.4.4.1 Description
A basic filter strip is flat with no side slopes (Figure V-7-23). Untreated stormwater is distributed as
sheet flow across the inlet width of a biofilter strip.
7.4.4.2 Applications/Limitations
The basic filter strip is typically used on-line and adjacent and parallel to a paved area such as
parking lots, driveways, and roadways. Where a filter strip area is compost-amended to a minimum
of 10% organic content in accordance with BMP L613; with hydroseeded grass maintained at 95%
density and a 4-inch length by mowing and periodic re-seeding (possible landscaping with
herbaceous shrubs), the filter strip serves as an Enhanced Treatment option.
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Figure V-7-23. Typical Filter Strip
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7.4.4.3 Design Criteria for Filter strips:
• Use the Design Criteria specified in Table V-7-9.
• Filter strips shall only receive sheet flow.
• Use curb cuts 12-inch wide and 1-inch above the filter strip inlet.
7.4.4.4 Sizing Procedure
1. Calculate the design flow depth using Manning’s equation as follows:
KQ = (1.49A R0.67 s 0.5)/n
Substituting for AR:
KQ = (1.49Ty1.67 s0.5)/n
Where:
Ty = Arectangle, ft
2
y = Rrectangle, design depth of flow, ft. (1 inch maximum)
Q = peak Water Quality design flow rate based on WWHM or an approved continuous
simulation model, ft3/sec
K = A ratio of the peak 10-minute flow predicted by SBUH to the water quality
design flow rate estimated using the WWHM. The value of K for off-line
systems is 3.5 and for on-line systems is 2.0.1
n = Manning’s roughness coefficient
s = Longitudinal slope of filter strip parallel to direction of flow
T = Width of filter strip perpendicular to the direction of flow, ft.
A = Filter strip inlet cross-sectional flow area (rectangular), ft2
R = hydraulic radius, ft.
Rearranging for y:
y = [KQn/1.49Ts0.5]0.6
y must not exceed 1 inch
2. Calculate the design flow velocity V, ft./sec., through the filter strip:
V = KQ/Ty
V must not exceed 0.5 ft./sec
3. Calculate required length, ft., of the filter strip at the minimum hydraulic residence time, t, of 9
minutes:
L = tV = 540 V
1 As in swale design, an adjustment factor of K accounts for the differential between the Water Quality design flow
rate calculated using WWHM and the SBUH design flow.
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7.4.5 BMP T950 Narrow Area Filter Strip
7.4.5.1 Description:
This section describes a filter strip design2 for impervious areas with flowpaths of 30 feet or less that
can drain along their widest dimension to grassy areas.
7.4.5.2 Applications/Limitations:
A narrow area filter strip could be used at roadways with limited right-of-way, or for narrow parking
strips. If space is available to use the basic filter strip design, that design should be used in
preference to the narrow filter strip.
7.4.5.3 Design Criteria:
Design criteria for narrow area filter strips are the same as specified for basic filter strips. The sizing
of a narrow area filter strip is based on the length of flowpath draining to the filter strip and the
longitudinal slope of the filter strip itself (parallel to the flowpath).
1. Determine the length of the flowpath from the upstream to the downstream edge of the
impervious area draining sheet flow to the strip. Normally this is the same as the width of the
paved area, but if the site is sloped, the flow path may be longer than the width of the
impervious area.
2. Calculate the longitudinal slope of the filter strip (along the direction of unconcentrated flow),
averaged over the total width of the filter strip.
• The minimum slope size is 2 percent. If the slope is less than 2 percent, use
2 percent for sizing purposes.
• The maximum allowable filter strip slope is 20 percent. If the slope exceeds
20 percent, the filter strip must be stepped so that the treatment areas between drop
sections do not have a longitudinal slope greater than 20 percent. Provide erosion
protection at the base and flow spreaders for the drop sections. Vertical drops along
the slope must not exceed 12 inches in height. If this is not possible, a different
treatment facility must be selected.
3. Select the appropriate filter strip length for the flowpath length and filter strip longitudinal slope
(Steps 1 and 2 above) from the graph in Figure V-7-24. Design the filter strip to provide this
minimum length L along the entire stretch of pavement draining into it.
2 This narrow area filter strip design method is included here because technical limitations exist in the basic design
method which results in filter strips that are proportionately longer as the contributing drainage becomes narrower (a
result that is counter-intuitive). Research by several parties is underway to evaluate filter strip design parameters.
This research may lead to more stringent design requirements that would supersede the design criteria presented
here.
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To use the graph, find the length of the flowpath on one of the curves (interpolate between
curves as necessary). Move along the curve to the point where the design longitudinal slope
of the filter strip (x-axis) is directly below. Read the filter strip length on the y-axis which
corresponds to the intersection point.
0.0
5.0
10.0
15.0
20.0
0%5%10%15%20%
Filter Strip Slope
Fi
l
t
e
r
S
t
r
i
p
L
e
n
g
t
h
(
f
e
e
t
)
Note: minimum allowable filter strip length is 4 feet
Flowpath
= 30 feet
20 feet
10 feet
Figure V-7-24. Filter Strip Lengths for Narrow Right-of-Way
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Chapter 8 Wetpool Facilities
8.1 Purpose
This Chapter presents the methods, criteria, and details for analysis and design of wetponds,
wetvaults, and stormwater wetlands. These facilities have as a common element, a permanent pool
of water - the wetpool. Each of the wetpool facilities can be combined with detention storage in a
combined facility.
8.2 Best Management Practices
The following wetpool facility BMPs are discussed in this chapter:
BMP T1010 – Wetponds - Basic and Large
BMP T1020 – Wetvaults
BMP T1030 – Stormwater Wetlands
BMP T1040 – Combined Detention and Wetpool Facilities
The specific BMPs that are selected should be based on the Treatment Facility Menus discussed in
Chapter 2.
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8.2.1 BMP T1010 Wetponds - Basic and Large
8.2.1.1 Description:
A wetpond is a constructed stormwater pond that retains a permanent pool of water ("wetpool") at
least during the wet season. The volume of the wetpool is related to the effectiveness of the pond in
settling particulate pollutants. As an option, a shallow marsh area can be created within the
permanent pool volume to provide additional treatment for nutrient removal. Flow control can be
provided in the "live storage" area above the permanent pool. Figure V-8-25 and Figure V-8-26
illustrate a typical wetpond.
8.2.1.2 Design Criteria:
For a basic wetpond, the wetpool volume provided shall be equal to or greater than the total volume
of runoff from the water quality design storm, which is the 6-month, 24-hour storm event.
Alternatively, the 91st percentile, 24-hour runoff volume predicted by WWHM can be used.
A large wetpond requires a wetpool volume at least 1.5 times larger than the total volume of runoff
from the 6-month, 24-hour storm event.
For private wetponds, a minimum three-inch (3”) diameter power conduit shall be grounded to the
edge of the pond for possible future installation of aerators.
Aerators are required to be installed on public ponds. Power provided to public ponds shall be
independently metered. Specific power and aeration requirements will be determined case by case
during plan review and will be based upon the size and configuration of the wetpond.
Design features that encourage plug flow and avoid dead zones are:
• Dissipating energy at the inlet.
• Providing a large length-to-width ratio.
• Providing a broad surface for water exchange using a berm designed as a broad-
crested weir to divide the wetpond into two cells rather than a constricted area such
as a pipe.
• Maximizing the flowpath between inlet and outlet, including the vertical path, also
enhances treatment by increasing residence time.
General wetpond design criteria and concepts are shown in Figure V-8-25 and Figure V-8-26.
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Figure V-8-25. Wetpond (top view)
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Figure V-8-26. Wetpond (side view)
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8.2.1.3 Sizing Procedure
Procedures for determining a wetpond's dimensions and volume are outlined below.
1. Identify required wetpool volume using the SCS (now known as NRCS) curve number
equations. A basic wetpond requires a volume equal to or greater than the total volume
of runoff from the 6-month, 24-hour storm event, or, alternatively, the 91st percentile,
24-hour runoff volume using WWHM. A large wetpond requires a volume at least
1.5 times the total volume of runoff from the 6-month, 24-hour storm event or 1.5 times
the 91st percentile, 24-hour runoff volume using an approved continuous runoff model.
SP
SPQd8.0
]2.0[2
+
-= for P 0.2S
and
0=Qd for P < 0.2S
Where:
Qd = runoff depth in inches over area
P = precipitation depth in inches over area
S = potential maximum detention, in inches over area, due to infiltration, storage, etc.
The area’s potential maximum depth, S, is related to its curve number, CN:
10)/1000(-=CNS
2. Determine wetpool dimensions. Determine the wetpool dimensions satisfying the design
criteria outlined below and illustrated in Figure V-8-25 and Figure V-8-26. A simple way
to check the volume of each wetpool cell is to use the following equation:
V = hAA(+)12
2
where: V = wetpool volume (ft3)
h = wetpool average depth (ft)
A1 = water quality design surface area of wetpool (sf)
A2 = bottom area of wetpool (sf)
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3. Design pond outlet pipe and determine primary overflow water surface. The pond outlet
pipe shall be placed on a reverse grade from the pond's wetpool to the outlet structure.
Use the following procedure to design the pond outlet pipe and determine the primary
overflow water surface elevation:
a. Use the nomographs in Figure V-8-27 and Figure V-8-28 to select a trial size for
the pond outlet pipe sufficient to pass the on-line WQ design flow Qwq as
determined using WWHM.
b. Use Figure V-8-29 to determine the critical depth dc at the outflow end of the pipe
for Qwq.
c. Use Figure V-8-30 to determine the flow area Ac at critical depth.
d. Calculate the flow velocity at critical depth using continuity equation
(Vc = Qwq /Ac).
e. Calculate the velocity head VH (VH =Vc
2 /2g, where g is the gravitational constant,
32.2 feet per second).
f. Determine the primary overflow water surface elevation by adding the velocity
head and critical depth to the invert elevation at the outflow end of the pond
outlet pipe (i.e., overflow water surface elevation = outflow invert + dc + VH).
g. Adjust outlet pipe diameter as needed and repeat Steps (a) through (e).
4. Determine wetpond dimensions.
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Figure V-8-27. Headwater Depth for Smooth Interior Pipe Culverts with Inlet Control
DI
A
M
E
T
E
R
O
F
C
U
L
V
E
R
T
(
D
)
I
N
I
N
C
H
E
S
ENTRANCE
TYPE
HW
D SCALE
(1)
(2)
(3)
Square edge with
Groove end with
Groove end
headwall
headwall
projecting
EXAMPLE
D = 42 inches (3.0 feet).
Q = 120 cfs
HW*
*D in feet
HW
D (feet)
(1)
(2)
(3)
2.5
2.1
2.2
8.8
7.4
7.7
EXAM
P
L
E
DI
S
C
H
A
R
G
E
(
Q
)
I
N
C
F
S
HE
A
D
W
A
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E
R
D
E
P
T
H
I
N
D
I
A
M
E
T
E
R
S
(
H
W
/
D
)
To use scale (2) or (3) project
horizontally to scale (1), then
use straight inclined line through
D and Q scales, or reverse as
illustrated.
1.0
2
3
4
5
6
8
10
20
30
40
50
60
12
15
18
21
24
27
30
33
36
80
100
200
300
400
500
600
800
1,000
42
48
54
60
2,000
3,000
4,000
5,000
6,000
8,000
10,000
72
84
96
108
120
132
144
156
168
180
ENTRANCE TYPE
SQUARE EDGE WITH
HEADWALL
GROOVE END WITH
GROOVE END
PROJECTING
HEADWALL
PLAN
PLAN
(1)(2)(3)
.5
.5 .5
.6
.6 .6
.7
.7 .7
.8
.8 .8
.9
.9 .9
1.0
1.0 1.0
1.5
1.5 1.5
2.
2.
3.
3.3.
4.
4.
4.
5.
2.
(3)
(2)
(1)
5.
5.
6.
6.
6.
New Design Manual
Figure 4.3.1.B Headwater Depth for Smooth Interior Pipe Culverts with Inlet Control
Revised 12-2-97/Mdev
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Wetpool Facilities Volume V
Chapter 8 698
Figure V-8-28. Headwater Depth for Corrugated Pipe Culverts with Inlet Control
ST
A
N
D
A
R
D
C
.
M
.
DI
A
M
E
T
E
R
O
F
C
U
L
V
E
R
T
(
D
)
I
N
I
N
C
H
E
S
ST
R
U
C
T
U
R
A
L
P
L
A
T
E
C
.
M
.
ENTRANCE
TYPE
HW
D SCALE
(1)
(2)
(3)
Headwall
Mitered to conform
to slope
Projecting
EXAMPLE
D = 36 inches (3.0 feet).
Q = 66 cfs
HW*
*D in feet
HW
D (feet)
(1)
(2)
(3)
1.8
2.1
2.2
5.4
6.3
6.6
EXAM
P
L
E
DI
S
C
H
A
R
G
E
(
Q
)
I
N
C
F
S
HE
A
D
W
A
T
E
R
D
E
P
T
H
I
N
D
I
A
M
E
T
E
R
S
(
H
W
/
D
)
To use scale (2) or (3) project
horizontally to scale (1), then
use straight inclined line through
D and Q scales, or reverse as
illustrated
1.0
2
3
4
5
6
8
10
20
30
40
50
60
12
15
18
21
24
27
30
33
36
80
100
200
300
400
500
600
800
1,000
42
48
54
60
2,000
3,000
4,000
5,000
6,000
8,000
10,000
72
84
96
108
120
132
144
156
168
180
ENTRANCE TYPE
HEADWALL PLAN
MITERED TO
CONFORM
TO SLOPE
SECTION
PROJECTING
SECTION
(1)
(2)
(3)
.5 .5
.5
.6 .6
.6
.7 .7
.7
.8 .8
.8
.9 .9
.9
1.0 1.0
1.0
1.5
1.5 1.5
2.
2.
3.
3.
3.
4.
4.
4.
5.
2.
(3)
(2)
(1)
5.
5.
6.
6.
6.
New Design Manual
Figure 4.3.1.C Headwater Depth for Corrugated Pipe Culverts with Inlet Control
Revised 11-24-97/Mdev
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Figure V-8-29. Critical Depth of Flow for Circular Culverts
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Figure V-8-30. Circular Channel Ratios
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8.2.1.4 Wetpool Geometry
Divide the wetpool into two cells separated by a baffle or berm. The first cell shall
contain between 25 to 35 percent of the total wetpool volume. The baffle or berm
volume shall not count as part of the total wetpool volume.
Provide sediment storage in the first cell. The sediment storage shall have a
minimum depth of 1-foot. Install a fixed sediment depth monitor in the first cell to
gauge sediment accumulation unless an alternative gauging method is proposed.
The minimum depth of the first cell shall be 4 feet, exclusive of sediment storage
requirements. The depth of the first cell may be greater than the depth of the second
cell.
The maximum depth of each cell shall not exceed 8 feet (exclusive of sediment
storage in the first cell). Plant pool depths of 3 feet or shallower (second cell) with
emergent wetland vegetation (see planting requirements).
Place inlets and outlets to maximize the flowpath through the facility. The ratio of
flowpath length to width from the inlet to the outlet shall be at least 3:1. The flowpath
length is defined as the distance from the inlet to the outlet, as measured at mid-
depth. The width at mid-depth can be found as follows: width = (average top width +
average bottom width)/2.
Wetponds with wetpool volumes less than or equal to 4,000 cubic feet may be single
celled (i.e., no baffle or berm is required). However, it is especially important in this
case that the flow path length be maximized. The ratio of flow path length to width
shall be at least 4:1 in single celled wetponds, but should preferably be 5:1.
All inlets shall enter the first cell. For multiple inlets, the length-to-width ratio shall be
based on the average flowpath length for all inlets.
Line the first cell in accordance with the liner requirements contained in Section 3.4.
8.2.1.5 Berms, Baffles, and Slopes
A berm or baffle shall extend across the full width of the wetpool, and tie into the
wetpond side slopes. If the berm embankments are greater than 4 feet in height, the
berm must be constructed by excavating a key with dimensions equal to 50 percent
of the embankment cross-sectional height and width. This requirement may be
waived if recommended by a geotechnical engineer for specific site conditions. The
geotechnical analysis shall address situations in which one of the two cells is empty
while the other remains full of water.
The top of the berm may extend to the WQ design water surface or be 1-foot below
the WQ design water surface. If at the WQ design water surface, berm side slopes
shall be 3H:1V. Berm side slopes may be steeper (up to 2H:1V) if the berm is
submerged 1-foot.
If good vegetation cover is not established on the berm, erosion control measures
shall be used to prevent erosion of the berm back-slope when the pond is initially
filled.
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The interior berm or baffle may be a retaining wall provided that the design is
prepared and stamped by a licensed civil engineer. If a baffle or retaining wall is
used, it shall be submerged one foot below the design water surface to discourage
access by pedestrians.
Criteria for wetpond side slopes are included in Section 3.3.
8.2.1.6 Embankments
Embankments that impound water must comply with the Washington State Dam Safety Regulations
(Chapter 173-175 WAC). If the impoundment has a storage capacity (including both water and
sediment storage volumes) greater than 10 acre-feet (435,600 cubic feet or 3.26 million gallons)
above natural ground level, then dam safety design and review are required by the Department of
Ecology. Contact Ecology for information about this regulation.
8.2.1.7 Inlet and Outlet
See Figure V-8-25 and Figure V-8-26 for details on the following requirements:
Submerge the inlet to the wetpond with the inlet pipe invert a minimum of two feet
from the pond bottom (not including sediment storage). The top of the inlet pipe shall
be submerged at least 1-foot, if possible.
Provide an outlet structure. Either a Type 2 catch basin with a grated opening (“jail
house window”) or a manhole with a cone grate (“birdcage”) may be used (see
Volume III, Figure III-2-9 for an illustration).
The pond outlet pipe (as opposed to the manhole or type 2 catch basin outlet pipe)
shall be back-sloped or have a down-turned elbow, and extend 1 foot below the WQ
design water surface.
Size the pond outlet pipe, at a minimum, to pass the on-line WQ design flow. The
highest invert of the outlet pipe sets the WQ design water surface elevation.
The overflow criteria for single-purpose (treatment only, not combined with flow
control) wetponds are as follows:
o The requirement for primary overflow is satisfied by either the grated inlet to
the outlet structure or by a birdcage above the pond outlet structure.
o The bottom of the grate opening in the outlet structure shall be set at or
above the height needed to pass the WQ design flow through the pond outlet
pipe. The grate invert elevation sets the overflow water surface elevation.
o The grated opening and downstream conveyance shall be sized to pass the
100-year design flow. The capacity of the outlet system shall be sized to pass
the peak flow for the conveyance requirements.
Provide an emergency spillway and design it according to the requirements for
detention ponds (see Volume III, Section 2.3.1).
The City may require a bypass/shutoff valve to enable the pond to be taken offline for
maintenance purposes.
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A gravity drain for maintenance is recommended if grade allows.
o The drain invert shall be at least 6 inches below the top elevation of the
dividing berm or baffle. Deeper drains are encouraged where feasible, but
must be no deeper than 18 inches above the pond bottom.
o The drain shall be at least 8 inches (minimum) diameter and shall be
controlled by a valve. Use of a shear gate is allowed only at the inlet end of a
pipe located within an approved structure.
o Provide operational access to the valve to the finished ground surface.
o The valve location shall be accessible and well marked with 1-foot of paving
placed around the box. It must also be protected from damage and
unauthorized operation.
o A valve box is allowed to a maximum depth of 5 feet without an access
manhole. If over 5 feet deep, an access manhole or vault is required.
All metal parts shall be corrosion-resistant. Do not use galvanized materials.
8.2.1.8 Access and Setbacks
All facilities shall be a minimum of 20 feet from any structure, property line, and any
vegetative buffer required by the local government, and 100 feet from any septic
tank/drainfield.
All facilities shall be a minimum of 50 feet from any slope greater than 20 percent. A
geotechnical report must address the potential impact of a wetpond on a slope steeper
than 20% or if closer than 50 feet.
Provide access and maintenance roads and designed them according to the
requirements for detention ponds. Access and maintenance roads shall extend to both
the wetpond inlet and outlet structures. An access ramp (7H minimum:1V) shall be
provided to the bottom of the first cell unless all portions of the cell can be reached and
sediment loaded from the top of the pond.
The internal berm of a wetpond or combined detention and wetpond may be used for
access if it is designed to support a loaded 80,000 pound truck considering the berm is
normally submerged and saturated.
8.2.1.9 Planting Requirements
Planting requirements for detention ponds also apply to wetponds.
Large wetponds intended for phosphorus control shall not be planted within the cells,
as the plants will release phosphorus in the winter when they die off.
If the second cell of a basic wetpond is 3 feet deep or shallower, the bottom area
shall be planted with emergent wetland vegetation. See Table V-8-15 for
recommended emergent wetland plant species for wetponds. The recommendations
in Table V-8-15 are for all of western Washington. Local knowledge should be used
to tailor this information to Auburn as appropriate.
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Cattails (Typha latifolia) shall not be used because they tend to crowd out other
species and will typically establish themselves anyway.
If the wetpond discharges to a phosphorus-sensitive lake or wetland, shrubs that
form a dense cover should be planted on slopes above the WQ design water surface
on at least three sides. Native vegetation species shall be used in all cases.
8.2.1.10 Recommended Design Features
The following features should be incorporated into the wetpond design where site conditions allow:
The method of construction of soil/landscape systems can cause natural selection of
specific plant species. Consult a soil restoration or wetland soil scientist for site-
specific recommendations. The soil formulation will impact the plant species that will
flourish or suffer on the site, and the formulation should be such that it encourages
desired species and discourages undesired species.
For private wetpond facilities, it is recommended that some form of recirculation be
provided, such as an aerator, to prevent stagnation and low dissolved oxygen
conditions. Recirculation is required for all public wetpond facilities.
A flow length-to-width ratio greater than the 3:1 minimum is desirable. If the ratio is
4:1 or greater, then the dividing berm is not required, and the pond may consist of
one cell rather than two. A one-cell pond must provide at least 6-inches of sediment
storage depth.
A tear-drop shape, with the inlet at the narrow end, rather than a rectangular pond is
preferred since it minimizes dead zones caused by corners.
A small amount of base flow is desirable to maintain circulation and reduce the
potential for low oxygen conditions during late summer.
Evergreen or columnar deciduous trees along the west and south sides of ponds are
recommended to reduce thermal heating, except that no trees or shrubs may be
planted on berms meeting the criteria of dams regulated for safety. In addition to
shade, trees and shrubs also discourage waterfowl use and the attendant
phosphorus enrichment problems they cause. Trees should be set back so that the
branches will not extend over the pond.
The number of inlets to the facility should be limited; ideally there should be only one
inlet. The flowpath length should be maximized from inlet to outlet for all inlets to the
facility.
The access and maintenance road could be extended along the full length of the
wetpond and could double as play courts or picnic areas. Placing finely ground bark
or other natural material over the road surface would render it more pedestrian
friendly.
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The following design features should be incorporated to enhance aesthetics where
possible:
o Provide pedestrian access to shallow pool areas enhanced with emergent
wetland vegetation. This allows the pond to be more accessible without
incurring safety risks.
o Provide side slopes that are sufficiently gentle (3H:1V or flatter).
o Create flat areas overlooking or adjoining the pond for picnic tables or seating
that can be used by residents. Walking or jogging trails around the pond are
easily integrated into site design.
o Include fountains or integrated waterfall features for privately maintained
facilities.
o Provide visual enhancement with clusters of trees and shrubs. On most pond
sites, it is important to amend the soil before planting since ponds are
typically placed well below the native soil horizon in very poor soils. Make
sure dam safety restrictions against planting do not apply.
o Orient the pond length along the direction of prevailing summer winds
(typically west or southwest) to enhance wind mixing.
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Table V-8-15. Emergent Wetland Species Recommended for Wetponds
Species Common Name Notes Maximum
Depth
INUNDATION TO 1-FOOT
Agrostis exarata1 Spike bent grass Prairie to coast 2 feet
Carex stipata Sawbeak sedge Wet ground
Eleocharis palustris Spike rush Margins of ponds, wet meadows 2 feet
Glyceria occidentalis Western mannagrass Marshes, pond margins 2 feet
Juncus tenuis Slender rush Wet soils, wetland margins
Oenanthe sarmentosa Water parsley Shallow water along stream and pond margins;
needs saturated soils all summer
Scirpus atrocinctus (formerly
S. cyperinus)
Woolgrass Tolerates shallow water; tall clumps
Scirpus microcarpus Small-fruited bulrush Wet ground to 18 inches depth 18 inches
Sagittaria latifolia Arrowhead
Inundation 1 to 2 feet
Agrostis exarata1 Spike bent grass Prairie to coast
Alisma plantago-aquatica Water plantain
Eleocharis palustris Spike rush Margins of ponds, wet meadows
Glyceria occidentalis Western mannagrass Marshes, pond margins
Juncus effuses Soft rush Wet meadows, pastures, wetland margins
Scirpus microcarpus Small-fruited bulrush Wet ground to 18 inches depth 18 inches
Sparganium emmersum Bur reed Shallow standing water, saturated soils
Inundation 1 to 3 feet
Carex obnupta Slough sedge Wet ground or standing water 1.5 to 3 feet
Beckmania syzigachne1 Western sloughgrass Wet prairie to pond margins
Scirpus acutus2 Hardstem bulrush Single tall stems, not clumping 3 feet
Scirpus validus2 Softstem bulrush
Inundation GREATER THAN 3 feet
Nuphar polysepalum Spatterdock Deep water 3 to 7.5 feet
Nymphaea odorata1 White waterlily Shallow to deep ponds 6 feet
1 Non-native species. Beckmania syzigachne is native to Oregon. Native species are preferred.
2 Scirpus tubers must be planted shallower for establishment, and protected from foraging waterfowl until established.
Emerging aerial stems should project above water surface to allow oxygen transport to the roots.
Primary sources: Municipality of Metropolitan Seattle, Water Pollution Control Aspects of Aquatic Plants, 1990.
Hortus Northwest, Wetland Plants for Western Oregon, Issue 2, 1991. Hitchcock and Cronquist, Flora of the Pacific
Northwest, 1973.
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8.2.1.11 Construction Criteria:
• Remove sediment that has accumulated in the pond after construction in the
drainage area of the pond is complete (unless used for a liner - see below).
• Sediment that has accumulated in the pond at the end of construction may be used
as a liner in excessively drained soils if the sediment meets the criteria for low
permeability or treatment liners in keeping with guidance given in Chapter 3.
Sediment used for a soil liner must be graded to provide uniform coverage and must
meet the thickness specifications in Chapter 3. The sediment must not reduce the
design volume of the pond. The pond must be over-excavated initially to provide
sufficient room for the sediments to serve as a liner.
8.2.1.12 Operation and Maintenance:
• All private drainage systems serving multiple lots shall require a signed Stormwater
Easement and Maintenance agreement with the City. The agreement shall designate
the systems to be maintained and the parties responsible for maintenance. Contact
the City to determine the applicability of this requirement to a project. A specific
maintenance plan shall be formulated outlining the schedule and scope of
maintenance operations.
• The pond may be inspected by the City. The maintenance standards contained in
Volume I, Appendix D are measures for determining if maintenance actions are
required as identified through the annual inspection.
• Trim site vegetation as necessary to keep the pond free of leaves and to maintain
the aesthetic appearance of the site. Revegetate sloped areas that have become
bare and regrade eroded areas prior to revegetation.
• Remove sediment when the 1-foot sediment zone is full plus 6 inches. Dispose of
sediments in accordance with current local health department requirements and the
Minimum Functional Standards for Solid Waste Handling. See Volume IV, Appendix
D Recommendations for Management of Street Waste for further guidance.
• Any standing water removed during the maintenance operation must be properly
disposed of. The preferred disposal option is discharge to a sanitary sewer at an
approved location. Other disposal options include discharge back into the wetpool
facility or the storm sewer system if certain conditions are met. See Volume IV,
Appendix D for additional guidance.
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Chapter 8 708
8.2.2 BMP T1020 Wetvaults
8.2.2.1 Description:
A wetvault is an underground structure similar in appearance to a detention vault, except that a
wetvault has a permanent pool of water (wetpool) which dissipates energy and improves the settling
of particulate pollutants (see the wetvault details in Figure V-8-31). Being underground, the wetvault
lacks the biological pollutant removal mechanisms, such as algae uptake, present in surface
wetponds.
8.2.2.2 Applications and Limitations:
A wetvault may be used for commercial, industrial, or roadway projects if there are space limitations
precluding the use of other treatment BMPs. The use of wetvaults for residential development is
highly discouraged. Combined detention and wetpools are allowed; see BMP T1040.
If oil control is required for a project, a wetvault may be combined with an API oil/water separator.
8.2.2.3 Design Criteria:
Sizing Procedure
The wetpool volume for the wetvault shall be equal to or greater than the total volume of runoff from
the 6-month, 24-hour storm event. Alternatively, the 91st percentile, 24-hour runoff volume estimated
by WWHM may be used.
Typical design details and concepts for the wetvault are shown in Figure V-8-31.
SURFACE WATER MANAGEMENT MANUAL
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Figure V-8-31. Wetvault
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8.2.2.4 Wetpool Geometry
Same as specified for wetponds (see BMP T1010) except for the following two modifications:
• The sediment storage in the first cell shall be an average of 1-foot. Because of the
v-shaped bottom, the depth of sediment storage needed above the bottom of the
side wall is roughly proportional to vault width according to the schedule below:
Vault Width Sediment Depth
(from bottom of side wall)
15’ 10”
20’ 9”
40’ 6”
60’ 4”
• The second cell shall be a minimum of 3 feet deep since planting cannot be used to
prevent re-suspension of sediment in shallow water as it can in open ponds.
8.2.2.5 Vault Structure
• Separate the vault into two cells by a wall or a removable baffle. If a wall is used,
provide a 5-foot by 10-foot removable maintenance access for both cells. If a
removable baffle is used, the following criteria apply:
o The baffle shall extend from a minimum of 1-foot above the WQ design water
surface to a minimum of 1-foot below the invert elevation of the inlet pipe.
o The lowest point of the baffle shall be a minimum of 2 feet from the bottom of
the vault, and greater if feasible.
• If the vault is less than 2,000 cubic feet (inside dimensions), or if the length-to-width
ratio of the vault pool is 5:1 or greater, the baffle or wall may be omitted and the vault
may be one-celled.
• Do not divide the two cells of a wetvault into additional subcells by internal walls. If
internal structural support is needed, it is preferred that post and pier construction be
used to support the vault lid rather than walls. Any walls used within cells must be
positioned so as to lengthen, rather than divide, the flowpath.
• Slope the bottom of the first cell toward the access opening. Slope shall be between
0.5 percent (minimum) and 2 percent (maximum). The second cell may be level
(longitudinally) sloped toward the outlet, with a high point between the first and
second cells. Sloping the second cell towards the access opening for the first cell is
also acceptable. Alternatively, access openings may be positioned a maximum of
10 feet from any location within the vault.
• Slope the vault bottom laterally a minimum of 5 percent from each side towards the
center, forming a broad "v" to facilitate sediment removal. Note: More than one "v"
may be used to minimize vault depth.
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Chapter 8 711
The City may allow the vault bottom to be flat if removable panels are provided over the entire
vault. Removable panels shall be at grade, have stainless steel lifting eyes, and weigh no
more than 5 tons per panel.
• The highest point of a vault bottom must be at least 6 inches below the outlet
elevation to provide for sediment storage over the entire bottom.
• Evaluate buoyancy of the vault with the results presented in design documentation.
• Wetvaults may be constructed using arch culvert sections provided the top area at
the WQ design water surface is, at a minimum, equal to that of a vault with vertical
walls designed with an average depth of 6 feet.
• Wetvaults shall conform to the "Materials" and "Structural Stability" criteria specified
for detention vaults in Volume III, Section 2.3.3.
• Where pipes enter and leave the vault below the WQ design water surface, they
shall be sealed using water tight seals or couplers.
• Galvanized materials shall not be used unless coated.
8.2.2.6 Inlet and Outlet
• Submerge the inlet to the wetvault. The inlet pipe invert shall be a minimum of 3 feet
from the vault bottom. Submerge the top of the inlet pipe at least 1-foot, if possible.
• Unless designed as an off-line facility, the capacity of the outlet pipe and available
head above the outlet pipe shall be designed to convey the 100-year design flow for
developed site conditions without overtopping the vault. The available head above
the outlet pipe must be a minimum of 6 inches.
• The outlet pipe shall be back-sloped or have tee section, the lower arm of which shall
extend 1 foot below the WQ design water surface to provide for trapping of oils and
floatables in the vault.
• Center the inlet and outlet pipes over the “V” portion of the vault.
8.2.2.7 Access Requirements
Same as for detention vaults (see Volume III, Section 2.3.3), except for the following additional
requirement for wetvaults:
• Provide a minimum of 50 square feet of grate over the second cell. For vaults in
which the surface area of the second cell is greater than 1,250 square feet, grate
4 percent of the top. This requirement may be met by one grate or by many smaller
grates distributed over the second cell area.
Note: A grated access door can be used to meet this requirement.
8.2.2.8 Access Roads, Right of Way, and Setbacks
Same as for detention vaults (see Volume III, Section 2.3.3).
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8.2.2.9 Recommended Design Features
The following design features should be incorporated into wetvaults where feasible, but they are not
specifically required:
• The floor of the second cell should slope toward the outlet for ease of cleaning.
• The inlet and outlet should be at opposing corners of the vault to increase the
flowpath.
• A flow length-to-width ratio greater than 3:1 minimum is desirable.
• Lockable grates instead of solid manhole covers are recommended to increase air
contact with the wetpool.
• The number of inlets to the wetvault should be limited, and the flowpath length
should be maximized from inlet to outlet for all inlets to the vault.
8.2.2.10 Construction Criteria
Remove sediment that has accumulated in the vault after construction in the drainage area is
complete.
8.2.2.11 Operation and Maintenance
• Vault maintenance procedures must meet OSHA confined space entry requirements,
which include clearly marking entrances to confined space areas.
• Facilities may be inspected by the City. The maintenance standards contained in
Volume I, Appendix D are measures for determining if maintenance actions are
required as identified through the inspection.
• Remove sediment when the 1-foot sediment zone is full plus 6 inches. Test
sediments for toxicants in compliance with current disposal requirements. Dispose of
sediments in accordance with current local health department requirements and the
Minimum Functional Standards for Solid Waste Handling. See Volume IV,
Appendix D Recommendations for Management of Street Waste for additional
guidance.
• Dispose of any standing water removed during the maintenance operation. The
preferred disposal option is discharge to a sanitary sewer at an approved location.
Other disposal options include discharge back into the wetpool facility or the storm
sewer system if certain conditions are met. See Volume IV, Appendix D for additional
guidance.
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8.2.2.12 Modifications for Combining with a Baffle Oil/Water Separator
If the project site is a high-use site and a wetvault is proposed, the vault may be combined with a
baffle oil/water separator to meet the runoff treatment requirements with one facility rather than two.
• The sizing procedures for the baffle oil/water separator (Chapter 9) shall be run as a
check to ensure the vault is large enough. If the oil/water separator sizing procedures
result in a larger vault size, increase the wetvault size to match.
• An oil retaining baffle shall be provided in the second cell near the vault outlet. The
baffle shall not contain a high-flow overflow, or else the retained oil will be washed
out of the vault during large storms.
• The vault shall have a minimum length-to-width ratio of 5:1.
• The vault shall have a design water depth-to-width ratio of between 1:3 and 1:2.
• The vault shall be watertight and shall be coated to protect from corrosion.
• Separator vaults shall have a shutoff mechanism on the outlet pipe to prevent oil
discharges during maintenance and to provide emergency shut-off capability in case
of a spill. Also, provide a valve box and riser.
• Wetvaults used as oil/water separators must be off-line and must bypass flows
greater than the off-line WQ design flow multiplied by 3.5. This will minimize the
entrainment and/or emulsification of previously captured oil during very high flow
events.
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8.2.3 BMP T1030 Stormwater Treatment Wetlands
8.2.3.1 Description
Stormwater treatment wetlands are shallow man-made ponds that are designed to treat stormwater
through the biological processes associated with emergent aquatic plants (see the stormwater
wetland details in Figure V-8-32 and Figure V-8-33).
Wetlands created to mitigate disturbance impacts, such as filling of wetlands, shall not be used as
stormwater treatment facilities.
8.2.3.2 Applications and Limitations
The most critical factor for a successful design is the provision of an adequate supply of water for
most of the year. Since water depths are shallower than in wetponds, water loss by evaporation is an
important concern. Stormwater wetlands are a good WQ facility choice in areas with high winter
groundwater levels.
8.2.3.3 Design Criteria
Stormwater wetlands use most of the same design criteria as wetponds (see above). However,
instead of gravity settling being the dominant treatment process, pollutant removal mediated by
aquatic vegetation and the microbiological community associated with that vegetation becomes the
dominant treatment process. Thus when designing wetlands, water volume is not the dominant
design criteria. Rather, factors which affect plant vigor and biomass are the primary concerns.
8.2.3.4 Sizing Procedure
1. The design volume is the total volume of runoff from the 6-month, 24-hour storm event or,
alternatively, the 91st percentile, 24-hour runoff volume using WWHM.
2. Calculate the surface area of the stormwater wetland. The surface area of the wetland shall
be the same as the top area of a wetpond sized for the same site conditions. Calculate the
surface area of the stormwater wetland by using the volume from Step 1 and dividing by the
average water depth (use 3 feet).
3. Determine the surface area of the first cell of the stormwater wetland. Use the volume
determined from Criterion 2 under "Wetland Geometry", below, and the actual depth of the
first cell.
4. Determine the surface area of the wetland cell. Subtract the surface area of the first cell
(Step 3) from the total surface area (Step 2).
5. Determine water depth distribution in the second cell. Decide if the top of the dividing berm
will be at the surface or submerged (designer's choice). Adjust the distribution of water depths
in the second cell according to Step 8 in Section 8.2.3.5. This will result in a facility that holds
less volume than that determined in Step 1 above. This is acceptable.
6. Choose plants. See Table V-8-15 for a list of plants recommended for wetpond water depth
zones, or consult a wetland scientist.
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8.2.3.5 Wetland Geometry
1. Stormwater wetlands shall consist of two cells, a presettling cell and a wetland cell.
2. The presettling cell shall contain approximately 33 percent of the wetpool volume
calculated in Step 1 above.
3. The depth of the presettling cell shall be between 4 feet (minimum) and 8 feet
(maximum), excluding sediment storage.
4. Provide one-foot of sediment storage in the presettling cell.
5. The wetland cell shall have an average water depth of about 1.5 feet (plus or minus
3 inches).
6. Shape the "berm" separating the two cells such that its downstream side gradually
slopes to form the second shallow wetland cell (see the section view in Figure V-8-
32). Alternatively, the second cell may be graded naturalistically from the top of the
dividing berm (see Step 8 below).
7. The top of the berm shall be either at the WQ design water surface or submerged
1 foot below the WQ design water surface. Correspondingly, the side slopes of the
berm must meet the following criteria:
a. If the top of berm is at the WQ design water surface, the berm side slopes
shall be no steeper than 3H:1V.
b. If the top of berm is submerged 1-foot, the upstream side slope may be up to
2H:1V.
8. Grade the bottom of the wetland cell in one of two ways:
a. Shallow evenly graded slope from the upstream to the downstream edge of the
wetland cell (see Figure V-8-32).
b. A "naturalistic" alternative, with the specified range of depths intermixed
throughout the second cell (see Figure V-8-33). A distribution of depths shall be
provided in the wetland cell depending on whether the dividing berm is at the
water surface or submerged (see Table V-8-16).
The maximum depth shall be 2.5 feet in either configuration.
8.2.3.6 Lining Requirements
In infiltrative soils, line both cells of the stormwater wetland. To determine whether a low-permeability
liner or a treatment liner is required, determine whether the following conditions will be met. If soil
permeability will allow sufficient water retention, lining may be waived.
1. The second cell must retain water for at least 10 months of the year.
2. The first cell must retain at least three feet of water year-round.
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3. Use a complete precipitation record when establishing these conditions. Take into
account evapo-transpiration losses as well as infiltration losses. Many wetland plants
can adapt to periods of summer drought, so a limited drought period is allowed in the
second cell. This may allow a treatment liner rather than a low permeability liner to be
used for the second cell. The first cell must retain water year-round in order for the
presettling function to be effective.
4. If a low permeability liner is used, place a minimum of 18 inches of native soil amended
with good topsoil or compost (one part compost mixed with 3 parts native soil) over the
liner. For geomembrane liners, a soil depth of 3 feet is recommended to prevent damage
to the liner during planting. Hydric soils are not required.
The criteria for liners given in Chapter 3 must be observed.
8.2.3.7 Inlet and Outlet
Same as for wetponds (see BMP T1010).
8.2.3.8 Access and Setbacks
Location of the stormwater wetland relative to site constraints (e.g., buildings, property
lines, etc.) shall be the same as for detention ponds (see Volume III). See Chapter 3 for
typical setback requirements for WQ facilities.
Provide access and maintenance roads and design them according to the requirements
for detention ponds (see Volume III). Extend access and maintenance roads shall to
both the wetland inlet and outlet structures. Provide an access ramp (7H minimum:1V) to
the bottom of the first cell unless all portions of the cell can be reached and sediment
loaded from the top of the wetland side slopes.
The internal berm of a stormwater treatment wetland may be used for access if it is
designed to support a loaded 80,000 pound truck considering the berm is normally
submerged and saturated.
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Figure V-8-32. Stormwater Wetland – Option 1
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Figure V-8-33. Stormwater Wetland – Option 2
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Table V-8-16. Distribution of Depths in Wetland Cell
Dividing Berm at WQ Design Water Surface Dividing Berm Submerged 1-Foot
Depth Range (feet) Percent Depth Range (feet) Percent
0.1 to 1 25 1 to 1.5 40
1 to 2 55 1.5 to 2 40
2 to 2.5 20 2 to 2.5 20
8.2.3.9 Planting Requirements
Plant the wetland cell with emergent wetland plants following the recommendations given in Table V-
8-15 or the recommendations of a wetland specialist. Cattails (Typha latifolia) are not allowed.
8.2.3.10 Construction Criteria
• Construction and maintenance considerations are the same as for wetponds.
• Construction of the naturalistic alternative (Option 2) can be accomplished by first
excavating the entire area to the 1.5-foot average depth. Then soil subsequently
excavated to form deeper areas can be deposited to raise other areas until the
distribution of depths indicated in the design is achieved.
8.2.3.11 Operation and Maintenance
• Inspect wetlands at least twice per year during the first three years during both
growing and non-growing seasons to observe plant species presence, abundance,
and condition; bottom contours and water depths relative to plans; and sediment,
outlet, and buffer conditions.
• Schedule maintenance around sensitive wildlife and vegetation seasons.
• Plants may require watering, physical support, mulching, weed removal, or replanting
during the first three years.
• Remove nuisance plant species and replant desirable species.
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8.2.4 BMP T1040 Combined Detention and Wetpool Facilities
8.2.4.1 Description:
Combined detention and WQ wetpool facilities have the appearance of a detention facility but contain
a permanent pool of water as well. The following design procedures, requirements, and
recommendations cover differences in the design of the stand-alone WQ facility when combined with
detention storage. The following combined facilities are addressed:
• Detention/wetpond (basic and large)
• Detention/wetvault
• Detention/stormwater wetland.
There are two sizes of the combined wetpond, a basic and a large, but only a basic size for the
combined wetvault and combined stormwater wetland. The facility sizes (basic and large) are related
to the pollutant removal goals. See Chapter 3 for more information about treatment performance
goals.
8.2.4.2 Applications and Limitations:
Combined detention and water quality facilities are efficient for sites that also have detention
requirements. The water quality facility may often be placed beneath the detention facility without
increasing the facility surface area. However, the fluctuating water surface of the live storage will
create unique challenges for plant growth and for aesthetics alike.
The live storage component of the facility shall be provided above the seasonal high water table.
8.2.4.3 Design Criteria:
Typical design details and concepts for a combined detention and wetpond are shown in Figure V-8-
34 and Figure V-8-35. The detention portion of the facility shall meet the design criteria and sizing
procedures set forth in Volume III.
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Figure V-8-34. Combined Detention and Wetpond (top view)
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Figure V-8-35. Combined Detention and Wetpond (side view)
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8.2.4.4 Sizing
The sizing for combined detention and wetponds are identical to those for wetponds and for detention
facilities. The wetpool volume for a combined facility shall be equal to or greater than the total volume
of runoff from the 6-month, 24-hour storm event or the 91st percentile 24-hour runoff volume
estimated by WWHM. Follow the standard procedure specified in Volume III to size the detention
portion of the pond.
8.2.4.5 Detention and Wetpool Geometry
• Do not include the wetpool and sediment storage volumes in the required detention
volume.
• The "Wetpool Geometry" criteria for wetponds (see BMP T1010) shall apply with the
following modifications/clarifications:
o The permanent pool may be shallower to comprise most of the pond bottom,
or deeper positioned to comprise a limited portion of the bottom. Note, having
the first wetpool cell at the inlet allows for more efficient sediment
management than if the cell is moved away from the inlet. Wetpond criteria
governing water depth must still be met. See Figure V-8-36 for two
possibilities for wetpool cell placement.
o The minimum sediment storage depth in the first cell is 1-foot. The 6 inches
of sediment storage required for a detention pond does not need to be added
to this, but 6 inches of sediment storage must be added to the second cell to
comply with the detention sediment storage requirement.
8.2.4.6 Berms, Baffles and Slopes
Same as for wetponds (see BMP T1010)
8.2.4.7 Inlet and Outlet
The Inlet and Outlet criteria for wetponds shall apply with the following modifications:
• Provide a sump in the outlet structure of combined ponds.
• Design the detention flow restrictor and its outlet pipe according to the requirements
for detention ponds (see Volume III).
8.2.4.8 Access and Setbacks
The same as for wetponds.
8.2.4.9 Planting Requirements
The same as for wetponds.
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8.3 Combined Detention and Wetvault
The sizing procedure for combined detention and wetvaults is identical to those outlined for wetvaults
and for detention facilities. The design criteria for detention vaults and wetvaults must both be met,
except for the following modifications or clarifications:
• The minimum sediment storage depth in the first cell shall average 1-foot. The
6 inches of sediment storage required for detention vaults do not need to be added
to this, but 6 inches of sediment storage must be added to the second cell to comply
with detention vault sediment storage requirements.
• The oil retaining baffle shall extend a minimum of 2 feet below the WQ design water
surface.
• If a vault is used for detention as well as water quality control, the facility may not be modified
to function as a baffle oil/water separator as allowed for wetvaults in BMP T1020.
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Figure V-8-36. Alternative Configurations of Detention and Wetpool Areas
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8.4 Combined Detention and Stormwater Wetland
8.4.1 Sizing Criteria
The sizing procedure for combined detention and stormwater wetlands is identical to those outlined
for stormwater wetlands and for detention facilities. Follow the procedure specified in BMP T1030 to
determine the stormwater wetland size. Follow the standard procedure specified in Volume III to size
the detention portion of the wetland.
Water Level Fluctuation Restrictions: The difference between the WQ design water surface and the
maximum water surface associated with the 2-year runoff shall not be greater than 3 feet. If this
restriction cannot be met, the size of the stormwater wetland must be increased. The additional area
may be placed in the first cell, second cell, or both. If placed in the second cell, the additional area
need not be planted with wetland vegetation or counted in calculating the average depth.
8.4.2 Design Criteria
The design criteria for detention ponds and stormwater wetlands must both be met, except for the
following modifications or clarifications The Wetland Geometry criteria for stormwater wetlands (see
BMP T1030) shall be modified as follows:
• The minimum sediment storage depth in the first cell is 1-foot. The 6 inches of
sediment storage required for detention ponds does not need to be added to this, nor
does the 6 inches of sediment storage in the second cell of detention ponds need to
be added.
8.4.3 Inlet and Outlet Criteria
The Inlet and Outlet criteria for wetponds shall apply with the following modifications:
• Provide a sump in the outlet structure of combined facilities.
• Design the detention flow restrictor and its outlet pipe according to the requirements
for detention ponds (see Volume III).
8.4.4 Planting Requirements
The Planting Requirements for stormwater wetlands are modified to use the following plants which
are better adapted to water level fluctuations:
Scirpus acutus (hardstem bulrush) 2 - 6' depth
Scirpus microcarpus (small-fruited bulrush) 1 - 2.5' depth
Sparganium emersum (burreed) 1 - 2' depth
Sparganium eurycarpum (burreed) 1 - 2' depth
Veronica sp. (marsh speedwell) 0 - 1' depth
In addition, the shrub Spirea douglasii (Douglas spirea) may be used in combined facilities.
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Chapter 9 Oil and Water Separators
9.1 Purpose
Oil and water separators remove oil and other water-insoluble hydrocarbons and settleable solids
from stormwater runoff. This chapter provides a discussion of their application and design criteria.
Best Management Practices (BMPs) are described for baffle type and coalescing plate separators.
9.2 Description
Oil and water separators are typically the American Petroleum Institute (API) (also called baffle type)
(American Petroleum Institute, 1990) or the coalescing plate (CP) type using a gravity mechanism for
separation. See Figure V-9-37 and Figure V-9-38. Oil and water separators typically consist of three
bays; forebay, separator section, and the afterbay. The CP separators need considerably less space
for separation of the floating oil due to the shorter travel distances between parallel plates. A spill
control (SC) separator (Figure V-9-39) is a simple catch basin with a T-inlet for temporarily trapping
small volumes of oil. The spill control separator is included here for comparison only and is not
designed for, or to be used for, treatment purposes.
9.3 Applications/Limitations
The following are potential applications of oil and water separators where free oil is expected to be
present at treatable high concentrations and sediment will not overwhelm the separator.
• Commercial and industrial areas including petroleum storage yards, vehicle
maintenance facilities, manufacturing areas, airports, utility areas (water, electric,
gas), and fueling stations. (King County Surface Water Management, 2005).
• Facilities that would require oil control BMPs under the high-use site threshold
described in Chapter 2 including parking lots at convenience stores, fast food
restaurants, grocery stores, shopping malls, discount warehouse stores, banks, truck
fleets, auto and truck dealerships, and delivery services. (King County Surface Water
Management, 2005).
For low concentrations of oil, other treatments may be more applicable. These include sand filters
and emerging technologies.
Without intense maintenance oil/water separators may not be sufficiently effective in achieving oil and
TPH removal down to required levels.
Pretreatment should be considered if the level of total suspended solids (TSS) in the inlet flow would
cause clogging or otherwise impair the long-term efficiency of the separator.
For inflows from small drainage areas (fueling stations, maintenance shops, etc.) a coalescing plate
(CP) type separator is typically considered, due to space limitations. However, if plugging of the
plates is likely, then a new design basis for the baffle type API separator may be considered on an
experimental basis (see Section 9.6).
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Figure V-9-37. API (Baffle Type) Separator
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Figure V-9-38. Coalescing Plate Separator
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Source: 1992 Ecology Manual
Figure V-9-39. Spill Control Separator (not for oil treatment)
9.4 Site Suitability
Consider the following site characteristics:
• Sufficient land area
• Adequate TSS control or pretreatment capability
• Compliance with environmental objectives
• Adequate influent flow attenuation and/or bypass capability
• Sufficient access for operation and maintenance (O&M)
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9.5 Design Criteria
9.5.1 General Considerations
The following are design criteria applicable to API and CP oil/water separators:
• If practicable, determine oil/grease (or TPH) and TSS concentrations, lowest
temperature, pH, empirical oil rise rates in the runoff, and the viscosity and specific
gravity of the oil. Also determine whether the oil is emulsified or dissolved.
(Washington State Department of Ecology, 2005) Do not use oil/water separators for
the removal of dissolved or emulsified oils such as coolants, soluble lubricants,
glycols, and alcohols.
• Locate the separator off-line and bypass the incremental portion of flows that exceed
the off-line 15-minute water quality design flow rate multiplied by 3.5. If it is
necessary to locate the separator on-line, try to minimize the size of the area
needing oil control, and use the on-line water quality design flow rate multiplied by
2.0.
• Use only storm drain pipes or impervious conveyances for routing oil contaminated
stormwater to the oil and water separator.
• Specify appropriate performance tests after installation and shakedown, and/or
certification by a professional engineer that the separator is functioning in
accordance with design objectives. Expeditious corrective actions must be taken if it
is determined that the separator is not achieving acceptable performance levels.
• Add pretreatment for TSS that could clog the separator, or otherwise impair the long-
term effectiveness of the separator.
• All piping entering and leaving the separator must be six inches (6”) minimum
diameter.
• If a pump mechanism is required to convey the discharge from the site to the
sanitary sewer system, the pump must be designed to discharge to a controlled
gravity outlet flow into the City system.
• Access points in the top of the vault must be provided to allow a minimum twelve-
inch (12”) diameter access for observation and maintenance to all chambers of the
separator.
• Access doors as applicable, should be galvanized spring-assisted diamond plate
with a penta-head bolt-locking latch and recessed lift handle.
• Doors must open a full one hundred and eighty degrees (180º).
• Access to the separator shall be maintained free for inspection at all times.
9.5.2 Criteria for Separator Bays
• Size the separator bay for the off-line 15-minute water quality design flow rate
predicted by WWHM multiplied by 3.5.
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• To collect floatables and settleable solids, design the surface area of the forebay at
20 ft² per 10,000 ft² of area draining to the separator. The length of the forebay
shall be 1/3-1/2 of the length of the entire separator. Include roughing screens for the
forebay or upstream of the separator to remove debris, if needed. Screen openings
shall be about 3/4 inch.
• Include a submerged inlet pipe with a down-turned elbow in the first bay at least
two feet from the bottom. The outlet pipe shall be a Tee, sized to pass the design
peak flow and placed at least 12 inches below the water surface.
• Include a shutoff mechanism at the separator outlet pipe.
• Use absorbents and/or skimmers in the afterbay as needed.
9.5.3 Criteria for Baffles
• Oil retaining baffles (top baffles) shall be located at least 1/4 of the total separator
length from the outlet and shall extend down at least 50% of the water depth and at
least 1 foot from the separator bottom.
• Baffle height to water depth ratios shall be 0.85 for top baffles and 0.15 for bottom
baffles.
9.6 Oil and Water Separator BMPs
Two BMPs are described in this section:
• BMP T1110 for baffle type separators
• BMP T1111 for coalescing plate separators
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9.6.1 BMP T1110 API (Baffle type) Separator Bay
9.6.1.1 Design Criteria
The criteria for small drainages is based on Vh, Vt, residence time, width, depth, and length
considerations. As a correction factor API's turbulence criteria is applied to increase the length.
9.6.1.2 Sizing Criteria
• Determine the oil rise rate, Vt, in cm/sec, using Stokes’ Law, or empirical
determination, or 0.033 ft./min. for 60µ (micron) oil droplet size. The application of
Stokes’ Law to site-based oil droplet sizes and densities, or empirical rise rate
determinations recognizes the need to consider actual site conditions. In those cases
the design basis would not be the 60 micron droplet size and the 0.033 ft/min. rise
rate.
Stokes Law equation for rise rate, Vt (cm/sec):
Vt = [(g)(w – o)(d²)] / [(18*w)]
Where:
Vt = the rise rate of the oil droplet (cm/s or ft/sec)
g = acceleration due to gravity (cm/s² or ft/s²)
w = density of water at the design temperature (g/cm³ or lbm/ft³)
o = density of oil at the design temperature (g/cm³ or lbm/ft³)
d = oil droplet diameter (cm or ft)
w = absolute viscosity of the water (g/cm s or lbm/ft s)
Use:
oil droplet diameter, D=60 microns (0.006 cm)
w =0.999 g/cm³ at 32° F
o: Select conservatively high oil density,
For example, if diesel oil @ o =0.85 g/cm³ and motor oil @ o = 0.90 can be present then
use o =0.90 g/cm³
w = 0.017921 poise, gm/cm-sec. at Tw=32 °F
Use the following separator dimension criteria:
o Separator water depth, 3 d 8 feet (to minimize turbulence)
o Separator width, 6-20 feet
o Depth/width (d/w) of 0.3-0.5
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For Stormwater Inflow from Drainages under 2 Acres
Ecology modified the API criteria for treating stormwater runoff from small drainage areas (fueling
stations, commercial parking lots, etc.) by using the design hydraulic horizontal velocity, Vh, for the
design Vh/Vt ratio rather than the API minimum of Vh/Vt = 15. The API criteria appear applicable for
greater than two acres of impervious drainage area.
Performance verification of this design basis must be obtained during at least one wet
season using the test protocol referenced in Chapter 10 for new technologies.
The following is the sizing procedure using modified API criteria:
1. Determine Vt and select depth and width of the separator section based on above criteria.
2. Calculate the minimum residence time (tm) of the separator at depth d:
tm = d/Vt
3. Calculate the horizontal velocity of the bulk fluid, Vh, vertical cross-sectional area, Av, and
actual design Vh/Vt.
Vh = Q/dw = Q/Av (Vh maximum at < 2.0 ft/min.)
Q = (k) . Use a value of 3.5 for K for the site location multiplied by the off-line 15 minute
water quality design flow rate in ft³/min determined by WWHM, at minimum residence
time, tm
At Vh/Vt determine F, turbulence and short-circuiting factor API F factors range from
1.28-1.74 (see Appendix D).
4. Calculate the minimum length of the separator section, l(s), using:
l(s) = FQtm/wd = F(Vh/Vt)d
l(t) = l(f) + l(s) +l(a)
l(t) = l(t)/3 + l(s) + l(t)/4
Where:
l(t) = total length of 3 bays
l(f) = length of forebay
l(a) = length of afterbay
5. Calculate V = l(s)wd = FQtm, and Ah = wl(s)
V = minimum hydraulic design volume
Ah = minimum horizontal area of the separator
For Stormwater Inflow from Drainages > 2 Acres:
Use Vh = 15 Vt and d = (Q/2Vh)¹/² (with d/w = 0.5) and repeat above calculations 3-5.
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9.6.2 BMP T1111 Coalescing Plate (CP) Separator Bay
9.6.2.1 Design Criteria
Calculate the projected (horizontal) surface area of plates needed using the following equation:
Ah = Q/Vt = [Q] / [(0.00386) * ((Sw - So)/(w))]
Where
Ah = horizontal surface area of the plates (ft²)
Vt = rise rate of the oil droplet (ft/min)
Q = design flowrate (ft³/min) The design flowrate is the off-line 15-minute
water quality design flowrate predicted by WWHM multiplied by 3.5.
Sw = specific gravity of water at the design temperature
So = specific gravity of oil at the design temperature
w = absolute viscosity of the water (poise)
• Plate spacing shall be a minimum of 3/4 inches (perpendicular distance between
plates).
• Select a plate angle between 45° to 60° from the horizontal.
• Locate plate pack at least 6 inches from the bottom of the separator for sediment
storage.
• Add 12 inches minimum head space from the top of the plate pack and the bottom of
the vault cover.
• Design inlet flow distribution and baffles in the separator bay to minimize turbulence,
short-circuiting, and channeling of the inflow especially through and around the plate
packs of the CP separator. The Reynolds Number through the separator bay shall be
<500 (laminar flow).
• Include forebay for floatables and afterbay for collection of effluent.
• The sediment-retaining baffle must be upstream of the plate pack at a minimum
height of 18 in.
• Design plates for ease of removal, and cleaning with high-pressure rinse or
equivalent.
9.6.2.2 Operation and Maintenance
• Prepare, regularly update, and implement an O & M Manual for the oil/water
separators.
• Inspect oil/water separators monthly during the wet season of October 1-April 30
(WEF & ASCE, 1998) to ensure proper operation, and, during and immediately after
a large storm event of 1 inch per 24 hours.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Oil and Water Separators Volume V
Chapter 9 736
• Clean oil/water separators regularly to keep accumulated oil from escaping during
storms. They must be cleaned by October 15 to remove material that has
accumulated during the dry season, after all spills, and after a significant storm.
Coalescing plates may be cleaned in-situ or after removal from the separator. An
eductor truck may be used for oil, sludge, and washwater removal. Replace wash
water in the separator with clean water before returning it to service.
• Remove the accumulated oil when the thickness reaches 1-inch. Also remove sludge
deposits when the thickness reaches 6 inches.
• Replace oil absorbent pads before their sorbed oil content reaches capacity.
• Train designated employees on appropriate separator operation, inspection, record
keeping, and maintenance procedures.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Emerging Technologies Volume V
Chapter 10 737
Chapter 10 Emerging Technologies
10.1 Background
Traditional best management practices (BMPs) such as wetponds and filtration swales may not be
appropriate in many situations due to size and space restraints or their inability to remove target
pollutants. Because of this, the stormwater treatment industry emerged and new stormwater
treatment devices are currently in development.
Emerging technologies are those new stormwater treatment devices that are continually being added
to the stormwater treatment marketplace. These devices include both permanent and construction
site treatment technologies. Many of these devices have not undergone complete performance
testing so their performance claims cannot be verified.
10.2 Emerging Technology and the City of Auburn
Typically devices with general use level designation (GULD) will be accepted for private stormwater
treatment. Devices with GULD status may be accepted in the right-of-way with preapproval.
10.3 Ecology Role in Evaluating Emerging Technologies
To aid local governments in selecting new stormwater treatment technologies the Department of
Ecology (Ecology) developed the Technology Assessment Protocol – Ecology (TAPE) and Chemical
Technology Assessment Protocol Ecology (CTAPE) protocols. These protocols provide
manufacturers with guidance on stormwater monitoring so they may verify their performance claims.
As a part of this process Ecology:
• Posts information on emerging technologies at the emerging technologies website:
http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html.
• Participates in all Technical Review Committee (TRC) and Chemical Technical
Review Committee (CTRC) activities which include reviewing manufacturer
performance data and providing recommendations on use level designations.
• Grants use level designations based on performance and other pertinent data
submitted by the manufacturers and vendors.
• Provides oversight and analysis of all submittals to ensure consistency with this
manual.
10.4 Evaluation of Emerging Technologies
Local governments should consider the following as they make decisions concerning the use of new
stormwater treatment technologies in their jurisdiction:
Remember the Goal:
The goal of any stormwater management program or BMP is to treat and release stormwater in a
manner that does not harm beneficial uses.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Emerging Technologies Volume V
Chapter 10 738
Exercise Reasonable Caution:
Before allowing a new technology for an application, the local government should review evaluation
information based on the TAPE or CTAPE.
An emerging technology cannot be used for new or redevelopment unless this technology has a use
level designation. Having a use level designation means that Ecology and the TRC or CTRC
reviewed system performance data and believe the technology has the ability to provide the level of
treatment claimed by the manufacturer.
To achieve the goals of the Clean Water Act and the Endangered Species Act, local governments
may find it necessary to retrofit stormwater pollutant control systems for many existing stormwater
discharges. In retrofit situations, the use of any BMP that makes substantial progress toward these
goals is a step forward and Ecology encourages this. To the extent practical, the performance of
BMPs used in retrofit situations should be evaluated using the TAPE or CTAPE protocols.
10.5 Assessing Levels of Development of Emerging Technologies
Ecology developed use level designations to assess levels of development for emerging
technologies. The use level designations are based upon the quantity, quality, and type of
performance data. There are three use level designations: pilot use level designation, conditional use
level designation, and general use level designation.
Pilot Use Level Designation (PULD)
For technologies that have limited performance data, the pilot use level designation allows limited use
to enable field testing to be conducted. Pilot use level designations may be given based solely on
laboratory performance data. Pilot use level designations apply for a specified time period only.
During this time period, the proponent must complete all field testing and submit a technology
evaluation report (TER) to Ecology and the TRC. Ecology will limit the number of installations to five
during the pilot use level period.
Local governments may allow PULD technologies to be installed if the manufacturer agrees to
conduct additional field testing based on the TAPE at all sites to obtain a general use level
designation. Local governments covered by a municipal stormwater NPDES permit must notify
Ecology in writing when a PULD technology is proposed. The form can be found:
http://www.ecy.wa.gov/programs/wq/stormwater/newtech/PULDNOI.pdf
Conditional Use Level Designation (CULD)
For emerging technologies that have considerable performance data that was not collected per the
TAPE protocol, the CULD was established. Conditional use level designations may be given if field
data has been collected by a protocol that is reasonably consistent but does not necessarily fully
meet the TAPE protocol. The field data must meet the statistical goals set out in the TAPE guidelines
Appendix D. Laboratory data may be used to supplement field data. Technologies that are granted a
CULD will be allowed continued use for a specified time period, during which the field testing
necessary to obtain a general use level designation (GULD) must be completed and a TER must be
submitted to Ecology and the TRC. Ecology will limit the number of installations to ten during the
CULD period.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Emerging Technologies Volume V
Chapter 10 739
General Use Level Designation (GULD)
The GULD confers a general acceptance for the specified applications (land uses). Technologies
with a GULD may be used anywhere in Washington, subject to Ecology conditions.
10.6 Examples of Emerging Technologies for Stormwater Treatment and
Control
Go to the Ecology Emerging Technologies website to obtain information on technologies that have
obtained a use level designation:
http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html
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SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Basic Treatment Receiving Waters Volume V
Appendix A 741
Appendix A Basic Treatment Receiving Waters
All Salt Waterbodies
Rivers Upstream Point for Exemption
Baker Anderson Creek
Bogachiel Bear Creek
Cascade Marblemount
Chehalis Bunker Creek
Clearwater Town of Clearwater
Columbia Canadian Border
Cowlitz Skate Creek
Elwha Lake Mills
Green Howard Hanson Dam
Hoh South Fork Hoh River
Humptulips West and East Fork Confluence
Kalama Italian Creek
Lewis Swift Reservoir
Muddy Clear Creek
Nisqually Alder Lake
Nooksack Glacier Creek
South Fork Nooksack Hutchinson Creek
North River Raymond
Puyallup Carbon River
Queets Clearwater River
Quillayute Bogachiel River
Quinault Lake Quinault
Sauk Clear Creek
Satsop Middle and East Fork Confluence
Skagit Cascade River
Skokomish Vance Creek
Skykomish Beckler River
Snohomish Snoqualmie River
Snoqualmie Middle and North Fork Confluence
Sol Duc Beaver Creek
Stillaguamish North and South Fork Confluence
North Fork Stillaguamish Boulder River
South Fork Stillaguamish Canyon Creek
Suiattle Darrington
Tilton Bear Canyon Creek
Toutle North and South Fork Confluence
North Fork Toutle Green River
Washougal Washougal
White Greenwater River
Wind Carson
Wynoochee Wishkah River Road Bridge
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Basic Treatment Receiving Waters Volume V
Appendix A 742
Lakes County
Washington King
Sammamish King
Union King
Whatcom Whatcom
Silver Cowlitz
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Procedure for Conducting a Volume V
Pilot Infiltration Test Appendix B 743
Appendix B Procedure for Conducting a Pilot
Infiltration Test
The Pilot Infiltration Test (PIT) consists of a relatively large-scale infiltration test to better approximate
infiltration rates for design of stormwater infiltration facilities. The PIT reduces some of the scale
errors associated with relatively small-scale double ring infiltrometer or “stove-pipe” infiltration tests. It
is not a standard test but rather a practical field procedure recommended by Ecology’s Technical
Advisory Committee.
Infiltration Test
1. 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.
2. 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.
3. Accurately document the size and geometry of the test pit.
4. Install a vertical measuring rod (minimum 5-ft. long) marked in half-inch increments in
the center of the pit bottom.
5. 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.
6. 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.
7. 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.
8. 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).
9. After the flow rate has stabilized, turn off the water and record the rate of infiltration in
inches per hour from the measuring rod data, until the pit is empty.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Procedure for Conducting a Volume V
Pilot Infiltration Test Appendix B 744
Data Analysis
1. 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.
NOTE: Use statistical/trend analysis to obtain the hourly flow rate when the flow stabilizes.
This would be the lowest hourly flow rate.
2. Apply appropriate correction factors for site heterogeneity, anticipated level of maintenance
and treatment to determine the site-specific design infiltration rate (see Table V-B-17).
Table V-B-17. Correction Factors to be Used with In-Situ Infiltration Measurements
to Estimate Long-Term Design Infiltration Rates
Issue Partial Correction Factor
Site variability and number of locations
tested
CFv = 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
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, the design long-term infiltration rate becomes
2 inches per hour, anticipating adequate maintenance and pre-treatment.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Geotextile Specifications Volume V
Appendix C 745
Appendix C Geotextile Specifications
Table V-C-18. Geotextile Properties for Underground Drainage Survivability Properties
(WSDOT 2008 Standard Specifications or most current version)
Geotextile Property Requirements1
Low Survivability Moderate Survivability
Geotextile Property Test Method Woven/Nonwoven Woven/Nonwoven
Grab Tensile Strength,
min. in machine and
x-machine direction
ASTM D4632 180 lbs/115 lbs min. 250 lbs/160 lbs min.
Grab Failure Strain, in
machine and x-
machine direction
ASTM D4632 <50%/>50% <50%/>50%
Seam Breaking
Strength (if seams are
present)
ASTM D4632
and
160 lbs/100 lbs min. 220 lbs/140 lbs min.
Puncture Resistance ASTM D6241 370 lbs/220lbs min. 495 lbs/310 lbs min.
Tear Strength, min. in
machine and x-
machine direction
ASTM D4533 67 lbs/40 lbs min. 80 lbs/50 lbs min.
Ultraviolet (UV)
Radiation stability ASTM D4355
50% strength retained
min., after 500 hrs. in a
xenon arc device
50% strength retained
min., after 500 hrs. in a
xenon arc device
1 All geotextile properties are minimum average roll values (i.e., the test result for any sampled roll in a lot shall meet
or exceed the values shown in the table).
Table V-C-19. Geotextile for Underground Drainage Filtration Properties
(WSDOT 2008 Standard Specifications or most current version)
Geotextile Property Requirements1
Geotextile
Property Test Method Class A Class B Class C
AOS2 ASTM D4751 0.43 mm max.
(#40 sieve)
0.25 mm max.
(#60 sieve)
0.18 mm max.
(#80 sieve)
Water
Permittivity ASTM D4491 0.5 sec -1 min. 0.4 sec -1 min. 0.3 sec -1 min.
1 All geotextile properties are minimum average roll values (i.e., the test result for any sampled roll in a lot shall meet
or exceed the values shown in the table).
2 Apparent Opening Size (measure of diameter of the pores in the geotextile)
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Geotextile Specifications Volume V
Appendix C 746
Table V-C-20. Geotextile Strength Properties for Impermeable Liner Protection
Geotextile Property Test Method Geotextile Property Requirements1
Grab Tensile Strength, min. in
machine and x-machine direction ASTM D4632 250 lbs min.
Grab Failure Strain, in machine
and x-machine direction ASTM D4632 >50%
Seam Breaking Strength
(if seams are present)
ASTM D4632 and
ASTM D4884
(adapted for grab test)
220 lbs min.
Puncture Resistance ASTM D4833 125 lbs min.
Tear Strength, min. in machine
and x-machine direction ASTM D4533 90 lbs min.
Ultraviolet (UV) Radiation ASTM D4355 50% strength stability retained min.,
after 500 hrs. in weatherometer
1 All geotextile properties are minimum average roll values (i.e., the test result for any sampled roll in a lot shall meet
or exceed the values shown in the table).
Applications
1. For sand filter drain strip between the sand and the drain rock or gravel layers specify
Geotextile Properties for Underground Drainage, moderate survivability, Class A, from Table
V-C-18 and Table V-C-19 in the Geotextile Specifications.
2. For sand filter matting located immediately above the impermeable liner and below the
drains, the function of the geotextile is to protect the impermeable liner by acting as a
cushion. The specification provided in Table V-C-20 should be used to specify survivability
properties for the liner protection application. Table V-C-19, Class C should be used for
filtration properties. Only nonwoven geotextiles are appropriate for the liner protection
application.
3. For an infiltration drain specify Geotextile for Underground Drainage, low survivability,
Class C, from Table V-C-18 and Table V-C-19 in the Geotextile Specifications.
4. For a sand bed cover a geotextile fabric is placed exposed on top of the sand layer to trap
debris brought in by the stormwater and to protect the sand, facilitating easy cleaning of the
surface of the sand layer. However, a geotextile is not the best product for this application. A
polyethylene or polypropylene geonet would be better. The geonet material should have high
UV resistance (90% or more strength retained after 500 hours in the weatherometer,
ASTM D4355), and high permittivity (ASTM D4491, 0.8 sec. -1 or more) and percent open
area (CWO-22125, 10% or more). Tensile strength should be on the order of 200 lbs grab
(ASTM D4632) or more.
Courtesy of Tony Allen, Geotechnical Engineer-WSDOT
Reference for Tables 1 and 2: Section 9-33.2 “Geotextile Properties,” 1998 Standard Specifications for Road,
Bridge, and Municipal Construction
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Turbulence & Short-Circuiting Factor Volume V
Appendix D 747
Appendix D Turbulence and Short-Circuiting Factor
VH/VT Turbulence Factor (FT) F = 1.2 (FT)
20 1.45 1.74
15 1.37 1.64
10 1.27 1.52
6 1.14 1.37
3 1.07 1.28
Figure V-D-40. Recommended Values of F for Various Values of vH/Vt
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Volume VI
i Table of Contents
Volume VI – Low-Impact
Development and On-Site
Stormwater Management
Table of Contents
Purpose of this Volume...................................................................................................................749
Content and Organization of this Volume.......................................................................................749
Chapter 1 General Requirements.....................................................................................750
1.1 Objectives..............................................................................................................................750
1.2 Site Assessment....................................................................................................................750
1.3 Site Planning and Layout.......................................................................................................750
1.4 Retain Native Vegetation.......................................................................................................751
1.5 Minimize Clearing and Grading Impacts...............................................................................751
Chapter 2 Low Impact Development Best Management Practices.................................752
2.1 Application.............................................................................................................................752
2.2 Best Management Practices..................................................................................................752
2.2.1 Dispersion and Soil Quality BMPs..................................................................................753
2.2.1.1 BMP L610 Downspout Dispersion........................................................................753
2.2.1.2 BMP L611 Concentrated Flow Dispersion............................................................759
2.2.1.3 BMP L612 Sheet Flow Dispersion........................................................................761
2.2.1.4 BMP L613 Post-Construction Soil Quality and Depth...........................................763
2.2.1.5 BMP L614 Full Dispersion.....................................................................................766
2.2.2 Site Design BMPs...........................................................................................................769
2.2.2.1 BMP L620 Preserving Natural Vegetation............................................................769
2.2.2.2 BMP L621 Better Site Design...............................................................................770
2.2.3 Low Impact Development BMPS....................................................................................771
2.2.3.1 BMP L630 Bioretention Areas (Rain Gardens).....................................................772
2.2.3.2 BMP L631 Vegetated Rooftops (Green Roofs).....................................................779
2.2.3.3 BMP L632 Rainfall Reuse Systems......................................................................780
2.2.3.4 BMP L633 Alternate Paving Surfaces...................................................................781
2.2.3.5 BMP L634 Minimal Excavation Foundations........................................................786
2.2.3.6 BMP L635 Reverse Slope Sidewalks....................................................................787
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SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Purpose Volume VI
Content and Organization Introduction 749
Volume VI:
Low Impact Development and
On-Site Stormwater Management
Purpose of this Volume
This volume focuses on the concept of low impact development. Low impact development is a
stormwater management strategy that emphasizes conservation and use of existing natural site
features integrated with engineered, small-scale stormwater controls to more closely mimic
predevelopment hydrologic conditions. On-site stormwater management techniques are a key
component of low impact development.
All sites required to meet Minimum Requirement #5 must use dispersion and soil quality BMPs
described under on-site management below to the maximum extent practicable unless their use
would cause flooding or erosion impacts.
This section should be used in conjunction with “Low Impact Development: Technical Guidance
Manual for Puget Sound”, which can be found on the Puget Sound Partnership website:
www.psp.wa.gov/documents.html
Content and Organization of this Volume
Volume VI contains two chapters:
• Chapter 1 provides an overview of the general requirements for low impact
development.
• Chapter 2 provides detailed information pertaining to Best Management Practices
(BMPS) for low impact development.
Volume
VI
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
General Requirements Volume VI
Chapter 1 750
Chapter 1 General Requirements
1.1 Objectives
The goal of low impact development is to manage stormwater generated from new development and
redevelopment on-site so there will be no negative impacts to adjacent or downstream properties and
no degradation to ground or surface waters.
The following are list of objectives for low impact design:
• Minimize the impacts of increased stormwater runoff from new development and
redevelopment by maintaining peak flow frequencies and durations of the site’s
undisturbed hydrologic condition.
• For residential projects, retain and/or restore 65% of the site’s native soils and
vegetation. For commercial projects, retain and/or restore 25% of the site’s native
soils and vegetation.
• Retain and incorporate natural site features that promote infiltration of stormwater on
the developed site.
• Utilize LID BMPs and minimize the use of traditional technologies to manage
stormwater quality and quantity.
• Manage stormwater as close to the source as possible.
1.2 Site Assessment
Before implementing LID practices it is necessary to perform a site assessment which includes an
assessment of both on-site and off-site conditions and features. See Chapter 2 of the “Low Impact
Development: Technical Guidance Manual for Puget Sound” for more information on steps for
performing a site assessment.
1.3 Site Planning and Layout
Sites should be configured to reduce impervious surfaces and utilize natural drainage features.
Chapter 3 of the “Low Impact Development: Technical Guidance Manual for Puget Sound” contains
information and techniques for site planning. City of Auburn codes must be adhered to.
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General Requirements Volume VI
Chapter 1 751
1.4 Retain Native Vegetation
Retain native vegetation to the maximum extent practicable in order to:
• Reduce total impervious surface coverage
• Provide infiltration areas for overland flows generated in adjacent developed portions
of the project
• Maintain the natural hydrology of the site.
See BMP L620 and Chapter 4 of “Low Impact Development: Technical Guidance Manual for Puget
Sound” for techniques on retaining native vegetation.
1.5 Minimize Clearing and Grading Impacts
• Conduct a soils analysis prior to clearing and grading to identify predevelopment soil
types and infiltration capabilities.
• Keep grading to a minimum by incorporating natural topography.
• Always use appropriate erosion and sediment control techniques when clearing and
grading. See Volume 2 of this manual for erosion and sediment control measures.
• Utilize techniques from Chapter 5 of “Low Impact Development: Technical Guidance
Manual for Puget Sound”.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Low Impact Development Volume VI
Best Management Practices Chapter 2 752
Chapter 2 Low Impact Development Best Management
Practices
This Chapter presents the methods for analysis and design of on-site stormwater management Best
Management Practices (BMPs). Design procedures and requirements for stormwater management
BMPs meeting Minimum Requirement #7, Flow Control, are contained in Volume 3.
2.1 Application
Most of the on-site BMPs serve to control runoff flow rate as well as to provide runoff treatment. Non-
pollution generating surfaces, such as rooftops and patios, may also use the infiltration BMPs
contained in Volume 3, which provide flow control only. Pollution generating surfaces, such as
driveways, small parking lots, and landscaping, must use on-site BMPs to provide some water quality
treatment.
The design criteria components in this manual must be used in order to obtain runoff credits. Runoff
credits are considered when determining project thresholds.
2.2 Best Management Practices
The following Low Impact Development BMPs are discussed in this Chapter:
2.2.1 Dispersion and Soil Quality BMPs
BMP L610 Downspout Dispersion
BMP L611 Concentrated Flow Dispersion
BMP L612 Sheet Flow Dispersion
BMP L613 Post-Construction Soil Quality and Depth
BMP L614 Full Dispersion
2.2.2 Site Design BMPs
BMP L620 Preserving Natural Vegetation
BMP L621 Better Site Design
2.2.3 Structural Low Impact Development BMPs
BMP L630 Bioretention Areas (Rain gardens)
BMP L631 Vegetated Rooftops
BMP L632 Rainfall Reuse
BMP L633 Alternate Paving Systems
BMP L634 Minimal Excavation Foundations
BMP L635 Reverse Slope Sidewalks
Infiltration BMPs can be found in Volume 3 and Volume 5.
SURFACE WATER MANAGEMENT MANUAL
NOVEMBER 2009
Low Impact Development Volume VI
Best Management Practices Chapter 2 753
Projects shall employ these BMPs to infiltrate, disperse, and retain stormwater runoff on site to the
maximum extent practicable without causing flooding or erosion impacts. Infiltration and dispersion
systems require approval by the City. Sites that can fully infiltrate (see Volume 3, Chapter 3 and
Volume 5, Chapter 5) or fully disperse (see BMP L614) are not required to provide runoff treatment or
flow control facilities. Full dispersion credit is limited to sites with a maximum of 10% effective
impervious area that is dispersed through 65% of the site maintained in natural vegetation.
Impervious surfaces that are not fully dispersed should be partially dispersed to the maximum extent
practicable and then hydrologically modeled. Minimum requirements still apply to those portions of a
site that exceed thresholds described in Volume 1.
2.2.1 Dispersion and Soil Quality BMPs
2.2.1.1 BMP L610 Downspout Dispersion
Purpose and Definition
Downspout dispersion BMPs are splashblocks or gravel-filled dispersion trenches that serve to
spread roof runoff over vegetated pervious areas. Dispersion attenuates peak flows by slowing entry
of the runoff into the conveyance system, allows for some infiltration, and provides some water
quality benefits.
Applications
Downspout dispersion may be used on all sites that cannot infiltrate roof runoff and that meet the
feasibility setback requirements.
Flow Credit for Roof Downspout Dispersion
If roof runoff is dispersed according to the requirements of this section, and the vegetative flowpath of
the roof runoff is 50 feet or greater through undisturbed native landscape or lawn/landscape area that
meets BMP L613, the roof area may be modeled as a grassed surface.
General Design Criteria for Dispersion Trenches and Splashblocks
Also see Additional Design Criteria sections.
Maintain a vegetated flowpath of at least 50 feet in length between the outlet of the
trench and the top of slopes steeper than 20% and greater than 10 feet high. A
geotechnical analysis and report must be prepared addressing the potential impact
of the facility on the slope.
Vegetated flowpaths must be covered with well-established lawn or pasture, landscaping
with well-established groundcover, or native vegetation with natural groundcover.
The groundcover shall be dense enough to help disperse and infiltrate flows and to
prevent erosion.
For sites with multiple dispersion trenches or splashblocks, a minimum separation of 10
feet is required between flowpaths. The City may require a larger separation based
upon site conditions such as slope, soil type, and total contributing area.
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NOVEMBER 2009
Low Impact Development Volume VI
Best Management Practices Chapter 2 754
• Place all dispersions systems 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.
• Place all dispersion systems at least 5 feet from any property line. If necessary,
setbacks shall be increased from the minimum 5 feet in order to maintain a 1:1 side
slope for future excavation and maintenance.
• Setback dispersion systems from sensitive areas, steep slopes, landslide hazard
areas, and erosion hazard areas as governed by Auburn City Code.
• No erosion or flooding of downstream properties shall result.
• Runoff discharged towards landslide hazard areas must be evaluated by a
geotechnical engineer or qualified geologist. Do not place the discharge point on or
above slopes greater than 20% or above erosion hazard areas without evaluation by
a geotechnical engineer or qualified geologist and City approval.
• For sites with septic systems, place the discharge point downgradient of the
drainfield primary and reserve areas. This requirement can be waived by the City's
permit review staff if site topography will clearly prohibit flows from intersecting the
drainfield.
Additional Design Criteria for Dispersion Trenches
• Design dispersion trenches as shown in Figure VI-2-1 and Figure VI-2-2.
• Maintain a vegetated flowpath of at least 25 feet in length between the outlet of the
trench and any property line, structure, stream, wetland, or impervious surface.
Sensitive area buffers may count towards flowpath lengths
• Maintain a vegetated flowpath of at least 50 feet in length between the outlet of the
trench and any steep slope.
• Trenches serving up to 700 square feet of roof area may be simple 10-foot-long by
2-foot-wide gravel filled trenches as shown in Figure VI-2-1. For roof areas larger
than 700 square feet, a dispersion trench with notched grade board as shown in
Figure VI-2-2 may be used as approved by the City. The total length of this design
must provide at least 10 feet of trench per 700 square feet of roof area and not
exceed 50 feet.
Additional Design Criteria for Splashblocks
In general, if the ground is sloped away from the building foundation, and there is adequate
vegetation and area for effective dispersion, splashblocks will adequately disperse storm runoff. If the
ground is fairly level, if the structure includes a basement, or if foundation drains are proposed,
splashblocks with downspout extensions may be a better choice because the discharge point is
moved away from the foundation. Downspout extensions can include piping to a
splashblock/discharge point a considerable distance from the downspout, as long as the runoff can
travel through a well-vegetated area as described below.
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The following conditions must be met to use splashblocks:
• Design splashblocks as shown in Figure VI-2-3.
• Maintain a vegetated flowpath of at least 50 feet between the discharge point and
any property line, structure, stream, wetland, lake, or other impervious surface.
Sensitive area buffers may count toward flowpath lengths.
• Do not direct flows onto sidewalks.
• A maximum of 700 square feet of roof area may drain to each splashblock.
• Place a splashblock or a pad of crushed rock (2 feet wide by 3 feet long by 6 inches
deep) at each downspout discharge point.
• No erosion or flooding of downstream properties may result.
• Splashblocks may not be placed on or above slopes greater than 20% or above
erosion hazard areas without evaluation by a geotechnical engineer or qualified
geologist and approval by the City.
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Figure VI-2-1. Typical Dispersion Trench
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Figure VI-2-2. Standard Dispersion Trench with Notched Grade Board
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Figure VI-2-3. Typical Downspout Splashblock Dispersion
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2.2.1.2 BMP L611 Concentrated Flow Dispersion
Purpose and Definition
Dispersion of concentrated flows from driveways or other pavement through a vegetated pervious
area attenuates peak flows by slowing entry of the runoff into the conveyance system, allows for
some infiltration, and provides some water quality benefits.
Applications and Limitations
• Any situation where concentrated flow can be dispersed through vegetation.
• Dispersion for driveways will generally only be effective for single-family residences
on large lots and in rural short plats. Lots proposed by short plats in urban areas will
generally be too small to provide effective dispersion of driveway runoff.
• Figure VI-2-4 shows two possible ways of spreading flows from steep driveways.
Design Criteria
• Maintain a vegetated flowpath of at least 50 feet between the discharge point and
any property line, structure, steep slope, stream, lake, wetland, lake, or other
impervious surface.
• A maximum of 700 square feet of impervious area may drain to each concentrated
flow dispersion BMP.
• Place a pad of crushed rock (2 feet wide by 3 feet long by 6 inches deep) at each
discharge point.
• No erosion or flooding of downstream properties shall result.
• Runoff discharged towards 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 20% or above erosion hazard areas without
evaluation by a geotechnical engineer or qualified geologist and approval by the City.
• For sites with septic systems, locate the discharge point downgradient of the
drainfield primary and reserve areas. This requirement may be waived by the City if
site topography clearly prohibits flows from intersecting the drainfield or where site
conditions indicate that this is unnecessary (see Volume 3, Chapter 2).
Flow Credits for Concentrated Flow Dispersion
Where concentrated flow dispersion is used to disperse runoff into an undisturbed native landscape
area or an area that meets the requirements of “Post-Construction Soil Quality and Depth”, and the
vegetated flow path is at least 50 feet in length, the impervious area may be modeled as landscaped
area.
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Figure VI-2-4. Typical Concentrated Flow Dispersion for Steep Driveways
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2.2.1.3 BMP L612 Sheet Flow Dispersion
Purpose and Definition
Sheet flow dispersion is the simplest method of runoff control. This BMP can be used for any
impervious or pervious surface that is graded so as to avoid concentrating flows. Because flows are
already dispersed as they leave the surface, they need only traverse a narrow band of adjacent
vegetation for effective attenuation and treatment.
Applications and Limitations
Flat or moderately sloping (<15% slope) impervious surfaces such as driveways, sport courts, patios,
and roofs without gutters; sloping cleared areas that are comprised of bare soil, non-native
landscaping, lawn, and/or pasture; or any situation where concentration of flows can be avoided.
Design Criteria
• See Figure VI-2-5 for details for driveways.
• Provide a 2-foot-wide transition zone to discourage channeling between the edge of
the driveway pavement and the downslope vegetation, or under building eaves. This
may be an extension of subgrade material (crushed rock), modular pavement, drain
rock, or other material acceptable to the City.
• Provide a vegetated buffer width of 10 feet of vegetation for up to 20 feet of width of
paved or impervious surface. Add an additional 5 feet of width for each additional
20 feet of width or fraction thereof.
• Provide a vegetated buffer width of 25 feet of vegetation for up to 150 feet of
contributing cleared area (i.e., bare soil, non-native landscaping, lawn, and/or
pasture). Slopes within the 25-foot minimum flowpath through vegetation should be
no steeper than 8 percent. If this criterion cannot be met due to site constraints, the
25-foot flowpath length must be increased 1.5 feet for each percent increase in slope
above 8%.
• No erosion or flooding of downstream properties may result.
• Runoff discharge toward landslide hazard areas must be evaluated by a
geotechnical engineer or a qualified geologist. Do not place the discharge point on or
above slopes greater than 20% or above erosion hazard areas without evaluation by
a geotechnical engineer or qualified geologist and approval by the City.
• For sites with septic systems, place the discharge point downgradient of the
drainfield primary and reserve areas. This requirement may be waived by the City if
site topography clearly prohibits flows from intersecting the drainfield or where site
conditions indicate that this is unnecessary (see Volume 3, Chapter 2).
Flow Credits for Sheet Flow Dispersion
Where sheet flow dispersion is used to disperse runoff into an undisturbed native landscape area or
an area that meets the requirements of “Post-Construction Soil Quality and Depth”, the impervious
area may be modeled as landscaped area.
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Figure VI-2-5. Sheet Flow Dispersion for Driveways
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2.2.1.4 BMP L613 Post-Construction Soil Quality and Depth
Purpose and Definition
Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions
including: water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant
biofiltration; water interflow storage and transmission; and pollutant decomposition. These functions
are largely lost when development strips away native soil and vegetation and replaces it with minimal
topsoil and sod. Not only are these important stormwater functions lost, but such landscapes
themselves become pollution generating pervious surfaces due to increased use of pesticides,
fertilizers, and other landscaping and household/industrial chemicals, the concentration of pet
wastes, and pollutants that accompany roadside litter.
Applications and Limitations
• Soil amendments are required for the disturbed areas of sites subject to Minimum
Requirement #5.
• Where Minimum Requirement #5 does not apply, and the site is proposing a traditional lawn
installation, compost-amended lawn soil is strongly encouraged.
• Use soil amendments in areas that will be incorporated into the stormwater drainage system
such as vegetated channels, rain gardens, bioretention areas, and lawn and landscaped
areas.
Design Criteria
Soil Retention
• The duff layer and native topsoil should be retained in an undisturbed state to the
maximum extent practicable. In any areas requiring grading, remove and
stockpile the duff layer and topsoil on site in a designated, controlled area, not
adjacent to public resources and critical areas, to be reapplied to other portions
of the site where feasible.
Soil Quality
• All areas subject to clearing and grading that have not been covered by impervious
surface, incorporated into a drainage facility or engineered as structural fill or slope
shall, at project completion, demonstrate the following:
o A topsoil layer with a minimum organic matter content of ten percent dry
weight in planting beds, and 5% organic matter content in turf areas, and a
pH from 6.0 to 8.0 or matching the pH of the original undisturbed soil.
o The topsoil layer shall have a minimum depth of eight inches except where
tree roots limit the depth of incorporation of amendments needed to meet the
criteria. Subsoils below the topsoil layer should be scarified at least 4 inches
with some incorporation of the upper material to avoid stratified layers, where
feasible.
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• Planting beds have been mulched with 2 inches of organic material
• Quality of compost and other materials used to meet the organic content
requirements:
o The organic content for “pre-approved” amendment rates can be met only
using compost that meets the definition of “composted materials” in WAC
173-350-220. The WAC is available online at:
http://www.ecy.wa.gov/programs/swfa/facilities/350.html
The compost must also have an organic matter content of 35% to 65%, and a carbon
to nitrogen ratio below 25:1.
The carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely
of plants native to the Puget Sound Lowlands region.
o Calculated amendment rates may be met through use of composted
materials as defined above; or other organic materials amended to meet
the carbon to nitrogen ratio requirements, and meeting the contaminant
standards of Grade A Compost.
• The resulting soil is conducive to the type of vegetation to be established.
Implementation Options:
Use one of the following options to meet the post construction soil quality and depth requirements.
Use the most recent version of “Guidelines for Resources for Implementing Soil Quality and Depth
BMP T5.13” to meet the requirements of this BMP. This guidance can be found online at:
www.soilsforsalmon.org
• Leave native vegetation and soil undisturbed, and protect from compaction during
construction.
• Amend existing site topsoil or subsoil either at default “pre-approved” rates, or at
custom calculated rates based on specifiers’ tests of the soil and amendment.
• Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled
topsoil must also be amended if needed to meet the organic matter or depth
requirements, either at a default “pre-approved” rate or at a custom calculated rate.
• Import topsoil mix of sufficient organic content and depth to meet the requirements.
More than one method may be used on different portions of the same site. Soil that already meets
the depth and organic matter quality standards, and is not compacted, does not need to be
amended.
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Maintenance
• Soil quality and depth should be established toward the end of construction and once
established, should be protected from compaction, such as from large machinery
use, and from erosion.
• Soil should be planted and mulched after installation.
• Plant debris or its equivalent should be left on the soil surface to replenish organic
matter.
• It should be possible to reduce use of irrigation, fertilizers, herbicides, and pesticides.
These activities should be adjusted where possible, rather than continuing to
implement formerly established practices.
Flow Reduction Credits for BMP L613
Flow reduction credits can be taken in runoff modeling when Post-Construction Soil Quality and
Depth is used as part of a dispersion design under the conditions described in:
BMP L610 Downspout Dispersion
BMP L611 Concentrated Flow Dispersion
BMP L612 Sheet Flow Dispersion
Volume 3, Appendix A
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2.2.1.5 BMP L614 Full Dispersion
Purpose and Definition
This BMP allows for "fully dispersing" runoff from impervious surfaces and cleared areas of
development sites that preserve at least 65% of the site (or a threshold discharge area on the site) in
a forest or native condition.
Applications and Limitations
• Full dispersion can be used as long as the developed areas draining to the native
vegetation do not have impervious surfaces that exceed 10% of the entire site.
• Other types of development that retain 65% of the site (or a threshold discharge area
on the site) in a forested or native condition may also use these BMPs to avoid
triggering the flow control facility requirement.
• Runoff must be dispersed into native areas.
Design Guidelines
• Roof Downspouts - Roof surfaces that comply with the downspout infiltration
requirements in Volume 3, Chapter 3, are considered to be "fully dispersed" (i.e.,
zero percent effective imperviousness). All other roof surfaces are considered to be
"fully dispersed" (i.e., at or approaching zero percent effective imperviousness) only
if they are within a threshold discharge area that is or will be more than 65% forested
(or native vegetative cover) and less than 10% impervious (total), and if they comply
with the downspout dispersion requirements of BMP L610, and have vegetated flow
paths through native vegetation exceeding 100 feet.
• Driveway Dispersion - Driveway surfaces are considered to be "fully dispersed" if
they are within a threshold discharge area that is or will be more than 65% forested
(or native vegetative cover) and less than 10% impervious (total), and if they comply
with the driveway dispersion BMPs – BMP L611 and BMP L612 - and have flow
paths through native vegetation exceeding 100 feet. This also holds true for any
driveway surfaces that comply with the roadway dispersion BMPs described below.
• Roadway Dispersion BMPs - Roadway surfaces are considered to be "fully
dispersed" if they are within a threshold discharge area that is or will be more than
65% forested (or native vegetative cover) and less than 10% impervious (total), and
if they comply with the following dispersion requirements:
o Roadway runoff dispersion is allowed only on rural neighborhood collectors
and local access streets. To the extent feasible, disperse driveways to the
same standards as roadways to ensure adequate water quality protection of
downstream resources.
o Design the road section to minimize collection and concentration of roadway
runoff. Use sheet flow over roadway fill slopes (i.e., where roadway subgrade
is above adjacent right-of-way) wherever possible to avoid concentration.
o When it is necessary to collect and concentrate runoff from the roadway and
adjacent upstream areas (e.g., in a ditch on a cut slope), concentrated flows
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shall be incrementally discharged from the ditch via cross culverts or at the
ends of cut sections. These incremental discharges of newly concentrated
flows shall not exceed 0.5 cfs at any one discharge point from a ditch for the
100-year runoff event. Where flows at a particular ditch discharge point were
already concentrated under existing site conditions (e.g., in a natural channel
that crosses the roadway alignment), the 0.5-cfs limit would be in addition to
the existing concentrated peak flows.
o Ditch discharge points with up to 0.2 cfs discharge for the peak 100-year flow
shall use rock pads or dispersion trenches to disperse flows. Ditch discharge
points with between 0.2 and 0.5 cfs discharge for the 100-year peak flow
shall use only dispersion trenches to disperse flows.
o Dispersion trenches shall be designed to accept storm flows (free discharge)
from a pipe, culvert, or ditch end, shall be aligned perpendicular to the
flowpath, and shall be minimum 2 feet by 2 feet in section, 50 feet in length,
filled with ¾-inch to 1½-inch washed rock, and provided with a level notched
grade board (see Figure VI-2-2). Manifolds may be used to split flows up to
2 cfs discharge for the 100-year peak flow between up to 4 trenches.
Dispersion trenches shall have a minimum spacing of 50 feet.
o After being dispersed with rock pads or trenches, flows from ditch discharge
points must traverse a minimum of 100 feet of undisturbed native vegetation
before leaving the project site, or entering an existing onsite channel carrying
existing concentrated flows across the road alignment.
o Flowpaths from adjacent discharge points must not intersect within the 100-
foot flowpath lengths, and dispersed flow from a discharge point must not be
intercepted by another discharge point. To enhance the flow control and
water quality effects of dispersion, the flowpath shall not exceed 15% slope,
and shall be located within designated open space.
o Ditch discharge points shall be located a minimum of 100 feet upgradient of
steep slopes (i.e., slopes steeper than 40%), wetlands, and streams.
o Where the City determines there is a potential for significant adverse impacts
downstream (e.g., erosive steep slopes or existing downstream drainage
problems), dispersion of roadway runoff may not be allowed, or other
measures may be required. See Volume 1, Chapter 2 for geographic specific
requirements.
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Cleared Area Dispersion BMPs
The runoff from cleared areas that are comprised of bare soil, non-native landscaping, lawn, and/or
pasture is considered to be "fully dispersed" if it is dispersed through at least 25 feet of native
vegetation in accordance with the following criteria:
• The contributing flowpath of cleared area being dispersed must be no more than 150
feet.
• Slopes within the 25-foot minimum flowpath through native vegetation should be no
steeper than 8%. If this criterion cannot be met due to site constraints, the 25-foot
flowpath length must be increased 1.5 feet for each percent increase in slope above
8%.
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2.2.2 Site Design BMPs
2.2.2.1 BMP L620 Preserving Natural Vegetation
Purpose
Preserving natural vegetation on-site to the maximum extent practicable will minimize the impacts of
development on stormwater runoff.
Applications and Limitations
On lots that are one acre or greater, preservation of 65 percent or more of the site in natural
vegetation will allow the use of full dispersion techniques presented in BMP L614. Sites that can fully
disperse are not required to provide runoff treatment or flow control facilities.
Design Criteria
• Situate the preserved area to maximize the preservation of wetlands, and to buffer
stream corridors.
• Place the preserved area in a separate tract or protect through recorded easements
for individual lots.
• If feasible, locate the preserved area downslope from the building sites, since flow
control and water quality are enhanced by flow dispersion through duff, undisturbed
soils, and native vegetation.
• Show the preserved area on all property maps and clearly mark the area during
clearing and construction on the site.
Maintenance
• Do not remove vegetation and trees from undisturbed areas, except for approved
timber harvest activities and the removal of dangerous and diseased trees.
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2.2.2.2 BMP L621 Better Site Design
Purpose
Fundamental hydrological concepts and stormwater management concepts can be applied at the site
design phase that are:
• More integrated with natural topography,
• Reinforce the hydrologic cycle,
• More aesthetically pleasing, and
• Often less expensive to build.
Design Criteria
Define Development Envelope and Protected Areas – The first step in site planning is to define
the development envelope. This is done by identifying protected areas, setbacks, easements and
other site features, and by consulting applicable local standards and requirements. Site features to be
protected may include important existing trees, steep slopes, erosive soils, riparian areas, or
wetlands.
By keeping the development envelope compact, environmental impacts can be minimized,
construction costs can be reduced, and many of the site’s most attractive landscape features can be
retained. In some cases, economics or other factors may not allow avoidance of all sensitive areas.
In these cases, care can be taken to mitigate the impacts of development through site work and other
landscape treatments.
Minimize Directly Connected Impervious Areas
Impervious areas directly connected to the storm drain system are the greatest contributors to urban
nonpoint source pollution. Minimize these directly connected impervious areas. This can be done by
limiting overall impervious land coverage or by infiltrating and/or dispersing runoff from these
impervious areas.
• Maximize Permeability - Within the development envelope, many opportunities are
available to maximize the permeability of new construction. These include minimizing
impervious areas, paving with permeable materials, clustering buildings, and
reducing the land coverage of buildings by smaller footprints. All of these strategies
make more land available for infiltration and dispersion through natural vegetation.
• Build Narrower Streets - More than any other single element, street design has a
powerful impact on stormwater quantity and quality. In residential development,
streets and other transportation-related structures typically can comprise between 60
and 70 percent of the total impervious area, and, unlike rooftops, streets are almost
always directly connected to the stormwater conveyance system.
• Maximize Choices for Mobility - Given the costs of automobile use, both in land
area consumed and pollutants generated, maximizing choices for mobility is a basic
principle for environmentally responsible site design. By designing residential
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developments to promote alternatives to automobile use, a primary source of
stormwater pollution can be mitigated.
• Use Drainage as a Design Element - Unlike conveyance storm drain systems that
hide water beneath the surface and work independently of surface topography, a
drainage system for stormwater infiltration or dispersion can work with natural land
forms and land uses to become a major design element of a site plan.
2.2.3 Low Impact Development BMPS
Low impact development BMPs are structural BMPs that can be used to manage stormwater on-site.
Using LID techniques can reduce surface runoff. For each category, basic design criteria is included.
The design criteria components in this manual must be used in order to obtain runoff credits. Runoff
credits are considered when determining project thresholds.
The guidance provided in “Low Impact Development: Technical Guidance Manual for Puget Sound”,
found on the Puget Sound Partnership website: www.psp.wa.gov, should also be used in design.
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2.2.3.1 BMP L630 Bioretention Areas (Rain Gardens)
Purpose and Definition
Bioretention areas are shallow stormwater retention systems designed to mimic forested systems by
controlling stormwater through detention, infiltration, and evapotranspiration. Bioretention areas
provide water quality treatment through sedimentation, filtration, adsorption, and phytoremediation.
Bioretention facilities are integrated into the landscape to mimic natural hydrologic conditions better.
Bioretention facilities may be used as a water quality facility or a water quality and flow control
(retention) facility.
Applicability and Limitations
• Rain gardens can be used as on-lot retention facilities.
• Rain gardens may be used to receive roof runoff in areas where traditional infiltration
is not feasible.
• Three feet of clearance is necessary between the lowest elevation of the bioretention
soil or any underlying gravel layer and the seasonal high groundwater elevation or
other impermeable layer if the area tributary to the facility meets or exceeds any of
the following:
o 5000 square feet of pollution-generating impervious surface
o 10,000 square feet of impervious area
o ¾ acre of lawn and landscape
• For bioretention systems with a contributing area less than the above thresholds, a
minimum of 1 foot of clearance is required from the seasonal high groundwater or
other impermeable layer.
• Bioretention facilities can be used in parking lots and any other type of development.
• Bioretention systems may meet the requirements for basic and enhanced treatment
when soil is designed in accordance with the requirements below and at least 91% of
the influent runoff volume using WWHM is infiltrated. Drawdown requirements must
also be met.
Setback and Site Constraints
• Assure that water movement through the surface soils and interflow will remain unobstructed
and soils will remain uncompacted.
• Locate bioretention facilities at least 10 feet from any structure or property line unless
approved in writing by the City.
• Locate bioretention facilities at least 50 feet back from slopes with a grade of 20% or greater.
A geotechnical analysis must be prepared addressing the potential impact of the facility on
the slope if closer than 50 feet or greater than 20%.
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Design Criteria
Flow Entrance/Presetting
• Flow velocity entering the facility should be less than 1 ft/sec.
• Use one of the four types of flow entrances:
o Dispersed, low velocity flow across a grade or landscape area.
o Dispersed flow across pavement or gravel and past wheel stops for parking
areas.
o Dispersed curb cuts for driveway or parking lot areas. Include rock or other
erosion protection material in the channel entrance to dissipate energy.
o Pipe flow entrance. Include rock or other erosion protection material at the
facility entrance to dissipate energy and/or provide flow dispersion.
• Do not place woody plants directly in the entrance flow path as they can restrict or
concentrate flows.
• A minimum 1-inch grade change between the edge of a contributing impervious
surface and the vegetated flow entrance is required.
• Install flow diversion and erosion control measures to protect the bioretention area
from sedimentation until the upstream area is stabilized.
• If the catchment area exceeds 2,000 square feet, a presettling facility may be
required.
Cell Ponding Area
• The ponding area provides for surface storage and particulate settling,
• Ponding depth and drawdown rate provide variable conditions that allow for a range
of appropriate plant species. Soil must be allowed to dry out periodically in order to:
o Restore hydraulic capacity of system.
o Maintain infiltration rates.
o Maintain adequate soil oxygen levels for healthy soil biota and vegetation.
o Provide proper soil conditions for biodegradation and retention of pollutants.
o Prevent conditions supportive of mosquito breeding.
• The ponding depth shall be a maximum of 12 inches.
• The surface pool drawdown time shall be 24 hours.
• The minimum freeboard measured from the invert of the overflow pipe or earthen
channel to facility overtopping elevation shall be 2” for drainage areas less than
1,000 square feet and 6” for drainage areas 1,000 square feet or greater.
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• If berming is used to achieve the minimum top elevation, maximum slope on berm
shall be 4H:1V, and minimum top width of design berm shall be 1 foot. Soil for
berming shall be imported bioretention soil or amended native soil compacted to a
minimum of 90% dry density.
Overflow
• Unless designed for full infiltration of the entire runoff volume, bioretention systems
must include an overflow.
• A drain pipe installed at the designed maximum ponding elevation and connected to
a downstream BMP or an approved discharge point can be used as the overflow.
• Overflow shall be designed to convey the 100-year recurrence interval flow.
Soils
• For bioretention systems to meet the requirements for basic and enhanced treatment
the following requirements must be met:
• The bioretention soil mix (BSM) shall meet the following requirements:
o Have an infiltration rate between 1 and 2.4 inches per hour.
o The CEC must be at least 5 meq/100 grams of dry soil.
o The soil mix should be about 40% by volume compost and about 60% by
volume aggregate component. The aggregate component shall meet the
specifications in Table VI-2-1
o The compost component shall be stable, mature, and derived from organic
waste materials including yard debris, wood wastes, or other organic matter.
Compost must meet the Washington State compost regulations in WAC 173-
350, which is available at http://www.ecy.wa.gov/programs/compost
Table VI-2-1. Bioretention Soil Mix Aggregate Component
Sieve Size Percent Passing
3/8” 100
#4 95-100
#10 75-90
#40 25-40
#100 4-10
#200 2-5
o Minimum depth of treatment soil must be 18 inches.
o Soil depths of 24” and greater should be considered to provide improved
removal of nutrients as needed, including phosphorus.
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• For facilities that infiltrate and do not have an underdrain, a soils report, prepared by
a soils professional, shall be required and must address the following for each
bioretention area:
o A minimum of one soil log or test pit is required at each facility.
o The soil log shall extend a minimum of 4 feet below the bottom of the
subgrade (the lowest point of excavation).
o The soil log must describe the USDA textural class of the soil horizon through
the depth of the log and note any evidence of high groundwater level, such as
mottling.
o Use infiltration rates of the native soil when sizing and modeling bioretention
systems.
Underdrain
Only install underdrains in bioretention areas if:
• Infiltration is not permitted and/or a liner is used, or
• Where infiltration rates are not adequate to meet the maximum pool drawdown
time.
• Where the facility is not utilized for infiltration.
Underdrain pipe diameter will depend on hydraulic capacity required, 6-inch minimum.
Use a geotextile fabric between the soil layer and underdrain.
Planting
• Plants must be tolerant of summer drought, ponding fluctuations, and saturated soil
conditions.
• Consider rooting depth when choosing plants. Roots must not damage underground
infrastructure.
• Locate slotted and perforated pipe at least 5 feet from tree roots and side sewer
pipes.
• Consider adjacent plant communities and potential invasive species.
• Consider aesthetics, rain gardens should blend into surrounding landscapes.
• The “Low Impact Development: Technical Guidance Manual for Puget Sound”
Appendix 3 contains information for selecting proper bioretention vegetation.
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Mulch Layer
• Bioretention areas should be designed with a mulch layer. Properly selected mulch
material reduces weed establishment, regulates soil temperatures and moisture, and
adds organic matter to soil. Mulch should be:
o Compost in the bottom of the facilities,
o Wood chip mulch composed of shredded or chipped hardwood or softwood
on cell slopes,
o Free of weed seeds, soil, roots, and other material that is not trunk or branch
wood and bark,
o A maximum of 3 inches thick for compost or 4 inches thick for wood chips.
• Mulch shall not include grass clippings, mineral aggregate or pure bark.
• A dense groundcover can be used as an alternative to mulch though mulch most be
used until the dense groundcover is established.
Modeling and Sizing
For sites with contributing area less than 2,000 square feet:
Table VI-2-2 provides the square footage of the bottom of the rain garden per 1000 square feet of
roof area.
The same method of sizing can be used for rain gardens receiving driveway runoff if the soils meet
the water quality treatment requirements outlined in Section 2.2.3.1 in the Soils subsection, or the
runoff passes through a basic treatment facility before reaching the rain garden. Collection areas
greater than 2000 square feet must be designed by a professional engineer.
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Table VI-2-2. Sizing Table for Rain Gardens
Soil Type Raingarden bottom
(square feet)
Coarse sands and cobbles 25
Medium sands 60
Fine sands, loamy sands 130
Sandy loam 160
Loam 225
For sites with contributing areas 2,000 square feet or more:
Use WWHM and model the facility as an infiltration facility with appropriate stage-storage and
overflow/outflow rates. Bioretention cells may be modeled as a layer of soil with infiltration to
underlying soil, ponding, and overflow. The tributary area, cell bottom area, and ponding depth
should be iteratively sized until the duration curves and/or peak volumes meet the flow control
requirements.
NOTE: WWHM Pro has the ability to model bioretention areas with or without underdrains so facility
will be sized differently than described above. Contact the Washington State Department of Ecology
for more information. Use the assumptions in Table VI-2-3 when sizing the facilities.
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Table VI-2-3. Modeling Assumptions for Rain Garden Sizing
Variable Assumption
Computational Time Step 15 minutes
Inflows to Facility Surface flow and interflow from drainage area routed to facility
Precipitation and Evaporation
Applied to Facility
Yes
Bioretention Soil Infiltration Rate For imported soils, for sites that have a contributing area of
less than 5,000 square feet of pollution generating surfaces,
less than 10,000 square feet of impervious area, and less than
¾ acre of landscaped area, reduce the infiltration rate of the
BSM by a factor of 2. For sites above these thresholds, a
reductions factor of 4 shall be applied.
For compost amended native soil, rate is equal to native soil
design infiltration rate.
Bioretention Soil Porosity 40%
Bioretention Soil Depth Minimum of 18 inches.
Native Soil Infiltration Rate Measured infiltration rate with applicable safety factors. See
Volume III for more information on infiltration rate
determination.
Infiltration Across Wetted Surface
Area
Only if sides slopes are 3:1 or flatter
Overflow Overflow elevation set at maximum ponding elevation
(excluding freeboard). May be modeled as weir flow over rider
edge or riser notch.
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2.2.3.2 BMP L631 Vegetated Rooftops (Green Roofs)
Purpose and Definition
Vegetated rooftops, also know as green roofs or eco-roofs, are veneers of living vegetation that are
installed on top of conventional roofs. A green roof is an extension of the existing roof, which involves
a special root repelling membrane, a drainage system, a lightweight growing medium, and plants.
Vegetated walls and slopes may also be allowed.
Applications and Limitations
Vegetated rooftops offer a practical method of managing runoff in densely developed urban
neighborhoods and can be engineered to achieve specific stormwater runoff control objectives.
Design Guidelines
• Soil or growth media must have a high field capacity.
• Soil or growth media must have a saturated hydraulic conductivity of 1 inch/hour.
• Drainage layer must allow free drainage under the soil/growth media.
• Vegetative cover must be both drought and wet tolerant.
• There must be a waterproof membrane between the drain layer and the structural
roof support.
• The maximum slope shall be 20%.
Flow Credits for Vegetated Roofs
Where vegetated roofs are used, the impervious areas may be modeled based on the thickness of
the soil media:
• For roofs with 3-8” of soil/growing media, model the roof as 50% till landscaped and
50% impervious.
• For roofs with 8” of soil/growing media, model the roof as 50% till pasture and 50%
impervious.
Resource Material
Miller, C. and Grantley Pyke. Methodology for the Design of Vegetated Roof Covers, Proceedings of
the 1999 International Water Resources Engineering Conference, Seattle, Washington.
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2.2.3.3 BMP L632 Rainfall Reuse Systems
Purpose and Definition
Rainfall reuse systems are designed to collect stormwater runoff from non-pollution generating
surfaces and make use of the collected water. Rainfall reuse systems are also known as rainwater
harvesting systems and rainfall catchment systems.
Applications and Limitations
Highly developed areas where large buildings encompass nearly all of the area.
Approval of the water reuse system requires approval of the appropriate state and local agencies as
required for any water right.
Design Guidelines
• Where captured water is solely for outdoor use, density shall be 4 homes per acre or
less can be used.
• Cisterns can be incorporated into the aesthetics of the building and garden.
• If a rainwater reuse system holds more than 6 inches depth of water, it should be
covered securely or have a top opening of 4 inches or less to prevent small children
from gaining access to the standing water.
• Design and maintain the system to minimize clogging by leaves and other debris.
Flow Credits for Rainfall Reuse Systems
The drainage area to the rainfall reuse system does not need to be entered into the runoff model
when:
• 100% of the annual average runoff volume (using WWHM) is reused, or
• Interior uses have a monthly water balance that demonstrates adequate capacity for
each month and reuse of all stored water annually.
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2.2.3.4 BMP L633 Alternate Paving Surfaces
Purpose and Definition
Alternate paving surfaces include porous asphalt pavement, porous concrete, grid and lattice rigid
plastic or paving blocks where the holes are filled with soil, sand, or gravel; and cast-in-place paver
systems. Porous surfaces are designed to accommodate pedestrian, bicycle, and auto traffic while
allowing infiltration and storage of stormwater.
Alternate paving systems may be designed with an underdrain to collect stormwater or without an
underdrain as an infiltration facility.
Applications and Limitations
• Appropriate application for alternative porous surfaces depend on the type of paving
system, but typically include parking overflow areas, parking stalls, low volume
residential roads, low-volume parking areas, alleys, driveways, sidewalks/pathways,
and patios.
• Porous paving surfaces can provide some attenuation and uptake of stormwater
runoff even on cemented till soils while still providing the structural integrity required
for a roadway surface to support heavy loads.
• Porous paving surfaces work well in concert with other LID BMPs such as porous
parking stalls adjacent to bioretention areas, and porous roadway surfaces bordered
by vegetated swales.
• Although there is a drop of infiltration rates over time, the long-term infiltration rate is
still substantial enough to provide significant reductions in runoff.
• Infiltration through pervious pavement surfaces shall not be allowed with land uses
that generate heavy pollutant loads. The potential sediment loading for each
application should be considered when determining if the application of alternate
surfaces is appropriate.
• Runoff generated from lawns or other pervious surfaces shall not be directed onto
porous surfaces.
• No point discharges may be directed to porous surfaces.
• Sheet flow runoff may be directed onto a porous surface provided that the length of
sheet flow across the paved section is no more than twice the length of sheet flow
across the porous pavement section.
Design Criteria
• Unless approved in writing by The City, maximum slopes for alternative paving
surfaces are:
o 5% for porous asphalt
o 6% for porous concrete
o 10% for interlocking pavers
o 5-6% grid and lattice systems
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• Follow manufacturer’s recommendations for design, installation, and maintenance.
• Subgrade infiltration rates less than 2.4 inches/hour and a cation exchange capacity
of 5 milliequivalents CEC/100 grams dry soil (or greater) will provide water quality
treatment.
• Typical cross-sections of porous paving systems consist of:
o A top layer with either porous asphalt, porous concrete, concrete block
pavers, or a plastic grid paver filled with sand topsoil or gravel.
o An aggregate subbase with larger rock at the bottom and smaller rock directly
under the top surface.
o For open-celled paving grids and blocks, a leveling course consisting of finer
aggregate.
o A geotextile fabric
• Both gravel and soil with vegetation can be used to fill the opening in paver and rigid
grid systems. Manufacturer recommendations should be followed to apply the
appropriate material.
• Porous systems that use pavers shall be confined with a rigid edge system to
prevent gradual movement of the paving stones.
• Subgrade layer:
o Compact the subgrade to the minimum necessary for structural stability. Do
not allow heavy compaction. The subgrade should not be subject to truck
traffic.
o Use on soil types A through C.
• Geotextile
o Use geotextile between subgrade and base material to keep soil out of base
layer.
o The geotextile must pass water at a greater rate than the subgrade soils.
• Separation or Bottom Filter Layer (optional but recommended)
o A layer of sand or crushed stone graded flat is recommended to promote
infiltration across the surface, stabilize the base layer, protect the underlying
soil from compaction, and serve as a transition between the base course and
the underlying geotextile.
• Base Material
Material must be free draining. Below are examples of possible base material specifications.
See Chapter 6 of the “Low Impact Development: Technical Guidance Manual for Puget
Sound” for more detailed information.
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Driveway base material:
o >4” layer of free-draining crushed rock, screened gravel, or washed sand.
o <5% fines (material passing the #200 sieve) based on fraction passing the #4
sieve.
Roads and Parking Lots
o Follow the standard material and quantities used for asphalt roads.
• Wearing Layer
o A minimum infiltration rate of 10 inches/hour is required though higher
infiltration rates are desirable.
o For porous asphalt, products must have adequate void space, commonly 12-
20%.
o For porous concrete, products must have adequate void space, commonly
15-21%.
o For grid/lattice systems filled with gravel, sand, or a soil of finer particles with
or without grass, fill must be at least 2”. Fill should be underlain with 6” or
more of sand or gravel to provide an adequate base. Locate fill at or slightly
below the top elevation the top elevation of the grid/lattice structure. Modular
grid openings must be at least 40% of the total surface area.
o For paving blocks, fill spaces between blocks with 6” of free draining sand or
aggregate material. Provide a minimum of 12% free draining surface area.
• Drainage Conveyance
o Design roads with adequate drainage conveyance facilities if the road surface
was impermeable.
o Design drainage flow paths to move water safely away from the road prism
and into the roadside drainage facility for roads with base courses that extend
below the surrounding grade.
• Acceptance Test
o Test all permeable surfaces by throwing a bucket of water on the surface. If
anything runs off the surface or puddles, additional testing is necessary prior
to accepting the construction.
o As directed by The City, test with a 6” ring infiltrometer or sprinkle
infiltrometer. Wet the road surface continuously for 10 minutes. Test to
determine compliance with 10 inches/hour minimum infiltration rate.
o For facilities designed to infiltrate, the bucket test shall be completed
annually.
o Test documentation shall be retained with maintenance records.
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Maintenance
• Follow manufacturer’s suggestions for maintenance.
• Inspect project upon completion to correct accumulation of fine material. Conduct
periodic visual inspections to determine if surfaces are clogged.
• Sweep non-planted surfaces with a high-efficiency sweeper twice per year, one in
autumn, and one in early spring. Sweeping frequency can be reduced if infiltration
rate testing indicates that a rate of 10 inches/hour or greater is being maintained.
• Maintenance records shall be retained and provided to the City upon request.
Flow Credits for Alternate Paving Systems
Flow credits for alternate paving systems are based on the base material and type of alternate
surface. The following lists the possible credits that can be achieved by using alternative paving
systems:
For porous asphalt or concrete systems used as public road or public parking lot
configurations:
Where base material is laid above surrounding grade:
• Without an underdrain, model the surface as grass over underlying soil type.
• With an underdrain either at or below the bottom of the base layer or elevated
within the base course, model the surface as impervious.
Where base material is laid partially or completely below surrounding grade:
• Without an underdrain, model the surface as grass over underlying soil type or
impervious surface routed to an infiltration.
• With an underdrain at or below bottom of base layer or elevated within the base
course, model the surface as impervious.
For porous asphalt or concrete systems used at private facilities such as driveways, parking
lots, walks, and patios:
Where the base material is laid below ground:
• Without an underdrain, model the surface as 50% grass on underlying soil and
50% impervious.
• With a pipe underdrain, model the surface as impervious.
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For grid/lattice systems and paving blocks used as public road or public parking lot:
Where base material is laid above the surrounding grade:
• Without an underdrain, model grid/lattice systems as grass on underlying soil and
model paving blocks as 50% grass on underlying soil with 50% impervious.
• With an underdrain, model the surface as impervious.
Where base material is laid partially or completely below surround ground:
• Without an underdrain, model grid/lattice systems as grass on underlying soil and
model paving blocks as 50% grass with 50% impervious or model both
grid/lattice systems and paving blocks as impervious surfaces routed to an
infiltration basin.
• With an underdrain, at or below bottom of the base layer, model the surface as
impervious.
With an underdrain elevated within the base course, model the surface as impervious routed to
an infiltration basin.
For grid/lattice systems and paving blocks used at private facilities (driveways, parking lots,
walks, patios, etc.):
Where base material is laid partially or completely below surrounding ground:
• Without an underdrain, model the surface as 50% grass and 50% impervious.
• With an underdrain, model the surface as impervious.
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2.2.3.5 BMP L634 Minimal Excavation Foundations
Purpose and Definition
Minimal excavation foundation systems are those techniques that minimize disturbance to the natural
soil profile within the footprint of the structure. This preserves most of the hydrologic properties of the
native soil. Pin foundations are an example of a minimal excavation foundation.
Applications Limitations
• Suitable for pier and perimeter wall configurations for residential or commercial
structures up to three stories high.
• Useful for elevated paths and foot-bridges in environmentally sensitive areas.
• Heavy equipment cannot be used within or immediately surrounding the building.
Terracing of the foundation area may be accomplished by tracked, blading
equipment not exceeding 650 psf.
Design Criteria
See Chapter 6 of “Low Impact Development: Technical Guidance Manual for Puget Sound” for
design information.
Flow Credits for Minimal Excavation Foundation Systems
• Where roof runoff is dispersed on the up gradient side of a structure in accordance
with the design criteria in “Downspout Dispersion”, model the tributary roof area as
pasture on the native soil.
• Where “step forming” is used on a slope, the square footage of roof that can be
modeled as pasture must be reduced to account for lost soils. In “step forming,” the
building area is terraced in cuts of limited depth. This results in a series of level
plateaus on which to erect the form boards.
The following equation can be used to reduce the roof area that can be modeled as pasture.
A1 – dC(.5) X A1 = A2
dP
A1 = roof area draining to up gradient side of structure
dC = depth of cuts into the soil profile
dP = permeable depth of soil (the A horizon plus an additional few inches of the
B horizon where roots permeate into ample pore space of soil).
A2 = roof area that can be modeled as pasture on the native soil
• If roof runoff is dispersed down gradient of the structure in accordance with the
design criteria and guidelines “Downspout Dispersion”, and there is at least 50 feet of
vegetated flow path through native material or lawn/landscape area that meets the
guidelines in BMP L613 of Volume 5, Chapter 5, model the tributary roof areas as
landscaped area.
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2.2.3.6 BMP L635 Reverse Slope Sidewalks
Definition and Purpose
Reverse slope sidewalks are sloped to drain away from the road and onto adjacent vegetated areas.
Design Criteria for Reverse Slope Sidewalks
• There must be 10 feet of vegetated surface downslope that is not directly
connected into the storm drainage system.
• Vegetated area receiving flow from sidewalk must be native soil or meet the
guidelines in BMP L613: Post-Construction Soil Quality and Depth.
Flow Credits for Reverse Slope Sidewalks
• Model the sidewalk area as landscaped area over the underlying soil type.
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