Snohomish County Drainage Manual Volume V Runoff Treatment BMPs September 2010 Table of Contents Chapter 1 - Introduction ................................................................................................... 1 1.1 Purpose of this Volume .................................................................................................. 1 1.2 Content and Organization of this Volume ..................................................................... 1 1.3 How to Use this Volume ................................................................................................ 1 1.4 Runoff Treatment Facilities ........................................................................................... 2 1.4.1 General Considerations ............................................................................................... 2 1.4.2 Maintenance ................................................................................................................ 2 1.4.3 Treatment Methods ..................................................................................................... 2 Chapter 2 - Treatment Facility Selection Process .......................................................... 4 Chapter 3 - Treatment Facility Menus ............................................................................ 5 Chapter 4 - General Requirements for Stormwater Facilities ...................................... 6 4.1 Design Volume and Flow .............................................................................................. 6 4.1.1 Water Quality Design Storm Volume ......................................................................... 6 4.1.2 Water Quality Design Flow Rate ................................................................................ 6 4.1.3 Flows Requiring Treatment ........................................................................................ 7 4.2 Sequence of Facilities .................................................................................................... 8 4.3 Setbacks, Slopes, and Embankments ............................................................................. 9 4.3.1 Setbacks ...................................................................................................................... 9 4.3.2 Berms .......................................................................................................................... 9 4.4 Facility Liners ................................................................................................................ 9 4.4.1 General Design Criteria ............................................................................................ 10 4.4.2 Design Criteria for Treatment Liners ........................................................................ 11 4.4.3 Design Criteria for Low Permeability Liner Options ............................................... 12 4.5 Hydraulic Structures .................................................................................................... 14 4.5.1 Flow Splitter Designs ................................................................................................ 14 4.5.2 Flow Spreading Options ........................................................................................... 16 4.5.3 Outfall Systems ......................................................................................................... 18 4.6 Maintenance Standards for Drainage Facilities and Catch Basins .............................. 22 Chapter 5 - On-Site Stormwater Management............................................................. 53 5.3.1 Dispersion BMPs for Pollution-Generating Impervious Surfaces (PGIS) ............... 55 BMP T5.11 Concentrated Flow Dispersion ......................................................................... 55 BMP T5.12 Sheet Flow Dispersion ..................................................................................... 58 5.3.2 Post-Construction Soil Quality ................................................................................. 61 BMP T5.13 Post-Construction Soil Quality and Depth ....................................................... 62 5.3.2 Site Design BMPs ..................................................................................................... 65 BMP T5.20 Preserving Natural Vegetation ......................................................................... 65 BMP T5.21 Better Site Design ............................................................................................ 67 5.3.3 Other Practices .......................................................................................................... 70 September 2010 Snohomish County Drainage Manual Volume V - Runoff Treatment BMPs i BMP T5.30 Full Dispersion ................................................................................................. 70 Chapter 6 - Pretreatment ................................................................................................ 73 6.1 Purpose .......................................................................................................................... 73 6.2 Application .................................................................................................................... 73 6.3 Best Management Practices (BMPs) for Pretreatment ................................................. 73 BMP T6.10 Presettling Basin .............................................................................................. 73 Chapter 7 - Infiltration and Bioinfiltration Treatment Facilities ............................... 74 7.1 Purpose .......................................................................................................................... 74 7.2 Application .................................................................................................................... 74 7.3 General Considerations ................................................................................................. 74 7.4 Best Management Practices (BMPs) for Infiltration and Bioinfiltration Treatment .... 75 BMP T7.20 Infiltration Trenches ......................................................................................... 75 BMP T7.30 Bioinfiltration Swale ........................................................................................ 75 Chapter 8 - Sand Filtration Treatment Facilities and Bioretention Facilities used for Treatment Only ....................................................................................................... 77 8.1 Purpose .......................................................................................................................... 77 8.2 Description .................................................................................................................... 77 8.3 [RESERVED] ............................................................................................................... 84 8.4 Applications and Limitations ........................................................................................ 84 8.5 [RESERVED] ............................................................................................................... 84 8.6 Design Criteria .............................................................................................................. 84 8.7 [Reserved] ..................................................................................................................... 85 8.8 Maintenance .................................................................................................................. 85 BMP T8.10 Sand Filter Vault .............................................................................................. 87 BMP T8.20 Linear Sand Filter............................................................................................. 88 Chapter 9 - Biofiltration Treatment Facilities .............................................................. 89 9.1 Purpose .......................................................................................................................... 89 9.2 Applications .................................................................................................................. 89 9.3 Site Suitability ............................................................................................................... 89 9.4 Best Management Practices .......................................................................................... 90 BMP T9.10 Basic Biofiltration Swale ................................................................................. 91 BMP T9.20 Wet Biofiltration Swale ................................................................................. 109 BMP T9.30 Continuous Inflow Biofiltration Swale .......................................................... 112 BMP T9.40 Basic Filter Strip ............................................................................................ 113 BMP T9.50 Narrow Area Filter Strip ................................................................................ 116 Chapter 10 - Wetpool Facilities .................................................................................... 118 10.1 Purpose ........................................................................................................................ 118 10.2 Application .................................................................................................................. 118 10.3 Best Management Practices (BMPs) for Wetpool