Stormwater Best Management Practice Design and Maintenance Manual For ...

County of Los Angeles Department of Public Works

Stormwater Best Management Practice Design and Maintenance Manual For Publicly Maintained Storm Drain Systems

August 2010

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Stormwater BMP Design and Maintenance Manual

Table of Contents

1. INTRODUCTION..............................................................................................................1-1

PURPOSE.................................................................................................................................... 1-1 OTHER STORMWATER BMP SELECTION AND DESIGN MANUALS.................................................................. 1-1 CONTENT AND ORGANIZATION OF THIS MANUAL.................................................................................... 1-2 GENERAL CONSIDERATIONS ............................................................................................................. 1-2

Maintenance Responsibility .................................................................................................... 1-2 Pretreatment ........................................................................................................................ 1-3 Infiltration ............................................................................................................................ 1-3 Biofiltration........................................................................................................................... 1-4 Bioretention.......................................................................................................................... 1-4 Filtration............................................................................................................................... 1-4 Wetpools .............................................................................................................................. 1-4 Oil/Water Separation ............................................................................................................. 1-5 "On-line" and "Off-line" Facilities ............................................................................................ 1-5 Hydromodification Control...................................................................................................... 1-6 Unit Process-Based BMP Selection and Design ......................................................................... 1-7

2. DRY EXTENDED DETENTION BASINS ............................................................................. 2-1

DEFINITION ................................................................................................................................ 2-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 2-1 MULTI-USE OPPORTUNITIES ............................................................................................................ 2-1 DRY EXTENDED DETENTION BASIN DESIGN SPECIFICATIONS .................................................................... 2-2

Basin Sizing and Geometry..................................................................................................... 2-2 Soils Considerations .............................................................................................................. 2-2 Energy Dissipation................................................................................................................. 2-3 Forebay................................................................................................................................ 2-3 Vegetation............................................................................................................................ 2-3 Outlet Structure and Drawdown Time ..................................................................................... 2-4 Emergency Spillway............................................................................................................... 2-6 Side Slopes........................................................................................................................... 2-7 Embankments....................................................................................................................... 2-7 Fencing ................................................................................................................................ 2-8 Right-of-Way ........................................................................................................................ 2-8 Maintenance Access .............................................................................................................. 2-8 Landscaping ......................................................................................................................... 2-9 Restricted Construction Materials ............................................................................................ 2-9 DRY EXTENDED DETENTION BASINS MAINTENANCE STANDARDS .............................................................. 2-14 General Requirements ......................................................................................................... 2-14 Maintenance Standards........................................................................................................ 2-15

3. VEGETATED SWALES ...................................................................................................... 3-1

DEFINITION ................................................................................................................................ 3-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 3-1 MULTI-USE OPPORTUNITIES ............................................................................................................ 3-1 VEGETATED SWALE DESIGN SPECIFICATIONS........................................................................................ 3-2

Geotechnical Considerations................................................................................................... 3-2 Sizing................................................................................................................................... 3-2 Swale Geometry.................................................................................................................... 3-5 Bottom Slope ........................................................................................................................ 3-6 Water Depth and Dry Weather Flow Drain ............................................................................... 3-6 Energy Dissipation................................................................................................................. 3-6

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Flow Spreaders ..................................................................................................................... 3-7 Check Dams ......................................................................................................................... 3-8 Underdrains.......................................................................................................................... 3-8 Swale Divider........................................................................................................................ 3-9 Soils..................................................................................................................................... 3-9 Vegetation............................................................................................................................ 3-9 Restricted Construction Materials .......................................................................................... 3-11 VEGETATED SWALE MAINTENANCE STANDARDS ................................................................................... 3-14 General Requirements ......................................................................................................... 3-14 Maintenance Standards........................................................................................................ 3-15

4. FILTER STRIPS ............................................................................................................... 4-1

DEFINITION ................................................................................................................................ 4-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 4-1 MULTI-USE OPPORTUNITIES ............................................................................................................ 4-1 FILTER STRIP DESIGN SPECIFICATIONS............................................................................................... 4-1

Location ............................................................................................................................... 4-1 Tributary Area Length............................................................................................................ 4-2 Sizing................................................................................................................................... 4-2 Geometry ............................................................................................................................. 4-3 Energy Dissipation / Level Spreading ...................................................................................... 4-3 Access.................................................................................................................................. 4-4 Water depth and Velocity....................................................................................................... 4-4 Soils..................................................................................................................................... 4-4 Vegetation............................................................................................................................ 4-5 Restricted Construction Materials ............................................................................................ 4-5 FILTER STRIP OPERATIONS AND MAINTENANCE ..................................................................................... 4-7 General Requirements ........................................................................................................... 4-7 Maintenance Standards.......................................................................................................... 4-7

5. BIORETENTION .............................................................................................................. 5-1

DEFINITION ................................................................................................................................ 5-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 5-1 MULTI-USE OPPORTUNITIES ............................................................................................................ 5-1 BIORETENTION DESIGN SPECIFICATIONS ............................................................................................. 5-1

Geotechnical and Landscape Considerations ............................................................................ 5-1 Pretreatment ........................................................................................................................ 5-2 Sizing Criteria ....................................................................................................................... 5-2 Geometry ............................................................................................................................. 5-5 Flow Entrance and Energy Dissipation ..................................................................................... 5-5 Underdrains.......................................................................................................................... 5-5 Overflow .............................................................................................................................. 5-7 Hydraulic Restriction Layers ................................................................................................... 5-7 Planting/Storage Media.......................................................................................................... 5-7 Plants................................................................................................................................... 5-9 Restricted Construction Materials .......................................................................................... 5-10 BIORETENTION OPERATIONS AND MAINTENANCE ................................................................................. 5-12 General Requirements ......................................................................................................... 5-12 Maintenance Standards........................................................................................................ 5-13

6. INFILTRATION FACILITIES ............................................................................................ 6-1

DEFINITION ................................................................................................................................ 6-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 6-1

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MULTI-USE OPPORTUNITIES ............................................................................................................ 6-2 INFILTRATION FACILITY DESIGN SPECIFICATIONS .................................................................................. 6-2

Geotechnical Considerations................................................................................................... 6-2 Site Geotechnical Investigation............................................................................................... 6-3 Setbacks .............................................................................................................................. 6-3 Pretreatment ........................................................................................................................ 6-3 Sizing Criteria ....................................................................................................................... 6-4 Facility Geometry .................................................................................................................. 6-6 Embankments....................................................................................................................... 6-7 Additional Control Functions ................................................................................................... 6-7 Drainage .............................................................................................................................. 6-8 Emergency Overflow ............................................................................................................. 6-8 Vegetation............................................................................................................................ 6-8 Maintenance Access .............................................................................................................. 6-9 Construction ....................................................................................................................... 6-10 Landscaping ....................................................................................................................... 6-10 Restricted Construction Materials .......................................................................................... 6-11 INFILTRATION FACILITY OPERATIONS AND MAINTENANCE ...................................................................... 6-14 General Requirements ......................................................................................................... 6-14 Maintenance Standards........................................................................................................ 6-16

7. STORMWATER WETLAND BASINS .................................................................................. 7-1

DEFINITION ................................................................................................................................ 7-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 7-1 MULTI-USE OPPORTUNITIES ............................................................................................................ 7-2 STORMWATER WETLAND BASIN DESIGN SPECIFICATIONS......................................................................... 7-2

Basin Sizing and Geometry..................................................................................................... 7-2 Water Supply ........................................................................................................................ 7-3 Soils Considerations .............................................................................................................. 7-3 Buffer Zone .......................................................................................................................... 7-4 Energy Dissipation................................................................................................................. 7-4 Vegetation............................................................................................................................ 7-4 Outlet Structure and Spillway ................................................................................................. 7-4 Side Slopes........................................................................................................................... 7-5 Embankments....................................................................................................................... 7-5 Fencing ................................................................................................................................ 7-6 Right-of-Way ........................................................................................................................ 7-6 Maintenance Access .............................................................................................................. 7-6 Landscaping Outside of the Facility ......................................................................................... 7-7 Restricted Construction Materials ............................................................................................ 7-7 STORMWATER WETLAND BASIN MAINTENANCE STANDARDS.................................................................... 7-11 General Requirements ......................................................................................................... 7-11 Maintenance Standards........................................................................................................ 7-12

8. SAND FILTERS ................................................................................................................ 8-1

DEFINITION ................................................................................................................................ 8-1 GENERAL CONSTRAINTS AND SITING CONSIDERATIONS ........................................................................... 8-1 SAND FILTER DESIGN SPECIFICATIONS ............................................................................................... 8-1

Basin Sizing and Geometry..................................................................................................... 8-1 Sizing Methodology ............................................................................................................... 8-2 Sand Specification ................................................................................................................. 8-4 Underdrains.......................................................................................................................... 8-5 Pretreatment ........................................................................................................................ 8-6

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