KITSAP COUNTY STORMWATER DESIGN MANUAL

KITSAP COUNTY

STORMWATER DESIGN MANUAL

Effective Date: December 31, 2016

prepared for

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Acknowledgements

Stormwater Design Manual Authors

Shawn Alire Jonathan Brown Scott Diener Robin Kirschbaum Jenifer Lawrence Christopher May Jeff Rowe David Tucker

Kitsap County Department of Community Development (DCD) HDR Kitsap County DCD Robin Kirschbaum, Inc. (RKI) Kitsap County DCD Kitsap County Public Works Kitsap County DCD Kitsap County Public Works

Kitsap County Code Title 12 (Stormwater) Update

Shawn Alire Wayne Carlson Scott Diener Robin Kirschbaum Jenifer Lawrence Christopher May Jeff Rowe David Tucker

Kitsap County Department of Community Development (DCD) AHBL Kitsap County DCD Robin Kirschbaum, Inc. (RKI) Kitsap County DCD Kitsap County Public Works Kitsap County DCD Kitsap County Public Works

Thanks to the Following for Their Examples and Contributions

Washington State Department of Ecology King County City of Seattle City of Bellevue

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CONTENTS

VOLUME I ? PROJECT MINIMUM REQUIREMENTS AND SITE PLANNING CHAPTER 1 INTRODUCTION....................................................................................... 1-1

1.1 Purpose of This Manual............................................................................................ 1-1 1.2 How to Use this Manual .......................................................................................... 1-1 1.3 Purpose of Volume I................................................................................................ 1-2 1.4 Organization of this Volume ................................................................................... 1-3 1.5 References ............................................................................................................... 1-3 CHAPTER 2 SITE ASSESSMENT AND PLANNING ..................................................... 2-1 2.1 Inventory and Analysis of Key Project Components.................................................. 2-1

2.1.1 Project Boundaries and Structures................................................................... 2-1 2.1.2 Soils................................................................................................................. 2-2 2.1.3 Critical Areas ................................................................................................... 2-2 2.1.4 Dewatering....................................................................................................... 2-2 2.1.5 Topography...................................................................................................... 2-2 2.1.6 Hydrologic Patterns and Features .................................................................... 2-3 2.1.7 Vegetation........................................................................................................ 2-4 2.1.8 Land Use Controls ........................................................................................... 2-4 2.1.9 Access ............................................................................................................. 2-5 2.1.10 Utility Availability and Conflicts......................................................................... 2-5 2.2 Site Design Considerations ...................................................................................... 2-6 2.3 Low Impact Development ......................................................................................... 2-7 2.4 Site Mapping ............................................................................................................ 2-8 2.5 Submittal Requirements ......................................................................................... 2-10 CHAPTER 3 DETERMINING MINIMUM REQUIREMENTS ......................................... 3-1 3.1 Step 1 ? Define the Boundaries of the Project Site.................................................. 3-1 3.2 Step 2 ? Identify the Receiving Water and Downstream Conveyance.................. 3-1 3.3 Step 3 ? Review Minimum Requirements Exemptions.............................................. 3-3 3.4 Step 4 ? Per form Site Assessment and Planning ................................................. 3-3 3.5 Step 5 ? Calculate New Plus Replaced Hard Surface and Native

Vegetation Conversion............................................................................................ 3-3 3.6 Step 6 ? Calculate New Plus Replaced Pollution Generating Surface ................. 3-6 3.7 Step 7 ? Determine Which Minimum Requirements Apply .................................. 3-6

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CHAPTER 4 MINIMUM REQUIREMENTS FOR NEW AND REDEVELOPMENT .......... 4-1 4.1 Project Applicability ................................................................................................... 4-2 4.1.1 New Development............................................................................................ 4-5 4.1.2 Redevelopment................................................................................................ 4-5 4.2 Minimum Requirements............................................................................................ 4-7 4.2.1 Minimum Requirement #1: Preparation of Stormwater Site Plans .................... 4-7 4.2.2 Minimum Requirement #2: Construction Stormwater Pollution Prevention (SWPP)............................................................................................................ 4-7 4.2.3 Minimum Requirement #3: Source Control of Pollution .................................. 4-15 4.2.4 Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls .......................................................................................................... 4-15 4.2.5 Minimum Requirement #5: On-site Stormwater Management ........................ 4-15 4.2.6 Minimum Requirement #6: Runoff Treatment................................................. 4-22 4.2.7 Minimum Requirement #7: Flow Control ........................................................ 4-24 4.2.8 Minimum Requirement #8: Wetlands Protection ............................................ 4-26 4.2.9 Minimum Requirement #9: Operation and Maintenance ................................ 4-27

VOLUME II ? DESIGN STANDARDS AND REQUIREMENTS CHAPTER 1 PLANS AND REPORTS ............................................................................ 1-1

1.1 Introduction............................................................................................................... 1-1 1.2 Drainage Review ...................................................................................................... 1-1

1.2.1 Projects Requiring Drainage Review................................................................ 1-1 1.2.2 Review Types and Requirements .................................................................... 1-2 1.2.3 Review Stages ................................................................................................. 1-5 1.2.4 Drainage Review Required by Other Agencies ................................................ 1-7 1.2.5 Drainage Design Beyond Minimum Requirements ........................................... 1-7 1.3 Submittal Processes and Requirements ................................................................... 1-8 1.4 Submittal Documents................................................................................................ 1-9 1.4.1 Site Assessment and Planning Packet............................................................. 1-9 1.4.2 Site Improvement Plans ................................................................................. 1-10 1.4.3 Construction Stormwater Pollution Prevention Plan ....................................... 1-18 1.4.4 Drainage Reports........................................................................................... 1-21 1.4.5 Geotechnical Analysis/Soils Reports.............................................................. 1-24 1.4.6 Other Reports ................................................................................................ 1-26

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Volume II ? Design Standards and Requirements

1.5 Permit Issuance...................................................................................................... 1-26 1.5.1 Pre-construction Meeting ............................................................................... 1-26 1.5.2 Final Project Approval .................................................................................... 1-27 1.5.3 Performance Bond for uncompleted Subdivision improvements..................... 1-29 1.5.4 Transfer of Engineering Responsibilities ........................................................ 1-29 1.5.5 Project Phasing.............................................................................................. 1-29

CHAPTER 2 CONSTRUCTION STORMWATER POLLUTION PREVENTION .............. 2-1 2.1 Introduction............................................................................................................... 2-1 2.2 Local Amendments to Volume II of the Ecology Manual ........................................... 2-1

CHAPTER 3 SOURCE CONTROL OF POLLUTION...................................................... 3-1 3.1 Introduction............................................................................................................... 3-1 3.2 Minimum Requirement #3 ? Source Control of Pollution........................................... 3-1 3.3 Operation and Structural Source Control BMPs........................................................ 3-1 3.4 Treatment BMPs for Specific Pollutant Sources........................................................ 3-2 3.5 Selection of Operational and Structural Source Control BMPs.................................. 3-2

CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS & DESIGN ..................................... 4-1 4.1 Introduction............................................................................................................... 4-1 4.2 Conveyance System Design Flow ............................................................................ 4-1 4.2.1 Rational Method............................................................................................... 4-1 4.2.2 Santa Barbara Urban Hydrograph Method ....................................................... 4-1 4.2.3 Western Washington Hydrology Model ............................................................ 4-2 4.2.4 MGS Flood Model ............................................................................................ 4-2 4.3 Route Design and Easement Requirements ............................................................. 4-2 4.3.1 Route Design ................................................................................................... 4-2 4.3.2 Discharge Type and Location .......................................................................... 4-2 4.3.3 Easement and Setback Requirements ............................................................. 4-3 4.4 Pipes, Outfalls, and Pumps ...................................................................................... 4-4 4.4.1 Pipe Systems ................................................................................................... 4-4 4.4.2 Outfall Systems.............................................................................................. 4-11 4.4.3 Pump Systems............................................................................................... 4-12 4.5 Culverts .................................................................................................................. 4-13 4.6 Open Channels....................................................................................................... 4-16 4.6.1 Natural Channels ........................................................................................... 4-16 4.6.2 Constructed Channels ................................................................................... 4-17

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4.7 Downstream Analysis ............................................................................................. 4-19 4.7.1 Level 1 Analysis ............................................................................................. 4-20 4.7.2 Level 2 Analysis ............................................................................................. 4-21 4.7.3 Level 3 Analysis ............................................................................................. 4-21

4.8 Hydraulic Structures ............................................................................................... 4-22 4.8.1 Flow Splitters ................................................................................................. 4-22 4.8.2 Flow spreaders .............................................................................................. 4-23

CHAPTER 5 STORMWATER MANAGEMENT BMPS ................................................... 5-1 5.1 Purpose of This Chapter........................................................................................... 5-1 5.2 Organization of This Chapter .................................................................................... 5-1 5.3 BMP Selection .......................................................................................................... 5-2 5.3.1 Determine Dispersion Feasibility...................................................................... 5-2 5.3.2 Determine Infiltration Feasibility ....................................................................... 5-3 5.3.3 Select BMPs for On-site Stormwater Management ........................................ 5-15 5.3.4 Select BMPs for Water Quality Treatment...................................................... 5-16 5.3.5 Select BMPs for Flow Control ........................................................................ 5-21 5.4 BMP Design ........................................................................................................... 5-21 5.4.1 Post Construction Soil Quality and Depth ...................................................... 5-22 5.4.2 Better Site Design .......................................................................................... 5-25 5.4.3 Preserving Native Vegetation......................................................................... 5-26 5.4.4 Dispersion BMPs ........................................................................................... 5-27 5.4.5 Rain Gardens................................................................................................. 5-31 5.4.6 Bioretention Cells, Swales, and Planter Boxes............................................... 5-33 5.4.7 Perforated Stub-out Connections ................................................................... 5-36 5.4.8 Permeable Pavement .................................................................................... 5-38 5.4.9 Tree Retention and Tree Planting .................................................................. 5-42 5.4.10 Vegetated Roofs ............................................................................................ 5-43 5.4.11 Reverse Slope Sidewalks .............................................................................. 5-47 5.4.12 Minimal Excavation Foundations.................................................................... 5-48 5.4.13 Rainwater Harvesting..................................................................................... 5-49 5.4.14 Pre-settling Basins ......................................................................................... 5-50 5.4.15 Infiltration Basins............................................................................................ 5-52 5.4.16 Infiltration Trenches ....................................................................................... 5-53 5.4.17 Drywells ......................................................................................................... 5-55 5.4.18 Compost-amended Vegetated Filter Strips (CAVFS) ..................................... 5-57

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