Volume 3: Project Stormwater Control CITY OF SEATTLE STORMWATER MANUAL AUGUST 2017 Note: Some pages in this document have been purposely skipped or blank pages inserted so that this document will copy correctly when duplexed. Volume 3 – Project Stormwater Control Table of Contents Table of Contents CHAPTER 1 – Introduction .............................................................................. 1-1 1.1. Purpose of this Volume ..................................................................... 1-1 1.2. How to Use this Volume .................................................................... 1-1 CHAPTER 2 – BMP Categories ........................................................................... 2-1 2.1. Introduction .................................................................................. 2-1 2.2. Soil Amendment ............................................................................. 2-2 2.3. Tree Planting and Retention .............................................................. 2-2 2.4. Dispersion BMPs ............................................................................. 2-2 2.5. Infiltration BMPs ............................................................................. 2-3 2.6. Rainwater Harvesting BMPs ................................................................ 2-3 2.7. Alternative Surface BMPs .................................................................. 2-4 2.8. Detention BMPs .............................................................................. 2-4 2.9. Non-infiltrating BMPs ....................................................................... 2-5 CHAPTER 3 – BMP Selection and Sizing Approach ................................................... 3-1 3.1. Determine Dispersion Feasibility ......................................................... 3-2 3.2. Determine Infiltration Feasibility ......................................................... 3-4 3.3. BMP Selection for On-site Stormwater Management .................................. 3-16 3.3.1. On-site List Approach ........................................................... 3-16 3.3.2. On-site Performance Standard ................................................ 3-18 3.4. BMP Selection for Flow Control .......................................................... 3-19 3.5. BMP Selection for Water Quality Treatment ........................................... 3-21 3.5.1. Selection Steps .................................................................. 3-21 3.5.2. Treatment Performance Goals and BMP Options ........................... 3-25 3.5.2.1. Oil Control Treatment .................................................... 3-25 3.5.2.2. Phosphorus Treatment .................................................... 3-26 3.5.2.3. Enhanced Treatment ...................................................... 3-26 3.5.2.4. Basic Treatment ........................................................... 3-28 CHAPTER 4 – General Design Requirements.......................................................... 4-1 4.1. Sizing Approach ............................................................................. 4-2 4.1.1. On-site List Approach ............................................................ 4-2 4.1.2. Pre-sized Approach .............................................................. 4-3 4.1.2.1. Pre-sized Facilities ......................................................... 4-3 4.1.2.2. Pre-sized Credits, Sizing Factors, and Equations ...................... 4-3 4.1.2.3. Pre-sized Calculator ........................................................ 4-5 4.1.3. Modeling Approach ............................................................... 4-6 4.1.3.1. On-site Performance Standard ........................................... 4-6 4.1.3.2. Flow Control................................................................. 4-6 4.1.3.3. Water Quality Treatment ................................................. 4-7 Stormwater Manual Directors’ Rule 17-2017, DWW-200 August 2017 i Table of Contents Volume 3 – Project Stormwater Control 4.2. Bypass General Design Requirements .................................................... 4-8 4.2.1. On-line vs. Offline Treatment BMPs ........................................... 4-8 4.2.2. Bypassing Flows Entering or Leaving a Site ................................... 4-8 4.2.2.1. Scenario 1 – Managing Flows Entering a Site ........................... 4-8 4.2.2.2. Scenario 2 – Managing Flows Entering a Site ........................... 4-9 4.2.2.3. Scenario 3 – Uncontrolled Flows Leaving the Site ..................... 4-9 4.3. Conveyance General Design Requirements ............................................. 4-10 4.3.1. Conveyance Design and Capacity Analysis ................................... 4-10 4.3.2. Approved Point of Discharge ................................................... 4-11 4.3.2.1. Requirements for Projects with No Off-site Point of Discharge .... 4-11 4.3.3. Conveyance Systems to Point of Discharge .................................. 4-13 4.3.4. Overflow Requirements ........................................................ 4-14 4.4. Presettling and Pretreatment Requirements........................................... 4-15 4.4.1. Description ....................................................................... 4-15 4.4.2. Performance Mechanisms ...................................................... 4-15 4.4.3. Applicability ...................................................................... 4-15 4.4.3.1. Presettling and Pretreatment ........................................... 4-15 4.4.3.2. Pretreatment ............................................................... 4-17 4.4.4. Site Considerations .............................................................. 4-18 4.4.5. Design Criteria ................................................................... 4-19 4.4.6. Operations and Maintenance Requirements ................................. 4-19 4.5. Infiltration BMPs ............................................................................ 4-20 4.5.1. Infiltration BMP Sizing .......................................................... 4-20 4.5.2. Soil Requirements for Water Quality Treatment ........................... 4-20 4.5.2.1. Underlying Soil Requirements for Infiltration BMPs .................. 4-21 4.5.2.2. Imported Soil and Sand ................................................... 4-21 CHAPTER 5 – BMP Design ................................................................................ 5-1 5.1. Soil Amendment BMP ....................................................................... 5-2 5.1.1. Description ........................................................................ 5-2 5.1.2. Performance Mechanisms ....................................................... 5-2 5.1.3. Applicability ....................................................................... 5-2 5.1.4. Site Considerations ............................................................... 5-3 5.1.5. Design Criteria .................................................................... 5-3 5.1.5.1. Implementation Options ................................................... 5-3 5.1.5.2. Soil Retention ............................................................... 5-4 5.1.5.3. Soil Quality .................................................................. 5-5 5.1.5.4. Soil Management Plan ..................................................... 5-6 5.1.6. BMP Sizing ......................................................................... 5-6 5.1.7. Minimum Construction Requirements ......................................... 5-6 5.1.8. Operations and Maintenance Requirements .................................. 5-6 5.2. Tree Planting and Retention .............................................................. 5-7 5.2.1. Description ........................................................................ 5-7 5.2.2. Performance Mechanisms ....................................................... 5-7 5.2.3. Applicability ....................................................................... 5-7 Directors’ Rule 17-2017, DWW-200 Stormwater Manual ii August 2017 Volume 3 – Project Stormwater Control Table of Contents 5.2.4. Site Considerations ............................................................... 5-7 5.2.4.1. Retained Trees .............................................................. 5-7 5.2.4.2. Newly Planted Trees ....................................................... 5-8 5.2.5. Design Criteria .................................................................... 5-8 5.2.5.1. Retained Trees .............................................................. 5-8 5.2.5.2. Newly Planted Trees ....................................................... 5-9 5.2.6. BMP Credits ...................................................................... 5-10 5.2.6.1. Credit for On-site List Approach ........................................ 5-10 5.2.6.2. Pre-Sized Approach for Flow Control ................................... 5-11 5.2.6.3. Modeling Approach for On-site Performance Standard and Flow Control ....................................................................... 5-11 5.2.7. Minimum Construction Requirements ........................................ 5-11 5.2.8. Operations and Maintenance Requirements ................................. 5-11 5.3. Dispersion BMPs ............................................................................ 5-13 5.3.1. Design Requirements for Dispersion BMPs ................................... 5-13