Pierce County Stormwater Management and Site Development Manual Volume III Hydrologic Analysis and Flow Control BMPs Prepared by: Pierce County Surface Water Management Ordinance No. 2015-48s October 28, 2015 Effective Date: December 5, 2015 PIERCE COUNTY STORMWATER and SITE DEVELOPMENT MANUAL Table of Contents Chapter 1 - Introduction ................................................................................................ 1-1 1.1 Purpose of This Volume .............................................................................................. 1-1 1.2 Content and Organization of This Volume .................................................................. 1-2 1.3 How to Use This Volume ............................................................................................ 1-3 Chapter 2 - Hydrologic Analysis and Design Standards ............................................ 2-1 2.1 Minimum Computational Standards ............................................................................ 2-2 2.1.1 Discussion of Hydrologic Analysis Methods Used for Designing BMPs........................................................................................................... 2-3 2.2 Western Washington Hydrology Model ...................................................................... 2-5 2.2.1 Limitations to the WWHM ......................................................................... 2-6 2.2.2 Assumptions Made in Creating the WWHM .............................................. 2-6 2.3 Closed Depression Analysis ........................................................................................ 2-9 2.4 Flow Control Criteria ................................................................................................. 2-10 2.5 Infiltration Facilities for Flow Control ...................................................................... 2-10 2.5.1 Purpose ...................................................................................................... 2-10 2.5.2 Procedures ................................................................................................. 2-11 2.5.3 General Criteria for Infiltration Basins and Trenches............................... 2-16 2.6 Underground Injection Control .................................................................................. 2-21 Chapter 3 - Flow Control Design .................................................................................. 3-1 3.1 Soil Preservation and Amendment (Ecology BMP T5.13) .......................................... 3-1 3.1.1 Description .................................................................................................. 3-1 3.1.2 Applications and Limitations ...................................................................... 3-1 3.1.3 Modeling and Sizing ................................................................................... 3-2 3.1.4 Soil Preservation and Amendment Design Criteria .................................... 3-2 3.1.5 Construction Criteria ................................................................................... 3-7 3.1.6 Operations and Maintenance Criteria ......................................................... 3-7 3.2 Dispersion Facilities..................................................................................................... 3-7 3.2.1 General Dispersion Facility Design Criteria ............................................... 3-7 3.2.2 65/10 Dispersion (Ecology BMP T5.30) .................................................... 3-8 3.2.3 Sheet Flow Dispersion (Ecology BMP T5.12) ........................................... 3-8 3.2.4 Concentrated Flow Dispersion (Ecology BMP T5.11) ............................... 3-9 3.3 Tree Planting and Tree Retention (for Flow Control Credit, Ecology BMP T5.16) . 3-13 3.3.1 Description ................................................................................................ 3-13 3.3.2 Applications and Limitations .................................................................... 3-13 3.3.3 Modeling and Sizing ................................................................................. 3-14 3.3.4 Tree Planting and Tree Retention Design Criteria .................................... 3-14 3.3.5 Construction Criteria ................................................................................. 3-18 3.3.6 Operations and Maintenance Criteria ....................................................... 3-19 December 2015 Volume III – Hydrologic Analysis and Flow Control BMPs i PIERCE COUNTY STORMWATER and SITE DEVELOPMENT MANUAL 3.4 Bioretention Cells, Swales, and Planter Boxes (Ecology BMPs T5.14 and T7.30) .. 3-20 3.4.1 Description ................................................................................................ 3-20 3.4.2 Applications and Limitations .................................................................... 3-21 3.4.3 Infeasibility Criteria .................................................................................. 3-23 3.4.4 Modeling and Sizing ................................................................................. 3-26 3.4.5 Field and Design Procedures .................................................................... 3-27 3.4.6 Bioretention Design Criteria ..................................................................... 3-30 3.4.7 Bioretention Construction Criteria ............................................................ 3-44 3.4.8 Operations and Maintenance Criteria ....................................................... 3-44 3.5 Permeable Paving (Ecology BMP T5.15) .................................................................. 3-44 3.5.1 Description ................................................................................................ 3-44 3.5.2 Applications and Limitations .................................................................... 3-45 3.5.3 Infeasibility Criteria .................................................................................. 3-47 3.5.4 Modeling and Sizing ................................................................................. 3-49 3.5.5 Field and Design Procedures .................................................................... 3-50 3.5.6 Paving Surface Design Criteria ................................................................. 3-52 3.5.7 Construction Criteria ................................................................................. 3-60 3.5.8 Operations and Maintenance Criteria ....................................................... 3-62 3.6 Infiltration Trenches (Ecology BMP T7.20) .............................................................. 3-62 3.6.1 Description ................................................................................................ 3-62 3.6.2 Applications and Limitations .................................................................... 3-63 3.6.3 Modeling and Sizing ................................................................................. 3-63 3.6.4 Infiltration Trench Design Criteria ........................................................... 3-63 3.6.5 Construction Criteria for Trenches ........................................................... 3-64 3.6.6 Operations and Maintenance Criteria for Trenches .................................. 3-65 3.7 Infiltration Basins (Ecology BMP T7.10) .................................................................. 3-65 3.7.1 Description ................................................................................................ 3-65 3.7.2 Applications and Limitations .................................................................... 3-65 3.7.3 Infiltration Basin Design Criteria .............................................................. 3-65 3.7.4 Construction Criteria ................................................................................. 3-66 3.7.5 Operations and Maintenance Criteria ....................................................... 3-66 3.8 Rain Gardens (Ecology BMP T5.14A) ...................................................................... 3-67 3.8.1 Description ................................................................................................ 3-67 3.8.2 Applications and Limitations .................................................................... 3-67 3.8.3 Infeasibility Criteria .................................................................................. 3-68 3.8.4 Modeling and Sizing ................................................................................. 3-68 3.8.5 Field and Design Procedures .................................................................... 3-69 3.8.6 Rain Garden Design Criteria ..................................................................... 3-70 3.8.7 Rain Garden Construction Criteria ........................................................... 3-72 3.9 Roof Downspout Controls ......................................................................................... 3-72 3.9.1 Selection of Roof Downspout Controls .................................................... 3-72 3.9.2 Application to Minimum Requirements ................................................... 3-72 3.9.3 Downspout Infiltration Systems (Ecology BMP T5.10A) ........................ 3-73 3.9.4 Downspout Dispersion Systems (Ecology BMP T5.10B) ........................ 3-79 3.9.5 Perforated Stub-Out Connections (Ecology BMP T5.10C) ...................... 3-84 ii Volume III – Hydrologic Analysis and Flow Control BMPs December 2015 PIERCE