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Draft District of Columbia Stormwater Management Guidebook Page 10 Chapter 2 STORMWATER MANAGEMENT GUIDEBOOK Prepared for: District Department of the Environment Watershed Protection Division District of Columbia Prepared by: Center for Watershed Protection 8390 Main Street Ellicott City, MD 21043 August 2012 i ii #!&,'!* ',$-0+2'-, 0#%0"',% $3230# 3."2#1 2- 2&# '120'!2 -$ -*3+ ' 2-0+52#0 ,%#+#,2 3'"# --) 5'** # 4'* *# 2 Q &22.S ""-#T"!T%-4 .3 *'!2'-, 12-0+52#0V%3'"# --) #-2'!#1 0#%0"',% $3230# 4#01'-,1 -$ 2&# +,3* 5'** # .-12#" 2 2&'1 5# 1'2#T $3230# 4#01'-,1 0# #6.#!2#" 2- -!!30Q 2 +-12Q -,!# 7#0T Acknowledgements A major undertaking such as this requires the dedication and cooperative efforts of many individuals. Dr. Hamid Karimi, Director of Natural Resources, Sheila Besse, Associate Director of Watershed Protection, Jeff Seltzer, Associate Director of Stormwater Management, and Timothy Karikari, Branch Chief of Technical Services each deserve credit for their overall leadership and support for this project. Their willingness to allow staff to pursue ideas to their fullest and provide necessary time, resources and managerial support, laid the foundation for much innovation. Project Manger Rebecca C. Stack, DDOE-technical services Lead Authors Greg Hoffmann, P.E., Center for Watershed Protection Rebecca C. Stack, DDOE-technical services Brian Van Wye, DDOE-stormwater Contributors and Peer Reviewers Joseph Battiata, P.E., Center for Watershed Protection Gerald Brock, Ph.D., George Washington University Josh Burch, DDOE-planning & restoration Collin R. Burrell, DDOE-water quality Walter Caldwell, DDOE-inspection enforcement Jonathan Champion, DDOE-stormwater Reid Christianson P.E., Ph.D., Center for Watershed Protection Laine Cidlowski, Office of Planning Richard DeGrandchamp, Ph.D., University of Colorado/Scientia Veritas Elias Demessie, DDOE-technical services Diane Douglas, DDOE-water quality Alex Foraste, Williamsburg Environmental Group, Inc. Timothy Karikari, P.E., DDOE-technical services Melissa Keeley, Ph.D., George Washington University Tim Klien, DDOE-inspection enforcement Sarah Lawson, Ph.D., Rainwater Management Solutions Leah Lemoine, DDOE-planning & restoration Susan Libes, Ph.D., Coastal Carolina University, Conway, SC. Kelly (Collins) Lindow, P.E., (Center for Watershed Protection)/RK&K Dennis Lye, Ph.D., US EPA Massoud Massoumi, DDOE-technical services Abdi Musse, P.E., DDOE-technical services Phetmano Phannavong, P.E., C.F.M., DDOE-technical services Philip C. Reidy, P.E., Geosyntec Consultants Stephen Reiling, DDOE-planning & restoration Emily Rice, DDOE-stormwater Tom Schueler, Chesapeake Bay Stormwater Network Jeff Seltzer, P.E., DDOE-stormwater Tanya Spano, MWCOG-water resources Bill Stack, P.E., Center for Watershed Protection Meredith Upchurch, DDOT-stormwater June M. Weintraub, ScD, San Francisco Department of Public Health iii iv Table of Contents Stormwater Management Guidebook Page Acknowledgments iii Chapter 1. Introduction to the Stormwater Management Guidebook 1.0 Introduction 3 1.1 Purpose and Scope 3 1.2 Impacts of Urban Runoff 4 Chapter 2. Minimum Control Requirements and Methods 2.0 District of Columbia Stormwater Management Requirements 11 2.1 Stormwater Retention Volume (SWRv) 13 2.2 Quantity Control Requirements (Qp2 and Qp15)14 2.3 Extreme Flood Requirements (Qf)15 2.4 Minimum Criteria for Determining Extreme Flood Requirements 16 2.5 Additional Stormwater Management Requirements 17 2.6 Hydrology Methods 18 2.7 Acceptable Urban BMP Options 18 Chapter 3. Stormwater Best Management Practices (BMP) 3.0 Performance Criteria for Stormwater Best Management Practices 25 3.1 Green Roofs 26 3.2 Rainwater Harvesting 40 3.3 Impervious Surface Disconnection 62 3.4 Permeable Pavement Systems 74 3.5 Bioretention 96 3.6 Stormwater Filtering Systems 126 3.7 Stormwater Infiltration 149 3.8 Open Channel Systems 165 3.9 Stormwater Ponds 190 3.10 Stormwater Wetlands 210 3.11 Storage Practices 230 3.12 Proprietary Practices 244 3.13 Tree Planting and Preservation 247 Chapter 4. Selecting and Locating the Most Effective Stormwater Management Practice System 4.0 Choosing between Stormwater Management Best Practice (BMP) 262 v 4.1 Stormwater Management Suitability 264 4.2 Land Use Factors 266 4.3 Physical Feasibility Factors 268 4.4 Community and Environmental Factors 270 4.5 Location and Permitting Considerations 274 Chapter 5. Administration of Stormwater Management Rules 5.0 Stormwater Management Plan 278 5.1 Administration 284 5.2 Inspection Requirements 285 5.3 Maintenance 289 5.4 Penalties 290 5.5 Appeals 290 5.6 Exemptions 291 Chapter 6. Use of Off-Site Retention by Regulated Sites 6.0 Off-Site Retention Overview 308 6.1 Off-Site Retention via Stormwater Retention Credits 309 6.2 Off-Site Retention via In-Lieu Fee 310 6.3 See Appendix C for Sample Forms 311 Chapter 7. Generation, Certification, Trading and Retirement of Stormwater Retention Credits 7.0 Stormwater Retention Credits Overview 313 7.1 Eligibility Requirements 313 7.2 Certification of Stormwater Retention Credits 315 7.3 Format of SRC Serial Numbers 316 7.4 Failure to Maintain Retention After Certification of Stormwater Retention 317 Credits 7.5 Buying and Selling Stormwater Retention Credits 317 7.6 Voluntary Retirement of Stormwater Retention Credit 318 7.7 Calculations of Stormwater Retention Credits 318 7.8 Stormwater Credit Calculation Scenarios 323 Appendix A. Compliance Calculations and Design Examples A.1 Appendix B. Maximum Extent Practical Process for Existing Public Right of B.1 Way Appendix C. Off-Site Retention Forms for Regulated Sites C.1 Appendix D. Stormwater Retention Credit Forms (Certification, Trading, and D1 Retirement) Appendix E. Relief for Extraordinarily Difficult Site Conditions E.1 vi Appendix F. Stormwater Conveyance Systems Design F.1 Appendix G. Flow Control Structures Design G.1 Appendix H. Hydrologic Methods and Models H1 Appendix I. Rooftop Storage Guidance and Criteria I.1 Appendix J. Green Area Ratio J.1 Appendix K. Soil Compost Amendment Requirements K.1 Appendix L. Construction Inspection Checklists L.1 Appendix M. Maintenance Inspection Checklists M.1 Appendix N. Tiered Risk Assessment Management (TRAM): water quality end N.1 use standards for harvested stormwater for non-potable uses Appendix O. Land Cover Designations O.1 Appendix P. Geotechnical Information Requirements for Underground BMPs P.1 Appendix Q. Stormwater Hotspots Q.1 Appendix R. Pollution Prevention through Good Housekeeping R.1 Appendix S. Stormwater Fee Discount Program S.1 Appendix T. Proprietary Practices Approval Process T.1 Appendix U. References U.1 Appendix V. Definitions V.1 vii viii &.2#0 S Introductiontothe Stormwater Management Guidebook Chapter 1. Introduction to the Stormwater Guidebook 0$2 '120'!2 -$ -*3+ ' 2-0+52#0 ,%#+#,2 3'"# --) Page 2 Chapter 1. Introduction to the Stormwater Guidebook 1.0 Introduction Inadequate management of increased stormwater runoff resulting from development places a burden on sewer systems and degrades the aquatic resources in waterbodies of the District of Columbia (District). By overloading the capacity of streams and storm sewers, unmanaged stormwater runoff is responsible for increased combined sewer overflow events and adverse downstream impacts such as flash flooding, channel erosion, surface and groundwater pollution, and habitat degradation. Recognizing this, the District first adopted stormwater management regulations in 1988. These regulations (in chapter 5 of title 21 of the District of Columbia Municipal Regulations) established requirements to manage both stormwater quality and quantity. Quality control focused on the removal of pollutants from up to the first 0.5 inches of stormwater runoff, often referred to as the first flush. Quantity control came in the form of detention requirements based on the 2-year, 24- hour event for stream bank protection, as this is widely accepted as the channel shaping flow, and the 15-year, 24-hour event for flood protection, as this is the typical design capacity of the District of Columbia’s sewer conveyance system. The revisions to the 1988 regulations, on which this Stormwater Management Guidebook provides technical guidance, have not significantly changed the detention requirements, but the focus on water quality treatment has shifted to volume retention. Major land-disturbing activities must retain the volume from a 1.2 inch storm event, and major substantial improvement activities must retain the volume from a 0.8 inch storm event. By keeping stormwater on site, stormwater retention effectively provides both treatment and additional volume control, significantly improving protection for District waterbodies. This volume can be managed through runoff prevention (e.g. conservation of pervious cover or reforestation), runoff reduction (e.g. infiltration, water reuse), and runoff treatment (e.g. plant/soil filter systems, permeable pavement, etc.). 1.1 Purpose and Scope The purpose of this Stormwater Management Guidebook (SWMG) is to provide the technical guidance required for compliance with the District’s stormwater management regulations, including the criteria and specifications to be used by design engineers and planners for the planning, design, and construction of sites and Stormwater Best Management Practices (BMPs). It is the responsibility of the design engineer to review, verify, and select the appropriate BMPs and materials for the specific project under design and to submit to DDOE, as required, all reports, design computations, worksheets, geotechnical studies, surveys, rights-of-way determinations,
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