VOLUME III – Hydrologic Analysis and Flow Control BMPs


1.1 Purpose of this Volume

Best Management Practices (BMPs) are schedules of activities, prohibitions of practices, maintenance procedures, managerial practices, or structural features that prevent or reduce adverse impacts to waters of Washington State. As described in Volume I of this stormwater manual, BMPs for long-term management of stormwater at developed sites can be divided into three main categories:

•    BMPs addressing the volume and timing of stormwater flows,

•    BMPs addressing prevention of pollution from potential sources.

•    BMPs addressing treatment of runoff to remove sediment and other pollutants.

This volume of the stormwater manual focuses mainly on the first category. It presents techniques of hydrologic analysis, and BMPs related to management of the amount and timing of stormwater flows from developed sites. The purpose of this volume is to provide guidance on the estimation and control of stormwater runoff quantity.

BMPs for preventing pollution of stormwater runoff and for treating contaminated runoff are presented in Volume IV and Volume V, respectively.

1.2 Content and Organization of this Volume

Volume III of the stormwater manual contains three chapters. Chapter 1 serves as an introduction. Chapter 2 reviews methods of hydrologic analysis, covers the use of hydrograph methods for designing BMPs, and provides an overview of various computerized modeling methods and analysis of closed depressions. Chapter 3 describes flow control BMPs and provides design specifications for roof downspouts and detention facilities. It also provides design considerations of infiltration facilities for flow control.

This volume includes three appendices. Appendix A has isopluvial maps for western Washington. Appendix B has information and assumptions on the Western Washington Hydrology Model (WWHM). Appendix C includes detailed information concerning how to represent various Low Impact Development (LID) techniques in continuous runoff models so that the models predict lower surface runoff rates and volumes.

Design considerations for conveyance systems are included in Appendix I-F. Stormwater conveyance systems shall be designed in accordance with Chapter 5 of the City of Olympia Engineering Design and Development Standards (EDDS).

1.3 How to Use this Volume

Volume I should be consulted to determine Core Requirements for flow management (e.g. Core Requirements #4, #5, #7, and #8 in Volume I, Chapter 2). After the Core Requirements have been determined, this volume should be consulted to design flow management facilities. These facilities can then be included in Drainage Control Plans (see Volume I, Chapter 3.)