Publication Summary

Draft version for public comment period that ended November 30, 2002. See 2003 publication list for revised final draft.
In this file you will find:
Appendix 4B – Precipitation Maps from Chapter 4 – Hydrologic Analysis and Design
See below for links to the other files. (Also see abstract below.)

Urban development causes significant changes in patterns of stormwater flow from land into receiving waters. Water quality can be affected when runoff carries sediment or other pollutants into streams, wetlands, lakes, and marine waters or into ground water. Stormwater management can help to reduce these effects. Stormwater management involves careful application of site design principles; construction techniques and source controls to prevent sediment and other pollutants from entering surface or ground water; treatment of runoff to reduce pollutants; and flow controls to reduce the impact of altered hydrology.

The objective of this Manual is to provide a commonly accepted set of technical standards and guidance on measures that will manage the quantity and quality of stormwater produced by new development and redevelopment in eastern Washington. The Department of Ecology believes that when the standards and recommendations of this Manual are properly applied, stormwater runoff should generally comply with water quality standards and protect beneficial uses of the receiving waters. Ecology recognizes that individual circumstances vary greatly, and in some instances compliance with the Manual may not ensure compliance with water quality standards.

Many guidance manuals for stormwater have been written to address national, regional and local characteristics and management needs. In the state of Washington, several guidance manuals have been prepared, used, and updated to address regional and local requirements. The Washington State Department of Ecology published the Stormwater Management Manual for Western Washington (SWMMWW) in August 2001 as an update to a predecessor manual prepared in 1992. Ecology initially proposed that the Manual could be updated to cover the entire State of Washington. In response to that proposal, representatives of eastern Washington requested that Ecology create a separate Stormwater Management Manual (SWMM) for the eastern portion of the state. Based upon these requests and upon recognition of the significantly different hydrology and geology of eastern Washington, Ecology agreed to create a separate Manual.

Discussions continued at various conferences, meetings and forums to determine the best method to accomplish this effort. A chartering meeting was held in June 2001 to formalize the structure and process for preparing the Manual for eastern Washington. David Evans & Associates, supported by EnviroIssues for meeting facilitation, was hired as a consultant to support the effort. The day long chartering meeting was attended by more than 70 representatives of 17 cities, 11 counties and five Federal and State agencies with interests in stormwater management in eastern Washington. The chartering meeting established a ten-person Steering Committee with several alternate members to lead the overall effort; it also created two Subcommittees: one for leading the preparation of the Technical Stormwater Manual, and another for leading the preparation of a Model Municipal Stormwater Program. Ecology agreed to fund the hiring of a consultant team to support the development and preparation of the documents and to assist the Steering Committee and Subcommittees with meeting coordination, public involvement and related project tasks. A Request for Qualifications was prepared and submittals were received by four consultant teams in October 2001; the team lead by Tetra Tech/KCM of Spokane was selected.

A project kick-off meeting was held on November 7, 2001 with members of the Steering Committee, Ecology, and the consultant team. The scope of work for the project and a proposed production schedule were prepared; a corresponding budget was prepared and the work began. A stakeholder workshop was held on November 29, 2001 to inform interested parties about the project efforts, the regulatory requirements, the schedule for meetings, and the document production format. After the introductory sessions, concurrent meetings of the Subcommittees were held to begin the development of the Technical Manual and the Model Municipal Stormwater Program. At least once per month (and sometimes twice per month) meetings were held in 2002 to review drafts and updates for each chapter of each document. Periodic presentations were made to address special issues such as: design storm hydrology, subsurface infiltration rates, cost estimates for facilities and stormwater management program needs. These efforts resulted in draft documents submitted for public review beginning in September 2002.