WSR 04-21-094


[ Filed October 20, 2004, 11:12 a.m. ]

Announcement of

an Update to the 2001 Stormwater Management Manual for Western Washington

The Department of Ecology is updating the 2001 stormwater management manual for Western Washington. The update is needed to correct errors, clarify statements, update design criteria and procedures, and apply recent research.

Proposed additions and deletions to the text of the 2001 manual are available at the Department of Ecology stormwater web page:

Public meetings to present and explain the significant changes and to receive public comments on all of the changes will be held at the following times and locations: On November 29, at 1:00 p.m. - 5:00 p.m., Pierce County Library, Administrative Service Center, 3005 112th Street S.E., Tacoma; on December 1, at 1:00 p.m. - 5:00 p.m., Washington Department of Ecology, 3190 160th Avenue S.E., Bellevue; and on December 2, at 1:00 p.m. - 5:00 p.m., WSU Cooperative Extension, Snohomish County, 600 128th Street S.E., Everett.

Directions to these locations are available at the Department of Ecology stormwater web page listed above.

Copies of proposed changes will be available at public meetings and upon request. Please contact Charlene Witczak at or (360) 407-7529.

A brief listing of the more significant changes being considered for the update includes:

Providing an exemption from the flow control requirement for discharges to certain rivers and large lakes.

Allowing the predevelopment land cover condition to be modeled as the existing site condition in basins that have been highly urbanized, and have had a stable land cover condition for an extended period of time. This proposal applies to sizing flow control facilities.

Deleting the instructions for development of single event hydrographs, and deleting single event hydrograph-based approaches for sizing flow control and most treatment facilities.

Updating the design procedures for sizing infiltration and filtration systems with references to use of the Western Washington Hydrology Model (WWHM), and to design criteria and steps developed by Dr. Joel Massman.

Adding interim guidance in regard to stormwater facilities that are regulated under the underground injection control (UIC) program.

Correcting design criteria for biofiltration swales.

Restricting the enhanced treatment required of arterials and highways to those which exceed a certain average daily traffic threshold.

Clarifying that action should be taken to improve erosion and sediment control and to update the site stormwater pollution prevention plan if inspections indicate that existing measures are not adequately effective.

Adding additional and new information on low impact development options.

Inserting guidance concerning how to represent certain low impact development techniques within the WWHM so that it predicts flow reduction benefits from use of those techniques.

If you are aware of errors in the manual, or statements whose intent is not clear to you, or if you have questions about the proposed changes please contact Ed O'Brien at Washington Department of Ecology, P.O. Box 47696, Olympia, WA 98504-7696, phone (360) 407-6438, e-mail

After considering the opinions and information provided through public input and making appropriate changes, ecology plans to publish the update before the end of the calendar year. The updated pages will be published separately at the website so that they can be printed and inserted into the paper copies of the manual that many users have. The full manual edition at the website will incorporate those updated pages.

Legislature Code Reviser 


Washington State Code Reviser's Office