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2009 Thurston County Drainage Design and Erosion Control Manual

The Thurston County Drainage Design and Erosion Control Manual sets standards for reducing and managing stormwater runoff on new development and redevelopment projects in the unincorporated areas of Thurston County. The manual also applies to projects within incorporated and urban growth areas that adopt the Thurston County manual.

red arrowThe drainage manual you see on this page was approved by Thurston County commissioners on August 17, 2009 and took effect November 16, 2009.  To view the adopting ordinance, click here. To see the previous, 1994 version, click here.

manual coverThe new manual is consistent with the 2005 Department of Ecology Stormwater Manual for Western Washington. The updates were required under Thurston County's NPDES Phase II permit.

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2016 Drainage Design and Erosion Control Manual Revision for Thurston County

The FINAL DRAFT of the Thurston County Drainage Design and Erosion Control Manual (DDECM) was adopted by the Thurston County Board of Commissioners on October 18, 2016. The 2016 DDECM will be effective on December 31, 2016. Projects submitted for permitting after December 31, 2016 will be subject to the 2016 DDECM. The Final Draft of the 2016 DDECM is available for download below. The Stormwater Utility staff is in the process of completing final formatting and editorial changes prior to publication. The final document will be posted prior to the effective date.   

 Final Draft Drainage Design and Erosion Control Manual September 2016

Comment Resolution

Preface and Table of Contents

Volume I

Volume II

Volume III

Volume IV

Volume V



Summary of Changes

Comment Form (downloadable)




2009 Drainage Design and Erosion Control Manual for Thurston County


Administrator Memo's


Modeling Information

  • The 2009 Drainage Design and Erosion Control Manual for Thurston County requires the use of the Western Washington Hydrologic Model, Version 3 (WWHM3) with Thurston County enhancements for BMP design. The Thurston County enhancements include the use of additional rain gage stations and 15-minute time steps and also include a recharge calculator to demonstrate minimum infiltration requirements for projects. The program is available for download by clicking here.
  • Guidance for Using the Recharge Calculation in WWHM3 with Thurston County Enhancements (PDF)
  • Department of Ecology memo approving Western Washington Hydrologic Model with Thurston County Enhancements (PDF)

Design Guides for 2009 Drainage Manual Best Management Practices

Fact Sheets

Related Items

Contact Us

 Staff contact: Pat Allen (360) 867-2078.  E-mail: AllenP@co.thurston.wa.us


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