Drainage Manual 

The Thurston County Drainage Design and Erosion Control Manual (DDECM) spells out regulatory standards for reducing and managing stormwater runoff on new development and redevelopment projects. It applies in unincorporated Thurston County, and in cities and urban growth areas that adopt it. 

Current Manual-Entire (PDF)   |   Individual Volumes of the Current Manual   |   Past Manuals  |   Supporting Documents   |   Subscribe to our email list for updates 


DDECM Online Open House

We invite you to review the proposed changes to the Thurston County Drainage Design and Erosion Control Manual. Comments will be accepted through October 29, 2021.

1. View the video presentation.
2. Read the changes summary documents below.
3. Review the 2021 changes in a Word document track changes format below
4. Comment on the proposed DDECM updates using the DDECM comment form.


DDECM Environmental Impact Review-Determination of Nonsignificance-Comments Open until November 8, 2021

Thurston County Public Works has completed the Environmental Impact Review for the 2022 DDECM update and determined it to have no significant impact on the environment.
Comment on the impacts to the human environment using the DDECM Determination of Nonsignificance Comment Form.

DDECM Environmental Impact Review-Determination of Nonsignificance:


2022 Drainage Manual Update

We’re updating the DDECM per Department of Ecology requirements. The updated version will become effective on June 30, 2022.

In addition to incorporating minor staff- and industry-requested changes, nine significant changes were included in this update: 

  • Continuous Simulation Modeling
  • Replaced Hard Surfaces Redevelopment Threshold
  • Equivalent Areas
  • Core Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP)
  • Core Requirement #5: Onsite Stormwater Management
  • Core Requirement #7: Flow Control
  • Concrete Washout Best Management Practices
  • Source Control Best Management Practices
  • Wetlands Guidance
Changes summaries:

2021 changes in a Word document track changes format:
Volume I (Word)
Volume II (Word)
Volume III (Word)
Volume IV (Word)
Volume V (Word)

Ecology Requirements:

DDECM-Timeline.PNG