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. 

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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
Watch this short video to learn about the proposed updates:

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:


For Questions: 
Shannon Peterson - Water Resources Utility Planner