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. 

The 2022 Drainage Manual is the current standard. Due to project vesting provisions contained in Thurston County's Municipal Stormwater Permit, the 2022 Drainage Manual requirements apply to projects located within the Municipal Stormwater Permit boundary with applications submitted:

1. On or after July 1, 2022.
2. Prior to January 1, 2017, that have not started construction by January 1, 2022.
3. Prior to July 1, 2022, that have not started construction by July 1, 2027. 

The 2022 Drainage Manual also applies to projects located outside the Municipal Stormwater Permit boundary with applications submitted on or after July 1, 2022. For projects with applications submitted prior to July 1, 2022, applicable County code will apply.

Read the 2022 DDECM Vesting Memorandum.

Current Manual-Entire   

Supporting Documents 

The 2022 Drainage Manual was updated in March, 2024 with minor clerical edits. View the list of changes.

Individual Volumes of the Current Manual

Volume 1: Minimum Technical Requirements and Site Planning

Volume 2: Construction Stormwater Pollution Prevention

Volume 3: Hydrologic Analysis and Stormwater Conveyance

Volume 4: Source Control

Volume 5: Stormwater Best Management Practices

Past Manuals

Subscribe to our email list for updates

2022 Drainage Manual Update

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

• View the 2022 DDECM Changes Table.
• Watch this short 2022 Drainage Manual Update video to learn about the updates.

For Questions: 

Zach Severs - Water Resources Civil Engineer
zach.severs@co.thurston.wa.us
360-867-2086