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Thurston County Building Codes

Drainage Design and Erosion Control Standards

When land is developed in Thurston County, the developer must meet certain requirements for managing runoff or erosion that might result from the project. The requirements are set forth in a document called the “2009 Drainage Design and Erosion Control Manual for Thurston County.”

In virtually all cases, the manual requires builders to submit a drainage plan along with their building applications. The drainage plan must either be an “engineered abbreviated” plan an “abbreviated” plan, or a full "drainage and erosion control plan" depending on the size of the project and its impact on drainage.

Drainage plans are reviewed and approved by the Development Review program in the Public Works Department.

red floatClick here to see summaries and fact sheets, or to see the entire manual.neighborhood storm drain

Frequently Asked Questions

Please click here to read answers to frequently asked questions about drainage plans.

Drainage Scoping Report - A Precaution

Drainage design and erosion control standards are complicated, and Thurston County has seen instances where developers have spent thousands designing stormwater ponds, swales, or other stormwater-control facilities, only to find out later that the designs did not meet state and local standards.

The 2009 Drainage Design and Erosion Control Manual includes a mandatory safety-check for planning purposes: It requires certain applicants to provide a conceptual design of their drainage plans before submitting the plans formally along with their building permit applications.

Generally if your project exceeds 5,000 square feet of new impervious surface; or converts more than ¾ acres of native vegetation to lawn or landscape; or converts more than 2.5 acres of native vegetation to pasture then a Drainage Scoping Report submittal is required.

Contact Us

For drainage plan information: Development Review Division of the Public Works Department, (360) 867-2050.
For scoping report information: Pat Allen, PE, (360) 867-2078 or e-mail allenp@co.thurston.wa.us.

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