2021 King County Surface Water Design Manual and Addendum Update

UPDATE: On October 4, 2022 the city adopted the 2021 King County Surface Water Design Manual and 2021 Sammamish Addendum.

To comply with the Western Washington Phase II Municipal Stormwater Permit, the City is required to implement and enforce a program to reduce pollutants. The program must control runoff from new development, redevelopment, and construction sites. City staff review development permits for compliance with stormwater codes and standards. 

Background

The Phase II Permit is effective from 2019 through 2024. It requires technical updates that are equivalent to Ecology’s 2019 Stormwater Management Manual for Western Washington (Ecology Manual).

The Ecology Manual incorporated a variety of updates, including:

• technical updates to modeling tools,
• clarifications for replaced hard surfaces and treatment trades,
• new and updated source control best management practices, and
• revised wetlands guidance.

The municipal permit allows Phase II permittees to adopt a Phase I permittee's program approved by Ecology to meet this requirement. City of Sammamish is a Phase II permittee, and King County is a Phase I permittee. The City used the 2016 King County Surface Water Design Manual (2016 KCSWDM) and the City of Sammamish Surface Water Design Manual Addendum. The City's Addendum outlines changes that the City made when adopting the 2016 KCSWDM.

Updates are required to be adopted in 2022 to comply with permit timelines.

Project Scope

King County, as a Phase I permittee, updated and adopted the 2021 KCSWDM, effective on July 23, 2021. Because Sammamish uses the KCSWDM with addendum, the City will review and update its code to allow for and be consistent with these changes. Additionally, we may update tools that developers and city reviewers use to efficiently comply with these changes.

These updates are not intended to change drainage review thresholds, requirements, or maps. More restrictive requirements implemented by the City of Sammamish Surface Water Design Manual Addendum were adopted by Ordinance O2016-428. These will be maintained in the new addendum.

The City selected AHBL (the Consultant) to assist in reviewing and comparing code to incorporate the requirements noted in Appendix 10 of the municipal permit. The consultant compared current City stormwater code (21.03.050), the 2021 King County Surface Water Design Manual, and City of Sammamish Addendum. City Council awarded the contract to AHBL at the January 4, 2022 regular meeting with AB-21-307.

Timeline