2021 King County Surface Water Design Manual and Addendum Update

For the latest updates, see the Connect Sammamish project page.

To comply with the Western Washington Phase II Municipal Stormwater Permit, the City is required to implement and enforce a program to reduce pollutants by controlling runoff from new development, redevelopment, and construction sites. The Phase II Permit (effective 2019 through 2024) requires technical updates that are equivalent to Ecology’s 2019 Stormwater Management Manual for Western Washington (Ecology Manual). Updates are required to be adopted in 2022 to comply with permit timelines.

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

City staff review development permits for compliance with stormwater codes and standards. The City uses the 2016 King County Surface Water Design Manual (2016 KCSWDM) and the City of Sammamish Surface Water Design Manual Addendum . The Addendum to the 2016 KCSWDM outlines changes that the City opted to make when adopting the 2016 KCSWDM.

The Ecology Manual incorporated 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. King County, as a Phase I permittee, updated and adopted the 2021 KCSWDM which was effective on July 23, 2021. The municipal permit allows Phase II permittees to adopt a Phase I permittee's program approved by Ecology to meet this requirement. Therefore, the City will review and update the 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. The more restrictive requirements implemented by the City of Sammamish Surface Water Design Manual Addendum, adopted by Ordinance O2016-428, will be maintained in the new addendum.

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