Surface Water Design Manual Revision
Surface Water Design Manual Revision
King County engineers and scientists are adding a new stormwater treatment option to the 2021 Surface Water Design Manual (SWDM). The amendment includes adding guidance for using High Performance Bioretention Soil Mixes, approved by the Washington State Department of Ecology, that can better protect fish.
Public comment period
March 25 - May 8
King County is amending its Surface Water Design Manual to include a soil mix option that is proving to be successful in removing the chemical called 6PPD-Q which could reduce fish mortality. The amendment offers an opportunity for public review and comment.
Drafts of the proposed public rules are available below:
Proposed SWDM Public Rule - 126 KB
Proposed 2021 SWDM 2024 Revision Draft Redlines All - 12 MB zip
Amended chapter 1 - 1.5 MB
Amended chapter 5 - 2.4 MB
Amended chapter 6 - 3.6 MB
Amended definitions - 614KB
Amended appendix C - 2.8 MB
Amended reference 8-M - 1KB
Amended reference 11-C - 621 KB (no redline version because of large degree of reorganization)
Amended reference 14-B - 753 KB (no redline version because of large degree of reorganization)
How to comment
Comments can be submitted by phone, email, postal service, at an online public comment meeting (date to be determined), or using the comment form below.
Email: Mark Wilgus
Voicemail: 206-477-4848
U.S. Postal Service:
ATTN: Mark Wilgus
Water and Land Resources Division
201 S. Jackson St, Suite 5600
Seattle, WA 98104
Online public comment meeting: Date to be determined.
About this revision
Surface water or stormwater is rainwater that picks up pollutants from building and land surfaces. With stormwater treatment that mimics nature, we can remove pollutants from stormwater.
Adding bioretention using the new soil mixes to the SWDM adds to our list of tools to help reduce pollutants such as sediment, heavy metals like copper and zinc, and phosphorus – a nutrient that promotes excess algae growth.
The chemical 6PPD-Q is released from tire particles into stormwater. The University of Washington and Washington State University researchers have identified it as the chemical that is killing Coho salmon.
More information can be found on the Surface Water Design Manual web page.
From the Keeping King County Green blog
Frequently Asked Questions
2021 Surface Water Design Manual
This is a technical manual detailing stormwater management requirements for parcel development in King County, WA and those municipalities that have adopted this manual. At present the most current version is the 2021 Surface Water Design Manual (SWDM). A complete update will take place in 2025.
6PPDQ is a chemical used to preserve tires that has been linked to coho salmon mortality. Stormwater washes the "tire dust" containing 6PPDQ off roads into salmon bearing streams across Puget Sound. Read more.
Early data from a King County study showing the effectiveness of the new High Performaning Bioretention Soil Mix in reducing coho mortality has prompted this revision to recommend its use in stormwater design. Read more.