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2021 Surface Water Design Manual (Amended 2024)
A technical manual detailing stormwater management requirements for parcel development in King County, WA. Also applicable in any other municipalities that have adopted this manual. At present this 2024 amendment to the 2021 SWDM is the most current version.
Important notice about projects permitted under earlier versions of the SWDM
Per requirements in King County's Phase 1 Municipal Stormwater Permit (effective date August 1, 2019), projects permitted under the 2016 SWDM that have not started construction by July 1, 2026, and projects permitted under earlier versions of the SWDM that have not started construction by July 1, 2021, must be revised to comply with the requirements of the 2021 SWDM, or subsequent versions as directed by state regulations. Revisions will need to be approved by the Permitting Division of the King County Department of Local Services (DLS-PD), or its successor agency prior to construction.*"Started construction" means, the site work associated with, and directly related to the approved project is at a stage where rough grading is complete or utilities are installed. For rough grading to be considered complete, elevations are within 1 foot of final design elevations.
The Department of Natural Resources and Parks has adopted a public rule (PUT 8-3-7-PR) that amends the 2021 King County Surface Water Design Manual (SWDM), in accordance with King County Code, Chapter 2.98, Rules of County Agencies and Executive Policy INF 7-6-1-EP "Drafting and Implementing Rules". The 2021 SWDM (Amended 2024) is effective as of November 14, 2024.
Explanation regarding this amendment
The 2024 amended SWDM is fundamentally the 2021 version, with the addition of bioretention designs using 'high performance bioretention mixes' (HPBSM) for a water quality treatment facility and as a flow control best management practice (BMP). No other substantive changes have been made.Printed copies of the SWDM are not being provided. Interested parties can download the electronic version of the manual and then print and bind their own manual.
The entire 2021 SWDM (amended 2024) including Chapters 1-6, Appendices A through D, Definitions, and Reference sections is available as file downloads from links below. The manual is searchable and the bookmarks within the navigation pane allow for quickly moving through document. Note that although some of the materials in the Reference section of the 2021 SWDM (amended 2024) may have been adopted by administrative rule or ordinance, the administrative rule that adopted the SWDM has not formally adopted any of the materials in the Reference section.
2021 SWDM (amended 2024) documents
Combined SWDM including all chapters, appendices, definitions, and reference sections (29.5 MB)
The combined SWDM PDF is markup capable; the individual Chapters and other sections are not at this point in time.
Individual SWDM Chapters, Appendices, References, and drawings
The changes between the 2021 SWDM and this 2024 amended version have to do with the addition of bioretention using "high performance bioretention soil mixes" (HPBSM).
Chapters, Appendices, and References that contain these amendments are labeled accordingly.
SWDM chapters
Chapter 1 - Drainage Review and Requirements (amended 2024) (1.1 MB)
Chapter 2 - Drainage Plan Submittal (867 KB)
Chapter 3 - Hydrologic Analysis and Design (2.1 MB)
Chapter 4 - Conveyance System Analysis and Design (2.5 MB)
Chapter 5 - Flow Control Design (amended 2024) (2.2 MB)
Chapter 6 - Water Quality Design (amended 2024) (3.4 MB)
Definitions (amended 2024) (579 KB)
Appendices
Appendix B - Master Drainage Plan Objective, Criteria, Components, and Review Process (80.8 KB)
Appendix C - Simplified Drainage Review Requirements (amended 2024) (2.4 MB)
Appendix D - Construction Stormwater Pollution Prevention Standards (1.7 MB)
Design Manual drawings:
Standard drawings from the SWDM can be downloaded individually in AutoCAD DWG format. Additionally, the AutoCAD DWG drawings can be downloaded all at once in a single zip file; or chapters can be downloaded as separate zip files.
Select a group of drawings to download and extract:
Standard drawings in AutoCAD DWG format
- Chapter 4 in AutoCAD format (5.3 MB) (zip file)
- Chapter 5 in AutoCAD format (10.6 MB) (zip file)
- Chapter 6 in AutoCAD (17.5 MB) (zip file)
- Appendix C in AutoCAD format (15.6 MB) (zip file)
- Appendix D in AutoCAD format (13.7 MB) (zip file)
- All AutoCAD drawings (62.8 MB) (zip file)
References
Combined References, Forms, and Worksheets (amended 2024) (5.5 MB)
*Note: References 8-M, 11-C and 14-B are the amended portions of the References collection; they contain content regarding addition of HPBSM in WQ treatment facility and Flow Control BMP design.
NEW as of 9/1/2022
Errata and updates
- Reference 8-H, Site Improvement Bond Quantities Worksheet:
- The web links on full SWDM pdf file pages 979 and 1030 are incorrect.
- The web link on the separate References pdf file page 56 is incorrect.
- The web link on full SWDM pdf file page 149 is correct as it is here.
- Modeling bioretention
- At time of publication of the 2021 SWDM, MGS Flood was not approved for modeling bioretention explicitly. Version 4.56 or later is now approved by Washington State Department of Ecology for this purpose.
SWDM cover pages
Full Manual binder cover (amended 2024) (2.7 MB)
Full Manual binder spine (amended 2024) (606 KB)
Appendix C and D covers (amended 2024) (3.8 MB)
Surface Water Design Manual application maps
Find the most up-to-date information posted in iMap
SWDM maps in PDF format:
Flow Control Application Map (1.4 MB)
Water Quality Application Map (1.4 MB)
Landslide Hazard Drainage Areas Map (1.5 MB)
SWDM signed public rule and 2016-2021 summary of changes
(for 2016-2021 changes see the 2021 SWDM web page)
SWDM Public Rule PUT-8-3-7-PR (268 KB)
SWDM Summary of Changes (142 KB)
Blanket adjustments and clarifications
None present at the present time.
If you have questions about these changes, contact Mark Wilgus at mark.wilgus@kingcounty.gov