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Stormwater services

Managing stormwater and waterborne pollution

Find comprehensive services, data and information related to stormwater and non-point source pollution in King County, Washington.

To offer a suggestion or report an error on the King County stormwater services and information website, please contact Fred Bentler, webmaster.

Stormwater Services and Information for King County, Washington

Stormwater services and information

Driving rain - stormwater in King County, Washington
Rainfall washes pollutants from yards and pavement into our local streams, lakes and Puget Sound.

Report a problem

Contacts: Stormwater Services Section

Stormwater Pollution Prevention Manual

King County Surface Water Design Manual

Learn about stormwater and how to reduce pollution

Introduction to stormwater in King County
Learn about the science of stormwater, common problems related to stormwater including causes and solutions, homeowner tips, regulations, and how to get help from King County.

Reduce stormwater and its pollution at home

Act to reduce local flooding

Proposed Changes to King County's 2009 Surface Water Design Manual and 2009 Stormwater Pollution Prevention Manual

King County is updating both the 2009 Surface Water Design Manual (SWDM) and the 2009 Stormwater Pollution Prevention Manual (SPPM). The changes to the manuals are approved through the adoption of public rules. To review the proposed changes and proposed public rules and to learn how to provide comments, please go to the links below.

Neighborhood Drainage Assistance Program (NDAP)
Get technical assistance for questions about construction, permitting, and storm drainage and request capital improvements or repairs to address flooding, erosion and sedimentation problems affecting private property in unincorporated King County.

Website mini-survey

Yes | No | Other

Drainage design resources for professionals

2009 Surface Water Design Manual
Documents and software to help engineers design storm drainage systems that reduce problems for downstream neighbors and minimize damage to our natural environment.

KCRTS hydrologic and KCBW hydraulic modeling software

  • The King County Runoff Time Series (KCRTS) hydrologic model is a required methodology under the 1998 King County Surface Water Design Manual for sizing detention and infiltration flow control facilities.
  • The King County Backwater (KCBW) Program is a standard step backwater analysis program for pipe networks, culverts, and simple open channels.

Drainage maintenance resources

Low impact development

Low impact development reports and resources (external link)
The EPA offers Low Impact Development information, reports, and resources.

Wastewater system

Combined Sewer Overflow Control Program - Wastewater Treatment Division
Combined sewer overflows - Dept. of Health, Seattle/King County
Some old segments of our regional sewers were designed to drain stormwater too, but these combined systems occasionally flood in big downpours.  Combined sewers were designed to let diluted wastewater overflow untreated into rivers, lakes or Puget Sound to avoid sewage backups in homes and businesses.

Regional Inflow and Infiltration Control Program
What King County is doing to reduce the unintended flow of surface water and groundwater into the sewer system.

King County Waterworks Garden stormwater facility
Example of a beautiful and functional stormwater treatment facility that integrates art, technology and nature.

Stormwater projects and plans

Green River stormwater retrofit planning project
Study modeling and comparing methods to manage stormwater and reduce pollution in developed areas, to establish best practices to manage stormwater.

King County Stormwater Management Program (SWMP)
Describes King County's action plan to comply with the National Pollutant Discharge Elimination System (NPDES) phase I municipal stormwater permit issued by the Washington State Department of Ecology.

King County Road Right-of-Way Drainage Trunk Line Assessment Project - Final Report
A condition assessment of the regional trunk drainage system located within the King County road Right-of-Way.

King County ROW Drainage Trunk Line Assessment -Technical Memo #1
Supporting documentation to the "King County Road Right-of-Way Drainage Trunk Line Assessment Project - Final Report"

King County climate change plan
How King County seeks to reduce greenhouse gas emissions, adapt to projected climate change impacts, and embed mitigation and adaptation into county policy decisions. The plan incorporates issues, goals and actions related to stormwater.

Background information

King County's surface water management fee
How your fees pay for these services in Unincorporated King County including Vashon Island.

Stormwater resources and references
Links to regulatory agencies, environmental organizations, King County stormwater staff directory, and other resources for information on stormwater and related water resource topics.

Interactive map - stormwater complaints, facilities, projects and studies
Note: this link will open a new browser window
Zoom in to view the full set of map layers including drainage complaints, the regional, commercial and residential stormwater facilities, neighborhood drainage projects, stormwater studies, and aerial imagery among other things. Viewing information about individual map features may take some practice: choose the "active" (or queryable) map layer from the layers list, select the Arc IMS Query Tool tool, and click on one of that layer's features to display associated data.

Annual reports to Ecology for the NPDES Municipal Stormwater Permit

Facility and permit compliance - King County performance measures
These performance measures track King County's compliance with its surface water management permit among other things.


Stormwater treatment and flow controls map - Post 1990 (17.5 Mb Acrobat pdf)
This map shows parcels in King County that were developed after 1990, when stormwater treatment controls and substantially more effective stormwater flow controls started to be required for new development. Thus the map presents a general graphic illustration of where modern stormwater controls are presumed to have been implemented and where development has occurred with no treatment control and little or no flow control. Note, the map is based on the year of latest development and does not represent actual stormwater control. In addition, the area presumed to have modern stormwater controls is likely smaller than what is shown because not all incorporated areas began requiring such controls when unincorporated King County did in 1990.

Related information

Related agencies

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News and announcements

May 6, 2016
May is ‘Puget Sound Starts Here’ month, stresses stormwater pollution prevention

Waterworks grants due May 25
Funding is available for projects that improve water quality. Pre-application workshop May 4 in Tukwila

Apr. 12, 2016
External article, Seattle Times
Stormwater detox: Pollution solutions are within reach

Nov. 5, 2015
External opinion, Seattle Times
Puget Sound recovery hinges on blocking polluted runoff

» Archived news and announcements