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Learn about stormwater and how to reduce pollution


How does stormwater impact us?

1/3 of the pollution in Washington State's water is from stormwater runoff.

Using Weed and Feed?

  • 23 pesticides can be found in our streams during any given rainstorm.
  • 5 of those exceed the level that is considered safe for aquatic life.

After 1 inch of rain...

  • 748 gallons of stormwater runs off a 1,200 square foot roof
  • 27,000 gallons of stormwater runs off a 1-acre parking lot

Other stormwater impacts

50% of all salmon and steelhead runs in Puget Sound are considered unhealthy.

Washington State is the number 1 producer of oysters, clams and mussels in the U.S.

32% of the certified commercial shellfish growing areas in Puget Sound, or 140,000 total acres, have been partially or completely closed due to water pollution since 1980.

Download infographic: How does stormwater impact us?

Reduce stormwater and its pollution at home

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.

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.

Permit-related drainage design resources

  • Surface Water Design Manual (SWDM)
    A technical manual detailing stormwater management requirements for parcel development in King County, WA. Also applicable in other municipalities that have adopted this manual. Designed to help engineers design storm drainage systems that reduce problems for downstream neighbors and minimize damage to our natural environment.
  • Stormwater Pollution Prevention Manual (SPPM)
  • Regulatorily-required best management practices (BMPs) for managing stormwater on commercial, multi-family, and residential properties. Where structural BMPs (e.g. area covers) are required, those are called out in the SWDM (above), citing specific sections in the SPPM.
  • Hydrologic and hydraulic modeling software
  • Runoff Time Series (RTS) and Backwater Analysis are required by the SWDM for sizing detention and infiltration flow control facilities and for conveyances. King County has historically provided required software for these, but in more recent years other models have been approved, and King County RTS is not approved as of the 2016 and later SWDMs. Approved models and vesting dates are noted at the Hydrologic and hydraulic modeling software web page.

Permit-related drainage maintenance resources

Stormwater infrastructure

What are stormwater facilities?

Interactive map of stormwater facilities (opens new window)
Stormwater facilities map

Wastewater system

Stormwater disposal
Dispose of liquids and solids from cleaning stormwater drains.

Combined sewer overflow status map
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.

Stormwater projects and plans

The Restore and Protect Project
Project to identify habitat stressors and recommend restoration and protection actions to improve freshwater quality and aquatic habitat in lowland Puget Sound streams.

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.

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.

Miller and Walker Creeks stormwater retrofit planning for improving streams
Project to reduce stormwater pollution in urban creeks that drain into Puget Sound.

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.

Stormwater documents

Low impact development

Low impact development resources (external link)
The EPA offers Low Impact Development guides, fact sheets and other resources.

Background information

Driving rain - stormwater in King County, Washington

Rainfall washes pollutants from yards and pavement into our local streams, lakes and Puget Sound.

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

Interactive map - stormwater complaints, facilities, projects and studies
Note: this link will open a new browser window
View map layers related to stormwater by clicking the layers icon iMap layers icon, expand the "Stormwater Services" group of layers and check those layers you want to view and query including drainage complaints, stormwater facilities, neighborhood drainage projects, and stormwater studies among other things. Click a symbol or area to display the data about it.

Annual reports to Ecology for the NPDES Municipal Stormwater Permit

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