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Technical guidance for stormwater projects

Find information and resources for stormwater projects in King County.

Stormwater permit-related drainage design resources

Surface Water Design Manual (SWDM)

A technical manual detailing stormwater management requirements for parcel development in King County, WA.

Applicable in other municipalities that have adopted this manual. It offers help for engineers designing storm drainage systems that reduce problems for downstream neighbors and minimize damage to our natural environment.

View the Surface Water Design Manual

Stormwater Pollution Prevention Manual (SPPM)

Regulatorily-required best management practices (BMPs) for managing stormwater on commercial, multi-family, and residential properties. 

Where structural BMPs (area covers, for example) are required, those are described the SWDM (above), citing specific sections in the SPPM.

View the Stormwater Pollution Prevention Manual

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.

The King County Runoff Time Series is not approved for use as of the 2016 and later SWDMs. Approved models and vesting dates are noted on the hydrologic and hydraulic modeling software page.

Stormwater infrastructure

Stormwater facilities

What are stormwater facilities?

Interactive stormwater facilities map

View stormwater facilities in King County with the interactive map 

View the interactive stormwater facilities map

Wastewater system

Stormwater disposal

Dispose of liquids and solids from cleaning stormwater drains.

Learn more about stormwater disposal

Combined sewer overflow status map

Older combined sewer and storm drain lines were designed to let diluted wastewater overflow untreated into rivers, lakes or Puget Sound to avoid sewage backups in homes and businesses.

Combined sewer overflow real-time map- Seattle area

Combined Sewer Overflow Control - Wastewater Treatment Division

Regional Inflow and Infiltration Control Program 

What we are doing to reduce surface water and groundwater leaks into the sewer system.

Learn more about the Regional Inflow and Infiltration Control program

King County Industrial Waste Program

We administer regulations to make sure the wastewater generated by industries is treated properly. 

Visit the King County Industrial Waste program

Stormwater legislation and regulations

The United States Environmental Protection Agency (EPA) develops the stormwater regulations, in accordance with the Clean Water Act (CWA).

The Washington State Department of Ecology (Ecology) develops stormwater regulations in Washington State, in accordance with Chapter 90.48 of the Revised Code of Washington, Water Pollution Control.

King County administers its Stormwater Management programs in accordance with King County Code Title 9 - Surface Water Management



Stormwater glossary of terms and abbreviations

Industrial stormwater terms and abbreviations can be confusing. We have compiled a helpful list of terms with definitions.

View the Stormwater Services glossary