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Barb Graff, Director



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Seattle has three distinct flooding hazards:

  • Riverine flooding – Heavy precipitation causes a river or stream to overflow its banks into the adjoining floodplain. Seattle’s creeks, especially Thornton and Longfellow, have flooded more often than the managed Duwamish River. Failure of the Howard Hanson dam or the release of large volumes of water from the dam could affect the Duwamish River. (See Infrastructure Failures for more on dam failures). These areas are small for a large urban area comprising 388 acres.
  • Coastal flooding – Associated with storms. High tides and wind can push water into coastal areas. Coastal flooding can erode the toes of bluffs and are one factor in landslides. Some areas, like South Park, can experience drainage problems under the same conditions.
  • Urban flooding – Happens suddenly when intense rain overwhelms the capacity of the drainage system. Low lying, bowl-shaped areas like Madison Valley and Midvale are the most likely to flood.

The area in the 100-year floodplains covers South Park and the drainage basins for Thornton and Longfellow Creeks. Flood control structures have been built in all of these areas. Small segments of two high-volume arterials cross the flood plain: SR99 crosses the South Park floodplain and SR 522 cross along three segments of Thornton Creek.

Seattle has fewer than ten buildings that have had more than one flood loss.

Major Incidents






Flash Flood


A rainstorm of record intensity flooded low-lying areas of city during rush hour.


Creek Flood / Watershed


Heavy rain in October flooded many basements. Watersheds heavily damaged.


Creek Flood


More basement flooding. Occurred in the same storm that caused many landslides.

Issues to Note

There have been other flooding incidents, including in 1990, 1986, 1978, but the impacts seems to have limited and localized. Local flooding often accompanies winter storms that cause landslides and wind damage that tend to eclipse flood damage.

Thornton Creek in Northeast Seattle and Longfellow Creek in West Seattle flood. A detention pond was built to mitigate damage on Thornton creek and a water quality channel in being built that will complement this work. Flooding on both creeks has been local events. Flooding on Longfellow Creek is being addressed in the Longfellow Creek Watershed Action Plan.

Regionally, flooding is the most frequent natural hazard in Western Washington. City can be affected when the Puget Sound Lowlands flood. Not only is the regional transportation network disrupted, but both Seattle City Light and Seattle Public Utilities own and operate facilities located outside the city limits on the Cedar, Tolt, Skagit River and Pend Oreille Rivers. Flooding can be a concern in these areas during times of heavy rains and extraordinary snowpack. Flooding in these areas can impact us by damaging facilities in these areas.

On the Web

Green River Flooding

How to Prepare for Floods (PDF) - Information from the City of Seattle, King County and Washington State.

Thornton Creek Water Quality Channel - Explains how the project will work

Longfellow Creek - Explains the restoration project.

Seattle Floodplain Development Code

During an emergency, go to for the latest information.

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