Floods

Key Points

• Nationally, floods are the most costly and destructive disasters.

• Most damage caused by Hurricane Katrina was caused by flooding.

• Western Washington is very prone to flooding. Seattle's flood profile is very different from rural areas of the state.

• Seattle has three distinct flooding hazards: 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: State Route 99 crosses the South Park floodplain and State Route 522 cross along three segments of Thornton Creek.

  • 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.

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

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