Landslides

Key Points

• Seattle has steep hills, wet winters, and geology that is prone to landslides. Landslides occur frequently, especially in the winter and early spring.

• 8.4% of the city's surface is covered by areas identified as slide prone in the city's Environmentally Critical Areas Ordinance. 81% of the slide-prone area is zoned for open space, the right of way, or single-family residential areas. The City of Seattle is the largest owner of landslide-prone slopes.

• The most common landslides in Seattle are shallow (less than 6 - 10 feet deep), fast moving (up to 60 km per hour) slides that occur on undeveloped slopes. Shallow slides can have run-outs that exceed 50 feet.209 Less common are deep-seated landslides that cover a wider area and have a depth of movement greater than 6 - 10 feet.210 Small movements of deep-seated slides occur gradually over weeks or months. They can be very destructive to property and infrastructure if this gradual movement is not identified before a large failure of the slope occurs.

• Landslides are more likely to occur when soils are saturated. Many landslides can occur within a few days when Seattle experiences heavy rainfall or rapid snowmelt.

• Response to landslides can be more difficult when they are triggered by an event like a winter storm, which is often associated with other hazards such as widespread flooding.

• Traditional home-owners insurance policies do not cover landslide damage, making property owners extremely vulnerable to economic loss.211

• Freight and passenger rail lines run along landslide-prone slopes. Landslides have disrupted or canceled passenger trains along the Puget Sound over 500 times from 2015 - 2018.212.

• Landslides can trigger secondary hazards like flooding and hazardous materials incidents.

• The City of Seattle has undertaken measures to mitigate vulnerability to landslides. They include inventorying and mapping landslide prone areas, requirements to stabilize building sites during construction, public education, and slope stabilization projects. Mitigation often requires cooperation between private land owners and the city.

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