Landslides

Key Points

  • Seattle has a lot of steep hills, very wet winters and geology that is prone to landslides. As a result slides occur frequently, especially in the winter. January has the most landslides.

  • 8.4 percent of the city's surface is covered by areas identified as slide prone in the city's Environmentally Critical Areas Ordinance.

  • Landslides can be any size. Most are shallow happen on undeveloped land and have minor consequences. A small percentage of landslides are deeper seated. Deep seated landslides are usually bigger and therefore more dangerous that shallow slides, but any slide can be deadly.

  • Occasionally swarms of landslides occur within a few days, especially during winter storms. While most of the slides in these swarms are small, some cause extensive damage.

  • The response to landslides becomes more difficult when they are part of a larger event like a winter storm.

  • Any landslide can be dangerous. A shallow 1997 slide on Bainbridge Island killed a family of four.

  • Most insurance policies do not cover damages from landslides making property owners extremely vulnerable to economic loss.

  • Most of the land use in potential slide areas is open space, single family residential and right of way. The City of Seattle is the largest owner of landslide prone slopes.

  • Landslides can precipitate secondary emergencies, most notably 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 of Seattle.

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