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Mapping Washington's Landslides

Washington State Department of Natural Resources
Lidar image showing landslides in the Cedar River Valley south of North Bend by Rattlesnake Ledge.

The last few weeks of heavy rains in Western Washington means the possibility of landslides in certain areas is higher. But this is only the beginning of the rainy season.  

If you look at Seattle’s landslide maps, you can pinpoint thousands of properties that are prone to sliding.

"It doesn't mean that we now have 20,000 properties and we're on pins and needles getting ready that we're going to have 20,000 landslides happen," said Susan Chang, geotechnical engineer for the city of Seattle.

"What it means is that we know the areas in the city that where we have heightened landslide risk because of geologic conditions, because of how steep the slopes may be, and so when the ground gets saturated because of the amount of rain that we've had, and then if you get more rain, we're likely to have landslides," said Chang.

Seattle is unique in that it has paper records dating back to the late 1800s documenting landslides. So far this season, only three shallow slides have been reported.  Beyond Seattle, the state is working with counties like Pierce to improve landslide maps using light detecting and ranging or lidar. Lidar is a technology that shoots a laser from the air and takes three-dimensional pictures of the terrain below.  

"You're able to measure then the elevation more accurately of the surface  of the earth and it allows us to basically strip away all of the vegetation which in Western Washington is a significant deterrent to mapping and identifying landslides," said Dave Norman, state geologist for the Washington State Department of Natural Resources

But it will take years to map the state using lidar, and that’s dependent on federal grants as well as money from the Legislature. Ultimately experts say it’s up to private property owners to be aware of their risks.