Ty Swenson
City of Seattle landslide expert Bill Benzer explains precautionary tips to the media at a West Seattle home on Nov. 15. Seattle's rainy season heightens the possibility of slides on steep slopes.

Landslide season looms in West Seattle

City warns: when rain falls, the earth may follow

A West Seattle house on s.w. Manning St. became a media magnet Monday morning, Nov. 15, as city officials used a city employee’s abode to exemplify precautions homeowners should take to minimize the threat of a landslide.

“As the rainy season begins in Seattle, residents and homeowners need to be alert that the chance of having landslides greatly increases,” Bill Benzer, geotechnical engineer and landslide expert for the City of Seattle, said in a press release. “The soil will be saturated, which reduces slope stability, but certain measures can be taken to help protect yourself and your property from landslides.”

“What we found looking in our records over the years is about 84 percent of the slides that occur in Seattle have some kind of a human influence …,” Benzer said.

“There have been quite a few slides in West Seattle, and when I say West Seattle I’m going to lump both sides of the hill. Pretty much wherever there are steep slopes there is a potential for slides,” he added.

Benzer gave a tour of the house, built on a steep slope overlooking the downtown skyline, and pointed out several landslide prevention tips.

“I think the most important thing a homeowner can do is maintain their drainage system to make sure it works and that it goes to a safe location and water is not concentrated on a slope,” Benzer said.

Maintaining a drainage system includes making sure drain pipes do not lead water to a slope, inspecting drains on the house to make sure they are not clogged with leaves and ensuring drainage grates on the ground are leaf free, he said. As the fall leaves fall, he also recommended keeping street drainage grates free of debris.

The house on display had a drainage catch basin in the driveway that kept water from flowing over the driveway and onto the steep slope to the side of the house.

Benzer said maintaining a thick mat of vegetation on the slopes is also important, preventing erosion and slowing water’s infiltration into the ground.

Keeping a critical eye out for changes to one’s property is the next step. Leaning trees or landscape walls, cracks or shifts in concrete steps and cracks opening up the yard are all possible indicators of slope movement that could lead to a landslide, Benzer said.

He recommends consulting a geotechnical engineer or other landscape professionals if these warning signs are present.

Whenever a tree is removed from steep property, Benzer recommended keeping the stump intact. “When you pull the stump out you weaken the surface soil and contribute to erosion and possibly slides,” he said.

Benzer also said landscape debris, such as lawn clippings, should be put on the curb for pickup instead of dumped on the property as they will aid in slope saturation.

Although Seattle is currently below the U.S. Geological Survey threshold for occurrence of landslides (you can monitor the graph here), Benzer said there are several storms predicted in the near future that could put us quickly over the threshold. The threshold only indicates that conditions are right for a landslide to occur, not that it will.

“November and December tend to be our rainiest months, however it takes some time for the ground to get saturated and most of our slides occur starting about the end of December and mostly in January,” Benzer said. “About 45 percent of the slides that happen in Seattle happen in January.”

The City of Seattle will hold two landslide awareness meetings open to the public on Nov. 20 from 10 a.m. to 12 p.m. at the Northgate Community Center and Dec. 4 from 10 a.m. to 12 p.m. at South Seattle Community College. The meetings are part of Western Washington’s “Take Winter By Storm” initiative to provide winter preparedness tips and resources.

For more information on the meetings and landslide prevention, visit www.seattle.gov/dpd/emergency/landslides.

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