Property owners urged to look out for potential landslides

With last week's soaking and more rain to come, city engineers are urging Seattle residents to examine steep slopes to prevent landslides.

"Eighty-six percent of landslides have some human influence," said Bill Benzer, manager of landscape mitigation for Seattle Public Utilities. "It could be grading and cutting, filling a slope or not controlling surface water."

November is the start of the city's landslide season, although most slides occur in January and February, after months of water has soaked into the ground. The areas most at risk are the bluffs of Magnolia and Alki.

In 1996, several homes were damaged on Perkins Lane West in Magnolia after mud slid off a city-owned hillside onto private property. Six people sued the city for damages and lost. The city has worked on controlling storm runoff on Perkins Lane by adding asphalt curbs and ditches.

In Alki, where the city held a news conference Monday, a slide closed parts of California Avenue Southwest for several months in 1996 and 1997. The city has since built retaining walls and rechanneled surface water on the hillside.

The city urges residents to examine steep slopes on their property now. A steep slope is any hillside with greater than a 40 percent incline, for instance, a 4-foot vertical rise spanning a 10-foot horizontal distance.

Risk of a slide increases if owners have altered a slope by adding fill at the top of the slope or by removing soil at the bottom. Fill soil, in particular, can weaken over time, and slide risk increases when the soil becomes saturated with water.

Property owners should pay attention to:

• Cracks in the ground and trees that have started to lean recently. If you see either of these conditions, the city recommends hiring a geotechnical engineer for an evaluation.

• Route rain-gutter downspouts so the water flows down a steep hillside. Pay attention to downspouts from rain gutters. Ideally, homeowners on a steep slope should route their stormwater to a storm drain, Benzer said.

Sharon Pian Chan: 206-464-2958 or schan@seattletimes.com