Week of Jan. 9, 2006

It’s official - landslide season is here

Last Friday’s landslide advisory for parts of King, Snohomish and Pierce counties, issued by the U.S. Geological Survey made it official – it’s turning out to be a wet season with an increased threat of damaging landslides. The advisory warned “that wet soil conditions and continuing rainfall are likely to result in additional landslides during the next several days.”

The advisory is proving to be right on the money this week. Given the wet soil conditions, there’s an increased likelihood the region could see more landslide activity during intense rainfall or when rainfall amounts exceed one inch in 24 hours. Steep coastal bluffs and other steep hillsides are particularly susceptible to landslides under present conditions.

The advisory reinforced something King County residents have long been familiar with. Slides can occur after several consecutive rainy days, when the ground becomes saturated and excess water seeks its own path downhill. Along the way, the water may loosen soil and hillsides, liquefying sections of earth, causing the unstable muck to slide. With houses built on steep hillsides, roads carved out of mountains, and the diverse geology of King County, a landslide can cause tremendous damage.

King County Road Services Division crews are well aware of the season of slides. Recent landslides are reminders of how quickly nature can get slippery.

There’s no way to predict when a slide or washout will occur, but an area’s history is a good indicator of where there is potential for a problem. Roads crews have maps showing slide-hazard areas, such as Holmes Point Drive Northeast on the Eastside and Auburn-Black Diamond Road in South County. These roads are located in areas with steep slopes and poor drainage, which have made them prone to slides in the past.

“These areas are so large, it’s impossible to stop slides from occurring,” said Roads Maintenance Superintendent Tony Ledbetter. “But, we monitor their drainage and try to prevent the situations from getting worse.”

Preventive actions to help minimize slides include covering hillsides with plastic to prevent the ground from soaking up rainwater, and diverting drainage flows to safer areas. Crews also make sure drainage pipes are clear, catch basins and roadside ditches are free of debris, and in some cases they add Jersey barriers at the bottom of slopes to prevent slides from spilling onto roadways.

Jersey barriers are often part of the solution in slide-repair work. They can be used to contain dirt sloughing off from the hillside and reduce the need for clearing roadways. It took two days to clear the slide because there was more than just mud to deal with. Trees can also be a challenge and sometimes there are utilities to work around. One of the goals in any slide situation is to evaluate all of the factors involved in what caused the soil to give way and what steps are necessary in the clean-up to avoid a situation that could make the problem worse.

Washouts such as the one on Dockton Road last month are equally difficult to predict, but again history is a good guide for areas to watch.

The old timber bulkhead along portions of Dockton Road has been deteriorating for years. High tides and high winds generated wave action that literally pulled material under the road out to sea, causing a washout under two sections of the roadway. Roads crews immediately closed that portion of the roadway to traffic. Crews then worked in conjunction with Maintenance Engineering and Environmental staff to complete the repairs to the two areas of the bulkhead and roadway, which included removing the undermined roadway and a portion of the old bulkhead.

Slides and washouts generally coincide with our stormiest and wettest months of the year, so roads crews will be on the watch for trouble for several more weeks. County residents can help out, be reporting any signs of slides in unincorporated areas to the King County Road Services 24-hour hotline at (206) 296-8100 or 1-800-KC ROADS.

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