SEATTLE – Seattle is considered one of the most landslide-prone cities in the country. It often involves view properties built on steep sloping terrain; during wet weeks like this one, soil becomes saturated and starts moving.
"We've got the geology that's conducive to landslides, we've got the topography and we obviously have the rain," said Jim Lee, a civil and geotechnical engineer for Seattle Public Utilities.
In the Northwest region of the Washington state Department of Transportation, that ranges from the Pierce/King County line to the south to the Canadian border to the north, and from Puget Sound east to the central Cascades, some 2,500 slide prone slopes are watched as they threaten roads during those same rains.
"We've got our maintenance folks who are out surveying their areas," said Chris Johnson, the region's maintenance and operations manager, who is also an engineering geologist as well as a civil engineer. "And they know where the unstable slopes are at."
Because of those risks, particularly in the urban areas, the U.S. Geological Survey developed an online tool to indicate when the saturation enters the danger zone based on rainfall totals measured at airports ranging from Tacoma Narrows to Everett's Paine Field, as well as Sea-Tac and Boeing Field. The City of Seattle has expanded on that, factoring in rain forecasts from the National Weather Service that look out five days.
"There's definitely more concern when we're above the landslide threshold and we've got storms continuously coming in," said Lee. "That's what we want to track. Do we want to put crews on call, do we need to put engineers on call?"
In Burien, a landslide wiped out one home, left a vehicle on the beach and damaged others. On U.S. Highway 2, 10 feet of mud blocked off traffic. WSDOT says one lane of Highway 18 near the Green River was blocked by debris.
"As time goes on and the rain continues, the slopes get saturated," said Johnson. "And the question is, how much more rain does it take before the soil loses its strength and down it comes?"
