Heavy rain brings increased risk of slides

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by GLENN FARLEY / KING 5 News

Bio | Email | Follow: @GlennFarley

KING5.com

Posted on November 20, 2012 at 7:56 PM

Updated Tuesday, Nov 20 at 8:00 PM

SEATTLE - A new statewide slide hazard map still in beta testing shows some coastal counties are now at extreme risk from rain driven landslides.  The state Department of Natural Resources, Division of Geology and Earth Resources manages the map, in conjunction with the National Weather Service.  
   
Those counties officially listed at extreme danger as of 2:15 Tuesday afternoon,  include Grays harbor, Jefferson, Mason, Pacific, Wahkiakum, Mason and Skamania.  Clallam,  Snohomish and Kitsap are listed at high risk…King, Thurston, Lewis, Skagit and Whatcom at moderate, the point where slides can start.  Western Washington counties at low risk include Pierce, San Juan and Island, along with all counties east of the Cascades.
    
Emergency managers in hard hit Grays Harbor County say parts of the county saw six inches of rain.
   
But even in more moderate risk King County, where Seattle saw record levels of rain of up to two inches in a six hour period, there are slides.  One is along the notorious Perkins Lane, which in 1996 and 1997 saw homes slide down hill, some crashing on the beach. 
   
“This is kind of ground zero in the city for slides,” says geotechnical engineer Jim Lee, with Seattle Public Utilities.
   
The slide risk goes up, as more and more rain is absorbed down into the soil.  Engineers say typically what happens, is that when enough of that water hits an impermeable later of clay, the soil above the clay wants to start moving.  Saturated soil is heavier, and the risk goes up with the steepness of the slope.  It will just get worse as we get further and further into the winter. 
   
Between Seattle and Everett in Snohomish County, Burlington Northern Santa Fe Railroad spokesman Gus Melonas says there are some 15 slides, some of which reached the tracks.  Slides ranging from small to fairly large.  

While freight trains are moving again after a several hour delay Tuesday morning. Amtrak and Sounder passenger trains are suspended for  48 hours after the slides have stopped for safety.

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