MAPPING OF POTENTIAL LANDSLIDE HAZARDS ALONG THE RIVER CORRIDORS OF KING COUNTY, WASHINGTON

The 2014 SR-530 landslide in Snohomish County, WA, focused attention on landslide hazards in the Pacific Northwest. King County, located immediately south of Snohomish County, is similar in topography, geology, and climate; and is subject to similar landslide hazards. King County determined that its landslide maps should be updated and initiated a new landslide mapping effort using the current information including Lidar-derived imagery and recent geologic mapping to more accurately locate and characterize potential landslide hazards. Study limits for this project were along the river corridors of Cedar, Green, Sammamish, South Fork Skykomish, Snoqualmie, and White Rivers and Issaquah Creek. The mapping identified the different landslide processes active in different parts of the county and considered a wide range of landslide processes including deep-seated landslides, shallow debris landslide susceptibility, depositional fans, potential rock fall locations, and rock avalanche deposits. Although, it was determined that no feasible method existed for delineating runout areas on a landscape scale, characteristics that might predispose a large landslide to have a long runout event were identified.