PUBLIC/PRIVATE PARTNERSHIP TO PRODUCE DIGITAL MAP OF LANDSLIDE HAZARDS FOR SEATTLE, WASHINGTON

The U. S. Geological Survey, the city of Seattle, Washington, and Shannon and Wilson, Inc., a geotechnical engineering firm, have entered into a partnership to produce a landslide hazard map for the city of Seattle, Washington. Having sustained major damage and loss of life within the Puget Sound Region during the 1996 and 1997 rainfall seasons, the city of Seattle requested assistance from Federal and private experts to generate a landslide hazard map that the city could use for future land-use policy changes that needed to be made to make the city more disaster resistant. Unique to Seattle is a database dating from the late 1800’s of landslide locations and other information from every significant storm that triggered landslides within the city boundaries. Maintained to the present day, the database contains locations and relevant data for more than 1,300 individual landslides of which the majority are debris flows or shallow landslides that commonly mobilize into debris flows. Using a digital elevation model derived from the 2-ft contour topographic map provided by the city, a digital geologic map, shear strength values for the different geologic units provided by Shannon and Wilson, Inc., and an infinite slope analysis to calculate the stability of each map cell, we have produced “factor-of-safety” maps for Seattle under a variety of soil moisture conditions using a rainfall infiltration model of actual storm rainfall. In addition to a landslide hazard map cast in terms of the statistical correlation between the landslide database and susceptibility defined by the “factor-of-safety”, we have derived a rainfall threshold for debris flows derived from a comparison of the landslide database with rainfall records since the 1950’s.