DEBRIS-FLOW HAZARD MAP OF SEATTLE, WASHINGTON

Landslides and, in particular, debris flows have long been a significant cause of damage and destruction to people and property in the Puget Sound region. Following the years of 1996 and 1997, the Federal Emergency Management Agency designated Seattle as a “Project Impact” city with the goal of encouraging the city to become more disaster resistant to the effects of landslides and other natural hazards. A major recommendation of the Project Impact council was that the city and the U. S. Geological Survey collaborate to produce a landslide hazard map of the city. A unique data set archived by the city containing more than 100 years of landslide data from severe storm events allowed comparison of actual landslide locations with those predicted by slope-stability modeling. An infinite-slope analysis, which models slope segments as rigid friction blocks, was used to estimate the susceptibility of slopes to debris flows, water-laden slurries that form from shallow failures of soil and weathered bedrock that can travel at high velocities down steep slopes. Data used for the analysis consisted of a digital slope map derived from recent Light Detection and Ranging (LIDAR) imagery of Seattle, recent digital geologic mapping of the city, and shear-strength test data for the geologic units found in the city. The combination of these data layers within a Geographic Information System (GIS) platform allowed the preparation of a debris-flow hazard map for the entire city area of Seattle.