AN OVERVIEW OF SIGNIFICANT LANDSLIDE EVENTS IN WASHINGTON STATE (Invited Presentation)

Landslides are a significant and common geologic hazard in Washington State. The diverse geology, extended periods of precipitation, and varied topography allow for all landslide types and processes to occur. Substantial storm events can trigger thousands of landslides as observed in December 2007 when more than 1,600 landslides occurred in just the Chehalis River headwaters alone. A single storm in January 2009 caused more than 1,500 landslides greater than 5,000 ft²in size. From 1984 to 2014, 45 significant deep-seated landslide have occurred with direct costs ranging from 1 million to 110 million dollars. In 1998, the Aldercrest-Banyon landslide destroyed 138 homes, making it the second costliest landslide in U.S. history. In Washington, the risk to human life and infrastructure has increased as a result of the rapidly increasing population advancing into areas recognized as landslide prone. This presentation will briefly discuss examples of significant landslide events in Washington State and how these events, some quite large and damaging, eventually led to the development of the Washington Geological Survey’s Landslide Hazards Program.