The Oso, Washington, USA Landslide: Lessons Learned Through Field Reconnaissance and Data Collection

The neighborhood of Steelhead Drive in Oso, Washington, USA was devastated by a massive landslide on Saturday, 22 March 2014, after a three-week period of intense precipitation. Debris flows from the Oso Landslide rushed down along the slope rapidly, suddenly and without much warning, destroying homes, burying a section of a major highway and claiming 43 lives. The landslide was initiated within an approximately 200 m high hill slope comprised of unconsolidated glacial and colluvial deposits. The overall size of the Oso Landslide was estimated at approximately 8 million cubic meters. This paper presents a brief summary of the event as documented by a team from the Geotechnical Extreme Events Reconnaissance (GEER) Association. The field observations and other available data suggest that the event occurred in two main stages. Through this work as well as looking at the historical landslide data in the immediate Oso region, including hydrology, geology, geomorphology and rainfall data, the GEER team identified a number of factors potentially contributing to the Oso Landslide and provide some broader lessons for those involved in assessing landslide safety.