Washington disaster subject of keynote at Geo15 conference.
Editor’s note: The 2014 Oso landslide is the subject of a shared plenary session at the Geosynthetics 2015/Environmental Connection 2015 conference Feb. 15-18 in Portland, Ore.
A new report released July 22 analyzes the March 22, 2014, landslide in Oso, Wash., and makes recommendations for predicting and mitigating the damage caused by such landslides.
The Oso event occurred Saturday morning, March 22, 2014. It killed 43 people, making it the deadliest landslide in U.S. history, according to the report prepared by a team from the Geotechnical Extreme Events Reconnaissance Association (GEER), released July 22. The report was delivered four months after the disaster, two months after the GEER team’s field work in Oso.
The landslide dimensions were measured at approximately 270 million cubic feet in size, with a debris flow that traveled more than half a mile across the north fork of the Stillaguamish River valley, destroying a neighborhood of 35 single-family residences and covering a section of State Highway 530 with as much as 20 feet of debris.
The report noted that the slope that collapsed had slid several times since the 1930s, and it is “also the site of an ancient landslide.”
“Records indicate no significant seismic activity in the days preceding the landslide and therefore it is unlikely that it had a siesmogenic origin,” the report said. “Instead, it is highly probable that the intense 3-week rainfall that immediately preceded the event played a major role in triggering the landslide.”
The GEER report also discusses additional factors, including the weakening and changing of the landslide mass by previous slides. “Given the size and depth of the  landslide, if timber harvest practices did influence the landslide, it was through modification of the groundwater recharge regime rather than by any shallow-depth loss of root mass reinforcement,” the report said.
The National Science Foundation sponsored the work by GEER in Oso.
-Ron Bygness, senior editor Geosynthetics magazine