Report Offers Lessons from the Oso Landslide

It recommends communicating landslide risks clearly and consistently to the public, implementing monitoring and warning systems, and making seismological recordings of landslides–especially large ones.

A report on the March 22, 2014, landslide in Oso, Wash., makes valuable recommendations for predicting and mitigating the damage caused by such slides. The Oso event occurred about 10:37 a.m. on a Saturday. 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 and published July 22, two months after the team's field reconnaissance. The National Science Foundation supported their work.

Eerily, the last missing victim's body was located on the day the report was released, the Snohomish County Sheriff's Office reported.

The landslide was 7.6 million cubic meters in size, or about 270 million cubic feet, and the debris flow traveled more than half a mile across a river valley, destroying an entire neighborhood of about 35 single-family residences and covering a section of State Highway 530 with as much as 20 feet of debris. The report notes that the slope that collapsed had slid several times since the 1930s and is the site of an ancient landslide.

The team concluded it is unlikely that seismic activity triggered the landslide and three weeks of intense rain that immediately preceded it, perhaps more than 30 inches in March 2014, played a major role. It 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," it states.

The report's recommendation section suggests communicating landslide risks clearly and consistently to the public, implement monitoring and warning systems, and making seismological recordings of landslides--especially large ones--to help in understanding failure sequences.

Visit www.geerassociation.org to read the full report.

This article originally appeared in the September 2014 issue of Occupational Health & Safety.

About the Author

Jerry Laws is Editor of Occupational Health & Safety magazine, which is owned by 1105 Media Inc.

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