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Team Seeks to Learn From Fatal Landslide
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Aerial view of mudslide March 22, 2014
A team from the Geotechnical Extreme Events Research Association will compare lidar data recorded before and after the deadly mudslide in Oso, Washington, in hopes of determining which factors played key roles in the disaster. U.S. Navy

A research team is assembled to investigate what factors caused the massive landslide and debris flow that claimed at least 39 lives outside of Oso, Washington.

April 22, 2014—A research team from the Geotechnical Extreme Events Reconnaissance (GEER) Association hopes to capitalize on an extraordinary coincidence to gain a better understanding of the causes of the massive landslide and unconfined debris flow outside of Oso, Washington, on March 22 that killed 39 and left 7 missing.

The team will be led by Jeffrey Keaton, Ph.D., P.E., F.ASCE, a principal engineering geologist for AMEC Americas, in Los Angeles, and Joseph Wartman, Ph.D., P.E, M.ASCE, an associate professor of civil and environmental engineering at the University of Washington.

“What’s unusual about this landslide is that this is very well documented, both before and after,” Wartman says. He explains that as part of a larger effort to record the topography of the Puget Sound region, the area around Oso was extensively documented shortly before the landslide via lidar, a remote sensing technology. Lidar has also been used to document the results of the slide and settling of the debris field since the event.

“It’s really extraordinary. We are often able to get lidar afterwards. But we very rarely have lidar before,” Wartman notes. “It will allow us to be much more certain in our models, in trying to tell a coherent story about what happened.”

“Often we speculate about what we would have expected the slope would have been—that the slope would have been this inclination, that the slope would have been this high, that the morphology would have been smooth or rounded. But now that information exists and is there,” he adds.

This lidar data will provide the critical surface “morphology” of the site before the landslide—the shape and topography of the ground surface. It will also document the postlandslide surface morphology where the debris field ran out. Researchers will also examine what was entrained in that debris flow and the patterns of movement.

“We know it’s big and we know it had some really extraordinary run-out,” Wartman says. “Sometimes landslides are initiated and they stall or their movement is arrested or suspended. There are a lot of good mechanical reasons why that happens. Others develop into these more viscous-flow-type movements. This one seems to really be at the extreme, when you compare it to similar flow slides that we have observed over the past several decades.”

The research team has not been able to visit the site, but hopes to gain access soon. The GEER, which is supported by the National Science Foundation, plans a short-term field reconnaissance investigation and plans to issue findings approximately one month after the team gains access to the site. The team will gather soil samples for classification, examine the lidar data, and prepare numerical models. The modeling will likely be an iterative process in which the team works backward from the known results of the tragedy to the slope immediately before the event.

“They will do some numerical modeling and they will have the baseline conditions to start,” Wartman says. “The question will be, can their models predict what we know happened? We know the result. What we don’t know is how it got from prelandslide to postlandslide. That’s where we will be using numerical models.”

Researchers will be looking at the roles of soil erosion at the toe of the landslide, heavy rainfall in the area, and the topography within the disaster zone. Precipitation in March was exceptional, with rain totals doubling the historical averages. Soil conditions also make the area prone to landslides. Once the models are completed and validated, the research team can then alter variables to examine the roles of various factors in the tragedy.

“We can start to ask questions such as, ‘If we didn’t put in water, what would have been the effect? If we didn’t have erosion at the toe, what would have been the effect?’ We can start to disaggregate and pull apart the different factors that may have contributed to this and look at their respective roles and do that in a very systematic way,” Wartman says.

The ultimate goal is to advance scientists’ understanding of landslide and debris-flow events to the point that engineers can develop detailed hazard and risk maps that include a new variable—the intensity of the potential triggers that might result in landslides.

“It’s that combination of the setting and the intensity of the trigger,” Wartman says. “The likelihood of that trigger must be factored in as well.”


 

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    Based on the picture that I have seen the failure most probably has been triggered by toe erosion in conjunction with unstable geotechnical conditions. What we have to understand is that erosion and deposition processes are rules not exceptions. For thousands and millions of years mountains and hillsides have been washed into the seas. The local agencies should have noticed the potential toe erosion and had taken preventive measures.
    Hasan Nouri, Hoover Medalist, President Fluvialtech Inc.

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