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Oso Landslide: Anatomy of a Landslide

September 22, 2014

GEER—Geotechnical Extreme Events Reconnaissance—is a team of volunteer scientists who visit natural disasters to determine causes and help prepare for or prevent future disasters.  UW geomorphology professor David Montgomery was part of the team that visited Oso and prepared a report on their findings.  We hear from Montgomery about what the team discovered.

Oso, Washington -

David Montgomery: The real question we were trying to answer while we were out there was how did the slide fail, sort of ‘What happened?’ and also how that might relate to ‘Why did it go so far this time?’

Feliks Banel: David Montgomery is a professor of geomorphology at the University of Washington. Geomorphology is the study of landforms and what shapes them.  Montgomery responded to the Oso slide as a member of something called GEER—Geotechnical Extreme Event Reconnaissance—a “SWAT” team of scientists and engineers brought in to study natural disasters.  The first thing they needed to know . . .

Montgomery: . . .what was the history of that site in sort of recent times?”

Banel: Montgomery says the site of the Oso landslide has been changing for the past 6,000 years. Scientists have had their eyes on the area for the last half-century.

Montgomery:  And it turns out that the site had been studied before; it had been failing a number of times back to at least the 1950s. And there are reports from 1952, reports in 1988, reports in ’96 and ’97 that looked at that site and the failures that have been happening on it so we could go back and read some of that material as background for what, as it turns out, helped set up the landslide in 2014.

Banel: Something was obviously very different about this event. But what? That’s where the scientific sleuthing of GEER comes in.

Montgomery: So what we know in the immediate aftermath of the failure is roughly how big the landslide was and how far it went. And a whole section of the hillside mobilized, mobilized rapidly and swept across the valley bottom. Literally sweeping what was on the valley bottom, the whole community off, and the trees that were growing and the houses, off to the far end.

Banel: But Montgomery had more than just some old reports to go on.  Big clues came from the U.S. Geological Survey and something called LiDAR.

Montgomery: When we use the new topographic data, the LiDAR data, laser-surveyed altimetry data, it’s like putting on a new pair of glasses for geologists and the U.S. Geological Survey colleague Ralph Haugerud mapped all the landslides in the vicinity of the Oso landslide in the immediate aftermath of the 2014 failure and used the LiDAR data to identify four different age classes of landslides.  He had here noted as "A" things that were active very recently; next oldest class "B"; then "C" and "D". One of the things you can infer from the LiDAR data is how these long runout landslides in this valley have pushed the river to the far side of the valley bottom and how that has influenced the next slide that happen on the far side.  

U.S. Geological Survey “LiDAR” (or “Light Detection and Ranging”) data shows the history of landslides along the Stillaguamish River near Oso.  Courtesy U.S. Geological Survey.

Banel: Even with the LiDAR images, Montgomery and the other GEER team members had to see the site for themselves.

Montgomery: So one of the big questions we had in going out into the field was, why did it fail so big and go so far this time when in the past it had been much smaller?

Banel: As it turns out, there was a lot for Montgomery and the GEER team to piece together. The big clues came from the stratigraphy—the complex layers of materials in the ground.

Montgomery: Essentially what you see in the basic stratigraphy of the valley is the oldest sediments at the bottom are pre-glacial sediments, old river floodplain sediments that are about 35,000 years old. And as the glaciers started to come down, started to dam the North Fork Stillaguamish River, the sequence it left behind were lake sediments at the bottom and then sandy outwash, or sand laid down by rivers in the front of the glaciers. Till, when the ice actually ran over the place, and then recessional outwash, or more sand and gravel on top. So you have this deep pile of fairly weak, relatively loose stuff.

Banel: Montgomery says it was only a matter of time until this “deep pile of fairly weak, relatively loose stuff,” just slid. But it needed a trigger.

Montgomery: So you have the long-term geological setup of this weak material on steep slopes with the river cutting back and forth along the valley bottom, occasionally getting in and sizing in at the toe—eating away at the toe, of the hillsides, which over the long run, sets the stage for repeated landsliding. But part of what drives it, and certainly helped drive the 2014 landslide, was the great amount of rain we had in March of that year. It was a record rainfall for the three weeks in March preceding it.

Feliks: And all that rain, he says, triggered a sort of one-two punch.

A graphic from GEER’s report on the Oso landslide shows the one-two punch of the disaster, as the lower and upper hillside gave way about 90-seconds apart. Credit: GEER

Montgomery: Based on our field work, one of the conclusions we came to was that the site failed in two phases, with the lower part being the first domino that fell due to being very saturated and fluidizing and running out across the valley bottom. But that domino falling triggered the next domino to fall, which happened a minute-and-a-half later when the upper part of the landslide came down as well.

Banel: Ultimately, David Montgomery hopes the work he and the rest of the GEER team have done in Oso will help others in Western Washington be aware of the invisible danger of landslides.

Montgomery:  The most basic thing that the public needs to know about landslide hazards, if they’re going to make informed decisions themselves about where to live, or if counties are going to make informed decisions about where they’ll let people develop, are what the risks really are out in the landscape. And I think the Oso landslide is an important example of a case where we can learn from a disaster by trying to say, ‘Well what really are the risks out there?’



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