Landslides occur when material like debris, rock, and soil become dislodged from the earth and slide downward at speeds that can approach 100 miles per hour. David Montgomery at the University of Washington studies past and present landslides to try to understand what causes them. "When Nature Strikes" is produced by NBC Learn in partnership with the National Science Foundation and The Weather Channel. For a classroom activity related to this video, please click the Links section below.
When Nature Strikes -- Landslides
MARSHALL SHEPHERD reporting:
Whether it's falling rocks or a giant wall of mud, landslides are a relatively common natural disaster that can strike anytime, and virtually anywhere. David Montgomery, a geologist at the University of Washington who is funded by the National Science Foundation, is studying past and present landslides to try and understand their causes.
It's been called the most devastating landslide in U.S. History. On the morning of March 22, 2014, just outside Oso, Washington, part of a mountainside collapsed without warning. Eighteen million tons of mud and debris engulfed more than 40 homes and structures and nearly a mile of highway. The disaster claimed 43 lives and cost millions of dollars in property damage.
DAVID MONTGOMERY (University of Washington): Immediately after the Oso landslide happened, there were a lot of questions about what happened, how did that hillside fail?
SHEPHERD: David Montgomery at the University of Washington is a professor of geomorphology, a branch of geology that studies the evolution of the earth's surface. As part of a six-person team investigating the Oso disaster, Montgomery is trying to unravel how and why the landslide happened.
MONTGOMERY: I was asked to join a team called the Geotechnical Extreme Event Reconnaissance team that went to look at and document data from that landslide to try and gather data that might not survive another year or so, stuff that would degrade over time so it's important to actually document it and share that information with other scientists.
SHEPHERD: Landslides occur when material, such as debris, rock, and soil become dislodged from the earth due to events like excessive rain or earthquakes. The displaced material forms something called slurry, a semi-liquid material that flows downward, sometimes reaching speeds up to 100 miles per hour.
MONTGOMERY: Landslides are gravitationally driven movements of stuff off the Earth's surface. They come in sort of deep varieties, sort of say like a whole side of a mountain may slide and move out in to a valley bottom or in shallow varieties where just the soil might slide off of the rock at the surface.
SHEPHERD: To analyze the Oso disaster, Montgomery used a combination of aerial photography and a mapping technique called LIDAR that scans the terrain using a laser in order to create a highly-detailed 3-dimensional map of the mountainside.
MONTGOMERY: So this image shows the site of the Oso landslide before it happened. There's a long history of landsliding at this site. For a failure that happened after the 1940’s and 50’s when a piece of the slope reactivated and started moving again.
SHEPHERD: By comparing historic views of the mountainside with current scans, Montgomery pinpoints a line of cliffs called scarps that indicate the risk for landslides in the area.
MONTGOMERY: This shows the head scarp from the pre-2006 landslide, the landslide sitting right in here, and then this other dashed yellow line shows that prehistoric head scarp of an even older landslide that had been at the same site and failed in the past. So there's a whole series of landslides that had happened at the same site that then failed again in 2014.
SHEPHERD: Knowing an area's history is only one method for determining if landslides could happen again. Another method is to create a digital representation of the area and use computer modeling to see what could happen under different conditions, such as heavy rainfall.
MONTGOMERY: You can then map how wet will different places be, how steep are they and you can put that kind of information into a slope stability model to predict the places where it'd take a little bit of rainfall to cause a landslide or it would take Noah’s flood to trigger a landslide. And that can give you an idea for where the higher risk areas and the lower risk areas are on a piece of topography.
SHEPHERD: Montgomery's research also takes him out in the field. On this day, he and his team are investigating conditions near Carbon Glacier, on the north slope of Washington's Mount Rainier, where a series of landslides and debris flows have recently struck.
MONTGOMERY: It's pretty amazing all the debris on that thing, that's just astounding.
SHEPHERD: The melting glacier has increased the amount of sediment flowing downward and overfilling the river, causing damage to the surrounding trees, roadways, and highways. The information they gather here may help determine when and why the next landslide in the area will be triggered.
MONTGOMERY: One of the things, the lessons from Oso I think is really that you have to understand the frequency with which they occur. And you don’t get that simply from the LIDAR. It takes focused geologic study to actually go out and understand how does this place work? What are the hazards it really presents?
SHEPHERD: Though disasters like the one in Oso, Washington will never be fully preventable, the research of Montgomery and his fellow geologists is essential for understanding the causes and effects of landslides.
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