Western Washington has all the conditions that make it prime territory for landslides: lots of loose material that our glaciers left behind as they carved steep slopes into a landscape that gets lots of heavy rainfall.
Still, predicting exactly when landslides will happen is extremely complex.
It’s been about eight months since a deadly landslide in Oso killed 43 people and swallowed up dozens of homes on the North Fork of the Stillaguamish River. Among the experts studying the causes of the Oso slide is David Montgomery, a prolific author, MacArthur Fellow and professor of Geomorphology at the University of Washington.
Montgomery says there are some very human reasons why geologists did not see the Oso slide coming.
1. We Think They're One-offs
Unlike a big earthquake that’ll shake the whole region all at once, landslide hazards tends to happen one hill at a time or a few hills at a time during one particularly wet storm. So the public perception of them and the public policies we make to manage landslides tend to underestimate their magnitude.
But Montgomery says there’s an awful lot of potentially unstable ground, “very little of which actually fails at any one time … if you add them up over the years, it’s sort of a steady drum beat of damage to property and tragically, every now and then, loss of life,” he says.
Consider, for example, that just the year before Oso, there was a huge sinkhole on Whidbey Island that devastated several properties. Then there are the nearly perennial interruption of train schedules on the tracks between Seattle and Everett due to mudslides.
2. It's Hard To Tell Just How Far The Land Will Slide
Montgomery says the Oso landslide was certainly sudden, but the hillside had also failed many times in the past. What geologists failed to predict was that that hill could potentially sweep all the way across the valley bottom.
“It ran out an awfully long way, much farther than prior slides had done at that same location,” Montgomery says.
But the past doesn’t necessarily predict the future, because there are so many geological factors at work. Scientists need to look at the broader context.
“There’s a very large landslide immediately next door (to the Oso slide) that shows up in the LIDAR data quite well that ran out pretty much across the whole valley bottom. But how old is it? How far ago in the past did it happen? Those aren’t questions you can answer simply by looking at the terrain and the topography.
"It involves a much more elaborate geological work and investigation to try and ferret out what the risk might actually be in a setting like that,” he says.
And Montgomery says even though he drove past the site numerous times, he himself was surprised by the Oso slide.
“Never once did I stop and think, ‘Oh, that hill might come across this highway,’” he says.
3. Massive Slides 'Boggle The Imagination'
Then there's the fact big landslides are much bigger than our imagination can easily embrace.
“To see half a mountain fall down and basically liquefy and run out across a valley bottom, that’s sort of the definition of stretching the human imagination,” Montgomery says.
He has seen bigger slides than Oso, but only rarely. When the flank of Mount St. Helens collapsed and triggered the eruption in 1980, it was a much larger failure, he says, but that, too, “boggled the imagination.”
“So, seeing events like this can help even geologists recognize the evidence in other areas for the same kind of processes having happened before,” Montgomery says.
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David Montgomery is featured in a new NOVA documentary titled “Killer Landslides”that premieres on KCTS-9 Wednesday, Nov. 19 at 9 p.m.
