Expert baffled by ferocity, distance of ‘freakish’ slide
Geologists were stunned at how far and how fast the Oso slide ran out — one expert estimated the slide hit the Stillaguamish River at 60 mph. But scientists also hope that detailed analysis will help prevent such tragedies in the future.
Seattle Times science reporter
Richard Iverson may be one of the world’s foremost landslide experts, but if he had been driving along the Stillaguamish River March 22 when the slope across from Steelhead Drive began to crumble, he wouldn’t have fled to high ground.
He would have pulled over to snap pictures.
That’s because Iverson never dreamed a slide from such a modest bluff could travel as far and as fast as it did, sweeping across the river, burying an entire neighborhood and engulfing a swath of Highway 530 in a minute’s time.
“It was a freakish thing,” said Iverson, of the U.S. Geological Survey’s Cascades Volcano Observatory. “I’m not sure anybody would have anticipated a slope like that would liquefy the way it did.”
Using a new computer model, Iverson estimated that the mass of mud, rocks and trees was traveling about 60 mph when it slammed into the river.
Imagine standing on the side of a freeway when a semi truck barrels past at 60 mph, Iverson said. Then imagine a battalion of eighteen-wheelers, half a mile wide, headed straight at you.
“It’s hard for me to imagine somebody being in the middle of that Steelhead neighborhood and having any chance of survival,” he said.
As of Tuesday, Snohomish County officials reported 35 people dead and an additional 11 missing.
An eyewitness Iverson spoke with described the slide literally shoving the river out of its banks.
Iverson spent several days at the site last week and is compiling the first scientific report on what he believes will be a “profoundly important” event in the understanding of landslide hazards.
When the recovery effort is over, he and other scientists hope to study the slide in detail, trying to figure out why it so quickly liquefied and ran out for nearly a mile — more than three times the distance Iverson would have expected based on comparison with hundreds of other slides of similar heights.
“This bluff is only about 180 meters (about 600 feet) tall, which by Northwest standards is sort of small potatoes,” Iverson said.
The hope is that by understanding what caused the slide to morph into a monster, scientists will be able to better identify the most dangerous hazard zones in the future, said Jonathan Godt, a Denver-based USGS expert who also visited the Oso slide.
“It’s one of these tragic events that unfortunately provides an opportunity to increase awareness,” he said.
While it’s fairly easy to identify landslide-prone slopes, it’s much harder to predict how far slides will travel, said University of Washington geomorphologist David Montgomery. Geologists who analyzed slide risks along the North Fork of the Stillaguamish warned in 1999 that a “large catastrophic failure” was possible. But in the worst-case scenario they deemed most likely, the runout was estimated at less than a quarter of a mile.
“Any geologist who went out there would say, yes, this situation is ripe for a landslide,” Iverson said. “But in my mind, the story isn’t that a landslide occurred, but the type of landslide that occurred.”
He blames the disaster on a combination of unusually wet weather, erosion at the toe of the slide and local geology.
The slope that failed is largely made up of loose, sandy soil deposited by retreating glaciers. But that porous material is underlain by a compacted layer of silt and clay, which blocks the flow of water and allows it to accumulate deep within the hillside.
The slide may have started at that clay layer, Iverson said, but more field work is needed to be sure.
He believes the first block of soil to start moving was the jumbled pile of debris at the foot of the slope from a previous slide in 2006. The movement of that chunk destabilized the upper slope, which then collapsed.
The witness, who was in his yard across the valley, said the first thing he noticed was his dogs running away. “They probably either heard it or felt it first,” Iverson said. Then the man heard a loud roar and saw the river being flung into the air as the debris raced across the landscape.
“In my view, the thing liquefied very quickly after beginning to move,” Iverson said.
As the sandy slope collapsed, the weight probably compressed the sodden soil, which would have increased water pressure between soil grains and turned the mass to soup.
A similar effect could have occurred as the slide thundered across the low ground of the Steelhead Drive neighborhood, squeezing the saturated soil like a sponge, Iverson said.
Images of the area produced in 2013 using a laser-scanning method called lidar show that the North Fork of the Stillaguamish is riddled with the scars of old landslides. At least one of those slides clearly ran out even farther than the Oso slide, said Montgomery.
Had the lidar images been available sooner, they would have shown that slopes along the river were capable of unleashing slides with a much longer reach than any of those previously recorded — information that could have been factored into land-use decisions, he pointed out.
Iverson hopes his new computational model, which he has been working on for 20 years, will help scientists and land-use planners estimate the probability of catastrophic landslides. In the meantime, geologic studies along the Stillaguamish are crucial to provide a more complete picture of the region’s landslide history, including how long ago some of the biggest slides occurred.
“One of the things I would love to see come out of this is really serious geologic investigation, not just of the Oso slide, but all those surrounding landslides,” he said.
Sandi Doughton at: 206-464-2491 or firstname.lastname@example.org