Scientists say previous slides set the stage for the deadly Oso landslide by destabilizing the upper hillside — and that an aerial mapping method called lidar could reveal other shaky slopes.
An aerial map compiled a year before the deadly Oso landslide shows that the upper portion of the hillside was being dangerously undercut, which could point the way to identifying other high-risk slopes, according to a new analysis.
University of Illinois engineering professor Timothy Stark says he and his colleagues are convinced that the slide originated high on the slope, not lower down as previous investigations suggested. And height alone could account for the destructive power, he argues.
“This is really the key to understanding why the 2014 slide was completely different” from previous slides that didn’t travel nearly as far, Stark said.
That insight could be used to guide future hazard analysis based on the aerial-mapping method called lidar, which reveals ground topography with unprecedented clarity, he said.
Lidar maps from 2013 show that previous slides and erosion were eating away at a shelf that had long served as a kind of doorstop to buttress the upper slope and the plateau, called the Whitman Bench. Stark and his team, who were among the first scientists to visit the site last year, say that’s where the collapse of the rain-soaked hillside started.
“By 2014, that doorstop had been cut back so much due to the prior sliding that it was marginally stable, so one more slide was what brought it down,” he said.
With lidar maps, geologists and land managers should be able to zero in on other slopes where the “upper deck” might be similarly vulnerable.
“We think this is a common-sense approach to mapping these valleys to try to figure out where we could have these large runouts,” said Stark, who worked with graduate student Ahmed Baghdady. “The key is looking at the lidar to determine whether the landslide will occur lower on the slope or higher up the slope.”
In the wake of the Oso slide, which killed 43 people and obliterated an entire neighborhood, Washington’s Department of Natural Resources proposed a $6.6 million effort to expand lidar mapping and improve analysis of landslide hazards.
After months of wrangling, the state Senate last week matched the House’s proposed $4.6 million in funding for the program, though the final budget has yet to be determined.
The hillside above the North Fork of the Stillaguamish had been the site of repeated, smaller slides — the most recent in 2006 — but many geologists were stunned by the ferocity and reach of the 2014 slide, which traveled nearly a mile in some directions.
Lidar maps examined after the fact clearly show that massive slides had occurred along the valley in the distant past. The question scientists and engineers are grappling with now is how to distinguish slopes that might unleash small slides from those capable of producing devastating slides that pose the greatest risk to people.
Height the key?
Stark’s approach may have merit, said Ralph Haugerud, a lidar expert with the U.S. Geological Survey. But testing it and doing the type of analysis he suggests would require high-quality lidar data and geologic expertise to interpret it — and it remains to be seen whether the state is willing to put up the money, he cautioned.
And height alone is not always a good indicator of how far a slide will travel, said USGS researcher Richard Iverson, who has also studied the Oso slide.
“To me that seems like an awfully broad statement,” he said. “There are lots of landslides that start from much higher and don’t run out nearly as far as the Oso slide.”
Stark’s is the third scientific analysis of the slide so far. All three teams agree it was a multi-step process, but they differ on exactly how it unfolded.
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Iverson and his colleagues, who have done the most extensive field work, believe a loose mass of saturated debris near the base of the hill slid first. Then a portion of the upper slope collapsed, pressurizing and liquefying the saturated debris and propelling it across the river. That interpretation agrees well with the pattern of vibrations recorded by seismometers in the area, Iverson said.
A team sponsored by the National Science Foundation’s Geotechnical Extreme Events Reconnaissance (GEER) program also concluded the slide originated low on the slope, then destabilized the upper reaches.
Stark and his colleagues argue that a big chunk of the upper slope and underlying clay slid first, shooting off the upper hillside as if it were going over a ski jump. Traveling at high speed, that chunk of material slammed into the loose debris left by earlier slides, bulldozing it across the river and turning it into a soupy mass that entombed the neighborhood in a matter of seconds.
In Stark’s scenario, the second step involved the collapse of the very top of the slope and a portion of the Whitman Bench — but he argues that material did not travel far and was not responsible for the destruction.
The debris left on the slope by the 2014 slide is now acting as a protective doorstop for the upper hillside and the Whitman Bench. While smaller slides are almost certain to occur in the future, Stark estimates it could be 300 years before that doorstop is destabilized enough to set the stage for another massive slide.
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One unresolved question is whether the Oso slide was surprisingly big given the modest dimensions of the slope where it originated. Stark and the GEER group argue that it was not, but Iverson and his colleagues compared it to similar slides and concluded it was an outlier.
It’s important to settle the debate in order to evaluate the risk posed by other slopes, said University of Washington engineer Joe Wartman, a member of the GEER team. So he and some of his colleagues are now comparing Oso against a database of nearly 1,000 slides from around the world.
Understanding the slide process is also vital to preventing similar disasters, said UW geomorphologist Dave Montgomery, also a GEER member.
“The degree to which we can sort out and understand how this one actually happened is going to affect our ability to understand — and maybe forecast — what might happen in the future,” he said.
The best way to resolve the different explanations would be to drill into the slide and the surrounding areas, to get a clearer picture of the geologic layers and the way water moves through the ground, Montgomery said.
The Washington State Department of Transportation (WSDOT) drilled two boreholes on top of the Whitman Bench and one south of State Route 530. Their findings are being compiled in a report that will released soon, said WSDOT spokesman Travis Phelps.
Drilling into the slide mass itself will be costly and difficult, Montgomery conceded. But he’s disappointed that there hasn’t been more of a coordinated effort by government and academic scientists to study the site.
One reason may be the multiple lawsuits filed by victims and their families. Montgomery, Wartman and Stark are not acting as expert witnesses in those cases, but some experts are.