The disaster was the deadliest landslide in U.S. history, but a new report says area has been hit by 15 similar slides in the past 6,000 years.
Editor’s note: This story was originally published July 22, 2014. The original copy has been republished below as part of our 10-year anniversary coverage in order to fix broken links, photos, graphics and related stories.
Previous slope failures set the stage for the deadly Oso landslide and could help explain why the hillside collapsed in such catastrophic fashion, says a new report issued on the four-month anniversary of the disaster.
A succession of slides dating to the 1930s had eaten away at the slope above the North Fork of the Stillaguamish River, the report points out. The most recent, in 2006, left a jumbled mass of soil piled against the lower part of the hill.
It was that jumble, saturated by rain and groundwater, that likely collapsed first on the morning of March 22, transforming into a slurry that became more liquid as it gained momentum, plowed across the river and buried the Steelhead Haven neighborhood.
A few minutes later — after most of the damage was already done — the upper slope collapsed in a series of massive blocks.
Forty-three people were killed, the greatest loss of life in any single landslide in U.S. history. On Tuesday, searchers found the body of 44-year old Molly Kristine “Kris” Regelbrugge, believed to be the last missing victim.
The report is the first published, scientific analysis of the slide. It was prepared by a team of six scientists and engineers convened by the Geotechnical Extreme Events Reconnaissance (GEER) Association, a federally funded program to conduct rapid surveys of natural disasters.
The team spent four days in May gathering field data and making observations at the site.
The scientists didn’t identify any single trigger that set off the slide but said several factors could have contributed — heavy rainfall foremost among them.
In the first 21 days of March, the report estimates, as much as 30 inches of rain may have fallen on the slide area, compared with the March average of 6 inches.
“That very clearly played an important role in triggering the event,” said University of Washington engineer Joseph Wartman, co-leader of the study.
The report says logging also may have contributed by increasing the amount of groundwater flowing into the slope — but the scientists stressed that their survey couldn’t determine whether that was the case.
“We’re not in a position to answer the question: To what degree did forest practices potentially contribute to this?” said UW geomorphologist David Montgomery, also a member of the team.
The team estimated that at least 15 similarly large landslides occurred up and down the river in the past 6,000 years. Carbon dating of buried trees showed evidence of a slide on the Oso hill 6,000 years ago — long before any logging in the area.
But that doesn’t mean timber practices couldn’t have been a factor in contemporary times.
“What it shows is that these are naturally very unstable slopes,” Montgomery said. “And where would you expect the effects of forest practices — if there are any — to be most manifest? On the least stable slopes.”
Changes to slope
Another unanswered question is whether the river might have eroded the base of the slope and destabilized it.
The scientists say it was obvious that groundwater was a major factor in the amount of moisture in the slope — particularly on the eastern edge of the slide zone. It’s possible that previous slides might have changed the topography and internal plumbing of the slope enough to channel more groundwater into the area that failed, the report says.
None of the previous slides crossed the river.
But the presence of so much water in the slope this year, along with the presence of crumbly soil from a 2006 slide, could help explain why the 2014 slide was so much more destructive.
“With a history of sliding, you’re basically churning the stuff up … and making it more prone to liquefaction,” Montgomery said. “Then if you are also cranking more water into it, that may help explain why this time was different.”
One major lesson from Oso is that unstable slopes can change and become more dangerous over time, the report says.
Several analyses of the hillside, beginning in 1952, warned that continued sliding could divert water from an adjacent drainage called Headache Creek into the slope, making it even more unstable and susceptible to collapse — which appears to have happened.
But it really shouldn’t have been such a surprise that the hillside was capable of generating a massive slide, the report says. A 2004 geologic map showed old landslide deposits under the Steelhead Haven neighborhood.
Comparison with landslides and debris flows around the world shows that the Oso slide, which totaled 10 million cubic yards and stretched nearly a mile in some places, was not an outlier.
“Perhaps the most striking finding is that, while the Oso landslide was a rare geologic occurrence, it was not extraordinary,” Wartman said.
Report will be in court
The GEER report is a good way to disseminate initial observations and measurements but is far from definitive, said Jonathan Godt, head of the U.S. Geological Survey’s landslide program. USGS scientists plan to study the site beginning next month, and the agency is trying to find funding for even more detailed field work, including wells to study subsurface flow.
Portland State University geologist Scott Burns, who was not involved in the study, praised it for underscoring the need for better landslide-hazard maps and risk communication.
“We should approach landslides just like we do floods, and say: There’s a risk here, like the hundred-year flood plain,” he said.
If the Oso hillside were located in Hong Kong or Australia, the report says, regulations would have required developers or local agencies to take steps to reduce the risk. One way to do that is by monitoring dangerous slopes.
“If we had been monitoring the slope, were there indications that could have been used to prepare and warn that this was about to happen?” asked team member Robert Gilbert, of the University of Texas. “I don’t have the answer to that question.”
The report is likely to wind up as an exhibit in court. At least 19 legal claims have been filed against Snohomish County and the state on behalf of slide victims. The claims, which are precursors to lawsuits, argue that residents weren’t adequately warned of the slide dangers and question whether logging aggravated the risk.