Scientists and experts are learning from the Oso landslide


Six months after the Oso landslide, our state and community are still in the process of learning and healing. There are multiple groups taking a deeper look at the slide, everything from its causes and its impact to the emergency response and how development is approved near steep slopes. A 12-member commission of experts enlisted by Gov. Jay Inslee and Snohomish County Executive John Lovick held its first meeting last month, and a team of geologists released a report about the causes of the landslide in July.

The job of Inslee and Lovick’s commission is to glean lessons from the disaster, but with the tragedy still so close and so powerful, it’s impossible for commissioners to do their job without strong emotion.

The commission toured the landslide site in August and later that day, held its first meeting in Everett.

The commission has nine more meetings ahead of them, the next one on Thursday evening. They’re under a mid-December deadline to produce a report with findings and recommendations.

The commissioners, by design, have varied backgrounds — and mostly come from outside Snohomish County. They include police and fire chiefs, emergency managers, scientists and a former longtime state real estate commissioner.

Their meeting Friday afternoon was in downtown Everett.

Their morning, however, began with a trip to the disaster zone.

“One of those things where you just get quiet and listen,” said Paul Chiles, the commissioner with the real estate background.

The team of geologists, meanwhile, had a more specific mission when it issued its report earlier this summer: what geologically caused the slide to happen? The team, organized by the Geotechnical Extreme Events Reconnaissance Association and co-led by University of Washington engineering professor Joseph Wartman, found that one of the largest causes of the slide was extreme rainfall. Thirty inches of rain fell in the three weeks leading up to the slide; the state’s average for the entire month of March is just six inches of rain.

The landslide’s area’s history of previous slides played another key role, the geologists said.

The researchers also looked further back in history, reviewing evidence from a number of large landslides in the Stillaguamish Valley around Oso during the previous 6,000 years. The team estimated that, based on a review of carbon dating and maps of 15 similar historic landslides nearby, slides such as the March event have happened in the same area as often as every 400 to 1,500 years historically.

“The real different thing about that particular spot was how much it had failed in the very recent past,” said Dave Montgomery, a geomorphologist with the University of Washington and co-author of the report. “It had been chewed on a lot by prior failures.”

The report authors said the landslide occurred in two phases. The first slope failure was a repeat of previous slides that had been documented as far back as the 1950s at that site. The most recent one to contribute to the March slide took place in 2006.

The second phase of the March 22 event tapped into a much deeper landslide history at the site.

“You have the really big ones from thousands of years ago,” Montgomery said, “But why did the piece of the slope fail that did? It was different from some other areas up and down the valley due to the history of failure in recent decades, which exacerbated the stability problem.”

Joseph Wartman, the co-leader of the GEER team, was back in the news this week with an op-ed in the Seattle Times, in which he discusses how future landslides can be avoided.

In Oso, local and state leaders had done enough due diligence to know that the landslide area, Steelhead Haven, was prone to slides, Wartman writes. But they were hampered by limitations in how the hills are examined.

Such was the case for the Oso region, where much of the nearby hillside terrain had been mapped as being susceptible to landslides. But these types of maps tell us little about the potential consequences of landslides. For example, does a hazard designation imply that a hillside will creep along at just a few inches each year, or does it instead suggest that a slope could collapse and run out over a community, as happened at Oso?

In fact, the most important issue — the risk to human life — is not explicitly considered in routine landslide-hazard assessments.

Other countries, such as New Zealand, are pioneering the use of new technologies that can map not just which hillsides may collapse and slide, but the path those landslides will likely travel. When you know where a potential landslide is going to go, you can limit development in those areas and keep people out of harm’s way, Wartman says.

Mapping landslide risks is not simple. But technologies such as remote sensing, combined with improved analytical and probabilistic models, are making it easier, cheaper and faster to do than ever before. Perhaps the biggest challenge is for public officials who must decide what risk level is tolerable in their communities, as well as what to do about established residences and communities where risk to life is deemed to be unacceptably high.

Landslide-risk mapping alone could not have prevented the Oso tragedy. Only direct actions such as investing in hillside stabilization or keeping individuals out of harm’s path could have done that. This would have required the will of both public officials and the community.

But unlike traditional landslide-hazard mapping used today, risk mapping could have identified Steelhead Haven as a place where the chance of losing one’s life was very high, and where mitigation actions would have been warranted. It also could have informed community residents about just how risky a place Oso was.