Guest: Map the runout risk for landslides like Oso
The long runout of the Oso landslide surprised geologists who knew of the slide risks. Guest columnist David Montgomery considers lessons from this Snohomish County tragedy.
Special to The Times
AS a geologist, I find the Oso landslide is all the more devastating when I look at the county hazard map and see that none of the homes on Steelhead Drive were mapped as part of a landslide-hazard zone.
As we all saw, the slide moved incredibly fast — and surprisingly far. It moved several times farther than one would expect for its size, according to U.S. Geological Survey landslide guru Richard Iverson.
I was asked a lot of questions in the days after the slide, usually starting with what caused it. Landslides are complex, with multiple potential causes. It will take some time to get solid answers as to the role of anything other than record rainfall on a steep slope with notoriously unstable geology undercut by decades of river erosion.
But whether you are a tree-hugging liberal or a chainsaw-wielding conservative, we should all agree that everyone in landslide country deserves access to the best available knowledge about where slopes are prone to fail in the future, how far they might travel, and how likely they are to occur. Local governments need to know these things to rationally decide whether and how to regulate land use (or not).
Did the people living on Steelhead Drive know it was such a dangerous place? Those who had lived there when the slope last failed in 2006 certainly knew the slope could fail and once again push the river toward them. Still, I doubt they imagined it could slide so catastrophically, or go so far.
I can’t imagine that Amanda Lennick knew any of this. She was the nurse who bought her house 11 days before the land slid. Hazard maps showed the steep slope across the river as the landslide zone, not the flat land where her dream house sat.
Back in 1999, geologist Daniel J. Miller warned of the potential for catastrophic failure of this slope. After the 2006 Oso landslide, he was shocked to find that the county continued to allow residential development on the flood plain right across the river from the foot of the slide.
Miller thinks this was crazy. So far every geologist I’ve talked to has agreed. But most have also said they were surprised at the long runout for this slide. So was I.
Yet the potential pops out in sharp relief in new topographic images made using laser technology known as lidar. Clues to the destructive capability of the Oso slide are evident in these images because they show how an ancient, larger slide ran out across and buried the valley bottom right next door to Steelhead Drive.
Gov. Jay Inslee is right to call for a review of landslide risk assessments. As part of that effort, we need to both assess the need for policy changes and address how to ensure that homeowners have access to up-to-date knowledge of geological hazards that might affect their property.
Without hazard assessments that realistically portray what landslides can do, we won’t really know. And new homeowners, like Amanda Lennick, might not find out until it is too late.
We’ve already learned one painful lesson. When mountains move, we need to better understand how far they’ll go. Given the unexpected length of the Oso slide, geologists need to re-evaluate the potential for such long runout slides using modern, available technology and expertise. It’s also clear that we need to better communicate knowledge of geologic hazards to the general public.
Finally, it’s worth stating what should appear obvious. We need to re-evaluate and revise landslide hazard-zone maps using new technologies, like lidar, to better identify potentially unstable ground — and include potential runout zones.
David R. Montgomery is the Dean’s Professor in Geomorphology at the University of Washington College of the Environment and author of “The Rocks Don’t Lie: A Geologist Investigates Noah’s Flood.”