Scientists Say Smaller 2006 Landslide Set The Stage For Oso Disaster

Jul 22, 2014

A small landslide in 2006 set the stage for the catastrophe that claimed 43 lives in Oso, Washington this past March, say a panel of scientists in a federally-funded study.

The hills above the North Fork of the Stillaguamish River had slid before, at least 15 times over the centuries, according to the study.

But one slide in particular left Oso vulnerable. In 2006, that smaller slide left a loosely-packed mass of debris perched dangerously above the Steelhead Haven development and its neighbors.

Joseph Wartman, an associate professor of civil and environmental engineering at the University of Washington, is the lead researcher on the study. He says last spring, rains saturated the unstable ground and let loose a huge slab. Within seconds, the earth liquefied, sending a torrent more than half a mile across the valley floor.

The length of its path of destruction turned out to be within the normal range for this kind of slide, Wartman says, but it still seems shocking.

“You know, when we were at the site and you take a look at it, even as someone who’s a trained professional in this area, it’s difficult to imagine that run-out could come that far, when you look up to the slope,” Wartman said. 

Minutes after that slide, a second, higher mass of earth slipped, filling the bald spot left by the first.

The study noted widespread timber harvests in the immediate area of the slide, but didn’t analyze exactly what role forest practices might have played. Researchers say they wouldn’t expect it to be a major factor, though, and note that landslides are evident in the valley for millennia before people started cutting down trees.

In all, researchers say the slide moved 270 million cubic feet of earth, and was the deadliest in American history.

Just as the team was delivering its findings, word came that the landslide’s final missing victim appears to have been recovered. Search crews believe they found the body of 44-year-old Molly Kristine Regelbrugge.