A deadly landslide that killed 44 and obliterated a riverside neighborhood in Washington state last March was fueled by rain soaking the site of an eight-year-old landslide, while logging in the area may have also played a role, according to a scientific report released Tuesday.
The first published scientific account of the mudslide, the report doesn't offer a definitive explanation but describes a devastating chain reaction sparked by rain.
The report comes from a team of university and private-sector researchers who are part of the Geotechnical Extreme Events Reconnaissance, a National Science Foundation-funded initiative to quickly dispatch scientists to evaluate natural disasters.
The slide plowed through part of the little mountain town of Oso, northwest of Seattle, shortly after 10:30 a.m. on Saturday, March 22. It demolished sections of State Highway 530, which remains closed today. The official death count is 43 because one body remains missing in the debris field.
The hillside that collapsed had been the scene of multiple smaller slides since the first housing development was built there in the 1960s. Government agencies repeatedly attempted to buttress the hillside, though engineers warned it might do little good.
Still, the scientists were shocked at the speed of the slide and the distance it traveled—more than half a mile (over a kilometer). The report found that a 2006 landslide likely set the stage for the much bigger slide, creating a loose bed of rocks and soil that soaked up water more readily and was more prone to failing.
The recent slide was really two interconnected events, the report said. First, the older landslide liquefied, a phenomenon in which solid earth becomes a liquid as the friction holding together the particles suddenly decreases. That sent a mass of mud and debris shooting across the valley, removing support for a large chunk of the mountain, which fell away and added to the slide.
In the tragedy's aftermath, suspicion fell on recent logging on the mountain just above the slide and erosion along the Stillaguamish River, which flows along the base of the slide. Heavy rains in the weeks leading up to the disaster had soaked the region, turning slide-prone areas into loaded guns. The question had been about just what pulled the trigger.
The new report suggests there was no single cause. "Our work did not conclude that only one path could have led to the Stage 1 failure," it says, referring to the first stage of the slide.
While the report doesn't definitively point a finger at the timber industry, it suggests that logging above the slide area might have changed the way rain soaked into the hillside, adding more water to the unstable slope.
"It is possible that in 2014, the location, size and maturity of growth in these [logging] tracts was such that groundwater discharge to the slope was greater in 2014," the report states.
The study could become fodder for legal claims already filed by survivors and families of some of the victims against Snohomish County and the state of Washington. Those focus on the state's role in regulating logging near the slide area, and the county's part in permitting homes to be built there.
The county recently imposed a six-month ban on construction in the area.
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