Landslide: A mountain in Oso, a hill at WSU
By Linda Weiford, WSU News
PULLMAN, Wash. – For all the people who pass by an innocuous-looking hill on the Washington State University campus each day, with a quick turn of the head they will see proof that one wet winter day 18 years ago, a mass of Palouse mud broke loose and flowed toward Grimes Way.
No one was threatened and no buildings were damaged when a wedge of the hillside let go in 1996, putting its impact a world apart from the devastation caused by the recent landslide in Oso, Wash. Even so, when it comes to the mechanics of what happened and why, the two slides share similar characteristics, according to a WSU geologist who researches these types of landslides.
“In both cases, an unstable slope containing fine-grained sediment became even less stable following a prolonged episode of high precipitation,” said geology professor David Gaylord of the WSU School of the Environment.
Excessive water decreased the cohesion that helps hold loose sedimentary particles together, he said, and, “Eventually, the force of gravity overcame the ability of the upper sediment to stay in place.”
In Oso, the slide produced a massive field of earth that carried trees, boulders and homes with it. At WSU, it was a tongue of mud that, in places, was four feet high. It flowed past the east campus power substation and stopped short of Grimes Way, Gaylord recalled, near the intersection of Olympia Way.
Like the Oso slide, it left a scarp, or gouge, on the slope where sediment was displaced.
Landslides – and there are different types – occur in all 50 states; the greatest numbers are in Washington, Oregon and northern California, according to the U.S. Geological Survey. Steep terrain, soil type and precipitation make the regions more prone, as do earthquakes, the agency states on its website.
While that combination of conditions affects western Washington more than the state’s eastern side, “slides do frequently happen here, but many tend to be relatively small and cause less damage,” said Gaylord.
The slide at WSU and the one in Oso evolved into debris and mudflows while moving downslope. It was a mudflow at WSU; a debris flow in Oso.
Debris flows contain more coarse material than mudflows, move quickly and can travel for miles, according to Washington state’s Department of Natural Resources. The debris avalanche that accompanied the 1980 Mount St. Helens eruption in the state evolved into an enormous debris flow.
As to how combined forces can trigger the collapse of a mountainside in Oso and yet a small hill at WSU: “It doesn’t take high topography for slides of any kind to occur,” said Gaylord.
David Gaylord, WSU geology professor, 509-335-8127, firstname.lastname@example.org
Linda Weiford, WSU News, 509-335-7209, email@example.com