What Creates a Deadly Mudslide?

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Was it a landslide, landslip, mudslide or mud flow, and was there any way to prevent the catastrophe that killed at least 14 people on Saturday along the Stilliguamish River in Washington State?

"This will be the worst landslide in the U.S.A. for many years," predicted David Petley, a landslide researcher at Durham University in the U.K. and author of The Landslide Blog. By worst he means in terms of deaths, not the size of the landslide. "The last event on a similar scale of which I am aware was the 25th December, 2003, debris flow in San Bernadino County, California, which killed 16 people. It looks likely that this landslide will be worse."

Petley also calls the disaster a "landslip." So which is it? Both, actually.

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Landslide and landslip are both terms used to cover a lot of different kinds of movements in the land, with landslide being the most popular word. But the movements encompass everything from a slump (a slow sagging of a hillside that can be measured on a calendar) to a rock fall (where chunks of rock can reach terminal velocity).

The term mudslide works too, except that the mud describes the bottom, deadly part more than the upper part of the event.

In the Stilliguamish case, the hillside is made of loose sandy material laid down by glacial lakes less than 16,000 years ago, according to Washington State geologist Dan McShane of Stratum Group who writes a blog called Reading the Washington Landscape. Since then the Stilliguamish River has cut down into that loose material, creating hillsides of the sandy material that have a history of collapsing.

Those hillsides were apparently saturated with water when the one collapsed on Saturday. As anyone knows who has tried to build a sand castle, too much water causes the sand to flow and not stand up. In microscopic terms, it's the grain-to-grain friction that holds sandy slopes together. When water fills the spaces between the grains and pushes the grains apart, the sand, and in this case an entire hillside, collapses.

The upper part of the Stilliguamish/Oso landslide appears to have slumped, and rotated -- like a person sliding down off a chair. The lower portions were even wetter and apparently behaved more like liquid -- a mudflow.

In fact, there's evidence that the landslide released a great deal of water at its base as it occurred. The National Weather Service tweeted an image of data from a nearby river gauge on the Stilliguamish River showing a spike in the water level at the time of the landslide, followed by a dramatic drop in the river. The latter was caused by the landslide damming the river.

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"This landslide area is well known and has been a big problem for the past roughly 20 years," explained McShane. "I looked at this slide area last summer. It was clearly bad news then and with very heavy multiple rain events the past six weeks landslides -- particularly big ones -- are not a surprise."

That history prompted Petley to wonder, in an email to Discovery News, whether the deaths from this landslide could have been avoided.

"Whilst I am referring to this as the Oso landslip, in fact it is a reactivation of an existing landslide, known as the Hazel Landslide," Petley reported in his blog. "This landslide is known to have moved in 1988, and went through a second phase of movement in 2006. It is well described in a blog post from 2009."

All of which suggests that there is a dangerous gap between the science and the way the land is being used in the Stilliguamish Valley, and perhaps elsewhere.

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