Major landslide in the state of Washington, USA: UPDATE(S)

Oso, Washington, is at the marker. Arlington is to
the left, Darlington is to the right by the magnifier bar.
The slide at Oso cut off Highway 530 between
Arlington an Darlington.

**Chapter 4 in my book "The Dynamics of Disaster" is titled "The Flying Carpet of Elm" and discusses the factors that influence landslides. It also discusses the mother of all landslides, a slide that occurred 50 million years ago in Wyoming, covering 1300 square miles and traveling more than 30 miles.

Monday a.m. Update and correction: The Seattle Times reports "108 reports of missing people." CNN.COM has reported "Washington landslide: 8 dead, 108 missing." Emergency managers are saying that they have a list of those reported missing but that it does not mean all of them were killed. I thank a Washington reader for pointing out this difference.

Cliff Mass has a post on March 24 that describes the meteorological conditions leading up to the landslide.

The SeattleTimes is providing excellent coverage.

Take I-5 north from Seattle about 50 miles through Everett toward Arlington, and turn east onto Highway 530, which takes you south of Mount Baker. Along this road is the small town of Oso, population about 200. At about 11:00 this morning, a massive slide of mud, rocks and trees travelled a mile down near Oso, taking down at least 6 homes, killing at least three people and trapping others. (As of Monday morning, 18 are still missing.) Three more are reported in critical condition.The slide was at the 29400 block of SR 530 near milepost 37, between the cities of Arlington and Darrington. It landed in the path of the Stillaguamish River, reducing its level at one spot from about 3.1 feet to 0.9 feet, indicating that the slide appreciably blocked the river. The state hydrologist reported that 15-20 feet of debris blocked the river, and that its flow had been reduced to about 1,000 cubic feet per second. Other reports have said that the slide is 135 feet and 180 feet deep.

Image of the landslide from Seattle Pi

     In situations like this where an earth slide blocks a river, the concern is that water will pond behind the blockage forming a lake, and that the blockage--a dam--will suddenly collapse and release the water catastrophically. The National Weather Service has said that for this to occur the water would have to be blocked for 36 hours, and then released within an hours time.
     Snohomish county officials have advised residents downstream of the slide to evacuate their homes as "It is going to break loose and the question is how and where" (John Pennington with the Department of Emergency Management (quoted from King5 news here).
     The National Weather Service has issued a flash flood watch through Snohomish County through Sunday afternoon. The ground is saturated with water from recent rains, flash flooding is possible, and the saturated ground combined with rain is believed to be the cause of the Oso mudslide. Flood alerts have been issued both upstream and downstream of the slide.

Image showing the abundance of water at the base of the slide.
From Seattle Pi

     UPDATE: Several serious hazards remain in this area. The first is the landslide material itself. It is composed of a lot of fine grained materials, sand and clay, that form a nasty, hazardous substance called "quick sand" or, depending on particle size, "quick clay." Quick clay was the cause of the landslide from which the title of my book chapter, The Flying Carpet of Elm, was taken. I'm not sure of the exact geology of the Oso area, but it looks like the materials are a watery mixture of very fine particles. These materials, if undisturbed, can be very strong, forming the slopes on which homes and other buildings were built in the area. But, when disturbed, in an instant under certain conditions of stress, the state of the materials changes to liquid. Although some of the water visible in the images (to the right) may be from the dammed river, it looks like a lot of the water came from within the landslide itself.
     The second major hazard arises from the partially blocked river. These landslide dams are not strong and eventually, sooner or later, the water will either erode through the toe of the dam (the best outcome) or the dam will break (worst outcome). Until equilibrium is restored, downstream residents and infrastructure are at risk. Apparently a major bridge on Highway I-5 is being watched carefully because the pilings holding it up are old and not as deep as would be built under newer bridges.

Here are more references from Dave Petley's AGU Landslide Blog.