Tuesday, March 25, 2014

The Oso, Washington Landslide

Monday, March 24, 2014
1:54 a.m.

Retrieved from WSDOT Flickr page

Sorry about the delay in postings. End-of-quarter finals unfortunately usually take higher priority over weather posts. This isn't always the case during the end of fall quarter, as we are in the thick of the storm season and lowland snow is a very real possibility, but in the middle of March, there's usually not too much in the way of meteorological mayhem happening. Our storm season has passed, and the threat of a major lowland snow storm is long gone. I'm afraid you'll just have to wait until November.

The weather's been pretty nice since I got out of finals. But it was rainy as hell earlier in the month, as I'm sure you are all aware. November, December, and January posted a paltry 9.15 inches of rain at Sea-Tac, whereas March has put on 7.71, and we still have a week to go! We could very well pick the 1.44 inches we need to tie the sum of these three months before March is up, and if that happens, that'll have to set some sort of record. When you factor this on top of a February that was also much wetter than average, you begin to really see how delayed our rainy season has been.

All of this rain has saturated the soils, and unfortunately, this saturated soil contributed to a massive landslide on the Stillaguamish River near SR 530. Outside of the Mt. St. Helens landslide (which was not due to soil saturation), this is the largest landslide I can recall ever seeing in the state of Washington. Take a look at the photos below... the size of this thing is colossal.

Aerial photo of the Oso mudslide taken by the Washington State Patrol on 3/23/2014. Retrieved from  http://en.wikipedia.org/wiki/2014_Oso_mudslide

Dave Norman, a state geologist, said that this is "one of the largest landslides he's seen." After surveying, the slide was found to be 4,400 feet wide, 1,500 feet long, and 600 feet tall, with a 30-40 feet debris field. In addition to wiping out a neighborhood of 30 homes and blocking Highway 530, the slide dammed the North Fork of the Stillaguamish River. There were serious concerns about the dam suddenly collapsing and unleashing biblical-like floods downstream, but the river eventually gradually poked through the dam without having any catastrophic collapse take place.

Take a look at the hydrograph below for the Stillaguamish River, and look how the flow increases sharply right after the slide, then decreases. I would think that this increase would be due to the material in the slide pushing a bunch of water downstream as the slide surged down the hill. Of course, once the river was dammed up, the flow decreased dramatically.

As I mentioned earlier, this dam finally gradually let some water through. The following pictures are from the Washington State Department of Transportation's Flickr page and were taken on Sunday the 23rd.

In the midst of the horror of this event, we can at least take solace in the fact that we did not have any flash flooding associated with a catastrophic dam failure, which would have certainly caused more death and destruction, especially if it had occurred at night when everybody was sleeping. 

By Monday, the dam was continuing to erode away and flows downstream were continuing to increase. The photo below is from the Associated Press (bad, I know...), but it was such a great picture that I had to show it here.

So why did this slide occur in the first place? Well, the saturated soil certainly contributed to the slide. The North Cascades have been the wettest place in the state over the last month, with some locations picking up as much as 30 inches of rain. That's pretty insane for March. But the thing is, when it comes to sheer precipitation amounts, 30 inches of rain in one month isn't all that unusual for some locales. I remember in November 2006 when Mt. Rainier National Park got 18 inches of rain in 36 hours. THAT, my friends, is extraordinary. 30 inches of rain in three weeks is wet, but not unprecedented. This hill has survived much wetter spells, so why did it decide to give during this one?

It turns out that this hill has actually had quite a history of instability. In fact, according to a recent Seattle Times article, it is known by some simply as "Slide Hill." The article mentions four major slides (1949, 1951, 1967, two slides... one in 1949, and one 1951 . The 1949 slide was 1000 feet long, 70 feet tall, and affected a 2,600-foot stretch of the Stillaguamish, and the 1951 slide formed a small dam, giving rise to creeks in the area known as "Slide Creek" and "Mud Flow Creek."

The main cause of these slides was found to NOT be soil saturation, but to be that the river is constantly eroding away at the hill. The more it erodes the hill, the less support there is at the bottom to hold the rest of the hill up, and you get a large landslide. Saturated soil no doubt speeds up the process, and it may have been the "straw that breaks the camel's back," but this slide was not due to soil saturation alone.

In 1999, the Army Corps of Engineers issued a report warning of "the potential for a large catastrophic failure" on Slide Hill. Risk mitigation efforts began, but these were delayed when a 2006 slide moved the course of the river 730 feet. During the summer of 2006, a 1,300-foot wall of boom logs and concrete was created to prevent the river from eroding, and although this was to protect fish from sediment erosion, it also acted to stabilize the hill and hopefully lower the risk of any landslides happening. Of course, it was no match for Saturday's disaster.

Here are some more pictures of the disaster from WSDOT's Flickr site.

Below is one of the most powerful and terrifying pictures of the disaster. Comparing the slide with the size of the trees really gives you an idea of its immensity.

This is one of the most powerful and terrifying pictures of the disaster. Comparing the slide with the size of the trees really gives you an idea of its immensity. 

Here, you can see how SR 530 is completely submerged under a pile of mud, and you can see a few lucky homes that just escaped the slide.

These fallen trees bear a resemblance to those that were felled due to the pyroclastic flows from the Mt. St. Helens eruption of 1980.

Finally, the picture below was not retrieved from the WSDOT, but from Twitter. I forget who specifically I got it from. It shows Oso before and after the slide, and it is a haunting photo.

There were 14 fatalities and 176 people unaccounted for last time I checked. I can only hope the latter goes down and the former doesn't go up. If you want to help, you can donate money to the Red Cross specifically for the Oso slide (they have enough food and clothes and don't need any more) by calling 1-800 RED-CROSS or visiting redcross.org. A Disaster Relief Account for the victims has been set up at Union Bank in Arlington (525 N Olympic Ave), and you can send checks addressed to the Cascade Valley Hospital Health Foundation or donate online at http://www.youcaring.com/nonprofits/cascade-valley-hospital-foundation-disaster-relief-fund/154422.

The biggest thing you can do to help right now outside of donating a million bucks is not going to the site of the slide. These people are working their tails off to find anybody who might be alive, and we don't want to interfere with them and put anything else on their plate.


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