Stillaguamish Landslide- Geologic Perspectives

Interpretation of landslide scarp on LiDAR image, by Dan McShane.

Interpretation of landslide scarp on LiDAR image, by Dan McShane. Click to enlarge.

Dan McShane has written some geologic perspectives about Saturday’s landslide into the Stillaguamish River. Dan is a consulting geologist based in Bellingham and author of ‘Washington Landscapes’ blog, and has some great insights into the geology and history of the slide area. Rather than trying to rewrite his excellent reports, I’ll just provide the links. It is not the first landslide in this location. In my view, it  is a tragedy that people are permitted to live in this location.

 

Dan’s Initial report:

http://washingtonlandscape.blogspot.com/2014/03/arm-waving-notes-on-stilliguamish.html

Geologic background:

http://washingtonlandscape.blogspot.com/2014/03/geology-of-silliguamish-blocking-slide.html

LiDAR images and slide history.

http://washingtonlandscape.blogspot.com/2014/03/aerial-history-and-lidar-of.html

Stay tuned for more posts from Dan. Consider subscribing to his blog.

 

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10 Responses

  1. Don’t worry about them Tuck—they will rebuild as soon as the mud dries and they find all the bodies.

    *Never underestimate the power of human stupidity Robert Heinlein* *or * “Never underestimate the power of stupid people in large groups.” — George Carlin

  2. I am concerned about comments like the one Mulevalley made …. sadly some might rebuild. But what about others living in the Stillaguamish River valley? One of my first thoughts was implementation of a warning system similar to tsunami networks at the beach. But, then, the realization these folks had no short-term warning. Is that always the case, or are there warning signs? And importantly…Did they know they were living in a well-documented slide area? I pose these questions and suggest that a longterm public education campaign could be indicated here. Certainly building permits in known slide areas should never be issued in the first place.

    • Dear JANE,
      Thanks for your comment. I don’t think any sort of warning system could be implemented in a case like this. There are many slopes in the western valleys of the Cascades that could fail, not to mention along the coast of the Salish Sea. The only relevant ‘warning sign’ would be to realize that landslides may be more likely after periods of heavy rain. But, those weather conditions are more common than large landslides. I don’t know if the people were aware of the landslide potential. As Dan McShane noted in one of the links I posted, there have been other recent slides in the immediate area: In 2006, for instance, a slide very nearby {The Hazel Slide] briefly dammed the Stilly but caused only minor damage and no deaths or even injuries so far as I know. Earlier slides in 1949 and a smaller one in 1951 also caused much less damage and no fatalities. As to building permits, different counties have different ordinances, and awareness of these rare landslides is a fairly recent phenomenon. I doubt any county would be able or be willing to evict people once they own the property.

    • Thank you for the thoughtful reply. I guess the bottom line is, we choose to live in the Northwest for its beauty. We are in a very geologically active area. Understanding the dynamic forces around us and the risks they potentially can cause is perhaps a realistic form of “preparedness.”,

      Good to know geologists are continually collecting and assessing data. Keeping the public informed is important.

  3. I heard someone say on the news today that there were yellow flags in this situation but not red flags. What do you make of that?

    Here are some ways to help the Oso community: http://wildninjablog.com/2014/03/25/oso-aid/.

    • The presence of unstable glacial deposits + heavy rainfall + river flowing at the bottom of the slope + long history of earlier slides + houses at the bottom = a pretty distinct red flag to a geologist. Meaning: This is not a good place to live.
      Dave

  4. The Seattle Times has an informative article in its March 25, 2014 issue detailing the history of the site where the deadly March 22 landslide occurred. To briefly summarize, the area’s mass movement potential was well-documented, complete with a history of several recent slides. According to geologist Daniel Miller, “We’ve known that it’s [the hillside] been failing. It’s not unknown that this hazard exists.” This is in stark contrast to what John Pennington, head of Snohomish County’s Department of Emergency Management, is quoted as saying on March 24: “It was considered very safe. This was a completely unforeseen slide. This came out of nowhere.”
    Acknowledging that Mr. Pennington’s remarks may have been taken out of context, and with due respect for his excellent leadership in the wake of this disaster, I strongly disagree! The factors are clearly recognizable and were known. The exact timing and size of the next slide was the only part that was not known.
    Re-read Dave Tucker’s excellent concise summary of factors (above) that made this a red flag area to anyone who considers both probability and consequences of an event (which is how risk is determined, by the way).

    • Here are some excellent aerial images of the slide:
      http://mashable.com/2014/03/25/washington-mudslide-photos/

      Also here is the link to the article in the Seattle Times, including a four minute video with geomorphologist Dan Miller, who had extensively studied the geology of the area several decades ago and concluded that the combination of pertinent factors result in a potential dangerous situation.
      http://seattletimes.com/html/localnews/2023218573_mudslidewarningsxml.html

      Doug

    • In looking at references for the Wikipedia article updated on 4/27 on the Stillaguamish River slide, I came upon this interesting website that contains topo maps and some aerial photos of the landslide site. One especially interesting….and alarming….image is one taken in 1933, where you can see a “whitish” curved zone in the vicinity of the recent slide. It isn’t clear to me whether the scarp (the slightly curved gash, whitish because it is free of vegetation) in the middle of the image is the head scarp from a previous slide, or the beginnings of a slide-to-come. But as geologists have known all along, this material in this setting is slide prone, especially when being undercut by a river cutbank and when lubricated and saturated in groundwater and soil moisture. Note that you can’t see any homes on the inside of the large meander. Why oh why were homes allowed to be built in the area that became Steelhead Drive?
      http://www.historicaerials.com/aerials.php?scale=6&lon=-121.85849656687999&lat=48.283683958846765&year=2006

  5. Holy smokes Dave, I had a close but non-calamitous personal reminder on Saturday March 22 that this geology stuff is dangerous. I was on my way to Ross lake for a hike, and we were just minutes past the Stilly landslide impact zone when it hit. I posted some details on NWhikers:

    http://www.nwhikers.net/forums/viewtopic.php?t=8009896&sid=e36d9baa3b6ebd0935cb743073050be0

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