What happened in Eastern Washington in Spring of 2017?!?  For those of you living out here, you understand that the higher than average snow with prolonged, higher than average rainfall in Spring caused widespread flooding, closing highways and causing many roads within NE region to become mud bogs instead of passable gravel roads.  Yet, there is a story here and a greater appreciation to the dynamics of Eastern Washington, when it comes to landslides.  This is a short presentation I did to summarize the event:

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The Spring of 2017 in Eastern Washington was anything but normal.  Snow came early to NE Region, but more than that, cold temperatures prior to the snow falling caused snow to fall onto frozen ground.  There is an interesting dynamic out here when it comes to landslides, how the ground freezes can play a role in how landslides fail.

Slide2-1Where to start.  First, there was a concerted effort to document this landslide event.  This wasn’t an easy task; as roads were failing around the area (there are a lot of gravel roads out here and as they became saturated, vehicles would break through the gravel road prism, potentially stranding the vehicle).  The second difficulty, there was an enormous amount of erosion, so discerning landslides from erosion became a difficult process (pictures to follow to better explain this).  This map (above) represents events that could be separated out from erosion events.  The landslides were about even in separation on land-use, about half were in areas of managed timber lands, the other, were urban areas (roads, residential type areas).

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For those who have spent any time in Eastern Washington, one of the most well known aspects of it, it is dry, much dryer than Western Washington.  When we think of landslides and regional areas where we have landslides, this can make a big difference.  In Western Washington, landslides typically fail when large storms come through (sometimes associated with lowland snow – or a rain on snow event, so I am thinking of December 1st to 3rd, 2007 or January 6th to 9th, 2009) or through long-term duration of rainfall (Westside winter of 2005-2006).  In Eastern Washington, it is rare, except perhaps in summer thunderstorms, to have any one rainstorm event to trigger landslides.  What perhaps makes sense, if not any one storm causes landslides, as there is just not enough precipitation to drive the antecedent moisture up high enough over time, we need to have long-duration, or concentrated water in others ways.  So, the winter of 2016-2017, higher than average snow, followed by higher than average spring rains, brought about the equivalent of long-term duration of precipitation needed to trigger landslides.

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This is a perfect chart to show just that, higher than average snow fall, followed by higher than average rainfall.  Or, to really drive it home:

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That wasn’t even the end of the event and for Eastern Washington, there was additional moisture after that as well.  Yet, what made this interesting are these maps showing soil moisture:

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We can see between the two maps, a progression of soil moisture migrating westward (in Eastern Washington).  I never thought to review soil moisture maps for landslides (on the westside of Washington, we have a calculation for antecedent moisture and additional rainfall, so it was easier that way, but that same model doesn’t work well in Eastern Washington).  When we look at these maps, we can see a correlation between landslides initiations and the progression and movement of soil moisture.  What does that mean, here is what came out of the timing (as an observation):

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It was touched upon earlier that frozen ground can play a role in landslide initiation.  Coming from Western Washington, I had never thought about frozen ground playing much of a role in landslide initiation (it is rarely seen in that area), yet here in NE Washington, it can be common during spring thaw.  Here is a simplified mechanic for how these landslides would occur.  We have melting snow, which increases groundwater.  As the groundwater starts to flow, it becomes impounded on a roadcut or sidecast that is frozen, resulting in increased pore-water pressure and eventually, causing the frozen material combined with the saturated material to fail and move.  Here are some examples where we can see that:

 

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Not a great picture I know, but we have a few more pictures of similar events, with similar scenarios:

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In each of these events, the landslide material was identified as frozen blocks of material on top of a slurry of wet, flowing material.

This was the beginning of the event, but as we progressed, landslides became more ‘typical’ on how we think of them, increase groundwater causing increase pore water pressure, often perching on bedrock or some sort of substrate that water can perch onto.

More on that next time (and onto the burned areas in 2015).

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Eastern Washington Landslides

December 22, 2018

*Brushing off some dust*

Welcome back!  Okay, I know it has been a long time since I posted, that adventure is a story within itself.  At this point, I am still working on landslides, but I have moved around a lot within that time and am now stationed in Eastern Washington and have been making my home and studying the area around here.

One of the things that I have been finding, news about landslides in Eastern Washington rarely reaches outside of the area and most people don’t realize that there are even landslides out here.

So, it seems about time that some pictures and data starts coming about with the landslides that occur out here.  Above are landslides that came out in Spring of 2017, which was a major landslide event out here (more to follow on that).