Eastern Washington Landslide Event Spring 2017 – Part 2

January 6, 2019

The spring of 2017 was in many ways a rare event for Eastern Washington.  Yet, it provided us with crucial data that helps us to understand our landscape and gain a better understanding on when and where we might expect failures.  One of the places that we do expect landslides is in burned areas.  Wildland fires can create ideal conditions for landslides to occur.  In a general sense, the removal of vegetation creates a scenario where water can easily concentrate on the landscape.  Large storms, such as thunderstorms can overwhelm the ground, depending on the intensity of the burn and soil, the availability for the water to soak into the ground can be very limiting.  The opposite can occur as well if the water can infiltrate into the ground, as the removal of vegetation can increase groundwater, as there is a drastic reduction of evapo-transpiration from trees and other vegetation.   Sometimes we will see a combination of both of these within a fire area as well.

In 2015, Washington witnessed on of its most severe fire seasons, with large fire complexes burning across Central and Eastern Washington.  Many of these fires were thousands of acres, with intense burning.  Many areas experienced what is referred to as a stand replacement fire, which is when the fire burns with such intensity (often experiencing widespread crown/canopy fire), that a species of trees over a wide area is wiped out.  If left to regrow naturally, a different species can become dominate within those areas.

One of the fire areas that experienced a significant number of landslides was the Carpenter Road Fire, which is between Springdale and Hunters.  The fire burned approximately 63,972 acres, give or take a few acres and varied in intensity.  However, a large portion of the area did experience high intensity crown fires, which caused a high mortality of tree survival within the area.  It was one of the areas that was observed for instability and through the winter of 2015 to 2016, it seemed that it had remained relatively stable.  However, in the spring of 2017, numerous landslides began to initiate throughout the fire area.  See the map below.

carpenter road fire landslides

In the field, most of these landslides occurred along road, commonly along road cuts.  During the observations, it was apparent most of these areas had elevated groundwater that helped to initialize the movement of the landslide.

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That makes sense, considering so much additional water was put into the system, as well as it was allowed to infiltrate into the ground through snow melt.  There is more to the story.  Lidar is relatively new for Eastern Washington, although there have been small pockets of it, overall coverage of the area has only been recent.  When looking at the lidar for the Carpenter Road Fire Area, we can see that there are pockets of areas where deep-seated landslides exist.  Additionally, areas where we observed landslides in the fire area correlate to a lot of these areas where deep-seated landslides are located.  That makes sense, as deep-seated landslides are like the sponges of the landscape (generally, not always).  In an event where higher levels of water is available in the landscape, deep-seated landslides potentially, have a higher chance of reactivating, or as we see in many areas in the Carpenter Road Fire Area, initiation of shallow landslides along the toes or areas where roads intercept the groundwater in the area.

img_0132

This is a good learning experience for fires, as the interaction with deep-seated landslides (and their reactivation) and/or initiation of shallow landslides within them can help us understand where we could expect instability in the future…

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