28 May 2014

Grand Mesa: an unusually large and mobile mudslide

Posted by dr-dave

The Grand Mesa mudslide

Mesa County in Colorado, USA suffered an unusually large and mobile mudslide this weekend, which appears to have resulted in the deaths of three men.  The landslide is surprising in many ways, not least the scale and the apparent mobility of the mass.  The Denver Post has a great set of images of the landslide, including this one:

Grand Mesa

Aaron Ontiveroz/The Denver Post

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The scale of the landslide can best be appreciated from this video, taken by the Mesa County Sheriff’s Office:

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The landslide is reportedly half a mile (900 m) wide and between 2 and 3 miles (3.5 – 5.1 km) long. The victims appear to have been investigating reports of a blocked irrigation canal, presumably from an earlier movement event.  The mobility of the landslide does seem to be unusual – note how the slide has overtopped a ridge on the right side (as seen in the photograph above).  The elevation reached by the mass on the upslope side of this ridge is quite surprising.

The landslide mass has some fascinating flow structures that with interpretation will tell us a great deal about the movement of the landslide.  For example, this Denver Post image of the landslide shows the structures in the toe area.  To me, this suggests that the landslide should really be classified as a mudflow rather than a mudslide, or in fact technically as a complex landslide consisting of a rotational landslide that transitioned into a mudflow:

Grand Mesa

Aaron Ontiveroz/The Denver Post

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This is the Google Earth perspective view of the site before the collapse (this image is dated 2011):

Grand Mesa..

The upper reaches of the landslide clearly consist of an old rotational landslide block – this is clearer on a zoomed in image:

Grand Mesa..

Compare this with the Denver Post photograph of the same area after the recent failure:

Grand Mesa

Aaron Ontiveroz/The Denver Post

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It is quite hard to interpret this, and my initial view (that there was a new rotated block) is not the case I think. My reading is that the forested rear scarp of the old landslide remains intact.  The rotated block appears to have undergone a second rotational failure, possibly on the same failure plane?  A part of this rotated block has remained intact, but most has disintegrated to form the mudflow. Rotated blocks do degrade and form mudflows – this is the case in many such slides.  But this catastrophic disintegration to form a high mobility mudslide looks really interesting, so will merit a detailed investigation.  Perhaps the whole mass underwent a rotational failure, which caused the lower part of the block to disintegrate and form the flow.

Given that the reported rainfall prior to the failure was nothing special, something quite unusual seems to have happened here.

Unfortunately, the site looks to be quite dangerous, especially in the next heavy rainfall.  I wonder also what the platform at the toe of the slide is for (see the second image above)?  There are several similar structures in the area around the landslide.