8 October 2020

Post wildfire mudslide potential in the Pacific West of the United States

Posted by Dave Petley

Post wildfire mudslide potential in the Pacific West of the United States

It is well-documented that 2020 has seen exceptional wildfire activity in California, spurred by drought, exceptional thunderstorm activity and, at least in the eyes of some, the legacy of management practices in forest areas.  As I type the August Complex wildfire is being named the first gigafire, a fire that has burnt over 1 million acres (about 407,200 hectares as of 6 October 2020).  This was started as 38 separate fires triggered by lightning in the middle of August.  They have now morphed to form a huge, complex wildfire.

A legacy of these massive wildfires is the potential for mudslides and mudflows triggered by heavy rainfall in the forthcoming winter.  There is good scientific evidence of this process, and such post wildfire mudflows have had a devastating impact in the past.

Loyal reader Ken has very kindly created a mosaic of eight recent Sentinel satellite infrared, atmospheric penetration images covering Washington, Oregon and California. The images are dated 29 September 2020, so do not capture the full impact of the August Complex wildfire.  In this image, the areas burnt by the fires are rusty red-brown, healthy forest is blue, grasslands yellow, urban areas grey:-

Wildfire burnt areas in the Pacific West

A mosaic of eight recent Sentinel satellite infrared, atmospheric penetration images covering Washington, Oregon and California. Image dated 29 September 2020, created by loyal reader Ken.

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This is of course a beautiful image, but the scale of the rusty red-brown (wildfire) areas is enormous, both in terms of the number of fire-burnt areas and in the size of many of them.

A month ago, the NOAA Climate Prediction Centre indicated that La Nina conditions are likely to prevail through winter 2020-2021:

A majority of the models in the IRI/CPC plume predict the continuation of La Nina (Niño-3.4 index less than -0.5°C) through the Northern Hemisphere winter 2020-21. The forecaster consensus supports that view, and favors a borderline moderate event (Niño-3.4 index near -1.0°C) during the peak November-January season. In summary, La Niña conditions are present and are likely to continue through the Northern Hemisphere winter (~75% chance).

La Nina conditions are quite variable, but in general they lead to drier than normal conditions in the southern part of this area, and wetter than normal in the northern part.  So, in parts of this area, post wildfire mudslide potential is high.

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Quickslide 1: The location of the Eyjafordur landslide in Iceland

In the comments to my post yesterday, loyal reader Fabien has successfully located the landslide at Eyjafordur.  This is at 65.404°, -18.251°. I note incidentally that the problem with comments not displaying is back.  I don’t know why the WordPress software does this.

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Quickslide 2: A 1.2 billion years old rockfall in Western Scotland

National Geographic has a nice article, based on a paper in the journal Geology, on the geological evidence for a 250,000 tonne rockfall that occurred 1.2 billion years ago at Clachtoll in northwest Scotland.