1 February 2021
Rat Creek: a large washout generated by an atmospheric river in California
Last week a large atmospheric river rainfall event struck California. It dumped large amounts of rainfall on several of the large areas affected by the terrible 2020 wildfire season. As I have noted previously, heavy rainfall shortly after a wildfire is a recipe for damaging mudflows. This time was no exception.
The most dramatic event occurred at Rat Creek in the Big Sur, on State Highway 1 between San Francisco and Los Angeles. The best image that I have found of this event is this one:-
China News has also published a good overview of the washout:-
The San Francisco Chronicle has a really high quality pictorial explanation for the washout at Rat Creek. Upstream of the site of the washout is a large area burnt by the fire:-
The Dolan Fire burnt an area of about 52,000 hectares, starting on 18 August 2020. It burnt for many weeks; on 8 September 2020 15 firefighters were severely injured whilst fighting the fire. There is a high probability that the fire was started deliberately. As the map above shows, Rat Creek drains an area partially burnt by the fire. The images of the aftermath of the washout show large volumes of timber, which would be as expected for a post-wildfire mudflow.
Of course the amount of overland flow generated in wildfire areas means that the gully will have dealt with a larger than normal volume of flow, with a higher density, generating the unusual level of erosion. The road bed was on erodible material, possibly weathered rock and some fill?
The road will be closed to allow repairs to be completed. No estimate has yet been provided as to how long this will last.