11 August 2020
Landslides after wildfires
Posted by Dave Petley
Landslides after wildfires
It is well established that one of the lingering affects of wildfires is increased sensitivity of the landscape to landslides. The primary cause is considered to be the loss of vegetation, although the actual mechanisms through which this generates increased landsliding are complex. Notable examples include the 2018 Montecito mudslides in California, but similar events can be found in many places,
In a paper just published in the journal Landslides (Rengers et al. 2020), which is open access, the occurrence of landslides following wildfires in Southern California is investigated. The authors have looked at a 70 km² area of the San Gabriel Mountains in Southern California, which was burned in the 2009 Morris fire, the 2014 Colby fire and the two wildfires that are collectively known as the 2016 San Gabriel Complex fire. None of the fires burned all of the study area, but some parts were burned by more than one of the fires.
The authors observed debris flows in the first year after a fire in the area that had been burnt. The example below, the Van Tassel watershed, highlighted by Rengers et al. (2020), underwent extensive debris flow activity in early 2017 having been burnt the year before:-
But the pattern of landslides in time is quite complex. Whilst debris flow activity was indeed high in the first year after a fire, triggered by heavy seasonal rainfall, debris flow activity rapidly declined with time. Three years after a fire, the primary type of landsliding had transitioned to shallow slips, with failures being triggered on both unburnt and burnt slopes. The density of landslides was highest in areas that had been burnt three years previously, whilst areas that had been burnt five years beforehand had a low density of landslides, similar to areas had not been burnt.
Where a second fire affected a previously burnt area, the density of landslides did not increase. A really interesting aspect of the landslides is they mostly occurred on slopes facing towards the south. Rengers et al. (2020) suggest that this may be because slopes facing away from the sun (i.e. to the north) in this semi-arid area regenerated vegetation quickly, reducing the likelihood of landslides. This is an important finding.
The study by Rengers et al. (2020) shows that wildfires do indeed lead to increased susceptibility to landslides triggered by rainfall. However, the response is more complex than might have been anticipated both in terms of the types of slides triggered and their spatial patterns. In this case the affect is quite shortlived. It is important to stress that this effect will be strongly controlled by local factors such as the topography, the geology and the climate, so responses in other locations might differ. I hope that this study will inspire similar investigations in other environments.
Quickslide 1: an update on the Rajmala landslide
The number of bodies recovered from the Rajmala landslide in India has increased to 49. The Indian Express has a good article about the background to the landslide. It is now clear that this occurred late on 6 August rather than on 7 August, as had been reported previously.
Quickslide 2: Further problems at Rest and be Thankful
The diversion route for the A83 at Rest and Be Thankful was closed overnight due to concerns about another landslide. The main road remains closed.
Rengers, F.K., McGuire, L.A., Oakley, N.S. et al. 2020. Landslides after wildfire: initiation, magnitude, and mobility. Landslides (2020). https://doi.org/10.1007/s10346-020-01506-3