Landslides after wildfire: initiation, magnitude, and mobility

Abstract

In the semiarid Southwestern USA, wildfires are commonly followed by runoff-generated debris flows because wildfires remove vegetation and ground cover, which reduces soil infiltration capacity and increases soil erodibility. At a study site in Southern California, we initially observed runoff-generated debris flows in the first year following fire. However, at the same site three years after the fire, the mass-wasting response to a long-duration rainstorm with high rainfall intensity peaks was shallow landsliding rather than runoff-generated debris flows. Moreover, the same storm caused landslides on unburned hillslopes as well as on slopes burned 5 years prior to the storm and areas burned by successive wildfires, 10 years and 3 years before the rainstorm. The landslide density was the highest on the hillslopes that had burned 3 years beforehand, and the hillslopes burned 5 years prior to the storm had low landslide densities, similar to unburned areas. We also found that reburning (i.e., two wildfires within the past 10 years) had little influence on landslide density. Our results indicate that landscape susceptibility to shallow landslides might return to that of unburned conditions after as little as 5 years of vegetation recovery. Moreover, most of the landslide activity was on steep, equatorial-facing slopes that receive higher solar radiation and had slower rates of vegetation regrowth, which further implicates vegetation as a controlling factor on post-fire landslide susceptibility. Finally, the total volume of sediment mobilized by the year 3 landslides was much smaller than the year 1 runoff-generated debris flows, and the landslides were orders of magnitude less mobile than the runoff-generated debris flows.

A data-driven evaluation of post-fire landslide susceptibility

“…This corresponds to the evidence from the bootstrap analysis suggesting that dried out soil or slow vegetation regrowth may be an important part of the post-wildfire landslide mechanism in this region. Vegetation regrowth as a main control of landslide susceptibility is supported by a study of landslide occurrence in the San Gabriel mountains of the US by Rengers et al (2020), in which the authors found that hillslopes with slower vegetation regrowth were more likely to have landslides.…”

Extreme Precipitation Across Adjacent Burned and Unburned Watersheds Reveals Impacts of Low Severity Wildfire on Debris‐Flow Processes

“…Root strength is generally hypothesized to be at a minimum at some intermediate time period following disturbance, once the roots of dead vegetation have decayed and before roots of new growth are well established (Rice, 1982;Schmidt et al, 2001). Rengers et al (2020), for example, documented a landslide density of 11.8 landslides/km 2 due to a rainstorm that impacted the 2016 San Gabriel Complex burn scar after 3 years of recovery compared with densities of 0.2 landslides/km 2 in a nearby unburned area and 0.8 landslides/km 2 in a nearby burn scar that had been recovering for 5 years. Root strength at our study area may not have been that different in the burned and unburned areas given that the rainstorm occurred less than 3 months after the fire was contained, leaving little time for roots to decay.…”

Internet of Things Geosensor Network for Cost-Effective Landslide Early Warning Systems

“…Landslide hazard is increasing year by year due to more intense and frequent rainfall as a consequence of climate change [1]. Additionally, secondary causes such as wildfires play an important role in promoting landslides [2]. The poorest residents are often the most vulnerable to landslides because in many cases they live in the most dangerous areas [3][4][5][6].…”