Landslides after wildfire: initiation, magnitude, and mobility
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Francis K. Rengers I Luke A. McGuire I Nina S. Oakley I Jason W. Kean I Dennis M. Staley I Hui Tang
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 longduration 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. Keywords Landslide . Wildfire . Geomorphology Introduction Wildfire disturbs normal hydrologic and soil conditions. Burned forest soils often repel water, causing fire-induced reductions in infiltration (DeBano et al. 1979; DeBano 2000; Ebel and Moody 2016; McGuire et al. 2018), leading to an increased likelihood of flooding and runoff-generated debris flows (Wells 1987). This is commonly described as the “fire–flood cycle” (Kotok and Kraebel 1935). Post-fire debris flows, in particular, can be lethal hazards, resulting in high numbers of fatalities (Dowling and Santi 2014; Eaton 1935; Kean et al. 2019a). Post-fire landslides have also been observed. Scott (1971) documented several landslides in areas that had burned the prior year, but the storm that triggered the landslides also triggered landslides in many unburned areas (Campbell 1975). Several additional authors have observed landslides in areas that burned 5 or more years beforehand (Benda and Dunne 1997; May and Gresswell 2003; Meyer et al. 2001; Rice and Foggin 1971). These landslides can subsequently transition into debris flows (De Graff 2018; Jackson and Roering 2009; W
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