Estimating post-fire debris-flow hazards prior to wildfire using a statistical analysis of historical distributions of fire severity from remote sensing data

Abstract

Following wildfire, mountainous areas of the western United States are susceptible to debris flow during intense rainfall. Convective storms that can generate debris flows in recently burned areas may occur during or immediately after the wildfire, leaving insufficient time for development and implementation of risk mitigation strategies. We present a method for estimating post-fire debris-flow hazards before wildfire using historical data to define the range of potential fire severities for a given location based on the statistical distribution of severity metrics obtained from remote sensing. Estimates of debris-flow likelihood, magnitude and triggering rainfall threshold based on the statistically simulated fire severity data provide hazard predictions consistent with those calculated from fire severity data collected after wildfire. Simulated fire severity data also produce hazard estimates that replicate observed debris-flow occurrence, rainfall conditions and magnitude at a monitored site in the San Gabriel Mountains of southern California. Future applications of this method should rely on a range of potential fire severity scenarios for improved pre-fire estimates of debris-flow hazard. The method presented here is also applicable to modelling other post-fire hazards, such as flooding and erosion risk, and for quantifying trends in observed fire severity in a changing climate.

Evolution of Debris‐Flow Initiation Mechanisms and Sediment Sources During a Sequence of Postwildfire Rainstorms

“…Since the modeled sediment yield compares favorably with the sediment yield estimated from the volumetric change within the debris basin, we conclude that the model is adequately simulating erosion volumes during these storms (Figure f). Similarly, the basin‐scale sediment yield resulting from the storm on 18 February 2017 was approximately 60 % of the 2,100 m 3 reported by Staley et al () or 1,260 m 3 . The model simulation results in a sediment yield of 740 m 3 , with 52% of that sediment coming from the hillslope and 48% coming from the channel (Figures b and f).…”

“…(a) Interrill erosion accounts for over 50% of hillslope erosion in the terrestrial laser scanner (TLS) area throughout all rainstorms based on model results; (b) hillslope erosion and channel erosion contribute approximately the same amount to total sediment yield at the basin scale during storms 1 and 7; (c) the minimum Manning coefficient (n 0 ) generally increased with each storm following the fire; (d) despite constant decreases in the amount of ravel deposited within the channel following each storm, the percentage of the total sediment yield attributable to channel erosion varied nonmonotonically with time after the fire. the basin-scale sediment yield resulting from the storm on 18 February 2017 was approximately 60% of the 2,100 m 3 reported by Staley et al (2018) or 1,260 m 3 . The model simulation results in a sediment yield of 740 m 3 , with 52% of that sediment coming from the hillslope and 48% coming from the channel (Figures 7b and 7f).…”

“…Despite not having poststorm DEMs covering the entire basin, we do have estimates of basin‐averaged sediment yields from the 0.2 km 2 drainage that contains our 0.12 km 2 study basin. The sediment yield from the 0.2 km 2 basin was obtained for several storms by using prestorm and poststorm estimates of the sediment volume within the Las Lomas debris basin (Staley et al, ) located downstream of the channel monitoring station (Figure a). Since our study basin composes 60 % of the 0.2 km 2 drainage that empties into the debris basin, we estimate the sediment yield from our study area as being 60 % of the total sediment volume within the debris basin.…”

“…Note that the actual erosion volumes from hillslopes and channels are unknown; (g) without flow-driven detachment processes, the modeled erosion volume is substantially less than the actual erosion volume, whereas the difference is less dramatic when only (h) raindrop-driven detachment processes are neglected. 0.2 km 2 basin was obtained for several storms by using prestorm and poststorm estimates of the sediment volume within the Las Lomas debris basin (Staley et al, 2018) located downstream of the channel monitoring station (Figure 1a). Since our study basin composes 60% of the 0.2 km 2 drainage that empties into the debris basin, we estimate the sediment yield from our study area as being 60% of the total sediment volume within the debris basin.…”

Debris‐flow‐dominated sediment transport through a channel network after wildfire

“…SE Australia; Smith et al, ; Nyman et al, ). There is a substantial body of literature on post‐fire debris flows focused on hazards (Cannon et al, ; Riley et al, ), the prediction of where and when they will occur (Gartner et al, ; Staley et al, ) and how much sediment they produce (Gartner et al, , ; Santi et al, ; Nyman et al, ; DeLong et al, ). A growing body of research is focusing on the physical mechanisms that give rise to post‐fire debris flows, and the development of process‐based debris‐flow models (Kean et al, ; Rengers et al, ; McGuire et al, ).…”