Postfire hydrologic response along the Central California (USA) coast: insights for the emergency assessment of postfire debris-flow hazards

Abstract

The steep, tectonically active terrain along the Central California (USA) coast is well known to produce deadly and destructive debris flows. However, the extent to which fire affects debris-flow susceptibility in this region is an open question. We documented the occurrence of postfire debris floods and flows following the landfall of a storm that delivered intense rainfall across multiple burn areas. We used this inventory to evaluate the predictive performance of the US Geological Survey M1 likelihood model, a tool that presently underlies the emergency assessment of postfire debris-flow hazards in the western USA. To test model performance, we used the threat score skill statistic and found that the rainfall thresholds estimated by the M1 model for the Central California coast performed similarly to training (Southern California) and testing (Intermountain West) data associated with the original model calibration. Model performance decreased when differentiating between “minor” and “major” postfire hydrologic response types, which weigh effects on human life and infrastructure. Our results underscore that the problem of false positives is a major challenge for developing accurate rainfall thresholds for the occurrence of postfire debris flows. As wildfire activity increases throughout the western USA, so too will the demand for the assessment of postfire debris-flow hazards. We conclude that additional collection of field-verified inventories of postfire hydrologic response will be critical to prioritize which model variables may be suitable candidates for regional calibration or replacement.

Rainfall intensification amplifies exposure of American Southwest to conditions that trigger postfire debris flows

“…The rainfall thresholds that we used were calculated for a likelihood of 0.5, a value that has been widely applied to estimate debris-flow triggering conditions in the western United States 32 . This likelihood-based rainfall threshold approach has been shown to generally perform as well, and in some cases better than, preexisting regional rainfall thresholds in Arizona, Colorado, Montana, and New Mexico 38 , and has also performed similarly well in areas with no preexisting regional thresholds 34 .…”

“…Our statewide analyses rely on the known recurrence interval of the shortduration rainfall intensities associated with these same debris-flow observations 8 ; therefore, our analyses could be extended beyond California and Colorado as part of a future study. The control of short-duration rainfall on debris-flow triggering soon after fire has also proven valid for the highly destructive debris flows that followed recent megafires (>100,000 acres) in central and northern California 26,34 . However, we refrain from expanding our analyses to the Pacific Northwest, where the effect of short-duration rainfall on debris-flow generation after wildfire is considerably more uncertain 61 and short-duration precipitation frequency data for the present climate 41 is unavailable (Supplementary Fig.…”

“…2). This includes areas where communities, emergency managers, and weather forecasters may be less accustomed to contending with postfire debris, flows (relative to those in southern California), in part because the hazard has only recently been systematically documented 26,34 . Therefore, this study highlights increased hazard potential and the benefits of prefire planning, education, and outreach for postfire hazards 36 .…”

“…As wildfire activity increases throughout the western United States 31 , so too has the demand for the emergency assessment of postfire debris-flow hazards 32 , including areas where our understanding of postfire debris-flow initiation is limited, such as in the Cascade ranges spanning northern California, Oregon, Washington, and British Columbia 33 . This demand, coupled with recent observations of postfire debris flows in regions where they were previously undocumented 26,34 , highlights a need to understand how rainfall intensification driven by climate change may heighten our exposure to this hazard. Despite progress in recognizing the potential effect of rainfall intensification on postfire debris flows 35,36 , we have not yet developed an approach to quantify how the frequency, magnitude, and seasonality of rainfall capable of triggering postfire debris flows could evolve.…”

How Long Do Runoff‐Generated Debris‐Flow Hazards Persist After Wildfire?

“…Rainfall of intensity sufficient to initiate debris flows must also occur in the years following fire, and threshold rainfall intensities necessary for debris flows are expected to increase with time as the burned area recovers (Ebel, 2020; Hoch et al., 2021). In addition, spatially variable recovery rates within many burned areas (Kinoshita & Hogue, 2011) and increasing numbers of very large fires burning across hydroclimaticly diverse terrain (e.g., Thomas, Kean, et al., 2023; Thomas, Lindsay, et al., 2023) imply the need for refining the relationship between recovery and triggering intensity. Measuring recovery at smaller spatial scales (e.g., individual catchments) could help to address these issues.…”