Post-Wildfire Debris-Flow Hazards

2025

1. Rainfall Thresholds for Postfire Debris‐Flow Initiation Vary With Short‐Duration Rainfall Climatology

   David B. Cavagnaro, Scott W. McCoy, Donald N. Lindsay, and 3 more authors

   Journal of Geophysical Research: Earth Surface, Jun 2025

   Abs DOI

   The size, frequency, and geographic scope of severe wildfires are expanding across the globe, including in the Western United States. Recently burned steeplands have an increased likelihood of debris flows, which pose hazards to downstream communities. The conditions for postfire debris‐flow initiation are commonly expressed as rainfall intensity‐duration thresholds, which can be estimated given sufficient observational history. However, the spread of wildfire across diverse climates poses a challenge for accurate threshold prediction in areas with limited observations. Studies of mass‐movement processes in unburned areas indicate that thresholds vary with local climate, such that higher rainfall rates are required for initiation in climates characterized by frequent intense rainfall. Here, we use three independent methods to test whether initiation of postfire runoff‐generated debris flows across the Western United States varies similarly with climate. Through the compilation of observed thresholds at various fires, analysis of the spatial density of observed debris flows, and quantification of feature importance at different spatial scales, we show that postfire debris‐flow initiation thresholds vary systematically with short‐duration rainfall‐intensity climatology. The predictive power of climatological data sets that are readily available before a fire occurs offers a much‐needed tool for hazard management in regions that are facing increased wildfire activity, have sparse observational history, and/or have limited resources for field‐based hazard assessment. Furthermore, if the observed variation in thresholds reflects long‐term adjustment of the landscape to local climate, rapid shifts in rainfall intensity related to climate change will likely induce spatially variable shifts in postfire debris‐flow likelihood.

2. Improved Prediction of Postfire Debris Flows Through Rainfall Anomaly Maps

   David B. Cavagnaro, Scott W. McCoy, Matthew A. Thomas, and 2 more authors

   Geophysical Research Letters, 2025

   Abs DOI

   Predicting where runoff-generated debris flows might occur during rainfall on steep, recently burned terrain is challenging. Studies of mass-movement processes in unburned areas indicate that event locations are well-predicted by rainfall anomaly, R*, in which peak observed rainfall is normalized by local rainfall climatology. Here, we use remote and field methods to map debris flows triggered within the 2020 Dolan Fire burn area in coastal California, demonstrate that a short-duration R* metric predicts debris-flow occurrence more effectively than absolute peak intensity or longer-duration rainfall metrics, and show that incorporating an R* criterion into an existing debris-flow likelihood model can reduce false positive predictions and improve accuracy. We test R* at three other climatically distinct fires in California, demonstrating its utility for mapping likely debris-flow locations in different climates. We also consider how R* can benefit postfire debris-flow prediction given recent increases in climatological variability within individual burn perimeters.

2024

1. A Robust Quantitative Method to Distinguish Runoff-Generated Debris Flows From Floods

   David B. Cavagnaro, Scott W. McCoy, Jason W. Kean, and 4 more authors

   Geophysical Research Letters, 2024

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2024GL109768

   Abs DOI

   Debris flows and floods generated by rainfall runoff occur in rocky mountainous landscapes and burned steeplands. Flow type is commonly identified post-event through interpretation of depositional structures, but these may be poorly preserved or misinterpreted. Prior research indicates that discharge magnitude is commonly amplified in debris flows relative to floods due to volumetric bulking and increased frictional resistance. Here, we use this flow amplification to develop a metric (Q*) to separate debris flows from floods based on the ratio of observed peak discharge to the theoretical maximum water discharge from rainfall runoff. We compile 642 observations of floods and debris flows and demonstrate that Q* distinguishes flow type to ∼92% accuracy. Q* allows for accurate identification of debris flows through simple channel cross-section surveys rather than through qualitative interpretation of deposits, and therefore should increase the performance of models and engineered structures that require accurate flow-type observations.

2. Post-Wildfire Debris Flows

   Joseph E. Gartner, Jason W. Kean, Francis K. Rengers, and 3 more authors

   In Advances in Debris-flow Science and Practice, 2024

   Abs DOI

   Post-wildfire debris flows pose severe hazards to communities and infrastructure near and within recently burned mountainous terrain. Intense heat of wildfires changes the runoff characteristics of a watershed by combusting the vegetative canopy, litter, and duff, introducing ash into the soil and creating water repellant soils. Following wildfire, rainfall on bare ground is less able to infiltrate into the fire-altered soils and overland flow is less impeded by vegetation. Rainfall runoff in recently burned areas can erode hillslopes owing to the removal of soil binding organic matter near the soil surface by fire. In channels, loose, dry-ravel deposits composed of sand and gravel are readily entrained by concentrated runoff in channels. Entrainment of soil on hillslopes and in channels bulks up the sediment concentration of the rainfall runoff to generate debris flows capable of transporting boulders and large woody debris. Post-wildfire debris flows can be triggered by rainfall conditions that would typically produce little runoff during unburned conditions. The primary rainfall trigger for post-wildfire debris flows is high intensity rainfall during short duration convective rainstorms or periods of high rainfall intensity embedded within a long-duration frontal storm. Numerous observations of debris flows triggered by storms lasting less than an hour following periods of little to no rainfall indicate that antecedent rainfall is not a requirement for initiation of post-wildfire debris flows. Post-wildfire debris-flow hazard assessment entails estimating probability and magnitude of debris flows in the burned area, estimating debris-flow runout and intensity, and defining rainfall intensity-duration thresholds for debris-flow initiation. In the United States, probability and magnitude is estimated using empirically derived models largely based on data collected in southern California. The models provide maps to identify watersheds and drainage paths where post-wildfire hazards are most pronounced. Rainfall intensity-duration thresholds can be incorporated into flood hazard forecasting tools. Currently, work is underway to identify how to best implement debris-flow runout models in burned areas with efficiency and accuracy. Post-wildfire debris flows have been a long-recognized process in the Transverse Ranges of southern California; however, climate change is driving more frequent wildfires to burn more mountainous terrain throughout the western United States and worldwide. As a result, post-wildfire debris flows are becoming a more common threat in areas where they were once infrequent. As the threat of post-wildfire debris flow expands into new areas, evaluating the hazard becomes challenging because the degree to which wildfire increases debris-flow susceptibility varies from region to region. This chapter summarizes the knowledge to date for evaluating post-wildfire debris-flow susceptibility and hazard assessment. We summarize the characteristics of wildfire burn severity, topography, underlying soil and geology, and rainfall conditions that contribute to making a watershed most likely to produce post-wildfire debris flows. Methods for hazard assessment in the United States and other countries are summarized. We highlight knowledge gaps for how post-wildfire debris-flow susceptibility varies throughout the western United States and worldwide and identify research needs to improve hazard assessment methods in different geographies.

2023

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

   Matthew A. Thomas, Jason W. Kean, Scott W. McCoy, and 8 more authors

   Landslides, Jul 2023

   Abs DOI

   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.

2. The Rainfall Intensity-Duration Control of Debris Flows After Wildfire

   Matthew A. Thomas, Donald N. Lindsay, David B. Cavagnaro, and 3 more authors

   Geophysical Research Letters, 2023

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2023GL103645

   Abs DOI

   Increased wildfire activity in the western United States has exposed regional gaps in our understanding of postfire debris-flow generation. To address this problem, we characterized flows in an unstudied area to test the rainfall intensity-duration control of the hazard. Our rainfall measurements and field observations from the northern Sierra Nevada (California, USA) show that debris flows resulted from a short burst rainfall during a low-accumulation storm. In contrast, a much higher accumulation storm (∼10 times more rainfall) with lower short-duration rainfall rates only produced low-hazard flooding. We conclude that total storm rainfall is not an ideal metric for identifying the rainfall conditions that initiate runoff-generated debris flows in the first year after wildfire. Rather, a focus on short-duration (\textless1 hr), high-intensity rainfall that can occur during localized thunderstorms, or bands of intense rainfall during prolonged rainstorms, is more beneficial for the purposes of hazard assessment and warning.

2022

1. Quantification of Wildfire-Induced Perturbations to Soil-Water Retention Curves and Their Impact on Rainfall Runoff

   Nathan R. Delgado

   University of Nevada, Reno, 2022

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Increased frequency and magnitude of wildfires can cause elevated post-fire rainfall-runoff and debris-flow hazards in and around steeplands. Improved prediction of the severity and spatial extent of post-fire increases to these hazards requires understanding which soil properties are most strongly affected by wildfire and what role these changes play in runoff generation and debris-flow initiation. Previous studies have shown significant post-fire perturbations to saturated hydraulic conductivity (Ks) and sorptivity, and in some cases these perturbations appear to exert a strong control on the observed increase in post-fire runoff and debris flow. However, a variety of other soil properties also influence rainfall infiltration rates into unsaturated soils. The soil-water retention curve (SWRC), also called the soil-water characteristic curve, is the relationship between matric suction and soil-water content and determines many critical unsaturated soil properties such as unsaturated hydraulic conductivity. The SWRC is commonly described using the Van Genuchten equation, in which key parameters are the residual water content, θr, saturated water content, θs, the pore-size distribution index, n, and the inverse of the air-entry pressure head, α. To date, the degree to which SWRCs are perturbed by wildfire is less clear, and as a result, they are commonly assumed to be unaffected by wildfire and obtained based on soil texture from databases composed of unburned agricultural soils. To quantify potential wildfire perturbations to SWRCs, we collected a suite of 43 unburned and 70 burned soil cores from seven recent wildfires in Nevada and California. We measured burned and unburned SWRCs and found best-fit Van Genuchten parameters θr, θs, n, α, using ASTM standard methods and the Meter HYPROP system. We found high quality fits to SWRCs from both unburned and burned samples using the Van Genuchten equation. While the magnitude of fire perturbation varies across fire sites, in most cases we found post-wildfire reductions in soil water retention at most values of matric suction, as quantified by increases in n and α, and decreases in θs and θr relative to the unburned state. To generalize the importance of the observed perturbations to SWRC parameters for rainfall runoff generation, we conducted a suite of numerical rainfall-runoff simulations in which we solved Richards equation with HYDRUS 1-D and with parameters constrained from burned and unburned SWRCs. These simulations suggest that the most commonly observed perturbations to SWRCs decrease time to ponding and strongly increase runoff generation at the sub-hourly timescales relevant to flash flooding and debris-flow initiation by rainfall runoff. These results highlight that field constraints on soil-hydraulic properties beyond just the saturated hydraulic conductivity are critical to more accurate prediction of rapid rainfall runoff following fire. The large database of such measurements presented here provides an important step towards increasing our understanding of the magnitude of expected post-fire perturbations to soil properties.

2021

1. Watershed Sediment Yield Following the 2018 Carr Fire, Whiskeytown National Recreation Area, Northern California

   Amy E. East, Joshua B. Logan, Peter Dartnell, and 4 more authors

   Earth and Space Science, Sep 2021

   Abs DOI

   Wildfire risk has increased in recent decades over many regions, due to warming climate and other factors. Increased sediment export from recently burned landscapes can jeopardize downstream infrastructure and water resources, but physical landscape response to fire has not been quantified for some at-risk areas, including much of northern California, USA. We measured sediment yield from three watersheds (13–29 km2) that drain to Whiskeytown Lake, California, within the area burned by the 2018 Carr Fire. Structure-from-Motion photogrammetry on aerial images combined with sonar bathymetric mapping of submerged areas indicated first-year post-fire sediment yields of 4,080 ± 598 t/km2 (Brandy Creek), 2,700 ± 527 t/km2 (Boulder Creek), and 305 ± 58.0 t/km2 (Whiskey Creek)—some of the first post-fire yields measured in northern California and 64, 42, and 4.8 times greater than pre-fire yields, respectively. These were measured during a wet year and resulted largely from rilling erosion and fluvial sediment transport, without post-fire debris flows. Rilling preferentially developed in contact with dirt roads, aided by thin soils and exposed bedrock, and on slopes vegetated by chaparral pre-fire. The second post-fire year (a dry year) was characterized by fluvial reworking and delta progradation of the first-year deposits and relatively little new sediment export. First-year sedimentation of 111,000 m3 represented minor loss of storage capacity in Whiskeytown Lake but would be detrimental to smaller reservoirs; in general, increased sediment yields from western US watersheds as fire and extreme rainfall increase will likely pose risks to water quality and storage.