Forecast, Monitor, Adapt: A Multi-Agency Effort to Protect People during Postfire Debris Flows

The Campania region, in Southern Italy, is affected by hundreds of wildfires every year, mainly during the summer season. Starting from the month of September, mountain watersheds including those hit by wildfires are impacted by even more frequent intense rainstorms. In such conditions, the high sediment availability, lack of recovered vegetation and a likely stronger soil water repellency increase the likelihood of surface runoff and soil erosion, leading to potential post-fire debris flows downstream.This work provides information on more than 100 post-fire debris flows (PFDFs) that occurred in Campania between 2001 and 2021, with a particular focus on the triggering rainfall conditions. Rainfall measurements at a high temporal resolution (10 min) were gathered from a dense rain gauge network, with an average distance between sensors and PFDFs initiation areas of 2.6 km. Information on the occurrence of PFDFs was obtained from web news, social networks, and reports produced by the Fire Brigades. The collection of accurate information related to the debris flow timing and location allowed retrieving and analyzing properties of the triggering rainfall inputs, by identifying the minimum triggering conditions with rainfall thresholds. Moreover, to evaluate the temporal structure and type of the storms associated with the PFDFs (e.g., convective or frontal systems), the standardized rainfall profiles of the triggering events were defined. The return times of the peak cumulative rainfall of the bursts in 10, 20, and 30 minutes were also calculated.Results show that the triggering rainfall events are very short (37 minutes on average), with high average intensity (73.2 mm/h and 49 mm/h in 10 and 30 minutes, respectively), and mostly associated with severe convective systems (i.e., thunderstorms). The estimated return times are quite low, with 75° percentiles of the related distribution ranging from 2.7 to 3.2 years, indicating that these rainfall events are neither rare nor extreme, as also observed by other authors worldwide. Differences are observed in return times and the spatial distribution of the events that occurred in July-September (higher rainfall magnitudes and longer return times) rather than in October-December. The time window in which PFDFs are more likely to occur in the study area has an extension of four months, from September to December. According to the defined triggering rainfall threshold, a rainfall of 11.4 mm in 30 minutes (corresponding to an average intensity of 22.8 mm/h) is likely sufficient to trigger a PFDF in the study area.These research outcomes provide reliable and effective support to inform decision-makers engaged in hazard assessment and risk management, in order to implement suitable countermeasures in terms of monitoring and early warning systems. It is worth noting that PFDFs often occur in small-scale watersheds characterized by very short concentration times, in response to intense bursts of less than 60 minutes. This means insufficient lead time to fully develop an effective emergency response. This and other criticalities represent serious challenges requiring additional work.

The post-fire debris flow is another special type of debris flows that closely related to forest fire and generate in burned area. Obvious difference in term of the initiation mechanism has been find between post-fire debris flows and general debris flows. Fire combusts the surface vegetation and furtherly destroys the texture of underlying soil for the high temperature of fire. The unit weight, pore porosity, permeability and other physical and hydrological properties of the fire-infected soil are changed dramatically in fire, and abundant residual ash layer and loose debris accumulated on the slope after fire. Therefore, post-fire debris flows usually have the characteristics of high unit weight and viscosity. A review of research on the generation mode, kinetic mechanism, influence factors, debris flow forecasting and mitigation measures of this kind of debris flows is put forward, referring to relevant literature at home and abroad. Post-fire debris flows are first studied in 1936, detailed study on which is mostly conducted in America, Australian and Spain. But few can been find in China, except the researches in agroforestry, in which the study are mainly about water and soil loss, vegetation combust and recovery. As another special type of debris flow hazards, systematic study of post-fire debris flows has not attracted enough attention in China, which reflects the poor study of this kind of debris flows. Based on the key problem mentioned above, some suggestions are came up with to enhance the research on the generation and spatial-temporal evolution mechanism and effective protection of post-fire debris flows.

Frequency–magnitude distribution of debris flows compiled from global data, and comparison with post-fire debris flows in the western U.S.

Wildfires are a growing threat to roadway infrastructure. While causing minimal direct damage, wildfires cause substantial indirect damage, including through post-fire debris flows (PFDFs), which occur when moderate intensity rainfall occurs over recently burned areas where soil has been destabilized. PFDFs damage pavement systems, block drains and culverts, and disrupt roadway services. Wildfire frequency and intensity and rainfall intensity are evolving with climate change, which may lead to increased PFDF threats. Public agencies managing roadways need tools to support decision-making that accounts for limited resources and appropriately mitigates fire and debris flow risk. While there have been targeted assessments of fire and PFDF threat in specific areas of Arizona, there is no comprehensive assessment of statewide threat. In this work, we create a statewide assessment of roadway vulnerability to wildfire and PFDFs in Arizona in current conditions and future climate scenarios. We use a state-of-the-art regression-based model to estimate debris flow likelihood on each roadway segment in the state. Our model adapts novel geological methods to assess PFDF threats and engineered infrastructure data to characterize roadway threats. PFDF threat is affected by terrain ruggedness, burn intensity, soil characteristics, and rainfall intensity. Vulnerability of roadways is assessed by overlaying PFDF threat, roadway criticality (using betweenness centrality), and traffic data. We identify roadways most vulnerable in current conditions and estimate how threats change in future climate scenarios. Our results indicate roadways facing the highest PFDF threat are concentrated in the rugged mountains of Southeastern Arizona, the White Mountains of Eastern Arizona, and the Mogollon Rim in Central Arizona. We find that projected changes in precipitation patterns lead to an increase in PFDF threat in much of the state. The framework and results will provide guidance for agencies and decision-makers on where to focus their resources to mitigate the evolving threats of PFDFs.

Post-fire debris flows pose a significant hazard in mountainous regions, but accurate assessment of the associated risk at large scales, particularly in the context of mega-fires, is limited. Although numerous models exist, few are functional at the landscape scale, and none integrate assessments of downstream effects with the likelihood of flow initiation to define both the frequency and magnitude of potential flows. The aim of this work, therefore, was to develop a post-fire debris flow model that could characterise both the occurrence frequency and associated runout of debris flow events and which could be rapidly applied to fire events at the landscape scale. We achieve this using a model integration approach that included the development of a reduced-complexity debris flow runout model, which was calibrated and independently tested for its application at large scales with a dataset of 1377 individual post-fire flows across south-eastern Australia. This model was then integrated with previously published methods determining debris flow source areas and likelihood of initiation specific to our study area. The developed model is intentionally designed for rapid response hazard management. Therefore, it predominantly incorporates only the fundamental, lowest parameter form drivers of debris flow initiation and runout termination, thus minimising computational complexity while ensuring prediction accuracy sufficient for hazard assessment even at application scales as large as mega-fires. Accounting for debris flows initiated from single headwaters and cases where flows simultaneously converge from multiple initiation points, our integrated model effectively characterises debris flow hazard across a geomorphologically diverse landscape encompassing >200,000km2 (capturing ~640,000 potential debris flow initiation locations across Victoria, Australia). Model testing using an independent validation dataset resulted in debris flow runout predictions with root mean square error (RMSE) of 176 and 308 m for non-convergent and convergent flows respectively. Crucially, these predictions can be computed rapidly for large (up to landscape scale) fire events with minimal input requirements and the use of a remotely-sensed observation dataset for domain calibration. As such, the model provides a critical means of addressing the capacity gap in assessments of post-fire debris flow hazard by efficiently integrating measures of occurrence frequency and runout for functional application at large scales. This new work empowers land managers to rapidly predict the hazard arising from post-fire erosion processes and represents an important applied step towards better understanding and mitigating the impacts of wildfire-induced debris flows.

Predicting sediment delivery from debris flows after wildfire

The 2017–2018 Thomas Fire burned 281,893 acres of land in southeastern Santa Barbara County and southwestern Ventura County. An atmospheric river storm impacted the region on January 9, 2018, producing intense rainfall in the western and northern portions of the burned area and triggering numerous post-fire debris flows (PFDFs). The most destructive and deadly flows inundated the town of Montecito, where 23 people died. Debris flow source and inundation mapping data across the fire provide a rare opportunity to assess the interplay between rainfall intensity, watershed characteristics, geologic conditions, and resulting PFDF occurrence. Mapped data are compared to spatially explicit analyses of 857 drainage basins modeled with the U.S. Geological Survey (USGS) logistic regression model (LRM) for PFDF prediction using 15-minute rainfall thresholds at 50 and 90 percent (P50 and P90) probabilities of exceedance. Results indicate that the LRM successfully predicted nearly every PFDF reaching the basin pour point. However, overall model accuracy was lowered by numerous false-positive responses, even where rainfall depths were far above LRM thresholds. Analyses of basins where rainfall was above P50 thresholds reveal a strong correlation between high false-positive responses and basins experiencing rainfall of less than about 150 to 200 percent of USGS thresholds. These false positives occurred in basins with small (0.02–0.05 km2), steep (≥23°) burned areas and in basins underlain by relatively weak geologic units that weather to produce few boulders. Identified relationships provide a basis for refining and improving existing PFDF hazard assessment modeling.

Debris flow occurrence under changing climate and wildfire regimes: A southern California perspective

Debris flows in a burned area, post-fire debris flows, are considered as one of the most dangerous geo-hazards due to their high velocity, long run-out distance, and huge destruction to infrastructures. The rainfall threshold to trigger such hazards is often reduced compared with normal debris flow because ashes generated by mountain fires reduce the permeability of the top soil layer, thus increasing surface runoff. At the same time, burnt material and residual debris have very poor geo-mechanical characteristics, e.g., their internal friction angle and cohesion are typically low, and thus an intense rainfall can easily trigger some debris flows. Studying post-fire debris flow enables us to get a deeper understanding of disaster management. In this paper, the debris flow that occurred in Montecito, California, USA, and was affected by the Thomas Fire was used as a case study. Five major watersheds were extracted based on the digital elevation model (DEM). Remote sensing images were used to analyze the wildfire process, the extent of the burned areas, and the burn severity. The hypsometric integral (HI) and short-duration rainfall records of the watersheds around Montecito when the post-fire debris flows occurred were analyzed. Steep terrain, loose and abundant deposits, and sufficient water supply are the important conditions affecting the formation of debris flows. Taking watersheds as the research objects, HI was used to describe the geomorphic and topographic features, open-access rainfall data was used to represent the water supply, and burn severity represented the abundance of material sources. An occurrence probability model of post-fire debris flow based on HI, short-duration heavy rainfall, and burn severity was developed by using a logistic regression model in post-fire areas. By using this model, the occurrence probability of the post-fire debris flow in different watersheds around Montecito was analyzed based on the precipitation with time. Especially, the change characteristics of occurrence probability of debris flows over time based on the model bring a new perspective to observe the obvious change of the danger of post-fire debris flows and it is very useful for early warning of post-fire debris flows.

Rainfall conditions leading to runoff-initiated post-fire debris flows in Campania, Southern Italy

Increases in wildfire activity and rainfall intensification are driving more postfire debris flows (PFDF) in many regions around the world. PFDFs are most common in the first postfire year and may even occur before a fire is fully controlled. This underscores the importance of assessing postfire hazards before a fire starts. Evaluation of PFDF hazards prior to fire can help strategize interventions lessening the negative effects of future fires. However, debris‐flow runout and inundation analyses are not routine in PFDF hazard assessments, partially due to time constraints and substantial uncertainties in boundary conditions. Here, we propose a prefire PFDF inundation assessment framework using a debris‐flow runout model based on the Herschel‐Bulkley (HB) rheology (HEC‐RAS v6.1). We constrain model inputs and parameters using Bayesian posterior analysis, rainfall‐runoff simulations, and a debris‐flow volume model. We use observations from recent PFDF incidents in northern Arizona, USA, to calibrate model components and then apply our prefire inundation assessment framework in a nearby unburned area. Specifically, we (a) identify yield stress as the most influential factor on inundation extent and arrival time in a HB model, (b) establish posterior distributions for model parameters suitable for forward modeling by leveraging uncertainties in field observations, and (c) implement a predictive forward analysis in an area that has not burned recently to evaluate PFDF inundation under several future fire scenarios. This study improves our ability to assess postfire debris‐flow hazards before a fire begins and provides guidance for future applications of single‐phase rheological models when assessing PFDF hazards.

Habitat fragmentation and degradation and invasion of nonnative species have restricted the distribution of native trout. Many trout populations are limited to headwater streams where negative effects of predicted climate change, including reduced stream flow and increased risk of catastrophic fires, may further jeopardize their persistence. Headwater streams in steep terrain are especially susceptible to disturbance associated with postfire debris flows, which have led to local extirpation of trout populations in some systems. We conducted a reach-scale spatial analysis of debris-flow risk among 11 high-elevation watersheds of the Colorado Rocky Mountains occupied by isolated populations of Colorado River Cutthroat Trout (Oncorhynchus clarkii pleuriticus). Stream reaches at high risk of disturbance by postfire debris flow were identified with the aid of a qualitative model based on 4 primary initiating and transport factors (hillslope gradient, flow accumulation pathways, channel gradient, and valley confinement). This model was coupled with a spatially continuous survey of trout distributions in these stream networks to assess the predicted extent of trout population disturbances related to debris flows. In the study systems, debris-flow potential was highest in the lower and middle reaches of most watersheds. Colorado River Cutthroat Trout occurred in areas of high postfire debris-flow risk, but they were never restricted to those areas. Postfire debris flows could extirpate trout from local reaches in these watersheds, but trout populations occupy refugia that should allow recolonization of interconnected, downstream reaches. Specific results of our study may not be universally applicable, but our risk assessment approach can be applied to assess postfire debris-flow risk for stream reaches in other watersheds.

The Transverse Ranges of southern California often experience fire followed by flood. This sequence sometimes causes post-fire debris flows (PFDFs) that threaten life and property situated on alluvial fans. The combination of steep topography, highly erodible rock and soil, and wildfire, coupled with intense rainfall, can initiate PFDFs even in cases of relatively small storm rainfall totals. This study identifies common atmospheric conditions during which damaging PFDFs occur in the Transverse Ranges during the cool season, defined here as November–March. A compilation of 93 PFDF events during 1980–2014 triggered by 19 precipitation events is compared against previous studies of the events, reanalysis, precipitation, and radar data to estimate PFDF trigger times. Each event was analyzed to determine common atmospheric features and their range of values present at and preceding the trigger time. Results show atmospheric rivers are a dominant feature, observed in 13 of the 19 events. Other common features include low-level winds orthogonal to the Transverse Ranges and other conditions favorable for orographic forcing, a strong upper level jet south of the region, and moist-neutral static stability. Several events included closed low-pressure systems or narrow cold frontal rain bands. These findings can help forecasters identify more precisely the synoptic-scale atmospheric conditions required to produce PFDF-triggering rainfall and thus reduce uncertainty when issuing warnings.

Quantifying sources of fine sediment supplied to post-fire debris flows using fallout radionuclide tracers

In 2020 the Grizzly Creek wildfire burned both sides of the narrow and deep Glenwood Canyon in Colorado, USA. Within the canyon there is a major Interstate Highway (I-70, the only east-west interstate highway across the state of Colorado), a major railroad (the Union Pacific), and a critical waterway (the Colorado River that supplies water to millions of downstream users). Within this canyon, there is a history of life-threatening postfire debris flows from two previous fires (the 1994 South Canyon Fire and the 2002 Coal Seam Fire) that both produced debris flows a few months following the wildfires. Based on this historical knowledge, several government agencies used their combined expertise to coordinate on life-safety decision-making following the Grizzly Creek Fire. After the Grizzly Creek Fire, nine large debris flows were triggered by rainstorms in the summer of 2021, followed by three small debris flows in the summer of 2023. Despite the disruptive postfire debris flow activity, there were no fatalities during these storms, which was largely due to a tiered strategy of hazard assessment/forecasting, monitoring, and adaptation. Many different government agencies worked together to share knowledge and inform decision-making to preserve life safety during these events, including: the U.S. Forest Service, U.S. Geological Survey, Colorado Department of Transportation (CDOT), and the National Weather Service (NWS). Weather forecasts and estimates of debris-flow likelihood, volume, and triggering rainfall thresholds were used to anticipate the location, triggering rainfall, and debris flow volume. These forecasts were compared with rainfall thresholds to determine when to deliver warnings to the public and advise canyon closures. After debris flow triggering rainstorms, the rainfall thresholds were re-evaluated. If a forecast was above the debris-flow rainfall threshold then the NWS would issue a watch or a warning. If the NWS issued a watch, CDOT staff would be positioned at either end of the canyon, and then if the NWS upgraded the watch to a warning CDOT staff would close the highway. This helped to make sure that the public was out of the canyon when there was a potential for debris flows. As the burn area recovered the warnings were adapted based on observations from monitoring. This collaborative model may be helpful in future wildfire situations in areas with critical infrastructure where the mandate for life safety falls across multiple jurisdictions.