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At present, debris flow warning uses precipitation threshold and issues regional warning throughout the world. Precipitation threshold warning is less accurate and in most of the time large portion of unaffected population are evacuated. More precise warning should use direct monitoring. There are many debris flow monitoring stations but no real time warning system in use. The main reason is that the identification and confirmation of debris flow occurrence requires human interaction and it is too slow. A debris flow monitoring and warning system has been installed in the midstream section of Yusui Stream, Taiwan. The monitoring station operates fully automatically, providing early warnings without the need for manual intervention. The system comprises two webcam cameras, two Micro-Electro-Mechanical Systems (MEMS), and a rain gauge. The arrival of debris flows is detected and confirmed through both webcam images and MEMS signals. Once debris flow is detected, the system automatically issues a warning to the affected areas via voice messages, line messages, broadcasts, and web-based alerts. The webcam cameras are also used to estimate debris flow velocity and flow height, while the MEMS sensors are utilized to determine the phase speed and flow rate. On July 24th, 2014, Typhoon Gaemi triggered several debris flows, and the system successfully issued several warnings automatically. The entire video record, along with depth variation data, was recorded automatically. [ABSTRACT FROM AUTHOR]

Forests play an important role in controlling the formation and movement processes of debris flows. They contribute to soil stabilization, regulation of soil water content, and act as robust structures impeding the downstream progression of debris flows. On the positive side, trees, to some extent, can intercept debris flows and effectively mitigate their velocity by increasing flow resistance. On the negative side, trees may suffer damage from debris-flow hazards, characterized by the generation of substantial quantities of wood fragments and consequential ramifications such as river channel blockage, resulting in backwater rise. In extreme cases, this blockage collapse can lead to instantaneous discharge amplification, thereby adversely impacting urban safety and impeding sustainable development. Therefore, in order to grasp the effects of tree characteristics on tree failure modes, the tree failure modes and corresponding parameters, diameters at breast height (DBH) and root-soil plate size, were identified and recorded through the post-event field investigation in Keze Gully, a region prone to debris-flow events in Sichuan, China, respectively. To investigate the impact of spatial variability in tree root distribution on tree failure modes, the root cross-sectional area ratio (RAR), root density (RD), root length density (RLD) and soil detachment rate (SDR) were obtained. The findings indicated that: (1) Tree characteristics reflect the interactions of debris flows and trees, and influence the tree failure modes ultimately. The root distribution characteristics influence the size and shape of the root-soil plate to affect the resistance of trees. (2) Compared to burial and abrasion, stem breakage and overturning are the predominant modes of tree failure in debris-flow hazards. Trees with a smaller DBH primarily experience stem breakage and bending, and trees with a larger DBH mostly experience overturning. (3) The root-soil plate shapes of overturned trees, affected by the root architecture and root growth range, are generally semielliptical or semicircular, and the horizontal and vertical radii increase with DBH, but the correlation between the root-soil plate's breadth-depth ratio and DBH is low. (4) The biomass and RAR decrease with distance. The RAR distribution exhibit the order of upslope direction > downslope direction > lateral direction. The coarse root biomass significantly increases with DBH, but no clear trend in fine root biomass. (5) The roots can significantly enhance the soil erosion resistance, but the erosion resistance of coarse roots is not as significant as that of fine roots. The erosion resistance increases with DBH, and follows the order of upslope direction > downslope direction > lateral direction. The results could provide new insights into the influences of tree and root distribution characteristics on tree failure modes during debris flows. [ABSTRACT FROM AUTHOR]

Debris flows are solid‐liquid mixtures originating in the upper part of mountain basins and routing downstream along incised channels. When the channel incises an open fan, the debris flow leaves the active channel and propagates downstream along a new pathway. This phenomenon is called an avulsion. We retrieve the most probable avulsion pathways leveraging a Monte Carlo approach based on using Digital Elevation Models (DEMs). Starting from LiDAR‐based DEMs, we build an ensemble of synthetic DEMs using a local Gaussian probability density function of local elevation values and obtain an ensemble of drainage networks using a gravity‐driven routing algorithm. The ensemble of drainage networks was used to obtain the most probable pathways of avulsions. We applied our methodology to a real monitored fan in the Dolomites (Northeastern Italian Alps) subjected to debris‐flow activity with avulsions. Our approach allows us to verify the consistency between the occurrence probability of a synthetic pathway and those that historically occurred. Furthermore, our approach can be used to predict future debris‐flow avulsions, assuming relevance in debris‐flow risk assessment and planning of debris‐flow control works. Plain Language Summary: Debris flows are impulsive routing of mixtures of water and solid material that start in the upper parts of mountain basins after intense rainfall events. These flows move downstream through channels, carved within the debris fan, which act as preferential pathways. Debris flows can leave original channels and may suddenly change direction and propagate downstream along pathwaysThis phenomenon, called avulsion, increases the difficulty for scientists to assess the most probable debris‐flow pathways and, much more, future possible ones. We answer this scientific call via a Monte Carlo approach based on an ensemble of synthetic DEMs obtained from available ones. Our framework involves running many simulations, leveraging synthetic possible terrains based on available data, and retrieving as many synthetic equiprobable debris‐flow pathway configurations to be wrapped up within a probabilistic framework. Key Points: The probabilistic approach we propose leverages a Monte Carlo framework using an ensemble of synthetic Digital Elevation Models (DEMs) obtained from available DEMs, to identify the most probable debris‐flow pathwaysWe can assess the probability of an avulsion by validating our approach on field data and consistently using available surveysOur approach can be used for predicting future most probable pathways following avulsions based on available DEMs [ABSTRACT FROM AUTHOR]

This paper provides a quantitative assessment of debris flows, the morphometric properties of high-mountain lakes, and their catchments in the Slovak part of the High Tatra Mts. Our database comprises 29 high-mountain lakes in 11 valleys. We utilised vector models to derive some maps, and for a detailed study, we employed a digital elevation model of the study area and applied map algebra within a geographic information system environment. Using ArcGIS software, we calculated 18 morphometric parameters (such as basin orientation, topographic slope factor, basin length, basin ruggedness, etc.). This study is valuable for conducting various analyses of hydrological and morphometric characteristics of catchments. It is based on a comprehensive approach that integrates Earth remote sensing with aerial image interpretation and detailed field research of the High Tatra Mts.' lakes. The study has shown a significant influence of morphometric parameters of catchments on the dynamics and frequency of morpho-dynamic processes. [ABSTRACT FROM AUTHOR]

Estimating flow velocities is key to assessing hazards associated with debris flows. One approach to post‐event velocity estimation is the superelevation method, which uses debris‐flow mudlines to measure the cross‐channel surface inclination, or superelevation, produced by centripetal forces acting on the flow in a bend. Flow velocities are then calculated using a subjective parameterization of the forced vortex equation modified to include a debris‐flow specific correction factor. Subjective parameterization of this equation leads to substantial variability and uncertainty in the resulting flow velocities. We present an analysis of the reliability of the superelevation method using a large UAV‐based data set of 14 debris flows with front velocities of ∼0.8–6.5 m s−1 and cross‐channel surface inclinations of ∼0.6–8.5°, as well as a validation for a single debris flow measured using high‐resolution, high‐frequency 3D lidar data fused to video imagery. The validation event indicates that when the flow surface inclination can be measured directly, the forced vortex equation produces excellent results without needing a correction factor for Froude numbers ranging from 0.7 to 1.5. This finding indicates that the main challenge with the superelevation method lies in obtaining accurate measurements of superelevation from the mudlines, and that a correction factor may serve to compensate for measurement difficulties rather than variable flow properties. For very small and highly subcritical flows, the superelevation method may generate a large overestimation of flow velocities. Plain Language Summary: Debris flows are hazardous, surging flows containing a mixture of water, sediment and other debris. Efforts to mitigate debris‐flow hazards depend on accurate estimates of flow velocity, which are difficult to obtain in the field. The superelevation method uses mudlines left by a debris flow in a bend, where it has been affected by centripetal acceleration, to estimate the cross‐channel surface tilt and uses this to achieve a post‐event estimate of flow velocity. We used an unusually large field‐based data set consisting of pre‐ and post‐event orthophotos and digital elevation models from 14 debris flows along with highly detailed laser scanning measurements of one debris flow to evaluate the accuracy and uncertainties of this method. We find that the equation used in the superelevation method produces excellent velocity estimates when the surface tilt can be directly measured, but that the post‐event mudlines are often not representative of the maximum surface tilt and therefore likely result in an underestimate of the peak flow velocity. Practitioners may need to use a correction factor to account for this underestimation when using the superelevation method for hazard mitigation. Key Points: 3D lidar scans (10 Hz) provide new insights into the superelevation method for post‐event debris‐flow velocity estimationResults indicate that the traditional superelevation method overestimates the front velocity and underestimates the peak flow velocityExcellent results generated with a known surface tilt indicate that the challenge lies in obtaining representative post hoc measurements [ABSTRACT FROM AUTHOR]

The rapid melting of glaciers and thawing of permafrost in mountainous regions have heightened the danger of rock‐ice avalanches. These avalanches pose a severe threat due to their potential to transform into water‐saturated debris flows. The catastrophic event in Chamoli, India, on 7 February 2021, illustrates the devastating consequences of such processes. Developing a model capable of predicting the dynamics and extent of these events is imperative for natural hazard science and disaster mitigation. In response, we propose a depth‐averaged rock‐ice avalanche model encompassing four distinct materials: rock, ice, snow, and water. The model integrates crucial physical processes, including frictional heating, phase changes, ground material entrainment, and air‐blast hazards. Through a system of mass and momentum balance equations extended with grain flow and internal energy equations, the model captures heat exchanges and resulting phase changes as the fragmented material flows. Focusing on identifying the primary water source in the flow and testing the model on the 2021 Chamoli event, we quantify water's influence on flow dynamics and regime transitions. However, uncertainties persist in heat transfer physics and quantifying the hydro‐meteorological state of the flow path. Our thermo‐mechanical model enables the simulation of complex avalanches and identifies key flow transitions: powder cloud formation and potential debris flow transformation. The study underscores the pivotal role of water in avalanche dynamics and the challenge of accurately quantifying water content within the flow, necessitating comprehensive ground assessments for effective disaster management. Plain Language Summary: In the context of global climate warming, the rapid melting of glaciers and thawing of permafrost lead to rock and ice mass collapses, forming rock‐ice avalanches. These can evolve into water‐saturated debris flows, increasingly endangering societies and infrastructures in hazard‐prone regions. The 2021 Chamoli rock‐ice avalanche, which transformed into a debris flow causing over 200 fatalities, exemplifies the severe consequences of such natural disasters. To address this issue, we developed a depth‐averaged model to predict the complex dynamics characteristics of rock‐ice events. The model considers various components like rock, ice, snow, and water, along with crucial processes such as frictional heating, phase changes, material entrainment, and air‐blast hazards; and captures heat changes between phases and resultant phase transitions. Using this model, we investigated the primary water source within avalanche flows and its effects on dynamics, focusing on the 2021 Chamoli avalanche. Our findings highlight the pivotal role of water in shaping the behavior of these rock‐ice events and flow transitions. Despite significant progress, challenges remain in refining our understanding of heat transfer physics and quantifying hydro‐meteorological conditions within the flow. Nevertheless, our thermo‐mechanical model represents a significant advancement in simulating and comprehensively analyzing the dynamics of rock‐ice avalanches. Key Points: New depth‐averaged model for rock‐ice avalanches, including rock, ice, snow, and water, phases changes, ground entrainment and air blastOur model identifies three key flow transitions: powder cloud formation, debris flow transformation, and hyperconcentrated flood occurrenceModel testing shows water's impact on flow transitions. Uncertainties remain in heat transfer and entrainment calling for more validation [ABSTRACT FROM AUTHOR]

Post-wildfire debris flows are a significant natural hazard that is becoming more prevalent because of climate change and increasing populations in the wildland-urban interface. In the United States, the US Geological Survey (USGS) conducts regular assessments of debris-flow probability and volume following major wildfires, but these assessments do not include estimates of potential runout length, which is needed for identifying infrastructure potentially at risk from post-wildfire debris flows. Runout can be simulated by physically based dynamic models, but these models require detailed local site information that necessitates field measurements. As an alternative, topographically based runout models, such as the Average Channel Slope (ACS) model, can be used to estimate the likely maximum runout length of debris flows using widely available existing geospatial datasets. Here, a geographic information systems (GIS)-based modeling workflow was developed to run the ACS model for rapid assessment of post-wildfire debris-flow runout. Modeled runout length was tested against 34 observed debris flows across six US regions, with regionally averaged modeled runout ranging from 86 to 107% of observed runout. The model was then used to estimate potential runout length for 16 recent wildfires in the western US and the results were displayed in an interactive web map. The code used to run the model is freely available, allowing for rapid assessment of debris-flow hazards following wildfire. [ABSTRACT FROM AUTHOR]

Debris‐flow volumes can increase along their flow path by entraining sediment stored in the channel bed and banks, thus also increasing hazard potential. Theoretical considerations, laboratory experiments and field investigations all indicate that the saturation conditions of the sediment along the flow path can greatly influence the amount of sediment entrained. However, this process is usually not considered for practical applications. This study aims to close this gap by combining runout and hydrological models into a predictive framework that is calibrated and tested using unique observations of sediment erosion and debris‐flow properties available at a Swiss debris‐flow observation station (Illgraben). To this end, hourly water input to the erodible channel is predicted using a simple, process‐based hydrological model, and the resulting water saturation level in the upper sediment layer of the channel is modelled based on a Hortonian infiltration concept. Debris‐flow entrainment is then predicted using the RAMMS debris‐flow runout model. We find a strong correlation between the modelled saturation level of the sediment on the flow path and the channel‐bed erodibility for single‐surge debris‐flow events with distinct fronts, indicating that the modelled water content is a good predictor for erosion simulated in RAMMS. Debris‐flow properties with more complex flow behaviour (e.g., multiple surges or roll waves) are not as well predicted using this procedure, indicating that more physically complete models are necessary. Finally, we demonstrate how this modelling framework can be used for climate change impact assessment and show that earlier snowmelt may shift the peak of the debris‐flow season to earlier in the year. Our novel modelling framework provides a plausible approach to reproduce saturation‐dependent entrainment and thus better constrain event volumes for current and future hazard assessment. [ABSTRACT FROM AUTHOR]

Fire facilitates erosion through changes in vegetation and soil, with major postfire erosion commonly occurring even with moderate rainfall. As climate warms, the western United States (U.S.) is experiencing an intensifying fire regime and increasing frequency of extreme rain. We evaluated whether these hydroclimatic changes are evident in patterns of postfire erosion by modeling hillslope erosion following all wildfires larger than 100 km2 in California from 1984 to 2021. Our results show that annual statewide postfire hillslope erosion has increased significantly over time. To supplement the hillslope erosion modeling, we compiled modeled and measured postfire debris‐flow volumes. We find that, in northern California, more than 50% of fires triggering the top 20 values of sediment mass and sediment yield occurred in the most recent decade (between 2011 and 2021). In southern California, the postfire sediment budget was dominated by debris flows, which showed no temporal trend. Our analysis reveals that 57% of postfire sediment erosion statewide occurred upstream of reservoirs, indicating potential impacts to reservoir storage capacity and thus increased risk to water‐resource security with ongoing climate change. Plain Language Summary: Large amounts of soil erode following wildfire, particularly if heavy rainfall occurs on the burned area within the first year after the fire. Climate change is increasing wildfires as well as extreme rain in many regions, including California. We investigated whether documented changes in fire and climate in California in the years 1984–2021 resulted in measurable changes in the amount or patterns of sediment produced from soil erosion following wildfire. Using a combination of models and measurements, we estimated annual postfire sediment erosion statewide, and separately for northern and southern California. We found that postfire erosion increased statewide over the time frame of interest. In southern California, postfire sediment erosion is dominated by debris flows, which are a liquefied mix of rock, mud, and soil, and no clear temporal patterns are apparent. In northern California, most cases of major postfire erosion occurred recently between 2011 and 2021. Most of the sediment mass eroded postfire statewide in the years 1984–2021 occurred upstream of reservoirs, rather than in places where sediment would move downstream directly to the ocean. These results indicate increasing pressure on water resources from postfire erosion with ongoing climate change. Key Points: First multi‐decadal assessment of postfire sediment mobilization in a region with rapidly intensifying fire and hydroclimate regimesThe mass of sediment eroded from hillslopes after wildfire has increased significantly in California since the 1980sMost postfire sediment erosion occurred upstream of reservoirs, indicating a growing risk to water security [ABSTRACT FROM AUTHOR]

Debris flows can be triggered by landslides and lake outbursts, constituting cascading hazards that pose threats to life and property. However, cascading mechanisms are poorly understood due to the complex energy transfer and material transformation among precipitation, landslides, floods, and debris flows. Here, we aim to identify the cascading mechanisms and key controls of the Gaojiashan debris flow and provide insights applicable to other cascading hazards. Our study integrates field measurements and numerical simulations, combining data on runoff discharge, landslide stability, overtopping volume, and debris-flow discharge. We show that gravity-driven high-altitude water sources control the weakening and entrainment of sediment sources in cascading. Water convergence and seepage due to intense precipitation, elevate pore pressure and weaken sediment sources, triggering landslides and initiating cascading hazards. Water from the overtopped landslide impulse wave, combined with runoff, entrained wet gully bed materials, and formed the debris flow. A physical approach that accounts for the strength of wet sediment sources and the paths of unstable water release in mountains may therefore be applicable for assessing cascading hazards. This work provides a reference for reducing the losses caused by cascading hazards. [ABSTRACT FROM AUTHOR]

Multiple barriers with basal clearance are usually adopted in engineering practice to mitigate large volumes of debris flows. This approach allows for sufficient discharge and minimizes the need for cleaning and maintenance over the service life of the barriers. Despite the effectiveness, the design of basal clearance is still based on empirical methods, which can lead to uncertainty in safety. In this study, a new dimensionless overflow number Nof is proposed for designing dual rigid barriers with basal clearance and validated with physical and numerical results. The physical tests were conducted using a 28 m long flume model. A finite element model calibrated using the physical test results is utilized for numerical parametric study to investigate the effects of basal clearance on debris flow impact force against dual barriers. Physical and numerical results reveal that Nof can serve to determine whether the impact force on the second barrier is governed by basal discharge or overflow from the first barrier. A threshold value of Nof=1 can be used to design dual barriers with basal clearance to achieve the lowest impact force on the second barrier. The proposed Nof also enables engineers to predict the impact force on the second barrier by only considering the impact force from basal discharge. [ABSTRACT FROM AUTHOR]

Post‐fire flooding and debris flows are often triggered by increased overland flow resulting from wildfire impacts on soil infiltration capacity and surface roughness. Increasing wildfire activity and intensification of precipitation with climate change make improving understanding of post‐fire overland flow a particularly pertinent task. Hydrologic signatures, which are metrics that summarize the hydrologic regime of watersheds using rainfall and runoff time series, can be calculated for large samples of watersheds relatively easily to understand post‐fire hydrologic processes. We demonstrate that signatures designed specifically for overland flow reflect changes to overland flow processes with wildfire that align with previous case studies on burned watersheds. For example, signatures suggest increases in infiltration‐excess overland flow and decrease in saturation‐excess overland flow in the first and second years after wildfire in the majority of watersheds examined. We show that climate, watershed and wildfire attributes can predict either post‐fire signatures of overland flow or changes in signature values with wildfire using machine learning. Normalized difference vegetation index (NDVI), air temperature, amount of developed/undeveloped land, soil thickness and clay content were the most used predictors by well‐performing machine learning models. Signatures of overland flow provide a streamlined approach for characterizing and understanding post‐fire overland flow, which is beneficial for watershed managers who must rapidly assess and mitigate the risk of post‐fire hydrologic hazards after wildfire occurs. [ABSTRACT FROM AUTHOR]

Wildfires and their long-term impacts on the environment have become a major concern in the last few decades, in which climate change and enhanced anthropogenic activities have gradually led to increasingly frequent events of such hazards or disasters. Geological materials appear to become more vulnerable to hazards including erosion, floods, landslides and debris flows. In the present study, the well-known 2017 wildfire and subsequent 2018 debris flows in the Montecito area of California are examined. It is found that the post-wildfire debris flows were initiated from erosion and entrainment processes and triggered by intense rainfall. The significant debris deposition in four major creeks in this area is investigated. Numerical modeling of the post-wildfire debris flows is performed by employing a multi-phase mass flow model to simulate the growth in the debris flows and eventual debris deposition. The debris-flow-affected areas estimated from the numerical simulations fairly represent those observed in the field. Overall, the simulated debris deposits are within 7% error of those estimated based on field observations. A similar simulation of the pre-wildfire scenario indicates that the debris would be much less significant. The present study shows that proper numerical simulations can be a promising tool for estimating post-wildfire erosion and the debris-affected areas for hazard assessment and mitigation. [ABSTRACT FROM AUTHOR]

Shallow landslides pose a widely growing hazard and risk, globally and particularly in Mediterranean areas. The implementation of adequate adaptation and mitigation measures necessarily requires the development of practical and affordable methodologies and technologies for assessing the shallow landslides hazard and its territorial impact. The assessment of shallow landslide hazard maps involves two different and sequential steps: the susceptibility and the runout analysis, respectively, aimed at the identification of the initiation and the propagation areas. This paper describes the application in the Giampilieri and Briga Villages area (Sicily, Italy) of a shallow landslide risk process at a basin scale with an innovative approach in the runout assessment segment. The runout analysis was conducted using specific GIS tools employing an empirical–geometric approach at a basin scale. The exposure and vulnerability values of the elements at risk were assigned using a qualitative and semi-quantitative approach, respectively. The results highlight the effectiveness of the procedure in producing consistent runout hazard and risk assessments in the valley areas where the more important and vulnerable exposed elements are located. This study contributes to addressing the public administration demand for valuable and user-friendly tools to manage and drive regional planning.

Debris flow can cause damage only when its discharge exceeds the drainage capacity of the prevention engineering. At present, most rainfall thresholds for debris flows mainly focus on the initiation of debris flow and do not adequately consider the magnitude and drainage measures of debris flows. These thresholds are likely to initiate numerous warnings that may not be related to hazardous processes. This study proposes a method for calculating the rainfall threshold that is related to a defined level of debris flow magnitude, over which certain damage may be caused. This method is constructed by using the transient rainfall infiltration analysis slope stability model (TRIGRS) and the fluid dynamics process simulation model (MassFlow). We first use the TRIGRS model to analyze slope stability in the study area and obtain the distribution of unstable slopes under different rainfall conditions. Afterward, the MassFlow model is employed to simulate the movement process of unstable slope units and to predict the depositional processes at the mouth of the catchment. Lastly a rainfall threshold is constructed by statistically analyzing the rainfall conditions that cause debris flows flushing out of the given drainage ditch. This method is useful to predict debris flow events of a hazardous magnitude, especially for areas with limited historical observational data. [ABSTRACT FROM AUTHOR]

Forests, serving as natural barriers in mountainous regions, can reduce surface erosion, enhance water flow resistance and facilitate sedimentation. While extensive research has been conducted for forests and their capacity to withstand debris flows, there has been relatively little investigation into the interactions between riparian forests and debris flows. In this paper, a field investigation of Keze gully was conducted through section surveys, quadrat surveys and unmanned aerial vehicle field investigations. Based on the flow calculation results, analysis of sediment characteristics, gully morphology change and analysis of stand characteristics, the interaction model between the riparian forest and debris flow in the complex channel was discussed. The results further emphasise the role of forests in mitigating debris flows. Furthermore, the results show that the identified critical slope for the study area provides a threshold to explain two different flow interaction dynamics with riparian forests. Above the critical slope, debris flows predominantly follow the main channel, where the root-soil complex of riparian forests reduces channel bank erosion. However, this erosion can also lead to the uprooting and destruction of trees. Below the critical slope, floodplains are vital in accommodating overflow. Debris flows interact not only with the roots of trees near the riverbank but also with the trunks of trees on floodplains. Trees with a diameter less than 30 cm may also suffer damage due to broken stems. [ABSTRACT FROM AUTHOR]

Debris flows are geomorphological processes that affect the landscape evolution process of any region. In this study, an integrated methodology is proposed to identify the chance of further debris flows and quantify the similarities between debris flow locations, materials and rheology, using field and laboratory investigations and remote sensing data. The method was tested for four failure‐triggered debris flow sites in the Western Ghats of India, using dimensionless parametric similarity values ranging from 0 to 1. The maximum parametric similarity was observed as 0.84 when comparing the flow accumulation values of Sites 3 and 4, and the maximum overall site similarity was 0.68. The calibrated rheological parameters of one site were found to be satisfactory in modelling the shape of debris flow at all other sites. The findings can be used to identify similar hotspots in the region and to simulate debris flows for quantitative hazard assessment. [ABSTRACT FROM AUTHOR]

Pore‐fluid pressure (PP) plays an important role in bed erosion, but the mechanisms that control PP evolution and the resulting feedbacks on flow dynamics are unclear. Here, we develop a general formulation, allowing quantification of the propensity for PP evolution of saturated and unsaturated bed sediments. We conduct erosion experiments by systematically varying grain composition and water content of beds, for investigating effects of PP evolution on flow erosion. With increasing water content, PP shows a slight rise in deforming beds with drained behavior but significant larger rise in undrained beds. Regardless of bed composition, the erosion rate of beds presents a synchronous change tendency with PP evolution due to the loss in basal friction. PP instigates positive feedback that induces a remarkable gain of flow velocity and momentum on wet beds with undrained behavior. Our results help explain observations of volume growth and long run out of debris flows. Plain Language Summary: Debris flows are common geophysical flows consisting of debris grains and muddy water. Debris flows can grow significantly in volume and mobility as they pick up loose sediment from gully bed and banks. The destructive potential of debris flows increases with increasing flow volume and run out. This brings about great challenges for effective early warning of debris flows, design of prevention measures and mapping of hazard zones related to human settlements. It is commonly believed that flow momentum is consumed by carrying static bed sediments. However, flows can gain momentum by overriding wet bed sediments. This can be explained by pore‐pressure generation as debris flows move across wet beds. The increase of measured pore‐fluid pressure is limited for beds with a low water content, but substantial for beds with a higher water content, which strongly affects the erosion rates of bed sediments. Flow velocity and momentum on wet beds are observed to increase significantly but slightly for dryer beds as a result of the pore‐pressure feedback. These findings indicate that the debris composition of the catchment, the water content of bed sediment and the pore‐pressure development should be evaluated when making predictions on debris‐flow hazard. Key Points: Propensity for pore pressure evolution of bed sediments during debris‐flow erosion is evaluated by a Deborah numberSignificant pore pressure and accompanying intense erosion occur for wet bed sediments with undrained behaviorEnhanced pore pressure of wet bed sediments reduces flow basal friction, increasing flow mobility and runout [ABSTRACT FROM AUTHOR]

Debris flow early warning is an effective method to prevent major disasters, so a multi-index fusion debris flow early warning model based on spatial interpolation and a support vector machine is designed. Aiming at the discrete rainfall data in the study area, the collaborative Kriging spatial interpolation method based on Kriging spatial interpolation is adopted to process the rainfall data into multi-index fused surface data. The rainfall data after spatial interpolation are used as the input sample data of the support vector machine early warning model, and the optimal parameters of the support vector machine are calculated by the sea squirt algorithm, and then the debris flow early warning results are output. After experimental analysis, the model can obtain rainfall surface data. After calculation by the model, the accuracy of the early warning probability of debris flow is improved, and the early warning result is consistent with the actual result of debris flow. [ABSTRACT FROM AUTHOR]

Shallow landslides pose a widely growing hazard and risk, globally and particularly in Mediterranean areas. The implementation of adequate adaptation and mitigation measures necessarily requires the development of practical and affordable methodologies and technologies for assessing the shallow landslides hazard and its territorial impact. The assessment of shallow landslide hazard maps involves two different and sequential steps: the susceptibility and the runout analysis, respectively, aimed at the identification of the initiation and the propagation areas. This paper describes the application in the Giampilieri and Briga Villages area (Sicily, Italy) of a shallow landslide risk process at a basin scale with an innovative approach in the runout assessment segment. The runout analysis was conducted using specific GIS tools employing an empirical–geometric approach at a basin scale. The exposure and vulnerability values of the elements at risk were assigned using a qualitative and semi-quantitative approach, respectively. The results highlight the effectiveness of the procedure in producing consistent runout hazard and risk assessments in the valley areas where the more important and vulnerable exposed elements are located. This study contributes to addressing the public administration demand for valuable and user-friendly tools to manage and drive regional planning. [ABSTRACT FROM AUTHOR]

Summary: The main approach to managing the volume of sediment transported by stony debris flows routing along channels is through retention basins and open check dams, usually built in the lower reach just upstream of inhabited areas where slopes are gentler. In some cases, these measures are not sufficient to retain all the volume of sediment transported by debris flows. Works for trapping the sediments should also be placed in the upper reach of debris‐flow channels. In this area, where the channel bed is characterized by very high slopes and vertical variability over time, constructing and maintaining transversal embankments for sediment retention is difficult. In addition, they could also be susceptible to failure, potentially increasing the magnitude of the phenomenon instead of mitigating it. On the other hand, other types of works, such as solid‐body check dams or net barriers, have a decreasing efficiency over time in reducing the volume of sediment transported by debris flows. An alternative solution could be a retention basin open on the downstream side, that is, without the ending transversal embankment or berm. Therefore, it can be designated as a deposition area because the retention effect of the downstream embankment is missing, and sediment deposition only occurs due to the flatness of the basin. For sizing purposes, here, we derive two relationships between the deposition area and the volume of the deposit. These two relationships are derived from a physically based geometrical approach and empirical approach, respectively. After analyzing the morphology of the depositional processes that occurred in an existing deposition area during debris‐flow events, we test the validity of the two relationships. Overall, these can be used independently or in combination for designing a deposition area. At last, we introduce the necessary geomorphological conditions for the construction of the deposition area and guidance for its placement. [ABSTRACT FROM AUTHOR]

Abstract The Karakoram highway (KKH) is renowned for its complex natural environment and geological conditions. The climate changes drastically and directly influences the frequency and magnitude of debris flows in this region, resulting in significant casualties and economic losses. However, the risk assessment of debris flows along the KKH in the context of climate change has been rarely explored. Therefore, in this study we used the debris flow data, historical meteorological data and future climate prediction data to assess the debris flow risk of the study region during the baseline period (2009–2018), 2025s (2021–2030), 2035s (2031–2040) and 2045s (2041–2050) under the Representative Concentration Pathway 8.5 scenario. The results show that the risk of debris flows increases with climate change, with the highest risk level in the 2025s. Among different parts of this highway, the upper reaches of the Ghez River and the second half of Tashkorgan-Khunjerab are the sections with the highest risk. These findings are helpful for debris flow prevention and can offer coping strategies for the existing line of the KKH. They also provide some reference for the renovation, improvement, operation, and maintenance of the KKH.

Abstract Pore‐fluid pressure (PP) plays an important role in bed erosion, but the mechanisms that control PP evolution and the resulting feedbacks on flow dynamics are unclear. Here, we develop a general formulation, allowing quantification of the propensity for PP evolution of saturated and unsaturated bed sediments. We conduct erosion experiments by systematically varying grain composition and water content of beds, for investigating effects of PP evolution on flow erosion. With increasing water content, PP shows a slight rise in deforming beds with drained behavior but significant larger rise in undrained beds. Regardless of bed composition, the erosion rate of beds presents a synchronous change tendency with PP evolution due to the loss in basal friction. PP instigates positive feedback that induces a remarkable gain of flow velocity and momentum on wet beds with undrained behavior. Our results help explain observations of volume growth and long run out of debris flows.

The Uttarakhand Himalaya is highly prone to geo-hydrological disasters—landslides, flash floods, cloudbursts, debris flows, rock falls, extreme weather, and land submergence. Cloudbursts and glacier bursts generally trigger these disasters, which are very active and frequent in the Uttarakhand Himalaya and occur mainly during the monsoon season. This study examines the geo-hydrological disasters in the Uttarakhand Himalaya, which occurred from 2020 to 2023 mainly during the three months of Monsoon season. Further, the study assesses the magnitude of the disaster and maps the disaster hotspot areas. Both qualitative and quantitative approaches were used to conduct this study. Data on geo-hydrological hazards that occurred from 2020 to 2023 in Uttarakhand were gathered from multiple sources such as the records of the State Disaster Management Department, Dehradun, various media reports—print and electronic, and personal observation of some affected areas through field visits. Further, an empirical study of the Bandal and Song River valleys was conducted. These valleys were severely hit by a cloudburst-triggered flash flood and debris flow. Household-level survey of the damage caused by huge flash floods and debris flows in three villages of Bandal Valley was carried out. The data were analyzed and mapped and the Uttarakhand Himalaya was divided into disaster hotspots and frequency zones. ArcGIS was used to construct maps and the Google Earth imagery and Sentinel 1-SAR were the major sources of maps. This study recommends that the construction of settlements, infrastructural facilities, and economic avenues should be banned in the most disaster-affected areas, along river valleys, and on fragile slopes. Eco-disaster risk reduction, such as conserving forests and wetlands, and plantation drives in the most degraded areas such as the river banks and fragile slopes, can help communities to prepare for disasters. [ABSTRACT FROM AUTHOR]

The stratigraphic architecture of fjords is complicated due to the delicate interplay between ice dynamics, sediment supply, relative sea‐level fluctuations and slope failures. Glaciogenic sediment is prone to failure and to be carried downslope to the fjord floor through the entire spectrum of mass movements and subaqueous density flows, as the unstable paraglacial submarine landscape moves towards stability. Palaeofjords formed by Gondwanan glaciers during the late Palaeozoic Ice Age contain a compelling record of gravitational resedimentation in fjord depositional systems. This paper showcases the geomorphology and depositional history of a glacial cycle in the Orutanda fjord in north‐western Namibia as an example of an overdeepened fjord basin fill dominated by products of subaqueous gravitational processes. During glaciation, the Orutanda glacier carved a 20 km long by 3.7 km wide glacial trough that embodies an overdeepened basin. Ice thickness during terminal glacial occupation of the fjord is estimated to had been up to 200 m based on the fjord geomorphology. The progressive retreat of the tidewater glacier, concomitant with marine flooding, increased accommodation space in the overdeepened basin during deglaciation. During this stage, proglacial sedimentation through iceberg rafting and settling of turbid plumes was outpaced by intense paraglacial downslope resedimentation of glacially‐transported debris. Successive failures from the fjord walls and downslope resedimentation resulted in coalescing debrite–turbidite lobes on the fjord floor. Slide deposits, composed entirely of deformed debrites and turbidites, indicate that these resedimented facies were prone to renewed mass wasting. As the Orutanda glacier melted, the fjord experienced the axial progradation of a fjord‐head delta registered only by turbidites and slide deposits derived from its collapse. The Orutanda fjord sheds light on the relevance of paraglacial mass wasting in overprinting glaciogenic deposits. This insight is key to understanding the role of glaciers versus non‐glacial processes in producing the glacial deep‐time record. [ABSTRACT FROM AUTHOR]

The loose material in the mountain gullies is prone to form serious debris flow disasters under appropriate hydrodynamic conditions. The various compositions of these loose materials often lead to very large differences in the runout distances and the inundated areas. However, the mechanisms of the influence of material composition on the deposition characteristics of debris flow remain unclear, and studies are lacking on the influence of material composition on the deposition characteristics of runoff-generated debris flows. This study conducted experiments using different water discharges, flume slopes, and grain size distributions. The deposition processes of debris flows were observed, and their deposition patterns were explored. The effects of material composition on flow regimes, runout distances, inundated areas, and deposit geometries are revealed. The results showed that there are three types of debris flow, and their deposition patterns are different. The relationships of the Bagnold number and Friction number concerning the volume solid concentration were exponential. The mean mobility angle, a function of the upstream terrain slope, was established to assess the runout distance of debris flows with different flow regimes. Additionally, a criterion for identifying debris flow types was established by combining dimensionless runout distance and dimensionless slope. This study not only improved the observation of experimental phenomena in the deposition process of runoff-generated debris flows but also deepened the understanding of the mechanism of the influence of material composition on the deposition characteristics. [ABSTRACT FROM AUTHOR]

The study area is located in the Ulaanbaatar basin and is characterized by complicated environmental conditions determined by the peculiarities of an extremely continental climate with high rainfall intensity, a structural–tectonic pattern, the geological structure of the Ulaanbaatar intermountain depression, high seismic activity reaching 8 points, and the occurrence of catastrophic flash floods. In the conditions of rapid urban expansion, the high population density and housing development of Ulaanbaatar, and the implementation of large investment projects, it is an urgent task to assess the exogeodynamic hazard of the Ulaanbaatar catchment basin and develop the concept of geoecological safety of the territory. These methods and approaches consist of combining a complex of methods of engineering geology and geomorphology: monitoring the dynamics of exogenous processes; environmental assessment; the analysis of development factors (climatic, geomorphologic, tectonic, and lithological) and interaction of processes; laboratory methods of analyzing the composition, structure, and properties of soils; a detailed morphometric analysis of catchment areas; and the application of the basin approach. Based on the analysis of topographic maps on a scale of 1 : 50 000 and of digital relief models in GIS programs, the parameters of the drainage network of the catchment areas and of energy and basin geometry were calculated and reflected on map layers, and specialized maps were compiled. A detailed assessment of erosion-prone soils (composition, structure, and specific properties) revealed a moderate resistance of sediments to geodynamic impacts. As a result, maps of flood hazard of catchments under different rainfalls—short intensive heavy rainfalls and long-term rains—were compiled. The elaborated concept of geoecological safety of territories includes successive stages of works aimed at (a) identifying the exogeodynamic hazard, (b) the organization and implementation of monitoring works, (c) assessing geoecological safety with respect to hazardous exogenous geological processes, and (d) the development of recommendations for further use and specification of information. [ABSTRACT FROM AUTHOR]

In many catchments prone to debris flows, prevention structures such as check dams and retention basins have been installed to prevent debris flows from impacting the nearby infrastructures. The SPH model adopting the Herschel–Bulkley–Papanastasiou (HBP) constitutive law has shown good potential in modeling the interaction between debris flow and prevention structures. However, the accuracy of this model is not fully satisfactory when modeling the deposition process of debris flow, because the original HBP law is a viscoplastic model which does not consider frictional dissipation. Therefore, in this paper, we proposed a novel SPH model for analyzing the interaction between debris flow and prevention structures, by incorporating a modified HBP law with frictional dissipation into the original SPH model. The proposed model is validated by column collapse and flume benchmark experiments first and then utilized to analyze a real debris flow and its interaction with the prevention structures in the Cancia catchment in northern Italian Alps. The results of the column collapse experiment show that our model exhibits a better performance in simulating the collapse process compared with the original SPH model, and the simulation results of the sand flume test illustrate that the proposed model can accurately predict the impact force of debris flow on the prevention structure. The simulation results of the Cancia debris flow demonstrate that the check dams can dramatically diminish the discharge and the frontal flow velocity of the debris flow, and the peak impact force of debris flow generally decreases with gentler channel slope. Furthermore, various prevention structures show different interaction mechanisms with debris flows: the flat deposition platform mainly dissipates the kinetic energy of the flow, the check dam mainly reduces the peak discharge of the debris flow and intercepts the debris mass, and the retention basin at the outlet contributes to the deposition of debris flow. The proposed novel SPH model is helpful for guiding the optimization design of multiple prevention structures in debris flow gullies. [ABSTRACT FROM AUTHOR]

Runoff‐generated debris flows are a potentially destructive and deadly response to wildfire until sufficient vegetation and soil‐hydraulic recovery have reduced susceptibility to the hazard. Elevated debris‐flow susceptibility may persist for several years, but the controls on the timespan of the susceptible period are poorly understood. To evaluate the connection between vegetation recovery and debris‐flow occurrence, we calculated recovery for 25 fires in the western United States using satellite‐derived leaf area index (LAI) and compared recovery estimates to the timing of 536 debris flows from the same fires. We found that the majority (>98%) of flows occurred when LAI was less than 2/3 of typical prefire values. Our results show that total vegetation recovery is not necessary to inhibit runoff‐generated flows in a wide variety of regions in the western United States. Satellite‐derived vegetation data show promise for estimating the timespan of debris‐flow susceptibility. Plain Language Summary: Debris flows caused by excessive surface‐water runoff during intense rainfall can be a deadly and destructive hazard in mountainous areas after wildfire. In some cases, debris flows have only occurred in the burned area in the weeks to months after the fire, while, in other cases, debris flows occurred over several years. Though the recovery of vegetation is important for stabilizing sediment and reducing debris‐flow likelihood, uncertainty remains about how much recovery is needed to inhibit debris flows and about how much time is needed to reach this level of recovery. Knowing for how long debris flows are likely to be a hazard is important for managing risks to residents and infrastructure. To investigate this issue, we assembled a data set of 536 debris flows from the western United States and used satellite‐derived vegetation data to calculate the recovery condition of the burned area when each debris flow occurred. We found that the vast majority of the debris flows initiated when the burned area had not yet reached two‐thirds of its prefire vegetation condition. Burned areas that were slower to recover tended to experience debris flows over more protracted timescales. Key Points: Majority (>98%) of western United States postfire debris flows occurred when leaf area index was less than 2/3 of typical prefire valuesTotal recovery of vegetation not necessary to inhibit debris flowsRemotely sensed postfire vegetation state useful to evaluate elevated debris‐flow susceptibility with time [ABSTRACT FROM AUTHOR]

Featured Application: This study presents an overview of the several mitigation measures, including several check dams and flexible net barriers constructed in a typical Alpine torrential catchment in northern Slovenia (Europe) in order to protect infrastructure (i.e., ski resort cabin lift station, access road to the station, downstream settlements) against sediment-related disasters due to torrential hazards, such as flash floods and debris floods, debris flows, and shallow landslides. This newly introduced combination of technical countermeasures in Slovenia withstood the extreme floods in August 2023. Different sediment-related disasters due to torrential hazards, such as flash floods, debris flows, and landslides, can occur in an Alpine torrential catchment. When protecting infrastructure and human lives, different structural and non-structural protection measures can be used to mitigate permanent and future risks. An overview of the mitigation measures constructed near the Krvavec ski resort in northwest Slovenia (Central Europe) is presented. In May 2018, an extreme debris flood occurred in this area, causing significant economic damage. After the May 2018 event, different field investigations (i.e., geological and topographic surveys) and modeling applications (e.g., hydrological modeling, debris flow) have been conducted with the purpose of preparing the required input data for the design of protection measures against such disasters in future—due to climate change, more disasters are expected to happen in this torrential watershed. The mitigation includes the restoration of local streams, the construction of a large slit check dam for sediment retention, the construction of several smaller check dams and the construction of 16 flexible net barriers with an estimated ~8000 m3 retention volume for controlling in-channel erosion in steep torrential streams. Additionally, in order to observe and monitor potential future extreme events, an extensive monitoring system has been established in the investigated area. This monitoring system will cover measurements of flexible net corrosion, the estimation of concrete abrasion at check dams, periodical geodetic surveys using small drones (UAV), hydro-meteorological measurements using rainfall gauges and water level sensors. The recent extreme floods of August 2023 also hit this part of Slovenia, and this combination of technical countermeasures withstood the event and prevented large amounts of coarse debris from being transported to the downstream section and devastating infrastructure, as was the case in May 2018 during a less extreme event. Therefore, such mitigation measures can also be used in other torrential catchments in the Alpine environment. [ABSTRACT FROM AUTHOR]

This dataset collects tabular and geographical information about all hydrogeological disasters (landslides and floods) that occurred in Italy from 2013 to 2022 that caused such severe impacts as to require the declaration of national-level emergencies. The severity and spatiotemporal extension of each emergency are characterized in terms of duration and timing, funds requested by local administrations, funds approved by the national government, and municipalities and provinces hit by the event (further subdivided between those included in the emergency and those not, depending on whether relevant impacts were ascertained). Italian exposure to hydrogeological risk is portrayed strikingly: in the covered period, 123 emergencies affected Italy, all regions were struck at least once, and some provinces were struck more than 10 times. Damage declared by local institutions adds up to EUR 11,000,000,000, while national recovery funds add up to EUR 1,000,000,000. The dataset may foster further research on risk assessment, econometric analysis, public policy support, and decision-making implementation. Moreover, it provides systematic evidence helpful in raising awareness about hydrogeological risks affecting Italy. Dataset: See annex in Supplementary Materials. Dataset License: CC-BY 4.0. [ABSTRACT FROM AUTHOR]

Adaptation decision-making in mountain regions necessitates dealing with uncertainties which are driven by the complex topography and the potential interconnections of stochastic events. Such events can lead to amplifying consequences for the exposed communities located at different elevations. In this study, we present a stylized application of stochastic dynamic programming for local adaptation decision-making for a small alpine community exposed to debris flows and floods. We assume that local decision-makers and planners aim at maximizing specific objectives by choosing from a feasible set of adaptation measures and under given constraints on these actions. Our results show that stochastic dynamic programming is a promising tool to address the underlying problem faced by local planners when evaluating the feasibility and effectiveness of adaptation measures. Furthermore, stochastic dynamic programming has some advantages compared to deterministic approaches which assume full knowledge of the system of interest in a world dominated by randomness. We provide an estimation of a best option and an appropriate metric to benchmark adaptation effectiveness for long time horizons. We show how multiple constraints, risk preferences, time horizons and decision periods all influence the decision-making and the overall success of adaptation responses over time. [ABSTRACT FROM AUTHOR]

The Karakoram highway (KKH) is renowned for its complex natural environment and geological conditions. The climate changes drastically and directly influences the frequency and magnitude of debris flows in this region, resulting in significant casualties and economic losses. However, the risk assessment of debris flows along the KKH in the context of climate change has been rarely explored. Therefore, in this study we used the debris flow data, historical meteorological data and future climate prediction data to assess the debris flow risk of the study region during the baseline period (2009–2018), 2025s (2021–2030), 2035s (2031–2040) and 2045s (2041–2050) under the Representative Concentration Pathway 8.5 scenario. The results show that the risk of debris flows increases with climate change, with the highest risk level in the 2025s. Among different parts of this highway, the upper reaches of the Ghez River and the second half of Tashkorgan-Khunjerab are the sections with the highest risk. These findings are helpful for debris flow prevention and can offer coping strategies for the existing line of the KKH. They also provide some reference for the renovation, improvement, operation, and maintenance of the KKH. [ABSTRACT FROM AUTHOR]

Three strong storms that occurred on July 21, 2012, July 20, 2016, and July 16, 2018 caused severe flooding and debris flows in the mountain areas of Beijing. The detailed records of these storms and the actual occurrence time of the debris flows provide an opportunity for evaluating the thresholds derived from different types of rainfall intensities. Herein, a new rainfall threshold of debris flows is derived from the real-time rainfall intensities in Beijing. In addition, the thresholds are estimated based on the average rainfall intensities over the interval from the beginning of the rainfall to the debris flow occurrence and over the entire rainfall duration. The results show that the various types of rainfall thresholds display significantly different capabilities in terms of separating the storms with positive debris-flow response from those with negative debris-flow response, and the real-time rainfall threshold exhibits the best performance. Moreover, our data indicate that the debris flows in Beijing are triggered by the combined work of rainfall intensity and cumulative precipitation. A debris flow is initiated only when the cumulative precipitation and rainfall intensity simultaneously reach a threshold level.

Debris flows are rapid mass movements with a mixture of rock, soil and water. High-intensity rainfall events have triggered multiple debris flows around the globe, making it an important concern from the disaster management perspective. This study presents a numerical model called debris flow simulation 2D (DFS 2D) and applicability of the proposed model is investigated through the values of the model parameters used for the reproduction of an occurred debris flow at Yindongzi gully in China on 13 August 2010. The model can be used to simulate debris flows using three different rheologies and has a user-friendly interface for providing the inputs. Using DFS 2D, flow parameters can be estimated with respect to space and time. The values of the flow resistance parameters of model, dry-Coulomb and turbulent friction, were calibrated through the back analysis and the values obtained are 0.1 and 1000 m/s2, respectively. Two new methods of calibration are proposed in this study, considering the cross-sectional area of flow and topographical changes induced by the debris flow. The proposed methods of calibration provide an effective solution to the cumulative errors induced by coarse-resolution digital elevation models (DEMs) in numerical modelling of debris flows. The statistical indices such as Willmott's index of agreement, mean-absolute-error, and normalized-root-mean-square-error of the calibrated model are 0.5, 1.02 and 1.44, respectively. The comparison between simulated and observed values of topographic changes indicates that DFS 2D provides satisfactory results and can be used for dynamic modelling of debris flows.

The geology of the Himalayas is intricated and intriguing. It features numerous tectonic bodies and structures too complex to interpret. Along with such mysteries it has too many common geohazards, such as landslides. In this study, a detailed geological investigation is carried out to overcome the discrepancies in structural interpretation, the nature of two crystalline bodies, and non-uniformity in geological mapping in the central Himalayan arc, in the Jajarkot district of Nepal. Along with the geological exploration and landslide characterization of the area, consequent landslide susceptibility mapping is performed considering 13 causative factors related to geology, topography, land use, hydrology, and the anthropogenic factor, using two bivariate statistical models. This study concludes that the two metamorphic crystalline bodies in the study area are most probably the klippen, due to the absence or erosion of the root zone. The field study revealed that haphazard road excavation without the consideration of geological and geotechnical features has caused shallow landslides. The results obtained from the susceptibility maps, with a varying range of susceptibility zones, are in good agreement with the spatial distribution of pre-historic landslides. The results of the susceptibility modeling are validated by calculating landslide density and plotting area under curves (AUC). The AUC value for the WOE, and the FR method, revealed an overall success rate of 79.42% and 77.62%, respectively.

Abstract Runoff‐generated debris flows are a potentially destructive and deadly response to wildfire until sufficient vegetation and soil‐hydraulic recovery have reduced susceptibility to the hazard. Elevated debris‐flow susceptibility may persist for several years, but the controls on the timespan of the susceptible period are poorly understood. To evaluate the connection between vegetation recovery and debris‐flow occurrence, we calculated recovery for 25 fires in the western United States using satellite‐derived leaf area index (LAI) and compared recovery estimates to the timing of 536 debris flows from the same fires. We found that the majority (>98%) of flows occurred when LAI was less than 2/3 of typical prefire values. Our results show that total vegetation recovery is not necessary to inhibit runoff‐generated flows in a wide variety of regions in the western United States. Satellite‐derived vegetation data show promise for estimating the timespan of debris‐flow susceptibility.

In the Campania region (southern Italy), sloping pyroclastic surfaces in partially saturated conditions are frequently affected by flow-like landslides and shallow slip. In this context, safeguarding human life is entrusted to early warning systems (EWSs), typically based on empirical precipitation intensity-duration thresholds that may suffer from missed and false alarms. Indeed, the consequences of heavy rainfall depend on the hydraulic conditions in the soil before the rainstorm. If this aspect is neglected, precipitation intensity-duration thresholds can fail. However, although the current hydraulic condition in the soil is a discriminating factor, it is rarely measured and taken into account. In this regard, the present study focuses on the assessment of the role of the groundwater conditions for the initiation of shallow landslides, and it may be the first step towards the implementation of an innovative early warning system. At this aim, a numerical approach to reproduce the current hydraulic condition in the slope is presented. This study is based on the back-analysis of two debris flows that occurred in the Campania region by means of uncoupled hydromechanical numerical modelling. The stratigraphy and hydromechanical characteristics of the soil layers are known due to surveys and investigations carried out at an experimental test site set up a few kilometres from the two debris flows that occurred. The results show that in the slopes examined, the hydraulic state in the soil before landslide triggering was correctly reproduced through an infiltration analysis starting a few months before, in which rainfall recorded on site was imposed as an entering flow at ground level. Moreover, it was shown that the water storage in the entire loose soil cover was an effective indicator of the average hydraulic state of the slope; more than local variables, such as matric suction and water content. [ABSTRACT FROM AUTHOR]

Debris flows are a hazard in mountainous regions. Cost‐effective, long‐term studies of debris‐flow torrents, however, are rare, leading to uncertainties in hazard assessment and hazard prevention. Here, we address the question of whether cost‐effective remote sensing techniques can be applied for assessment of mountain torrents and possibly further gather accurate, long‐term information on the evolution of the catchment. Torrents prone to debris flows are often devoid of vegetation in the near channel area and hence can be well captured with photogrammetrically derived methods using uncrewed aerial vehicle (UAV) surveys. The possibility of automatically extracting specific torrent parameters from high‐resolution terrain models, such as cross‐section area or gradient, is investigated. The presented methodology yields continuous and automatically derived geometrical parameters such as torrent bed width, inclination and cross‐section area, which is a major advantage compared with point‐based, often dangerous field surveys. Their cross‐validation with field measurements shows strong agreement. Those parameters are accurate along sharply incised sections with strong limitations along sections with steep adjacent slopes and/or dense vegetation. The information along the torrent allows fast identification of key sections and weak spots which can be precisely evaluated in the field. The study highlights that proper classification of real ground points poses the key challenge. We show that photogrammetric routines to derive a high‐resolution digital terrain model (DTM) are limited in the case of dense vegetation coverage. In such cases, LiDAR surveys have clear advantages even though they are also limited by very dense vegetation. We find that UAV data can be used for an objective method of estimating debris‐flow torrent geometric properties. And the introduced approaches therefore build a stepping stone towards a more comprehensive, reproducible and objective assessment of torrent processes and predispositions. However, ground‐referencing fieldwork remains essential, and further research on remote sensing supported hazard assessment of debris‐flow‐prone torrents is indispensable. [ABSTRACT FROM AUTHOR]

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The 2019 Museum Fire burned in a mountainous region near the city of Flagstaff, AZ, USA. Due to the high risk of post‐fire debris flows and flooding entering the city, we deployed a network of seismometers within the burn area and downstream drainages to examine the efficacy of seismic monitoring for post‐fire flows. Seismic instruments were deployed during the 2019, 2020, and 2021 monsoon seasons following the fire and recorded several debris flow and flood events, as well as signals associated with rainfall, lightning and wind. Signal power, frequency content, and wave polarization were measured for multiple events and compared to rain gauge records and images recorded by cameras installed in the study area. We use these data to test the efficacy of seismic recordings to (a) detect and differentiate between different energy sources, (b) estimate the timing of lightning strikes, (c) calculate rainfall intensities, and (d) determine debris flow timing, size, velocity, and location. We then calculate forward models of seismic signals associated with debris flows and rainfall to better interpret our results and characterize these events. Our observations and modeling show that we can differentiate between these sources and that seismic data can provide insight into post‐fire debris flow characteristics, including relative particle sizes and velocity. Plain Language Summary: Wildfires are a growing hazard as the size and frequency of high‐severity fires are growing globally. Following containment, post‐fire flooding and landslides can put downstream communities at risk, particularly as communities expand within the wildland‐urban interface and close to fire‐prone mountains. In this work, we use seismic instruments to measure ground vibrations created by rainfall, lightning, landslides and floods as they move downstream and compare these recordings to game camera photos and rainfall records. This data set allows us to detect and better understand hazards in post‐fire areas. Observations from these instruments show a high potential for detecting these events and the validity of using seismic data as a tool for understanding debris flow behavior. Key Points: Rainfall, wind, and lightning can be detected and distinguished with seismic dataSeismic observations and analysis are effective for constraining debris flow characteristics such as velocity in post‐wildfire environmentsTheoretical models of debris flows provide a framework to evaluate the sensitivity of seismic data to changing debris flow characteristics [ABSTRACT FROM AUTHOR]

Empirical/geometric methods rely on simple geometrical connections between some landslide parameters and the runout distance reached by the displaced material. Despite the extreme simplification of the dynamic of this landslide typology, those methods can provide useful information about the propagation of this shallow and fast landslide typology, joining the reliability of the results with easiness of use. The objective of this work is to compare the efficacy of different geometric relationships for the identification of the runout distances in a debris- and mud-flows-prone test area located in Sicily, southern Italy, where several events were analyzed, and a consistent set of data was collected and processed. Notwithstanding some uncertainties in the methodological approach and not negligible scattering between expected and observed runout distances, the use of such geometric approaches, together with the evaluation of kinematic parameters such as velocity and kinetic energy, can significantly boost the implementation of site-specific analyses for a more detailed definition of landslides susceptibility at a local scale. [ABSTRACT FROM AUTHOR]

Alpine mass wasting events have impacts that extend past their headwater origins, sometimes reaching populated lowlands. Understanding the processes driving these sediment pulses, and how they contribute to basin‐scale sediment fluxes, is important for hazard assessment and aquatic habitat management. The Suiattle River, which drains Glacier Peak stratovolcano in Washington State, is a dominant contributor of suspended sediment in the region. Normalized for drainage area, it supplies more suspended sediment than nearly any other river in the area and more than twice as much as the White Chuck River, which drains the opposite flank of the volcano. Despite its importance to the regional sediment budget, geomorphic processes in the basin have received relatively little attention in the literature. In this study, we build on previous work to explore the magnitude, timing and triggering mechanisms of sediment loading events in the basin. We find that outburst flood‐triggered debris flows from Chocolate Glacier are of widely varying magnitude and coincide with high temperatures in the late summer. Major debris flow activity initiated in the late 1930s, with at least eight valley‐filling debris flows since then. Smaller, more recent debris flows, likely also driven by outburst floods, occur in five of seven years of complete data. In total, the small debris flows and the subsequent autumn flushing events explain ~21% of the 'anomalous' sediment load in the basin, while reworking and abrasion of the historic events may explain another ~26%. We speculate that some of the remaining unexplained 'anomalous' load could be the result of a feedback between channel lateral instability (originally triggered by the valley‐spanning debris flows) and bluff erosion. [ABSTRACT FROM AUTHOR]

Shallow water equations (SWEs) have been traditionally derived by integrating fundamental flow equations over a flow profile above a single point in a horizontal or nearly horizontal plane, with the main assumptions that the profile thickness is much smaller than other two dimensions and it contains only water. This paper presents the derivation of generalized SWEs (GSWEs) obtained for a finite plan area, allowing for the presence of phases other than water, such as air, grains, vegetation, and debris, which can be either stationary or mobile. The derivation provides a rigorous basis for various applications of layer-averaged models and opens numerous research questions, some of which are highlighted in the paper. [ABSTRACT FROM AUTHOR]

Debris flows can grow dramatically in volume and mobility as they override bed sediment due to the reduction in bed friction resistance caused by high pore fluid pressure (PP). However, the mechanisms that control PP evolution in overridden bed sediment are still unclear. Here, a new mathematical model clarifies how diverse styles and magnitudes of PP evolution can result from regulation of the flow diffusion and shear contraction of bed sediment. Normalization of the model equations shows that the propensity for PP generation depends on timescales of PP diffusion and deformation of the bed grains. The PP of saturated bed sediment under immobile conditions is equal to the fluid pressure at the bottom of the flow due to a non‐flux basal boundary. However, the PP of unsaturated bed sediment is lower than that of the overlying flow. PP diffusion from a debris flow into an unsaturated bed increases with the bed's permeability and water content. The shear deformation behavior changes from undrained to drained with increasing permeability or decreasing shear velocity of the saturated bed sediment, leading to a reduced magnitude of pore pressure. By contrast, the shear deformation transitions from drained to undrained behavior with increasing permeability and water content of unsaturated bed sediment. The entrainment rate and erosion pattern of bed sediment are closely related to the PP evolution and liquefaction ratio of the bed sediment due to Coulomb‐friction shear tractions. Our models can be used to interpret the feedback of PP on the flow momentum during debris flow entrainment. [ABSTRACT FROM AUTHOR]