Linking rainfall-induced landslides with debris flows runout patterns towards catchment scale hazard assessment

Debris flows are frequently occurring natural processes in geologically complex terrains of the Indian Himalayas. Debris flow runout modelling leading to hazard assessment is essential for planning, designing, and execution of mitigation measures. In the present context, debris flow hazard assessment has been carried out for the Tangni debris flow in Garhwal Himalayas, India. Runout modelling was carried out using a Voellmy model-based 3D numerical simulation for estimation of flow intensity parameters such as runout distance, flow velocity, height and pressure along the propagation path. For calibration of the model inputs, back analysis of Tangni debris flow event that occurred in 2013, with its known runout length and deposition volume as the criteria, has been conducted. The best calibrated values of frictional parameters are obtained at μ = 0.10 and ξ = 400 m/s2. Using the best calibrated values of frictional parameters, hazard assessment was carried out for two potential release areas separately, with different initial volumes, and in combination to derive the probable runout distance along with other flow intensity parameters for different scenarios that may happen in the future. It has been observed that this debris flow scenario including the combination of two potential release areas will block the Alaknanda River, forming a landslide dam with a probable height of 6 m. Debris flow runout modelling-based hazard assessment will be helpful in determining quantitative information on flow intensity parameters, where complete data on past events are generally not available or were not possible to capture for the Indian Himalayan region.

Predicting debris flow runout is of major importance for hazard mitigation. Apart from topography and volume, runout distance and area depends on debris flow composition and rheology, but how is poorly understood. We experimentally investigated effects of composition on debris flow runout, depositional mechanisms, and deposit geometry. The small‐scale experimental debris flows were largely similar to natural debris flows in terms of flow behavior, deposit morphology, grain size sorting, channel width‐depth ratio, and runout. Deposit geometry (lobe thickness and width) in our experimental debris flows is largely determined by composition, while the effects of initial conditions of topography (i.e., outflow plain slope and channel slope and width) and volume are negligible. We find a clear optimum in the relations of runout with coarse‐material fraction and clay fraction. Increasing coarse‐material concentration leads to larger runout. However, excess coarse material results in a large accumulation of coarse debris at the flow front and enhances diffusivity, increasing frontal friction and decreasing runout. Increasing clay content initially enhances runout, but too much clay leads to very viscous flows, reducing runout. Runout increases with channel slope and width, outflow plain slope, debris flow volume, and water fraction. These results imply that debris flow runout depends at least as much on composition as on topography. This study improves understanding of the effects of debris flow composition on runout and may aid future debris flow hazard assessments.

The predictive hazard analysis at a detailed scale for debris flow runout analysis can be improved significantly through reliable estimation of the input parameters. In this study, a method for database establishment of input parameters at a site-specific scale was laid out for the predictive-based debris flow hazard assessment under extreme rainfall. The adoption of the DAN-3D code necessitated the estimation of three main input parameters: initial volume, bulk basal frictional angle, and growth rate. The initial volume was assessed using a 3D coupled finite element seepage and limit equilibrium-based slope stability analysis. An artificial neural network-based model was developed using 27 debris flow events for predicting the basal bulk frictional angle and consisted of eight factors: plan curvature, profile curvature, percentage of fine content, D50, initial unit weight, initial volume, relative relief ratio, and channel length. Finally, the growth rate was estimated using the previously assessed initial volume, soil depth, and the approximate runout length. The proposed method was validated by application to the Raemian slope in the Woomyeon mountain region, Seoul, for the extreme rainfall event of 27 July 2011. The analysis yielded a final volume of 53,067.9 m3, a velocity upon arrival on the road of 26.81 m/s, and an approximately 0.5-m debris thickness concentrated near the Raemian apartments. The comparison of the predicted debris flow path and debris flow velocity with the actual event demonstrates good similarity and provides a conservative estimate of the volume. This study therefore illustrates the importance of an input parameter database in providing a reliable debris flow runout hazard assessment.

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. Plain Language SummaryWildfires and increased rainfall are causing more dangerous postfire debris flows around the world.These are most likely within the first year after a fire, sometimes even while the fire is ongoing.Recognizing this, it's important to assess these hazards before a fire starts to minimize damage.Current methods for predicting where and how debris flow will happen after a fire are not used often enough, mainly because they take a lot of time and there are many uncertainties involved.Our study proposes a new approach using a model to predict debris flow behavior before a fire occurs.This model, tested with data from northern Arizona, helps us understand factors like the mud's "stickiness" which influences how far and fast it travels.We also improve the way we estimate model parameters by incorporating uncertainties from real-world observations.Finally, we use this model to foresee the impact of debris flow in different fire scenarios that could happen in the future.This work not only gets us better prepared for postfire debris flow but also offers guidance on how to use these models in future hazard assessments.

Due to their potentially long runout, debris flows are a major hazard and an important geomorphic process in mountainous environments. Understanding runout is therefore essential to minimize risk in the near‐term and interpret the pace and pattern of debris flow erosion and deposition over geomorphic timescales. Many debris flows occur in forested landscapes where they mobilize large volumes of large woody debris (LWD) in addition to sediment, but few studies have quantitatively documented the effects of LWD on runout. Here, we analyze recent and historic debris flows in southeast Alaska, a mountainous, forested system with minimal human alteration. Sixteen debris flows near Sitka triggered on August 18, 2015 or more recently had volumes of 80 to 25 000 m3 and limited mobility compared to a global compilation of similarly‐sized debris flows. Their deposits inundated 31% of the planimetric area, and their runout lengths were 48% of that predicted by the global dataset. Depositional slopes were 6°–26°, and mobility index, defined as the ratio of horizontal runout to vertical elevation change, ranged from 1.2 to 3, further indicating low mobility. In the broader southeast Alaskan region consisting of Chichagof and Baranof Islands, remote sensing‐based analysis of 1061 historic debris flows showed that mobility index decreased from 2.3–2.5 to 1.4–1.8 as average forest age increased from 0 to 416 years. We therefore interpret that the presence of LWD within a debris flow and standing trees, stumps, and logs in the deposition zone inhibit runout, primarily through granular phenomena such as jamming due to force chains. Calibration of debris flow runout models should therefore incorporate the ecologic as well as geologic setting, and feedbacks between debris flows and vegetation likely control the transport of sediment and organic material through steep, forested catchments over geomorphic time. © 2020 John Wiley & Sons, Ltd.

Epiclastic volcanic debrites-evidence of flow transformations between avalanche and debris flow processes, Middle Ordovician, Baie Verte Peninsula, Newfoundland, Canada

Abstract. The occurrence of debris flows has been recorded for more than a century in the European Alps, accounting for the risk to settlements and other human infrastructure that have led to death, building damage and traffic disruptions. One of the difficulties in the quantitative hazard assessment of debris flows is estimating the run-out behavior, which includes the run-out distance and the related hazard intensities like the height and velocity of a debris flow. In addition, as observed in the French Alps, the process of entrainment of material during the run-out can be 10–50 times in volume with respect to the initially mobilized mass triggered at the source area. The entrainment process is evidently an important factor that can further determine the magnitude and intensity of debris flows. Research on numerical modeling of debris flow entrainment is still ongoing and involves some difficulties. This is partly due to our lack of knowledge of the actual process of the uptake and incorporation of material and due the effect of entrainment on the final behavior of a debris flow. Therefore, it is important to model the effects of this key erosional process on the formation of run-outs and related intensities. In this study we analyzed a debris flow with high entrainment rates that occurred in 2003 at the Faucon catchment in the Barcelonnette Basin (Southern French Alps). The historic event was back-analyzed using the Voellmy rheology and an entrainment model imbedded in the RAMMS 2-D numerical modeling software. A sensitivity analysis of the rheological and entrainment parameters was carried out and the effects of modeling with entrainment on the debris flow run-out, height and velocity were assessed.

A physically-based multi-hazard risk assessment platform for regional rainfall-induced slope failures and debris flows

Debris flows are typically saturated mixtures of debris grains and interstitial slurry consisting of water and clay. Pore pressure in slurry plays a crucial role in the characteristic behavior and runout of debris flows. However, the mechanisms that cause pore pressures to diffuse are still uncertain. Here we report consolidation tests to investigate the effects of debris composition (uniformly graded and widely graded) and slurry density on the dissipation of pore pressure. Pore pressures of debris flows were found to dissipate at contrasting rates ranging from a few seconds to dozens of hours and dissipation time strongly depended on slurry density. We observed an abrupt change in pore-pressure dissipation time at a critical slurry density. We propose that a critical pore throat controls the permeability of a debris material and regulates the pore-pressure dissipation based on percolation theory. This hypothesis is verified by pore-network models from a micro-computed tomography analysis with high resolution. The critical pore throat of the uniformly graded debris material is larger than that of widely graded debris material due to the difference in the porosity. The permeabilities and hydraulic diffusivities of debris flows significantly decrease once critical pore throats are blocked, resulting in a change of pore-pressure dissipation. Critical slurry density is approximately linearly correlated with the porosity of debris flows. Our results highlight opportunities to use micro-structural properties for interpreting debris-flow behavior.

Gravitational mass movements, such as debris and mud flows, are among the most destructive natural hazards, leading to substantial fatalities and extensive economic damage worldwide. Improving our understanding and modeling of these processes is essential for developing effective risk management and early warning strategies. When debris and mud flows pass through a curved channel, centrifugal forces may cause a difference in flow height between the inner and outer banks of the channel. This height difference, known as superelevation, can be described using analytical models that establish a relationship between the superelevation height and the flow velocity.Analytical models often employ a forced vortex approach, incorporating parameters such as the cross-sectional slope of the flow surface, flow width, and bend radius. These models, however, rely on assumptions such as a linear flow surface between mud deposits on the banks, a rectangular cross-section, and neglect both complex rheological behaviors and solid-fluid interactions. As a result, an empirically determined correction factor is required within the formula. The absence of a clear mechanical rationale for this correction factor presents challenges, as it is currently derived only through field investigations and laboratory experiments.This study presents an enhancement to the existing forced vortex approach by incorporating insights from numerical modeling. A coupled SPH-DEM numerical model is employed, where DEM particles represent coarse solid particles, and SPH accounts for the fluid phase, comprising fines and water. The SPH-DEM coupling is based on the no-slip interaction model, with simulations performed using a GPU-based solver to ensure enhanced computational efficiency. To validate the approach, a parametric test is conducted, initially back-calculating laboratory-scale experiments. The study further involves varying the water content in debris and mud flows to examine its impact on flow behavior and superelevation. Larger water contents lead to an increased superelevation angle. Results from the parametric test reveal a clear correlation between water content and the flow surface shape in curved channels. Specifically, mud flows are characterized by convex upward surface shapes, whereas more granular debris flows typically exhibit concave downward shapes.The distribution of material within the cross-section of the flow is governed by the equilibrium between boundary forces and centrifugal forces acting on the flow, which directly influences the superelevation. Numerical investigations are conducted to determine a correction factor and assess the extent to which inertial effects contribute to this correction factor for different material mixtures. Furthermore, we demonstrate that the effect of the flow surface shape is significant and is currently only accounted for by the empirical correction factor. This study offers new physical insights for the back-calculation of debris flow velocities in curved sections marked by mud deposits. Large-scale SPH-DEM simulations of a real debris flow event at Illgraben (Switzerland) are performed, showing good agreement with field data and its potential for further real-scale modeling.

We analyzed multi-time satellite images of the Central Caucasus glacial zone and interpreted more than thirty rock avalanche events in the 21st century with a total damage area of more than 25 km2 (including the collapse zone of the Kolka Glacier disaster). The highest rock and rock-ice avalanche activity is detected in the section of The Greater Caucasus range (northern and southern slopes) with a length of about 20 km between the Bashkara and Kulaktau peaks (16 rock avalanches) and in the section of the Kazbek-Dzhimaray Massif (series of rock avalanches to the surface of Kolka, Suatisi and Devdoraki glaciers). The feature of the rock and ice-rock avalanches is the large runout distance. For 12 events (about 40%) the distance was more than 2000 m. One ice-rock avalanche from the Mount Kazbek (excluding the Kolka Glacier disaster in 2002) reached the runout distance more than 10 km. In some areas, the rock avalanches occurred several times. In particular, a large number of avalanches were in the cirque of the Kolka Glacier; the last of them at the end of 2019. Thrice шт each case, rock avalanches originated from Mount Bashkara, in the cirques of the Murkvam Glacier, the East Shtulu Glacier, and the Devdoraki Glacier. Ice and rock avalanches were the initial stage of the complex process of the Kolka Glacier disaster and following catastrophic glacial debris flow in the Genaldon/Gizeldon River valley in 2002. Also, they were causes of glacier surges, formation of dammed lakes, and debris flows. As a result of the collapse of the hanging glacier and bedrock, the former right tributary of the Kolka Glacier surged to 200 m in 2006. Ice-rock avalanche from Mount Kazbek in 2014 load up the former right tributary of the Devdoraki Glacier and caused its advancing in 2015–2019, at a distance of more than 400 m. The avalanches caused catastrophic debris flows in the Amilishka/Kabakhi River valley in 2014, the Mestiachala River valley in 2019. Rock avalanches can cause outbursts of lakes and debris flows. Two dammed lakes formed as a result of the rock avalanche from the cirque above the Seri Glacier in the Tviberi River valley of the in May 2016. The lakes (total area was more than 0.05 km2) have outburst at the end of August 2017 after heavy rains. Rock avalanches of the 20th century led to an abrupt deceleration in the retreat of the Yusengi, Bartuytsete, East Shtulu and Mosota glaciers. The formation of rock avalanches in the 21st century took place at high altitudes (an average of about 3900 m). Possibly, the reason was associated with an increase of the «0» isotherm and of the high border of the zone of intense frost weathering due to climate warming. Some rock avalanches in the section of the Kazbek-Dzhimarai Massif have been caused by endogenous factors (seismicity and volcanism).

A probabilistic model of debris-flow delivery to stream channels, demonstrated for the Coast Range of Oregon, USA

Mountain catchments are highly sensitive to the impacts of global warming, which affects seasonal weather patterns, glacier retreat, permafrost thawing, and snow cover duration. These changes drive rapid transformations in their ecosystems and alter their hydrological and sedimentological regime, exacerbating their susceptibility to various hazards, including floods, shallow landslides, and debris flows, all closely tied to sediment dynamics. Consequently, sediment management plays a key role in developing watershed management strategies that lead to programme interventions for mitigating potential losses for the mountain communities.In this context, an integrated approach combining field surveys and aerial imagery analysis is essential for evaluating the effectiveness of existing countermeasures (mainly torrent control structures) and for finding innovative solutions, especially in absence of sediment transport monitoring systems. This approach enables the collection of observations on lithology, geology, channel cross-section shape, longitudinal profiles, land use, active soil movements, and grain size distribution within sediment source areas and along the channel network. The field inspections of torrent control structures further provide a detailed assessment of their condition and functionality.All these observations are essential for geomorphological approaches, statistical formulae, and hybrid methods to estimate potential debris flow volumes at both reach and catchment scales. Additionally, simplified rainfall-runoff modelling, such as the SCS-CN method, is employed to assess critical runoff thresholds that could trigger water-sediment flows. The outputs include the spatially distributed assessments of in-channel and hillslope sediment storage volumes, and the delineation of sediment source areas. Aerial imagery complements this process by verifying the spatial distribution and extent of sediment source areas and tracking land cover changes over time.The proposed methodology was developed and applied to the Rovina Torrent basin, located in the Central Alps (Lombardy, North Italy). The basin is characterized by coniferous forest cover (37.2%) and extensive debris accumulations and lithoid outcrops (36.7%). In the study case, the results provide the typologies in terms of driving the triggering event mode within the catchment, the minimum critical rainfall for designing an early warning system, and the potential debris flow volume for adjusting the sediment trapping basin.The outcome significantly enhances the accuracy of hazard mitigation strategies and supports the adaptation or redesign of the torrent control structures to better address evolving sediment dynamics. The combination of the scientific experience in similar mountain context and the additional field observations can quantitatively provide a robust diagnosis of the current scenario for planning maintenance operations, for proposing and designing alternative solutions to reduce the natural hazard and for effectively supporting the decision-making process including the strategic allocation of human and financial resources.

Abstract. Many shallow landslides and debris flows are precipitation initiated. Therefore, regional landslide hazard assessment is often based on empirically derived precipitation intensity-duration (ID) thresholds and landslide inventories. Generally, two features of precipitation events are plotted and labeled with (shallow) landslide occurrence or non-occurrence. Hereafter, a separation line or zone is drawn, mostly in logarithmic space. The practical background of ID is that often only meteorological information is available when analyzing (non-)occurrence of shallow landslides and, at the same time, it could be that precipitation information is a good proxy for both meteorological trigger and hydrological cause. Although applied in many case studies, this approach suffers from many false positives as well as limited physical process understanding. Some first steps towards a more hydrologically based approach have been proposed in the past, but these efforts received limited follow-up. Therefore, the objective of our paper is to (a) critically analyze the concept of precipitation ID thresholds for shallow landslides and debris flows from a hydro-meteorological point of view and (b) propose a trigger–cause conceptual framework for lumped regional hydro-meteorological hazard assessment based on published examples and associated discussion. We discuss the ID thresholds in relation to return periods of precipitation, soil physics, and slope and catchment water balance. With this paper, we aim to contribute to the development of a stronger conceptual model for regional landslide hazard assessment based on physical process understanding and empirical data.

Heavy rainfall brought by typhoons has been recognised as a major trigger of landslides in Taiwan. On average, 3.75 typhoons strike the island every year, and cause large amounts of shallow landslides and debris flow in mountainous region. Because landslide occurrence strongly corresponds to the storm dynamics, a reliable typhoon forecast is therefore essential to landslide hazard management in Taiwan. Given early warnings with sufficient lead time, rainfall-induced shallow landslide forecasting can help people prepare disaster prevention measures. To account for inherent weather uncertainties, this study adopted an ensemble forecasting model for executing precipitation forecasts, instead of using a single-model output. A shallow landslide prediction model based on the infinite slope model and TOPMODEL was developed. Considering the detailed topographic characteristics of a catchment, the proposed model can estimate the change in saturated water table during rainstorms and then link with the slope-instability analysis to clarify whether shallow landslides can occur in the catchment.Two areas vulnerable to landslide in Taiwan were collected to test the applicability of the model for landslide prediction. Hydrological data and landslide records derived from 15 typhoons events were used to verify the applicability of the model. Three indices, namely the probability of detection (POD), false alarm ratio (FAR), and threat score (TS), were used to assess the performance of the model. The results indicated that for landslide prediction through the proposed model, the POD was higher than 0.73, FAR was lower than 0.33, and TS was higher than 0.53. The proposed model has potential for application in landslide early warning systems to reduce loss of life and property.