Debris Flow Hazard Research Articles

Towards a better understanding of debris flow sediment sources: Monitoring of an active rock slope at Spitze Stei, Switzerland

Rapid process cascades can lead to destructive debris flows. Identifying and characterizing the processes conditioning debris-flow occurrence will strongly contribute to the mitigation of debris-flow hazards. In recent years, the rock slope near “Spitze Stei”, in the Kandersteg region, Switzerland, has exhibited elevated displacement rates exceeding 10 cm per day, suggesting a growing instability up to 20 million m3. The accumulated sediments at the bottom of the Spitze Stei slope are mobilized as debris flows by melting snow and heavy summer precipitations. Here, we use seismology combined with an intelligent algorithm to automatically detect rockfall and landslides at the Spitze Stei rock slope. These mass movements act as primary sediment sources delivering sediments to a debris-prone channel. Our initial results quantify mass movement activity before two debris flow events that occurred in 2022 and identify their triggers. Such analysis can contribute towards mitigating debris flow hazards and extending warning time, especially for debris flows triggered by factors other than precipitation.

Mitigation Strategy for Low-frequency Large-magnitude Debris Flows in Hong Kong

Potential hazards arising from low-frequency large-magnitude debris flows have become apparent under the adverse impact of climate change and extreme weather. Hong Kong has systematically managed landslide risks arising from man-made slopes and natural hillsides since 1977, owing to its unique geographic location, steep terrain, deep weathering profile and post-war rapid urban development. In recent years, evaluation of potential hazards arising from low-frequency large-magnitude debris flows and their possible dire consequences has developed into a key part of the landslide mitigation effort in Hong Kong in light of the occurrence of more frequent and more severe extreme rainfall events. Valuable experience is gained in identifying potential low-frequency large-magnitude debris flow hazards, assessing their potential impacts to the public located at the densely populated foothills and developing a holistic strategy to mitigate against potentially catastrophic consequences. This paper sets out the mitigation strategy for mitigating low-frequency large-magnitude debris flows in Hong Kong and gives a case study to demonstrate the approach.

Integration of meteorology and geomorphology for enhanced understanding of post-fire debris-flow hazards

Through precipitation, the fields of meteorology and geomorphology are fundamentally linked, thus interdisciplinary efforts are needed to advance understanding and warning of rainfall-driven geohazards. With a focus on recent efforts specific to post-fire debris flows in California, our presentation provides an overview of the benefits and challenges of working in an interdisciplinary team of meteorologists and geomorphologists, as well as results of a recent project demonstrating advancement through the integration of these fields. In this project, we combine high-resolution ensemble precipitation forecasts with post-fire debris-flow models to explore the feasibility and potential value of providing probabilistic post-fire debris-flow hazard information over a burn scar. In sharing these examples, we emphasize the multi-benefit nature of these efforts, and encourage future interdisciplinary efforts that improve warning and mitigation of rainfall-driven geohazards.

Monitoring and modelling of debris-flow erosion in the Rebaixader catchment (Pyrenees, Spain)

Debris flow and other torrential processes can entrain large volumes of sediments along their runout path. Since the debris-flow hazard strongly depends on the volume, the research on the entrainment is relevant and has been analysed by multiple approaches in the past. In this study, the erosion volume due to torrential processes has been monitored in the Rebaixader catchment (Spain) by digital elevation models obtained from Uncrewed Aerial Vehicle (UAV) surveys and by the instrumental monitoring system installed in the channel reach. In addition, the entrainment of debris flows has been modelled by the numerical code FLATModel. The results of both approaches show that debris flows in the studied catchment are characterised by a large entrainment and the assumption of landslide-triggered debris flows has been refuted. Important erosion in the gullies of the initiation area has been detected by the UAV data and the numerical modelling. An average annual erosion volume of about 6100 m3/y has been determined.

Predicting burn severity for integration with post-fire debris-flow hazard assessment: a case study from the Upper Colorado River Basin, USA

Background Burn severity significantly increases the likelihood and volume of post-wildfire debris flows. Pre-fire severity predictions can expedite mitigation efforts because precipitation contributing to these hazards often occurs shortly after wildfires, leaving little time for post-fire planning and management. Aim The aim of this study was to predict burn severity using pre-fire conditions of individual wildfire events and estimate potential post-fire debris flow to unburned areas. Methods We used random forests to model dNBR from pre-fire weather, fuels, topography, and remotely sensed data. We validated our model predictions against post-fire observations and potential post-fire debris-flow hazard estimates. Key results Fuels, pre-fire weather, and topography were important predictors of burn severity, although predictor importance varied between fires. Post-fire debris-flow hazard rankings from predicted burn severity (pre-fire) were similar to hazard assessments based on observed burn severity (post-fire). Conclusion Predicted burn severity can serve as an input to post-fire debris-flow models before wildfires occur, antecedent to standard post-fire burn severity products. Assessing a larger set of fires under disparate conditions and landscapes will be needed to refine predictive models. Implications Burn severity models based on pre-fire conditions enable the prediction of fire effects and identification of potential hazards to prioritise response and mitigation.

Assessment of Physical Vulnerability and Uncertainties for Debris Flow Hazard: A Review concerning Climate Change

Global climate change has increased severe torrential hazards, particularly debris flows in mountainous regions. After floods and earthquakes, debris flows are the most devastating natural hazard in the world. The effects of debris flow on human life and built environments necessitate reconsidering current infrastructure planning, engineering, and risk management practices. Hence, the vulnerability of elements at risk is critical for effective risk reduction systems. Therefore, this article reviews the existing physical vulnerability approach of infrastructure, particularly building toward debris flow hazards in the last 20 years. Furthermore, uncertainties associated with the vulnerability assessment and their quantification techniques have also been discussed in detail. It was found that matrices, curves, and indicators have been extensively used for vulnerability assessment approaches in the last two decades. However, if aleatory and epistemic uncertainties are not quantified or conserved in the vulnerability assessment process, it makes the system inefficient and unreliable. Moreover, data requirements, strengths, and weaknesses of approaches presented herein are highlighted with case studies. Finally, a thorough discussion on future needs in the field of risk assessment methodologies is highlighted by considering uncertainties into account.

A multi-step hazard assessment for debris-flow prone areas influenced by hydroclimatic events

Hazard assessment studies are fundamental to identifying disaster-prone areas, especially in locations with high environmental and socioeconomic vulnerability. This study proposes a multi-step debris-flow hazard assessment, based on the combination of Logistic Regression (LR) analysis, numerical simulation and rainfall back-analysis. A landslide-prone area of 84 km2 is chosen as test-site, including 20 river catchments and one of the largest petrochemical plants in Latin America. Rainfall is the main influencing factor in debris-flow initiation, as highlighted by the LR analysis, followed by soil cover and slope. The analysis also indicated the catchments more susceptible to debris flows and the simulation results show that the average runout distance in these catchments is 470 m, with an average peak flow height of 5 m and a peak velocity of 23 m s−1. Debris flows are triggered by short duration (<48 h), high-intensity (>200 mm) precipitation, with return periods that vary from 3 to 10 years. Five levels of hazard (very low to very high) are, then, proposed for the study site, based mainly on 48-h accumulated rainfall and flow properties. Industrial and residential areas in the projected debris-flow route generally exhibit the highest overall hazard levels, as many were developed in the depositional area of debris flows and near fluvial courses, where associated floods and flash floods may occur. As pointed out by recent studies, an increase in the frequency of extreme precipitation events is projected in the Serra do Mar region and when the general short return period of the debris-flow triggering rainfall is considered (<10 years), large magnitude (>105 m3) debris flows are likely to occur in the near future.

Characteristics, causes, and risk reduction of a catastrophic debris flow hazard on 05 July 2021 at the Xiangjiao Gully of Muli County, China

Characteristics, causes, and risk reduction of a catastrophic debris flow hazard on 05 July 2021 at the Xiangjiao Gully of Muli County, China

Effects of Bed Hydrophobicity on Post‐Fire Debris Flow Entrainment and Momentum Growth

AbstractForest wildfires leave vast areas with burned topsoil that is water repellent, susceptible to erosion by surface runoff and a potential source of debris flow. To mitigate post‐fire debris flow hazards, debris resisting barriers are usually constructed in the flow path. However, the fundamental interaction mechanisms of debris flow on a water repellent bed and the momentum exchange process that governs the destructive potential of debris flows have yet to be elucidated. This study investigates the influence of bed sediment hydrophobicity on entrainment by debris flows and the impact force on a downstream barrier using a series of physical flume tests. Bed sediment with increasing contact angles (50°–130°) were used to model bed hydrophobicity from wettable to hydrophobic conditions, and volumetric water content (0%–30%) to simulate the natural unsaturated condition after rainfall in fire‐burned hillslopes. Compared to a wettable bed, a hydrophobic bed exhibits a failure pattern with slab‐by‐slab entrainment and en masse failure with a sixfold increase in the average erosion depth. A new dimensionless number is proposed to quantify the effects of contact angle on soil erosion. The flow momentum increases up to 21% after entrainment of hydrophobic bed compared to non‐erodible bed. The peak impact force on the downstream barrier can increase up to 80% for hydrophobic bed as a consequence of momentum gains after entrainment. The hydrodynamic coefficient for estimating impact force reveals the inadequacy of current design criteria of debris resisting barriers for fire‐prone vegetated hillslopes, suggesting further investigations are needed.

Debris Flow Hazard Assessment in District Chitral, Eastern Hindu Kush, Pakistan

The aim of this study is debris flow risk zonation using geological and hydrometeorological indicators in district Chitral, Hindu Kush Region Northwest Pakistan. The research is based on secondary data. Multi criteria Analysis (MCA) in Geographical Information System (GIS) environment was used to achieve the objective of the study. The geological and hydrometeorological parameters were analyzed by making five classes of each parameter. The classes are ranked as most favorable and least favorable with numerical weights. The weights were assigned in accordance to their importance in debris flow occurrence. Then weighted overlay analysis techniques were applied to develop composite map representing the importance of each factor. Debris flow risk zonation map was resulted into four classes very high risk zones, high risk zone, moderate zone, low risk zone. The geology of the study area is diverse with frequents earthquakes. Similarly the forest cover is decreasing due to anthropogenic activities. The area is also characterized by long cold winters with frost action. These factors are destabilizing the slope. During summer season rain storm event results high surface runoff and peak discharge in the perennial and non-perennial channels which results flood and debris flow. These events result human life loss and disruption. The main villages located in very high risk zone are Mulkoh, Mastuj, Reshun, Shegram, Terich Gol, Rogar, Asurat, Boni, Brep and Rech Tockhow. They have been frequently affected by hazard in the past decade. Out of the total area, very high risk zone is expanded over 8%, high risk zone is expanded over16%, moderate risk zone is 29% and the rest is low risk zone. This study has highlighted the risk zones which will help disaster management authorities and policy makers to reduce the risk of debris flow in future.

Debris Flow Hazard Assessment in District Chitral, Eastern Hindu Kush, Pakistan

Debris Flow Hazard Assessment in District Chitral, Eastern Hindu Kush, Pakistan

A Study on the Potential Vulnerability of Debris Flow Hazard in Sukabumi Regency

Debris-flow hazards are a hydrometeorological disaster that often occurs in parts of Indonesia. The intensity of Debris-flow hazards increases Debris-flow hazards are a Debris-flow Debris-flow hazards are a hydrometeorological disaster that often occurs in parts of Indonesia. The intensity of Debris-flow hazards increases in various parts of Indonesia in line with climate change and environmental degradation. Sukabumi Regency has a diverse topography with a relatively high annual rainfall rate of 2,805 mm/year with 144 rainy days. In 2020, Sukabumi Regency was affected by Debris-flow hazards in three Districts. In a study by the National Disaster Management Agency (BNPB), Sukabumi District is prone to land movements. There has been no study of Debris-flow hazards in the Sukabumi Regency area. This study aims to determine regions of the Sukabumi Regency that have the potential for Debris-flow hazards. It is hoped that the study results will also complement the disaster studies that BNPB has made. The method used is a weighted multi-criteria analysis. The parameters used are rainfall, location of the 2020 flood, slope, land cover, altitude, and soil type. The results showed that the level of vulnerability is very prone to be located in the northern part of Sukabumi.

Influence of the modern climatic setting in the mountains of Central Asia on the state of glaciation, glacier runoff and glacial lake outbursts

The climatic setting of Central Asia in the second half of the 20th century and in the first decade of the 21st century is characterized by a change of horizontal and vertical temperature gradients at the western periphery of the Central Asian Mountain system. The growth of summer air temperatures led to the decrease of mountain glaciations. In the last 45 years the glaciers in selected catchments of Gissar-Alay lost about 16% in area while the glaciers of the Pskem river catchment lost 27% in area. The river regimes, primarily the annual distribution of runoff react to the glacier decrease. The share of glacial runoff is over 25% in years with little precipitation, while it considerably decreases in years with abundant precipitation. One of the negative consequences of the glacier decrease is the formation of lakes in the periglacial area. The state of glacial lakes should be monitored, because they may pose outburst debris flow hazard.

A Scalable Framework for Post Fire Debris Flow Hazard Assessment Using Satellite Precipitation Data

AbstractWildfire is a global phenomenon that has dramatic effects on erosion and flood potential. On steep slopes, burned areas are more likely to experience significant overland flow during heavy rainfall leading to post fire debris flows (PFDFs). Previous work establishes methods for PFDF hazard assessment, often relying on regional‐scale parameterizations with in‐situ rainfall measurements to categorize hazard as a function of meteorological and surface properties. We present a globally scalable approach to extend the benefit these models provide to new areas. Our new model relies on publicly available satellite‐based inputs with a global extent to provide first order hazard assessments of recently burned areas. Our results show it is possible to identify the conditions relevant for PFDF‐initiation processes across a variety of physiographic settings. Improvements to satellite‐borne rainfall intensity data and increased availability of PFDF occurrence data worldwide are expected to enhance model skill and applicability further.

Debris-Flow Channel Headwater Dynamics: Examining Channel Recharge Cycles With Terrestrial Laser Scanning

Debris-flows present a natural hazard to the safe operation of linear infrastructure in mountainous environments. The most significant contributor to debris-flow occurrence is a supply of readily erodible material, often created by rockfalls and other shallow landslides. The spatial distribution and total volume of storage are also critical factors, controlling the initiation location, predominant flow type, and termination location of debris-flow surges. Therefore, there is a need to be able to systematically incorporate debris recharge processes and timeframes into the monitoring and characterization of debris-flow hazards. In this work, the authors present the results of 7 years of terrestrial laser scanning (TLS) captured at the White Canyon. The White Canyon represents an analog to large scale, steep catchments to investigate the role of sediment supply on debris-flow processes. The TLS dataset was collected at monthly to quarterly intervals, providing a basis for analysis of debris transfer processes occurring on the study slope. A rockfall database of over 72,000 events was generated from 52 change detection analyses and is linked to catchment recharge and transfer processes. The results indicate that the 17 channels analyzed in the White Canyon do not directly match the conceptual models proposed from the supply theory. The channels display a variety of behaviors when exposed to the same climate signature. The temporal data acquisition rate was found to have a significant influence on the dynamics of movement that can be interpreted from TLS change detection analysis. The work highlights the need for higher frequency monitoring and the integration of climate data into the analysis, in order to better understand these dynamic processes.

Experimental study on the mitigation effects of deflection walls on debris flow hazards at the confluence of tributary and main river

Experimental study on the mitigation effects of deflection walls on debris flow hazards at the confluence of tributary and main river

Risk Assessment of Debris Flow in a Mountain-Basin Area, Western China

Debris flow risk comprehensively reflects the natural and social properties of debris flow disasters and is composed of the risk of the disaster-causing body and the vulnerability of the carrier. The Bailong River Basin (BRB) is a typical mountainous environment where regional debris flow disasters occur frequently, seriously threatening the lives of residents, infrastructure, and regional ecological security. However, there are few studies on the risk assessment of mountainous debris flow disasters in the BRB. By considering a complete catchment, based on remote sensing and GIS methods, we selected 17 influencing factors, such as area, average slope, lithology, NPP, average annual precipitation, landslide density, river density, fault density, etc. and applied a machine learning algorithm to establish a hazard assessment model. The analysis shows that the Extra Trees model is the most effective for debris flow hazard assessments, with an accuracy rate of 88%. Based on socio-economic data and debris flow disaster survey data, we established a vulnerability assessment model by applying the Contributing Weight Superposition method. We used the product of debris flow hazard and vulnerability to construct a debris flow risk assessment model. The catchments at a very high-risk were distributed mainly in the urban area of Wudu District and the northern part of Tanchang County, that is, areas with relatively dense economic activities and a high disaster frequency. These findings indicate that the assessment results provide scientific support for planning measures to prevent or reduce debris flow hazards. The proposed assessment methods can also be used to provide relevant guidance for a regional risk assessment of debris flows in the BRB and other regions.

Analysis of Debris Flow Triggering Conditions for Different Rainfall Patterns Based on Satellite Rainfall Products in Hengduan Mountain Region, China

Rainfall is the main factor that induces debris flow. Satellite rainfall products provide a new source of data in terms of debris flow-triggering conditions to overcome the lack of rainfall data coverage from ground-based rainfall gauges in large-scale mountainous regions. In this study, the applicability of four satellite rainfall products (CMORPH, GPM, MSWEP, and PERSIANN) in the Hengduan Mountain region (HMR) was evaluated with reference to ground observation data from 2000 to 2020. The critical rainfall and rainfall thresholds under different rainfall patterns and warning levels that trigger debris flows were analyzed according to the empirical cumulative distribution function (ECDF) and cumulative probability. The results showed that CMORPH (comprehensive indicator score (CI = 0.72) and GPM (CI = 0.70) performed better in the simulation of daily rainfall sequence consistency and extreme rainfall conditions in the study area. CMORPH also had the highest reconstruction rate for correctly capturing rainfall events that triggered debris flows, with a value of 89%. Approximately half of the rainfall patterns that cause debris flows are antecedent-effective-rainfall-dominated. Both intraday-rainfall-dominated and intraday-antecedent-rainfall-balanced patterns were below 30%. There were evident differences in the critical rainfall for different rainfall patterns under the same warning level. By comparing the results of previous studies on rainfall thresholds, it is believed that the results of this study confirm the application of satellite rainfall products; in addition, the calculated rainfall thresholds can provide a reference for the early warning of debris flows in the HMR. In general, this work is of great significance to the prediction and early warning of debris flow hazards.

A progressive flow-routing model for rapid assessment of debris-flow inundation

Debris flows pose a significant hazard to communities in mountainous areas, and there is a continued need for methods to delineate hazard zones associated with debris-flow inundation. In certain situations, such as scenarios following wildfire, where there could be an abrupt increase in the likelihood and size of debris flows that necessitates a rapid hazard assessment, the computational demands of inundation models play a role in their utility. The inability to efficiently determine the downstream effects of anticipated debris-flow events remains a critical gap in our ability to understand, mitigate, and assess debris-flow hazards. To better understand the downstream effects of debris flows, we introduce a computationally efficient, reduced-complexity inundation model, which we refer to as the Progressive Debris-Flow routing and inundation model (ProDF). We calibrate ProDF against mapped inundation from five watersheds near Montecito, CA, that produced debris flows shortly after the 2017 Thomas Fire. ProDF reproduced 70% of mapped deposits across a 40 km2 study area. While this study focuses on a series of post-wildfire debris flows, ProDF is not limited to simulating debris-flow inundation following wildfire and could be applied to any scenario where it is possible to estimate a debris-flow volume. However, given its ability to reproduce mapped debris-flow deposits downstream of the 2017 Thomas Fire burn scar, and the modest run time associated with a simulation over this 40 km2 study area, results suggest ProDF may be particularly promising for post-wildfire hazard assessment applications.

Torrential Hazard Prevention in Alpine Small Basin through Historical, Empirical and Geomorphological Cross Analysis in NW Italy

Debris flow is one of the most dangerous natural processes in mountain regions and it occur in a wide variety of environments throughout the world. In the Italian Alps, some tens of thousands of damaging debris flow and, in general, torrential floods associated to intense sediment transport in secondary catchments have been documented in the last 300 years. These have caused socio-economic damage, damage to anthropogenic structures or infrastructures and in many cases casualties. Often, in the same basins, the occurrence of debris-flow processes recurs many years later. Prediction can often be spatial and based on the magnitude of the largest known process, while the temporal forecast is the most uncertain. It is also possible to increase the resilience of the population and of the territory. The present study aims at investigating different levels of debris-flow hazard in urban areas on Alpine alluvial fans and proposes a strategy for debris-flow prevention based on historical research and on a simplified analytical approach, methods that also involve relatively low costs. For such analysis, Ischiator stream catchment (ca. 20 km2) and its alluvial fan (NW Italy) were selected. This area was partly affected by historical torrential flood associated to intense sediment transport and debris-flow processes. Present-day instability conditions along the slope and the stream network were detected and synthesized through surveys and aerial photo interpretation integrated by satellite images (period 1954–2021). An estimation of the potential amount of moving detritus, referred to as debris flow, was carried out regarding the June 1957 debris-flow event, based on the predictive models. The individual hazard index value was estimated based on different methods. The results indicate that 56% of the area is exposed to flood associated to intense sediment transport hazard, which fluctuates from high to very high levels; such results are supported by debris-flow historical records. Since today almost half of the settlement (Bagni di Vinadio) is located on potentially risk-exposed areas, the urban evolution policy adopted after the 1957 event failed to manage the risk connection to debris-flow activity.