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

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 physically-based distributed cell model for predicting regional rainfall-induced shallow slope failures

Shallow slope failures are one of the most frequent geological hazards in mountainous areas, which pose great danger to people and properties in affected areas. This paper presents a cell-based platform for quantitative risk assessment for shallow slope failures at a regional scale. There are five components in this platform; namely, a digital terrain module, a spatial rainfall distribution module, a slope stability and reliability evaluation module, a movement prediction module, and a risk assessment module. The locations and volumes of the slope failures are estimated using the slope stability and reliability evaluation module. The movement prediction module is used to predict the potential movement traces of the detached material, which moves along the steepest path. The risks of the slope failures are finally assessed using the risk assessment module, which can be used as an indicator for early warning. The vulnerability of a person to slope failure is determined by the volume of slope failure and angle of reach. The platform is applied to a 164.5 km2hilly terrain in the Wenchuan earthquake zone to test its performance in quantitative risk assessment for regional shallow slope failures. The platform evaluates the risks of shallow slope failures at a regional scale efficiently.

Debris flows represent one of the most prevalent and impactful natural hazards in mountainous areas, posing significant risks to both human life and infrastructure. Alpe di Succiso (2017 m a.s.l.) located in Northern Apennines, Italy, is an area where numerous occurrences of debris flows have been identified in this area encompassed by a National Park, thus densely traversed by touristic routes and infrastructures. Understanding the spatial and temporal patterns of these debris flows is critical for assessing the hazard and managing the associated safety risks for mountaineers, hikers, tourists, and, more generally, for the communities and infrastructure in the area.This research integrates field observations of debris flows, dendrochronological analysis, geomorphological mapping (Rashid et al., 2024), and satellite imagery to reconstruct the history of debris flow events and is partially comprised in the DECC project (2023). A key focus is the 1987 debris flow, triggered by an intense rainfall event on August 25, which recorded 179 mm of rainfall in a single day, including 133 mm within a 6-hour period.This study investigates the dynamics of debris flows, through channel-specific analysis, GIS-based zonation, and statistical evaluation. Slope angle and elevation data were analyzed to delineate source, transport, and deposition zones across four channels of debris flows. Channel 1 was identified as a debris flood channel, while Channels 2, 3, and 4 exhibited typical debris flow characteristics.To account for the wide variation in grain sizes in the study area (0.0005 mm to 5 meters), four techniques were employed. Sieve analysis was used for grains between 2 mm and 32 mm, while laser granulometry measured finer particles below 2 mm. Direct field measurement was applied to intermediate grains (32 mm to 1000 mm), and particle counting was used for large particles above 1 meter. This multi-method approach ensured accurate representation of sedimentary material across the broad grain size spectrum.Geomorphological analysis indicates that rockfalls and rock weathering significantly contribute to the material on the slopes. During debris flow events, these deposits are triggered at the first stage like debris/rock slide and then as flow creating channels and deposits. A geological survey of rock outcrops feeding these debris flow channels revealed that the Rock Quality Designation (RQD) ranges from 44 to 65 (poor to fair quality), while the Rock Mass Rating (RMR) falls between 45 and 56 (fair quality).Using the RAMMS Debris Flow software (RAMMS, 2017), a scenario-based modeling approach was employed to better understand the interactions between debris flow and material constituting the slope deposits. Simulation results are currently being compared with the geometry of levees and lobes to refine the model and ensure accuracy. This comprehensive approach aims to improve the understanding of debris flow dynamics and the influence of rockfalls, thereby aiding hazard assessment and management.

<p>Debris flows are among the most widespread natural hazards in mountain areas. They play an important role in sediment transfer from low order channel networks to downstream areas and, thus, they can pose a serious threat to infrastructure. For debris flows to be released, in addition to a considerable amount of rainfall, sediment availability and a peculiar morphological setting (e.g. inclined terrain) are required. However, areas prone to trigger debris flows are not always structurally connected to the main channel, and thus do not necessarily provide sediments to the draining channel network. Despite the relatively high number of published research in the fields of debris flow susceptibility mapping and sediment connectivity analyses, to our knowledge, no attempts have yet been made to combine these two aspects, explicitly by means of data-driven procedures.</p><p>This work builds upon a novel data-driven approach developed by Steger et al. (under review), which allows to categorize areas based on both aspects, debris flow release susceptibility and structural connectivity. The original method was tested within three mountain catchments in the South Tyrolean Alps (max area 140 km²). This study aims at upscaling this approach at the regional scale and to this end, the whole South Tyrolean Dolomites (1,120 km²) has been selected as study area. In summary, the methodical framework comprised (i) modelling of debris flow release susceptibility using an interpretable machine learning algorithm (generalized additive model) and derivation of a quantitative threshold (susceptible vs. not-susceptible areas), (ii) training a logistic regression model to calculate the probability of an area being structurally connected in terms of debris flow release and subsequent thresholding (connected vs. disconnected areas), and (iii) combination of both previously generated and validated binary maps to create a joint susceptibility-connectivity map.</p><p>The results are represented via a variety of maps and associated statistics. It is shown that only 1.7% of the Dolomites were determined to be both susceptible to debris flow initiation and structurally connected to a target channel system, whereas areas that are susceptible but disconnected cover 8.2% of the whole study area. 16.5% of the area represents hillslopes that are potentially connected to the river network but result not susceptible in terms of debris flow release.</p><p>The straightforward implementation of this approach allows it to be an effective tool to assist hazard mapping and management. This study shows that the presented method provides spatial information useful to preliminary identify areas of interest for connected debris flows at the regional scale, but at the same time, it also highlights the need for further ground truth data to validate the results quantitatively beyond single catchment analyses.</p><p>Stefan Steger, Vittoria Scorpio, Francesco Comiti, Marco Cavalli. <em>Data-driven modelling of joint debris flow release susceptibility and connectivity</em>. Earth Surface Processes and Landforms. Under review.</p>

In this study, the spatial characteristics of the factors influencing the occurrence and scale of multiple slope failures in areas with smaller grid sized than the unit size for the current emergency alert system for sediment disasters in Japan were investigated by using high-resolution data for the spatial distribution of topography and rainfall. The study areas were located in southern Hiroshima Prefecture and southeastern Ehime Prefecture of Japan, where multiple slope failures occurred due to heavy rainfall in July 2018. With an increase in slope gradient, the frequency of smaller-scale slope failures increases significantly in southeastern Ehime Prefecture, whereas this tendency is not as noticeable in southern Hiroshima Prefecture. This indicates that other indices (e.g., geological feature, large total precipitation) also influence the occurrence of slope failure. A slope-facing direction is not seem to affect the occurrence of slope failure in both study areas. Total precipitation (i.e., long-term rainfall index) influence the frequency of the occurrence of slope failure in southern Hiroshima Prefecture. Whereas, maximum hourly precipitation (i.e., short-term rainfall index) relatesto the occurrence of slope failure in both study areas. These indicate that the occurrence of slope failures depend on different rainfall indices in each study area. Combining the topographic indices (e.g., slope gradient) and rainfall indices (e.g., hourly precipitation, total precipitation) may lead to a highly accurate estimation of the occurrence of slope failure on a smaller grid size than the unit size for the current system.

Dynamic movement history of the Iiyama slope failure revealed from seismic data

Erosion is concentrated in steep landscapes such that, despite accounting for only a small fraction of Earth’s total surface area, these areas regulate the flux of sediment to downstream basins, and their rugged morphology records transient changes (or lack thereof) in geologic and climatic forcing. Steep landscapes are geomorphically active; large sediment fluxes and rapid landscape evolution rates can create or destroy habitat for humans and wildlife alike, and landslides, debris flows, and floods common in mountainous areas represent a persistent natural and structural hazard. Despite the central role that steep landscapes play in the geosciences and in landscape management, the processes controlling their evolution have been poorly studied compared to lower-gradient areas. This thesis focuses on the basic mechanics of sediment transport and bedrock incision in steep landscapes, as these are the fundamental processes which set the pace and style of landscape evolution. Chapter 1 examines the spatial distribution of slow-moving landslides; these landslides can dominate sediment fluxes to river networks, but the controls on their occurrence are poorly understood. Using a case-study along the San Andreas Fault, California, I show that slow-moving landslides preferentially occur near the fault, suggesting a rock-strength control on landslide distribution. Chapter 2 provides the first field-measurements of incipient sediment motion in streams steeper than 14% and shows a large influence of slope-dependent flow hydraulics and grain-scale roughness on particle motion. Chapter 3 presents experimental evidence for bedrock erosion by suspended sediment, suggesting that, in contrast to prevailing theoretical predictions, suspension-regime transport in steep streams can be the dominant erosion agent. Steep streams are often characterized by the presence of waterfalls and bedrock steps which can have locally high rates of erosion; Chapters 4 and 5 present newly developed, experimentally validated theory on sediment transport through and bedrock erosion in waterfall plunge pools. Finally, Chapter 6 explores the formation of a bedrock slot canyon where interactions between sediment transport and bedrock incision lead to the formation of upstream-propagating bedrock step-pools and waterfalls.

Debris flows play an important role among natural hazards in mountainous areas of Italy. This paper provides an overview on the recent research on debris flows conducted in Italy, taking into account both hydraulic and geomorphological studies. Moreover, the most encouraging research perspectives in this field are briefly presented, such as the monitoring of debris flow in instrumented basins and the geotechnical analysis of the processes leading to debris flow initiation. Finally, two study cases are reported, with the aim of outlining the main characteristics of these phenomena and consequent risk conditions in representative sites in the Italian mountains.

On 27 December 2011, a rock avalanche in the upper Val Bondasca in the southern Swiss Alps deposited 1.5–1.7 million m3 of rock debris. The following summer, debris flow activity in Val Bondasca was unusually high with four events after a 90‐year period of debris flow inactivity. This was an exceptional situation for the valley. Analysing the 2012 events, the long‐term record of meteorological conditions such as rainfall intensity and duration, in comparison with debris flow activity, suggests that the meteorological conditions in summer 2012 would not have triggered the high intensity debris flow events without additional sediment input. Consequently, the suddenly increased debris availability can be considered a major factor in these events. Interestingly, rainfall events of similar magnitude in the subsequent years 2013–2015 did not trigger additional debris flow events, indicating that debris flow initiation thresholds are increasing again, back towards pre‐rock avalanche levels. This study aims to help in understanding the so far poorly understood temporal evolution of debris flow triggering thresholds and the effect of sudden changes in sediment availability.

Spatial and temporal evolution of long-term debris flow activity and the dynamic influence of condition factors in the Wenchuan earthquake-affected area, Sichuan, China

The debris flow that occurred in the upper stream of the Gamahara-zawa killed 14 construction workers and injured 9 others. The debris flow was triggered by a small slope failure. It flowed down 2.7 km downstream with a grade of 20 degrees. The volume of the slope failure was about 33,000m3. According to the chemical analyses of the water, calcium and sulfate iron components are predominant. It showed a lot of groundwater discharged through the volcanic products. The precipitaion of the day before disaster was comparatively small even if the melting snow was considered. Conclusively, it is supposed that the slope failure was easy to occur due to the existence of big slope failures caused in the preceding year and due to the geologic conditions.

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.

We model the rainfall-induced initiation of shallow landslides over a broad region using a deterministic approach, the Transient Rainfall Infiltration and Grid-based Slope-stability (TRIGRS) model that couples an infinite-slope stability analysis with a one-dimensional analytical solution for transient pore pressure response to rainfall infiltration. This model permits the evaluation of regional shallow landslide susceptibility in a Geographic Information System framework, and we use it to analyze susceptibility to shallow landslides in an area in the eastern Umbria Region of central Italy. As shown on a landslide inventory map produced by the Italian National Research Council, the area has been affected in the past by shallow landslides, many of which have transformed into debris flows. Input data for the TRIGRS model include time-varying rainfall, topographic slope, colluvial thickness, initial water table depth, and material strength and hydraulic properties. Because of a paucity of input data, we focus on parametric analyses to calibrate and test the model and show the effect of variation in material properties and initial water table conditions on the distribution of simulated instability in the study area in response to realistic rainfall. Comparing the results with the shallow landslide inventory map, we find more than 80% agreement between predicted shallow landslide susceptibility and the inventory, despite the paucity of input data.

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.