Unravelling the evolution of the Frébouge polygenetic cone in Val Ferret (Mont Blanc Massif)

Landslide deposits dam Lake Oeschinen (Oeschinensee), located above Kandersteg, Switzerland. However, past confusion differentiating deposits of multiple landslide events has confounded efforts to quantify the volume, age, and failure dynamics of the Oeschinensee rock avalanche. Here we combine field and remote mapping, topographic reconstruction, cosmogenic surface exposure dating, and numerical runout modeling to quantify salient parameters of the event. Differences in boulder lithology and deposit morphology reveal that the landslide body damming Oeschinensee consists of debris from both an older rock avalanche, possibly Kandertal, as well as the Oeschinensee rock avalanche. We distinguish a source volume for the Oeschinensee event of 37 Mm3, resulting in an estimated deposit volume of 46 Mm3, smaller than previous estimates that included portions of the Kandertal mass. Runout modeling revealed peak and average rock avalanche velocities of 65 and 45 m/s, respectively, and support a single-event failure scenario. 36Cl surface exposure dating of deposited boulders indicates a mean age for the rock avalanche of 2.3 ± 0.2 kyr. This age coincides with the timing of a paleo-seismic event identified from lacustrine sediments in Swiss lakes, suggesting an earthquake trigger. Our results help clarify the hazard and geomorphic effects of rare, large rock avalanches in alpine settings.

Large rock avalanches play a key role in shaping alpine landscapes. However, the complex interplay between mass movement and other surface processes poses challenges in identifying these deposits and understanding the underlying process controls. Here, we focus on the rock avalanche deposit of the Lurnigalp valley in the Bernese Alps (Switzerland), originally mapped as till. The Lurnigalp valley is a U-shaped tributary valley located in the southwest of Adelboden, Canton Bern. To explore the timing and dynamics of the rock avalanche event, we employed detailed remote and field mapping, sedimentary petrology, surface exposure dating with cosmogenic 36Cl, and runout modelling with DAN3D®. For the reconstruction of the chronology, we analyzed cosmogenic 36Cl in surface samples from 15 boulders of the rock avalanche deposit. We developed three distinct scenarios to investigate the dynamics and contextual conditions of the rock avalanche event. In the first scenario, we consider a rock avalanche depositing 1 Mm3 of sediment in a valley devoid of ice. The second scenario uses the same deposit volume but introduces a hypothetical glacier occupying the uppermost part of the valley. Finally, the third scenario, similar to the first scenario with a glacier-free valley, assumes a substantially larger volume of collapsed rock mass. We consider the third scenario the most plausible, in which approximately 6 Mm3 of rock mass, composed of limestone and sandstone, was released from a limestone cliff around 12 ± 2 ka during the Younger Dryas. The collapsed rock mass fell into the ice-free valley floor, ran up the opposite valley side and was deflected towards the northeast following the valley orientation. The rock mass stopped after 2.2 km leaving approximately 6.4 Mm3 deposits spread across the entire valley floor. Subsequently, most of the rock avalanche deposit have been reworked by periglacial activity. We suggest that structural features, lithology and glacial erosion and debuttressing were involved in the weakening of the in-situ bedrock that finally led to the collapse. Our study not only enhances the understanding of rock avalanche mechanisms and their profound impact on Alpine landscape evolution but also elucidates the complex interplay of geological processes that led to the collapse and altered the rock avalanche deposit afterwards.

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).

The Wenchuan earthquake triggered 15,000 rock avalanches, rockfalls and debris flows, causing a large number of causalities and widespread damage. Similar to many rock avalanches, field investigations showed that tensile failure often occurred at the back edge. Some soil and rock masses were moved so violently that material became airborne. The investigation indicates that this phenomenon was due to the effect of a large vertical seismic motion that occurred in the meizoseismal area during the earthquake. This paper analyses the effect of vertical earthquake force on the failure mechanism of a large rock avalanche using the Donghekou rock avalanche as an example. This deadly avalanche, which killed 780 people, initiated at an altitude of 1,300 m and had a total run-out distance of 2,400 m. The slide mass is mainly composed of Sinian limestone and dolomite limestone, together with Cambrian slate and phyllite. Static and dynamic stability analysis on the Donghekou rock avalanche has been performed using FLAC finite difference method software, under the actual seismic wave conditions as recorded on May 12, 2008. The results show that the combined horizontal and vertical peak acceleration caused a higher reduction in slope stability factor than horizontal peak acceleration alone. In addition, a larger area of tensile failure at the back edge of the avalanche was generated when horizontal and vertical peak acceleration were combined than when only horizontal acceleration was considered. The force of the large vertical component of acceleration was the main reason rock and soil masses became airborne during the earthquake.

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

The 12 research papers and two summaries of conference discussion sessions contained in this Theme Issue build upon presentations and dialogue at the Third Johnston–Lavis Colloquium held at University College London in September 2009. The meeting brought together delegates from the UK, Europe and

Three groups of factors, topography, geology and hydrology have influence on the formation of gully-type debris flows triggered by shallow landslides. In this paper, a single representative factor (T factor) for the topography, and a single representative factor (R factor) for the rainfall (hydrology) are proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Dayi area, Guizhou Province, China. During a heavy rainfall event on June 5 and 6, 2011, 37 gully-type debris flows caused by shallow landslides were triggered. In some catchments no such debris flows were triggered even though these catchments were in the vicinity of gullies with debris flows. The triggering mechanism for gully-type debris flows is the transport of sediment provided by shallow landslides into the gully. We isolated and analyzed the influence of the topography on the formation of debris flows in gullies under almost identical hydrological and geological conditions and propose a T factor as a topographical indicator which is a combination of the catchment surface area, the ratio of the catchment area with a slope sensitive to trigger debris flows, and the average gradient of the drainage channel in the catchment. Additionally an R factor is proposed as a rainfall indicator which is a combination of the rainfall in 1 h before the debris flow was triggered, the cumulative rainfall before the debris flow was triggered, and the annual rainfall. Higher T factor values and higher R factor values are generally related to higher probabilities of debris flow formation. The primary probability factor P, which is the combination of T and R, gives an indication of the probability of debris flow formation. The T factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.

The 12 May 2008 Wenchuan earthquake (Ms 8.0) in China, produced an estimated volume of 28 × 108 m3 loosened material, which led to debris flows after the earthquake. Debris flows are the dominant mountain hazards, and serious threat to lives, properties, buildings, traffic, and post-earthquake reconstruction in the earthquake-hit areas. It is very important to understand the debris flow initiation processes and characteristics, for designing debris flow mitigation. The main objective of this article is to examine the different debris flow initiation processes in order to identify suitable mitigation strategies. Three types of debris flow initiation processes were identified (designated as Types A, B, and C) by field survey and experiments. In “A” type initiation, the debris flow forms as a result of dam failure in the process of rill erosion, slope failure, landslide dam, or dam failure. This type of debris flow occurs at the slope of 10 ± 2°, with a high bulk density, and several surges following dam failure. “B” type initiation is the result of a gradual increase in headward down cutting, bank and lateral erosion, and then large amount of loose material interfusion into water flow, which increases the bulk density, and forms the debris flow. This type of debris flow occurs mainly on slopes of 15 ± 3° without surges. “C” type debris flow results from slope failures by surface flow, infiltration, loose material crack, slope failure, and fluidization. This type of debris flow occurs mainly on slopes of 21 ± 4°, and has several surges of debris flow following slope failure, and a high bulk density. To minimize the hazards from debris flows in areas affected by the Wenchuan earthquake, the erosion control measures, such as the construction of grid dams, slope failure control measures, the construction of storage sediment dams, and the drainage measures, such as construction of drainage ditches are proposed. Based on our results, it is recommend that the control measures should be chosen based on the debris flow initiation type, which affects the peak discharge, bulk density and the discharge process. The mitigation strategies discussed in this paper are based on experimental simulations of the debris flows in the Weijia, Huashiban, and Xijia gullies of old Beichuan city. The results are useful for post-disaster reconstruction and recovery, as well as for preventing similar geohazards in the future.

Investigation of erosion characteristics of debris flow based on historical cases

Debris flows are one of Brazil's most frequent mass movement processes, triggered by extreme rainfall events and initial volume provided by shallow landslides. Despite the recurrence of catastrophic occurrences, Brazil still lacks basic data containing the main characteristics of previous events. In this way, this research aimed to make a morphometric characterization of the event and to provide debris-flow and shallow landslides inventories. For the morphometric analysis, a Digital Elevation Model (DEM) was used. For the inventory map of debris flow runout and shallow landslides scars, a post-event free access image from Google Earth Pro and satellite images from RapidEye were used. The results show that debris flows had two main flows that affected different areas of the city of Itaóca. Also, one single shallow landslide contributed as initial volume to the debris flows that reach the city downtown, demonstrating the importance of entrainment. Shallow landslides analysis shows its concentration in slopes between 20.1 – 30°, with orientation South and Southeast, elevation between 600 – 800m, and in concave curvatures. The results helped to better understand debris flows in Brazil, highlighting their relationship with the occurrence of shallow landslides as one of the main triggering factors. Those data are crucial to mitigation action of possible new events.

Massive rock avalanches form some of the largest landslide deposits on Earth and are major geohazards in high-relief mountains. This work reinterprets a previously reported glacial deposit in the Alai Valley of Kyrgyzstan as the result of an extremely long-runout, probably coseismic, rock avalanche from the Komansu River catchment. Total runout of the rock avalanche is ~28 km, making it one of the longest-runout subaerial non-volcanic rock avalanches thus far identified on Earth. This runout length appears to require a rock volume of ~20 km3; however, the likely source zone in the Trans Alai range likely contained just ~4 km3 of rock, and presently, the deposit has a volume of only 3–5 km3; a pure rock avalanche volume of >10 km3 is therefore impossible, so the event was much more mobile than most non-volcanic rock avalanches. Explaining this exceptional mobility is crucial for present-day hazard analysis. There is unequivocal sedimentary evidence for intense basal fragmentation, and the deposit in the Alai Valley has prominent hummocks; these indicate a rock avalanche rather than a rock-ice avalanche origin. The event occurred 5,000–11,000 yr B.P., after the region’s glaciers had begun retreating, implying that supraglacial runout was limited. Current volume—runout relationships suggest a maximum runout of ~10 km for a 4-km3 rock avalanche. Volcanic debris avalanches, however, are more mobile than non-volcanic rock avalanches due to their much higher source water content; a rock avalanche containing a similarly high water content would require a volume of about 8 km3 to explain the extreme runout of the Komansu event. Rock and debris avalanches can entrain large amounts of material during runout, with some doubling their initial volume. The best current explanation of the Komansu rock avalanche thus involves an initial failure of ~4 km3 of rock debris, with high water content probably deriving from large glaciers on the edifice that subsequently entrained ~4 km3 of valley material together with further glacial ice, resulting in a total runout of 28 km. It is as yet unclear whether glacial retreat has rendered a present-day repetition of such an event impossible.

Both glaciers and debris flows can shape the landscape in high mountain areas close to drainage divides. As the glacier erodes the landscape, it leads to drainage divide migration and an asymmetric landscape. During divide migration, catastrophic mass movement events, such as rock avalanches and debris flows, may intensify. The intensive erosion ability induced by debris flow could trigger effects on the landscape as well. However, we still cannot quantify the effects of debris flow on divide migration in glacier-dominated regions. Here, we propose a new numerical framework combining erosion from glaciers, fluvial processes, and debris flows in a long-term landscape evolution framework. Our preliminary results show that debris flow processes can slow down divide migration speed within the glacier-dominated regions. An intensive erosion ability of debris flow can make the divide move to the glacier side. Under the effects of debris flow, the effects trigger a longer glacier response time. Debris flow and glacier work together to decrease the divide’s elevation. Our new model can help us to understand the effects of debris flows and glaciers on long-term landscape evolution under climate changes.

This paper aims to systematically study kinematic characteristics of Wenjiagou rock avalanche triggered by the M8.0 2008 Wenchuan earthquake and one of subsequent debris flows that occurred on August 13, 2010 (8.13 debris flow), using a numerical approach. The kinematic characteristics of the rock avalanche are back-calculated using an energy-based runout model. The erosion process of the subsequent rainfall-induced debris flow is analyzed using a progressive scouring entrainment model incorporated in the energy-based runout model. The results indicate that Wenjiagou rock avalanche traveled farther than most other rock avalanches with a maximum velocity of approximately 73 m/s detected at about 1700 m away from the source zone. The erosion of the 8.13 debris flow mainly occurred right after the dam failure which contributes to more than 97% of the total eroded volume. Besides, four different scenarios, including no blockage of the runoff (scenario I), no dam failure (scenario II), dam failure at the end of precipitation (scenario III), and no erodible material (scenario IV), are supposed to study the effect of the failure of a check dam on the erosion characteristics of the debris flow. The results reveal that the blockage of the channel without failure could reduce 86% of the total volume and the failure of check dams is the main reason for the destructive consequence of the 8.13 debris flow. This paper proposes an approach to explore kinematic characteristics of a rock avalanche and erosion process of the subsequent rainfall-induced debris flows and also innovatively provides a method to indirectly assess mitigation strategies that can be used for the design of mitigation projects.

Numerical modelling of entrainment/deposition in rock and debris-avalanches

Catastrophic collapse of large rock slopes ranks as one of the most hazardous natural phenomena in mountain landscapes. The cascade of events, from rock-slope failure, to rock avalanche and the near-immediate release of debris flows has not previously been described from direct observations. We report on the 2017, 3.0×106m3 failure on Pizzo Cengalo in Switzerland, which led to human casualties and significant damage to infrastructure. Based on remote sensing and field investigations, we find a change in critical slope stability prior to failure for which permafrost may have played a destabilizing role. The resulting rock avalanche traveled for 3.2km and removed over one million m3 of glacier ice and debris deposits from a previous rock avalanche in 2011. Whereas this entrainment did not lead to an unusually large runout distance, it favored debris flow activity from the 2017 rock avalanche deposits: the first debris flow occurred with a delay of 30s followed by ten debris flows within 9.5h and two additional events two days later, notably in the absence of rainfall. We hypothesize that entrainment and impact loading of saturated sediments explain the initial mobility of the 2017 rock avalanche deposits leading to a near-immediate initiation of debris flows. This explains why an earlier rock avalanche at the same site in 2011 was not directly followed by debris flows and underlines the importance of considering sediment saturation in a rock avalanche’s runout path for Alpine hazard assessments.