NH3.5

Several rock avalanches with significant consequences have taken place in Norway during the last centuries. This has caused a high awareness with respect to this natural hazard. As a result, mapping of unstable slopes was initiated in 2006 and several high-risk unstable rock slopes have been identified and investigated in detail and today are monitored. Furthermore, the mapping program of unstable rock slopes has become systematic. Under this initiative, so far five out of eleven Norwegian counties have been analysed systematically for unstable rock slopes and the mapping has been completed for one of these counties. Registered slopes are mapped and classified based on a systematic hazard and risk classification system, established in 2012. This process is time intensive, and currently attention might not be given to the highest risk objects.

In order to get a rapid, complete national overview of potential large rock slope failures, as well as their total hazard and consequence potential, a national overview mapping project has been started. This will make it possible to better prioritize high risk objects in the systematic mapping program. The project will be divided into several steps: (1) systematic analysis of remote sensing data (e.g. detailed DEM, orthophoto and InSAR data) to locate potential unstable rock slopes; (2) a simplified hazard ranking; (3) semi-automated volume estimation; (4) automated run-out assessment; (5) and empirical displacement wave run-up height assessment.

In order to minimize the area that needs to be analysed in Step 1, presently known unstable rock slopes have been analysed. Results indicate that the study area can be restricted based on available relief, presence of inhabitants and distance to the shorelines (fjords and lakes). This makes it possible to reduce the study area significantly, from the total land area of Norway down to roughly one third of this. Furthermore, for this quick overview assessment we use a simplified hazard ranking that is based on signs of activity, visible grade of development and its volume.

The resulting susceptibility map will serve as a source to prioritize mapping and mitigation efforts, with respect to other natural hazards in Norway as well.

How to cite: Böhme, M., Morken, O. A., Oppikofer, T., Hermanns, R. L., Penna, I., Nicolet, P., Bredal, M., Pullarello, J., and Noël, F.: Towards a national susceptibility map for rock avalanches, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12124, https://doi.org/10.5194/egusphere-egu22-12124, 2022.

The trans-Himalayan highway, between Gangtok and Yumthang, winds along steep valley sides, including a long section above the Teesta River. Many villages are precariously perched above the V-shaped valley bottoms. The highway is subject to frequent rainfall-triggered landslide events during monsoon season, disrupting transport and destroying infrastructure. The area has also experienced at least three large rock slope failures (RSF) within the past 40 years and many smaller RSF after the 2011 Sikkim earthquake (Martha et al, 2015). Earlier RSF, many prehistoric, have left at least 30 large boulder deposits along the valley. Several of those such as the Lanta Khola landslide get reactivated each monsoon season (Sengupta et al., 2011). A number of villages are located on these deposits, as they are frequently found in shallower sections of the valley slopes.

In the present study, Persistent Scatterer InSAR (PSI) has been employed, using Sentinel-1A and -1B Synthetic Aperture Radar (SAR) images acquired between 2015 and 2021 for selected historical landslides and landslide-prone areas along the Dzongu and Yumthang Valleys. Among them are the massive translational Dzongu landslide that occurred in 2016 near Mantam village forming a landslide dam (Morken et al., 2020), a large rock avalanche that occurred in 2015 in Yumthang valley (Penna et al., 2021), and several slope instabilities in the cities of Mangan and Mangshila.

Despite the challenges of dense vegetation and winter snow, we detected sufficient targets within the landslides, mainly over the scar areas, rock outcrops, building roofs, and landslide deposits. In this study, we compare the movement/settlement of these historic deposits with ongoing movement in prehistoric deposits. We look at linear vs seasonal components of ongoing deformation within the settlements built upon RSF deposits and discuss the implications with respect to possible catastrophic reactivation.

Martha, T. R., Govindharaj, K. B., & Kumar, K. V. (2015). Damage and geological assessment of the 18 September 2011 Mw 6.9 earthquake in Sikkim, India using very high-resolution satellite data. Geoscience Frontiers, 6(6), 793-805.

Morken, O. A., Hermanns, R. L., Penna, I., Dehls, J. F., & Bhasin, R. (2020, June). The Dzongu landslide dam: high sedimentation rate contributing to dam stability. In ISRM International Symposium-EUROCK 2020. OnePetro.

Penna, I. M., Hermanns, R. L., Nicolet, P., Morken, O. A., Dehls, J., Gupta, V., & Jaboyedoff, M. (2021). Airblasts caused by large slope collapses. Bulletin, 133(5-6), 939-948.

Sengupta, A., Gupta, S., and Anbarasu, K., 2010, Rainfall thresholds for the initiation of landslide at Lanta Khola in north Sikkim, India: Natural Hazards, v. 52, no. 1, p. 31-42.

How to cite: Aslan, G., Dehls, J., Hermanns, R., Penna, I., Sengupta, A., and Gupta, V.: Sentinel-1 InSAR Time-series Monitoring of the Unstable Rock Slopes in North Sikkim, India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11320, https://doi.org/10.5194/egusphere-egu22-11320, 2022.

Technology advances and rising population has led to the establishment of geoengineering projects such as dams, tunnels, bridges, road network, etc. in the mountainous terrain which causes slope destabilization. National Highway-5 connects Shimla, Kinnaur, Kullu, and China border to the rest of the country. The route is of paramount importance for defense and security purposes. The area encompasses complex geomorphological and geological terrain and often encounters road cut slopes susceptible to failure. In the present study, a detailed geotechnical investigation is carried out around Dhalli Landslide (September, 2017) and Malyana Landslide (August, 2018) along NH-5, Shimla, Himachal Pradesh. RMR, SMR, kinematic analysis and numerical modeling using the finite element modelling (FEM) technique is applied for the aforementioned two slopes and its nearby area. Kinematic analysis of joint data shows that rocks are prone to mainly wedge and planar failures. The RMR results show that the slopes belong to fair (Class III) and weak (Class IV) category. The SMR results for the slopes show that slopes lie in the completely unstable (Class V) category, unstable (Class IV) category and in the partially stable (Class III) category. The Strength Reduction Factor (SRF) was calculated using RS2 module of Rocscience. The SRF for both the slopes was less than 1 which shows that the slopes are completely unstable. Dominating factors responsible for the slope instability are identified and accordingly, some suggestions are proposed to strengthen the stability of road cut slope.

How to cite: Singh, J., Thakur, M., and Kishore, N.: Slope Stability Assessment of Rock Slopes Using Finite Element Modelling Along National Highway-5, Shimla, Northwestern Himalaya, India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12612, https://doi.org/10.5194/egusphere-egu22-12612, 2022.

This work presents an approach to identify the rockfall triggering mechanism from video employing Optical Flow Technique. The video was captured by phone camera on 3rd, October 2017 when the massive rockfall happened at a quarry in Le Locle Jura mountains, Switzerland. Time-series frames were extracted from the video and registered using SIFT (Scale-Invariant Feature Transform), kNN (k-nearest neighbor classification) and affine transformation algorithm, which efficiently eliminate the video jitters. After that, the transformation of pixels in the time-series image sequence and the correlation between adjacent frames are used to find the correspondence, so as to calculate the motion data of the object between adjacent frames by Optical Flow Technique. The instantaneous velocity of pixel movement of failure rock mass or debris on the video frames during rockfall dynamic behavior can be obtained. The basal failure surfaces and two main phases of the failure have been anlayzed for the rockfall triggering mechanism. The workflow proposed here can be applied in a slope disaster monitoring and early warning system to identify and track rockfall events effectively.

How to cite: Sun, C., Baumann Traine, V., Derron, M.-H., and Jaboyedoff, M.: Rockfall triggering mechanism analyzed from video using optical flow technique, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8504, https://doi.org/10.5194/egusphere-egu22-8504, 2022.

Rockfalls are major natural hazards for road users and infrastructures in northern Gaspésie (Eastern Canada). In the last 30 years, more than 17 500 rockfalls have reached the two major road servicing the area. Rockfalls come from 10 to 100 m high flysch rockwall conducive to differential weathering. The retreat and settlement of weak rock strata (shale, siltstone) causes the gradual cantilevering of stronger rock strata (sandstone, greywacke), contributing to the development of tension cracks. The block, separated from the cliff, will eventually slide or topple on the eroding rock strata. These dynamics have been observed, but rarely studied with the objective of 1) determining the mechanical stresses and weathering conditions that promote rock cracking and 2) identifying the geometric conditions that control the final failure mode. We use the cantilever beam theory to model critical cantilever length (block size) and rock tensile strength. A frost cracking model (Rempel et al., 2016) was then used to explain the overestimation of the critical cantilever length and to verify whether the development of microfractures caused by frost damage can explain the decrease of the rock tensile strength over time. The results show that the areas of frost damage concentration correspond to those of maximum stress in the overhanging blocks. In order to identify the type of failure of these blocks, tests using a tilting table were carried out in laboratory. 405 tests were performed on 10 blocks characterized by different roughness coefficients and geometric ratios (height / length ratio, overhang length / total length of the block). The results, validated on natural blocks in the field, were used to identify the geometric conditions for stability, sliding, and toppling failure of overhanging block on an inclined plane. Such stability criteria could support the development of rock instability detection algorithm using high resolution 3D model.

How to cite: Gauthier, F., Birien, T., and Meloche, F.: Rock slope dynamics in flysch formation under cold climate (part 1) : rock cracking and failure mechanism, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3023, https://doi.org/10.5194/egusphere-egu22-3023, 2022.

Since 1987, more than 17 500 rockfalls reaching a 70 km stretch of road have been reported by the Québec Ministry of Transport (MTQ) in northern Gaspésie. This natural hazard represents a nearly permanent danger for users. Earthquake, rainfall and freeze-thaw cycles are considered to be the main rockfall triggering factors. Although these events are well correlated with rockfall occurrences, it is not clear how they affect the failure mechanism. The first step in managing the risk rockfalls pose is to better understand the pre-failure processes that contribute to their development. The second step is to improve our ability to predict and anticipate rockfalls. This study aims to better understand the influence of climate-dependent variables on (1) the mechanical deformations of stratified sedimentary rock and (2) the climatic conditions conducive to rockfalls. Meteorological instruments including a 550 cm thermistor strings have been installed directly on a vertical rockwall located in northern Gaspésie. Mechanical deformations of the flysch sequence composed of sandstone, siltstone and shale was monitored using crack-meters. In addition, rockwalls were scanned with a terrestrial laser scanner (TLS) during specific pre-targeted meteorological conditions. Over a period of 18 months, 17 LiDAR surveys have allowed to identify 1287 rockfalls with a magnitude above 0.005 m³ on a scanned surface of 12 056 m². Irreversible deformations are mainly induced by rainfall and snowmelt (shrink-swell process in porous and clayey rock and/or hydrostatic pressure variations in discontinuities), by freeze-thaw cycles and to a lesser extent, by large thermal variations. Gradual settling measured in the siltstone strata causes destabilization of sandstone strata and the eventual fall of sandstone blocks. In winter, rockfall frequency is 12 times higher during a superficial thaw than during a cold period in which temperature remains below 0°C. In summer, rockfall frequency is 22 times higher during a heavy rainfall event than during a period mainly dry. Superficial freeze-thaw cycle (< 50 cm) causes mostly a high frequency of small magnitude events while deeper spring thaw (> 100 cm) results in a high frequency of large magnitude events. Influence of meteorological conditions on mechanical deformations and on rockfall frequency and magnitude is crucial in order to improve risk management since large magnitude events represent higher potential hazards. This study provides a classification of meteorological conditions based on their ability to trigger rockfalls of different magnitudes which could be used to implement an adequate preventive risk management.

How to cite: Birien, T. and Gauthier, F.: Rock slope dynamics in flysch formation under cold climate (part 2): rock deformations and rockfall triggering factors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3207, https://doi.org/10.5194/egusphere-egu22-3207, 2022.

Rockfalls are major natural hazards for road users and infrastructures in northern Gaspésie (Eastern Canada) where nearly 15 kilometers of road runs along 10 to 100 m high flysch rockwall. The Ministère des Transports du Québec (MTQ) has recorded more than 17 500 rockfalls that have reached the roadway since 1987, which represents a nearly permanent danger for users. In the late 90s, protective berms were erected to reduce the number of rocks reaching the roadway. Despite the efficiency of these infrastructures, more than a hundred events are still recorded each year. Based on previous studies showing that rock instabilities in this type of geology is strongly correlated with meteorological events, we used different machine learning methods (logistic regression, classification tree, random forest, neural network) to design the best operational rockfall prediction model. Three event variables based on different rock fall frequency and magnitude thresholds were created. Nearly one hundred weather variables were used to explain and predict events. Preliminary results show that thawing degree-days is one of the most effective variables explaining the occurrence of winter and spring rockfall events. In summer, rainfall intensity is the most potential explanatory variable. Finally, fall events appear to be more responsive to rain events and freeze-thaw cycles. In order to optimize the percentage of predicted events and reduce the false alarm ratio, it remains important to evaluate the impact of each parameter on the performance of the models. These models can be used operationally as decision support tools to predict days with high event probability.

How to cite: Laliberté, J., Gauthier, F., and Tom, B.: Rock slope dynamics in flysch formation under cold climate (part 3) : rockfall forecasting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3079, https://doi.org/10.5194/egusphere-egu22-3079, 2022.

Understanding the magnitude-frequency relationship of rock falls is one of the most important issues for both geomorphologists assessing sediment budgets as well as public stakeholders evaluating rock fall hazards. Multi-temporal Terrestrial Laser Scanning (TLS) surveys, or more general LiDAR, is often applied to produce rock fall inventories of event magnitudes and their frequency. However, LiDAR-based rock fall inventories systematically miss or underestimate both ends of the magnitude bandwidth.

Here we present the first attempt of a complete rock fall inventory including the full spectrum of magnitudes, ranging from fragmental rock falls (cm³) to Bergsturz-sized events (10⁶ m³). We combine rock fall inventories derived from multi-temporal TLS campaigns over six years, rock fall collectors and the historic record in a previously intensely investigated study area (Reintal, German Alps). We investigate which factors – such as structural geology, systematic sampling limitations or different rock fall processes – can lead to possible misinterpretation of rock fall inventories regarding geomorphic systems.

The study shows that (i) LiDAR-based rock fall inventories do not cover the full spectrum of rock fall magnitudes due to their limitations in temporal and spatial resolution, (ii) structural geological features control the magnitude/frequency relation beyond the roll-over of these inventories and (iii) taking fragmentation as well as a clear distinction between rock fall processes into account when analysing rock fall inventories is crucial.

How to cite: Jacobs, B., Huber, F., and Krautblatter, M.: A complete rockfall inventory across twelve orders of magnitude., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12639, https://doi.org/10.5194/egusphere-egu22-12639, 2022.

A realistic quantification of rockfall risk is crucial for an effective and efficient prevention of damages. The estimation of realistic block and event volumes as well as their release frequencies remain a major challenge and are often based on mere expert estimation. Based on the analysis of the rockfall frequency and volume of a wide range of rock cliffs, Hantz et al. (2020) proposed a power law based model for the determination of rockfall magnitude-frequency aiming at a more objective approach for practitioners. It assumes that both, the released masses of rockfall events as well as the individual blocks of a rockfall event follow a power law distribution. The parameters of these distributions are determined using a simple classification of rock structure in combination with field measurements of blocks. In this study, we applied and tested the proposed rockfall frequency model (RFM) at 8 different sites at 7 locations in the Swiss Alps. The calculated frequencies of rockfall events and the derived block volumes were compared to release scenarios of official hazard assessments as well as inventory data. Block volume distributions of all sites could be well fitted by power law distributions (fitted b values between 0.69 to 1.69). The rockfall event and block volumes are in a comparable range as the scenarios of the official hazard assessments, but generally slightly larger. The differences increase with the return period. For all sites, the parameter sensitivity of the RFM is relatively large, in particular for return periods of 100-300 years. Nevertheless, the method proposed in this study allows for a more objective and consistent estimation of rockfall scenarios and thus has the potential to substantially improve the mostly opaque determination of rockfall scenarios. The results further show that the block volume scenarios for pre-defined return periods strongly depend on the considered cliff size, which does not appear to be consistently taken into account in current hazard assessments. However, the study should be extended to additional sites and the parameter estimation has to be optimised to come up with a consistent and transparent method to estimate rockfall frequencies in practice.

How to cite: Moos, C., Dorren, L., Jaboyedoff, M., and Hantz, D.: Estimating rockfall release scenarios based on a straightforward rockfall frequency model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11107, https://doi.org/10.5194/egusphere-egu22-11107, 2022.

The case study is located in the municipality of Les Fréaux, France. The site consists of Cambrian-Ordovician of amphibolite and gneiss rock with complex structural geology that formed in mountainous and large valley with steep slopes and even overhanging rock walls. In this site, rockfall is a major hazard for access roads and houses.

To assess rockfall hazard in the vicinity of the elements at risk, LiDAR data have been analysed and field work done on site from 2020 to 2021.  Rockfall source areas were identified directly on 3D point clouds (PC) based on two criteria, which are large slope angles and kinematic analysis from structural identification of fault, folds and joints. Based on these source areas, several 3D point cloud trajectory models were processed using the freeware stnParabel, for various block diameters (d1, d2, d3) in order to determine the propagation and the probability of reaching the settlements or roads.

Preliminary simulation of trajectories based on several method of simulations results showed some potential directions are reaching the road and also leading to settlements.

How to cite: Choanji, T., Noël, F., Fei, L., Sun, C., Wolff, C., Derron, M.-H., Bourrier, F., Jaboyedoff, M., and Gaucher, R.: Assessment of Rockfall Hazard and 3D Trajectography based on Slope and Structural Settings: Case Study in Les Fréaux, France, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9344, https://doi.org/10.5194/egusphere-egu22-9344, 2022.

Rockfalls are caused by preparatory processes (weathering and crack propagation) that gradually degrade bedrock and by triggering g processes (freeze-thaw activity, precipitation events, earthquakes, snow avalanches, animals, or anthropogenic activities) that eventually release a rock block. Both processes are controlled by several factors representing the internal (geology), external (meteorology), and surface and near-surface conditions (topography, vegetation, snow cover, thermal conditions, chemical weathering, and hydrology) of the bedrock. In this paper, electronic geotechnical monitoring is developed to detect the rockfall activity due to rock freezing and thawing on two separate steep cliffs composed of igneous and carbonate rocks in the eastern part of Slovenia. The monitoring programme includes automatic recordings of rock temperatures and meteorological influencing factors (air temperature, humidity, and precipitation), tiltmeters, kit for measuring rock stress and deformability, laser distance meters, and crackmeters. During the 2020 field investigation, cracks and discontinuities were mapped and Rock Mass Rating (RMR) was estimated. The Hoek-Brown Geological Strength Index was determined to qualitatively assess surface conditions in inaccessible areas using visual assessments of tectonic ruptured walls. We will present the first preliminary results of the parameters monitored for 10 months, which will help interpret rockfall activity and identify freeze-thaw cycles.

Acknowledgement:  The research was funded by the Slovenian Research Agency (Research project J1-3024). The electronic geotechnical sensors were founded by Project »Development of research infrastructure for the international competitiveness of the Slovenian RRI Space – RI-SI-EPOS« The operation is co-financed by the Republic of Slovenia, Ministry of Education, Science and Sport and the European Union from the European Regional Development Fund.

How to cite: Jemec Auflič, M., Šegina, E., Peternel, T., Zupan, M., and Vihtelič, A.: Detection of rockfall activity due to rock freezing and thawing by electronic geotechnical sensors in Slovenia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2623, https://doi.org/10.5194/egusphere-egu22-2623, 2022.

More than ten years ago, Swiss-wide rockfall modelling was carried out to indicate potential hazard areas and rockfall protection forests within the framework of the SilvaProtect-CH project. The forest effect itself was not included in these models and only one block size (1 m³) was calculated. The aim of our study was to model rockfall runout zones using Rockyfor3D for block size scenarios ranging from 0.05 – 30 m³ with explicit inclusion of the protective effect of the forest for an area of approx. 7200 km² in Switzerland and Liechtenstein with a 2m-resolution. For the determination of the start cells as well as the slope surface characteristics, we used the terrain morphometry derived from a 1m-resolution digital terrain model as well as the Swiss TLM geodata and information from geological maps. The forest structure was defined by individual trees with their coordinates, diameters and tree type (coniferous or deciduous). These were generated on the one hand from detected individual trees and on the other hand from statistical relationships between the detected trees, remote sensing-based forest structure type definitions and stem numbers from field inventory data. Based on the latter, we generated forest strata in addition to the detected individual trees. The delimited rockfall runout zones automatically derived from the simulated reach probability maps were validated with 1554 mapped historical rockfall events. The results of the more than 78 billion simulated trajectories showed that 94% of the mapped silent witnesses could be reproduced by the simulations and 78% were within the delimited runout zones. The median of the volume of the non-reproduced silent witnesses was 0.1 m³, which led us to a hypothesis, that these mapped blocks could partly be deposited fragments from larger blocks. We conclude that a rockfall simulation with explicit consideration of the forest effect at 2m-resolution with plausible results is possible for very large areas.

How to cite: Dorren, L., Moos, C., and Schaller, C.: Automated delimitation of rockfall runout zones using high resolution trajectory modelling at regional scale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12412, https://doi.org/10.5194/egusphere-egu22-12412, 2022.

The dynamics of rock fragmentation during the collapse of a rock avalanche, a rockfall, or an extremely energetic rockfall, is insufficiently known (De Blasio et al., 2018). Fragmentation especially at the base of a rock avalanche may affect on the one hand the dynamics of the rock avalanche and the geometry of the final deposit. On the other hand, fragmentation in the upper layers produces a dust of rock particles which: i) impacts energetically with the surrounding areas, and in a later stage, ii) propagates as a dust cloud. Although such dynamics are commonly observed, they are still inadequately addressed.

Recently, a rock avalanche in the Italian Alps occurred in November 2017, giving us the possibility to investigate these phenomena in better detail. In particular, we analysed a  8,000 m³ collapse of serpentinites and metabasics (Grivola-Urtier metaophiolitic Unit) from the Pousset peak (Aosta Valley Region in Western Italian Alps). The peak collapsed from an average height of 2800 m a.s.l. to the foot of the slope 800 m below, where it completely disintegrated. The impact on the ground produced a rock dust cloud which subsequently flowed downstream over the successive few minutes.  The site was visited immediately after the event, and it was possible to investigate the fresh deposit of rock dust before alteration by climate or weathering. This collapse thus represents an interesting case study for trying to determine the energy threshold required for fragmentation and dust cloud formation, the redistribution of the kinetic energy after impact and the amount related to cloud generation within the energy balance.

After identifying in situ the main characteristics of the collapse, we then concentrated our efforts on a more quantitative understanding of the event via numerical calculations. We reproduced the blocks trajectories and computed the impact points where a strong energy dissipation occurred by using the 3D rockfall simulator code HY-STONE (Crosta & Agliardi 2004; Frattini et al. 2012). In these points, the block fragmentation has been taken place and the formation of dust occurred. Through laboratory analysis of dust samples collected from the few centimetres thick deposits on trees and paths, we determined the particle size frequency curves for each location. The fragmentation energy was then estimated by integrating the spectrum of the grains assuming that the fragmentation energy is proportional to the area just created.

Once obtained the fragmentation energy, we estimated the maximum speed and runout of the dust cloud and the settling time using a simple model for suspension flows. From the analysis of the results obtained in the three described procedures, the fragmentation energy was found to be a relatively small fraction of the initial energy of the landslide, and the calculated flow rate of the suspended powder was found to be compatible with the one observed, even though flowage parameters for the cloud still need to be understood from first principles. In conclusion this case study, even if volumetrically small (or perhaps because of it), may add interesting information on the ongoing debate about rock fragmentation in catastrophic events.

How to cite: Crosta, G., Dattola, G., De Blasio, F., Lanfranconi, C., and Bertolo, D.: Collapse, fragmentation, high-speed boulders, and dust cloud: analysis of the 2017 Pousset (Cogne, Val D’Aosta) rockslide in Northern Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12804, https://doi.org/10.5194/egusphere-egu22-12804, 2022.

Slow rock-slope deformations are widespread in orogenic belts and pose significant threats to critical infrastructures, due to continuing slow movements and potential evolution to collapse. The analysis of related risks requires realistic models, accounting for the 3D complexity of both large landslides and infrastructures, often hampered by over-simplification of geological aspects.

We propose an integrated workflow for the 3D modeling of a complex system of deep-seated landslides affecting the N slope of Mt. Palino (Valmalenco, Italian Central Alps). The slope was carved by glacial and fluvial erosion in a complex metamorphic sequence including layers of metapelite, serpentinite, gabbro and gneiss with a regional foliation deformed in two folding stages. The slope hosts a hydroelectric power plant and related structures, affected by deformations observed since 1972. Site investigations (field surveys, full-core borehole drilling, seismic surveys) and deformation monitoring (EDM, GNSS, structural monitoring, GB-InSAR) show that the slope is affected by a deep-seated gravitational slope deformation, probably active before the LGM and partially collapsed, and by a system of nested large landslides, including a toe failure up to 200 m deep and two suspended rockslides affecting some of the structures.

We performed an accurate 3D geomodelling to provide sound constraints on the geometry, lithology, and mechanisms of the active landslides. By integrating all available geological data we reconstructed longitudinal and transversal cross-sections in MOVE^TM and performed implicit-surface interpolation in SKUA-GOCAD^TM, eventually obtaining solid objects corresponding to tectono-stratigraphic units that are dissected by the nested landslides. These volumes are populated with their rock mass properties, interpolated from boreholes and surface surveys. The geomodel shows a complex dome-and-basin folded structure, strongly constraining the spatial distribution and anisotropy of weaker rocks (e.g. serpentinites), and thus the geometry, kinematics, rock strength and shear zone properties of active landslides.

Based on the geomodel, we set up a continuum-based 3DFEM elasto-plastic model in MIDAS GTS-NX^TM. Individual solids in the analysis domain were discretized into a 3D mesh of 150000 hybrid finite elements with variable size in the range 20-200 m. Rock masses were considered as Mohr-Coulomb materials with tensile cut-off and post-peak dilatancy, while shear zones were included explicitly. After stress initialization, the model was ran with a Shear Strength Reduction (SSR) technique. Model parameters were calibrated using a quantitative back-analysis approach, optimizing the fit between normalized GB-InSAR measured displacements and computed displacements, projected in the radar LOS. The calibrated model was validated against field evidence and effects on man-made structures, and provided a starting point for forward modeling of the slope response to groundwater perturbations. We considered the effects of groundwater changes for 5 scenarios of perched aquifers, and assessed critical conditions corresponding to different instability scenarios with different impacts on the hydropower facilities.

Our results show that an explicit account for 3D geometrical and geological complexities is key to a realistic modeling of large slope failure mechanisms, their impacts on critical infrastructures and the evaluation of related risks.

How to cite: Agliardi, F., Carnevale, A., Andreozzi, M., Bistacchi, A., Spreafico, M. C., Franzosi, F., Crippa, C., Ceriani, M., Rivolta, C., Crosta, G. B., and Castellanza, R.: Integrated 3D geological and Finite Element modeling of slow rock-slope deformations affecting hydropower facilities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11756, https://doi.org/10.5194/egusphere-egu22-11756, 2022.

Granular slides can be defined as gravity-driven rapid movements of granular particle assemblies mixed with air and often also water. This ubiquitous phenomenon is not only observed in industrial applications such as hoppers, blenders and rotating drums, but also in natural contexts in the form of landslides, rockslides and avalanches. These granular slides in nature may cause devastation and human losses in their run-out path and indirect effects such as landslide-tsunamis, landslide dams and glacial lake outburst floods. The investigation of granular slides in nature is challenging due to the dangers in accessing the landslide locations in a timely manner and the challenges in predicting when and where they occur. Here, we use well defined and controlled three-dimensional (3D) laboratory experiments, building up on own (Kesseler et al., 2020*) and other studies, which were commonly limited to two dimensions (2D). The primary aim of the current study is to extend the scale effects investigation of Kesseler et al. (2020) to 3D and to provide new physical insight into 3D granular slides.

The experimental setup from Kesseler et al. (2020) has been upgraded from 2D to 3D by extending the side of the ramp and runout zone. The upgraded versatile 3 m long and 1.5 m wide ramp transitions via a curved section into a 3 m long and 2 m wide runout area. The measurement system, consisting of cameras recording the slide evolution and for general observations and a photogrammetry system to investigate the slide deposit shape including the runout, has been complemented with two laser distance sensors measuring the slide thickness along its centreline at two distinct positions during slide propagation.

In this initial study, we explore two different slide volume limits and, surprisingly, found a negative correlation between the slide volume and runout distance. Moreover, we identified a positive correlation between the slide thickness and slide volume. A positive correlation has also been identified between the maximum deposit height and the initial slide volume. Further, the good test repeatability is demonstrated with a detailed quantification and presentation of the characteristic variation plot at different time instances, involving the slide centroid and front velocities, the maximum slide thickness, the slide side expansion ratio and the locations of the slide deposit front- and backlines.

These findings may ultimately contribute to landslide and avalanche hazard assessments by providing an efficient and improved prediction of the slide kinematics, the slide evolution and the slide deposition features such as the runout distance. Moreover, once all experiments are conducted at different scales, we hope to be able to quantify and understand scale effects of granular slides and to improve the upscaling procedure from laboratory scale to nature.

*Kesseler, M., Heller, V., Turnbull, B. (2020) Grain Reynolds number scale effects in dry granular slides. Journal of Geophysical Research-Earth Surface 125(1):1-19.

How to cite: Begam, S. and Heller, V.: Experimental study towards the investigation of scale effects in 3D granular slides, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11877, https://doi.org/10.5194/egusphere-egu22-11877, 2022.

In the last 30 years, rockfall barriers made of steel wire nets have become established worldwide as a protective solution, are meanwhile CE certified and the question inevitably arises as to the effect of natural impacts, i.e. impacts from boulders that strike the net at any point, possibly also rotating as they do so. In 2019 an Innosuisse-sponsored research project was granted to the WSL Institute for Snow and Avalanche Research SLF together with the industry partner Geobrugg, for testing fully instrumented rockfall barriers, in natural terrain in the Swiss Alps, aiming at finding improvements to the capacity of a rockfall barrier outside of the certification standards. The awareness that the capacity of a rockfall barrier is different depending on the impact location, and how to deal with the so-called remaining capacity of rockfall barriers, in load cases outside the approval tests, differ worldwide. In some countries, specialized designers are aware of this fact and solve the problem by over-dimensioning the rockfall barriers to ensure the availability of residual capacity outside of the middle field. In other countries however, authorities and/or designers assume that a 1000kJ rockfall system absorbs this energy even in marginal areas or in case of an eccentric hit. Protective solutions are consequently not necessarily designed properly. This research project tries to assess the performance and the residual capacity of rockfall barriers, after being impacted by various load cases, to improve the current knowledge. Several field campaigns were conducted, in which rocks of different shapes and sizes are projected into the netting of the rockfall barrier and its structure (cables and posts). The barrier is equipped with sensors to measure the loading on different elements of the protection system. In addition, the test blocks (up to 3’200 kg) are also equipped with sensors that measure the rotation and the acceleration during the fall and on impact with the barrier. In combination with high-resolution drone recordings and video recordings from different viewing angles, the trajectories and velocities of the individual blocks can be reconstructed in detail, enabling further insights into the interaction of all parameters. The barrier was left in place since construction and is enduring its third winter without maintenance. A field survey (snow depth and density, loads on cables, posts, etc) was undertaken in the winters 19/20 and 20/21, and further surveys will take place this current winter. This contribution will present the evaluation of the rockfall test data. It allows an understanding of the remaining capacity of a barrier, the influence of rockfall rotation onto the protection system itself as well as the importance of the impact location. Forces measured in the system show a variation of up to 40% when compared to the standard testing results. The goal is then to assess if additional tests can be carried out to the standardized tests, to better prepare a rockfall barrier for the field.

How to cite: Hofmann, H., Eicher, M., Lanter, A., and Caviezel, A.: The Innonet project: understanding the capacity of flexible protection systems against rockfall in natural terrain, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7454, https://doi.org/10.5194/egusphere-egu22-7454, 2022.

Deep Seated Gravitational Slope Deformation (DSGSD) are gravitational processes damaging slopes over long periods of time. These processes may be reactived with the occurrence of smaller, shallow gravitational events. Thus, a better understanding of DSGSDs, from their formation to more catastrophic phases of activity, is an important goal  for natural hazard prevention in mountainous areas. .A first inventory of DSGSD in the Western Alps has been proposed by Crosta et al. (2013) with 1057 DSGSDs identified. A similar work has been conducted more recently at the scale of the French Alps by Blondeau (2018) who identified nearly 460 DSGSDs. Despite the importance of these works, there are still many Alpine sub-massifs where high concentrations of DSGSDs (Blondeau., 2018) have been recognized but where no detailed studies have been conducted. This is the case of the Queyras Massif (South French Alps). It is in this context that this study is carried out, with both the objectives of locating and characterizing the DSGSDs observed in this area and identifying their recent activity.

The proposed approach is based on quantitative geomorphological studies combining photo-interpretation of multi-date aerial imagery, analysis of DSMs and field observations. Quantitative description criteria are proposed to identify DSGSDs and discriminate them from large deep-seated landslides. Thirty DSGSDs are inventoried and their lithological and structural setting is analyzed. Analysis of multi-date aerial photographs and InSAR derived landslide velocities (NSBAS processing of Sentinel-1 observations; e.g. André et al., XX?) allow characterizing their gravitational activity.

How to cite: Boivin, C., Malet, J. P., Bertrand, C., and Thiery, Y.: Inventory and characterization of recent (<100 years) gravitational activity of the Queyras DSGSDs - South French Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12009, https://doi.org/10.5194/egusphere-egu22-12009, 2022.

The ca. 80 km long trans-Himalayan highway between Gangtok and Yumthang has experienced at least three large rock slope failures (RSF) within the past 40 years and tens of smaller RSF related to the 2011 Sikkim earthquake. More than 30 conspicuous boulder deposits suggest that similar failures happened in the past. Since the largest of these deposits are located within the shallowest sections of otherwise 60 – 75° steep slopes, they are often the location of settlements. We have used Terrestrial Cosmogenic Nuclide (TCN) dating to understand better where and how often these events are likely to occur.

The trans-Himalayan highway connects the Lesser Himalaya, with a tropical to subtropical climate, with the cold-temperate climate in the Higher Himalaya north of the Main Central Thrust (MCT). This highway also crosses the orographic barrier, with rainfalls exceeding 3000 mm/yr in the south and less than 500 mm/yr in the north. On September 10^th, 1983, a large RSF was triggered by “exceptional” rainfall and impacted the settlement of Manul, with an estimated life loss of 200 persons. Today, the deposit is covered by a dense tropical forest 30-m high that restricts detailed analysis. However, boulder size and boulder density on the surface suggest that it was a rock avalanche.

The second reported RSF is a rock avalanche with a volume of 12 million m³ that occurred close to the village of Yumthang on March 11^th, 2015. This deposit overlies two generations of prehistoric rock-avalanche deposits. No trigger was reported.

The last reported RSF involved a volume of 8.7 million m³, occurred on August 13^th, 2016 at Dzongu, NW of Mangan. While no trigger for the collapse was reported, satellite footage indicates at least ten years of pre-failure rock-slope deformation. The deposit has the typical carapace of a rock avalanche, but videos posted on social media instead suggest that it was a collapse that took place over several hours.

RSF deposits are found in similar numbers in both the Higher and Lesser Himalaya, with the highest concentration in the vicinity of the MCT and a second cluster close to the village of Yumthang. We sampled ten of the deposits for TCN dating, including two of the historic events. Both historic events returned zero ages. The two older deposits overlain by the 2015 Yumthang rock avalanche returned equally young ages, suggesting multiple recent events at that site within a short time. The zero ages of both historical events suggest that inheritance of nuclides prior to failure in the samples can be ruled out. The ages of the remaining deposits range from 0.2 to ~12 kyr. Several deposits have bimodal age distributions. Others have three different ages in different sectors of the deposit. These results show that multiple RSF similar to the Yumthang site often can affect the same slope sector, leaving deposits on the same slope sections. Thus, the 30 identified deposits by far are the lower limit of RSF failures in the study area and that the threat of RSF is high.

How to cite: Hermanns, R., Penna, I., Gupta, V., Linge, H., Bhasin, R., Dehls, J., Morken, O. A., and Sengupta, A.: Spatial/temporal distribution of rock slope failures along the trans-Himalaya highway between Gangtok and Yumthang (Sikkim, India), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5318, https://doi.org/10.5194/egusphere-egu22-5318, 2022.

With global warming, geological hazards such as rockfalls, rockslides and rock avalanches have increased in alpine areas recently. In many studies, this increase has been attributed to the redistribution of the slope stress field, fluctuations in the temperature field (surface layer thaws during summer), and changes in the seepage field (infiltration of snow and ice melting water), which are led by permafrost degradation and glacier retreat. On the other hand, it is necessary to assess the long-term effects of these changes on rock mass fatigue, which could lead to rock instability. The GB-InSAR technique can detect deformation in the mm range. It is ideal for monitoring small deformations caused by daily physical weathering or other factors in high mountains.

A GB-InSAR campaign was performed from 12 August 2020 to 19 October 2020 in the Brenva glacier basin to assess the displacement of the Brenva rockslide scar. We found a daily cycle of expansion and shrinkage on the scar surface during the summer after examining the movement of different control points along the line-of-sight (LOS). Consequently, we explored possible causes behind such displacement. In this case, we realized that the crest and trough of the displacement curve occurred at a certain period of each day. For instance, in the cases of control points 2, 7, and 8, most crests in the displacement curve occurred in the early morning of each day and the troughs in the late afternoon or evening of each day during 06 September and 13 September, with amplitudes of displacement around 0.15mm, 0.25mm, and 0.4mm, respectively. The preliminary correlation between air temperature and daily deformation shows that point 7 moves towards SAR as the air temperature increases, and away from SAR as the temperature decreases. This phenomenon means that such displacement could be caused by the daily changes in temperature (leading to thermal expansion and contraction of materials, and movement of ice in micro-macro cracks) in the rock mass and air.

However, a comprehensive analysis of the LOS displacement that consists of checking the raw data of GB-InSAR (i.e., radar signal comparison), setting more specific control points at locations with various dip directions, and clear correlation between meteorological data and displacement is undergoing to verify and explain such kind of displacement.

In conclusion, continuous daily physical weathering (behaving as cyclic movement) that led to rock mass fatigue probably exists on the surface of alpine slopes, and GB-InSAR could be an effective technique to detect such movement. Despite only slight daily displacement fluctuation on the surface, it could play a crucial role in the initiation of geo-disasters.

How to cite: Fei, L., Wolff, C., Bertolo, D., Rivolta, C., Choanji, T., Derron, M.-H., Jaboyedoff, M., Troilo, F., Thuegaz, P., and Vicari, J. H.: Preliminary analysis of potential daily cyclic movements on the surface of Brenva rockslide scar based on the GB-InSAR monitoring (Mont-Blanc massif, Aosta Valley, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9929, https://doi.org/10.5194/egusphere-egu22-9929, 2022.

As a key part of landscape evolution and hazard to people in Alpine terrain, rock weathering leads to the breakdown and weakening of rock, causing rock fall and ultimately slope failure. Rock moisture availability is a major factor in these processes. It is understudied, partly due to a lack of reliable measurement techniques. Most frost weathering tests in the laboratory to date have been conducted with fully saturated specimens, which is often not the case under natural conditions.

As part of the DFG-funded CLIMROCK project, we performed laboratory based experiments in a climate cabinet looking at rock moisture movement during frost cracking cycles and its relation to rock weathering. A selection of Wettersteinkalk (limestone) blocks of 40 x 40 x 20 cm size were used, some of which were compact and some of which were highly fractured. The blocks saturated with water to different degrees (0%, 50%, 100%) and were insulated on the side faces. In different test runs, the base of the individual blocks were either left uncovered to allow water seeping through, also isolated at the base to create Different sensor types including Time Domain Reflectometry (TDR), Electrical Resistivity (ER) and Microwave sensor (MW) were used to quantify rock moisture levels and movement during freeze-thaw cycles of different duration. As a measure of relative rock weathering contact Acoustic Emissions (AE) loggers were used to detect subcritical cracking. Calibration of these instruments will be individual to each block.

Initial findings show marked movement of rock moisture at the beginning of the cycles with possible evidence of cryosuction down to 36cm depth from rock surface. Particularly strong moisture migration is seen in 50% and 100% samples at 25cm depth, though not when the sample is initially dry. There is also evidence of migrations to the freezing front and probable subsequent refreezing events.

Further test runs with different saturation levels (75%, 90%) are planned. Observations of moisture movements and weathering effects from the laboratory experiments will be applied to the interpretation of field rock moisture data from ongoing CLIMROCK studies in the Bavarian and Austrian Alps.

How to cite: Mitchell, A. and Sass, O.: Movement of moisture during frost cracking cycles: First laboratory results from the CLIMROCK project., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12182, https://doi.org/10.5194/egusphere-egu22-12182, 2022.

The sudden impact of a large slope collapse on the ground can cause a high degree of comminution of rocks and trigger an extreme rush of air loaded with particles, called an airblast. The airblast can expand the destructive capacity of a large slope collapse far beyond the run-out of the rock mass. The first airblast event documented in detail occurred in 1881 as consequence of a large collapse at Elm in the Unthertal valley (Switzerland). People being blown over by the air pressure wave were reported. In 2015, two rock avalanche related airblasts occurred in the Himalayas. In March 2015, an airblast in Yumthang valley (Sikkim, India) knocked down and snapped trees 1.4 km away from the impact zone of a rock avalanche. In April 2015, an avalanche triggered by the Gorkha earthquake induced a violent airblast that caused several casualties in Langtang valley. The destruction of stone and wooden houses can be observed in video footage. The damage on trees can be traced over a distance of 3.5 km and 400 m above the impact zone of the avalanche on the opposite slope. The most recent documented event occurred in February 2021 in Chamoli (India), where the flattened forest extends over 20 hectares.

This work presents a back analysis of the April 2015 airblast in the Sikkim Himalayas (India) and compares it with several other airblasts documented around the world. We review the conditions a large slope collapse should meet to cause a significant airblast. We also formulate an equation that links the potential energy of collapses having airborne trajectory to the extent of the related airblast.

How to cite: Penna, I., Hermanns, R., Nicolet, P., Morken, O. A., Dehls, J., Gupta, V., and Jaboyedoff, M.: Airblast caused by large slope collapses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11604, https://doi.org/10.5194/egusphere-egu22-11604, 2022.

Granite is distributed all over the world and one of the rock types that are very susceptible to various kinds of mass movements including rockfall, rock slide, debris slide and debris avalanche. For example in Japan, Hiroshima rainstorm disasters in 1999, 2014, and 2018, and southern Miyagi rainstorm disaster induced by typhoon 19 in 2019. This is because its special characteristics of formative processes and weathering behavior. The primary structures of granite have long been believed as orthogonal cooling joints since the pioneer work of Cloos (1921, 1922), but we found that a granite body has columnar joints near its roof using UAV and SfM. Whether granite has columnar joints or not leads to different mass movement types. Rock columns separated by columnar joints form high unstable rock towers or tors, which are susceptible to rockfalls. When rock columns are weathered under the ground, they form boulders surrounded by saprolite; when they are eroded to form hills they frequently fail during rainstorms and transform to debris avalanche or debris flow with high destructive potential because of large mass of boulders. Granite without columnar joints is not suitable for spheroidal weathering but is sheeted by unloading; sheeting forms dip slopes, on which rock slides occur. Some granite is micro-sheeted by unloading and micro-sheeted granite is weathered to form a loose soil layer beneath slope surfaces. Such soil layers are very prone to heavy rainfalls and frequently slide, transforming debris avalanches and debris flows.

Primary structures of granite and following weathering schemes thus define landslide behavior in granite areas.

How to cite: Chigira, M.: Primary structures of granite and following weathering schemes define landslide behavior in granite areas., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10843, https://doi.org/10.5194/egusphere-egu22-10843, 2022.

Deglaciation of mountain ranges promotes landslides of various scales and types, and many of them may present a major hazard. Traditionally, it is assumed that landslides are concentrated in the steepest, wettest, and most tectonically active parts of the orogens, where glaciers reached their greatest thickness. Based on our mapping of large landslides (>1km²) over an extensively large area of Southern Patagonia (~305,000 km²), we show that the distribution of landslides can have the opposite trend. The largest landslides within the limits of the former Patagonian Ice Sheet (PIS) cluster along its eastern margins occupying lower, tectonically less active, and arid part of the Patagonian Andes. In contrast to the heavily glaciated, highest elevations of the mountain range, the peripheral regions have been glaciated only episodically. However, a combination of glaciation, weak volcanic and sedimentary rocks, sufficient relief, and presence of large glacial lakes in the past, created favourable conditions for huge number of large landslides along eastern margin of PIS. We explain the scarcity of large landslides in the highest parts of the PIS by presence of strong granitic rocks and long-term glacial modification, that adjusted topography for efficient ice discharge. Our model is applicable only for large bedrock landslides, not for shallow slides and rock falls, which are abundant in the highest and western part of the Andes.

How to cite: Břežný, M., Pánek, T., Harrison, S., Schönfeldt, E., and Winocur, D.: Large landslides cluster along Patagonian Ice Sheet margin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4199, https://doi.org/10.5194/egusphere-egu22-4199, 2022.

Although ice retreat is widely considered to be an important factor in landslide origin, many links between deglaciation and slope instabilities are yet to be discovered. Here we focus on the origin and chronology of an exceptionally large landslides situated along the eastern margin of the former Patagonian Ice Sheet (PIS). Accumulations of the largest rock avalanches in the former PIS territory are concentrated in the Lago Pueyrredón valley at the eastern foothills of the Patagonian Andes in Argentina. Long-runout landslides have formed along the rims of sedimentary and volcanic mesetas, but also on the slopes of moraines from the Last Glacial Maximum. At least two rock avalanches have volumes greater than 1 km³ and many other landslide accumulations have volumes in the order of tens to hundreds of million m³. Using cross-cutting relationships with glacial and lacustrine sediments and using OSL and ¹⁴C dating, we found that the largest volume of landslides occurred between ~17 and ~11 ka BP. This period coincides with a phase of rapid PIS retreat, the greatest intensity of glacial isostatic uplift, and a fast dropping of the glacial lakes along the foothills of the Patagonian Andes. The position of paleoshorelines in the landslide bodies and, in many places, the presence of folded and thrusted lacustrine sediments at the contact with rock avalanche deposits indicate that the landslides collapsed directly into the glacial lake. Although landslides along the former glacial lobe of Lago Pueyrredón continue today, they are at least an order of magnitude smaller than the rock and debris avalanches that occurred before the drainage of the glacial lake around 10-11 ka BP. Numerical modeling results indicate that large postglacial landslides may have been triggered by a combination of rapid sequential glacial lake drawdowns and seismicity due to glacial isostatic adjustment. We conclude that in addition to direct links such as glacial oversteepening, debuttressing and permafrost degradation, the retreat of ice sheets and the subsequent formation of transient large glacial lakes can fundamentally alter slope stability, especially if the slopes are built by weak sedimentary and volcanic rocks.

How to cite: Pánek, T., Břežný, M., Schönfeldt, E., Kapustová, V., Winocur, D., and Smedley, R.: Large rock avalanches into a glacial lake(s): a new chapter of the Patagonian Ice Sheet story, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2810, https://doi.org/10.5194/egusphere-egu22-2810, 2022.

Giant rock avalanche is extremely rare worldwide, while giant rock avalanche developed in suture zone has presented unique development characteristics. The suture zone is a product of plate moving and strong tectonic activity, where the appearance of a giant avalanche not only plays a barometer role for the regional disaster development environment but an indicator role for the complicated geological environment. In 2019-2021, the author has found a giant paleo-rock avalanche (name Basu avalanche) in the Bangonghu-Nujiang suture zone of the Tibetan Plateau. Some infrequent characteristics such as huge volume, development in nappe structure, and hyper-mobility (debris impact height > 600m) appeared for this giant rock avalanche. In this paper, based on the detailed investigation, ³⁶Cl dating, and reconstructing the pre-avalanche terrain methods, the development, failure, and hyper-mobility of this giant rock avalanche have been analyzed. The result shows that: (1) The volume of the Basu avalanche is about 3.5×10⁹m³, the residue is about 1.4×10⁹m³ now. The avalanche occurred at 205.70±7.71ka B.P.(ka: millennium), subsequent the accumulation body occurred two times secondary landslides (name Duolasi landslide) at 17.57±0.72ka B.P. and 7.01±0.32ka B.P., respectively; (2) The nappe structure, formed from the uplift and orogeny process of the suture zone, controls the development and volume size of the Basu avalanche, while the strong earthquake is the biggest likely to trigger the avalanche finally failure because of the dense active faults distribution; (3) Because of the rich Ultrabasic clasts derived from the F2 fault and fine particles produced by cataclastic rock mass, the Basu avalanche formed the slide belt that thickness from centimeters to meters during the motion. The lubrication effect of the slide belt has dominated the avalanche debris's high-speed motion and hyper-mobility, the mechanism is that: due to the huge avalanche volume and induced the high pressure and closed slide belt environment, the slide belt fine-grain formed the lubrication layer with certain water involved, and the friction force sharply decreased; (4) Because of the Basu rock avalanche and the debris flow successive blocked the Leng River, the Leng River valley has experienced diversions process and the river valley from the ’S’ shape to approximate straight-line shape.

How to cite: Gao, Y. and Zhao, S.: A new perception in the development, failure, and hyper-mobility of a giant rock avalanche in the suture zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13034, https://doi.org/10.5194/egusphere-egu22-13034, 2022.

Over the last decades climate has warmed up worldwide and changes have occurred in the general weather patterns. Where the increase in temperature has rapidly been gathering pace in the last decade. These changes have also been observed in Iceland. From 1980 to 2015 the average temperature increase has been 0,47°C per decade and the average precipitation has increased from 1500 mm/year to around 1600-1700 mm/year. The increased temperature changes have also resulted in more frequent thawing periods and rainfall events during winter months, especially in the lowlands.

Mass movements, including rock falls, rock avalanches, debris flows and debris slides, are common geomorphological processes in Iceland and thus present a significant and direct threat to many towns, villages, and farmhouses. Weather conditions, e.g. precipitation and temperature variations, and earthquake activity are the most common triggering factors for such activity in Iceland. During the last decades several, somewhat unusual, mass movements events have occurred in the island. These events have been unusual both regarding their size, increased frequency, their triggering factors and not at least the timing within the year they have occurred.

One of the most visible consequence of temperature rise in Iceland is the fast retreat and thinning of outlet glaciers and formation of proglacial lakes. The frequency of mass movements on outlet glaciers have increased considerably from the turn of the century compared to the last 4 decades of the 20^th century. New discoveries of unstable slopes above outlet glaciers have also increased considerably from 2000.

In recent years, there has been an increasing interest worldwide in the influence of climate warming and possible decline of mountain permafrost on the occurrence of mass wasting phenomena. The rising frequency of rapid mass movements, such as debris flows, debris slides, rock falls and rock avalanches, in mountainous areas have been linked with mountain permafrost degradation. Several mass movements, which can be connected to thawing of mountain permafrost, have occurred in central N and NW parts of the island during the last decade.

Majority of landslides in Iceland in the past century have either occurred in relations with low-pressures systems that pass-through Iceland from August to November, bringing in high winds with heavy rainfall, or during spring snowmelt in May and June. But in the past two decades snowmelt and thawing periods are becoming more frequent and longer during wintertime resulting in higher frequency of slope failures during that time of year. Over the past 20 years’ large landslides events (> 300.000 m³) have become more frequent compared to the second half of the 20^th century. 

Climate change certainly seems to be affecting slope stability in Iceland and is an increasing risk. Especially slopes close to retreating glaciers and those affected by thawing of mountain permafrost. Changes in temperature and precipitation patterns in late fall and during winter months are causing slope failures that were not as common in the past. 

How to cite: Saemundsson, T. and Helgason, J. K.: Climate change and slope stability in Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11330, https://doi.org/10.5194/egusphere-egu22-11330, 2022.