NH3.5

EDI
Rockfalls, rockslides and rock avalanches

Rockfalls, rockslides and rock avalanches are among the primary hazards and drivers of landscape evolution in steep terrain. The physics of rock slope degradation and dynamics of failure and transport mechanisms define the hazards and possible mitigation strategies and enable retrodictions and predictions of events and controls.

This session aims to bring together state-of-the-art methods for predicting, assessing, quantifying, and protecting against rock slope hazards across spatial and temporal scales. We seek innovative contributions from investigators dealing with all stages of rock slope hazards, from weathering and/or damage accumulation, through detachment, transport and deposition, and finally to the development of protection and mitigation measures. In particular, we seek studies presenting new theoretical, numerical or probabilistic modelling approaches, novel data sets derived from laboratory, in situ, or remote sensing applications, and state-of-the-art approaches to social, structural, or natural protection measures. We especially encourage contributions from geomechanics/rock physics, geodynamics, geomorphology and tectonics to better understand how rockfall, rockslides and rock avalanches act across scales.

Co-organized by EMRP1/GI5/GM3
Convener: Michael Krautblatter | Co-conveners: Anne VoigtländerECSECS, John Clague, Benjamin Campforts, Axel Volkwein
Presentations
| Mon, 23 May, 08:30–11:50 (CEST), 13:20–14:50 (CEST)
 
Room 1.61/62

Presentations: Mon, 23 May | Room 1.61/62

Chairpersons: Michael Krautblatter, John Clague
08:30–08:32
08:32–08:38
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EGU22-12124
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On-site presentation
Martina Böhme, Odd Andre Morken, Thierry Oppikofer, Reginald L. Hermanns, Ivanna Penna, Pierrick Nicolet, Marie Bredal, José Pullarello, and Francois Noël

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.

08:38–08:44
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EGU22-3456
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ECS
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On-site presentation
Jaspreet Singh, Sarada Prasad Pradhan, and Mahendra Singh

The rock mass is strongly influenced by the presence of discontinuities and their role is also strongly regarded in rock mass characterization. Different traditional methods were developed for accessing the rock mass condition for safely designing engineering projects such as slopes, tunnels, foundations, etc. The progress in computational techniques has led to a significant understanding of rock mass related problems. Among them, the discrete fracture network (DFN) technique based on statistical distribution gains significant importance in examining the rock mass. The applicability of remote sensing techniques such as photogrammetry has made it easy to collect the essential data, which otherwise was difficult to acquire using scanline survey or window mapping. The study aims application of DFN in estimating block volume distribution and Rock Quality Designation (RQD) for finding the Geological strength index (GSI) of the rock mass. The results also compare the aggregate and disaggregate DFN with GSI estimated using traditional methods in the field. Along with the estimation of GSI using the existing chart method, the work also proposed the applicability of machine learning (ML) in predicting the GSI value. It is easy and handy to use a chart but becomes time-consuming when dealing with a larger dataset. We have developed a ML inbuilt python-based GUI tool to estimate the GSI value from block volume and joint condition parameters quickly.

How to cite: Singh, J., Pradhan, S. P., and Singh, M.: Characterization of a fractured rock mass using Geological Strength Index (GSI): A Discrete Fracture Network (DFN) and Machine learning (ML) approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3456, https://doi.org/10.5194/egusphere-egu22-3456, 2022.

08:44–08:50
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EGU22-11320
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On-site presentation
Gökhan Aslan, John Dehls, Reginald Hermanns, Ivanna Penna, Aniruddha Sengupta, and Vikram Gupta

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.

08:50–08:56
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EGU22-11939
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Virtual presentation
Alessandro De Pedrini, Christian Ambrosi, Cristian Scapozza, Andrea Manconi, and Federico Agliardi

The evolution of rockslide processes towards failure events depends on the combination of geological and geomorphological properties, structural setting, and the glacial history of each site. The identification and analysis of the dominant factors affecting the spatial distribution and the temporal evolution of such massive phenomena are relevant not only for scientific purposes but also have large impacts on hazard assessments. Several large rockslide phenomena are located between five valleys north of Bellinzona, southern Swiss Alps, including the Riviera, Leventina and Blenio valleys in Canton Ticino, and the Calanca and Mesolcina valleys in Canton Grisons. The distribution of such phenomena is highly variable and appears to be higher along the eastern side of the Leventina Valley and the western side of the Blenio valley rather than in the rest of the region. Furthermore, the observed failure events range from 13.50 ka cal BP to 2002 CE, and many rockslides have not yet collapsed despite visible signs of surface deformation. The reasons for these differences in spatial and temporal distribution are yet unknown.  
Our research aims to define the influence and relationship of regional and local factors on the spatial and temporal rockslides distribution in this study area. We rely on an exceptional dataset including (i) detailed geological and geomorphological mapping of the area of study, (ii) a collection of historical data and scientific research on the activity of the large rock slope failures in Ticino and Grisons Cantons, (iii) detailed knowledge of the timing of deglaciation for several valleys of the Canton Ticino, (iv) a catalog of instabilities of the Canton of Ticino finalized in 2016, and (v) several results of current surface deformation activity constrained with satellite radar interferometry. Here we present the preliminary results of the activities performed to extend the rockslides catalog in the Calanca and Mesolcina valleys (Canto Grisons) obtained through the evaluation of stereo-photogrammetry datasets and evaluating the state of activity with satellite radar interferometry. Moreover, we will detail the approach used to set upslope stability modeling attempts at selected locations, combining techniques such as slope exposure dating, analysis of morphological parameters from digital elevation models, and analysis of structural data providing the dominant orientations of rock mass discontinuities.

How to cite: De Pedrini, A., Ambrosi, C., Scapozza, C., Manconi, A., and Agliardi, F.: Investigation of rock slope failure processes in the Southern Swiss Alps , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11939, https://doi.org/10.5194/egusphere-egu22-11939, 2022.

08:56–08:58
08:58–09:08
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EGU22-748
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ECS
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solicited
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On-site presentation
Sophie Lagarde, Michael  Dietze, Conny Hammer, Martin Zeckra, Anne Voigtländer, Luc Illien, Anne Schöpa, Jacob Hirschberg, Niels Hovius, and Jens M. Turowski

In order to reduce the societal impact of mass-wasting events, we need observations to investigate the factors that control slope failure, such as the state of crack propagation along a failure plane. However, usually the failure plane is not accessible in-situ. Hence, cracks have to be monitored indirectly, for example using seismic methods.

We analysed the data from a seismometer array in the Illgraben catchment, Switzerland, that had registered a series of crack propagation and mass-wasting events, leading to a main event that happened on 2 January 2013. We used a state-of-the-art machine learning technique based on hidden Markov models to detect and classify the seismic signals of crack events. We obtained the temporal evolution of three signal types: (1) single crack signal, (2) rock avalanche and (3) rockfall activity due to debris remobilization. The temporal evolution of the number of cracks showed a linear trend in the weeks prior to the main mass-wasting event and, in the hours preceding the main event, a sigmoidal exponential growth. Using these observations, we propose a mechanistic model to describe the rupture of the failure plane. The model considers the internal parameter of the total crack boundary length as the primary control on failure plane evolution, in addition to the previously suggested crack propagation velocity control parameter. According to this model, internal parameters appear to be the dominant control for the failure plane growth at a slope scale.

 

How to cite: Lagarde, S.,  Dietze, M., Hammer, C., Zeckra, M., Voigtländer, A., Illien, L., Schöpa, A., Hirschberg, J., Hovius, N., and Turowski, J. M.: Insights on factors controlling rockslope failure from pre-event cracking, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-748, https://doi.org/10.5194/egusphere-egu22-748, 2022.

09:08–09:14
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EGU22-4641
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ECS
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Virtual presentation
Goran Vlastelica, Ana Duhović, and Marija Relota

Excavations in soft rocks usually have to be performed by blasting with explosives or with heavy pneumatic hammers. However, in a certain period after excavation, their physical and mechanical properties begin to change to a level where even manual excavation can be used. These changes can be significant during the building design life, where the initial design solution of the slope cut may prove inappropriate, sometimes resulting in collapse. In this context, it is necessary to define the causes of changes in the soft rock physical and mechanical properties, and determine all the necessary parameters (primarily strength parameters, but also all others relevant to describe the change in rock properties over time) in all phases of expected change during construction or other applications (such as use of slope area, in case of abandoning the site in certain time period, etc.).

Furthermore, when preparing project documentation for construction, in the part where the calculations of the global stability of the building on the slope are performed, the possibility of significant changes in the shape of the slope during the structure/building design life are usually neglected. Therefore, this paper also presents the Fisher Lehmann model of the change of slope geometry during the period of construction use, and explains the influences of weathering factors on parameters of the soft rock over time by using laboratory simulation of weathering.

Combined changing the geometry of the slope and the properties of the rock can have a negative impact on the safety of the structure, which is explained and shown through an example of an abandoned construction pit at Bračka Street in Split, where the stability of neighboring residential houses is endangered. By using appropriate mathematical models of the slope morphology change, results of long term slope monitoring by TLS and appropriate software for slope stability analysis (Slide 2, RocScience), the time span in which the instability can occur for Bračka Street case study is determined for multiple possible future intervention scenarios.  

How to cite: Vlastelica, G., Duhović, A., and Relota, M.: Long term stability of an abandoned construction pit in Eocene flysch rock mass: case study of Bracka street construction site (Split, Croatia), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4641, https://doi.org/10.5194/egusphere-egu22-4641, 2022.

09:14–09:20
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EGU22-12612
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ECS
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Virtual presentation
Jugraj Singh, Mahesh Thakur, and Naval Kishore

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.

09:20–09:22
09:22–09:28
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EGU22-1866
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ECS
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Virtual presentation
Alexander R. Beer, Nikolaus Krumrein, Sebastian G. Mutz, Gregor M. Rink, and Todd A. Ehlers

Rockfall both is a major process in shaping steep topography and a hazard in mountainous regions. Besides increasing thread due to thawing permafrost-stabilization in high-elevation areas, there are abundant permafrost-free over-steepened rockwalls releasing rockfall due to other triggers. General rockfall event susceptibility is addressed to frost cracking, earthquake shacking and hydrologic pressure in the walls, and to geotechnical rock properties. Spatial rockwall surface surveys or scans (delivering 3D point clouds) have been used to both deduce rock fracture patterns and to measure individual rockfall events from comparing subsequent scans. Though, the actually measured rockwall topography data has rarely been used as a general predictor of rockfall susceptibility against the background of observed events.

In this study, we use a series of dm-resolved annual (2014 to 2020) terrestrial laser scan surveys along 5km2 of limestone cliffs in the Lauterbrunnen Valley, Switzerland. The annual scan data were hand-cut to remove vegetation and fringes, and then referenced to detect subsequent topographic change in the direction of the wall. From the change-detection point clouds individual rockfall event volumes were detected from cluster and filtering analyses. One surveyed rockwall section of 2014 was used as training data for our Bayesian classification model of rockfall susceptibility, while the adjacent remaining section served for model validation. We rasterized their 3D data points and calculated several surface parameters per cell, including roughness, topography, mean distances for the three main fracture systems, fracture density, local dip, percent of overhang area, normal vector change rate (called edge) and percentage of overhang area. For various parameter sets and different cell sizes (32m2, 52m2, 102m2, 152m2, 252m2, and 402m2), we trained Naïve-Bayes-Classifier models. These were then used to predict rockfall susceptibility per cell, based on our observations of surface parameters, and assessed using Kullback-Leibler Divergence analysis and the misclassification cost score.

Results indicate the overall best model (accounting for the parameters roughness, edge, topography and overhang area) and for the lowest cell size (32m2) could predict rockfall cells with a probability of 0.73 (against a mean of 0.3 for all cells). Predictions on another rockwall section with observed rockfall, located on the opposite side of the valley, verified the model’s applicability by both comparable probabilities (0.6 vs 0.25) and visual surveys on overhangs. We find our approach could reliably extend this spatial rockfall susceptibility classification to all Lauterbrunnen rockwalls. The classification model generally identified overhang areas and fractured zones as high rockfall risks, matching the general insight of these zones to be of major susceptibility. Interestingly, our method is based only on orientation-independent variables that are directly calculated from the 3D point cloud. Thus, it should be principally transferable to other sites of fractured limestone walls. Specifically, there is no need to determine fracture sets from the point cloud as is generally done for susceptibility studies, since we account for topography that would anyway be used to calculate fracture planes (facets). Hence, this method provides a simple means to predict spatial rockfall susceptibility, applicable for both hazard mapping and landscape evolution studies.

How to cite: Beer, A. R., Krumrein, N., Mutz, S. G., Rink, G. M., and Ehlers, T. A.: Spatial rockfall susceptibility prediction from rockwall surface classification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1866, https://doi.org/10.5194/egusphere-egu22-1866, 2022.

09:28–09:34
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EGU22-8504
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Virtual presentation
Chunwei Sun, Valérie Baumann Traine, Marc-Henri Derron, and Michel Jaboyedoff

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.

09:34–09:40
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EGU22-1718
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ECS
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On-site presentation
Andreas Aspaas, Pascal Lacroix, Lene Kristensen, Bernd Etzelmüller, and François Renard

Slow creeping landslides move at rates of millimeters to several meters per year. They can cause extensive damage to infrastructure and pose a major threat to human lives if failing catastrophically. Landslides can progressively weaken over time by rock mass damage processes that may occur by constant slow creep or sudden transient slips. Eventually, damage can lead to strain localization along the basal shear plane and catastrophic failure of the landslide. When observed, transient slip events, also called creep bursts, may induce short-term loading and hence can control landslide stability. These creep bursts correspond to short periods that can last several days where the displacement of a landslide accelerates and then decelerates. Here, we compiled and analyzed extensive multiphysics data series of the Åknes landslide, Norway. This landslide is moving at a slow rate of 6 cm per year and could generate a large tsunami wave in a fjord if it would rupture catastrophically. Based on the time series of an array of eight seismometers, five extensometers, seven borehole inclinometers and piezometer strings, and ten continuous GPS stations sampled with time resolutions down to 5 minutes over several years, we detected creep bursts in this landslide. These events interact with a distinct creep trend related to seasonal variations of rainfall and snowmelt. We analyze the creep bursts in regards to micro-earthquake activity and water pressure levels, to study their origin.

How to cite: Aspaas, A., Lacroix, P., Kristensen, L., Etzelmüller, B., and Renard, F.: What causes transient deformations in the Åknes landslide, Norway?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1718, https://doi.org/10.5194/egusphere-egu22-1718, 2022.

09:40–09:42
09:42–09:48
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EGU22-3023
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ECS
Francis Gauthier, Tom Birien, and Francis Meloche

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.

09:48–09:54
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EGU22-3207
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ECS
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Virtual presentation
Tom Birien and Francis Gauthier

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.

09:54–10:00
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EGU22-3079
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ECS
Jacob Laliberté, Francis Gauthier, and Birien Tom

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.

Coffee break
Chairpersons: Axel Volkwein, Benjamin Campforts
10:20–10:22
10:22–10:28
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EGU22-12639
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ECS
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Virtual presentation
Benjamin Jacobs, Florian Huber, and Michael Krautblatter

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.

10:28–10:34
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EGU22-11107
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ECS
Christine Moos, Luuk Dorren, Michel Jaboyedoff, and Didier Hantz

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.

10:34–10:40
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EGU22-9344
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ECS
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On-site presentation
Tiggi Choanji, François Noël, Li Fei, Chunwei Sun, Charlotte Wolff, Marc-Henri Derron, Franck Bourrier, Michel Jaboyedoff, and Romain Gaucher

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.

10:40–10:46
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EGU22-7249
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ECS
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On-site presentation
Philip Süßer, Teemu Hagge-Kubat, Ansgar Wehinger, Michael Rogall, and Frieder Enzmann

Rockfall events, due to toppling or sliding rock slope failure are a common phenomenon within the Rhine-, Ahr- and Moselle-valley of the Rhenish Massif. Due to the dense traffic infrastructure, significant cases of damage with far-reaching economic and infrastructural consequences regularly occur in these areas. Therefore, there is a specific need for precautionary risk analysis in order to prevent further damage and to implement preventive measures. The research approach presented here aims to identify rockfall endangered zones for adjacent infrastructure in the valleys. It is assumed, that the main reason for these frequent occurrences are the high number of exposed rock faces and a complex fabric of intersecting foliation-, fracture- and cleavage- networks and faults. By using an index, calculated from the slope and real-surface area of high-definition LIDAR based DEM it is possible to extract areas with exposed rock faces as possible sources for rockfall modelling. To single out which parts of the outcrop are more likely to fail, we compute the aspect of natural occurring outcrops, characteristic of fabric orientations along which failure preferably takes place and pinpoint locations with highly varying directions. These intersection points, representing weakened areas within the outcrops serve as sources for our rock fall models using the Gravitational-Process-Path-Model by Wichmann (2017). Through the precise identification of the rockfall source areas and further input data like vegetation and relief energy numerous cases in the valley were modelled. By intersecting with real infrastructure data, it is possible to carry out risk assessments of specific sections of roads and railway lines. Validation using the mass movement database of the Rhineland-Palatinate Geological Survey and numerous ground checks show, that concrete rockfall events were plausibly simulated.

How to cite: Süßer, P., Hagge-Kubat, T., Wehinger, A., Rogall, M., and Enzmann, F.: Modelling Rockfall Source areas and hazard zoning along the Rhine-, Ahr- and Moselle-valleys in the Rhenish Massif, Rhineland-Palatinate, Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7249, https://doi.org/10.5194/egusphere-egu22-7249, 2022.

10:46–10:48
10:48–10:54
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EGU22-2623
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On-site presentation
Mateja Jemec Auflič, Ela Šegina, Tina Peternel, Matija Zupan, and Andrej Vihtelič

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.

10:54–11:00
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EGU22-12230
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ECS
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On-site presentation
François Noël, Thierry Oppikofer, Michel Jaboyedoff, Reginald Hermanns, Martina Böhme, and Synnøve Flugekvam Nordang

Working with 3D point clouds offers many benefits for reducing the subjectivity of rockfall simulations at a local scale. Indeed, many “dynamic” rockfall rebound models are strongly affected by the topography and the perceived surface roughness, which can be objectively represented with detailed terrain models. This reduces the need for complex time intensive back analyses and associated sensitive adjustments of parameters used for subjectively adjusting the simulations to the desired runout distances.

Predictable and reproductible simulations from a constrained set of parameters while still managing to reproduce observed runouts on a wide range of sites could be time saving for practitioners and their clients, ultimately improving quality at lower costs to the society. This could speed up the process for practitioners to deliver concise reports easier to interpret and quality-check by a wider range of employees on the client side.

However, working with 3D point clouds can have a steep learning curve and quickly becomes impractical at a larger scale for regional analysis, partially obscuring some of the previously mention advantages. To explore potential ways to circumvent these issues, a prototype of an algorithm that runs the stnParabel rockfall simulation freeware in batch was quickly implemented in 2020. It was developed to expand such dynamic simulation capabilities to larger regions and up to potentially national-covering capabilities.

Slight modifications were done on the impact detection algorithm to also work with high resolution gridded terrain models (DTMs) with a focus at not sacrificing the benefits of working on 3D point clouds. The sources biases due to the stretched grided cells underrepresenting the steep cliffs are worked around by randomly distributing the sources based on the 3D stretched surface occupied by the cells.

Preliminary results were produced regionally over 6000 km2, involving 115 000 000 simulated rockfalls with 10 m3 blocks of dimensions 3.8x3.2x1.8 m. The simulations were performed on the Norwegian national 1 m DTM from airborne LiDAR, up sampled to 50 cm cells for future proofing the approach. They were produced at a rate of about 15 000 000 simulated 3D trajectories per hour when ran on a small Ultrabook laptop with fast SSD.

The preliminary results from the dynamic rockfall model were then combined with databases of observed deposited blocks from previous rockfall events to act as a calibration guide for FlowR model. This simpler model based on gridded topographic-hydrologic spreading and sliding block approaches can be adjusted to produce a wide range of desired runouts envelopes from numerous processes, like rockfalls. The simpler simulations on 10 m DTM were used as a candidate for the revision of the national rockfall susceptibility mapping methodology.

The prototype approach to run detailed dynamic rockfall simulations regionally would require validations. Such potentially useful approach with objective dynamic simulations for hazard mapping as well as for the design of mitigation measures could then be shared through publications and be implemented in the distributed rockfall simulation freeware stnParabel. 

How to cite: Noël, F., Oppikofer, T., Jaboyedoff, M., Hermanns, R., Böhme, M., and Flugekvam Nordang, S.: Toward national-covering dynamic rockfall simulations: adapting stnParabel with efficiency in mind, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12230, https://doi.org/10.5194/egusphere-egu22-12230, 2022.

11:00–11:06
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EGU22-12412
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Virtual presentation
Luuk Dorren, Christine Moos, and Christoph Schaller

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.

11:06–11:08
11:08–11:14
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EGU22-12804
Giovanni Crosta, Giuseppe Dattola, Fabio De Blasio, Camilla Lanfranconi, and Davide Bertolo

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.

11:14–11:20
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EGU22-11756
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On-site presentation
Federico Agliardi, Antonio Carnevale, Matteo Andreozzi, Andrea Bistacchi, Margherita C. Spreafico, Federico Franzosi, Chiara Crippa, Massimo Ceriani, Carlo Rivolta, Giovanni B. Crosta, and Riccardo Castellanza

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 MOVETM and performed implicit-surface interpolation in SKUA-GOCADTM, 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-NXTM. 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.

11:20–11:30
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EGU22-11877
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ECS
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solicited
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On-site presentation
Sazeda Begam and Valentin Heller

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.

11:30–11:36
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EGU22-7454
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On-site presentation
Helene Hofmann, Manuel Eicher, Andreas Lanter, and Andrin Caviezel

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.

11:36–11:38
11:38–11:44
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EGU22-12009
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ECS
Clément Boivin, Jean Philippe Malet, Catherine Bertrand, and Yannick Thiery

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.

11:44–11:50
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EGU22-5318
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On-site presentation
Reginald Hermanns, Ivanna Penna, Vikram Gupta, Henriette Linge, Rajinder Bhasin, John Dehls, Odd Andre Morken, and Aniruddha Sengupta

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 10th, 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 11th, 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 13th, 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.

Lunch break
Chairpersons: Anne Voigtländer, Benjamin Campforts
13:20–13:22
13:22–13:28
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EGU22-9929
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ECS
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On-site presentation
Li Fei, Charlotte Wolff, Davide Bertolo, Carlo Rivolta, Tiggi Choanji, Marc-Henri Derron, Michel Jaboyedoff, Fabrizio Troilo, Patrick Thuegaz, and Joëlle Hélène Vicari

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.

13:28–13:34
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EGU22-2954
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ECS
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On-site presentation
Marius L. Huber, Luc Scholtès, and Jérôme Lavé

Deep-seated failures of rock slopes are partly controlled by structural, lithological and topographical factors. Among structural factors, layering, schistosity and foliation in rock material, which could be described as inherent anisotropy of the material, affect initiation and evolution of deep-seated rock slope deformation, especially in slow moving landslides.

In order to document such an influence of material anisotropy on slope stability, we carry out a parametric study using discrete element modelling (DEM). After a validation exercise for fully isotropic material, where we compare our numerical approach to an analytical slope stability solution, we introduce anisotropy (transverse isotropy) in our DEM model by inserting preferentially oriented and weakened bonds between discrete elements (weakness plane) to simulate two typical transverse isotropic lithologies, claystone and gneiss respectively. Considering these two lithologies, we then explore the influence of the weakness plane’s orientation with respect to the slope angle for both ridge and valley geometries.

We show that certain orientations of the weakness plane relative to the topographic slope favour deep-seated deformation. We also observe significant disparities in failure initiation, failure surface localisation, and mobilized volume depending on the weakness plane orientation. For instance, most unstable slopes occur when the weakness plane rises 10° to 30° less than the hillslope angle. These instabilities are associated with well-localized deformation at depth that when intersecting the surface mimic some of the morphological features (such as counter-slope scarps) that are commonly described along mountain ridges in association with slow-moving and deep-seated rock slope failures.

Our results help explain the appearance or absence of deep-seated failure in mountainous areas and allow to better assess slope failure hazard induced by anisotropic rock strength.

How to cite: Huber, M. L., Scholtès, L., and Lavé, J.: How does anisotropy control rock slope deformation? A discrete element modelling investigation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2954, https://doi.org/10.5194/egusphere-egu22-2954, 2022.

13:34–13:40
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EGU22-12182
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ECS
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Virtual presentation
Andrew Mitchell and Oliver Sass

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.

13:40–13:42
13:42–13:48
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EGU22-4554
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ECS
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On-site presentation
Symeon Makris, Matteo Roverato, Alejandro Lomoschitz, Paul Cole, and Irene Manzella

Debris avalanches (DA) are large landslide events characterised by long runouts and high mobility that poses a great hazard to communities close to volcanoes. Although many theories have been proposed to explain the excessive runout phenomenon, the mechanisms enabling the mobility remain unresolved and poorly constrained. As a result, it is still challenging for models and theoretical concepts to encompass DA deposit field observations.

DA deposits are complex; however, detailed study of their sedimentary architecture can provide information regarding their propagation processes. In this study, the deposits of two DAs in the Canary Islands: Tenteniguada DA, located on the east of Gran Canaria; and Abona DA on the southeast of Tenerife have been examined. Although they are located in nearby volcanic islands they occurred in different environments with different triggering processes, scale, material and their deposits suggest different propagation rheology. A detailed field study of the deposits was carried out in September 2021, mapping their facies and feature distribution and sedimentology. Structure from motion photogrammetry methodology has been used to generate high accuracy 3D models of outcrops and sample windows to quantify facies distribution. The data collected allow for evaluation of the effects of material properties, substrate and its geometry, and to assess aspects of the dynamics of the DAs. Therefore, it was possible to generate conceptual models for the transport and emplacement mechanisms of the two events corresponding to the observations and to relate them to the two debris avalanche distinctive characteristics by comparison.

In the Tenteniguada DA deposit, the degree of disaggregation is low, with large portions of the original edifice preserved along with their original stratigraphy, although displaced relative to each other by brittle deformation. In contrast, Abona DA is much more disaggregated. Monolithological blocks are microfractured and cataclased, and original stratigraphy is not preserved. There is no evidence of brittle deformation. The highly comminuted material has been elongated in a fluidised spreading flow, achieving a long runout on an erodible pumice substrate. Conversely, the Tenteniguada DA did not fully transition from a slide to a flow and has not generated a long runout while propagating in an active fluvial ravine. These findings suggest that the behaviour and the distribution of stresses was very different during propagation, owing to the properties and volume of the material in the flow and potentially the substrate properties and triggering mechanisms.

The present study highlights how the field examination of sedimentological, morphological, and structural features is vital in fully understanding DA propagation and emplacement mechanisms.

How to cite: Makris, S., Roverato, M., Lomoschitz, A., Cole, P., and Manzella, I.: Evidence of volcanic debris avalanche propagation dynamics from sedimentological analysis of the Tenteniguada and Abona deposits, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4554, https://doi.org/10.5194/egusphere-egu22-4554, 2022.

13:48–13:58
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EGU22-9134
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solicited
Jan Beutel and the PermaSense GNSS Team

Slope movements in mountain areas are abundant and diverse phenomena, with an extreme range in size and velocity, and constituted from different materials such as bedrock, debris, and ice. In the past two decades, many studies have observed accelerating trends in the surface velocities of these landforms, often attributed to global warming and its amplified impact on high mountains. Detailed data needed for quantitative analysis and modelling, however, remain scarce due to logistic and technical difficulties. In particular, state-of-the-art monitoring strategies of surface displacement in high-mountains rely either on geodetic terrestrial surveys or on remote sensing techniques. While these methods are beneficial for the establishment of long-term time series and distributed datasets of surface displacements, they lack high temporal resolution and are sensitive to data gaps. These characteristics limit their potential for underpinning detailed process understanding and natural hazard management procedures. By contrast, in-situ permanent instruments allow high temporal resolution without observation gaps, providing unprecedented information w.r.t. the processes at hand. Furthermore, continuous observations with short transmission delays are suitable for applications in real-time, essential for many aspects of natural hazard monitoring and early warning systems.

Here, we present a decadal dataset consisting of continuously acquired kinematic data obtained through in-situ global navigation satellite system (GNSS) instruments that have been designed and implemented in a large-scale multi field-site monitoring campaign across the Swiss Alps. The monitored landforms include rock glaciers, high-alpine steep bedrock as well as landslide sites, most of which are situated in permafrost areas. The dataset was acquired at 54 different stations between2304 and 4003 m a.s.l and comprises ~240’000 daily positions derived through double-difference GNSS post-processing. Apart from these, the dataset contains down-sampled and cleaned time series of weather station and inclinometer data as well as the full set of GNSS observables in RINEX format. Furthermore, the dataset is accompanied by tools for processing and data management in order to facilitate reuse, open alternative usage opportunities and support the life-long living data process with updates. To date, this dataset has seen numerous use cases in research as well as natural-hazard mitigation and adaptation measures. Some of those are presented in order to showcase the fidelity and versatility of the monitoring network.

How to cite: Beutel, J. and the PermaSense GNSS Team: Observations of slope movements in mountain landforms using permanent in-situ GNSS instruments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9134, https://doi.org/10.5194/egusphere-egu22-9134, 2022.

13:58–14:04
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EGU22-11604
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On-site presentation
Ivanna Penna, Reginald Hermanns, Pierrick Nicolet, Odd Andre Morken, John Dehls, Vikram Gupta, and Michel Jaboyedoff

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.

14:04–14:06
14:06–14:12
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EGU22-10843
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Virtual presentation
Masahiro Chigira

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.

14:12–14:18
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EGU22-3128
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ECS
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On-site presentation
Toussaint Mugaruka Bibentyo, Olivier Dewitte, Josué Mugisho Bachinyaga, Toussaint Mushamalirwa, Florias Mees, Charles Nzolang, and Stijn Dewaele

Tropical environments favour chemical weathering and regolith development. Weathering induces textural, mineralogical and chemical changes in rocks, modifying their strength and thus affecting slope stability. Degree of weathering is, however, not only a function of climatic conditions, but is also influenced by e.g. bedrock composition and structure, exposure length and intensity, and slope angle. To investigate the role of weathering and rock type on landslide occurrence, we focus on the Ruzizi Gorge in the Kivu Rift segment of the western branch of the East African Rift System. Stretching along the border between the DR Congo and Rwanda, development of this 40-km long bedrock river began about 10,000 years ago, rejuvenating the landscape at a very high rate, with rather invariant slope angles outside of the landslides. The gorge stretches across a region where two main types of rocks constitute the geological substrate, i.e. late Miocene to Pleistocene volcanic rocks and Mesoproterozoic metasedimentary rocks. The gorge is a hotspot of deep-seated landsides in the region, with slope failures of up to 2 km². For the present study, we sampled weathering profiles developed on both mentioned rock types, in each case with sampling points within and outside the landslides as well as within and outside the rejuvenated landscape. The chemical composition of rock and regolith samples was determined by Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP–OES) analysis, and their mineralogical composition by X-Ray Diffraction (XRD) analysis and thin section observations. Geotechnical tests were used to determine mechanical properties. Overall, we observe that lithological aspects alone control regolith characteristics, and that slope angle and exposure to landscape rejuvenation hence play no significant role. In areas with volcanic rock substrate, where the largest, mostly slide-type, landslides develop, stratified weathering profiles are observed. These profiles show a greater weathering depth than those over metasedimentary rocks, where flow- and avalanche-type landslides are more common. The regolith derived from volcanic rocks has higher clay content, greater plasticity and stronger cohesion than the sandy to silty weathering material that overlies the metasedimentary rocks. These preliminary results show that weathering and rock type are more important than landscape rejuvenation in controlling the type of deep-seated landslides.

How to cite: Mugaruka Bibentyo, T., Dewitte, O., Mugisho Bachinyaga, J., Mushamalirwa, T., Mees, F., Nzolang, C., and Dewaele, S.: Weathering, rock type, bedrock incision and landslides in a tropical environment: the Ruzizi gorge in the Kivu Rift, Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3128, https://doi.org/10.5194/egusphere-egu22-3128, 2022.

14:18–14:24
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EGU22-4199
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ECS
Michal Břežný, Tomáš Pánek, Stephan Harrison, Elisabeth Schönfeldt, and Diego Winocur

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.

14:24–14:26
14:26–14:32
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EGU22-2810
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On-site presentation
Tomáš Pánek, Michal Břežný, Elisabeth Schönfeldt, Veronika Kapustová, Diego Winocur, and Rachel Smedley

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.

14:32–14:38
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EGU22-13034
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ECS
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On-site presentation
Yunjian Gao and Siyuan Zhao

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.

14:38–14:44
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EGU22-11330
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On-site presentation
Thorsteinn Saemundsson and Jon Kristinn Helgason

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 20th 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 20th 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.

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