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Rockfalls, rockslides and rock avalanches are fundamental modes of erosion on steep hillslopes, and among the primary hazards in steep alpine terrain. To better understand the processes driving rock slope degradation, mechanisms contributing to the triggering, transport, and deposition of resulting rock slope instabilities, and mitigation measures for associated hazards, we must develop insight into both the physics of intact and rock mass failure and the dynamics of transport processes. This session aims to bring together state-of-the-art methods for predicting, assessing, quantifying, and protecting against rock slope hazards. 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.

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Co-organized by GM4
Convener: Michael Krautblatter | Co-conveners: Axel Volkwein, Anne VoigtländerECSECS, Matthew WestobyECSECS, Andreas EwaldECSECS
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| Attendance Thu, 07 May, 14:00–15:45 (CEST)

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Chat time: Thursday, 7 May 2020, 14:00–15:45

Chairperson: Michael Krautblatter, Axel Volkwein, Matt Westoby, Anne Voigtländer, Andreas Ewald
D1802 |
EGU2020-6793
Greta Bajni, Corrado Camera, and Tiziana Apuani

Due to climate change and the strong relationships between landslides and atmospheric variables, the concept of a stationary landslide susceptibility appears limited. However, relating landslides with climatic predisposing and triggering factors is challenging, due to the lack of multitemporal event datasets. Rockfalls are even more challenging in this context, as their reaction to meteorological events is connected to widely variable characteristics (e.g. rock type, in situ stress, fracture network).

By exploiting and homogenizing a multitemporal rockfall inventory and meteorological datasets of the Aosta Valley Region (Western Italian Alps), the general goal of our study was to develop a procedure to decipher the effects of both the short- and long-term action of rainfall and freeze-thaw cycles on rockfalls occurrence, recognized as main forcing climatic variables in the classic literature. Our specific objective was to define synthetic and effective meteorological variables that can act as predictors in statistical landslide susceptibility models.

We analysed 168 rockfall events and meteorological data from 17 stations from 1990 to 2018 (reference period) distributed on an area 670 km2. The analysis was performed considering:

  • Short term (hourly) precipitation expressed both by the intensity-duration characteristics of the single rockfall associated rainfall(1) and by the maximum cumulated rainfall in time intervals from 0.5 to 24 hours before the event(2);
  • Long term precipitation (multiple episodes) expressed both by cumulated rainfall in time interval of 1 day to 60 days (3) and by the number of rainfall episodes occurred in 1- to 12-month time intervals before the event(4);
  • Number of Freeze-thaw cycles in the year before the event, identified as temperature variation crossing the 0°C value(5).  

By comparing the statistical distribution, for the whole reference period, of the above mentioned climatic variables and the meteorological conditions before each rockfall event, we recognized four types of not ordinary climatic conditions. All conditions resulted to be associated to long term conditions of any time interval, while hourly intervals did not result significant. Type-a is associated to cumulated rainfall overcoming the 90th percentile of the historical time series(69 out of 168); Type-b to a number of rainfall episodes higher than the 75th percentile value(70 rockfalls out of 168); Type–c to a number to a number of freeze-thaw cycles higher than the 75th percentile value(66 out of 168); Type-d to a combination of these factors (47 out of 168). Only 5 rockfalls occurred during ordinary meteorological conditions, whereas the remaining 37 rockfalls could not being analysed due to the absence of complete meteorological data.

Based on these results, we defined a long term Intensity-duration and two episode-duration thresholds, each expressed by a power law equation. The number of times, in the reference period, of exceedance of the selected thresholds represent the synthetic variables to be spatialized by means of geostatistical techniques and tested within a statistical landslide susceptibility model.

How to cite: Bajni, G., Camera, C., and Apuani, T.: Deciphering Rainfall and Freeze thaw cycles as long-term preparatory factors for alpine rockfalls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6793, https://doi.org/10.5194/egusphere-egu2020-6793, 2020

D1803 |
EGU2020-11955
Ingo Hartmeyer, Robert Delleske, Markus Keuschnig, Michael Krautblatter, Andreas Lang, Lothar Schrott, and Jan-Christoph Otto

Over the past 150 years almost half of the glacier volume disappeared in the European Alps. Besides glacier retreat, ice surface lowering reflects much of the volume loss and uncovers the adjacent rockwalls. In steep glacial cirques, this process exposes rock to atmospheric conditions for the very first time in many millennia. Instability of rockwalls has long been identified as one of the direct consequences of deglaciation, but so far cirque-wide quantification of rockfall at high-resolution is missing and the proportional contributions of low-, mid- and high magnitude rockfalls have remained poorly constrained. 
We use terrestrial LiDAR to establish a rockfall inventory for the permafrost-affected rockwalls of two rapidly deglaciating cirques in the Central Alps of Austria (Kitzsteinhorn). During six-year monitoring (2011-2017) 78 rockwall scans were acquired. Overall, we registered 632 rockfalls ranging from 0.003 to 879.4 m³, which concentrate along pre-existing structural weaknesses. 60 % of the rockfall volume detached from less than ten vertical meters above the glacier surface, indicating enhanced rockfall over tens of years following deglaciation. 
Antecedent rockfall preparation is assumed to start when the rockwall is still ice-covered: Inside the Randkluft (gap between cirque wall and glacier) sustained freezing and sufficient water supply likely cause enhanced weathering and high plucking stresses. Following deglaciation, pronounced thermomechanical strain is induced and an active layer penetrates into perennially frozen bedrock, likely contributing to the observed paraglacial rockfall increase close to the glacier surface. 
Observed mean cirque wall retreat of 1.9 mm a-1 ranks in the top range of reported values and is mainly driven by enhanced rockfall from the lowermost, freshly deglaciated sections of the investigated rockwalls. Rockfall magnitude-frequency distribution, which has never been quantified before for deglaciating cirques, follows a distinct negative power law distribution over four orders of magnitude. Magnitude-frequency distributions in glacier-proximal and glacier-distal rockwall sections differ significantly due to an increased occurrence of large rockfalls in recently deglaciated areas. The present study thus demonstrates how recent climate warming shapes glacial landforms, controls spatiotemporal rockfall variation in glacial environments and indicates an exhaustion law over decades for rockfall activity immediately following deglaciation crucial for future hazard assessments.

How to cite: Hartmeyer, I., Delleske, R., Keuschnig, M., Krautblatter, M., Lang, A., Schrott, L., and Otto, J.-C.: Paraglacial responses in deglaciating cirque walls: Implications for rockfall magnitudes/frequencies and rockwall retreat, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11955, https://doi.org/10.5194/egusphere-egu2020-11955, 2020

D1804 |
EGU2020-7303
Yu-Hsuan Chang, Cheng-Han Lin, and Ming-Lang Lin

Joint persistence and groundwater are critical factors that influence the stability of rock slope. Persistence dominates the extent of pre-existing potential failure surfaces. Under certain conditions, slope instability may vary with time, as the propagation of existing joints leads to the development of fully persistence failure surfaces. At the same time, groundwater may travel through the fracture network and provides an external force to unstable rock masses, resulting in the damage of rock slope failure hard to predict. In general, when a rock slope consists of two or more sets of joints, the wedge failure often becomes the initial structurally controlled failure of a progressive large landslide. A classic case, which was occurred at a steep cut rock slope on 32.5k, Provincial Highway 7, Taiwan, had been completely recorded with UAV-surveys, field investigations and witness. The landslide first occurred on 13th May 2019 as a wedge failure with the magnitude of the volume of 892 m3 and resulted in a large landslide on 29th July 2019 with the magnitude of the volume of 37234 m3, destroyed the protection measures and roads. According to the field investigation, groundwater was discovered flowing out from the line of intersection of persistence joints, which could be the main reason leads to the wedge failure and the progressive large rockslide. Hence, the couple mechanics-hydraulic behavior in a rock slope should be studied in more detail to mitigate such hazards.

In this study, sandbox model was applied to clarify the effects of the groundwater and joint friction on failures of single rock wedge. In addition, the software 3DEC, which is based on Distinct Element method, was carried out to extent the analysis conditions. The results of sandbox simulations were used to calibrate the performance of the numerical model, especially the coupled hydro-mechanical analysis. The stability of jointed rock slopes under different persistence and various water pressure conditions has been studied. It is believed that the study can enhance the way for stability analysis and monitoring of the potential failure of jointed rock slopes.

Keywords: Wedge failure; Joint persistence; Groundwater; Rock slope stability.

 

How to cite: Chang, Y.-H., Lin, C.-H., and Lin, M.-L.: Influences of Joint Persistence and Groundwater on Wedge Failure Potential of Jointed Rock Slope, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7303, https://doi.org/10.5194/egusphere-egu2020-7303, 2020

D1805 |
EGU2020-19352
Gerard Matas, Nieves Lantada, Jordi Corominas, Josep Antoni Gili, Roger Ruiz-Carulla, and Albert Prades

Consideration of fragmentation during rockfalls is relevant for the assessment of hazard since it affects the number of generated blocks, their trajectories and impact energies, which also depends on the topography. Recently many scholars have paid attention to these phenomena since there are still many uncertainties around fragmentation regarding how mass and energy are distributed after fragmentation and how trajectory dispersion affects risk analysis. We developed a specific fragmentation model (Rockfall Fractal Fragmentation Model), as well as a 3D trajectory simulator called RockGIS with the fragmentation module implemented. In this contribution, we present the calibration of our rockfall trajectory simulator, based on real scale fragmentation tests performed on a quarry.

The RockGIS model considers a lumped mass approach and accounts block fragmentation upon impact with the terrain. Some improvements have been made on the simulator code regarding the consideration of rotation inside the kinematics of the model and restitution factors. The block size distributions obtained from natural rockfall events inventoried, as well as from the real scale fragmentation tests in a quarry, shows a fractal behaviour. On this way, the fractal fragmentation model implemented in the RockGIS simulator is able to reproduce the observed block size distributions.

To calibrate the model we used data gathered from a real scale rockfall test performed in a quarry. We calibrate the relations between the impact energy conditions and the fragmentation model parameters to generate the measured fragments size distribution. The initial volume of the tested blocks were measured manually using a tape and the release positions of the blocks were obtained with terrestrial photogrammetry. Both, the volume and spatial distribution of the fragments after each release were measured on the orthophotos obtained from UAV flights. Three calibration criteria were considered: runout distribution, volume distribution and cumulative volume as a function of the runout. Finally, the degree of fragmentation can be adjusted in the simulations allowing the comparison between different possible hazard scenarios (null, moderate, or severe fragmentation).

Finally, the results of the calibration shows that the RockGIS is able to reproduce the fragmentation behaviour in terms of block size distribution after breakage, as well as the spatial propagation, being a new tool with capabilities to assess the hazard related with fragmental rockfalls and the consequently risk associated.

The RockGIS tool and the fragmentation model based on the data collected from recent rockfall events have been developed within the RockRisk (2014-2016, BIA2013-42582-P) and RockModels (2016-2019, BIA2016-75668-P, AEI/FEDER, UE) projects. Both projects were funded by the Spanish Ministerio de Economía y Competitividad.

How to cite: Matas, G., Lantada, N., Corominas, J., Gili, J. A., Ruiz-Carulla, R., and Prades, A.: Rockfall fragmentation simulations of real scale tests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19352, https://doi.org/10.5194/egusphere-egu2020-19352, 2020

D1806 |
EGU2020-13810
Sibylle Knapp, Philipp Mamot, Bernhard Lempe, and Michael Krautblatter

Rock avalanches destroy and reshape landscapes within only few minutes and are among the most hazardous processes on earth. Water in the travel path may accelerate the rock avalanche, with longer runouts as a result. So far no study has aimed at proving the existence of a paleolake pushed out by a rock avalanche and further analysing the interaction of the moving mass with the former lake. Especially for ancient long-runout mass movements this could be the key to explain exceptional runout lengths.

In this study at the Zugspitze / Eibsee rock avalanche we prove the existence of, and the impact onto a paleolake inside the rock-avalanche trajectories. We assume that there has been a paleo-Lake Eibsee which was displaced by the ~200 mio. m³ rock avalanche. Our approach shows a complementary application of geomorphological mapping (over ~5 km²) and Electrical Resistivity Tomography (ERT) measurements (8 profiles with in total ~9.5 km length), combined with sedimentological analysis in outcrops and drillings. The geoelectrical profiles give us up to ~120 m deep insights into the structure, thickness and distribution of the rock-avalanche deposits, the interactive processes with the lake water and sediments, and the paleotopography. Sediments exposed in outcrops show water-escape structures at the front of the rock avalanche. The data further allow for ERT-calibration at 7 different sites, where it is possible to distinguish materials (rock avalanche, bedrock, lake clay, mixed sediments) and interactive processes of the rock avalanche with the lake and substrate (bulldozing, bulging, overriding of secondary lobes). Here we show how complementary geophysical, geomorphological and sedimentological applications on terrestrial deposits provide detailed insights into multiple effects of impacting of a rock avalanche onto a lake.

How to cite: Knapp, S., Mamot, P., Lempe, B., and Krautblatter, M.: Lake pushed out by 200 m³ rock avalanche (Zugspitze / Lake Eibsee, D) - New geophysical and sedimentological insights into interactive processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13810, https://doi.org/10.5194/egusphere-egu2020-13810, 2020

D1807 |
EGU2020-9675
| Highlight
Federico Agliardi, Marco M. Scuderi, Nicoletta Fusi, and Cristiano Collettini

Giant rockslides creep for centuries and then can fail catastrophically posing major threats to society. There is growing evidence that creeping landslides are widespread worldwide and extremely sensitive to hydrological forcing, especially in climate change scenarios. Rockslide creep is the results of progressive rock failure processes, leading to rock damage accumulation, permeability enhancement and strain localization within basal shear zones similar to tectonic faults. As shear zone accumulate strain, they become thicker and less permeable, favoring the development of perched aquifers. Since then, the creep behavior of mature rockslides becomes dominated by hydro-mechanical interaction with external triggers, e.g. rainfall and snowmelt. However, the mechanisms regulating the slow-to-fast transition toward their catastrophic collapse remain elusive, and statistical and simplified mathematical models used for collapse prediction are usually unable to account for the full spectrum of observed slip behaviors.

Here we couple laboratory experiments on natural rockslide shear zone material, sampled from high quality drillcores, and in situ observations (groundwater level and surface displacement) to investigate the mechanism of rockslide response to short-term pore pressure variations within basal shear zones at the Spriana rockslide (Italy). Using a biaxial apparatus within a pressure vessel, we characterized the strength and permeability of the phyllosilicate-rich shear zone material at in situ stress, as well as the rate and state frictional properties for shear rates typical of the slow-to-fast transition of real rockslides. Then we carried out non-conventional pore pressure-step creep experiments, in which shear stress is maintained at subcritical levels and pore pressure is increased stepwise while monitoring shear zone slip and dilatancy until runaway failure.

Our results, that are quantitatively consistent with in situ monitoring observations, provide a scale-independent demonstration that short-term pore pressure variations originate a full spectrum of creep styles, modulated by slip-induced undrained conditions. Shear zones respond to fluid pressure increments by impulsive acceleration and dilatancy, causing spontaneous deceleration followed by sustained steady-rate creep. Increasing fluid pressure results in high creep rates and eventual collapse. Laboratory experiments quantitatively capture the in situ behavior of giant rockslides, providing physically-based basis to improve forecasting models for giant mature rockslides in crystalline rocks.

How to cite: Agliardi, F., Scuderi, M. M., Fusi, N., and Collettini, C.: Undrained loading in basal shear zones modulates the slow-to-fast transition of giant creeping rockslides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9675, https://doi.org/10.5194/egusphere-egu2020-9675, 2020

D1808 |
EGU2020-13369
Enea Storni, Simon Loew, Marc Hugentobler, and Andrea Andrea Manconi

Valley glaciers have traditionally been expected to significantly influence the stability and movement rates of adjacent paraglacial landslides. However, detailed studies related to the mechanical and displacement interactions between glacier ice and unstable rock slopes are essentially non-existing. This project deals with a detailed in-situ investigation of the spatial variations of the displacement field of the Great Aletsch Glacier in the surroundings of a large active instability, called Moosfluh Landslide. The goals of this project are to assess the mechanical interactions between an active rockslide and an abutting valley glacier based on real field measurements and infer the impacts of glacier ice deformation on landslide dynamics. As most valley glaciers are currently strongly retreating due to global warming, uncovering significant numbers of pre-LIA slope instabilities, this detailed investigation has implications going far beyond academic interest.

The Moosfluh landslide is a Deep-Seated Gravitational Slope Deformation (DSGSD), with superimposed  large (1-5 million m3) secondary rockslides formed in fall 2016, located near the currently retreating tongue of the Great Altesch Glacier (Kos et. al. 2016, Glüer et al. 2018, 2019). In August 2018 we have performed repeat UAV-based photogrammetric surveys during 74 hours and applied Digital Image Correlation (DIC) techniques to record high-resolution surface displacement vector fields of the landslide, stable slopes and adjacent glacier. DIC results show that the landslide toe is composed of two sectors with significant differences in displacement mean velocities (0.5 and 1.5 m in 74 hours, excluding rapid movements from detached blocks). Both landslide sectors induce clear deflections of the glacier vector field, moving with a velocity  of about 0.3 to 0.4 m in 74 hours. This influence tends to be higher near the ice-contact boundary and decrease within a distance of about 100 m and 200 m from the rock slope instability. We investigate the viscous forces at the landslide/glacier contact using the multiphysics simulation software COMSOL and simplified analytical solution, assuming a vertical interface. These forces are then applied to a limit equilibrium landslide stability model representing the real geometry at the interface boundary, and quantitatively explore the true buttressing effects of valley glaciers on a fully developed slope instability. We show that a slope in critical stability conditions can respond strongly to a minor buttressing effect posed by a valley glacier occupying the landslide toe.

How to cite: Storni, E., Loew, S., Hugentobler, M., and Andrea Manconi, A.: Monitoring and Analysis of Landslide-Glacier Interactions at the Great Aletsch Glacier (Switzerland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13369, https://doi.org/10.5194/egusphere-egu2020-13369, 2020

D1809 |
EGU2020-1474
Da Zheng and Hua Zhao

To study the toppling deformed body before construction of the dam at the Gushui hydropower station, we developed here a physical model of the slope on the basis of known local geology and of similarity theory. We simulated valley trenching by a method using prior produced block modules and three levels of excavation, and we studied key hazard factors of deep toppling deformation and the disaster pattern related to anti-dip, layered-rock slope under gravity by a five-stage centrifuge-model test and Universal Distinct Element Code numerical-simulation analysis. The results show the following: (1) The occurrence, development and destruction of deep toppling deformation of anti-dip layered rock slopes must have gone through a long geological history; the accumulation of energy and deformation is a very long process, and accelerated-deformation is closely related to changes in external conditions (such as excavation, earthquake, etc.); (2) lithologic conditions (relatively weak rock mass), structural conditions (appropriate layer thickness and dip angle), and external conditions (valley trenching or excavation of slopes) are key factors for deep toppling deformation, while the free-surface condition is the key hazard factor; (3) deep toppling deformation can lead to multilevel bending zones at different depths inside the slope after the several stages of valley trenching (multilevel excavation); the bending zone is gradually connected from the foot of the slope all the way to the top, which eventually becomes the failure boundary; and the development and connection of the bending zone may result in the overall shear failure of the slope along the bending zone; (4) for deep toppling deformation, we propose a qualitative-judgment index and quantitative-judgment indicators of the degree of toppling deformation. We derived quantitative-judgment formulas for the degree of toppling deformation and the calculation formulas were used for the maximum depth of toppling deformation, and we established a system for discrimination of destruction patterns for deep toppling deformation of anti-dip slope.

How to cite: Zheng, D. and Zhao, H.: Centrifuge-model test study of key hazard factors of deep toppling deformation and disaster pattern of anti-dip layered-rock slope under gravity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1474, https://doi.org/10.5194/egusphere-egu2020-1474, 2019

D1810 |
EGU2020-2545
Chia-Ming Lo, Chen-Han Chu, and Yi-Xiang Su

In this study, the small-scale physical modeling tests have considered the impact of the infiltration of rainfall in order to investigate the processes involved in wedge slope deformation and failure. We are conducted under controlled conditions of the intersection angle and half wedge angle. Observations obtained during each stage of deformation and failure were used to explain how gravity deformation varies on wedge slopes, and infer how rainfall influence slope failure. The results indicate that half wedge angle is a crucial factor in the deformation failure of slopes. The failure mechanisms of low intersection angle slopes (sliding model) differ considerably from those of high intersection angle slopes (free falling or toppling model). The infiltration of surface water can have a significant influence on rock layer deformation and the speed of failure. Details of the failure characteristics of wedge slope models are discussed in this paper.

Keywords: physical modeling, rainfall, wedge slope, the intersection angle, half wedge angle.

How to cite: Lo, C.-M., Chu, C.-H., and Su, Y.-X.: Investigation of rainfall-induced failure processes and characteristics of wedge slopes using physical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2545, https://doi.org/10.5194/egusphere-egu2020-2545, 2020

D1811 |
EGU2020-10313
| Highlight
Oliver Sass

Rock moisture is an understudied factor governing weathering and rockfall. Many weathering processes like hydration, shrinking/swelling and thermal cracking are governed by moisture availability, and a high degree of saturation is a precondition for frost cracking. However, weathering studies have primarily focussed on temperatures. The role of moisture supply has not been given the same attention, also because there is no humidity sensor that meets all requirements for application in rock.

In the sandstone area of Saxony in eastern Germany ('Saxonian Switzerland'), climbing on wet rock poses a safety problem as the sandstone loses stability when saturated. Voluntary visitor guidance measures ('rock traffic lights') were implemented to temporarily stop climbing at rocks that are too wet. To accompany this measure scientifically, we carried out a pilot study at the approx. 70 m high Gohrisch sandstone massif, involving moisture measurements in the four cardinal directions (N, E, S, W) at the rockwall base, and at N and S near the summit of the massif. We used a combination of (a) electrical resistivity electrode pairs, combined with wind-driven rain (WDR) collectors; (b) 2D-electrical resistivity (ERT); (c) handheld microwave sensors with four sensor heads for different penetration depth; (d) numerical simulations and (e) Schmidt Hammer measurements to assess rock stability. All techniques were accompanied by laboratory measurements at rock samples.

WDR was registered at two of six sites, the distribution being due to micro-topography rather than wind direction. At these sites a clear response of (decreasing) resistivity on driving rain was registered. ERT profiles using adhesive electrodes showed good reliability (RMS error 5-14%). Most sites were slightly wet at the surface, drier at 5-15 cm depth (which might be due to surface-parallel zones of weakness) and moderately wet at 20-30 cm depth (1000 – 8000 Ohmm). The site Bottom North was much wetter than all others, and the two top positions were dried out at the surface probably due to wind.

This distribution was confirmed by microwave sensor data: Moisture contents show little differences between the sites except of the North site which was wetter at all depths. Schmidt Hammer data was very consistent with microwave moisture in the lab (lower rebound at wetter surfaces); however not in the field, where the wetter Bottom North site showed highest rebound values. The summit positions showed significantly lower rebound which we attribute to stronger weathering (more dry-wet cycles).

Lab results show that the sandstone loses stability (SH rebound) mainly between 60% and 100% pore saturation. Currently we cannot reliably determine if this saturation was actually reached in the field. According to ERT calibration, saturation >60% was only reached near the surface at North Bottom, while at some decimetres depth, saturation rarely exceeded 50%. Calibration from electrical resistivity to moisture and microwave reflectance to moisture was successful in the lab; however, the measured resistivity and microwave range did not match the values measured in the field. Calibration needs to be achieved directly at the field site which remains an open task.

How to cite: Sass, O.: Measuring rock moisture using different techniques in the sandstone area of Saxony, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10313, https://doi.org/10.5194/egusphere-egu2020-10313, 2020

D1812 |
EGU2020-14338
Verena Stoll, Riccardo Scandroglio, and Michael Krautblatter

One of the most important but still unknown destabilizing factors of rock faces in periglacial environments is the contribution of water in terms of hydrostatic pressure (e.g. Piz Cengalo in 2017). Its presence has often been registered in major rock failures, but it has never been quantified. Perched water table >>20m above virtually impermeable permafrost bedrock can cause excessive hydrostatic stress on affected rockwalls. Climate change related intensification of rainstorms as well as permafrost degradation promote water accumulation. An increase in rockfall activity due to higher water pressure peaks is therefore expected, thus intensifying the risk for humans and infrastructures.

Here we conduct a hydromechanical stability analysis at two study sites in the Northern Calcareous Alps where this effect has been observed. We use the distinct element method developed in the software UDEC (Itasca); the required geometric and mechanical model input parameters were obtained from previous studies with direct investigations and laboratory tests in frozen/unfrozen conditions. Infiltration from rainfall or snow/ice melting is expected to create extreme pressure peaks, especially when permafrost seals fractured rock.

Here we present results from:

  1. the permafrost affected Zugspitze summit (Wetterstein Range), where sealing permafrost allows the meltwater to accumulate in the active layer. This causes high hydrostatic pressure, evaluated by relative gravimetry methods and with the help of a fracture mapping.
  2. a preparing high-magnitude rock fall at the Hochvogel (Allgäu Alps), where perched water could destabilize up to 260’000 m³. Displacement measurements on the summit showed acceleration following intense precipitation.

Our model proves that a column of water can bring the Zugspitze north face to instable equilibrium. This happens with different intensities according to frozen/unfrozen conditions and various depth of the active layer, if the hydrostatic pressure is adequate (0.2-0.4 MPa = 20-40 m water column).

Water could also increase the destabilization rates of the south-east face of Hochvogel by adding hydrostatic pressure. A Factor of Safety < 1 is reached when other water-related factors are considered, like: (i) reduction of cohesion in saturated joints, (ii) decrease of the interface friction angle in fractures and (iii) accelerates weathering along the shear plane

How to cite: Stoll, V., Scandroglio, R., and Krautblatter, M.: Modelling rock walls destabilization caused by hydrostatic pressure in frozen/unfrozen bedrock (Hochvogel & Zugspitze, Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14338, https://doi.org/10.5194/egusphere-egu2020-14338, 2020

D1813 |
EGU2020-5292
Emilie Lemaire, Anne-Sophie Mreyen, and Hans-Balder Havenith

The stability of rock slopes is often guided by the structural geology of the rocks composing the slope. Geological structures, such as ductile folds, discontinuities as well as brittle faults and fractures, are known factors contributing to a decrease in slope stability according to their orientation in space - with respect to the general orientation of the main slope and its (seismo-) tectonic damage history. Additionally, a rock slope may undergo many forms of gravitationallyinduced, erosional and/or weathering-induced destabilisation.

Rock slope failures may be classified and described according to several factors, such as their volume, displacement mechanisms and velocity. In this work, especially deep-seated and very large failures (with a volume of >107 m3) are analyzed with regard to their structural characteristics.

Giant rockslides originate as planar, rotational, wedge, compound, or irregular slope failures. Most of them convert into flow-like rock avalanches during emplacement. Here, we will not detail the evolution of rock slope failures but rather focus on their origin. The main goal is to identify features allowing to distinguish seismic trigger modes from climatic ones, notably on the basis of the source zone rock structures. We will present examples of classical anti-dip slope (and along-strike) rock structures that hint at a seismic origin, but we will also consider a series of mixed structural types, which are more difficult to interprete. This morpho-structural study is supported by numerical modelling results showing that seismic shaking typically induces deeper seated deformation in initially ‘stable’ rockslopes.

For failures only partially triggered by dynamic shaking, these study results could help to identify the seismic factor in slope evolution. Especially in less seismically active mountain regions, such as the Alps and the Carpathian Mountains, these analyses can be used for paleoseismic studies – provided that dating the seismic initiation of mass movement is possible. For instance, we will show that the “Tamins” and the “Fernpass” rockslides in the Alps present structural and morphological features hinting at a partly seismic origin. Furthermore, we present study cases of ancient rockslides in the SE Carpathians (“Balta” and “Eagle’s Lake”), where a pure seismic origin is most probable and currently under discussion (supported by numerical analyses).

How to cite: Lemaire, E., Mreyen, A.-S., and Havenith, H.-B.: Structural geology of large (ancient) rockslides - an indicator for a seismic or climatic origin?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5292, https://doi.org/10.5194/egusphere-egu2020-5292, 2020

D1814 |
EGU2020-12633
Hsin-Tien Lee, Guo-Zhang M. Song, Li-Wan Chang, Cang-wei Chen, and Hung-Yen Hu

ABSTRACT    The above-ground (shoot) system of trees can affect slope stability through effects of infiltration facilitation, surcharge and wind loading. The amount of stem flow that infiltrates into soils is determined by diameter at root collar (DRC) of trees. Tree weight (surcharge) is a function of their heights (H) and diameters at breast height (DBH). Wind loading is related to crown area (CA) of trees. To save efforts for measuring all of these traits, we aimed to build regression models which allow researchers to estimate the other three traits with DBH. The study site was located in the Lienhuachih Forest Dynamics Plot, central Taiwan. DBH, DRC, CA and H of 20-30 individuals for the 18 most dominant tree species were measures. Trees which have been snapped off were excluded. Results showed that the regression models between DRC and DBH were linear. The models of CA against DBH and H against DBH was best built with allometric models, indicating that CA and H stop to increase with DBH once DBH reach to a certain size. In terms of model performance, the models of DRC against DBH was best (r2= 0.48- 0.97), followed by those of H against DBH (r2= 0.32- 0.89). The relatively poor performance of CA against DBH models (r2= 0.15- 0.93), especially for light-demanding tree species, indicated the need of incorporating light environment (i.e. crown illumination index) into regression analysis.

 

Key word:allomeric model, broad-leaved forest, diameters at breast height, landslide, Lienhuachih

How to cite: Lee, H.-T., Song, G.-Z. M., Chang, L.-W., Chen, C., and Hu, H.-Y.: Building regression models to estimate tree traits influential to slope stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12633, https://doi.org/10.5194/egusphere-egu2020-12633, 2020

D1815 |
EGU2020-13512
Chunwei Sun, Marc-Henri Derron, Michel Jaboyedoff, and Sixiang Ling

The water-rock chemical interaction of black shale interbedded with limestone along the bedding slip zone and its deterioration to the surrounding rock mass in Xujiaping rockslide is studied. As an important rock-forming mineral in black shale, pyrite is known for being easily oxidized to produce sulfuric acid in water, and sulfuric acid is a significant factor that leads to the dissolution of minerals. Significant number of erosion pits on the limestone were found and many geochemical phenomenon such as extremely low pH fissure water and the secondary mineral phases were investigated. Rock and water samples from this site were analyzed to determine mineralogy, chemical composition and hydrochemistry. The results indicate that many major elements and heavy elements are dissolved, such as Fe, Mn, Si, Zn, Ni, Al, S, Mg, Ca, Na, K, Co and Sr, because of the strong dissolution ability of acid water from black shale.The acid water migrates along the slip zone to exposed surface of cliff and fractures, where it evaporates to form the secondary mineral phases including melanterite, rozenite, szomolnokite, and gypsum etc. The water-rock chemical interaction in Xujiaping rockslide is a combination of dissolution, oxidation, dehydration, and neutralization reactions. Besides, the deterioration mechanism is expanded on two aspects: (1) rock-forming minerals, carbonate minerals especially are prone to be dissolved by sulfuric acid from oxidation of black shale in the slip zone; (2) the crystallization volume expansion of minerals precipitated, which leads to the further expansion and deformation of fractures.

How to cite: Sun, C., Derron, M.-H., Jaboyedoff, M., and Ling, S.: Oxidation of black shale and its deterioration mechanism in Xujiaping rockslide, Southwestern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13512, https://doi.org/10.5194/egusphere-egu2020-13512, 2020

D1816 |
EGU2020-13540
Li Fei, Marc-Henri Derron, Tiggi Choanji, Michel Jaboyedoff, and Chunwei Sun

The weathering posing a significant influence on the rock wall retreat has been widely recognized. In this paper, multi-methods monitoring is designed to detect the erosion and rockfall activity on a rockslide cliff composed of marl-sandstone (maybe mixed with limestone) in Western Switzerland. The monitoring program includes weekly SfM and monthly LiDAR scanning measurements of rock cliff surface, hourly time-lapse imaging of the rock cliff, manual measurement of rock surface moisture, automated recordings of rock temperature and influencing meteorological factors (air temperature, humidity, wind, and precipitation) collected by a weather station. Sequential 3D Points Clouds acquired by LiDAR and SfM from December 2019 are used to visually identify the location of erosion and rockfall at monthly resolution. According to the rock wall structural analysis, the rock mass consists of a network of discontinuities mainly oriented nearly parallel and perpendicular to the direction of the layers. Some fractures are filled with calcite which might lead to a zone of weakness in the rock mass. During the field survey, we saw some calcite crystals covering on the rock block surface in the deposit area and exposed on rock cliff outcrop. We suppose that some rockfalls are generated along those discontinuities filled with calcite where the chemical reaction is active when there is constant water infiltrating during rainfall season. According to the preliminary panoramic thermal image of the cliff surface shot by DJI Mavic 2 Enterprise on 19 December 2019, some weathered and fresh surface areas show different temperatures in the same rock layers which suggest the thermal imaging monitoring may help us to identify the weathering spatial characteristics. In this study, we try first to reveal the effect of temperature variations (thermal stress) on crack deformation from rock temperature values extracted from thermal images and the deformation measured by the crack meter during 24h in winter and summer. Secondly, we explore the role of freeze-thaw cycle playing in the rock fall initiation and rock face erosion. Thirdly, we make clear the link between surface weathering spatial distribution and location of erosion, rockfalls. This provides a model of weathering and rockfall estimation.

How to cite: Fei, L., Derron, M.-H., Choanji, T., Jaboyedoff, M., and Sun, C.: The effect of weathering on rock wall erosion and rockfall generation at La Cornalle, Switzerland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13540, https://doi.org/10.5194/egusphere-egu2020-13540, 2020

D1817 |
EGU2020-13961
Lidia Loiotine, Marco La Salandra, Gioacchino Francesco Andriani, Giovanni Barracane, Marc-Henri Derron, Michel Jaboyedoff, Antonella Marsico, and Mario Parise

Improving the methods for the characterization of rock masses by integrating traditional field surveys with remote sensing techniques is fundamental for practical and realistic discontinuous modelling, in order to identify the failures and kinematics, develop landslide susceptibility assessment and plan prevention and mitigation measures.

A 20 m-high cliff at Polignano a Mare (Southern Italy) was selected as case study for the presence of well-developed discontinuities (bedding and joints) and due to the local morphology, consisting of a valley with opposite slopes at a distance of 150 m, and a pocket beach at their toe. This configuration allowed to perform both traditional and remote sensing surveys. First, photogrammetry methods were carried out on the ground and with the help of a boat. Structure from Motion (SfM) technique was then used to process and combine the pictures, in order to elaborate a raw point cloud of the case study. Secondly, high resolution Terrestrial Laser Scanning (TLS) and Unmanned Aerial Vehicle (UAV) techniques were conducted after positioning Ground Control Points (GCPs) all over the rock mass, with the aim of obtaining a more detailed point cloud. Eventually, a unique and optimized georeferenced point cloud was obtained by combining the previous models, also removing the non-geological objects. Furthermore, Infrared Thermography (IT) was carried out in order to investigate the fracture pattern, the areas of concentrated stress, and the presence of humidity and voids.

The structural analysis of the rock mass was performed directly on the point cloud, by testing procedures and algorithms for the automatic identification of discontinuity sets and of their orientation, spacing, persistence and roughness.

The next step of this research will concern the evaluation of the instability mechanisms with the help of kinematic analyses, by means of stereographic projections. Finally, the reliability of the procedure for a complete rock mass characterization, which is expected to be obtained as the final result, will be tested by means of numerical stability solutions, after calibrating the geomechanical model and importing the fracture system in an appropriate software.

 

How to cite: Loiotine, L., La Salandra, M., Andriani, G. F., Barracane, G., Derron, M.-H., Jaboyedoff, M., Marsico, A., and Parise, M.: Geomechanical characterization of rock masses by means of remote sensing techniques, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13961, https://doi.org/10.5194/egusphere-egu2020-13961, 2020

D1818 |
EGU2020-16503
Jona Schlegel, Annemarie Grass, and Florian Fuchs

Gravitational mass movements like rockfalls or landslides pose a sincere threat to human population and infrastructure in particular in densely populated alpine regions such as the European Alps. Comprehensive identification of such events is challenging since they may occur spontaneously and at previously unknown places in remote areas. Small mass movements in remote areas may even completely evade our attention. Remote sensing surveys may also miss small-scale events in unfavorable conditions such as e.g. high-altitude rocky landscapes. However, comprehensive knowledge and reliable event data are of particular importance for the assessment of hazards imposed by rapid gravitational mass movements.

Consequently it is highly desired to expand our event databases and be open to new ways of data collection. We suggest that hikers and other enthusiasts can contribute to building a scientific database of gravitational mass movements by reporting events they witness or discover in the field. We developed a prototype of a mobile web application that allows anyone to report mass movements and to attach photographs and crucial event information such as location and time. Additional features may be implemented in the future, such as retrieving event information from social media posts. Future versions may also teach enthusiasts to characterize mass movements (e.g. type, volume) so they can contribute valuable information themselves. Ultimately, we are envisioning to form a citizen science community of interested enthusiasts that jointly create a valuable scientific database.

How to cite: Schlegel, J., Grass, A., and Fuchs, F.: CrowdSlide – a mobile web application for building a database of gravitational mass movements using volunteer field reports, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16503, https://doi.org/10.5194/egusphere-egu2020-16503, 2020

D1819 |
EGU2020-19073
Johannes Leinauer, Benjamin Jacobs, and Michael Krautblatter

Costs for (re)installation and maintenance of protective structures are increasing while alpine hazards progressively threaten alpine communities, infrastructure and economics. With climatic changes, anticipation and clever early warning of rock slope failures based on the process dynamics become more and more important. The imminent rock slope failure at the Hochvogel summit (2592 m a.s.l., Allgäu Alps) offers a rare possibility to study a cliff fall at a high alpine carbonate peak during its preparation and until failure. In this real case scenario, we can develop and test an operative and effective early warning system.

The main cleft is two to six metres wide at the summit and at least 60 metres deep at the sides. Several lateral cracks are opening at faster pace and separate different instable blocks. 3D-UAV point clouds reveal a potentially failing mass of 260,000 m³ in six subunits. However, the pre-deformation is yet not pronounced enough to decide on the expected volume. Analysis of historical ortho- and aerial images yields an elongation of the main crack length from 10 to 35 m from 1960 until now. Discontinuous tape extensometer measurements show 35 cm opening of the main cleft between 2014 and 2020 with movement rates up to 1 cm/month. Since July 2018, automatic vibrating wire gauges deliver high-resolution data to an online server. In October 2019, we transferred the system into LoRa with data transmission every 10 min. Automatic warnings via SMS and email are triggered when crossing specific thresholds.

Here we demonstrate long-term process dynamics and 2-years of high-resolution data of a preparing alpine rock slope failure. Corresponding geodetic, photogrammetric, seismic and gravimetric measurements complete the comprehensive measurement design at the Hochvogel. This will help to decipher anticipative signals of initiating alpine rock slope failures and improve future event predictions.

How to cite: Leinauer, J., Jacobs, B., and Krautblatter, M.: Process dynamics, real time monitoring and early warning at an imminent cliff fall (Hochvogel, Allgäu Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19073, https://doi.org/10.5194/egusphere-egu2020-19073, 2020

D1820 |
EGU2020-22287
| Highlight
Samuel Weber, Jan Beutel, Mauro Häusler, Paul R. Geimer, Donat Fäh, and Jeffrey R. Moore

Reliable rock slope stability assessment depends on the ability to characterize and quantify stability relevant properties as for example the internal structure of a rock slope. So far, to our knowledge, no study successfully determined the stiffness of a whole mountain. Here, we evaluate the structural characteristics of the Matterhorn (Swiss Alps) based on ambient vibration measurements using three seismometer stations (Nanometrics Trillium Compact 20s). We identified the fundamental resonant mode which consists of polarized horizontal ground motion at the summit of the Matterhorn. Based on that, we aim to infer the stiffness of the Matterhorn by reproducing field data in 3D numerical eigenfrequency simulation with Young's modulus that vary with strain magnitude.

How to cite: Weber, S., Beutel, J., Häusler, M., Geimer, P. R., Fäh, D., and Moore, J. R.: Can we infer the stiffness of the Matterhorn (CH) based on ambient vibrations?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22287, https://doi.org/10.5194/egusphere-egu2020-22287, 2020

D1821 |
EGU2020-6450
Bo Xu

This study focued on the case of rockfall in the Keelung Mountain Area in the northeastern part of Taiwan. To explore the different trajectories and range including free fall, bouncing and rolling when the rocks fall down, this research tried to analyze the local geomorphological characteristics, distribution of geological materials, and the extension of the discontinuities.

In the results, "coefficient of restitution " and "coefficient of friction" are the most important factors which affect the movement trajectory of bouncing and rolling. The coefficient of restitution is mainly affected by the three factors, such as the strength of slope surface’s material, incident angle, and collision speed. In the situation when falling rocks descend from 2m height, and setting the incident angles as 30°, 45°and 60°, we observed the coefficient of normal restitution as 0.18, 0.12, and 0.10. These results showed that, the coefficient of normal restitution of the rockfall inversely decreased with the incident angle. When fixing the incident angle at 90°, the coefficients of restitution were observed as 0.41, 0.35, and 0.31 when the rockfall from 1 m, 2 m, 3 m. This research found that the coefficient of restitution inversely decreased with the collision speed of rockfall. The size of the falling rocks which was related to the size of the block on the slope, also affected the path of the rockfall based on the bouncing movement. When the size of the rock was smaller than the size of the block at the bottom of the slope, the trajectories were influenced by undulation. When the size of the rock was larger than deposited one, the rock was hard to be affected by slope fluctuation, and continue to keep scrolling. At this situation, the movement of the rockfall was mainly affected by the coefficient of friction rather than the coefficient of restitution’s impact. The simulation is carried out using the Rocscience Rocfall program, which depicts the path and energy of rockfall, these data can be used as important reference basis of prevention of rockfall hazards.

Keywords: Rockfall, Coefficient of Restitution, Coefficient of Friction, Free Fall, Bouncing , Rolling

How to cite: Xu, B.: The study of Rockfall in Keelung Mountain Area of Northeastern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6450, https://doi.org/10.5194/egusphere-egu2020-6450, 2020

D1822 |
EGU2020-11826
Axel Volkwein, Florian Hofstetter, and Marc Hauser

Temporary rockfall protection measures are often implemented by using so-called steel palisades. Such elements can described as a steel surface that is supported perpendicular to the slope surface. In the present case, several sheet piling sections are welded onto a steel frame to form an area 1.5m high and 3m long. At the lateral edges of the surface, steel sections, welded together to form a triangle, create the support of the front surface, so that one side of the triangle is parallel to the impact surface and another side is parallel to the slope surface. At the corners close to the ground, massive steel spikes allow penetration into the ground. The weight of a palisade is about 900kg. An example of such a palisade can be found in [1].

The above barriers are in usage since many years. However, their rockfall energy retention capacity has never been evaluated yet. For that reasons, the Swiss Federal Railways launched a project for a deeper understanding of the performance of the palisades; for an adequate selection of the protection measures and a reliable risk analyses with respect to the variety of rockfall events that can be expected at a specific construction site and might cause failure of a structure.

Failure limits of the palisades are expected regarding the following failure scenarios:

  • tilting of the barrier over the valley side steel spikes
  • displacement of the barrier due to insufficient action of the steel spikes
  • failure of the front surface

In this contribution, the above mechanisms are evaluated by means of 1:1 field tests.  A detailed analysis of performance and failure states will be provided. Furthermore, potential solutions for simple but effective reinforcement of the barriers are discussed.

The field tests were carried out on a slope inclined at an angle of about 30 degrees. Test blocks with a minimum weight of 240kg are thrown onto the palisades with the help of a forestry cableway reaching impact speeds of up to 25m/s. The impact energies vary from 12 to 100 kJ. Impact location and impact speed are determined by means of laterally taken high-speed video records with a frame rate of up to 1000fps and a resolution of 800x600pxs. Furthermore, the accelerations in the test body were measured at 1000Hz and – for some of the tests - the acting anchorage forces at 5000Hz.

 

How to cite: Volkwein, A., Hofstetter, F., and Hauser, M.: Full scale field testing of temporary rockfall protection measures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11826, https://doi.org/10.5194/egusphere-egu2020-11826, 2020

D1823 |
EGU2020-11244
Back-analysis of rockfalls for the definition of an empirical vulnerability function for buildings
(withdrawn)
Sandra Melzner, Paolo Frattini, Federico Agliardi, and Giovanni Battista Crosta
D1824 |
EGU2020-9902
| Highlight
Alexander Preh, Thomas Glade, Arben Koçiu, Emmanouil Fleris, Mariella Illeditsch, Martin Mergili, Nina Marlovits, Joachim Schweigl, and Michael Bertagnoli

For regions with distinct rock cliffs, rock fall represents a serious hazard due to high propagation velocities. In order to pursue territorial planning with an awareness of rock fall hazard, it is necessary to identify those areas that are or may be affected by this process. A detailed analysis of rock fall hazard (at regional or municipal scale) represents a great challenge, as many parameters that are difficult to quantify in the field must be considered (e.g. block sizes, surface conditions, etc.).

The aim of the ongoing “NoeTALUS – Rock fall hazard modelling in Lower Austria” research project is to evaluate and suggest methods, applicable to different scales, which will enable the production of reliable rock fall hazard maps at a justifiable amount of human and financial resources.

Rock fall hazard maps are being prepared for two pilot areas in Lower Austria: the municipality of “Dürnstein” and the western part of the municipality of “Waidhofen an der Ybbs”. In order to answer questions regarding the required quality and effort in collecting data relevant to numerical modelling, investigations under two topographic scales are being conducted. The entire project area is processed at a regional scale (M ≤ 1:10.000). Additionally, ten selected domains within the project area are investigated at a slope scale (M ≥ 1:5.000). In this context, remote sensing methods (LiDAR, photogrammetry) are to be evaluated with regard to their benefits.

Two different simulation models, Rockyfor3D and WURF3D, are used to model rock fall spreading and magnitude. Both models differ in their calculation approach with regard to surface-roughness, energy-damping and rock fragmentation.

Rock fall simulations are being evaluated by comparing observed and calculated deposits. Relevant indicators such as the Critical Success Index, Factor of Conservativeness, or area under ROC are being employed for this task.

The selected approach is intended for identifying those methods that can contribute to the creation of reliable rock fall hazard maps at a reasonable cost. Finally, “recommendations for action” concerning the production of rock fall hazard maps are to be made based upon the comparison of different methodologies.

How to cite: Preh, A., Glade, T., Koçiu, A., Fleris, E., Illeditsch, M., Mergili, M., Marlovits, N., Schweigl, J., and Bertagnoli, M.: NoeTALUS - Methods for producing rock fall hazard maps of different scales in Lower Austria , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9902, https://doi.org/10.5194/egusphere-egu2020-9902, 2020

D1825 |
EGU2020-20353
Nieves Lantada, Jordi Corominas, Josep A. Gili, Gerard Matas, Roger Ruiz-Carulla, Albert Prades, Càrol Puig-Polo, M. Amparo Núñez-Andrés, Jose Moya, Felipe Buill, and Olga Mavrouli

A rockfall is a rapid mass movement generated by the detachment of a rock volume from the slope that falls, rolls and bounces during its propagation downhill. Rockfalls have great destructive potential due to the high kinetic and impact energies that may reach during the propagation. Rockfalls are frequent instability processes in road cuts, open pit mines and quarries, steep slopes and cliffs. The initial mobilized mass can be either a single massive block or a set of blocks defined by the joints present in the massif. During the propagation, the block or blocks detached may break when impacts against the terrain, producing a distribution of fragments with independent trajectories. Knowledge of the size and trajectory of the blocks resulting from fragmentation is critical for the assessment of the potential damage and the design of protective structures.

In this contribution, we summarise the main achievements of the RockModels project (BIA2016-75668-P, AEI/FEDER,UE). This project aims at quantifying the risk induced by fragmental rockfalls, by developing quantitative risk assessment methodologies and providing tools to improve its prevention and mitigation. It has three general objectives: i) Explicit identification of unstable rock volumes and stability assessment; ii)Development and validation of a fragmentation model, iii) Rockfall propagation analysis by means of the development of a 3D simulator tool and its calibration.

The use of geomatic techniques such as terrestrial photogrammetry or from UAV allow the generation of high-resolution 3D models of cliffs and the joint system characterization based on 3D point clouds. The orientation and persistence of joints within the rock mass define the kinematically unstable rock volumes and determine the initial block size distribution.  We inventoried fragmental rockfalls occurred in Spain by obtaining a 3D model, the orthophoto, specific cartographies and detailed volumes measurements to obtain the block size distribution in the deposits of each event. The fragmental rockfalls inventory have been collected in a spatial database using PostGIS and following the INSPIRE directive for natural hazards. This data can be consulted at different scales with a developed Web Map Service (WMS) (https://rockdb.upc.edu/). The inventory is the empirical data used to developed, calibrate and validate the Rockfall Fractal Fragmentation Model proposed, as well as the 3D trajectory simulator RockGIS that incorporates the fragmentation module.

More empirical data has been obtained by performing 4 real scale fragmentation test in a quarry. The impact of each block and trajectories of the fragments were recorded by several high speed cameras from different points of view. A program has been implemented to measure the kinematics of each tested block using the high-speed videos. The obtained kinematic parameters have been used for the calibration of the RockGIS simulator. An additional essay was carry out at laboratory to study the effect of the comminution among blocks. The distribution of fragments obtained confirms that the blocks undergoing greater confinement generate a greater number of fragments decreasing their maximum volume.

How to cite: Lantada, N., Corominas, J., Gili, J. A., Matas, G., Ruiz-Carulla, R., Prades, A., Puig-Polo, C., Núñez-Andrés, M. A., Moya, J., Buill, F., and Mavrouli, O.: The RockModels Project: Rockfalls Characterization and Modelling , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20353, https://doi.org/10.5194/egusphere-egu2020-20353, 2020

D1826 |
EGU2020-18427
Benjamin Jacobs, Andreas Grabmaier, and Michael Krautblatter

The Höllentalklamm (Höllental Gorge) in Grainau is part of the main mountaineering route to the Zugspitze and with up to 2000 daily visitors a major tourist attraction in the Bavarian Alps. Following several recent rock fall events (up to 300 m³) the TU Munich collaborates with the local Alpine Club (DAV-GAP) to detect, assess and monitor rock fall hazards and to develop a benchmark safety concept for the Höllentalklamm. We combine multi-temporal terrestrial laser scanning, field mapping and the use of wireless sensor networks and evaluate the applicability of these methods for deeply incised alpine gorges.

In this study, we investigate a deeply incised and tectonically shaped alpine gorge in a well-researched mountain range (Wetterstein). In visibly accessible areas, multi-temporal terrestrial laser scanning is applied to (a) detect active rock fall areas, (b) identify hazardous objects pre-failure and (c) monitor potentially unstable parts of the rock face. Additionally, larger objects, such as a 600 m³ rock tower located directly above the track, are equipped with a redundant crackmeter system implemented in a wireless sensor network. Together with the DAV Garmisch-Partenkirchen, we are working on the development of safety procedures and the implementation of an automated early warning system. The first results show that terrestrial laser scanning is well-suited to detect post- and pre-failure rock falls above the level of detection, however, monitoring of small deformations remains a challenge. The crackmeters provide sub-millimetre deformation data of the rock tower and show generally stable conditions but a significant sensitivity towards external triggers such snow blasting in spring. Aside from that, direct rock fall hits hinder the sensor maintainace.

Here we show a benchmark rock fall hazard assessment and safety concept for Alpine gorges with high safety demands providing four years of data. This work helps to evaluate the applicability of well-established monitoring techniques in confined and inaccessible terrain (deeply incised gorges).

How to cite: Jacobs, B., Grabmaier, A., and Krautblatter, M.: Benchmark rock fall hazard assessment and safety concept for touristically developed alpine gorges (Höllentalklamm, Bavarian Alps)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18427, https://doi.org/10.5194/egusphere-egu2020-18427, 2020

D1827 |
EGU2020-5783
Laura Blanco, David Garcia-Sellé, Nicolas Pascual, Anna Puig, Maria Salamó, Marta Guinau, Òscar Gratacós, Josep Anton Muñoz, Marc Janeras, and Oriol Pedraza

In recent years, different techniques and devices (LIDAR, photogrammetry, UAVs or hyperspectral sensors….) have been used to acquire large amounts of data for the study of the earth’s surface offering high temporal, spatial and spectral resolutions. However, a problem lies on the availability of an efficient methodology to extract the desired information with geological signification from these large datasets. Minimal intervention of the experienced users and automatic or semi-automatic data processing are mandatory to avoid dilatory processes and to obtain productive results.

Our aim is to develop a new methodology for the identification and classification of changes in the surface of cliffs from consecutive point clouds. The new algorithms implemented recognize the different orientations of the point cloud and then, compare each point respect to a previous one in the normal direction isolating clusters of displaced points. Thereafter, these clusters of points are classified according to geometrical and raw data parameters in a) rockfalls, b) small movements of the rock surface and c) non-interest clusters of vegetation or noise like edge effects. The methodology is focused on creating more geometrical features which serve as criteria to identify and classify the differences between two point clouds. Actually, the number of clusters remains slightly high for manual processing. In this regard, the aim is to minimize the interaction of the user and take advantage of the large volume of data generated from high temporal resolution associated with the monitoring. The high number of events collected along years of monitoring allows the use of Machine Learning techniques to improve the classification of clusters automatically.

Montserrat Massif (Catalonia, Spain) is a singular case study of rockfall risk to apply the developed methodology due to the high presence of visitors, whose security conflicts with natural heritage preservation. For a correct design of infrastructures protection measures, a rockfall monitoring plan is under development including Terrestrial Laser Scanner from 2007.

How to cite: Blanco, L., Garcia-Sellé, D., Pascual, N., Puig, A., Salamó, M., Guinau, M., Gratacós, Ò., Muñoz, J. A., Janeras, M., and Pedraza, O.: Methodology for rockfall activity identification and Machine Learning classification based on Point Clouds monitoring in Montserrat Massif (Spain) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5783, https://doi.org/10.5194/egusphere-egu2020-5783, 2020

D1828 |
EGU2020-19353
Gianluca Sala, Camilla Lanfranconi, Paolo Frattini, and Giovanni B. Crosta

In mountainous areas, rockfall phenomena cause damages and safety problems in residential areas and along transportation facilities. Forests that lay upslope the elements at risk can mitigate rockfall hazard by reducing the kinetic energy of blocks and the probability of impact. Nevertheless, the effects of rockfall protection forests is usually quantified only at local scale.

In order to assess the forest efficiency for different combinations of forest (tree size, forest density, forest position, forest length), morphological (slope gradient) and lithological (expected block volume) conditions, we performed a large set of parametric simulations by using the HY-STONE rockfall simulator (Crosta et al, 2004) with a tree impact algorithm that allows calculating the probability of impact, the loss of energy and the lateral deviation of the trajectories based on forest density, tree size and block volume. For each simulation, we therefore quantified the forest efficiency by using a new energy-based efficiency index (EEI) that measure the reduction of rockfall kinetic energy along the forest.

The results of the parametric simulations show that the block volume, the slope inclination, the tree size, and the forest density are, in decreasing order of relevance, the most sensitive parameters for rockfall efficiency. Due to its importance, the volume of blocks associated to different lithologies found in Central Italian Alps have been analysed through a statistical analysis of talus deposits. This allowed to obtain volume frequency distributions for the different lithologies, and the associated percentiles of expected block volume.

Starting from the parametric simulations, we developed a multiple linear regression that allows to predict an EEI index value (efficiency of protection forest) as a function of forest, morphological and lithological parameters. This regression function has been eventually applied to all the protection forest of Central Italian Alps, providing regional scale maps of rockfall-protection forest efficiency for different block volume percentiles.

 

Crosta, G. B., and F. Agliardi. (2004) Parametric evaluation of 3D dispersion of rockfall trajectories.” Natural Hazards and Earth System Science 4.4: 583-598.

How to cite: Sala, G., Lanfranconi, C., Frattini, P., and Crosta, G. B.: Regional scale mapping of rockfall-protection forest efficiency, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19353, https://doi.org/10.5194/egusphere-egu2020-19353, 2020

D1829 |
EGU2020-11446
François Noël, Synnøve Flugekvam Nordang, Michel Jaboyedoff, and Marc-Henri Derron

When planning for the implantation of transport infrastructures or buildings, it is necessary to identify the land zones that can be reached by rockfalls. These zones should then be avoided if possible, or stabilisation and risk mitigation measures must be considered. 3D preliminary rockfall simulations can be used to help finding the areas where inspections should be prioritised. Using orthophotos, a detailed shaded representation of the terrain and field work, geologists can then note the position of the deposited blocks and sources from past events, among other things. Collecting this information can however be complex, and the blocks can sometimes be mistaken for glacial deposits.

To increase the accuracy of this inspection task, the land can be analysed using a 3D detailed terrain model with artificial colors based on its aspect orientation and slope steepness and artificial shadows based on the ambient occlusion and eye dome lighting methods. Scars left by past rockfall events are then highlighted and some trajectories can be reconstituted. This method can help isolating identified rockfall deposited boulders from erratic blocks and help finding where is the source from. It can also draw attention to the location where a block has settled by showing parts of its trajectory. A relative aging can also be attributed based on the sharpness of the scar edges, with older events appearing smoother or partly erased. This can help estimating the activity of the site when no other information is known.

We applied this method to the Mel de la Niva site in Switzerland while analysing the two main rockfalls from the 2015 event. The 3D model used was created from SfM photogrammetry using pictures acquired on the field by manually flying a DJI Phantom 4 drone over the terrain. The method allowed to identify 1 rockfall that followed the main 2015 event and 7 rockfalls that preceded it, which is quite interesting. Indeed, if activity is observed on a site, inspection of the source cliff should be done to try to identify if a larger event is about to occur.

These identified rockfalls trajectories were validated using a time series of available orthophotos from SWISSIMAGE. Two paths were present before the oldest photo from 1983. Three appear on the 1999 photo. They then happened in between the previous photo from 1995 and the 1999 one. One happened in between the 1999 and 2005 photos. One happened in between the 2010 and 2013 photos and one in between the 2016 and 2017 photos.

The 8 identified trajectories combined with the 2 from 2015 also have an interesting shape. They tend to not directly follow the steepest path of the terrain. This behavior seems to be frequent, especially when the blocks are disk-shaped, and it has also been observed and partly quantified from the rockfall experiment we presented here last year (2019). Data from the Mel de la Niva site has been added to our rockfall database and it will used for the calibration and further developments of our rockfall simulation model.

How to cite: Noël, F., Flugekvam Nordang, S., Jaboyedoff, M., and Derron, M.-H.: Identifying past rockfall trajectories and runout distances from detailed 3D terrain model: The case of the Mel de la Niva mountain, Switzerland., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11446, https://doi.org/10.5194/egusphere-egu2020-11446, 2020

D1830 |
EGU2020-10469
Mauro Rossi, Roberto Sarro, Paola Reichenbach, and Rosa María Mateos

Rockfalls are the most frequent and dangerous instability phenomena in mountainous areas, causing high economic and social damages. Rockfalls are triggered by complex instability mechanisms and the source areas are controlled by environmental factors like geology, the presence of discontinuities and slope angle. Modeling rockfall phenomena is complex and requires diversified input including parameters controlling the boulders trajectories and the source areas identification.

In the Canary Islands, the steep topography and the geological complexity influence the activation of slope dynamics and the occurrence of slope failures. In particular, rockfalls are very common and they represent a major threat to society, costing lives, disrupting infrastructures and destroying livelihoods. In 2011 the volcanic crisis in El Hierro Island triggered numerous rockfalls that affected the road network causing a great social alarm.

After the recent event, we have attempted to identify rockfall source areas using different approaches including probabilistic modeling. The probabilistic approach applies a combination of multiple statistical models and requires a map of the observed source areas as dependent variable and a set of thematic information as independent variables (e.g., morphometric parameters derived from DTM, lithological information that considers the mechanical behavior of the rocks). For the purpose, we have identified various scenarios selecting different training and validation zones and evaluating for each scenario the associated errors. The maps resulting from the models, provide for the whole El Hierro Island, the probability of a pixel being a source area and can be used as input for the rockfall modeling.

How to cite: Rossi, M., Sarro, R., Reichenbach, P., and Mateos, R. M.: Probabilistic identification of rockfall source areas: an example from El Hierro island (Canary Island, Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10469, https://doi.org/10.5194/egusphere-egu2020-10469, 2020