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NH3.1

Large slope instabilities have been recognised in mountainous areas in different lithological and geological domains, and on other planets. Slow to extremely fast moving, complex mass movements have been recognized and sometimes described as strongly interrelated. Many types of slope instabilities can be grouped within this broad class, each presenting different types of hazard and risk. Some major aspects of these slope instabilities are still debated:
- regional distribution and relevance;
- presence, distribution and significance of phenomena on other planets;
- triggering and controlling factors;
- dating of initial movements and reactivation episodes;
- style and state of past and present activity;
- passive and/or active control by structural-tectonic elements;
- hydrological boundary conditions;
- possible evolution and modelling;
- assessment of related hazard;
- influence of anthropogenic factors and effects on structures;
- role on the erosional and sediment yield regime in drainage catchments and mountain belts;
- technologies for monitoring and warning systems, and the interpretation of monitoring data.
Study of these instabilities involves geology, geomorphology, geomechanics, hydro-geochemistry, and geophysics. For landslides on other planets a few of these approaches can be adopted making more difficult the interpretation of the phenomena, the identification of triggerings and controlling factors.
Trenching and drilling can be used for material characterization, recognition of activity episodes, which can be combined with monitoring data for establishing of warning thresholds and systems.
Geophysical survey methods can describe both the geometrical and geomechanical characteristics of the unstable mass. Dating techniques can be applied to determine the age of movements. Modelling can be applied to evaluate instability and failure, triggering (rainfall, seismicity, volcanic eruption, deglaciation), failure propagation, collapse (rock avalanches, debris avalanches and flows), and secondary failures (rockfall, debris flows).
Different hydraulic and hydrologic boundary conditions and hydrochemistry are involved, both at failure and during reactivations. The impacts of such instabilities on structures and human activities can be substantial and of a variety of forms. Furthermore, the local and regional sediment yield could be influenced by the landsliding activity and different landslides (e.g. type, size) can play different roles.

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Convener: Giovanni Crosta | Co-conveners: Federico Agliardi, Masahiro Chigira, Fabio Vittorio De BlasioECSECS
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D1940 |
EGU2020-15211
Larissa de Palézieux, Kerry Leith, and Simon Loew

Large rock slope instabilities affect river channels both due to catastrophic failures and long-term creep. The relationship between rock slop instabilities and processes in the adjacent river system are typically assessed in terms of channel profile perturbations and cross-sectional morphology, e.g. excess topography. However, such relationships can also be evident in planform changes of the channel alignment, e.g. in landslide dams and long-term channel migration. Large scale creeping rock slope instabilities can be considered point sources which introduce sediment laterally to a river channel. In cases in which sediment production from one side of the channel exceeds that of the opposing side, the course of the river can be shifted towards the less active hillslope. The deviation of the channel from its original course may therefore be used as a proxy for relative sediment input of the two opposing hillslopes.

In order to characterize the planform morphology of the river channels, we treat them as signals fluctuating around a smoothed channel and use a fast Fourier transform to extract characteristic wavelengths and amplitudes of the stream network. We observe a consistent increase in amplitude of planform deviation with increasing wavelength with a variability of two orders of magnitude at the shortest wavelength (101 m) and less than one order of magnitude at longer wavelengths (103 m).

When comparing characteristic channel morphologies based on these analyses to the deviation of channels adjacent to mapped landslides, the amplitude of the deviation appears higher than those naturally occurring in the river system at wavelengths similar to twice the landslide width.

How to cite: de Palézieux, L., Leith, K., and Loew, S.: Planform deviations in river channel alignment due to active landsliding in the High Himalaya of Bhutan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15211, https://doi.org/10.5194/egusphere-egu2020-15211, 2020

D1941 |
EGU2020-12411
Saskia de Vilder, Chris Massey, Garth Archibald, and Regine Morgenstern

Large landslides can result in significant geomorphic impacts to fluvial systems, via increased sediment input and subsequent changes to channel behaviour. We present a case-study of the actively moving  ̴65 M m³ Alpine Gardens Landslide in the Fox Glacier Valley, West Coast, New Zealand, to analyse the ongoing geomorphic impacts within the valley floor. Debris flows, sourced from the toe of the landslide, travel down Mill’s Creek and deposit sediment on the debris fan at its confluence with the Fox River. This debris flow activity and associated changes in sediment flux and fluvial behaviour have resulted in re-occurring damage to, and current closure of roads and tracks within the Fox Glacier Valley floor, impacting access to the Westland Tai Poutini National Park, the Fox Glacier, associated tourism, and the Fox Glacier township economy.

Initial movement of the Alpine Gardens landslide was detected in 2015, with aerial imagery analysis between March 2017 and June 2018 indicating that the landslide may be accelerating. This acceleration may potentially result in increased debris flow activity within the landslide complex and sediment flux into the Fox River. To monitor and understand the controls on movement rate, we installed a continuous GPS monitoring station along with rainfall gauges on the landslide in February 2019. On average, the landslide moves at a rate of 0.12 m/day ± 0.13 m/day, however this rate of movement of the landslide is closely correlated to and fluctuates with rainfall. Significant accelerations of 0.5 m/day have occurred after heavy rainfall, with these rainfall events also resulting in large debris flows.

We document and investigate the geomorphic impact of the Alpine Gardens landslide on the Mill’s Creek debris fan and Fox Glacier Valley floor via terrestrial laser scanning, airborne LiDAR, UAV surveys and aerial imagery. From this, we derive a time-series of nine surface change models to document the sediment flux within the Alpine Gardens Landslide and Mill’s Creek debris fan complex. Our initial results reveal that between March 2017 and June 2019, approximately 14.7 M m³ was eroded from the landslide, of which 3.7 M m³ was deposited directly on the debris fan. A further 9.6 M m³ has been transported downstream into the fluvial system. Upstream aggradation has also occurred, with 1.1 M m³ deposited in the river valley immediately upstream of the debris fan between June 2018 and June 2019. Continued monitoring of the Alpine Gardens Landslide and volumetric changes of the landslide complex allows us to understand the controls on the movement and sediment flux within the landslide and the geomorphic impact of large actively moving landslides on the valley floor, particularly within alpine and glacial environments. 

How to cite: de Vilder, S., Massey, C., Archibald, G., and Morgenstern, R.: The geomorphic impact of large landslides: A case-study of the actively moving Alpine Gardens Landslide, Fox Glacier Valley, West Coast, New Zealand., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12411, https://doi.org/10.5194/egusphere-egu2020-12411, 2020

D1942 |
EGU2020-391
Anja Dufresne, Gabriel Wolken, Clément Hibert, Erin Bessette-Kirton, Jeffrey Coe, Marten Geertsema, and Göran Ekström

In Glacier Bay Park and Preserve, Alaska, at least 25 rock avalanches occurred since the mid-1980s. The 2016 Lamplugh rock avalanche, with roughly 70 Mm3 deposit volume, is one of the larger events within the park. It originated from a north-facing bedrock ridge without any obvious trigger, and spread 10 km down Lamplugh Glacier. Based on field surveys, high-resolution digital elevation models, and continuous seismic data, we show that the emplacement dynamics of this supraglacial rock avalanche can be described by two distinct stages. Clear long-period seismic signals during Stage-1 record strong interactions of the rock avalanche debris with the ground, suggesting dynamic processes such as grain collisions and fragmentation ('active or dynamic emplacement' of a granular flow). During this first stage, the debris traveled about 5 km from the base of the slope; its deposit is thin and stretched with a dominant dry and flat area in the center, and has narrow raised margins. Stage-2 was essentially aseismic at long periods and dominated by low-friction sliding at slow deceleration rates ('passive sliding'). This sliding produced the distal roughly third of the total runout length where the deposit has a higher density of flowbands and more prominent, raised margins from entrainment and bulldozing of snow. The higher apparent mobility of supraglacial landslides (relative to their counterparts in other runout environments) may be explained by this two-stage model.

How to cite: Dufresne, A., Wolken, G., Hibert, C., Bessette-Kirton, E., Coe, J., Geertsema, M., and Ekström, G.: Emplacement dynamics of supraglacial rock avalanches: details from the 2016 Lamplugh event, Alaska, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-391, https://doi.org/10.5194/egusphere-egu2020-391, 2019

D1943 |
EGU2020-8040
Anne-Laure Argentin, Günther Prasicek, Jörg Robl, Daniel Hölbling, Lorena Abad, and Zahra Dabiri

In mountain landscapes, landslides often block river courses. Although landslides are well-known threats, the risks imposed by landslide dams are sometimes neglected. The impeding of a river can lead to the submergence of parts of the upstream valley and a failure of the dam can flood downstream terrain in a catastrophic event.

Our aim is two-fold: we are interested in creating a landslide dam susceptibility map relying on modelled landslides and resulting damming of valleys and formation of lakes, and in studying the relation between the occurrence of landslide dams and lithology.

Landslide susceptibility maps are a common tool for natural hazard mitigation, but landslide dam susceptibility maps are rarely produced. Several simple indices (Blockage Index, Backstow Index) have been developed to predict the obstruction capacity and stability of landslides on a river from landslide and catchment characteristics (landslide volume, catchment area, dam height etc.). However, those methods were applied on observed landslides, and did not consider landslide susceptibility. Here, we created a first modelling-based landslide dam susceptibility map and compared it to the results provided by the indices.

Although the relation between lithology and landsliding has been thoroughly studied, no connection with dam formation has been highlighted so far. Lithology has an impact on various characteristics of the landslide, including its volume, and also influences valley geometry. We investigated if some alpine lithological units are more prone to landslide dam formation than others.

In our modelling approach we used a 10 m DEM of the Austrian Alps and stochastically triggered landslides based on slope thresholds. We then simulated the runout of the landslides using a fluid flow solver. For each landslide deposit we computed the maximum dammed volume by filling the landslide-dammed DEM, and compared those volumes to the lithology. We also tested the different theoretical geomorphological indices to predict the impounding of the river and compared them to the actual results provided by our method.

How to cite: Argentin, A.-L., Prasicek, G., Robl, J., Hölbling, D., Abad, L., and Dabiri, Z.: Landslide dam susceptibility in the Austrian Alps inferred from modelled landslides, potential valley damming and lake formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8040, https://doi.org/10.5194/egusphere-egu2020-8040, 2020

D1944 |
EGU2020-10169
Delgado Fabrizio, Zerathe Swann, Schwartz Stéphane, and Benavente Carlos

The western flank of the central Andes presents an exceptional concentration of large paleo-landslides (v> 100*106 m3), most of which being well-preserved morphologies due to low erosion and weathering related to the hyper-arid climate of the Atacama Desert since the Miocene. First order questions are pending about the triggering of those mass-movements, their dynamics, their locations and their roles on the Andean relief evolution. Previous studies included geomorphological analysis and few dating on individual landslides (e.g. in Peru: Margirier et al., 2015; Crosta et al., 2014; Zerathe et al., 2017; Delgado et al., 2020; e.g. in Chile: Strasser and Schlunegger , 2005; Pinto et al., 2008; Crosta et al., 2017). Preliminary regional mapping have been attempted in Peru (Geocatmin-INGEMMET and Audin & Bechir 2006) and in Chile (Matther et al., 2014 and Crosta et al., 2014).

Here we proposed a new and exhaustive mapping of large landslides of the Western Andes updating and homogenizing the previous works. The considered area locates between latitude 15° and 20°S, from the coast to the mean elevation of the Altiplano (~5000 m a.s.l). The landslide mapping was done by using Google Earth and DEMs (TanDEM-X and Pléiades). We mapped polygons (surface area > 0.1 km²) corresponding to destructured areas and strictly including the evidence of major landslide scarps (cliffs, unusual slope-breaks, etc.) and its sliding mass (offset lithology, boulders fields, etc.).

We identified more than 700 landslides, distributed into three main typologies: (1) deep-seated rockslide (DSR) showing “in-mass” displacement; (2) rock-avalanche (RA) with typical granular-flow morphologies (e.g. levees, boulders fields) and (3) destabilizations associated with both dynamics. This GIS database allows statistical analysis and interpretations crossing the landslide distribution and typologies versus relief properties, geology-lithology, long-term uplift, dating, etc. Preliminary analysis of this database shows that spatial distribution of mass-movements is not homogeneous. Instead, we observed cluster of mass-movements following the main valleys or canyons. They mainly located at elevation between 1500 and 2000 m a.s.l. Interestingly, the largest landslides (surface area > 50 km2) are disconnected to fluvial incision. They occurred within interfluve areas. Few of the largest landslides cover alone more than 30 % of the total cumulated landslide area in this region and, on their own, might contribute at a first order to the relief erosion.

How to cite: Fabrizio, D., Swann, Z., Stéphane, S., and Carlos, B.: Large landslides database along the Central Western Andes (15º - 20º S): constraints on mass-movement development and implications on relief evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10169, https://doi.org/10.5194/egusphere-egu2020-10169, 2020

D1945 |
EGU2020-14611
Patrick Oswald, Jyh-Jaan Steven Huang, Stefano Fabbri, Markus Aufleger, Christoph Daxer, Michael Strasser, and Jasper Moernaut

Catastrophic, pre-historic rockslides are generally well studied in terms of geological controls on slope instabilities, dating of failure events and characterization of the transported mass. Regarding their triggering mechanism, however, either changing climatic forces or strong seismic shaking are discussed in literature, since such mechanisms cannot be unambiguously inferred by directly studying the transported mass or the failure scarp.

Here, we present two independent Holocene lacustrine archives in the Eastern Alps (Lake Plansee and Lake Piburger See), both situated within a spatial cluster of seven large and mostly well-dated rockslides that occurred between 4.2 to 3.0 ka cal BP, comprising the Tschirgant, Eibsee and Fernpass rockslides with up to 1 km³ rock mass volume.

To evaluate a potential seismic trigger for these rockslides, we investigated the lacustrine archives of Lake Plansee and Lake Piburgersee with multiple geophysical (multibeam bathymetric mapping, subbottom profiling) and sedimentological methods (e.g. XRF- & CT scanning) on up to 15m long sediment cores. In the deep Lake Plansee (2,87 km²; 77m deep), earthquakes are expressed by coeval, multiple subaqueous mass wasting deposits, while in the small and shallow Lake Piburger See (0,14 km²; 29 m deep), earthquakes have generated soft-sediment deformation structures such as intraclast breccias and folded strata.

The paleoseismic records derived from the investigated lakes contain 13 event deposits most likely induced by strong earthquakes in the Holocene. Comparison to seismic intensities of historical earthquakes reveals that the investigated lake sediments only record earthquakes exceeding the seismic intensity threshold of VI (EMS-98 scale) at the lake site. At least three earthquake-induced deposits at ~6.8, ~4.0 and ~3.0 ka cal BP are found in both lakes suggesting to be stronger than the region’s maximum documented earthquake (1930 M5.3 in Namlos). Most of the 13 identified pre-historic earthquakes concentrate in the timeframe around 7.0 – 3.0 ka cal BP coinciding with the majority of rockslide events (6.5 – 3.0 ka cal BP). Conspicuously, two strong earthquakes coincide within age uncertainties with two (Tschirgant and Haiming rockslides; ~3.0 ka cal BP) and at least three potentially simultaneous, large rockslides (Eibsee, Fernpass and Stöttlbach rockslides; ~4.0 ka cal BP), respectively. Moreover, an extraordinarily large earthquake-related deposit at 4.0 ka cal BP in Plansee coincides with rockslides in the lake’s vicinity. The same is true for the 3.0 ka cal BP event in Piburger See, pointing also at a spatial coincidence of rockslides and earthquakes.

Our new findings support the interpretation of earthquakes being the major triggering mechanism for large rock slope failures in the Eastern Alps such as e.g. the historically-known Dobratsch rockslide triggered by the AD 1348 Villach earthquake in Carinthia. Changing climatic forces during the Holocene such as heavy rainfall periods may play a significant role in pre-conditioning rock slopes for failure. However, the quiescence in rockslide activity despite a changing climate since 3.0 ka cal BP together with the striking coincidence of the rockslide cluster and the strong earthquakes corroborate the importance of earthquakes as ultimate trigger for large rockslides.

How to cite: Oswald, P., Huang, J.-J. S., Fabbri, S., Aufleger, M., Daxer, C., Strasser, M., and Moernaut, J.: Strong earthquakes as main trigger mechanism for large pre-historic rock slope failures in Western Tyrol (Austria, Eastern Alps): constraints from lacustrine paleoseismology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14611, https://doi.org/10.5194/egusphere-egu2020-14611, 2020

D1946 |
EGU2020-15636
Þorsteinn Sæmundsson, Páll Einarsson, Joaquin Belart, Ásta Rut Hjartardóttir, Eyjólfur Magnússon, Halldór Geirsson, Finnur Pálsson, Gro Pedersen, and Vincent Drouin

A large slow rock mass deformation has been detected in a mountain side north of the Tungnakvíslarjökull outlet glacier, located in the western part of the Mýrdalsjökull glacier in Iceland. A group of scientist from the University of Iceland, National Land Survey and Icelandic GeoSurvey have worked on collecting data from several sources and installed monitoring equipment at the site. According to observations, which were based on comparison of DEM from aerial photographs from 1945 to 2019, the slope has been showing slow rock mass deformation since at least 1945. The rate of movements has been estimated for the period from 1945 to 2019. The data show that the total displacement since 1945 is around 200 m. The data also indicate that the deformation rate has not been constant over this time period and the data shows that the maximum deformation was between 1999 and 2004 of total of 94 m or about 19 m/year.

The mountain slope north of the Tungnakvíslarjökull outlet glaciers reaches up to around 1100 m height. The head scarp of the slide, which is almost vertical, is around 2 km wide rising from about 4-500 m in the western part up to the Mýrdalsjökull glacier at 1100 m in the east. The total sliding from the head scarp down to the present day ice margin is around 1 km2. The total volume of the moving mass is not known as the sliding plane is not known, but the minimum volume might be between 100 to 200 million m3. The entire slope shows signs of displacement and is heavily fractured and broken up. A GPS station that was installed in the uppermost part of the slope in August shows that the slope is moving about 3-9 mm per day, at a constant rate since installation.

There are two main ideas of the causes for this slow rock mass deformation. One is the consequences of slope steepening by glacial erosion, followed by unloading and de-buttressing due to glacial retreat. Another proposed cause for the deformation is related to its location on the western flank of the Katla volcano. Persistent seismic activity in this area for decades may be explained by a slowly rising cryptodome, which may also explain the slope failure.

How to cite: Sæmundsson, Þ., Einarsson, P., Belart, J., Hjartardóttir, Á. R., Magnússon, E., Geirsson, H., Pálsson, F., Pedersen, G., and Drouin, V.: Slow rock mass deformation in the mountain side north of the Tungnakvíslarjökull outlet glacier in western part of the Mýrdalsjökull glacier , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15636, https://doi.org/10.5194/egusphere-egu2020-15636, 2020

D1947 |
EGU2020-8331
Klaus Voit, Christina Rechberger, Christine Fey, Volkmar Mair, and Christian Zangerl

Deep-seated rockslides in Alpine areas are common phenomena, especially if geological and tectonic conditions enable a disintegration of the rock mass extending deep into the ground. Furthermore, the failure process usually is controlled by groundwater flow, permafrost degradation and rock weathering mostly by input of surface water along geological discontinuities as well as by temperature fluctuations. Thereby, extensive slope areas can become unstable and – in the worst case – can endanger population and infrastructure.

At the valley entrance of the Münstertal at the stream Rambach (South Tyrol, Italy), close to the national road SS41 ca. road kilometres 6.5, a deep-seated rockslide was formed at a south-facing mountain slope with a gradient of ca. 30 - 50°. The U-shaped valley was formed by glaciers, whereby the valley floor is filled with alluvial sediments. The rockslide is approx. 400 m wide, measures approx.  700 m in height at its longest extension and comprise a total rock volume of approx.  500,000 m³. The geological bedrock consists of foliated metamorphic rocks (mainly orthogneisses) which partially is covered by talus and glacial sediments. In the past and still continuing, the area was exposed to major tectonic stress due to its close range to the Vinschgau and Schlinig fault zones generating a dense fracture system in the rock mass.

Since several years, the highly active rockslide shows displacements of several metres per year. In 2014, the road SS41 was relocated over a length of ca. 800 m to the other side of the Rambach due to ongoing rock fall events. Field surveys conducted at that time already showed clear geomorphological indications for the destabilization of a large area at the mountain ridge by the presence of primary and secondary scarps, tension cracks, and up-hill facing scarps in the slope area ranging up to the mountain ridge.

Geological field studies in 2018 and 2019 were carried out to investigate the rockslide geometry and kinematics as well as deformation and failure processes. Quantification of the deformation rates was carried out by multi-temporal terrestrial laser scanning (TLS). From a kinematic point of view, the rockslide can be divided into different slabs of varying activity showing actual deformation rates between approx. 0.3 to 3.6 m per year. The individual slabs show a translational movement behaviour with minor internal deformation. However, also a rotational kinematics along polygonal slip surfaces was observed. Disintegration and formation of slabs mostly takes place along pre-existing steeply dipping joint surfaces.

In this contribution, a preliminary geological, geometrical and kinematical model of the current rockslide is presented by the detailed analyses of field mapping and deformation monitoring data.

How to cite: Voit, K., Rechberger, C., Fey, C., Mair, V., and Zangerl, C.: Engineering-geological characterisation and activity analysis of a deep-seated rockslide near Laatsch (South Tyrol), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8331, https://doi.org/10.5194/egusphere-egu2020-8331, 2020

D1948 |
EGU2020-6211
Kuo-Lung Wang, Ching-Weei Lin, Meei-Ling Lin, Rou-Fei Chen, Ya-Ju Hsu, Chih-Yu Kuo, Chien-Chih Chen, Hsin-Hua Huang, Kuo-Jen Chang, Li-Wei Kuo, Chuen-Fa Ni, Bo-Hung Lin, Yi-Hsuan Lee, Hsiao-Yuan Yin, and Mei-Chen Feng

It is always tricky to definite deep-seated or massive scale landslide investigation and monitoring. The scars could map from a high-resolution digital elevation model. However, the activity or sliding depth is merely difficult to define before installing a monitoring system. Lantai potential landslide area is selected for testing and demonstrating newly developed scientific investigation and monitoring techniques. Possible landslide scars have mapped from airborne lidar data, which provided a reference area for DInSAR analysis. More than ten years of DInSAR analysis shows an active/fast-moving area. The sliding plane and geological structure defined from customized earthquake stations and UAV LiDAR following with field verification. The background noise detection can define potential sliding planes from various precipitation events or earthquakes. GPS/leveling stations are installed to monitor ground deformation and verification from DInSAR results providing single point information to the whole area. The drilling holes’ depth is determined from earthquake stations analysis result, geological data, and sliding model from preliminary numerical analysis. Resistivity poles are installed at two holes from 100m beneath the ground surface with connected poles between these two holes to form a window shape monitoring system. The window shape Resistivity Image Profiling system can measure continuously providing not only geological structure variance and groundwater passing this window. New developed optical-fiber water pressure gauges are installed at different depths to verified groundwater pressure and water flow. The deformation system including extensometer, MEMS inclinometer, In-Plane Inclinometer, and Shape Acceleration Array are installed to provide direct displacements from the ground surface to underground. The sliding threshold is thus defined with various measurements from different monitoring methods and with different scales.

How to cite: Wang, K.-L., Lin, C.-W., Lin, M.-L., Chen, R.-F., Hsu, Y.-J., Kuo, C.-Y., Chen, C.-C., Huang, H.-H., Chang, K.-J., Kuo, L.-W., Ni, C.-F., Lin, B.-H., Lee, Y.-H., Yin, H.-Y., and Feng, M.-C.: Scientific Investigation and Monitoring Result of Potential Large Scale Landslide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6211, https://doi.org/10.5194/egusphere-egu2020-6211, 2020

D1949 |
EGU2020-6568
Nikola Toshkov, Jordan Aaron, Simon Loew, Franziska Glueer, and Valentin Gishig

Glacial retreat is often cited as a cause of rock slope instabilities in mountain regions. Until recently, glacial debuttressing was thought to be the main mechanism by which glaciers influence slope stability, however recent work has questioned the efficacy of this mechanism.  It appears that other mechanisms, including slope kinematics and hydro-mechanical interactions between the glacier and slope are important drivers of paraglacial rock slope instabilities.  In the present work, we use discontinuum numerical models to investigate the interaction between rock slope kinematics, slope/glacial hydrology and glacial retreat. 
We perform both a theoretical analysis using a simplified slope geometry, as well as a back-analysis of the Moosfluh Landslide.  For the theoretical analysis, we investigate the response of both toppling and sliding slopes to two factors: the weight of the ice, assumed to be applied as a ductile load acting normal to slope topography, and the variation of the slope water table, which is linked to the ice level and lowers as the glacier retreats. We then apply the insights from the theoretical analysis to investigate the Moosfluh Landslide.  This landslide, which is located at the left flank of the Great Aletsch Glacier Valley (Valais, Switzerland), at the present-day glacial terminus, underwent a dramatic acceleration in 2016 in response to glacier retreat.  The landslide was extensively monitored during this acceleration, and analysis of this data has revealed that the kinematics of movement changed from toppling to secondary sliding.  We simulate the behaviour of the Moosfluh Landslide by implementing a structural model determined from field mapping, and systematically lowering the ice level and slope water table, to simulate glacial retreat.
We find that the interaction between slope kinematics and glacial retreat leads to a complex slope response.  For sliding slopes, the stability of the slope is relatively insensitive to glacial ice loss.  For toppling slopes, the slope response is highly sensitive to ice loss, and the slope is the most unstable at a critical ice level, before ice has completely retreated.  For the Moosfluh instability, we are able to simulate the initial toppling kinematics of this landslide, as well as the transition to sliding triggered by the ice reaching a critical elevation.  Our analysis has important implications for understanding rock slope response to glacial retreat, and highlights the disparate behaviour of toppling and sliding slopes. 

How to cite: Toshkov, N., Aaron, J., Loew, S., Glueer, F., and Gishig, V.: Numerical Investigation of the Stability of Toppling Rock Slopes Subjected to Glacier Retreat, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6568, https://doi.org/10.5194/egusphere-egu2020-6568, 2020

D1950 |
EGU2020-17616
Michael Krautblatter, Benjamin Jacobs, Philipp Mamot, Regina Pläsken, Riccardo Scandroglio, Julian Groß, and Tanja Schröder

This paper discusses mechanical modelling strategies for instable permafrost bedrock. Modelling instable permafrost bedrock is a key requirement to anticipate magnitudes and frequency of rock slope failures in a changing climate but also to forecast the stability of high-alpine infrastructure throughout its lifetime.  

High-alpine rock faces witness the past and present mechanical limit equilibrium. Rock segments where driving forces exceed resisting forces fall of the cliff often leaving a rock face behind which is just above the limit equilibrium. All significant changes in rock mechanical properties or significant changes in the state of stress will evoke rock instability which often occurs with response times of years to 1000 years. Degrading permafrost will act to alter (i) rock mechanical properties such as compressive and tensile strength, fracture toughness and most likely rock friction, (ii) warming subcero conditions will weaken ice and rock-ice interfaces and (iii) increased cryo- and (iv) hydrostatic pressures are expected. We have performed hundreds of laboratory experiments on different types of rock that show that thawing and warming siginficantly decreases both,  rock and ice-mechanical strength between -5°C and -0.0°C.  Approaches  to calculate cryostatic pressure (ad iii) have been published and are experimentally confirmed. However, the importance and dimension of extreme hydrostatic forces (ad iv) due to perched water above permafrost-affected rocks has been assumed but has not yet been quantitatively recorded.

This paper presents data and strategies how to obtain relevant (i) rock mechanical parameters (compressive and tensile strength and fracture toughness, lab), (ii) ice- and rock-ice interface mechanical parameters (lab), (iii) cryostatic forces in low-porosity alpine bedrock (lab and field) and (iv) hydrostatic forces in perched water-filled fractures above permafrost (field).

We demonstrate mechanical models that base on the conceptual assumption of the rock ice mechanical model (Krautblatter et al. 2013) and rely on frozen/unfrozen parameter testing in the lab and field. Continuum mechanical models (no discontinuities) can be used to demonstrate permafrost rock wall destabilization on a valley scale over longer time scales, as exemplified by progressive fjord rock slope failure in the Lateglacial and Holocene. Discontinuum mechanical models including rock fracture patterns can display rock instability induced by permafrost degradation on a singular slope scale, as exemplified for recent a recent ice-supported 10.000 m³ preparing rock at the Zugspitze (D). Discontinuum mechanical models also have capabilities to link permafrost slope stability to structural loading induced by high-alpine infrastructure such as cable cars and mountains huts, as exemplified for the Kitzsteinhorn Cable Car and its anchoring in permafrost rocks (A). 

Over longer time scales, the polycyclicity of hydro- and cryostatic forcing as well as material fatigue play an important role. We also introduce a mechanical approach to quantify cryo-forcing related rock-fatigue. This paper shows benchmark approaches to develop mechanical models based on a rock-ice mechanical model for degrading permafrost rock slopes.

How to cite: Krautblatter, M., Jacobs, B., Mamot, P., Pläsken, R., Scandroglio, R., Groß, J., and Schröder, T.: Towards a benchmark mechanical model for warming permafrost rock slopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17616, https://doi.org/10.5194/egusphere-egu2020-17616, 2020

D1951 |
EGU2020-11869
Mark Reid and Brian Collins

Enhanced landslide mobility can project devastation across extensive areas, greatly affecting hazard and risk. Despite this importance, assessing potential mobility can be challenging as underlying causes of enhanced mobility vary. Liquefaction can dramatically decrease shear resistance and promote mobility, and pervasive liquefaction is well known to boost the mobility of debris flows and other flow slides. However, liquefaction’s potential effect on more coherent slide masses can be difficult to identify in the field. The 2014 Oso, Washington (USA) debris avalanche provides an exceptional opportunity to understand specific causes of liquefaction and enhanced mobility. The slide was more mobile than typical debris avalanches, sweeping over 1 km across a flat alluvial plain to the opposite side of the river valley and killing 43 people as it travelled. Following the 2014 event, we performed detailed investigations aimed at illuminating the event sequence and the mechanisms promoting mobility, with a strong focus on the role of liquefaction.

The landslide initiated in stratified glacial materials and created a variety of landslide deposit types, including a widespread debris-avalanche hummock field covering much of the formerly flat river valley. Our field investigations revealed clear and widespread evidence for sub-bottom (basal) liquefaction as the cause for the slide’s long reach. Soon after the slide event, we mapped more than 350 sand boils – classic indicators of liquefaction – as both isolated vents and groups of multiple vents within the hummock field. We found sand boils in the depressions between hummocks; the hummocks themselves were not liquefied and commonly contained rafted materials such as intact pieces of glacial stratigraphy and forest floor on their surfaces. The sand boils erupted through a variety of glacial sediments, including lacustrine clays. Sand boil grain-size characteristics most closely matched the underlying alluvial sands, rather than the overriding glacial sediments. Evidence of sand boils was transient; most features were eroded from the landscape within a year.

Liquefaction can be induced by several mechanisms, including rapid loading, shearing of loose contractive sediment, and cyclical loading during ground shaking. Given these plausible mechanisms, we used a fully coupled fluid-sediment elastic deformation analysis, as well as triaxial geotechnical testing of the alluvium, to assess potential liquefaction of the materials overrun by the Oso slide. Our results demonstrate that the large failure rapidly loading loose, already wet alluvial sediments likely resulted in their liquefaction. The greatly reduced shear strength of the liquefied alluvium enabled enhanced mobility of the overriding landslide mass on a liquefied base. This process differs from liquefaction of the slide material itself and is therefore not directly dependent on slide-mass properties. Liquefaction of underlying sediments, similar to that observed at Oso, may have enhanced the mobility of other large, coherent landslides in Europe and Asia.

How to cite: Reid, M. and Collins, B.: Enhanced landslide mobility promoted by liquefaction of underlying sediments: Evidence from detailed field, lab, and modelling investigations of the deadly Oso, USA landslide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11869, https://doi.org/10.5194/egusphere-egu2020-11869, 2020

D1952 |
EGU2020-12509
Fang-Cing Liu, Chih-Hsuan Liu, and Ching Hung

  In slope stability analysis, two-dimensional (2D) analysis techniques are usually applied due to its simplicity and extensive applicability. Given that slope failures are three-dimensional (3D) in nature, especially in the slope with complex geometry, a 3D slope stability analysis could lead to more reasonable results [1]. In slope stability analyses, limit equilibrium method (LEM) and finite element method (FEM) are widely used. Note that LEM only satisfies equations of statics and does not consider strain and displacement compatibility; FEM may encounter significant mesh distortion during large deformations where convergence difficulty and the analysis may be terminated before the slope reaches failure [2]. In the study, a Coupled Eulerian-Lagrangian (CEL) method, which allows materials to flow through fixed meshes regardless of distortions, was utilized to investigate 3D slope stability [3]. Validation of the numerical modeling was first presented using a typically assumed 3D slope. After the validation, various types of slopes (i.e. turning corners, convex- and concave-shaped surfaces) with various boundary conditions (unrestrained, semi-restrained, and fully restrained) are carefully conducted to examine the 3D slope stability. It is anticipated the 3D analyses can shed some light on the slope stability analysis with extreme or complex geometry cases and provide more reasonable results.

 

REFERENCE

  1. T.-K. Nian, R.-Q. Huang, S.-S. Wan, and G.-Q. Chen (2012): Three-dimensional strength-reduction finite element analysis of slopes: geometric effects. Canadian Geotechnical Journal, 49: 574–588.
  2. C. Hung, C.-H. Liu, G.-W. Lin and Ben Leshchinsky (2019): The Aso-Bridge coseismic landslide: a numerical investigation of failure and runout behavior using finite and discrete element methods. Bulletin of Engineering Geology and the Environment. doi: 10.1007/s10064-018-1309-3.
  3. C. Han. Lin, C. Hung and T.-Y. Hsu (2020): Investigations of granular material behaviors using coupled Eulerian-Lagrangian technique: From granular collapse to fluid-structure interaction. Computers and Geotechnics (under review).

 

 

How to cite: Liu, F.-C., Liu, C.-H., and Hung, C.: Three-dimensional slope stability study using a Coupled Eulerian-Lagrangian method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12509, https://doi.org/10.5194/egusphere-egu2020-12509, 2020

D1953 |
EGU2020-6470
Chen-Hsun Tang and Guan-Wei Lin

An earthquake-induced large-scale landslide could lead to catastrophic disasters. In order to understand the characteristics of a coseismic landslide, the numerical simulation is a method worth using to reconstruct the movement process of the landslide. The study uses the coupled Lagrangian-Eulerian (CEL) method to simulate the processes of the Aso-Bridge landslide triggered by the 2016 Kumamoto Earthquake (ML 6.5) in Japan. Simulation results are consistent with terrain changes after the collapse and can be used to deduce the ground motion caused by the mass movement.

First of all, the mass movement changed from gradual deformation to rapid displacement when the earthquake acceleration exceeded 0.1 g. Second, the maximum velocity of the landslide reached 35 m/s, and the affected area was successfully estimated. Third, the ground motions induced by the simulated landslide at the ground surface revealed that sliding mass impacted the downslope channel at 40 s after the earthquake occurred. The amplitude of simulated landslide-induced ground motions was more significant than that of ambient noise after the main earthquake ended. Because the ground motions caused by the coseismic landslide were hidden in the vibration of the earthquake, it is difficult to distinguish it from the earthquake's shakes. The results in the study indicated that when the earthquake ended, unfinished landslide-induced ground motions may be identified from the records of nearby seismic stations. The CEL simulation provided valuable information to evaluate the impact of a coseismic landslide.   

Keywords: coseismic landslide, coupled Eulerian-Lagrangian approach, Aso-bridge landslide

How to cite: Tang, C.-H. and Lin, G.-W.: Modeling the coseismic landslide using coupled Eulerian-Lagrangian approach: a case study of 2016 Aso-Bridge landslide, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6470, https://doi.org/10.5194/egusphere-egu2020-6470, 2020

D1954 |
EGU2020-9058
Sahil Sardana, Rabindra Kumar Sinha, Mamta Jaswal, Amit Kumar Verma, and Trilok Nath Singh

The highways in the Himalayas region have an important concern as these are the only connecting corridors to the nearby land area. Manali-Leh highway is one such important route in India which is interrupted frequently by landslides and rockslides events due to freeze-thaw activity, earthquake, heavy rainfall and anthropogenic activities are major triggering factors. In the freeze-thaw activity, water enters into the cracks in rocks during rainfall, subsequently, it freezes, leads to enlargement of cracks and/or the initiation of new cracks due to the volumetric expansion of ice. In the summer season, the ice melts and water migrates to the newly generated cracks and later freezes in the winter season. This, in turn, weakens the rock structure that leads to the reduction of the rock mass strength which promotes instability in the rock slopes. This study focuses on the stability assessment of rock slope along the highway from Solang Valley in Himachal Pradesh, India. This highway connects the Solang Valley to the south portal of the Rohtang tunnel and provides all-weather connectivity, as the Manali-Leh highway shut down during the winter season due to heavy snowfall.

An extensive geotechnical survey was carried out on the studied slope and the rock samples were collected from the field. The artificial freeze-thaw environment was created in the laboratory for the rock specimens to account the natural freeze-thaw effect. Laboratory tests were conducted on the rock specimen conditioned with freeze-thaw to determine the physico-mechanical parameters of intact rock prior to the numerical simulation. The results indicate the significant loss in compressive and tensile strength of rock as the number of freeze-thaw cycles increases. A three-dimensional numerical modelling was performed to assess the stability of the rock slope using the Distinct Element Code (3DEC software). Slope geometry was prepared to represent the actual slope and the various discontinuity sets observed at the field was mapped on the model. The behaviour of the discontinuity sets was modelled using a Mohr-Coulomb slip with residual strength. Normal stiffness of the joints was calculated from rock mass deformation modulus, intact rock young’s modulus and joint spacing. Similarly, the shear stiffness was calculated. The results of numerical modelling show that the displacement of blocks increases and the factor of safety of the slope decreases as the number of freeze-thaw cycles increases.

How to cite: Sardana, S., Sinha, R. K., Jaswal, M., Verma, A. K., and Singh, T. N.: Instability in Himalayan Rock Slope under Recurrent Freeze-Thaw , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9058, https://doi.org/10.5194/egusphere-egu2020-9058, 2020

D1955 |
EGU2020-4016
Masahiro Chigira, Michel Jaboyedoff, Andrea Pedrazzini, and Satoru Kojima

Uphill-facing scarps develop in the central Swiss Alps, particularly along the Upper Rhone valley, the Urseren valley, the Upper Rhine valley, and Bedretto valley. It has been argued whether they have gravitational origin, tectonic origin, or differential uplift after the deglaciation. We made geological survey and topographic interpretation in the Bedretto valley, in which the Ticino River flows from the west-southwest to east-northeast. The Bedretto valley slopes have shoulders on both sides of the valley at elevations of 1500 to 1900 m, below which is a lower U-shaped valley. Uphill-facing scarps develop more on the southern side slopes of the Bedretto valley, where is underlain mainly by mica schist of the Bedretto zone, than on the northern side slopes, where is underlain mainly by gneiss and slate. In addition, they develop much more on slopes higher than the slope shoulders, and the uphill-facing scarps on the lower U-shaped valley are much smaller in scale. Tributary valleys on the south side of the Bedretto valley go down into this lower U-shaped valley from the southeast with intervening ridges, and we surveyed along the valleys of Ri di Cristallina, Ri di Valleggio, and Val Cavagnolo. We found that steeply-dipping schistosity in the ridges is toppled valleyward with brittle fractures along the hinge zones, which are approximately along or slightly higher than the tributary valley bottom. Rock mass as thick as 300 m thus toppled. Flexural toppling of mica schist developed uphill-facing scarps, which were mostly along high-angle faults, some of which were recognized to have brittle crush zones. The flexural toppling generated extension field in the upper ridges, where rock mass apparently settled down along normal faults. The reason why the northern side slopes of the Bedretto valley have much smaller uphill facing scarps may be due to the rocks are mainly gneiss and also due to the numbers of faults are possibly much less than in the southern slopes . The facts that uphill facing scarps are mainly developed above the lower U-shaped valleys may be related to the longer time intervals of the exposure of slopes higher than the slope breaks to the atmosphere during the glacial age.

How to cite: Chigira, M., Jaboyedoff, M., Pedrazzini, A., and Kojima, S.: Flexural toppling and the development of uphill-facing scarps along the Bedretto valley, Swiss Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4016, https://doi.org/10.5194/egusphere-egu2020-4016, 2020

D1956 |
EGU2020-8288
Margherita C. Spreafico, Federico Agliardi, Matteo Andreozzi, Alessandro Cossa, and Giovanni B. Crosta

Large-scale creeping landslides are widespread in alpine areas. Associated long-term, slow deformations threaten urban settlement, railways, main roads and hydropower facilities, on which our society is strictly dependent. Over the next decades, the continuous growing of the global population, the resulting increase in the urbanization (also closer to hazard-prone areas), and the climate change (e.g. melting of alpine glaciers) will increase these interactions and the related risk. Nevertheless, assessing the vulnerability of different types of elements at risk to this kind of hazard is not obvious, especially when hydropower structures (including dams, tunnels, penstocks, etc.) are involved. Large rockslides complexity often results in a variety of different evolutionary trends, making their forecasting and risk reduction a challenge. While catastrophic collapse can cause huge instantaneous damages, slow movements along long periods may lead to progressive damage of structures and infrastructures.
In the alpine and pre-alpine areas of Lombardia (Central Italian Alps), slow rock-slope deformations affect an area of 750 km2, threatening more than 10 km2 of urban areas and about 100 km of penstocks or tunnels related to hydropower facilities. Here we focus on the Mt. Palino slope (Valmalenco, Italian Central Alps), that is affected by a complex, apparently long-lived DSGSD (Deep seated Gravitational Slope Deformation) with a relief exceeding 1000 m. The slope hosts hydropower facilities and a tourist resort. In order to recognize dominant processes and their possible evolution (internal deformation, low-rate steady activity, progressive behaviour, seasonal effects) for better risk assessment and mitigation, we investigated the volume and depth of displaced rock mass and the possible localization of deformations along a basal shear zone. 
Geomechanical and geomorphological surveys, seismic tomography, deep borehole logs and monitoring data (borehole instrumentation, precise levelling, topographic and GB-InSAR) allowed recognizing different sectors with different evolutionary stage and activity degree. The DSGSD which affect the entire Mt. Palino was probably active before the last LGM (Last Glacial Maximum), while only the northern slope sector is now considered as active. We recognized multiple nested phenomena faster than the main mass, identified as large rockslides. They are suspended over the valley floor and may evolve into fast rock avalanches. One of them is located in correspondence with the hydropower penstock, causing differential deformation to the structure. Borehole evidence of localization along cataclastic shear zones was found, motivating a petrographic geomechanical characterization of both rock masses and shear zone samples. Integrated 3D analysis of different information permitted to reconstruct displacement patterns, long-term mechanisms and the controlling factors of possible future evolution. 

How to cite: Spreafico, M. C., Agliardi, F., Andreozzi, M., Cossa, A., and Crosta, G. B.: Large slow rock-slope deformations affecting hydropower facilities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8288, https://doi.org/10.5194/egusphere-egu2020-8288, 2020

D1957 |
EGU2020-5578
Alessandro De Pedrini, Christian Ambrosi, and Cristian Scapozza

The Monte Crenone rock avalanche of 30 September 1513 is one of the most catastrophic natural events in Switzerland and throughout the Alps. The enormous mass of rock that broke away from the western slope of Pizzo Magn or Monte Crenone, estimated at 50-90 million cubic metres, caused the complete damming of the course of the Brenno river, leading to the formation of a basin that extended from Biasca to the Castello di Serravalle in Semione (De Antoni et al. 2016). On 20 May 1515 the basin formed behind the dam overflowed, giving rise to a wave of more than 10 meters high that led to devastation in the territories downstream to reach Lake Maggiore (Scapozza et al. 2015).

In this project, we analyze the dynamics of the 1513 rock avalanche, trying to reconstruct the event through a numerical model, calculated with the software RAMMS::Debrisflow (RApid Mass Movement Simulation) provided by the Federal Institute for the Study of Snow and Avalanches (SLF/WSL).

The realization of the numerical model was preceded by the reconstruction of the topography before the landslide. This first phase of work, included a geological survey of the landslide body, the analysis of digital data (orthophotos, digital topographic maps, shaded model derived from swissALTI3D) and the collection of previous historical data.

The observation of the stratigraphic data obtained from the 701.27, 701.30 and 701.31 boreholes (part of the geotechnical studies for the Chiasso-San Gottardo highway) of the GESPOS database (GEstione Sondaggi, POzzi e Sorgenti) of the Institute of Earth Sciences SUPSI was essential to understand the landslide body thickness and volume in the deposition zone.

From the first phase of data collection and interpretation, we then moved on to the actual reconstruction of the digital model of the terrain before the landslide. This operation was carried out using ESRI's ArcGIS software, which made it possible recreating multiple models of the pre-event topography and thus finding the most realistic solution applicable to the subsequent RAMMS model.

How to cite: De Pedrini, A., Ambrosi, C., and Scapozza, C.: The 1513 Monte Crenone Rock Avalanche. Numerical model and geomorphological analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5578, https://doi.org/10.5194/egusphere-egu2020-5578, 2020

Chat time: Tuesday, 5 May 2020, 16:15–18:00

D1958 |
EGU2020-4890
Clément Boivin

"LONG AND SHORT TIME EVOLUTION OF DEEP SEATED GRAVITATIONAL SLOPE DEFORMATION: CONTRIBUTION TO KNOWLEDGE OF PHENOMENA FOR THE MANAGEMENT OF ALEA IN THE ALPINE MOUNTAINS"

 

C.Boivin a, J.P. Malet a, C. Bertrand b, F. Chabaux c, J. van der Woerd a, Y. Thiery d, F. Lacquement d

a  Institut de Physique du Globe de Strasbourg – IPGS/DA - UMR 7516 CNRS-Unistra

b  Laboratoire Chrono-Environnement – LCE / UMR 6249 CNRS – UFC

c  Laboratoire d’Hydrologie et de Géochimie de Strasbourg – BISE / UMR 7517 – Unistra

d  Bureau de Recherches Géologiques et Minières

 

          The Deep Seated Gravitational Slope Deformation (DSGSD) are defined like a set of rock mass characterized by a generally slow movement and which can affect all the slopes of a valley or a mountain range (Agliardi and al., 2001, 2009; Panek and Klimes., 2016). The DSGSD is identified in many mountains (ex: Alps, Alaska, Rocky Mountains, Andes…) and it can affect both isolated low relief and very high mountain ranges (Panek and Klimes., 2016). This deep instability are identified in many case like the origin zone for important landslide like the example of La Clapière landslide in the Alpes Maritimes (Bigot-Cormier et al., 2005). The DSGSD represent an important object we must understand to anticipate catastrophic landslides.

          Actually, many factors that could be at the origin or controlling the evolution of DSGSD have been identified such as for example the structural heritage, the climate or the tectonic activity (Agliardi 2000; 2009; 2013; Jomard 2006; Sanchez et al., 2009; Zorzi et al., 2013; Panek and Klimes., 2016; Ostermann and Sanders., 2017; Blondeau 2018). The long-term and short-term evolution of DSGSD is still poorly understood but represents an important point to characterize in order to predict future major landslides. A first inventory of DSGSD began to be carried out by certain studies such as Blondeau 2018 or Crosta et al 2013 in the Alps. These same studies have also started to prioritize the factors controlling the evolution of DSGSD.

          It is in order to better understand the short-term (<100 years) and long-term (> 100 years) evolution of the DSGSD of the French Alpine massifs and the link with the occurrence of landslides, that this thesis project is developed. The main objective of this project, will be proposed models of the evolution of DSGSD since the last glaciations. But also to propose key interpretations of the future evolution to locate the areas likely to initiate landslides. Two study areas in the French Alpine massifs were chosen because they represent areas of referencing and localization gaps in DSGSD: Beaufortain and Queyras. They have the advantage of having a low lithological diversity making it possible to simplify the identification of the factors influencing the evolution of DSGSD. A geomorphological analysis on satellite data and on the ground is carried out to locate the DSGSD. Several dating (14C, 10Be or 36Cl) will be carried out to reconstruct the history of these objects and understand the factors that controlled their evolution.

How to cite: Boivin, C.: Long and short time evolution of deep seated gravitational slope deformation: contribution to knowledge of phenomena for the management of alea in the Alpine mountains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4890, https://doi.org/10.5194/egusphere-egu2020-4890, 2020

D1959 |
EGU2020-13851
Monia Calista, Valeria Menna, Enrico Miccadei, and Nicola Sciarra

According to their structural-geomorphological features, different types of landslides, with variable areal extension, largely affect the Abruzzo region (Central Italy) from the mountains to the coastal areas, contributing to the geomorphological evolution of the landscape.

In this work, we present the results of integrated investigations carried out in recent years in the Abruzzo piedmont and the coastal areas. In detail, we investigated the role of the morphostructural setting, seismic and meteorological factors in the development of piedmont landslides, and the geomorphological evolution, erosion and retreat processes widespread along clastic soft rock coasts of the region.

We investigated Ponzano landslide (Civitella del Tronto, Teramo), a large translational slide-complex landslide, affecting the Miocene–Pliocene pelitic-arenaceous bedrock, and the Castelnuovo landslide (Campli, Teramo) a complex (topple/fall-slide) landslide, which involved conglomerate rocks pertaining to terraced alluvial fan deposits of the Pleistocene superficial deposits. Both these landslides occurred in the NE Abruzzo hilly piedmont in February 2017, causing severe damage and evacuees. Regarding the coastal area, we analyzed rockfalls, topples and translational landslides which characterize the active cliffs of Torre Mucchia, Punta Lunga, Punta Ferruccio (Ortona, CH) and Punta Aderci (Vasto, CH), composed of clayey-sandy-arenaceous-conglomeratic marine sequence (Early-Middle Pleistocene) covered by continental deposits (Late Pleistocene-Holocene). These coastal areas are popular tourist destinations, included in natural reserve areas with high tourism, natural and cultural landscape value.

Through this multidisciplinary approach, the lithological, geomorphological and structural-jointing features were estimated. Focusing on their role on the stability, processes and dynamics affecting Abruzzo piedmont and coastal sectors, it was possible to analyze the triggering factors, the landslide mechanisms and types, as well as the most critical and/or failure areas.

The obtained results outline how field and remote investigations combined with FLAC3D numerical modeling provide an effective approach in the analysis of landslides, strongly improving the identification and prediction of landscape changes and supporting a new geomorphological hazards assessment.

How to cite: Calista, M., Menna, V., Miccadei, E., and Sciarra, N.: A multidisciplinary approach to investigate the geomorphological evolution induced by landslides in the piedmont and coastal sectors of Abruzzo region (Central Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13851, https://doi.org/10.5194/egusphere-egu2020-13851, 2020

D1960 |
EGU2020-22360
Valentino Demurtas, Giacono Deiana, and Paolo Emanuele Orrù

Some cases of deep-seated gravitational slope deformations (DSGSDs) and paleo-landslides in central-eastern Sardinia are presented. This study focuses on the Quaternary landslide deposits preserved on the flanks of the Rio Pardu and Rio Ulassai valleys. The area is characterized by a wide
plateau with a prominent Jurassic limestone scarp overlying Palaeozoic metamorphites. The Plio-Pleistocenic uplift, linked to the Tirrenian basin opening and the consequent basalt volcanism, generated high slopes. In the middle-lower Pleistocene, deepening of the valley has been accelerated by
river capture processes. This litho-structural setting is prone to the development of rock falls, toppling and deep-seated gravitational slope deformations. During the upper-middle Pleistocene the gravitational and fluvial dynamics were dominated by the eustatic phases. The aim of this study is to determine the morpho-stratigraphy and main characteristics of the Quaternary landslide deposits using geomorphic, sedimentological and morphotectonic analysis. The use of high resolution UAV (Unmanned aerial vehicle) photogrammetry and geological, structural, geomorphological surveys allowed a detailed morphometric analysis and the creation of interpretative 3d models. This analysis allowed to recognize new morphostructural elements linked to a compound landslide with lateral spreading and sackung characteristics which involves giant carbonate blocks and the underlying foliated metamorphites. This high-resolution data allowed the formulation of new hypotheses about evolution and kinematics of DSGSD and landslides. The results of field surveys, geomorphological and sedimentological analysis of actual and paleo-landslide deposits show morphostratigraphic framework encompasses three order of rockfalls and three order of DSGSD. Cemented, quiescent and active landslide deposits were tentatively attributed to the Pliocene, Pleistocene and Holocene tectonic and climatic events, and compared with the traditional Quaternary stratigraphy of eastern Sardinia.

How to cite: Demurtas, V., Deiana, G., and Orrù, P. E.: Relations between DSGSDs, morphostratigraphy of landslide deposits, tectonic and climatic events in central-eastern Sardinia., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22360, https://doi.org/10.5194/egusphere-egu2020-22360, 2020

D1961 |
EGU2020-21622
Danhui Li and Shibiao Bai

Abstract: The upper reaches of the Yellow River is located in the transition zone between the Tibetan Plateau and the Loess Plateau, of which a large area is covered by extensive loess deposits. The Tibetan Plateau uplift has resulted in a high geomorphic activity. One landslide inventory of this region is compiled, which includes about 100 giant ancient landslides. Furthermore, their positions, boundaries, area, volume and so on are managed in geographic information system (GIS). The determinations of those giant ancient ages are an important step towards understanding the causes, frequency, hazards, the earth surface uplift and landscape-lowering rate. Development of OSL techniques has provided another alternative means of dating landslide and colluvial sediment. There are many challenges and some problems of luminescence dating of landslide and colluvial deposits because of the insufficiently bleached sediments condition. There are also some controversial issues existing in present studies of landslide dating by using Cosmic Ray Exposure (CRE) method. The study use the landslide pond sediments and the dammed lake deposits to dating the giant ancient landslide using OSL techniques, the surface of landslide scarp and boulders to dating the giant ancient landslide using CRE. The two dating results based on different datable landslide elements were be cross-validated using the typical giant ancient landslides in the upper reaches of the Yellow River, China.

Keywords: Giant fossil landslide; cosmogenic nuclides chronology; luminescence dating, the upper reaches of the Yellow River

 

How to cite: Li, D. and Bai, S.: Exposure dating of the giant fossil landslide in the upper reaches of the Yellow River, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21622, https://doi.org/10.5194/egusphere-egu2020-21622, 2020

D1962 |
EGU2020-7622
Andrea Werner, Philip Süßer, and Frieder Enzmann

In order to assess landslide susceptibility, the selection of the controlling factors (i.e., the predictor variables) is crucial. The most important factors for deep-seated landslides are geological settings such as the bedding conditions of rock formations and the distance to faults. We developed a GIS-based semi-automatic method to extract information on the orientation of bedding planes. This method uses information captured by the interpretation of high-resolution digital terrain models (DTMs). In order to calculate dip and dip direction of the bedding planes we have developed the Morpho-Line concept, which uses geometrical information captured by a detailed interpretation of DTMs. To increase the number of data points, additional field measurements were added to the morpho-line data. We have implemented the "accumulated cost" tool, which is similar to thiessen polygons, to interpolate between the data points. This method takes valleys and faults as break lines into account when interpolating bedding orientation values. Dip and dip direction data has been used, in combination with the slope and aspect, to calculate an extended TOBIA model. TOBIA classifies slopes into anaclinal, cataclinal and orthoclinal classes. To obtain a more accurate picture of orthoclinal bedding conditions and their connection to landslides in these areas, we have subdivided the orthoclinal classes. The angle difference between topography and bedding dip has been calculated and divided into classes. According to that model, the highest abundance of landslides is found in slopes classified as cataclinal and orthoclinal. This means that landslides preferably occur where the geological layers are inclined with the slope (cataclinal) or the dip direction is perpendicular to the slope direction (orthoclinal).

How to cite: Werner, A., Süßer, P., and Enzmann, F.: Extended TOBIA model for the assessment of deep-seated landslides , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7622, https://doi.org/10.5194/egusphere-egu2020-7622, 2020

D1963 |
EGU2020-8149
Che-Ming Yang, Wei-An Chao, Jyh-Jong Liao, and Yii-Wen Pan

Luhu landslide occurred at April 13 2018 and locates in Luhu village, Miaoli county, Taiwan during intermittent rainfall. A sequence of rockfall events has been documented also by the local government in early April. Frequent rockfalls and gully erosion possibly resulted failure of a deep-seated landslide (DSL). The estimated maximum thickness, collapsed area and volume of the landslide are about 60 m, 65,000 m2 and 2 million m3, respectively. The purpose of this study is to clarify the failure mechanism and dynamic process of Luhu landslide, which is practically critical case to understanding the susceptibility of deep-seated landslide without direct triggered factors (thereafter uses the term ‘non-triggered DSL’), including earthquake and intense rainfall. Study site is a steep anaclinal slope consisting of thick sandstone, interbedded of sandstone and shale. Multi-temporal ortho-images and digital elevation (surface) models from 1980 to 2019 are collected for geological investigation and geomorphological interpretation. The study area contains three sub-regions: the north, the northwest and the west slopes. The slope failure occurred repeatedly inside the north and the northwest slopes in the early stage. Gully erosion in the west slope progressed to a landslide in early April first and expanded to cover the DSL failure in the northwest slope eventually, blocking the Luchang River and forming a natural dam. In order to further investigating landslide dynamics, seismic records generated by landslide are collected from the broadband seismic network. A series of time-frequency analysis shows that the spectral power distributed in a wide frequency range. Low-frequency seismic signals, which are generated by the unloading/reloading cycle of landslide mass, would be helpful for force history inversion. We propose that the relative high-frequency (HF) signals contains the information about the small block particles interacting with the topographic barriers. The automatic scheme of HF signal detection was adopted to find out the activity of collision/impact of rock block, which can be an indicator of increasing instability. Aforementioned results combined with numerical simulations provide not only the better understanding of failure mechanism of Luhu landslide but also crucial for the identification of non-triggered DSLs and their hazard assessment.

How to cite: Yang, C.-M., Chao, W.-A., Liao, J.-J., and Pan, Y.-W.: Failure and dynamic process of Luhu landslide inferred from the geologic investigation, numerical modeling and seismic signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8149, https://doi.org/10.5194/egusphere-egu2020-8149, 2020

D1964 |
EGU2020-8264
Chiara Crippa, Federico Agliardi, Paolo Frattini, Margherita C. Spreafico, Giovanni B. Crosta, and Elena Valbuzzi

Slow rock slope deformations are widespread in alpine environments. They affect giant volumes and evolve over thousands of years by progressive failure, resulting in long-term slow movements threatening infrastructures and potential evolution into massive collapses. In the alpine sector of Lombardia (Italian Central Alps), 208 mapped slow rock slope deformations affect a total area exceeding 580 km2 and interact with a variety of elements at risk including settlements, hydroelectric facilities and lifelines characterized by different vulnerability to both slow and progressive deformations. In this context, a systematic, reliable and cost-effective approach is required to classify slow rock slope deformations on the regional scale for landplanning, prioritization and analysis of interactions with elements at risk, depending on their style of activity, including not only mean deformation rate, but also their kinematics and spatial complexity. In this work, we implemented a toolbox that integrates different approaches to classify a large dataset of slow rock slope deformations in discrete groups, according to the deformation style and morpho-structural expression of individuals, mapped on regional scale and characterized through remote sensing techniques. The landslide dataset used in this study was obtained by a “semi-detail” geomorphological and morpho-structural mapping on aerial imagery and DEM, performed on regional scale yet including local-scale information (e.g. tectonic lineaments, morpho-structures, landforms, nested deep-seated landslides) and a full set of geological and morphometric attributes. To characterize landslide activity, we use Persistent-Scatterer Interferometry (PSI) data, including PS-InSARTM and SqueeSARTM acquired by different sensors (ERS, Radarsat, Sentinel 1A/B) over different time periods from 1992 to 2017. Since Line-of-Sight velocity of point like data can hamper a correct evaluation of both landslide kinematics and deformation rates, for each phenomenon we automatically selected the most complete PSI datasets. From these, through a 2DSAR decomposition procedure, we derived 2D velocity components and computed the magnitude and orientation of the 2D total displacement vector T.  We then applied a supervised machine learning procedure to automatically classify the kinematics of each landslide (i.e. translational, roto-translational, rotational) depending on the statistical distribution of the T vector orientation. As the evaluation of a representative landslide mean deformation rate is strongly affected by spatial heterogeneity and landslide mass segmentation, we implemented an original peak analysis of the velocity distribution in each landslide to calculate a modal velocity of the main body and automatically outline nested sectors with differential displacement rates. Finally, we classified landslides in types, representative of different styles of activity and potential interaction with elements at risk, by combining PSI analysis results with geological, morpho-structural and morphometric variables in a multivariate statistical analysis framework including sequential Principal Component and K-medoids Cluster Analysis. The entire analysis workflow runs in a semi-automated way through a set of GIS and MatlabTM tools. Our procedure can be applied to different large landslide datasets, providing a fast and cost-effective support to landslide classification, risk analysis, landplanning and prioritization of local-scale studies aimed at granting safety and infrastructure integrity.

How to cite: Crippa, C., Agliardi, F., Frattini, P., Spreafico, M. C., Crosta, G. B., and Valbuzzi, E.: Semi-automated regional classification of slow rock slope deformations integrating kinematics, activity and spatial complexity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8264, https://doi.org/10.5194/egusphere-egu2020-8264, 2020

D1965 |
EGU2020-6536
Jyr-Ching Hu and Chiao-In Lu

The main objective of this study is to present the progress and state-of-the-approaches of PSInSAR with Sentinel-1 radar images to detect the creeping activity of the potential large landslides revealed by LiDAR in the mountainous area of the slate belt in central Taiwan. We choose Qingjing and Lushan area to process the Multi-Temporal InSAR (MTI) to capture the signals the creeping activity associated with the heavy rainfall events. First, we carry out the feasibility analysis to predict whether the MTI analysis is suitable for detecting the potential persistent scatterers (PS) and test the sensitivity with the effect of layover and shadowing resulted from mountainous topography in central Taiwan. In addition, we also take the effect of land cover on PS distributions into account. Second, we set a threshold of LOS (line of sight) velocity of creeping activity to assess the state of activity. Then we make a Vslope for projection of the LOS velocity along the down-slope direction for steep slope located in the potential landslide area. Furthermore, both the ascending and descending orbits are used to get two LOS velocities which allows us to resolve the E–W and vertical velocity components in order to compare with the tectonic motion due to the mountain building process in slate belt. Finally, the analysis in time series of PSInSAR is carried out for the evolution of creeping events in study area. In this study, we also want to improve the efficiency of remote sensing products for operational monitoring with integration of SAR/InSAR products with numerical and analytical geotechnical models for stability analysis of large potential landslide area detected by geomorphological features from LiDAR-derived DEM.

How to cite: Hu, J.-C. and Lu, C.-I.: Detecting recent creeping landslide activity in central Taiwan by multi-temporal InSAR technique, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6536, https://doi.org/10.5194/egusphere-egu2020-6536, 2020

D1966 |
EGU2020-10046
Rou-Fei Chen, Federico Agliardi, Chiara Crippa, De-Cheng Yi, and Ching-Weei Lin

Deep-seated gravitational slope deformations (DSGSD) gains new attention in Taiwan due to their catastrophic impacts on lives and infrastructures during Typhoon Morakot in 2009. As the main Taiwan island is located on a complex convergent plate boundary, conventional observations and analyses suggest that the island’s strong tectonic activity has, along with its subtropical climate and intense human activity in mountain areas, contributed to the formation of deep-seated landslides. It is especially so for high-altitude areas featuring Miocene to Eocene meta-sandstone and slate successions, where reactivations of landslide terrains are observed from field observation and some GPS sites after specific events. Among them, Tienchih, located in Lalong River of Kaohsiung, and Yakou, few km east in Taitung County were assessed as highly landslide-prone area after the heavy precipitation of Typhoon Morakot (over 2700 mm of rainfall within only 5 days). In this areas, several deep-seated landslides were identified according to geomorphological features seen in the 1-m resolution LiDAR DEM and InSAR preliminary results. In Tienchih area, a catastrophic 240-mm displacement sized 6.7 ha was recorded by a continuous GPS site, TENC, in 2016 after a heavy rainfall occurred on June 2. The correlation in the temporal variation of continuous GPS displacement time series and rainfall suggests that the movement is possibly related to gravitational load overlying water-saturated sediments. In addition, the average annual displacement rate of this downslope movement was measured at 20-40 mm/yr using the recently developed temporarily coherence points InSAR (TCPInSAR) technique based on ALOS/PALSAR imagery collected between 2007 and 2011. Apart therefrom, the high-angle thrust with highly fractured metamorphosed sandstone on the hanging wall; and the river incision and lateral river bank erosion are considered as the triggering factor of this catastrophic landslide. Similar triggering factors are responsible for Yakou landslide, where the 2018 landslide event exposed an outstanding cross section of the predisposing geological setting characterized by a tightly folded sequence of metamorphosed sandstone and slates. Spectacular gravitational deformation structures (i.e. kink folds and shear zones) are also found along this slope testifying a long-term displacement history and shedding light on possible kinematic mechanisms controlling its evolution. Through field data, remote sensing techniques and optical methods (i.e. digital image correlation, 3D LiDAR point cloud comparison), we compared the two landslide sites unravelling different deformation styles and identifying nested sectors possibly evolving to collapse. Our primary results demonstrate that valley erosion and deep-seated gravitational creep are significant to the deformation of slate, indicating a block movement with shear concentration at the basal sliding surface with a mainly rotational-translational movement in Tienchih and a translational failure mechanism in Yakou.

How to cite: Chen, R.-F., Agliardi, F., Crippa, C., Yi, D.-C., and Lin, C.-W.: Deformation characteristics, activity and kinematics of deep-seated landslide in the Tienchih and Yakou areas (S Taiwan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10046, https://doi.org/10.5194/egusphere-egu2020-10046, 2020

D1967 |
EGU2020-18278
Van Trung Chu, Shou-Hao Chiang, and Tang-Huang Lin

The arm of this study to analyze the effect of landslide sample position with point-based approaches for landslide susceptibility modeling which were conducted in the hotspot of the land sliding area located downstream of Nam Ma watershed (Sin Ho, Lai Chau, Viet Nam). Seven hundred fifty-nine landslide polygons that occurred in 2018 were mapped by using google earth integrated with field survey and 84 landslide points extracted from the inventory map conducted in 2013. The state-of-the-art sampling techniques and sample partition approach were applied to produce three subsets of training and testing point-based. Such as the highest position point within landslide polygon (SUB1), the centroid of landslide polygon (SUB2) and the point at the highest position within the seed cell area of the landslide polygon (SUB3). Along with that, the optimal strategy in selecting non-landslide samples was also applied and was first explicitly introduced in this study. Besides, multiple landslide conditioning factors were considered including topographic, geomorphological and hydrological groups. Especially beside of commonly used factors such as slope, elevation, curvature, land use land cover, aspect, etc. the unusual variables also considered such as high above the nearest drainage (HAND - the state-of-the-art terrain) or time series disturbance of land surface index was the first use in this study for landslide analysis and other cutting-edge data processing were proposed in this research arming to optimize the most vital part of whole procedure. The next stage of the analysis is landslide susceptibility modeling. In order to have a more objective judgment about the main issue mentioned above, instead of using only one model, we applied three different models namely Random forest (RF), Logistic regression (LR) and Decision tree (DT) to perform three kinds of scenarios by difference subsets of landslides with five folds of training phase. Subsequently, to compare the abilities of those cases, the model performance was assessed by using the area under the receiver operating characteristic curve both in model success rate (AUCSR) and model predictive rate (AUCPR). Finally, based on the results of this study, all three models performed consistent with three scenarios means the SUB2 and SUB3 are quite similar and much higher than the contribution of SUB1. And the model ability analysis indicated that RF can obtain higher accuracy following by LR and the lowest is DT.

Keywords: Sample position, Landslide Susceptibility, Logistic regression, Random forest, Decision tree, Viet Nam.

How to cite: Chu, V. T., Chiang, S.-H., and Lin, T.-H.: Sample Position Affect Landslide Susceptibility Models in Hotspot Area of Nam Ma Basin, Lai Chau, Viet Nam, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18278, https://doi.org/10.5194/egusphere-egu2020-18278, 2020

D1968 |
EGU2020-12166
Man-Il Kim, Jaehwan Kwak, and Namgyun Kim

The forest development has consistently increased that Korea is composed of almost 64% mountain area. The large-scale facilities, like a wind power system foundation, are planned along the top of mountain. As installation of the large-scale facilities, there is a potential risk in the mountain area like landslide, debris flow and so on. Therefore, we has performed some assessments to slopes and streams at mountain areas and roads of a wind power system foundation, which is being a large-scale change topography (1. Risk assessment using GIS analysis and design data, 2. Basic investigation research and detailed investigation research based on a standard from authorities, 3. Vulnerability analysis using a numerical analysis and a quantitative criteria). As a result, we are able to investigate a primary cause for a mountain disaster risk, and establish a planning of disaster mitigation facilities, which are consistent with a local and a geographical characteristic, for the mountain area involved potential risk.

How to cite: Kim, M.-I., Kwak, J., and Kim, N.: Establishment a planning of disaster mitigation facilities by the impact of large-scale artificial structures on mountain slope, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12166, https://doi.org/10.5194/egusphere-egu2020-12166, 2020

D1969 |
EGU2020-12213
Namgyun Kim, Jaehwan Kwak, and Man-Il Kim

In Korea, occurrence frequency of mountain disasters like landslide, debris flow, rock fall are increasing due to the extreme weather such as localized heavy rainfall and typhoon during the summer season. The Korea government is investigating and discovering vulnerable areas of mountain disaster to mitigate the damage of people’s lives and property. In this study, we selected the mountain slope with high probability of collapse among the vulnerable areas of mountain disaster and suggested reinforcement method through risk assessment. The slope safety factor was calculated using the limit equilibrium analysis for risk assessment of mountain collapse. The risk of collapse was determined by comparing the calculated slope safety factor with Korea government (Ministry of Land, Infrastructure, and Transport) restrict slope safety factor. The Slope safety factor suggested by the government (Ministry of Land, Infrastructure and Transport) is divided into three conditions: dry season, rainy season, and earthquake. Geotechnical parameters for limit equilibrium analysis were obtained by soil test. However, the results of the soil test could be different depending on soil sampling location or the weather condition. Therefore, geotechnical parameters were determined by comprehensive analysis such as comparing literature data, reviewing existing design data, and applying empirical formula of N value by standard penetration test. As a result of risk assessment, it was analyzed that there was a risk of mountain collapse in all conditions except dry season, and it was determined that slope stabilization is necessary.

How to cite: Kim, N., Kwak, J., and Kim, M.-I.: Determination method of the geotechnical parameters for assessing the collapse risk of mountain slope, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12213, https://doi.org/10.5194/egusphere-egu2020-12213, 2020

D1970 |
EGU2020-12977
Emmanuel Wyser, Yury Podladchikov, Marc-Henri Derron, and Michel Jayboyedoff

A granular collapse can be regarded as an idealized case of slumping, e.g., landslides. It consists in a sudden release, by the mean of an apparatus, of a dry granular mass initially contained which elasto-plastically collapses under its self weight and flows upon it reaches a new equilibrium.

We investigated such process by, i) performing numerical simulations and observing experimental evidences thanks to a newly designed apparatus that minimizes initial influences of the retaining walls over the collapse dynamic and, ii) developing an analytical formulation for the run-out distance of the granular mass in agreement with both experimental evidences and numerical solutions obtained by a home-made Material Point Method (MPM) implementation in Matlab based on the Generalized Interpolation Material Point (GIMP) variant. Finally, we further iii) showcase the suitability of the MPM solver to study strain localization problems and associated deformations considering homogeneous or inhomogeneous material properties for dry slumping processes.

We report an excellent agreement of the analytical solution with the experimental data. However, numerical solutions are in a similar range of validity but tend to overestimate the runout distance of the collapse. Nevertheless, large deformations induced by the elasto-plastic collapse are well captured by the solver. In addition, we report similar runout distances regardless for elasto-plastic constitutive relation. We finally demonstrate the importance of heterogeneities over the strain localization and the role of initial geometry in the non-linear behavior of the slumps. Moreover, this also establishes MPM as a relevant numerical framework to address fundamental issues for the geomechanics of slumping.

How to cite: Wyser, E., Podladchikov, Y., Derron, M.-H., and Jayboyedoff, M.: From experimental granular collapses to a three-dimensional numerical solver for landslides , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12977, https://doi.org/10.5194/egusphere-egu2020-12977, 2020

D1971 |
EGU2020-5423
Lucas Pelascini, Philippe Steer, Laurent Longuevergne, and Dimitri Lague

Landslides are a complex phenomenon which triggering depends on both intrinsic properties of soils and rocks and external influences such as the action of weather conditions, or earthquakes. Around 6,000 landslides failed the 6th of September 2018 during the Mw 6.6 Hokkaido Eastern Iburi earthquake (Japan), one day after the typhoon Jebi hit the region. If the ground acceleration induced by the seismic waves likely played a major role in the triggering of these landslides, it is unclear how it compares to the respective role of rainfall and atmospheric pressure drop induced by the typhoon. The aim of this work is therefore to investigate the influence of weather conditions on landslide triggering, and more specifically to characterize the relative contributions of rainfall and atmospheric pressure changes on slope stability.

For this purpose, a simple model is developed to describe the two mechanisms and to compare their respective impact on slope stability. The model considers a homogeneous isotropic tilted infinite half-space in one dimension. Slope stability is estimated using a safety factor and a Mohr-Coulomb criterion. In the static case, groundwater is accounted for by adding an unconfined aquifer into the model. Analytical models based on diffusion equations have been used to describe the impact of rainfall and atmospheric pressure changes on slope stability (Iverson, 2000; Schulz, 2009). Extracting a response function from these models allows us to compute the stability change due to any rainfall or pressure time series. The model parameters are taken for a typical slope in Taiwan tilted with a 25° angle and with characteristics of a fully saturated loamy soil at 4 m depth and put under conditions similar to the Morakot typhoon, with more than 240 mm of rain on a 24 h period and an associate atmospheric pressure drop of 4 kPa.

Atmospheric pressure change and rainfall impacts the media in a very different way despite being associated to the same physical phenomenon, pressure diffusion. The atmospheric effect is instantaneous and directly affects the effective stress with a maximum of 4 kPa. This effect decreases over time while the pore pressure is adjusted to the atmosphere. The rainfall effect is delayed in time but has a greater impact on the effective stress, reaching 11.7 kPa almost 2 days after the end of the rainfall event. While atmospheric pressure does not change significantly the safety factor, it can exacerbate the effect of rainfall and advance the failure in time because of the respective temporal lag between the 2 processes.  Therefore, this study may lead to a better understanding of the effect of weather events such as typhoons on landslide triggering and slope stability. Our results call for revisiting in a more systematic approach the role of atmospheric pressure change on landslide triggering during extreme weather events. Because a 1D model may hide some effects associated to hillslope geometry, we then consider 2D numerical models which allow us to offer some first insights on slope stability during weather events, accounting for topography.

How to cite: Pelascini, L., Steer, P., Longuevergne, L., and Lague, D.: The impact of atmospheric pressure change and rainfall for triggering landslides during weather events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5423, https://doi.org/10.5194/egusphere-egu2020-5423, 2020

D1972 |
EGU2020-21441
Giuseppe Dattola, Giovanni Battista Crosta, and Thomas Stewart

The prediction of a landslide behavior is fundamental for the design of early warning system (EWS) as well for the hazard and risk assessment. The evaluation of expected landslide volume (or extent), displacement, velocity and acceleration is mandatory. Very often empirical formulas are used for landslide volume determination whereas semi-empirical methods like the inverse velocity approach are used for time to failure definition.

Various approaches have been proposed in the literature to reproduce the landslide behavior in terms of displacement for landslides which are already in an active state or for which displacement data are available for calibration. Some approaches introduce the material viscosity to reproduce the slow motion of the landslide when the driving factor is the fluctuation of the ground water table. Another strategy consists in using numerical methods in which the material strength reduction is introduced. In other cases more sophisticated constitutive models are employed to reproduce the material behavior.

In this work, we propose an extension of a simple one dimensional mathematical model which reproduces the post failure behavior considering the landslide as an assembly of blocks interacting between each other and moving along the bedrock. In particular, the model takes into account the shear band mechanical behavior by means of a viscous-plastic model based on the Perzyna’s approach with strain-hardening. The interactions between blocks are modelled by formulating an interaction law which takes into consideration also the tangential effects due to friction along the lateral block boundaries. The forcing factors can be the piezometric level oscillation, the seismic shaking and the oscillation of external water reservoir level.

To validate the mathematical model the numerical results are compared with the Little Chief Landslide located in the North Western Canada along the upper Columbia River valley. The landslide involves a mass of about 800 million of m3 with the stable bedrock depth ranging between 100 and 300 meters. This is an extremely slow landslide which has been investigated since 1960’s and for which displacements, piezometric levels and their evaluation in time are available for long time out-wards allowing to test the model. The landslide shows a periodic trend for displacements with cyclic accelerations and stable creeping. This allows for the calibration of the model parameters.

How to cite: Dattola, G., Crosta, G. B., and Stewart, T.: An application of the MIBSA model to the Little Chief Landslide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21441, https://doi.org/10.5194/egusphere-egu2020-21441, 2020

D1973 |
EGU2020-21814
Fabio Vittorio De Blasio
F.V. De Blasio, G. Dattola and G.B. CrostaDept. of Earth and Environmental Sciences, University of Milano-Bicocca, Milan, Italy

Rock avalanches are initially intact rock masses that collapse catastrophically and that during sliding are subjected to a severe fragmentation processes reducing progressively the clasts diameter. The potential energy is so dissipated by friction and fragmentation, in addition to other energy sinks. During the motion of a rock avalanche, particles of tens of micrometers size are generated from crushing, grinding, or chipped off the rock and released to the air generating a suspension hereafter called dust cloud.  

The dust cloud moves away from the rock avalanche sliding path, partly thrust by the energy of impact of the avalanche against obstacles, and partly inheriting the speed of the rocky mass. Moreover, having density slightly higher than air, the cloud is responding to downward thrust exerted by the gravity field. Thus, the cloud velocities may be variable depending on the geometry of collapse and on the initial rock avalanche speed. At high cloud speed, hazards include severe abrasion  and air blast. Also after the high velocity phase the cloud may be hazardous, reducing visibility for hours until dust particles are completely settled. If this process takes place for example in proximity of facilities and transportation lines, problems may arise to traffic flow.

For this reason the prediction of the cloud formation and further motion is an important, albeit poorly developed subject. We are developing a simple physical model which describes cloud formation and motion. Firstly, the cloud is assumed to form by high-energy chipping of the rocks. To calculate the cloud movement, the shape is split up in a set of deformable sub element. By initially imposing the strongly limiting condition of incompressibility, namely, that cloud density does not change, the equations of motion for a deformable cloud can be written. The equations are then solved numerically. Several situations are considered, including (i) a change in the slope inclination, (ii) the presence of an obstacle, (iii) initial high cloud speed inherited by the travelling rock avalanche, in comparison with zero initial speed. So far, the model is capable to reproduce the cloud motion and the increase in the pressure when it strikes an obstacle.

Case studies considered in conjunction with this theoretical work include the recent events of the Pousset and Gallivaggio rock avalanches both in Northern Italy, where rock dust could be recovered from different locations along the cloud path, promptly after the event.

How to cite: De Blasio, F. V.: Generation and propagation of dust cloud during high energy rock-avalanches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21814, https://doi.org/10.5194/egusphere-egu2020-21814, 2020

D1974 |
EGU2020-3817
Seboong Oh, Sungjin Kim, and Kwang Ik Son

In unsaturated soils, the soil water retention curve (SWRC) is most important in the fundamental hydraulic properties. In order to measure SWRCs through an alternative method in Korea, high air entry disks were replaced by micro membranes. Micro membranes are thin in which the air entry value is around 100kPa. Tests with the membrane are fast to reduce the duration of infiltration through the high air entry disk.

The water retention curves using the membrane were compared with the data using high air entry disks from the volumetric pressure plate extractor and Tempe pressure cell for samples of various sites. As a result, the SWRCs using the membrane were very similar for most cases and the micro membrane was verified as a useful tool to measure SWRCs.

The unsaturated hydraulic behavior could be measured easily using the membrane than ceramic disks and the huge amount of data could have been obtained in Korea. Using DB of SWRCs, the hydraulic properties were interpreted based on the parameters of the van Genuchten SWRC model. The void ratio and density are correlated to SWRCs under the same classification soil.

Acknowledgements This research is supported by grant from Korean NRF (2019R1A2C1003604) and MOE (79608), which are greatly appreciated.

How to cite: Oh, S., Kim, S., and Son, K. I.: Experiments and interpretations on unsaturated hydraulic properties using water retention tests based on the membrane in Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3817, https://doi.org/10.5194/egusphere-egu2020-3817, 2020