NH3.9

Large slope instabilities: characterisation, dating, triggering, monitoring and modelling

Large rock slope instabilities have been recognised under very different geological and environmental conditions, lithological and geological domains, and on other planets. Slow to extremely fast moving, complex mass movements have been recognized, sometimes described as interrelated or as evolution stages of a same phenomenon. Many types of slope instabilities can be grouped within this broad class, presenting different types of hazard and risk. This phenomena, triggered by earthquakes, rainfall, snowmelt or deglaciation can originate relevant cascade events (e.g. tsunamis, landslide dams and overtopping, flooding).
Major aspects of these instabilities are still debated:
- distribution both on Earth and other planets;
- triggering and controlling factors and events;
- dating of initial movements and reactivation episodes;
- style and state of past and present activity;
- passive and/or active control of structural features;
- possible displacement evolution and modelling;
- hazard assessment inclusive of cascade events;
- influence of anthropogenic factors and effects on structures;
- role on the erosional and sediment yield regime;
- technologies for monitoring and warning systems, and the interpretation of monitoring data.
Study of these instabilities is interdisciplinar and multidisciplinar. Site investigation, geophysical survey and dating techniques can support geometrical and geomechanical characterization, recognition of activity episodes, monitoring data interpretation for warning thresholds. Different hydrologic boundary conditions and hydrochemistry are involved, both at failure and during reactivations. Modelling is a key element for understanding and evaluating instability and failure (initiation, propagation), triggering (rainfall, seismicity, volcanic eruption, deglaciation), collapse, and secondary failures as well as the effect on the local and regional geomorphological evolution (e.g. sediment yield). Cascade-like events are definitively a possible result and advanced modeling techniques are requested for studying these phenomena and for reliable and robust hazard zonation. Size and evolution of large instabilities require major efforts when assessing the potential impacts on structures and infrastructures, and human activities enforcing a deep understanding and modeling. On the other hand, instabilities on other planets can support indirect environmental and geomechanical characterization.

Co-organized by GM3
Convener: Giovanni Crosta | Co-conveners: Federico Agliardi, Masahiro Chigira, Irene Manzella
vPICO presentations
| Mon, 26 Apr, 15:30–17:00 (CEST)

vPICO presentations: Mon, 26 Apr

Chairpersons: Giovanni Crosta, Federico Agliardi, Irene Manzella
15:30–15:35
Regional studies
15:35–15:40
|
EGU21-1663
|
solicited
Janusz Wasowski, Maurice McSaveney, Luca Pisanu, Vincenzo Del Gaudio, Yan Li, and Wei Hu

Large earthquake-triggered landslides, in particular rock avalanches, can have catastrophic consequences. However, the recognition of slopes prone to such failures remains difficult, because slope-specific seismic response depends on many factors including local topography, landforms, structure and internal geology. We address these issues by exploring the case of a rock avalanche of >3 million m3 triggered by the 2008 Mw7.9 Wenchuan earthquake in the Longmen Shan range, China. The failure, denominated Yangjia gully rock avalanche, occurred in Beichuan County (Sichuan Province), one of the areas that suffered the highest shaking intensity and death toll caused by co-seismic landsliding. Even though the Wenchuan earthquake produced tens of large (volume >1 million m3) rock avalanches, few studies so far have examined the pre-2008 history of the failed slope or reported on the stratigraphic record of mass-movement deposits exposed along local river courses. The presented case of the Yangjia gully rock avalanche shows the importance of such attempts as they provide information on the recurrence of large slope failures and their associated hazards. Our effort stems from recognition, on 2005 satellite imagery, of topography and morphology indicative of a large, apparently pre-historic slope failure and the associated breached landslide dam, both features closely resembling the forms generated in the catastrophic 2008 earthquake. The follow-up reconstruction recognizes an earlier landslide deposit exhumed from beneath the 2008 Yangjia gully rock avalanche by fluvial erosion since May 2008. We infer a seismic trigger also for the pre-2008 rock avalanche based on the following circumstantial evidence: i) the same source area (valley-facing, terminal portion of a flat-topped, elongated mountain ridge) located within one and a half kilometer of the seismically active Beichuan fault; ii) significant directional amplification of ground vibration, sub-parallel to the failed slope direction, detected via ambient noise measurements on the ridge adjacent to the source area of the 2008 rock avalanche and iii) common depositional and textural features of the two landslide deposits. Then, we show how, through consideration of the broader geomorphic and seismo-tectonic contexts, one can gain insight into the spatial and temporal recurrence of catastrophic slope failures  in Beichuan County and elsewhere in the Longmen Shan. This insight, combined with local-scale geologic and geomorphologic knowledge, may guide selection of suspect slopes for reconnaissance, wide-area ambient noise investigation aimed at discriminating their relative susceptibility to co-seismic catastrophic failures. We indicate the feasibility of such investigations through the example of this study, which uses 3-component velocimeters designed to register low amplitude ground vibration.

How to cite: Wasowski, J., McSaveney, M., Pisanu, L., Del Gaudio, V., Li, Y., and Hu, W.: Recurrent rock avalanches progressively dismantle mountain ridges in the Longmen Shan, China, most recently in the 2008 Wenchuan earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1663, https://doi.org/10.5194/egusphere-egu21-1663, 2021.

15:40–15:42
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EGU21-10944
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ECS
Yu-Chung Hsieh, Mien-Ming Chen, Tung-Lin Tai, and Chung-Chi Chi

In the mountain area, Deep seated gravitational slope deformation (DSGSD) is a phenomenon that causes rock mass deformation under long-term gravity. In the Slate Belt of the Backbone Range where mainly slate distributed, it is more susceptible to develop DSGSD. After Typhoon Morakot, the high-resolution LiDAR digital terrain data of the entire island of Taiwan could be applied to visual interpretation with the potential landslide area. In this study, we used existing high-resolution LiDAR data and the latest computerized 3D environments to conduct and explore preliminary geological information at the regional scale and potential large-scale landslide distribution with detailed topographical characteristics. Through field investigations and UAV application in Lusan area of central Taiwan, the features caused by regional tectonic effects or DSGSD could be clarified and discussed possible mechanism of rock mass failure caused by these DSGSD. The results help to understand the deformation mechanism of the slate area in the Central Range of Taiwan. In the future, we could further explore the possible causes of why DSGSD transform into catastrophic landslides.

How to cite: Hsieh, Y.-C., Chen, M.-M., Tai, T.-L., and Chi, C.-C.: Study on Deep Seated Gravitational Slope Deformation in the Slate Belt of the Backbone Range, Central Taiwan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10944, https://doi.org/10.5194/egusphere-egu21-10944, 2021.

15:42–15:44
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EGU21-5370
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ECS
Luca Del Rio, Marco Moro, Michele Fondriest, Stefano Gori, Emanuela Falcucci, Michele Saroli, Fawzi Doumaz, Andrea Cavallo, and Giulio Di Toro

Abstract

Active faulting and Deep-seated Gravitational Slope Deformation (DGSD) constitute common geological hazards in mountain belts worldwide. In the Italian central Apennines, km-thick carbonate sedimentary sequences are cut by major active normal faults which shape the landscape generating intermontane basins. Geomorphological observations suggest that the DGSDs are commonly located in the fault footwalls.

          We selected five mountain slopes affected by DGSD and exposing the footwall of active seismic normal faults exhumed from 2 to 0.5 km depth. We combined field structural analysis of the slopes with microstructural investigation of the slipping zones from the slip surfaces of both DGSDs and major faults. The collected data show that DGSDs exploit pre-existing surfaces formed both at depth and near the ground surface by tectonic faulting and, locally, by gravitational collapse. At the microscale, the widespread compaction of micro-grains (e.g., clasts indentation) forming the cataclastic matrix of both normal faults and DGSDs is consistent with clast fragmentation, fluid-infiltration and congruent pressure-solution mechanisms active at low ambient temperatures and lithostatic pressures. These processes are more developed in the slipping zones of normal faults because of the larger displacement accommodated.

          We conclude that in carbonate rocks of the central Apennines, DGSDs commonly exploit pre-existing tectonic faults/fractures and, in addition, localize slip along newly formed fractures that accommodate deformation mechanisms similar to those associated to tectonic faulting. Furthermore, the exposure of sharp slip surfaces along mountain slopes in the central Apennines can result from both surface seismic rupturing and DGSD or by a combination of them.

How to cite: Del Rio, L., Moro, M., Fondriest, M., Gori, S., Falcucci, E., Saroli, M., Doumaz, F., Cavallo, A., and Di Toro, G.: Active faulting and deep-seated gravitational slope deformation in carbonate rocks (Central Apennines, Italy) , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5370, https://doi.org/10.5194/egusphere-egu21-5370, 2021.

15:44–15:46
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EGU21-8392
|
ECS
Chiara Crippa, Elena Valbuzzi, Paolo Frattini, Giovanni B. Crosta, Margherita C. Spreafico, and Federico Agliardi

Large slow rock-slope deformations are widespread in alpine environments and mountainous regions worldwide. They evolve over long time by progressive failure processes, resulting in slow movements that impact infrastructures and can eventually evolve into catastrophic rockslides. A robust characterization of the activity of these phenomena is thus required to cope with their long-term threats.

Displacement rates measured by remote sensing and ground-based techniques only provide a snapshot of long-term, variable trends of activity and are insufficient to capture the behavior of slow rock slope deformations in a long-term risk management perspective. We thus propose to adopt a more complete approach based on a re-definition of “style of activity”, including displacement rate, segmentation/heterogeneity, kinematics, internal damage and accumulated strain. To this aim, we developed a novel approach combining persistent-scatterer interferometry (PSI) and systematic geomorphological mapping, to obtain an objective semi-automated characterization and classification of 208 slow rock slope deformations in Lombardia (Italian Central Alps). Through a peak analysis of displacement rate distributions we characterized the degree of internal segmentation of mapped slow rock slope deformations and highlighted the presence of nested sectors with differential activity. Then, we used an original approach to automatically characterize the kinematics of each landslide (translational, compound, or rotational) by combining a 2DInSAR velocity vector decomposition and a supervised machine learning classification. Finally, we combined Principal Component and K-medoid Cluster multivariate statistical analyses to classify slow rock slope deformations into groups with consistent styles of activity. We classified DSGSDs and large landslides respectively in five and two representative groups described by different degree of internal segmentation and kinematics that significant influence the evolutionary behavior and affect the definition of representative displacement rates. Our results provide a statistical evidence that phenomena classified as “Deep-Seated Gravitational Slope deformations” (DSGSD) and “large landslides” actually have different mechanisms and/or evolutionary stages, mirrored by different morphological features that testify higher accumulated internal deformation for large landslides with respect to DSGSDs. Our statistical classification of rock-slope deformation style of activity further highlighted the different risk potentials associated to each one of the seven descriptive groups in a practical perspective, taking into account the most significant parameters (rate, volume and heterogeneity) to assess risks related to the interaction between slow movements and sensitive elements.

Our analysis benefits from both deterministic and statistical components to perform a complete regional screening of slow rock slope deformations and to prioritize site-specific, engineering geological analyses of critical slopes depending on the most important factors conditioning their long-term style of activity. Our methodology is readily applicable to different datasets and provides an objective and cost-effective support to land planning and the prioritization of local-scale studies aimed at granting safety and infrastructure integrity.

How to cite: Crippa, C., Valbuzzi, E., Frattini, P., Crosta, G. B., Spreafico, M. C., and Agliardi, F.: Semi-automated regional classification of the style of activity of slow rock slope deformations using PS InSAR and SqueeSAR velocity data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8392, https://doi.org/10.5194/egusphere-egu21-8392, 2021.

15:46–15:48
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EGU21-8394
|
ECS
Federico Franzosi, Chiara Crippa, Mattia Zonca, Andrea Manconi, Giovanni B. Crosta, Luca dei Cas, and Federico Agliardi

Spaceborne radar interferometry is a powerful tool to characterize landslide activity. However, its application to very slow rock slope deformations (displacement rates < 5 cm/yr) in alpine environments remains challenging due to low signal-to-noise ratio, severe atmospheric and snow cover effects, and heterogeneous deformation patterns related to complex landslide mechanisms in space and time.

In this study we combine available SqueeSARTM data (Sentinel 1A/B ascending and descending, 2015-2017), ad hoc multi-temporal baseline DInSAR processing (2016-2019), GPS data (2015 to 2019) and detailed field mapping to unravel the kinematics, internal segmentation and style of activity of the Mt. Mater deep-seated gravitational slope deformation (DSGSD) in Valle Spluga (Italy). The high relief slope (1500-3000 m.a.s.l.) is made of dominant micaschist and paragneiss of the Stella-Timun complex (Suretta nappe) and ranges in inclination between 33° (< 2500 m a.s.l.) and 25° (> 2500 m a.s.l.). At 2900 m a.s.l. the slope is cut by a sharp triangular headscarp with a vertical downthrow of about 40 m, moving downslope, shallower arcuate scarps mark the transition to two nested large landslides, affecting the slope between 2400 m a.s.l. and 1550 m a.s.l; with highly deformed toes.

Through 2DInSAR decomposition, we highlight the global translational kinematics of the DSGSD. However, regional scale processed PSI data result unsuitable to capture the spatial complexity of the phenomenon at the local scale. To obtain a spatially-distributed characterization of the DSGSD displacement patterns, we process several multi-temporal interferograms and retrieve unwrapped phase and displacement maps according to a process-oriented, targeted approach based on variable temporal baselines (from 24-days to 1-year). In this context: a) 1-year interferograms provide a picture of long-term background DSGSD displacement signals; b) seasonal interferograms highlight displacement trends suggesting a complex response of different slope sectors to hydrological input; c) 24 days interferograms outline a triangular shaped active sector extending between 2500 m a.s.l. and the main DSGSD headscarp, corresponding to the movement of extensive debris cover and overlying periglacial features.

Our analyses clearly outline a composite slope instability and a strong spatial heterogeneity with different nested sectors possibly undergoing different evolutionary trends towards failure. The combined analysis of seasonal interferograms and GPS data further confirm a sensitivity of the different slope sectors to hydrological forcing modulated by snowmelt and rainfalls. The herein results outline the potential of a targeted use of DInSAR, carefully constrained by field geological and morpho-structural data, for the detailed investigation of a complex very slow rock slope deformation successfully unravelling its mechanisms, temporal trends of activity and forcing factors. Ground-truthing by means of GPS data further prove that, in the context of very slow rock deformations, PSI data are useful for a first-order characterization of slope activity and kinematics, but often fail to capture local scale spatial segmentation, temporal trends and associated mechanisms.

Our approach prove to be effective in providing key information for the definition of possible evolutive scenarios for risk analysis and mitigation of a widespread, yet challenging class of slope instabilities.

How to cite: Franzosi, F., Crippa, C., Zonca, M., Manconi, A., Crosta, G. B., dei Cas, L., and Agliardi, F.: Unraveling spatial and temporal heterogeneities of very slow rock-slope deformations with targeted DInSAR analyses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8394, https://doi.org/10.5194/egusphere-egu21-8394, 2021.

Case studies
15:48–15:50
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EGU21-154
|
ECS
Nicola Fullin, Monica Ghirotti, Davide Donati, and Doug Stead

Geological structure and kinematics are often the most important factors controlling the stability of high rock slopes; their characterization can provide insights that are instrumental in understanding the behaviour of a slope in addition to its evolution with time. In this research, we used a combined remote sensing-numerical modelling approach to characterize the Joffre Peak landslides (British Columbia, Canada), two rock avalanche events that occurred on May, 13th and 16th 2019. The May 13th event involved a volume of 2-3million m3, and resulted in a runout distance of 6 km. The May 16th event involved a volume of 2-3 million m3, and a runout distance of 4 km. The failure was likely promoted by permafrost degradation and reduction in shear strength along geological structures (in our simulation checked in dry condition). Using a wide range of techniques, including Structure-from-Motion photogrammetry, virtual outcrop discontinuity mapping, GIS analysis, and 3D distinct element numerical modelling, we investigated the important role that structural geology and slope kinematics played prior to and during the Joffre landslide events. In particular, we demonstrate that i) a very persistent, sub-vertical geological structures formed the lateral and rear release surfaces of the rock mass volume that failed as two discrete landslide events. The landslide blocks were separated by one such sub-vertical structure, which remains visible in the fresh landslide scar; ii) the first block, failed on May 13th 2019, involving planar sliding failure mechanism, possibly promoted by progressive failure and propagation of discontinuities along the basal surface. The detachment of this block enhanced the kinematic freedom of the second landslide block, which, on May 16th, failed as wedge/toppling mechanism; iii) the first landslide block acted as a key block; its displacement and failure provided the kinematic freedom for the occurrence of the second landslide.  In this paper we show that combining remote sensing mapping and 3D numerical modelling allows for the identification of the structural geological features controlling the stability and evolution of high rock slopes in alpine environments. We also show that constraining and validating the numerical modelling results using historical data is of paramount importance to ensure that the correct failure mechanism of the landslides is simulated.

 

How to cite: Fullin, N., Ghirotti, M., Donati, D., and Stead, D.: Characterising the kinematics of the Joffre Peak landslides using a combined numerical modeling-remote sensing approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-154, https://doi.org/10.5194/egusphere-egu21-154, 2021.

15:50–15:52
|
EGU21-9581
|
ECS
Johannes Leinauer, Manfred Meindl, Benjamin Jacobs, Verena Stammberger, and Michael Krautblatter

Climatic changes are exacerbating the risk of alpine mass movements for example through more frequent and extreme heavy precipitation events. To cope with this situation, the monitoring, anticipation, and early warning of rock slope failures based on process dynamics is a key strategy for alpine communities. However, only investigating the release area of an imminent event is insufficient, as the primary hazard can trigger or increase secondary hazards like debris flows or the damming of a river. Nevertheless, recent case studies dealing with successive hazards are rarely existent for the Calcareous Alps. In this study, we precisely investigate the cascading effects resulting from an imminent rock fall and perform a pre-event analysis instead of back-modelling of a past event.

The Hochvogel summit (2592 m a.s.l., Allgäu Alps, Germany/Austria) is divided by several pronounced clefts that separate multiple instable blocks. 3D-UAV point clouds reveal a potentially instable mass of 260,000 m³ in six main subunits. From our near real time monitoring system (Leinauer et al. 2020), we know that some cracks are opening at faster pace and react differently to heavy rainfall, making a successive failure of subunits likely. However, pre-deformations are not yet pronounced enough to decide on the exact expected volume whereas secondary effects are likely as the preparing rock fall mass will be deposited into highly debris-loaded channels. Therefore, we developed different rock fall scenarios from the gathered monitoring information, which we implemented into a RAMMS modelling of secondary debris flows. To obtain best- and worst-case results, each scenario is calculated with different erosion parameters in the runout channel. The models are calibrated with a well-documented debris flow event at Roßbichelgraben (10 km NW and similar lithology) and are supported by field investigations in the runout channel including electrical resistivity tomography profiles (ERT) for determination of the depth of erodible material as well as a drone survey for mapping the area and the generation of an elevation model.

Here we show a comprehensive scenario-based assessment for anticipating cascading risks at the Hochvogel from initial rock failure volume estimation to debris flow evolution and potential river damming. This recent case study from an alpine calcareous peak is an excellent and rare chance to gain insights into cascading risks modelling and an improved hazard evaluation.

How to cite: Leinauer, J., Meindl, M., Jacobs, B., Stammberger, V., and Krautblatter, M.: Anticipating cascading risks at the imminent Hochvogel peak failure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9581, https://doi.org/10.5194/egusphere-egu21-9581, 2021.

15:52–15:54
|
EGU21-14225
Thorsteinn Saemundsson, Pall Einarsson, Halldor Geirsson, Joaquin Belart, Asta Rut Hjartardottir, Eyjolfur Magnusson, Finnur Palsson, Gro Pedersen, Vincent Drouin, and Daniel Ben-Yehoshua

A large deep seated gravitational slope deformation has been detected in a mountain slope north of the Tungnakvíslarjökull outlet glacier, in the western part of the Mýrdalsjökull ice cap in South Iceland. Mýrdalsjökull also hosts the Katla central volcano, which erupted spectacularly last in 1918. Based on comparison of Digital Elevation Models (DEMs) obtained from aerial photographs, lidar and Pléiades stereoimages, the slope has been showing slow gravitational slope deformation since 1945 to present. The total vertical displacement in 1945-2020 is around 200 m. The deformation rate has not been constant over this time period and the maximum deformation occurred between 1999 and 2004 of total of 94 m or about 19 m/year.

The mountain slope north of the Tungnakvíslarjökull outlet glacier 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 400-500 m in the western part up to the Mýrdalsjökull glacier at 1100 m in the east. The area of deformation, 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 depth of the sliding plane is not known, but the minimum mobile rock volume is between 100 to 200 million m3. The entire slope shows signs of displacement and is heavily fractured. Continuous GNSS stations which were installed in the uppermost part of the slope in August 2019 and in the lower part of the slope in 2020 provide real-time displacements. The GNSS time series show evidence of seasonal motion of the landslide, with highest deformation rates occurring in late summer or fall. Historically, seismicity in the area has been at maximum in the fall, although little seismicity has been observed since the GNSS stations were installed.

There are two main ideas of the causes for this 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 into the base of the slope, which may also explain the slope failure.

How to cite: Saemundsson, T., Einarsson, P., Geirsson, H., Belart, J., Hjartardottir, A. R., Magnusson, E., Palsson, F., Pedersen, G., Drouin, V., and Ben-Yehoshua, D.: Deep seated gravitational slope deformation north of the Tungnakvíslarjökull outlet glacier, in western Mýrdalsjökull ice cap, S-Iceland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14225, https://doi.org/10.5194/egusphere-egu21-14225, 2021.

15:54–15:56
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EGU21-10606
Maria Luisa Putignano, Emiliano Di Luzio, Luca Schilirò, Andrea Pietrosante, and Salvatore Ivo Giano

In the last two decades large clastic deposits in Central Apennines with specific morphological and sedimentological features have been interpreted as the result of Quaternary rock avalanche events (e.g., Di Luzio et al., 2004; Bianchi Fasani et al., 2014; Schilirò et al., 2019; Antonielli et al., 2020). The analysis of such deposits, that are located within intermontane basins and narrow valleys bounded by high mountain ridges, have improved the knowledge about this kind of massive rock slope failures, also clarifying their relationship with Deep-seated Gravitational Slope Deformations.

The present study then describes a multidisciplinary analysis carried out on a huge rock block deposit which crops out within the Pretare-Piedilama Valley, in the piedmont junction area of the Sibillini Mountain range (Central Italy), where Mesozoic basinal carbonates overthrust Miocene foredeep deposits.

Specifically, we performed sedimentological, stratigraphical and morphometric analyses on the clastic deposit; results support the interpretation of the event as a rock avalanche body. The accumulation area shows a T-like shape with a wide, E-W-oriented, proximal part and a N-S channelization in the central and lower sectors. The evidence suggests erosional events and tectonics as controlling factors on rock flow deposition. In this respect, the area was involved in the 2016 central Italy seismic sequence and was tectonically active during Quaternary times (Tortorici et al., 2009).

As regards on the deposit genesis, considering the geometric characteristics of a sub-rectangular detachment area located on the southern edge of the Sibillini Range, an original mechanism of rockslide failure involving about 8·106m3 of Early Jurassic limestone was inferred. Here, the post-failure geomorphic features behind the main scarp are considered for the evaluation of hazard conditions.

Finally, well-log analysis of the clastic sequence filling the Pretare-Piedilama Valley evidenced additional Quaternary landslide events occurred before the rock avalanche, thus testifying to a long history of large slope instabilities in the area controlling the landscape development.

 

REFERENCES

 

  • Antonielli B., Della Seta M., Esposito C., Scarascia-Mugnozza G., Schilirò L., Spadi M., Tallini M. (2020). Quaternary rock avalanches in the Apennines: New data and interpretation of the huge clastic deposit of the L'Aquila Basin (central Italy). Geomorphology, 361, 107-194. doi:10.1016/j.geomorph.2020.107194.
  • Bianchi Fasani G., Di Luzio E., Esposito C., Evans S.G., Scarascia-Mugnozza G. (2014). Quaternary, catastrophic rock avalanches in the Central Apennines (Italy): relationships with inherited tectonic features, gravity-driven deformations and the geodynamic frame. Geomorphology, 21, 22–42. doi:10.1016/j.geomorph.2013.12.027.
  • Di Luzio E., Bianchi-Fasani G., Saroli M., Esposito C., Cavinato G.P., Scarascia-Mugnozza G. (2004). Massive rock slope failure in the central Apennines (Italy): the case of the Campo di Giove rock avalanche. Bullettin of Engineering Geology and the Environment 63, 1-12. doi:10.1007/s10064-003-0212-7.
  • Schilirò L., Esposito C., De Blasio F.V., Scarascia-Mugnozza G. (2019). Sediment texture in rock avalanche deposits: insights from field and experimental observations. Landslides, 16, 1629-1643. doi: 10.1007/s10346-019-01210-x.
  • Tortorici G., Romagnoli G., Grassi S. et al. (2019). Quaternary negative tectonic inversion along the Sibillini Mts. thrust zone: the Arquata del Tronto case history (Central Italy). Environ Earth Sci 78: 37. doi:10.1007/s12665-018-8021-2.

How to cite: Putignano, M. L., Di Luzio, E., Schilirò, L., Pietrosante, A., and Giano, S. I.: The Pretare-Piedilama rock block deposit: evidence of a further case of quaternary rock avalanche in Central Apennines, Italy , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10606, https://doi.org/10.5194/egusphere-egu21-10606, 2021.

Physical and mechanical controlling factors
15:56–16:06
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EGU21-9938
|
solicited
Simon Loew, Clement Roques, Andrea Wolter, Kevin Schöngrundner, and Timon Blöchliger

The amount of internal deformation or damage created in a mature rockslide depends primarily on the basal rupture plane geometry and total amount of accumulated displacement. We present results from a 65 million m3 compound rockslide (Cerentino, Switzerland), which started to creep along a compound sliding surface about 5000 years ago. Investigations of the landslide body over the past 40 years include 8 deep boreholes, diverse monitoring systems, and geophysical as well as geomorphological investigations. The data set generated is unique and allows the quantitative linking of damage to hydrostratigraphy, groundwater recharge, and groundwater flow dynamics.   

The long-term creep of this crystalline rock landslide body along a stepped and bowl-shaped main rupture surface has led to a total displacement of about 500 m. Damage of the landslide body has been studied in great detail using a high quality triple tube core drilled in 2017 through the landslide body and into the stable bedrock down to 228 m depth. Inclinometer and fiber optic displacement measurements along this borehole suggest that the main sliding surface is located at 107 m and that significant distributed deformation occurs in the coarse-grained blocky carapace of the over-steepened landslide toe. In addition, several secondary sliding surfaces could be detected down to a depth of up to 207 m.

The landslide mass is heavily damaged and consists of variably broken cataclastic rock down to 140 m depth with grain sizes dominated by cobbles, gravel, sand and silt. From 140 to 170 m depth we observe a fractured rock mass with thinner kakirite sections. Below 170 m the rock mass quality is good in terms of RQD (40-90) and fracture density. 20 samples from cataclastic layers have been analyzed in detail with respect to grain size distribution, water content, and mechanical properties. Combining grain size analyses with a heating test conducted after borehole completion, we derive a detailed hydrostratigraphic profile through the entire landslide mass.

Groundwater discharge monitored at the landslide suggests high recharge rates for an alpine catchment (772 mm per year on average, or 0.7 Mm3), and can be balanced if we consider that there are no significant regional contributions from surrounding systems. Groundwater storage-discharge relationships were quantified based on spring recession analysis and a simple rainfall-runoff model (GR4J) that was coupled with a Snow Accounting Routine (SAR). Results allowed estimation of bulk landslide properties which are typical for strongly damaged rock (porosity 1%, hydraulic conductivity of 1-4·10-6 m/s). A transient groundwater flow model was then developed to study the impact of the stratified (variably damaged) geometries on recharge, groundwater flow partitioning and pore pressure distribution. We could notably show the importance of state of saturation in the unsaturated zone to allow effective recharge and pore pressure increase at the main sliding surface, especially during snowmelt and summer/fall rainstorms. The pore pressure response to major recharge events ranges from one to 20 days; such variability in pressure diffusion in the vadose zone highlights the importance of the saturation history, typically known for soil slides.

How to cite: Loew, S., Roques, C., Wolter, A., Schöngrundner, K., and Blöchliger, T.: Impact of Damage on Groundwater Flow Dynamics in a Compound Rockslide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9938, https://doi.org/10.5194/egusphere-egu21-9938, 2021.

16:06–16:08
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EGU21-14242
Chunwei Sun, Marc-Henri Derron, Michel Jaboyedoff, and Xiyong Wu

This work investigated the oxidative weathering deterioration of black shale along a bedding slip zone and how it affects the bedding shear failure in the Xujiaping landslide, southern Sichuan Province in China. Many dissolved pits were found on the limestone, and part of the black shale in the slip zone is mud-like and clastic, showing local shear failure, which can be one of the main reasons of slope instabiliy. The microstructure of black shale under oxidative weathering condition was observed by scaning electron microscopy (SEM), characterized by dissolved pores, weathering crust (iron sulfate) of pyrite crystals, and the filling gypsum crystal in the bedding foliation. The deterioration mechanism was expanded: (i) rock-forming and carbonate minerals were especially prone to dissolution by sulfuric acid from black shale oxidation in the slip zone, and (ii) volume expansion due to the crystallization force of precipitated minerals caused further fracture expansion and deformation. Therefore, two theoretical models were developed that use stoichiometric calculations of pyrite and calcite to determine the dissolution rate and the rock structure after chemical weathering; and establish a rock structure model characterized by foliation weakening of gypsum crystallization. In order to analyze the landslide failure, discrete element method (DEM) is used to analyze the black shale shear failure mechanism of the two degradation models after oxidative weathering. It will be useful to better understand how these oxidative weathering deterioration contribute to bedding shear failure in natural hazards.

How to cite: Sun, C., Derron, M.-H., Jaboyedoff, M., and Wu, X.: Oxidative weathering deterioration of black shale and its bedding shear failure modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14242, https://doi.org/10.5194/egusphere-egu21-14242, 2021.

16:08–16:10
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EGU21-9788
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ECS
Tesfay Kiros Mebrahtu, Thomas Heinze, and Stefan Wohnlich

Landslides and ground failures are among the common geo-environmental hazards in many of the tectonically active hilly and mountainous terrains of Ethiopia, such as in the western margin of the Main Ethiopian Rift in Debre Sina area. Besides the geological preconditioning, bi-modal monsoon and seismic events in the tectonically highly active region are usually suspected triggers. In order to minimize the damage caused by the slope failure events, a detailed investigation of landslide-prone areas using numerical modelling plays a crucial role. The aim of this study is to assess the stability of slopes, to understand the relevant failure mechanisms, and to evaluate and compare safety factors calculated by the different available numerical methods. The stability was assessed for slopes of complex geometry and heterogeneous material using the limit equilibrium method and the shear strength reduction method based on finite elements. Furthermore, numerical analysis was done under static and pseudo-static loading using the horizontal seismic coefficient to model their stability during a seismic event. The slope stability analysis indicates that the studied slopes are unstable, and any small scale disturbance will further reduce the factor of safety and probably causing failure. The critical strength reduction factors from the finite element method are significantly lower than the factor of safety from the limit equilibrium method in all studied scenarios, such as Bishop, Janbu Simplified, Spencer and Morgenstern-Price. The difference is especially evident for heterogeneous slopes with joints, which often are initiation points for the failure planes. The simulations show that slope stability of landslide prone hills in the study area strongly depends on the saturation conditions and the seismic load. The studied slopes are initially close to failure and increased pore-pressure or seismic load are very likely triggers.

How to cite: Mebrahtu, T. K., Heinze, T., and Wohnlich, S.: Comparing slope stability analysis using limit equilibrium and finite elements simulations of deep-seated landslides along the western margin of the Main Ethiopian Rift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9788, https://doi.org/10.5194/egusphere-egu21-9788, 2021.

16:10–16:12
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EGU21-16043
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ECS
Alberto Previati, Giuseppe Dattola, Gabriele Frigerio, Flavio Capozucca, and Giovanni B. Crosta

A reliable modeling of a landslide activation and reactivation requires a representative geological and engineering geological characterization of the affected materials. Beyond the material strength, landslide reactivation is sensitive to groundwater pressure distributions, that are generated by some external perturbation (recharge) and by the hydraulic properties of the materials. Drainage stabilization works generally involve drilling of a large number of drains and, therefore, minimize the total length is of primary concern to reduce the costs.

Aim of this work was the calibration of material properties for the optimization of drainage elements to be built for the slope stabilization and the construction of a shallow tunnel crossing a landslide. The case study is represented by the 4.0 · 105 m3 Carozzo landslide (La Spezia, Liguria, Italy) which affects some marly and sandstone formation. During the tunnel excavation a monitoring network consisting of five DMS columns for displacements and piezometric head multilevel measurements was installed. The monitoring provided a series of piezometric head recession curves following some recharge events. The series of data generated in response of a unique perturbation (rainfall recharge event) were chosen to calibrate the material properties through a multi-step approach, starting from a 1D model and progressively approaching a complete 3D model.

The 1D simplified approach applies the solution by Troch et al. (2003) that considers a homogeneous landslide material, with constant slope and a progressive change in the slope width. In this model a storage function considers the amount of water stored in a slope section. By imposing the continuity equation and the Darcy law a second order of partial differential equation is solved by integration in space and time. By taking the initial conditions from piezometric measurements and assuming a constant rainfall recharge, the piezometric level and the outflow rate were computed and compared with the local piezometric level time history, by changing the hydraulic conductivity and the storage function value.

Successively, a groundwater flow FEM numerical model (in 2D and 3D) was developed considering the landslide geometry and internal zonation, including the presence of the excavated part of the tunnel. The model domain was divided into sub-zones according to the available geological surveys to account for internal variations of the material properties. The steady-state simulation of the water flow allowed to estimate the equivalent hydrogeological parameters of each subdomain. The hydraulic head distribution obtained under steady-state conditions was used as initial condition for the transient-state simulation. The recharge from precipitation was also included in the water balance by means of daily rainfall time-series. Finally, the model parameters were calibrated in transient state by comparing measured data and simulated results.

The minimum error between simulated and measured piezometric heads under transient conditions was obtained through the 3D configuration. Calibrated hydraulic conductivities in the 3D solution are up to an order of magnitude lower than the 1D solution because of the homogenous assumption of the model. The internal zonation of the landslide body and the modeling of a low-conductivity shear zone were essential to explain the pressure differences inside the body.

How to cite: Previati, A., Dattola, G., Frigerio, G., Capozucca, F., and Crosta, G. B.: Hydraulic properties calibration by high resolution monitoring data and 1D to 3D groundwater flow modeling of the Carozzo Landslide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16043, https://doi.org/10.5194/egusphere-egu21-16043, 2021.

16:12–16:14
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EGU21-12658
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ECS
Lucas Pelascini, Philippe Steer, and Laurent Longuevergne

Landslides are one of the sources of natural hazards that cause damages and losses but also shapes the landscape. A better understanding the factors triggering or pre-conditioning landslide occurrence is therefore critical for risk assessment, with implications for hillslope erosion and landscape dynamics Triggering of catastrophic landslides is generally associated with events such as earthquakes or intense rainfalls. In Taiwan, a minimum of 22,705 landslides were reported during the typhoon Morakot in 2009 (Lin et al., 2011). Landslides triggered during storms are generally associated to the intensity and cumulated amount of rainfall, as water infiltration destabilize slopes (Iverson, 2000). However, a correlation has also been reported between slope stability and the change in atmospheric pressure (Schulz, 2009). Indeed, a change in air-pressure can lead in a readjustment in pore pressure, and cause fluid movements normal to the surface. The aim of this study is to characterize the effect of atmospheric pressure changes and define its specific contribution on slope stability when combined with rainfall

A 2-dimensional analytical model has been developed based on diffusion equations to describe the destabilization induced by water infiltration and atmospheric pressure changes induced by typhoons. As both mechanisms are function of pore pressure, and especially groundwater pore pressure, the water table within a finite-length hillslope is modelled using Townley’s (1995) analytical expression of water flow in a unconfined aquifer. The hillslope itself is a simple tilted half-space with a water divide at the top and a river at the toe forcing the water table to the surface. Slope stability is inferred through a safety factor computed using the coulomb criterion. Both rainfall infiltration and air pressure modify pore pressure through a diffusion process. While rainfall increases water table height and induce large increases in pore pressure within days or hours, , we show that atmospheric-induced pore pressure change is instantaneous and can occur even if the hillslope is fully saturated.

The model allows to separate the hillslope response into two regimes, upslope or downslope, where the destabilization is mainly linked to rainfall or to atmospheric pressure change, respectively.  Our results suggest that landslide occurring during storms in the downstream part of the hillslope are likely candidate for being triggered by atmospheric pressure change, in particular if the storm occurs with a humid initial condition. We show that the effect of atmospheric pressure changes is not negligible. On contrary, it is crucial to define the amplitude, timing and geometry of the hillslope instability, especially when combined to rainfall.

How to cite: Pelascini, L., Steer, P., and Longuevergne, L.: Atmospheric pressure compared to rainfall as landslide triggering factors along a hillslope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12658, https://doi.org/10.5194/egusphere-egu21-12658, 2021.

16:14–16:16
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EGU21-9549
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ECS
Symeon Makris, Irene Manzella, Paul Cole, and Matteo Roverato

Debris avalanches and lahars are among the most destructive and hazardous mass flows in volcanic environments making them important to understand from a hazard assessment perspective. Sedimentological characteristics of their deposits are important for assessing their propagation and emplacement mechanisms. Here, we compare the sedimentology of nine volcanic debris avalanches and eight lahars, by the descriptive statistics: median grain size, sand, gravel and finer particle proportion, skewness, and sorting.

Results suggest that lahars and debris avalanches diverge in their grain size distribution evolution during propagation, even when sourced from the same material. Increasing bimodality, evolution to negative skewness, with decreasing sediment size, accompanied by very poor sorting suggest comminution of particles due to particle-particle interactions in debris avalanches. Instead, preferential deposition of the coarsest particles and improved sorting suggest that the decrease in grain size of lahars is the result of debulking. The divergence is mainly caused by the high water content in lahars, which introduce different processes during propagation. This suggests, in agreement with previous studies, that debris avalanches can be considered as dense granular flows where the effect of inertial collisions of solid fragments are more important than fluid effects.

Present findings and previous sedimentological studies suggest that both volcanic and non-volcanic debris avalanches exhibit bimodal grain-size distributions, at least locally, in areas of high shear accommodation. Following these results, an experimental campaign has been carried out to test the effect of bimodality on the propagation of granular flows. These experiments are flows of bidisperse granular material on an initial inclined plane, with a horizontal accumulation surface at the bottom.  Findings confirm that the bimodality of the grain size distribution generates a more efficient shearing arrangement, which can increase the mobility of granular flows in the same way recorded in debris avalanche deposits.

How to cite: Makris, S., Manzella, I., Cole, P., and Roverato, M.: The role of sedimentology in the mobility of debris avalanches: Evidence from their deposits and granular flow experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9549, https://doi.org/10.5194/egusphere-egu21-9549, 2021.

Dating and activity
16:16–16:21
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EGU21-13580
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ECS
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solicited
Fabrizio Delgado, Swann Zerathe, Stephane Schwartz, Carlos Benavente, Xavier Robert, Krzysztof Gaidzik, Julien Carcaillet, and Aster Team

The western flank of the Central Andes shows a high concentration of giant landslides (Strasser and Schlunegger, 2005; Audin & Bechir 2006; Pinto et al., 2008; Matther et al., 2014; Crosta et al., 2014, Margirier et al., 2015; Zerathe et al., 2017; Delgado et al., 2020) related to specific characteristics such as a strong local relief (canyons, structural-flexures, etc.), strong and recurrent seismo-tectonic activities, and atypical climate combining long-term hyper-aridity and punctual extreme precipitation events. In this context, ongoing studies inventorying more than one-thousand giant paleo-landslides in this region underline their spatial clustering that is controlled by coupled conditioning factors including high topographical gradients and specific lithology (Delgado et al., 2020).

The purpose of this study is to constrain now the kinematics of landsliding and ultimately to get time-frequency law of the gravitational slope destabilizations of this Andean region. For this, we focus on the Locumba valley (south Peru) where more than 30 giant landslides are clustered and distributed in two main typologies (rockslide and rock-avalanche). We applied cosmogenic nuclide dating to 8 paleo-landslides, sampling 52 boulders. We used alternatively 10Be/quartz or 10Be/feldspar depending on the available lithology.

Our dataset opens an unprecedented opportunity for paleolandslides studies and reconstructions. Indeed, the exposure-ages obtained range from the Holocene to the Pleistocene, the oldest ages reaching one-million years. This new temporal-scale allows to address and discuss triggering processes in the context of seismo-tectonic activities and Quaternary climate changes. Exposure-ages distribution shows several time-frequency peaks suggesting that gravitational destabilizations are episodic phenomena with time recurrence on the range of ~100 ka. Additionally, our time-constraints indicate that most of the current landscapes along this Western Andean flank are older than one-million years. Especially, fluvial incision and valley deepening processes are currently very low as testified by relicts of landslide dams and associated lacustrine sediments of hundred’s thousand years old that are preserved along the main canyons and still not fully re-incised.

How to cite: Delgado, F., Zerathe, S., Schwartz, S., Benavente, C., Robert, X., Gaidzik, K., Carcaillet, J., and Team, A.: 10Be dating reveal a one-million-years records of landslide activities in the Central Western Andes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13580, https://doi.org/10.5194/egusphere-egu21-13580, 2021.

16:21–16:23
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EGU21-11994
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ECS
Clément Boivin, Jean Philippe Malet, Catherine Bertrand, Yannick Thiery, Fréderic Lacquement, and Jérome van der Woerd

Deep Seated Gravitational Slope Deformation (DSGSD) is defined as a set of rock mass characterized by a very slow movement (mm.yr-1) affecting large portions of slopes of a mountain range. These typical slope instabilities must not be neglected and need to be better identified and characterized to anticipate related hazard (e.g. landslides). Characterize them requires first of all to locate them as for example recently the inventories carried out for the European Alps or in France. These specific processes, which can lead to hazard and disasters (e.g. La Clapière landslides), should not be neglected and need to be better identified and characterized to anticipate related hazard). Documenting the DSGSDs requires first of all to locate them as for example the recently published inventories initiated for the European Alps and France. These studies initiated approaches aiming at defining the factors controlling their evolution in time and space.

The research developed in this study targets a better understanding the short- (<100 yrs) and long-term (> 100 yrs) evolution of DSGSDs developed in the sedimentary rocks of the Queyras Massif (South-East French Alps). The main objective is to propose models of DSGSDs evolution with key interpretations of future developments to locate possible new landslide prone areas. The Queyras Massif was chosen because it represents an under-studied area of DSGSDs. The massif is characterized by Cenozoic marine sedimentary rocks accreted and metamorphized by the Alpine orogen. The massif is characterized by a regional schistosity plunging to the West and complex and active fault networks mark the landscape (Tricart et al., 2004). The highest summits reach an altitude of 2500m a.s.l. and are separated by deep valleys incised by the Riss and Würm glaciers and currently by torrential streams.

The method is based on a geomorphological analysis of the landscape and landforms, field observations and image interpretation of remote sensing data. Results allow locating the DSGSD, estimating their degree of activity, and characterizing their structure. Several dating methods (14C, 10Be or 36Cl) complete the interpretations in order to reconstruct the history of the slopes and understand the factors that control their evolution.

At the scale of the massif, the DSGSDs were first identified using the approach proposed by Blondeau (2018). Visual remote sensing revealed the occurrence of thirty DSGSDs. These slopes were detected as they associate six common features commonly observed in DSGSDsEight DSGSDs were selected in order to investigate at the local scale their geomorphology, geology and hydrogeology and reconstruct their historical (millennial) and recent (last 50 years) evolution from dating methods and field observations. Through this multidisciplinary approach, present the observed bedrock and gravitational structural features and determine the predisposing factors of the formation of DSGSD. The research is part of the Program “Référentiel Géologique de la France / RGF – Chantier Alpes” which targets to update the geological knowledge of the Alpine basement, surficial formations and associated hazards in three dimensions and in digital format.

How to cite: Boivin, C., Malet, J. P., Bertrand, C., Thiery, Y., Lacquement, F., and van der Woerd, J.: Long and short time evolution of deep seated gravitational slope deformation int the Queyras massif / south east France, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11994, https://doi.org/10.5194/egusphere-egu21-11994, 2021.

16:23–17:00