Predisposing, triggering and runout processes at a permafrost‐affected rock avalanche site in the French Alps (Étache, June 2020)

• Published in: Earth Surface Processes and Landforms
• Main Authors: Cathala, Maëva, Bock, Josué, Magnin, Florence, Ravanel, Ludovic, Ben Asher, Matan, Astrade, Laurent, Bodin, Xavier, Chambon, Guillaume, Deline, Philip, Faug, Thierry, Genuite, Kim, Jaillet, Stéphane, Josnin, Jean‐yves, Revil, André, Richard, Jessy
• Other Authors: Environnements, Dynamiques et Territoires de Montagne (EDYTEM), Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS), Alpes Ingé SARL, Institut des Géosciences de l’Environnement (IGE), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Université Grenoble Alpes (UGA)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP ), Université Grenoble Alpes (UGA), De la Préhistoire à l'Actuel : Culture, Environnement et Anthropologie (PACEA), Université de Bordeaux (UB)-Centre National de la Recherche Scientifique (CNRS), Naga geophysics, ANR-19-CE01-0018,WISPER,Processus thermo-mécaniques liés à l'eau et à la glace dans les fractures des parois alpines à permafrost(2019)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2024
• Subjects: electrical resistivity tomography; ground surface temperature monitoring; periglacial geomorphology; permafrost modelling; propagation modelling; rock avalanche; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]; [SHS.GEO]Humanities and Social Sciences/Geography; [SPI.GCIV.RISQ]Engineering Sciences [physics]/Civil Engineering/Risques; Ice; permafrost
• Online Access: https://hal.science/hal-04644646; https://hal.science/hal-04644646/document; https://hal.science/hal-04644646/file/Cathala_et_al_2024_ESPL_Etache%20-%20Predisposing%20triggering%20and%20runout%20processes%20at%20a%20permafrost%E2%80%90affected%20rock-1.pdf; https://doi.org/10.1002/esp.5881

International audience Although numerous recent studies have explored the relationship between permafrost degradation and rock slope failure, there is still a need for in-depth investigations to develop relevant hazard assessment approaches. We investigate the predisposing, triggering and propagation processes of a rock avalanche (c. 225,000 m3) that occurred in Vallon d'Étache (France) on 18 June 2020, whose scar was coated by ice and water. Weather records and energy balance models show that the rock avalanche occurred right after the warmest spring and winter since at least 1985, but also right after the spring with the highest water supply anomaly (snowmelt and rainfall). Measured ground surface temperature and geoelectrical surveys reveal that relatively ice-rich permafrost could exist in the NW face (release area) while it is inexistent below the SE face, contradicting certain permafrost maps. Heat transfer simulations suggest that the rock avalanche occurred during a transition from cold to warm permafrost conditions at failure depth (30 m), with a temperature increase of up to 0.6°C per decade since 2012 (when considering potential snow cover effect), and current temperature ranging between −3 and −1°C, depending on the applied model forcing. This warming certainly contributed to predispose slope to failure. In addition, the shift towards warm permafrost and water infiltration potentially enhancing permafrost degradation along fractures through heat advection or favouring the development of high hydrostatic pressures may have played as triggering factors. Finally, propagation simulations show that the rock avalanche involved several phases with different rheological properties due to the incorporation of snow and material segregation within the deposit. These new insights at various scales highlight the complexity of the triggering and propagation processes of rock slope failure occurring in high mountains, a significant part of which can be linked to snow effects on ground temperature, water supply ...