Predisposing, Triggering and Runout Processes at a Permafrost‐affected Rock Avalanche Site in the French Alps (étache, June 2020)

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'& Eacute;tache (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 degrees C per decade since 2012 (when considering potential snow cover effect), and current temperature ranging between -3 and -1 degrees 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 and rheological properties. This paper investigates the triggering and propagation conditions of a recent rock avalanche (c. 225,000 m3, French Alps, 2020) at a permafrost site. Several methods including temperature monitoring, geophysical survey, energy balance and heat transfer modelling show a permafrost warming with a transition from cold to warm permafrost in the release area and significant water infiltration weeks and days prior to the event. Runout simulations point out the complexity of the propagation mechanism, involving several phases and different rheological properties. image