In high mountains environments, permafrost degradation causes rockwall instabilities, sometimes leading to rock slope failures. These processes could be a threat for human lives and activities, it is therefore essential to improve our knowledge about their triggering and propagation mechanisms.

This study focuses on two rock slope failures which occured in the Vallon d'Étache (Maurienne Valley, France) and the Crête des Grangettes (Écrins massif, France) in 2020. These events have respectively volumes around 270 000 m³ and 35 000 m³. In both cases, ice in the rockfall scars suggests the presence of permafrost, but its local distribution and its role in the triggering of the events remain to be confirmed. The aims of this study are (i) to assess thermal conditions in which both events were triggered and (ii) to model their propagation. To do so, we first propose a method that combines rockwalls temperature monitoring, statistical modelling of permafrost distribution, geophysical surveys (Electrical Resistivity Tomography) and laboratory measurements (petrophysical model) to explain the thermal conditions under which the rockfalls occurred. Then, 3D models acquired by photogrammetry are used to obtain high-resolution DEMs in order to measure their volumes, and to provide consistent inputs for runout modelling.

This multi-method approach allows to understand the context and the processes leading to rockfall triggering of two events in the French Alps. We also propose depth-averaged flow simulations to model the runout characteristics of the Vallon d’Étache rock slope failure.