Timing, volume and precursory indicators of rock‐ and cliff fall on a permafrost mountain ridge (Mattertal, Switzerland)

International audience

English. High mountain environments are increasingly affected by rockfall-related hazards, driven by climate change. Studying rockfall in these environments is, however, challenging due to the inaccessibility of mountain ridges and the complex interaction between controlling factors. In this study, the rock wall of Grosse Grabe North Pillar in the Matter valley (Western Swiss Alps) was studied in detail over a timespan of 4 years (2017–2021). Data was collected from time-lapse photography, terrestrial laser scanning, unmanned aerial vehicle photogrammetry and seismic measurements. The presented dataset is unique because data collection started before the onset of the rock wall destabilization, allowing us to understand precursory indicators of large-scale events. In total, we recorded 382 rock- and cliff fall events (100–31 300 m3), with a total volume of 204 323 ± 8173 m3, resulting in a scar depth of ~40 m. An associated rock wall retreat rate of 71.2 ± 2.8 mm year−1 was calculated for the 1991–2021 period. Highly fractured south-exposed gneiss lithology is viewed as the main predisposition for the observed rock- and cliff fall events, allowing high-temperature oscillations to cause irreversible movements at fracture level. Cliff falls (104–106 m3) were preluded by an outward movement of the rock wall that started to increase 1.5 years before any significant collapse of the rock wall, reaching locally up to 30 cm. All cliff fall events occurred in summer, exposing ice in the clefts. This is assumed to be the base of the permafrost from the north side. Rapid permafrost degradation is viewed as a triggering factor after its exposure, causing progressive failure of the rock wall, leading to very high rock wall retreat rates on a decadal timescale.