Moosfluh, towards a rock slope failure?

Abstract

Landslides and rockfalls are critical hazards to take into account in mountainous areas and understanding their mechanisms is crucial for mitigation purposes. Due to climate change, permafrost degradation and glacial retreat are becoming more and more relevant to consider in steep terrain. Indeed critically stressed slopes might react to these rapid changes and generate an increasing number of rock failure events. In the Great Aletsch region (Switzerland) glacial retreat has been considerable in the last decade and the adjacent southern slope (Moosfluh) has suffered progressive debuttressing, as well as increased deep-seated gravitational slope deformation (DSGSD) and enhanced rock failure phenomena. In this study, we investigate rockfall events occurring in the lower portions of the Moosfluh DSGSD in the 2017 summer season. We used a local network of three low-cost seismometers (raspberry shakes), webcam photos and field data to detect rockfalls and generate an event’s catalogue. Rockfall parameters as duration, amplitude, volume, runout, energy, as well as their spatial and temporal distribution, provided insights into the mechanisms driving the slope deformation. An increase in rockfall activity was detected at the beginning of August, corresponding to 3 days of heavy rainfalls. The weeks later followed a change in the displacement rate of the entire DSGSD. We also found that the majority of rockfalls occurred during night, pointing out a potential effect of daily surface temperature changes leading to rock contraction and eventually failure events. The peculiar spatial pattern of rockfall suggested temporally and spatially dependent activity for displacement in different domains of the moving mass. Finally, we calibrated a magnitude frequency relationship, which provides hints for mitigation strategies in Moosfluh as well as in other similar alpine areas.