Annual surface elevation changes of rock glaciers and their geomorphological significance: Examples from the Swiss Alps

Due to the ongoing degradation of permafrost and the associated impacts on the stability of periglacial mountain slopes, there is a need to improve our understanding of the dynamics of rock glaciers over relatively short timescales. There is also a growing interest in the hydrological role of rock glaciers and other ice-debris landforms, as they can potentially store valuable amounts of ice. Despite the acceleration in rock glacier velocities observed primarily in the European Alps, annual and inter-annual changes in surface elevation and volume have so far escaped detailed quantification in monitoring programs. In this study, annual and biannual uncrewed aerial vehicle (UAV) surveys combined with structure-from-motion (SfM) photogrammetric techniques are used to derive high-resolution surface kinematics, elevation and volume changes of three rock glaciers in the Western Swiss Alps. Between 2016 and 2021, detailed digital elevation model (DEM) differencing revealed a high variability of mean annual elevation changes, reaching up to −0.19 ± 0.02 m/yr (2020−2021) at the Yettes Condjà B rock glacier, while elevation changes at the Les Cliosses rock glacier ranged from −0.05 ± 0.02 m/yr (2016–2017) to +0.014 ± 0.008 m/yr (2021−2021). In addition, the multi-temporal and very high-resolution topographic datasets revealed the presence of distinct rock glacier features, including superimposed lobes, lateral levees and flow divides, that could not be identified previously with traditional in-situ measurements and observations. Our results in terms of elevation and surface velocity changes highlight the importance of detailed and regular high-resolution monitoring for understanding the rapid evolution of active glaciers under the current degradation of mountain permafrost.