Geomorphic responses at the permafrost margins: observations from the Swiss Alps

The warming and thawing of permafrost creates a multitude of geomorphic responses. Warm permafrost areas, with temperatures between -2° and 0°C, are especially affected because of the occurrence of pressurized water at the bounding of the ice/rock contact, which is very sensitive to any temperature change. In mountain permafrost regions, this implies that geomorphic response will first be observed at lower elevations, close to the permafrost margins, before shifting upwards as the climate changes. In addition, an increased surface summer runoff related to the rising elevation of rain precipitation, more severe rainfall events and a reduced extent of snow patches can be observed. Therefore, there is a need for a detailed monitoring of these critical areas, where climate change induced processes will first occur, to improve our understanding of the landscape evolution in mountainous regions.

For this purpose, four common mountainous periglacial landforms, a rock wall, a debris flow affected talus slope, a rock glacier and a rockslide are monitored in high temporal and spatial resolutions. These landforms are important steps in the alpine sediment cascade, potentially acting as a sediment source or sink depending on their connectivity within the landscape. Several close range sensing techniques were combined (GNSS data, archival aerial photographs, uncrewed aerial vehicles, terrestrial laser scanning, time-lapse photography and seismic data), providing multiple lines of evidence. Limitations related to the sensor and monitoring intervals were overcome by the integration of the different datasets. Especially in the European Alps, where monitoring activities have been ongoing for decades with an increased instrumentation, this approach unlocks interesting research paths.

All four studied landforms show a clear response to the present-day climate change. We observed a 2-year rock wall destabilisation with an unprecedented level of detail, including a precursory deformation of the rock wall, a process already ongoing before the start of the monitoring. The deep permafrost bedrock that was exposed after large cliff falls (10⁴-10⁶ m³) has already been out of equilibrium with the surface temperature for three decades. On the studied talus slope, a high magnitude debris flow event (3 x 10⁴ m³, various surges) was recorded in summer 2019 as a result of several convective thunderstorms, exceeding all historical debris flow events since 1946. Rock glacier acceleration (up to 15 m yr^-1) and destabilisation has been observed, in this case delivering a considerable volume of debris to steep torrential gullies where it can be mobilised again in the form of debris flows. The Grabengufer rockslide, one of the only permafrost-affected active rock slide accurately monitored in the Alps, is continuously accelerating (from 0.3 to > 1 m y^-1 in a bit more than a decade). Although all our observations are study area specific, similar observations have been made elsewhere in the European Alps. Therefore, the high resolution spatial and temporal data collected in this study deepens the insight in processes increasingly occurring throughout the Alps. By doing so, this research contributes to the understanding of high mountain geomorphology in a changing climate.