Thermal conditions and kinematics of steep bedrock permafrost

Steep mountain flanks with heterogeneous micro-topography and surface characteristics are typical for high alpine mountain ranges. Permafrost, defined as the subsurface volume remaining at or below 0 °C throughout the year, modifies the hydrological and mechanical conditions of these flanks. Rock fall from permafrost bedrock and stability problems of high-alpine infrastructure raise the demand for understanding how these rock faces evolve in response to climate change and where most critical situations may emerge. Diverse studies address this topic and advances in the knowledge of the general permafrost distribution in steep rock faces and in the documentation and investigation of high-alpine rock instabilities have been made in the past decade. However, the processes linking climate change and rock fall are still poorly understood and empirical data is limited. This thesis addresses the corresponding knowledge gap. It is an exploratory study of the thermal, hydrological and mechanical processes in steep bedrock permafrost and its active layer. A large part of the thesis focuses on distributed and extensive in-situ measurements of temperatures, rock movements and hydrological parameters. For this a data acquisition infrastructure based on wireless technology was developed within the consortium PermaSense, which was operated at two field sites (Matterhorn and Jungfraujoch, Swiss Alps) over the past 2–3 years and acquired datasets of novel quality and with new contents. The hydrothermal processes in fractured rock are, however, difficult to observe with field measurements alone. Therefore, laboratory experiments and numerical simulations of a single ice-filled cleft where set up and applied as complementary methods. The main findings of these investigations are: i) a significant cooling effect with respect to snow-free surfaces in radiation-exposed rock faces caused by thin snow cover and air ventilation in clefts; ii) melt water warmed-up before percolation causes high erosion rates of the cleft ice but little warming of the surrounding rock; iii) two temporal patterns of rock movements have been described of which one is novel and restricted to the period with percolation of melt water into the cleft system. These findings are synthesized in a conceptual process model of climate-related rock fall in permafrost bedrock. This hypothetical model suggests that both, rock thermal conditions and melt-water production should be considered as drivers of warming-related rock fall. The formulated hypotheses may be challenged or refined with the continuation of the present measurements and additional datasets of combined thermal and geotechnical investigations. Further, the deployment of the developed measurement infrastructure in an active instability may be a training case for a rock-fall monitoring and provide tools for future early-warning systems. More detailed analysis of the hydrothermal – mechanical interaction in rock clefts at subzero temperatures need to be considered to apply existing rock-mechanical models for permafrost bedrock. However, the application of such coupled numerical models to the field is limited to some case studies and requires a transfer to simple rules that may be applied based on the available information over large areas.