Topographic and Geologic Controls on Frost Cracking in Alpine Rockwalls
Creators: name: Draebing, Daniel
name: Mayer, Till
Publisher: name: FID GEO
Year: 2021

description: Frost weathering is a major control on rockwall erosion in Alpine environments. Previous frost cracking model approaches used air temperatures as a proxy for rock temperatures to drive frost weathering simulations on rockwall and on mountain scale. Unfortunately, the thermal rockwall regime differs from air temperature due to topographic effects on insolation and insulation, which affects frost weathering model results and the predicted erosion patterns. To provide a more realistic model of the rockwall regime, we installed six temperature loggers along an altitudinal gradient in the Swiss Alps, including two logger pairs at rockwalls with opposing aspects. We used the recorded rock surface temperatures to model rock temperatures in the upper 10 m of the rockwalls and as input data to run four different frost cracking models. We mapped fracture spacing and rock strength to validate the model results. Our results showed that frost cracking models are sensitive to thermal, hydraulic and mechanical parameters that affect frost cracking magnitude but frost cracking patterns in terms of peak location and affected rock mass remained consistent between varying input parameters. Thermo‐mechanical models incorporate rock strength and hydraulic properties and provided a frost cracking pattern at the rockwall scale that better reflects the measured fracture spacing. At the mountain scale, these models showed a pattern of increasing frost cracking with altitude, which is contrary to purely thermal models but consistent with observations of existing rockfall studies.
description: Plain Language Summary: Frost weathering is an important mechanism in shaping rockwalls in Alpine environments. Previous studies developed either purely thermal or thermo‐mechanical models incorporating mechanical and hydraulic parameters to simulate this process. Both model types provide valuable insights about a process that is hard to measure. Previous model approaches used air temperature as input data. However, rock temperatures differ from air temperatures due to topography that changes the insolated surface of rockwalls and insulating snow cover. We measured rock temperatures directly at six rockwalls with different aspects along a large range of altitude. We used our data to run four existing frost weathering models. Our results show that rock type, the strength of rocks and water availability influence the frost weathering magnitude, but the location of cracking and the rockwall depth affected does not change. The frost cracking pattern should be reflected by the fracture network and the strength of rockwalls. We mapped fractures and measured rock strength and our results correspond better to thermo‐mechanical model results. Thermo‐mechanical model results show an increase in frost weathering with increasing altitude. This pattern is consistent with rockfall observations. In contrast, purely thermal models showed an inverse relationship with higher frost cracking at lower altitudes.
description: Key Points: Temperature loggers provide rock temperature data that incorporates topographic effects on insolation and insulation. Sensitivity tests on frost cracking models showed differences of frost magnitude while frost cracking depth patterns were consistent. Thermo‐mechanical models incorporating rock strength and hydraulic properties produced more realistic altitudinal frost cracking patterns.
description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659