Walter Steinkogler
Snow avalanches are a direct thread to many mountain communities around the world and already small avalanches can endanger traffic routes and result in loss of life or property. Their destructive power depends, among other things, on the overall mass and the properties of the flowing snow. The variety of flow regimes in avalanches, ranging from powder clouds to slush flows, is mainly controlled by the properties of the snow released and entrained along the path. So far, the knowledge on how snow conditions affect avalanche behavior is limited and hypotheses are not supported by data. This study aims to provide a first step towards the successful link between snow cover properties and the internal granular composition which in turn affects the flow dynamics of an avalanche. In a first step, the snow cover properties with most relevance for avalanche dynamics, such as run-out distance and front velocity, are identified. For selected large-scale avalanches, the snow conditions were reconstructed using the three-dimensional surface processmodel Alpine3D and the snow cover model SNOWPACK. The data shows that the total mass, mainly controlled by entrained mass, defines run-out distance but does not correlate with front velocity. A direct effect of snow temperature on front velocity, development of a powder cloud and deposition structures could be observed. As a next step field experiments with multiple artificially released avalanches were conducted to quantify the temperature of the flowing snow more accurately and to discuss the magnitudes of different sources of thermal energy. Measured snow temperature profiles allowed quantifying the temperature of the eroded snow layers. Infrared radiation thermography was used to assess the surface temperature before, during and just after the. This data set allowed to calculate the thermal balance, from release to deposition. We found that, for the investigated dry avalanches, the thermal energy increase due to friction was mainly depending on the elevation drop of the avalanche with a warming of approximately 0.5°C per 100 height meters. Contrary, warming due to entrainment was very specific to the individual avalanche and depended on the temperature of the snow along the path and the erosion depth ranging from nearly no increase to 1°C. Furthermore, we could observe that the warmest temperatures are located in the deposits of the dense core. Especially in cases where the described warming processes cause the temperature of the flowing snow to approach the melting point significant differences in the granular composition of an avalanche can occur. Consequently, the granular structures in the deposition zone of avalanches are often intuitively associated with cold or warm avalanches. We conducted experiments on the temperature-dependent granulation of snow and demonstrated that temperature has a major impact on the formation of granules. The experiments showed that granules only formed when the snow temperature exceeded -1°C. Depending on the conditions, different granulation regimes were obtained, which were qualitatively classified according to their properties. All experimentally observed granule classes were reproduced by a discrete element (DE) model that mimicked the competition between cohesive forces, which promoted aggregation, and impact forces, which induced fragmentation.
EPFL2015
