Influence of snow cover properties on avalanche dynamics

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 effect avalanche behavior is limited and hypotheses are not supported by data.

As part of the project STRADA we 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.

Identification of relevant snow cover parameters

To takle this challenging task initially, the snow cover properties with most relevance for avalanche dynamics, such as run-out distance and front velocity, were identified. Five avalanches with similar initial mass and topography, but different flow dynamics were selected from the Vallée de la Sionne test site (Western Swiss Alps) database. For each of these avalanches, the snow conditions were reconstructed using the three-dimensional surface process model Alpine3D and the snow cover model SNOWPACK. For the investigated avalanches the data shows that the total mass, mainly controlled by entrained mass, defines run-out distance but does not correlate with front velocity. Yet, a direct effect of snow temperature on front velocity, development of a powder cloud and deposition structures could be observed. A temperature of the flowing snow warmer than approximately -2°C was identified as critical value for changes in flow dynamics.

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 investigate the magnitudes of different sources of thermal energy. Manually measured snow temperature profiles along the avalanche track and in the deposition area allowed quantifying the temperature of the eroded snow layers. Infrared radiation thermography (IRT) was used to assess the surface temperature before, during and just after the avalanches with high spatial resolution. This data allowed the calculation of the thermal balance, from release to deposition. We found that, for the investigated dry avalanches, the thermal energy increase due to friction was mainly dependent 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 associated with cold or warmavalanches. We therefore conducted experiments on the temperature-dependent granulation of snow. An ordinary concrete tumbler provided a suitable setup to investigate snow granulation. In a set of experiments at constant rotation velocity with varying temperatures and water content, we 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 and thus support the earlier made observations. Depending on the snow temperature and moisture content, different granulation regimes were obtained, which were qualitatively classified according to their properties and size distribution. All experimentally observed granule classes were reproduced by a discrete element model (DEM) that mimicked the competition between cohesive forces, which promoted aggregation, and impact forces, which induced fragmentation. Furthermore, the DEM simulations reproduced the grain size distribution and general behavior observed in the different granule regimes.