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 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.

Thermal energy in dry snow avalanches

Michael Lehning, Walter Steinkogler

Avalanches can exhibit many different flow regimes from powder clouds to slush flows. Flow regimes are largely controlled by the properties of the snow released and entrained along the path. Recent investigations showed the temperature of the moving snow to be one of the most important factors controlling the mobility of the flow. The temperature of an avalanche is determined by the temperature of the released and entrained snow but also increases by frictional processes with time. For three artificially released avalanches, we conducted snow profiles along the avalanche track and in the deposition area, which allowed quantifying the temperature of the eroded snow layers. This data set allowed to calculate the thermal balance, from release to deposition, and to discuss the magnitudes of different sources of thermal energy of the avalanches. For the investigated dry avalanches, the thermal energy increase due to friction was mainly depending on the effective elevation drop of the mass of the avalanche with a warming of approximately 0.3 degrees C per 100 vertical metres. Contrarily, the temperature change due to entrainment varied for the individual avalanches, from -0.08 to 0.3 degrees C, and depended on the temperature of the snow along the path and the erosion depth. Infrared radiation thermography (IRT) was used to assess the surface temperature before, during and just after the avalanche with high spatial resolution. This data set allowed to identify the warmest temperatures to be located in the deposits of the dense core. Future research directions, especially for the application of IRT, in the field of thermal investigations in avalanche dynamics are discussed.

Copernicus Gesellschaft Mbh2015

Granulation of snow: From tumbler experiments to discrete element simulations

Johan Alexandre Philippe Gaume, Michael Lehning, Betty Sovilla, Walter Steinkogler

It is well known that snow avalanches exhibit granulation phenomena, i.e., the formation of large and apparently stable snow granules during the flow. The size distribution of the granules has an influence on flow behavior which, in turn, affects runout distances and avalanche velocities. The underlying mechanisms of granule formation are notoriously difficult to investigate within large-scale field experiments, due to limitations in the scope for measuring temperatures, velocities, and size distributions. To address this issue we present experiments with a concrete tumbler, which provide an appropriate means to investigate granule formation of snow. In a set of experiments at constant rotation velocity with varying temperatures and water content, we demonstrate that temperature has a major impact on the formation of granules. The experiments showed that granules only formed when the snow temperature exceeded -1(degrees)C. No evolution in the granule size was observed at colder temperatures. Depending on the conditions, different granulation regimes are obtained, which are qualitatively classified according to their persistence and size distribution. The potential of granulation of snow in a tumbler is further demonstrated by showing that generic features of the experiments can be reproduced by cohesive discrete element simulations. The proposed discrete element model mimics the competition between cohesive forces, which promote aggregation, and impact forces, which induce fragmentation, and supports the interpretation of the granule regime classification obtained from the tumbler experiments. Generalizations, implications for flow dynamics, and experimental and model limitations as well as suggestions for future work are discussed.

2015

Influence of snow cover properties on avalanche dynamics

Michael Lehning, Betty Sovilla, Walter Steinkogler

The destructive power of an avalanche depends, among other things, on the overall mass and the snow conditions in the avalanche path. So far, the knowledge on the effect of snow conditions on avalanche behavior is limited and largely qualitative. We investigate the effects of snow cover properties on avalanche dynamics, such as run-out distance and front velocity. Therefore, five avalanches with similar initial mass and topography but different flow dynamics were selected from the Vallee 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. A direct effect of snow temperature on front velocity, development of the powder cloud and deposition structures could be observed. A snow temperature warmer than approximately 2 C was identified as critical value for changes in flow dynamics. No direct correlations of flow dynamical parameters with snow density and type of entrained snow were observed. (C) 2013 Elsevier B.V. All rights reserved.

Elsevier Science Bv2014