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Powder snow avalanches (PSAs) are a major threat to people and infrastructure in many mountainous regions of the world. Their force is enough to easily destroy mature forests and any infrastructure located along their path. This is to some extent related to the high velocities and flow heights they can develop, but the basic physical mechanisms controlling their destructiveness remain unclear. Part of this insecurity is related to the fact that the structure of a PSA is very complex. In fact, it can be visualized as the superposition of three distinct layers: (i) a dense granular basal layer, (ii) a transition layer in the form of a turbulent flow with strong density stratification, and (iii) a dilute turbulent suspension of finer particles covering the whole.Experimental data collected at the Vallée de la Sionne avalanche test site (VdlS) in Switzerland suggests that the destructive capacity of PSAs is largely related to high-energy pulses within this complex stratification. This data indicates that there are at least three different physical processes at the origin of the pulses, specifically: (i) waves at the surface of the dense basal layer (e.g roll-waves, erosion-deposition waves), (ii) coherent vertical structures within the transition layer, and (iii) coherent turbulence structures in the suspension layer. All of these processes, which can also coexist, can be associated with maxima in the dynamic pressure measured over a fixed obstacle.Although the origin of these pulses still remains largely speculative, with this contribution we aim to present experimental evidence for the existence of various mechanisms of pulsing in PSAs and discuss their relevance in terms of dynamic pressure calculations.

The effect of climate change becomes more and more perceptible in mountain areas. For instance, in low to medium elevations, the avalanche activity is already impacted because of a warmer snow cover. This warming has an important effect on the mechanical properties of snow, especially close to 0°C, where the temperature and the presence of liquid water greatly affects the cohesion and friction. In snow avalanches, the transition from cold (T < -2°C) to warm (T = 0°C) snow generates a variety of dense flow regimes which differ drastically in terms of velocity and shearing profiles. For a cold and loose snow, one can typically observe fast flows with Bagnold-shaped profiles of a few tens of meters per second, while a warm and wet snow exhibits low- or zero-sheared velocity profiles with a magnitude of a few meters per second, over the same topography.The present work aims to investigate the rheology of flowing snow as a function of its physical properties, with a view to bring more physics into continuum avalanches models based on empirical coefficients. We use a 2D Discrete Element Modeling (DEM) to model snow as a cohesive granular material. The simulated distinct particles interact through a contact model that can be tuned in terms of cohesion and friction, in order to satisfy the four dense flow regimes: cold dense regime, sliding slab regime, warm shear regime and warm plug regime.First, we calibrate the contact model to find the adequate ranges of cohesion and friction corresponding to the four flow regimes. We also highlight the particular boundary conditions that are required for specific flow regimes to occur, particularly the importance of the ground friction and the initial cohesion of the snow. Second, we extract rheological features such as the friction law μ(I) and the values of θstop for each flow regime and discuss their relevance regarding avalanche dynamics. Finally, the interaction of the flow regimes with an erodible snow cover is explored and discussed qualitatively.

Erosion significantly affects the dynamics of gravity-driven mass flows. In snow avalanches, the snow cover can be substantially eroded but only partially entrained, however, there are very limited investigations to substantiate this difference. Here, we study various erosion and entrainment behaviors in snow avalanches using the material point method (MPM), finite strain elastoplasticity and critical state soil mechanics. With varied snow properties, distinct erosion patterns are obtained and analyzed with the mass change rate. When there is significant eroded and entrained mass, properties of released snow and erodible bed snow have clear correlations with the eroded mass, but not with the entrained mass, disclosing the difference in erosion and entrainment. Both enhanced and inhibited avalanche mobilities due to erosion and entrainment are captured under different conditions of snow properties and lengths of release and erodible zones. These new insights may stimulate the development of advanced erosion and entrainment models for large-scale avalanches.

Erosion and entrainment are critical processes in gravity-driven mass flows like snow avalanches, as they can significantly change the flow mass and momentum and thus affect the flow dynamics. In snow avalanches, snow cover can be considerably eroded but only partially entrained into the flow. Differentiating erosion and entrainment gives more accurate prediction of the increased flow mass and offers information on eroded snow cover remaining on the slope, but is challenging in practice. This study investigates snow avalanche erosion and entrainment with the material point method, focusing on exploring various erosion mechanisms, differences in erosion and entrainment, and their possible influences on runout distance. By using different mechanical properties for the flowing snow, distinct erosion patterns are observed and the corresponding temporal evolutions of entrainment, erosion, and deposition in the erodible bed are examined. Erosion and entrainment require an appropriate combination of snow friction and cohesion of the bed. If cohesion and/or friction are too low, the bed will naturally be unstable. On the other hand, highly cohesive and frictional bed will prevent erosion. For intermediate values, erosion and entrainment can be notable, and the amount of eroded snow shows a clear negative correlation with snow friction and cohesion while the entrained snow does not demonstrate a strong tendency. Furthermore, the release and erodible bed lengths are varied to study their effect on erosion and entrainment propensity. It is found that the increase in the lengths of the release zone and erodible bed leads to more erosion and entrainment as expected, but not necessarily to a longer runout distance. In our simulations, the release and erodible bed lengths are positively and negatively correlated with the runout distance, respectively. This implies that the runout distance can have opposite trends with erosion and entrainment, which might be closely related to the energy change of the simulated avalanches from the outlet of the erodible bed to the final deposit. Our results shed more light into the erosion and entrainment mechanisms and may contribute to improve related parametrizations in large-scale avalanche dynamics models.

Measurements of snow avalanche impact pressures are performed at the Vallée de la Sionne test site since winter 1999. In these years of operation, we recorded the impact pressure of around 60 avalanches characterized by different flow regimes and dimensions.Pressure measurements were performed, simultaneously, on three different structures which are spatially distributed with a maximum distance of 30 m, in the run-out zone of the Vallée de la Sionne test site. The structure widths range from 0.25 to 1 m. On these structures pressure sensors ranging from small cells with 0.10 to 0.25 m in diameter to large pressure plates with area of 1m2 are mounted at different heights.A systematic analysis of all 60 avalanche data sets shows that the pressure measured at the different obstacles varies considerably, even within the same avalanche, both in space and time. Part of these differences can be attributed to different drag coefficients and dependence on obstacle size, but a large part of these differences can only be explained by the spatial variability of the flow properties and the temporal variability of the physical processes governing the interaction of the avalanche and the structures.In this contribution we show how spatial and temporal impact pressure variabilities correlate to avalanche dimension and flow regimes and we discuss the implication of such variations for structural design and hazard mapping.

In the near future, climate change will impact the snow cover in Alpine regions. Higher precipitations and warmer temperatures are expected at lower altitude, leading to larger gradients of snow temperature, snow water content and snow depth between the top and the bottom of slopes. As a consequence, climate change will also indirectly influence the behavior of snow avalanches.The present work aims to investigate how changes in snow cover properties will affect snow avalanches dynamics. Experimental studies allowed to characterize different avalanche flow regimes which result from particular combinations of snow physical and mechanical properties. In particular, expected variations of snow temperatures with elevation will cause more frequent and more extreme flow regime transitions inside the same avalanche. For example, a fast avalanche characterized by cold and low-cohesive snow in the upper part of the avalanche track will transform more frequently into a slow flow made of wet and heavy snow when the avalanche will entrain warm snow along the slope. A better understanding of these flow regime transitions, which have already been largely reported, is crucial, because it affects both daily danger assessment (e.g. forecasting services, road controls) and hazard mapping of avalanches.To date, most avalanche modeling methods are not considering temperature effects and are then unable to simulate flow regime transitions and unprecedented scenarios. Our goal is then to develop a model capable of simulating reported flow regimes, flow transitions and the interactions between the snow cover and the flow (erosion, entrainment). Since these elements are not yet fully understood, we firstly model these mesoscopic processes using a 2D Discrete Element Model (DEM) in which varying particle cohesion and friction mimic the effect of changes in snow temperature. Flow regimes are simulated by granular assemblies put into motion by gravity on an inclined slope, which interact with a granular and erodible bed surface. Simulations are calibrated using experimental data coming from the avalanche test site located in Vallée de la Sionne, which record avalanches since more than 20 years. This modeling will then be used as an input to improve slope-scale models and make them more appropriate for avalanche risk management in the context of climate change.