On the rheology of dense flow regimes in snow avalanches

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 θ_stop 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.