A model of powder-snow avalanches

What is it about?

Powder-snow avalanches (more precisely: mixed snow avalanches) show three different flow regimes simultaneously: a dense core, an intermediate-density part, and the prominent "powder snow cloud", the suspension layer. For a complete description of the phenomenon, all three flow regimes and the transitions between them should be taken into account, but this is a complex task. The majority of earlier avalanche models (except those by Eglit and co-workers) describe either the dense core or the suspension layer. This paper proposes a model describing the intermediate-density and suspension regimes in terms of a depth-averaged two-layer model. The model thus neglects the dense core, which often gets left behind by the faster intermediate-density layer. When this paper was written, the existence of an intermediate-density flow regime was not widely recognized in the avalanche research community, even though a number of experiments had revealed it. In particular, the mechanism responsible for fluidization of the dense core was unknown. The basic assumption made in this paper is that the intermediate-density layer is analogous to the saltation layer observed near the ground in blowing sand or snow; it was therefore called 'saltation layer'. The friction can then be modeled in terms of the momentum lost by saltating particles as they hit the snow cover and may mobilize (erode) additional snow or deposit part of the saltating mass. Turbulence is responsible for suspension of fine snow particles from the saltation layer, but is counteracted by gravity. The model contains a fairly large number of parameters. In contrast to earlier simple models like the Voellmy–Salm model, these parameters have a clear physical significance and can be constrained by general phyiscal considerations. Detailed process studies allow in principle to determine their values so that so-called class-1 predictions would become possible.

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Why is it important?

The model proposed here represents one of the first examples of a description of avalanche flow in terms of more fundamental processes like splashing impacts of particles onto the snow cover and turbulent suspension. It is also the first paper modeling the suspension layer in conjunction with the intermediate-density, fluidized flow regime. A somewhat simplified version has been made available for hazard mapping work as the component SL-1D of the software package AVAL-1D, distributed by SLF. Subsequent experimental and theoretical work has shown that the basic modeling assumption, namely the analogy of the fluidized flow with saltating snow, is too simple because it disregards collisions between particles. The latter are important in this flow regime where the mean free path between collisions is only one to two particle diameters. Nevertheless, it appears that explicit modeling of the fluidized flow regime is advantageous in models of mixed snow avalanches and more realistic than the approach chosen in other models where only the dense core and the suspension layer are considered.