INTRODUCTION



 
 

The aim of this session was first of all an opportunity for us to discover the free-surface flow code Telemac2d.
The subject we have chosen is kind of original: simulations of wet snow avalanches. Indeed, it is also the subject of our BEI (Bureau d'Etudes Industrielles Sciences de l'Eau et de l'Environnement). The first idea of the group in BEI was to study natural risks in mountains and we have separated the work into two parts: study of avalanches and hydraulics study. We are more precisely in charge of the global and theorical definition of avalanches, as well as the simulations of wet snow avalanches. The idea is then to work with an other group, who will run the same simulations but with a protective system on the flow channel in order to study the influence of protective systems on avalanches parameters.

We have chosen to simulate wet snow avalanches as it was the easiest kind to simulate with Telemac2d. Indeed, there are three different kind of avalanches:

  • Powder-snow avalanches
  • A powder-snow avalanche can be considered as the flow of a turbulent buoyant volume of heavy fluid (air-snow suspension) in an ambient fluid, the air.  They can be particularly dangerous because of the high quantity of snow which is involved in such an avalanche, as well as the high increasement of pressure due to its passage. The effects of the depression which follows the passage of the avalanche (due to the return to a normal pressure after the high pressure field within the avalanche) can be considered as the most devastating of this kind of avalanches.
  • Slab avalanches
  • Hard slabs are often created by the action of wind and are constituted of compact snow, with a high density (200 to 400 kg per meter cube). The slab can be more or less cohesive and there is a danger of rupture when for exemple, the slab is upper a layer with a slower cohesion. The slab isn't well consiladated with the sublayer. An important factor for the triggering-off of this kind of avalanche is the configuration of the area.
  • Wet-snow avalanches
  • The risk of triggering-off is directly linked to the presence of liquid water in the snow cover. Indeed, the water has the tendancy to flow in the snow and to accumulate within certain layers of the cover. If the water meets a non-permeable layer or a less permeable layer, the water can play the role of lubrificiating and can constitute a sweep plan.
    Those avalanches can particularly occur in spring, when the temperature gets higher. They can be very devastating because of their high power of erosion. This kind of phenomenon can be compared to flows of lava, because of the high viscosity of the "fluid" and of the configuration of the flow.

    Those brief descriptions show that wet-snow avalanches are the ones which are the closest of a free-surface flow. That's why, in order to be able to use Telemac2d, we have chosen those avalanches for our simulations. It is clear that the simulations are very corse because a good simulation of an avalanche requires an appropriated code, specially developed for that (some research codes can simulate avalanches but they are not available). In consequence, we had to do a lot of simplifications in the model, knowing that anyway, avalanches constitute a very difficult problem to modelize because of the great quantity of phenomena that must be taken into account (meteorology, snow metamorphosis, topography...).