Modélisation des avalanches mixtes de neige sèche

Two-layers model for dry mix snow avalanches

U.R., ETNA, CEMAGREF, Saint-Martin-d'Hères

Abstract

In terms of extension and damages, the most dangerous avalanches are the ones who mobilize the fresh and dry snow. These avalanches occur in cold weather and succeed great storms. Modeling suggested here, for this type of avalanches, treats the flow from its release to its run-out zone. The avalanche is seen to be a two layers. The lowest layer, most concentrated, is considered as a dense flow. According to the shear rate and the Froude number, the flow is described either by the Mohr-Coulomb frictional model, or by an inertial model resulting from the kinetic theory applied to granular material. The dense model was validated using experimental measurements in scale model carried out in the Col du Lac Blanc (Isère France). The second layer, of weak concentration, accounts for the development of the aerosol; the flow is turbulent diphasic there and the energy dissipation is dominated by the turbulence of the interstitial fluid. The initiation and the development of the aerosol result from the erosion at the top of the dense layer. The interface between the two layers is treated by a model resulting from the saltation theory. The mass flux exchanged is proportional to the difference between the constraints exerted by the aerosol and the threshold strength of the partides in the dense flow. The model of erosion is completed by a deposit model, which operates when the turbulence decreases. These two models (erosion and deposit) were validated by wind tunnel tests. Finally the two-layers model was used to reproduce real events.