Interaction between dams and powder avalanches: determination of simple friction laws for shallow water avalanche models

One of the goals of the European project CADZIE was the determination of simple friction laws for shallow water avalanche models in order to describe the interaction between defense structures and powder as well as dense avalanches. In this paper, we will focus on powder avalanches. We carried out physical model experiments in a water tank, in which the powder snow was simulated by a “heavy” salt solution. Respecting both the densimetric Froude number and the density ratio to meet the laws of similarity required a prohibitively large tank. Practically, a distortion of the density ratio must be accepted. A ratio of 1.2 can be achieved in the water tank with salt suspensions, whereas powder avalanches attain ratios of the order of 10. Quasi-two and three-dimensional experiments with several configurations of dams were conducted in the water tank and the internal velocity of the flow was measured using Doppler ultrasonic velocimetry. Numerical simulations of such experiments were done using a turbulent “heavy gas” model: two mass balances, one for mixture and one for the salt solution are considered, but only one momentum balance for the entire mixture implying that both water and salt solution are assumed to move with the same velocity. The obtained results were compared with measurements obtained in the water tank and the effect of density ratio was numerically studied. As it was shown that the entrainment of air is modified by the density ratio, friction coefficients were determined for different density ratio (2.5 and 10) and for different values of damʹs height (from 0.2 to 0.6 H0, where H0 is the height of the reference avalanche). The practical use of such coefficient tables is shown.