A comparison of powder-snow avalanches at Vallee de la Sionne, Switzerland, with plume theories

Powder-snow avalanches are natural hazards which affect the way populations live in mountainous areas. Field measurements from avalanches remain one of the most significant and useful sources of information about their dynamics and behaviour. In this paper, we consider all the video data from the Swiss Vallee de la Sionne test site from the years 2003-05. General scaling laws are sought for the avalanche front velocity based on plume theories. Avalanche Froude numbers are found, comparing three different length scales: the cube root of the fracture volume; the avalanche height; and the depth of entrained snow cover. We discuss the difficulties in defining the volume of a powder-snow avalanche: should we include just the head or also the turbulent wake that extends back to the starting zone? This relates to whether we use a compact model for the avalanche, such as the KSB model (Ancey, 2006; Turnbull and others, 2006) or a plume model (Turner, 1973). Observations are made regarding the lateral spreading behaviour of the avalanches. We show that the slow lateral spreading can be explained by large internal velocities and anisotropic turbulence generated by the large-scale motion in the avalanche head.