On the origin of the pulsing activity in powder snow avalanches

Powder snow avalanches (PSAs) are a major threat to people and infrastructure in many mountainous regions of the world. Their force is enough to easily destroy mature forests and any infrastructure located along their path. This is to some extent related to the high velocities and flow heights they can develop, but the basic physical mechanisms controlling their destructiveness remain unclear. Part of this insecurity is related to the fact that the structure of a PSA is very complex. In fact, it can be visualized as the superposition of three distinct layers: (i) a dense granular basal layer, (ii) a transition layer in the form of a turbulent flow with strong density stratification, and (iii) a dilute turbulent suspension of finer particles covering the whole.

Experimental data collected at the Vallée de la Sionne avalanche test site (VdlS) in Switzerland suggests that the destructive capacity of PSAs is largely related to high-energy pulses within this complex stratification. This data indicates that there are at least three different physical processes at the origin of the pulses, specifically: (i) waves at the surface of the dense basal layer (e.g roll-waves, erosion-deposition waves), (ii) coherent vertical structures within the transition layer, and (iii) coherent turbulence structures in the suspension layer. All of these processes, which can also coexist, can be associated with maxima in the dynamic pressure measured over a fixed obstacle.

Although the origin of these pulses still remains largely speculative, with this contribution we aim to present experimental evidence for the existence of various mechanisms of pulsing in PSAs and discuss their relevance in terms of dynamic pressure calculations.