Velocity profiles and sheare rates for the different avalanche flow regimes

Abstract

Snow avalanches are a well-known hazard in alpine regions such as the Alps, the Himalaya, or the Rocky Mountains, and thus, a better understanding of the avalanche dynamics can contribute to risk mitigation for the alpine environment. In this context, it is important to analyse real-scale snow avalanches to further simulate avalanche motion downward different slopes to predict and prevent the impact and damage that avalanche can pose. In this thesis we analysed the data from 5 real-scale avalanche eventsregistered at the full-scale avalanche experimental site “Vallée de la Sionne” in Switzerland, with the purpose of better understanding of the various flow regimes that characterize snow avalanches. To this aim, we interpreted the data collected on a 20 meters height pylon instrumented with velocity, pressure, and temperature sensors, situated before the avalanche run-out zone in VdlS. This pylon was now equipped with more sensors allowing us to extract more accurate velocity profiles from the avalanche flow than previous studies. The implementation of more optical sensors also made possible to be the first data analysis extracting conclusion from the avalanche boundary conditions. An appropriate selection and interpretation of the avalanche velocity profiles, and flow parameters were essential for obtaining reliable results. Here is presented an individual analysis for the five avalanches to introduce and understand each avalanche event. These avalanches were discretized in their different flow regimes and transitions from the interpretation of their main parameters and velocity profiles. We compared the avalanche behaviours and their variables time-evolution, therefore the different flow regimes that each avalanche experiments were contrasted. The very high vertical resolution of the velocity measurements has allowed, for the first time, to observe the basal sliding layer for the different flow regimes. In particular we could observe that there is an existent slip velocity different from zero, at the boundary between the avalanche flow and the base surface once the avalanche has developed a smooth basal sliding surface. This sliding surface induces the avalanche to persist longer on warm plug regimes due the low energy dissipation. In this regime, the velocity is almost linearly correlated with the flow depth. On the other hand, the sheared flow regimes are normally shorter and with higher deceleration, because of the higher kinetic energy dissipation caused by the velocity differential in the whole velocity profile. We also found out that avalanche flow depth has a direct impact on the flow velocity for plug regimes but not for sheared regimes.