Retrieving avalanche basal friction law from high rate positioning of avalanches

ABTRACT: The Voellmy avalanche basal friction parameters are retrieved from high rate positioning of artificially released avalanches. Two dense snow avalanches were triggered at the Lautaret full-scale test site on December 19th 2012 and February 13th 2013. Their fronts were tracked thanks to an accurate photogrammetric system. Couples of images were acquired at 1 frame per second with 2 APS-C DSLR synchronized cameras set at 800 meters from the avalanche track. 9 control points are used for image orientation and the typical precision for this measurement is around 0.2m for any point of the avalanche front. For the two avalanches, velocities are determined on several points of the front along different directions orthogonal to the flow. In order to retrieve rheological parameters of the avalanche, the front is considered as a hydraulic intumescence and the relation between its velocity and the avalanches properties is developed in the sliding block model framework. The Voellmy friction parameters in the accelerating, steady and decelerating phases are quantified and discussed