Fractures in weak snowpack layers in relation to slab avalanche release

The vast majority of recreational avalanche incidents are caused by human-triggering of the slab avalanche. Specific snowpack characteristics, including hardness difference and difference in crystal size across the failure layer, associated with skier-triggered dry slab avalanches were identified. The relation of these snowpack variables with fracture initiation and fracture propagation, both of which are required for skier-triggering, was investigated. The properties of the slab overlying the weak layer, as well as the layer above the weak layer, were found to be important for fracture propagation in that the slab supplies the energy necessary to propagate the fracture through the weak layer. A classification system for fractures in stability tests was assessed. It was shown that incorporating such a descriptive classification system can improve the interpretation of these test results. Sudden fractures are more often the failure layer of slab avalanches than other fractures. Specific snowpack characteristics associated with the different fracture characters showed that sudden fractures are typically associated with snowpack conditions favouring both fracture initiation and fracture propagation. In-situ fractures in weak snowpack layers were photographed at 250 frames per second in 39 field tests. Displacement measurements of markers placed in the snow above weak layers showed that slope normal displacement (due to crushing of the weak layer) was observed in each fracture. The speed of propagating fractures was measured, ranging from 17 to 26 m/s. These observations were used to assess theoretical slab release models, suggesting that the fracture of the weak layer is coupled to a propagating flexural wave in the overlying slab that controls the speed of propagation.