The underestimated role of the stauchwall in full-depth avalanche release

During the winter 2011/12 the Swiss Alps experienced repeated cycles with high activity of snow gliding and full-depth avalanches. Up to 8 m deep glide cracks formed and full-depth avalanches caused significant damage to infrastructure. Long-standing questions reappeared: Are full-depth ava- lanches predictable? Are there optimal preventive measures? We use field observations to develop a model that divides the snowcover into a full-depth, gliding zone and a non-gliding deformable snowcover. We assume a glide crack opens and calculate how the lost tensile force is transferred to the non-gliding zone, the stauchwall. The model reveals how the interaction between the stauchwall and the instable glid- ing mass depends on the visco-elastic properties of the snowcover. We first demonstrate why the open- ing of a glide crack does not automatically lead to avalanching. However, under certain conditions mate- rial failure in brittle compression is imminent and full-depth avalanches can release. Failure depends not only on the snow properties of the stauchwall, but also on the frictional properties of the gliding interface. Even small length gliding zones can initiate avalanches, depending on the slope angle and the glide vari- ability of the interface. The model underscores the importance of the stauchwall and reveals why the for- mation of full-depth avalanches is depth invariant (although the consequences are not). We use the model to draw conclusions concerning the forecasting and mitigation of full-depth avalanches. The con- clusions stress the difficulties and limitations of forecasting as the necessary and relevant data will seldom be available.