Slab avalanches occur as a consequence of a failure in a weak snow layer – a layer within the snowpack in which the bonding between individual ice crystals is poor. Where this bonding is especially weak, a local overload – typically caused by a skier passing over the surface – can trigger a sequence of fracture processes that culminate in the release of a slab avalanche. The initial breaking of individual bonds between ice grains leads to the formation of an initial crack in the weak layer. Once the damage reaches a certain size – the so-called critical crack length – the crack suddenly and rapidly begins to propagate along the boundary of the weak layer underneath the slab and across the entire slope a sequence of events like a domino effect. Within a few seconds large areas of the snowpack can be released as a slab avalanche.
2015 - 2017
For a long time snow researchers have been seeking to describe this type of avalanche formation with physical models in order to improve avalanche forecasting. The initial models described the failure as a shear fracture while disregarding key properties of the weak layer itself. A more recent model - the so-called anticrack model, focused on the collapse of the weak layer, which improved the understanding of remotely triggered avalanches in flat terrain. However, this new model suggests that slope angle has very little influence on the critical crack length – a puzzling outcome in view of contrary observations indicating that the steeper the slope, the more likely avalanches are to occur.
Improving stability assessments
Scientists working at the SLF and at EPFL have now succeeded in developing a new model which reconciles these two earlier approaches. Their theoretical deliberations are founded on a numerical model. This new model is the first model to give consideration to the complex interaction of the weak layer's mechanical behaviour, in particular its microstructure, with the elasticity of the overlying layer of snow – the snow slab. This new modelling approach allows crack propagation on flat terrain to be simulated. In addition, the critical crack length for avalanche release decreases as the terrain becomes steeper – as one would expect.
Tests conducted with a wealth of data from field studies give reason for optimism – the model described the real situation with a good degree of accuracy. The researchers are therefore confident that the new approach can improve the assessment of snowpack stability, which will ultimately benefit those who issue and rely on avalanche warnings.