Avalanches also on the flat slope

New model challenges previous explanations for slab avalanches

Example of a remotely triggered slab avalanche. The break release was initiated by a field worker on top of the ridge. Although the frictional forces were overcome only in the steep slope, the break could still spread through the intermediate section. This fact can be explained in a natural way with the proposed model. © A. Duclos, www.data-avalanche.org
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Until now it was said: "The flatter the terrain, the lower the risk of a slab avalanche." Unfortunately, this rule of thumb does not necessarily apply, according to a recent study in Science magazine. The new model is based on the interaction of the formation and propagation of cracks and friction within snowfields and contradicts previous theories.

Every year, around 100 winter athletes die in avalanche accidents. In many cases, the avalanches are triggered by the winter athletes themselves. One differentiates between loose snow avalanches, which emanate from a point and slab avalanches, with which a contiguous "slab" slips down the slope. Due to precipitation and heat flow, snow fields are usually built up of several layers of snow, which are different firm.

Cracks in the snow as an indication

New tests on the behavior of snow under load by Joachim Heierli and Michael Zaiser from the University of Edinburgh in Scotland and Peter Gumbsch from the Karlsruhe Institute of Technology (KIT) and from the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg. At the same time, material researchers have looked at new mechanisms for how cracks in snow develop and grow.

As snow consists of ice crystals and a lot of void, ice crystals that break off in a deeper layer or separate from each other, can move closer together. This enlarges the cavity. If this cavity spreads out under the snowpack, the snowpack collapses afterwards. Where now the two fresh contact surfaces meet now

Frictional forces and decide on the departure of an avalanche. display

Cavity with zipper effect

"Once such a cavity has been created, there is a risk that it will spread quickly by itself. The effect is similar to opening a zipper: The expansion of the cavity separates the bound snow layers from each other. It can, for example, within seconds

Run up a slope from the flat terrain and trigger an avalanche there. It comes to a so-called remote release, which is particularly insidious for skiers, "explains Joachim Heierli of the University of Edinburgh.

Responsible for the emergence of slab avalanches is thus the interaction of cracks, which spread out between layers of snow and the friction that exists between the now separated snow layers. The surprising thing is that the collapse, which causes two layers of snow to break away from each other, is just as likely to arise in flat terrain as in steep terrain. In contrast, the friction deficit, which leads to the slipping of a snowboard and thus to the avalanche, prevails primarily on steep terrain.

More than just shearing

With our findings, the widely held belief that slab avalanches are only caused by shear forces has to be called into question, "explains Michael Zaiser from the University of Edinburgh. "The key to the emergence of slab avalanches is better understood

of the material snow. We have shown how this material collapses in itself and can thus better understand the avalanche development and contribute to safety, "explains Peter Gumbsch, head of the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg and hall and

Head of the Institute for Reliability of Components and Systems at the University of Karlsruhe.

(Fraunhofer Institute for Mechanics of Materials IWM, 21.07.2008 - NPO)