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Saturday, 09 August 2008 23:28

Avalanche! It's all about the anti-crack

The discovery of a new theory behind avalanches by a team of English-German scientists could produce better understanding on avalanche formation.

The researchers include Joachim Heierli, P. Gumbsch, and M. Zaiser. They are associated with the Centre of Materials Science and Engineering, University of Edinburgh, United Kingdom; Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe, Germany; and Fraunhofer Institut für Werkstoffmechanik, Germany.

They state, within the abstract to their August 11, 2008 Science journal paper, “Snow slab avalanches are believed to begin by the gravity-driven shear failure of weak layers in stratified snow. The critical crack length for shear crack propagation along such layers should increase without bound as the slope decreases. However, recent experiments show that the critical length of artificially introduced cracks remains constant or, if anything, slightly decreases with decreasing slope.” [Science: “Anticrack Nucleation as Triggering Mechanism for Snow Slab Avalanches”]

According to the Science News article “It all began with a single crack” (August 2, 2008, page 12),  “Scientists had previously thought that the gravity tugging on the slope drives the avalanche and that the critical crack size to start an avalanche should increase as the slope angle decreases. But, field experiments suggest this is not the case. Heierli’s team addressed the discrepancy by modeling gravity’s tug on the snow along the slope and the downward pull of gravity perpendicular to the slope, finding the perpendicular pull was more important.”

The Heierli team adds, “This surprising observation can be understood in terms of volumetric collapse of the weak layer during failure, resulting in the formation and propagation of mixed-mode anticracks, which are driven simultaneously by slope-parallel and slope-normal components of gravity. Such fractures may propagate even if crack-face friction impedes downhill sliding of the snowpack, indicating a scenario in which two separate conditions have to be met for slab avalanche release.” [Science News (subscription required): “Recipe for an avalanche”]

Thus, the researchers are stating that the theory behind predicting when avalanches first form are due to many factors and not only to the angle of the mountain slope. Additional factors include how the snow cracks internally and when it shifts and eventually shears off.

The scientists break down the structure of snow on page two. So, are avalanches more dangerous or less, according to these scientists? Check out the answer.

The researchers classify the structure of a snow pile as generally consisting of three layers.

The top layer is characterized as a solid, dense snow slab. The second, or middle layer, is structured as a brittle, collapsible section, while the third (bottom) layer consists of a rigid snow base.

The middle layer is the most important layer when it comes to forming an avalanche. The way the middle layer is fractured controls whether a snow pile shears off (thus, forming a slab avalanche) or simply collapses under its own weight.

In fact, Heierli states, “Some layers inside the snow are a very frail network of ice grains with lots of space in between. Some arrangements may crumble like a house of cards.” [Science News]

The research team discovered this new information as they modeled slab avalanches, which are the most dangerous type of avalanche. They are very dangerous because the slab of snow can unexpectedly break loose and quickly flows downward to the bottom of the mountain slope.

In fact, the Heierli team found that the scientific belief that the fracture itself is not easily created is not true. The fracturing process is actually very easily to create.

They suggest that even very small layers of snow could fracture; in fact, much more easily that previous thought. The researchers suggest that skiers, for instance, traveling over flat snow or even across gentle slopes are not completely safe.

According to Hoachim Heierli (School of Engineering and Electronics, University of Edinburgh, U.K.), the lead author of the study, “The new theory could be a breakthrough in understanding what is going on at the very moment when an avalanche begins.” [Science News]


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