How a deadly avalanche can be triggered

Bryn Nelson Columnist

Look out above!

A recent European study and follow-up experiments are suggesting how cross-country skiers or snowmobilers can easily trigger deadly slab avalanches hundreds of yards uphill on steeper slopes.

The study’s surprising findings suggest that a massive collapse of a snow layer can be initiated regardless of a slope’s inclination — in contrast to previous hypotheses that slab avalanches were almost entirely dependent upon gravitational forces — meaning that the process can begin in a valley below or even on a flat snowfield.

“If you as a skier triggered the collapse, even if it is flat land, it can travel uphill until it gets to a steep portion,” said Peter Gumbsch, director of the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Germany, and a co-author of a study published July 11 in the journal Science. “So skiers may release avalanches from a distance from where they are. If it’s below you, then you’re fine, but it’s not always the case. If it’s above you, then you’re in deep trouble.”

Heavy snowfall across North American mountain ranges over the past month has led to a series of deadly avalanches, including two slides that claimed the lives of eight snowmobilers this week in British Columbia.

The study's findings could lead to better warnings for recreational enthusiasts based on signals of snow instability. In addition, safety officials could use directed blasts to proactively “release little avalanches rather than wait for a big one,” Gumbsch said.

Jane Blackford and Chris Jeffree / University of Edinburgh The fragile connections, or interstices, between ice crystals play a significant role in the formation of cracks in the snow, especially in a weak frost layer between older and newer snow accumulations.

The mechanics of a slab avalanche, in which an upper snow layer breaks free and plunges downhill, have long been a source of confusion for scientists. Researchers have traditionally held that such avalanches require only gravity-driven shear failure in the brittle connection between an upper and lower layer of snow, eventually shifting the upper layer if the slope is steep enough.

But numerous observations from groups such as the International Avalanche Association suggest that a slab avalanche can be remotely triggered from dozens or even hundreds of yards downslope or from an adjacent slope.

To explain the avalanche conundrum, Gumbsch teamed with materials scientist Michael Zaiser and physicist Joachim Heierli at The University of Edinburgh in Scotland.

“What we found out is that all previous descriptions were focusing on shear forces — a steep incline would be required to get any fracturing event started,” Gumbsch said. “When we examined this in more detail, we found that this was insufficient. The force, the weight pushing down the snow even in flat land, this forms an important contribution in triggering the release of a slab avalanche.”

Instead of a one-step process dependent on gravity’s shear forces alone, the team’s physical model suggests that slab avalanches are two-step processes that first require a triggering event. The model suggests that a weak boundary between the older snow below and newer snow above — a boundary usually composed of frost or hoar crystals — is a critical source of such avalanches.

“To release the slab and have it slide down the hill, you need to break this weaker layer in between,” Gumbsch said.

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