— 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.
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.
When snow goes ‘whumph’
Unfortunately for snow enthusiasts, the model found that the fragile needle-like ice crystals connecting the two layers can be broken easily and separated by the weight of new snow or a skier.
The upper layer may collapse by as little as a few millimeters (or less than one-tenth of an inch), often with an audible “whumph” of settling snow. This relatively minor shift, the model found, still produces enough energy to initiate a wave-like pulse of crystal-shattering snow separation that quickly propagates uphill and leads to a major release of the upper layer.
Once the layers of snow lose their cohesion, friction is the final barrier. For a cross-country skier in a mountainous area, then, the “whumph” sound and feel of settling snow “is the highest possible warning for you,” Gumbsch said. “I would recommend not to try to proceed any further but to get back.”
Over the past few months, he said, scientific colleagues in Switzerland have been testing his team’s model by triggering avalanches from below with small detonations. In agreement with the model, Gumbsch said, the testing is suggesting that releasing slab avalanches is much easier than previously thought.
Dave Gauthier, a researcher in the Applied Snow and Avalanche Research Centre at the University of Calgary in Canada, said the center’s field data have both informed and benefited from Gumbsch’s studies.
“I see their research as a very important step toward formulating an explanation for many different behaviors of snow and avalanches that field workers have been observing for a long time, a number of which were inexplicable using the 'traditional’ models,' ” he said in an e-mail.
Gauthier and his collaborators visited as many avalanche sites as they could to conduct their own tests. Essentially, they isolated a rectangular column from the surrounding snowpack and used a snow saw to start an artificial fracture by cutting through the weak underlying frost layer, then watched what happened.
“We found that often in snow we knew to be unstable, the weak layer fracture took off on its own before we had cut half the column, and ran across the column right to the end,” he said.
After many tests, the group concluded that an avalanche forecaster who sees such behavior in the field could expect conditions to favor the release of an avalanche, “provided a weak layer fracture is triggered or initiated by a skier or explosives, or some other natural cause.”
Other researchers responded less positively. Dave McClung, a snow and avalanche expert at the University of British Columbia in Vancouver, in an e-mail called the European study “full of flaws and errors.”
“The authors don’t seem to know anything about snow, and the fracture mechanics in the paper is incorrect,” he said. “In addition to the physically inadmissible assumptions, there are field data which show the model is incorrect.”
Gumbsch, however, said the study suggests such avalanches can begin with a deceptively minor settling — perhaps as little as a few millimeters in a snowpack of 20 centimeters, or less than one-tenth of an inch in a depth of eight inches.
The energy required to break such a weak middle layer is as little as one-tenth of a joule, or unit of energy, per square meter, he said. By comparison, an apple falling from a height of about 3.3 feet would release 10 times that amount of energy.
How far away from a skier can an avalanche be triggered?
“We certainly know it goes from tens of meters to hundreds of meters,” Gumbsch said.
One important determinant may be the elastic strength of snow, a property that can change dramatically as snow ages. Nevertheless, he conceded that researchers still do not understand what triggers or stalls the collapse wave that extends the break between snow layers, a factor that could literally make the difference between life and death for anyone below.