Swiss pro snowboarder, researcher shares new information on ‘super critical crack length’ at annual state avalanche workshop
DILLON — As part of the avalanche release and mapping session at this week’s Colorado Snow and Avalanche Workshop, former pro snowboarder Johan Gaume shared with Colorado’s backcountry snow community recent progress in snow and avalanche modeling.
Namely, Gaume — a professor who currently serves as the head of the SLAB Snow and Avalanche Simulation Laboratory based out of Lausane, Switzerland — shared research and findings regarding new insights into crack propagation in snow slab avalanche releases to better understand snow mechanics.
Gaume explained two years ago he went to the University of California at Los Angeles to work with people who did the snow graphics for the Disney movie “Frozen.” Gaume said that work led to expanded computer modeling work in the past two years to understand the mechanical behavior of snow.
Though the research has applied to various avalanche elements — including better calculating avalanche impact pressures and simulating avalanche dynamics on forested slopes, among other variables — the lab’s work was focused around scaling the mechanics of an avalanche all the way from the failure in a weak layer to the concluding slide. At the workshop, Gaume and participants centered their discussion around a “super critical crack length,” or the length at which a crack in the snow will slide a snow slab and accelerate crack propagation from subsonic to supersonic speeds.
Gaume said the super critical crack length is larger than the length needed to trigger propagation, or the spontaneous spreading of a crack within the snowpack without the addition of any external force. Essentially, there’s a transition in the crack propagation process and something called super critical crack growth where the speed is driven by the snow’s specific slab properties.
“After this length,” Gaume said, “the crack will propagate way faster.”
Gaume explained the research supported that avalanches are rarely a simple, immediate physics reaction where the sheer force on a slope overtakes the friction holding a slab in place leading to a slide.
“In reality, it doesn’t happen like this,” Gaume said. “There is a certain delay, a speed associated to the sliding of the material. Basically what we show here is there is a first critical crack length to start propagating failure, which rises slow. But then you need a second critical crack length for the slab to slide. The sliding of the slab is associated with the second phase of crack propagation, which is called super sheer crack propagation, something also observed in earthquakes.”
Gaume said depending on slope angle the super critical crack length is between 3-5 meters. Gaume said the super critical crack always happens unless a slab is too weak.
Gaume said because the lab’s model did not account for grain types the properties it took into account were cohesion and friction. That said, he believes grain does influence the super critical crack length, though it’s not modeled.
In terms of the “practical relevance” of what the super critical crack length means to members of the backcountry community, Gaume said it is not clear yet, though he said he hopes the lab’s research will help improve the evaluation of the run-out distance of avalanches, will help develop next generation avalanche dynamics models for engineering, will help to study the effects of climate change and will help improve hazard mapping and risk management procedures in a climate change context.
“For us it is very exciting mechanical results, but for me the fact that the speed increased a lot has a practical relevance,” Gaume said of the super critical crack length findings.
“I was really struck by just how the science of this is progressing,” said CAIC Executive Director Ethan Greene.
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