2020 Avalanche News

Swiss pro snowboarder, researcher shares new information on ‘super critical crack length’

News News | October 16, 2020

Former pro snowboarder Johan Gaume — now a professor serving as the head of the SLAB Snow and Avalanche Simulation Laboratory based out of Lausane, Switzerland — has new insights into crack propagation in snow slab avalanche releases to better understand snow mechanics.

Two years ago Gaume went to the University of California at Los Angeles to work with people who did the snow graphics for the Disney movie “Frozen.” That work led to expanded computer modeling work to understand the mechanical behavior of snow.

Though the research has applications to various avalanche elements — including better calculating avalanche impact pressures and simulating avalanche dynamics on forested slopes, among other things — 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.

Gaume has recently been studying the “super critical crack length,” or the length at which a crack in the snow will accelerate crack propagation from subsonic to supersonic speeds.

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.”

The research supports the idea that avalanches are rarely a simple, immediate physics reaction where the 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 with the sliding of the material. Basically what we show is that there is a first critical crack length to start propagating failure, which is 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 that the lab’s model did not account for grain types but the properties it did take into account were cohesion and friction. That said, he believes grain does influence the super critical crack length, even though it’s not currently modeled.

In terms of the “practical relevance” of what the super critical crack length means to people in the backcountry, Gaume said it is not clear.

He did say he hopes the lab’s research will help improve the evaluation of the run-out distance of avalanches, help develop next generation avalanche dynamics models for engineering, help studies of the effects of climate change, and 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.

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