Year by year, more backcountry skiers travel to mountain ranges across America. There is some worry that avalanche accidents involving multiple groups of people will grow as a result. A recent paper published on the open access website arXiv suggests that this risk increases with the density of skiing parties in a given area. Put simply, the closer together multiple groups of skiers are on a slope, the higher their chances are of endangering one another with an avalanche.
As their name suggests, inter-party avalanches involve at least two people or groups: one inadvertently triggers the avalanche, and the others are swallowed up by it. This type of hazard—and its increasing likelihood in the age of an overpopulated backcountry—has been discussed by the skiing community before, but this paper’s author, Charlie Hagedorn, has put it in a mathematical context so it can be measured. In the paper, he builds a model that attempts to predict at what point the risk of an avalanche starts to increase—and by how much—as the density of skiing groups ticks up in a given area.
Hagedorn, a physics researcher at the University of Washington and a backcountry skier himself, was inspired to pursue the topic after a backcountry traveller disappeared on a slope he has skied many times. The paper is written as a discussion piece, to spur a professional conversation among avalanche experts on inter-party accidents.
“The idea with this paper is to create a mathematical framework and start a rigorous professional conversation about inter-party incidents while the rate remains small,” Hagedorn told GlacierHub.
His model indicates that the key variable affecting the likelihood of an inter-party avalanche is the density of ski groups (consisting of one or more people). The likelihood of one of these avalanches increases as the square of the density of groups in the area.
The incident that stirred Hagedorn to make this model occurred on December 19, 2015. Two parties of backcountry travelers were skiing just below the ridgeline on Kendall Peak in Mt. Baker-Snoqualmie National Forest in the Cascades in Washington State. This area is considered “avalanche terrain”—a tract of land where avalanches occur. The region had been hit by record breaking snowfall, and deep powdery snow had piled up on the peak.
After a run, one of the groups recalled passing a solo skier further down the slope. As they ascended through the glades for another go, they witnessed a series of avalanches triggered by the other party of skiers further up by the ridgeline.
Later that day the solo skier they had passed was reported missing. A 3,000-man-hour search and rescue effort ended six months later, when the skier’s body was found face down, ski poles scattered 40 to 50 feet above him, and some personal belongings strewn about further downhill. According to the accident report, an avalung pack—a device meant to help people breathe while buried under snow—lay next to his body, the mouthpiece visible.
Hagedorn was in the vicinity on the day when the skier disappeared and was involved with the six-month search and post-accident investigation. The subsequent report concluded it was possible the skier was killed by one of the human-caused avalanches. The experience, he told GlacierHub, was impactful. “Friends and I were easily within half a kilometer of him the day he disappeared,” he said.
There are many kinds of avalanches, all of which depend on a particular recipe of conditions to occur, but snow and avalanche scientist Jordy Hendrikx of Montana State University told GlacierHub that slab avalanches typically require four ingredients: a slab of cohesive snow, a weak layer (below the slab), a slope of 30 degrees or greater, and a trigger.
Triggers come in many forms: new snow, wind, a cornice drop; but when fatal accidents occur they’re usually triggered by people. “Research shows that ninety percent of avalanche accidents are triggered by either a victim or a member of the victim’s group,” said Hendrikx.
Hagedorn described trigger points as being “not unlike a minefield.” The more groups of people passing through avalanche terrain, the higher the chances of someone plodding on one. “There have been many accidents, where a slope that has been skied many times has avalanched when a person has hit just the right spot at just the right time.”
In addition to the Kendall Peak accident, Hagedorn chronicled 12 other incidents of inter-party avalanches, including one from the Nisqually and Wilson glaciers on Mt. Rainier. That one, from 2008, was triggered by a descending party of skiers and caught a splitboarder and skier on its way down. It nearly swallowed a party of ice climbers as well, as it plowed passed the convergence of the two glaciers.
There are discussions in the backcountry community of skiers using radios to communicate with each other, or mapping/sharing information on their ski runs in order to keep an awareness of who is above and below on the slope.
Safe behavior cannot always protect you from someone else’s unsafe behavior, however. One study found that overconfident skiers expose themselves to more “black swan” events like avalanche pileups. Even experienced skiers, according to another study, are susceptible to skiing risky terrain because of social pressures.
The best action, said Hagedorn, is to avoid densely skied areas altogether. He has sharply curtailed visits to such places and instead skis regions that are harder to access or have lower quality snow. In his paper, he writes that there are still “lonely places with great skiing” out there.
Hendrikx expressed a similar sentiment. “We have plenty of mountains and space—it’s just that we all crowd into certain spots, in increasingly marginal conditions” he said. And it’s mostly because those spots have “easy access and good snow.”
Hagedorn hopes this paper can spark quantitative study of inter-party avalanches—especially in Europe which has dealt with high mountain population densities for some time. But, perhaps more than that, “this paper,” he said, “is an attempt, among several, to push back at entropy a little, to find something helpful from within a tragedy.”