Examining the myth of skier-stabilized slopes

With increased skier traffic at resorts and revolutionary touring gear becoming the norm, more people are skiing in the backcountry than ever. In fact, certain off-piste zones have become so popular that I often hear comments about runs being ‘skied out’ or ‘mogul fields’. While this might be a slight dramatization, there is no question that increased skier traffic has an effect on the snowpack. The question is, can enough skier traffic and the subsequent compaction actually improve snow stability in uncontrolled avalanche terrain?

As a resident of Vail, I hear this rhetoric often as a justification for skiing in East Vail. This popular backcountry area adjacent to the resort is easily accessed via lift and a fifteen minute hike. As a result, hundreds of riders can make turns in this uncontrolled terrain on any given day. The popularity of this zone gives people confidence in the snow stability, and the idea of skier compaction certainly holds weight in people’s decision making process. In my opinion, while this heavy skier traffic probably does improve snow stability, I believe it’s a risky proposition to treat a backcountry zone any differently because of skier traffic.

According to ‘The Effects of Compaction on Snow Stability’ a paper presented at ISSW 2016, compaction can improve snowpack stability if it impacts a weak layer, disrupts the failure plain, or affects slab cohesion. In any of these scenarios, the probability of fracture arrest, and therefore an avalanche not happening, is greater because of the increased slab or weak layer heterogeneity. In other words, when skiers track up a slope, they are reducing the snow’s continuity and therefore its ability to avalanche. The hundreds of skiers that ski East Vail storm after storm are certainly having this effect, but to what extent? Are they impacting every weak layer in the snowpack? Are they impacting the entire slab let alone the underlying weak layer?

Even on great snow years, the poma lift which offers the quickest access to East Vail won’t start running until early December. By this time, our continental snowpack usually consists of weak basal facets buried underneath more recent storms. Without skiers to interrupt these layers, this snow stays undisturbed and will remain lurking for the remainder of the season. In fact, persistent weak layers have the opportunity to stick around any time skier traffic is restricted or limited. This is pretty intuitive that when skiers aren’t skiing a slope that stability is not improving. However, what isn’t so intuitive is that even when skiers do have consistent access to a slope, there are still limitations of their impact.

“We’ve all seen slopes slide that have had tons of tracks on them,” says Brian Lazar of the Colorado Avalanche Information Center. “For skier compaction to be effective, it needs to break up the whole slab and it also needs to disrupt the underlying weak layer. Oftentimes this doesn’t happen no matter how many tracks you put on a slope.”

Also, in the modern age of wide rockered skis, there’s a line of thought that skiers are having a broader yet shallower impact on the snow. As a result, it is less likely for skiers to impact the entire slab, let alone the underlying weak layer. While there is a chance that skier compaction might make these certain types of avalanches less likely at certain times- think loose dry and storm slab instabilities- it is really hard to know whether or not skier traffic is actually having a positive impact on slope stability.

The only way to feel confident about a slope’s stability is to consistently break up weak layers, storm after storm, throughout the entire season. This technique is the foundation of any resort snow safety program. Colorado’s Aspen Highlands offers a great example of this.

On the resort, the northeast facing Highland Bowl tops out at 3774 meters and features 40-45 degree slopes. In other words, it’s ideal avalanche terrain in an unstable continental snowpack. However, since the ski patrol implemented an intensive ‘boot packing’ program, they have been able to keep this terrain open most days of the season. Every winter at the beginning of the season, hoards of people hike shoulder to shoulder up and down the snow covered bowl. This strenuous technique is incredibly effective at breaking up the weak Colorado basal snow and gives patrol a stable base to work with. From then on, the snow is controlled storm by storm with a mix of mitigation techniques and skier traffic.

Amazingly enough, as thorough as this strategy is, it still is not 100% effective. Peter Carvelli summed it up perfectly in his paper for the ISSW, “..there are reports in the ski area industry of avalanches occurring in previously boot packed terrain … these reports serve to remind us that no method is 100% successful and redundant control work, monitoring, and testing are necessary to ensure the best chance for success.” For all of the effort and disruption they put into their snow safety program at Highlands and for them still not being 100% certain — that says a lot to me.

Despite the inevitable uncertainty in the avalanche world, we know that when we break up weak layers throughout the season from the first snowfall, it is a largely effective mitigation tool. Unfortunately, there is not a backcountry area that see’s consistent traffic throughout the entirety of the season to break up every slab and weak layer. Even in heavily trafficked zones like East Vail, we can’t even be certain that skier compaction is successfully stabilizing the upper snowpack. So while skier influence can improve stability in certain conditions, I don’t believe it holds the weight to impact the decision making process. “I don’t treat sidecountry accessed terrain any different than backcountry,” says Lazar, “unless you know the intimate history of a slope — being out there every day, knowing where every track has been skied, whether every slab and weak layer has been disrupted to a sufficient degree — you can’t make that call with confidence.”