World of Science | The legend of the busy slope

If you leave the secured pistes and ski routes in the Alps, this is a decision for which you are responsible, along with the potential consequences. Even if you are buried just 3 meters from the piste in the open ski area, it is generally not the ski resort's fault. Although the ski resorts go to great lengths to secure the slopes and infrastructure and, of course, also break avalanches in the open ski area, no guarantees are given for safety beyond the boundaries of the slopes and ski routes.

In the USA, things are different. The responsibility of the ski resorts here does not end at the edge of the piste, but at the boundary between "inbounds" and "out of bounds" terrain. The latter is not part of the ski area and is not secured. The former, on the other hand, is part of the ski area and is secured, but not necessarily groomed. Many resorts have a lot of inbounds terrain, some of it very challenging, which is secured against avalanches and does not require pistes. Often everything that can be reached from the lift without an ascent is inbounds. This terrain can be closed by the ski resort (if you ski in anyway, you could have your ski pass revoked, for example), but if it is open, the ski resort largely assumes responsibility for ensuring that no one is buried there.

The bootpackers of Highland Bowl

In 1994, the Aspen Snowmass ski resort in Colorado decided to expand the ski area to include Highland Bowl. Highland Bowl is an extensive basin near the lifts with large, open slopes and wooded terrain on the sides. Colorado's climate is very continental - cold and relatively dry. Long-lasting old snow problems are inevitable and occur almost every winter.

In order to integrate Highland Bowl into the ski area, an area of around 49 hectares with average gradients of between 37° and 42° and the most unfavorable snowpack structure imaginable had to be secured in such a way that visitors do not have to worry about avalanches. Levelling the entire basin to create pistes was out of the question. The terrain was to offer secured deep snow fun.

After the first large-scale bootpacking operation in late fall, the "systematic application of explosives" (SEA) follows in a second step. This involves detonating 1 kilo of explosives every 10 meters (considerably more explosives are used in avalanche blasting during operation, but only at strategic points, not every 10 meters). This in turn destroys weak layers. In addition, the snow in the craters is heavily compacted by the explosives and "pillars" of snow are created that are significantly firmer than the snow around them. Potential weak layers, in which the break for a snow slab can be created, are thus interrupted.

The next time it snows, the local skiers are again asked to help and are gradually released onto the terrain under strict supervision. This step serves to compact the snow that has already been trampled and treated with explosives and to consolidate the fresh snow on top.

This means that Highland Bowl is then ready for the opening, which usually takes place in mid-December - a little later than in the rest of the ski area. After each subsequent snowfall, blasting takes place again (at strategic points, with larger blasting sets). Smaller snow packs are removed by the ski patrol.

The bootpacking program in Aspen is probably the largest and most traditional, but the method is also used successfully in other areas. Since the introduction of systematic bootpacking in Aspen - in contrast to previous years - no more avalanches have been recorded in terrain treated in this way.

The Arlberg effect

There is also extremely popular off-piste terrain on the Arlberg, which plays a central role in ski resort marketing. Unlike in Aspen, however, no one here tramples the future old snow of the high winter in the fall. Instead of inbounds powder, there are pistes, ski routes and the unsecured, open ski area. The Schindlerkar may be skied just as much as Highland Bowl, but the general conditions are fundamentally different.

In spite of this, you keep hearing about the proverbial Arlberg effect, which supposedly makes everything a little safer here. What is meant is that there is so much terrain on the Arlberg, so regularly and so quickly after every snowfall, that the risk of avalanches is not as great as elsewhere in comparable terrain.

Avalanche forecasters in the USA are also aware of touring and off-piste terrain that is used so much that avalanche conditions there are significantly different from those in less tracked areas:

"A concern for some backcountry forecasters is the development of a false sense of avalanche knowledge and confidence in recreationalists who learn and progress in such high-usage backcountry areas. As users progress and explore, they eventually leave such high use areas and move into less-compacted terrain, where the regional avalanche forecast is more representative and the snowpack more variable. A major challenge presented to avalanche forecasters is how to communicate avalanche hazard in a region that has both areas with minimal usage and compaction and areas that have extremely high usage and a heavily disturbed snowpack." (Saly et al., 2016)

Munter's reduction method allows an additional reduction factor for "constantly used slopes", whereby these are defined as "numerous tracks after each new snowfall, even in the starting zone". The situation report, on the other hand, repeatedly warns: "Pay particular attention to shaded slopes that have not been tracked much so far"

So if many skiers are enough to make a slope safer, the question arises: why does so much more effort have to be made in Aspen if the Arlberg effect can be achieved without laborious bootpacking? Or rather: when and how reliably does the Arlberg effect work?

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Can this be measured?

In principle, there are the following ways to increase the stability of the snowpack:

• Destroy, reduce or interrupt weak layers

• Increase the strength of the snowpack by compacting it

The first point concerns fracture propagation, which cannot take place or can only take place to a limited extent if the weak layer has been crushed or otherwise destroyed. The second point concerns the fracture itself. The firmer the snowpack, the more force has to be applied to create a break.

The Aspen method with bootpacking and SEA has a compacting effect on the one hand, and on the other, weak layers created in the early season are destroyed before they become a dangerous old snow problem. New snow or drift snow problems that arise during the season are kept in check by constant skiing and targeted blasting.

The Arlberg effect eliminates bootpacking in the early season and the extremely small-scale SEA blasting. What remains are the many skiers and blasting operations, the primary aim of which is not to secure the open ski area, but to protect the slopes. The last few winters in the Alps have shown that groomed slopes - and even those that have been blasted several times - do not necessarily offer safety from old snow avalanches.

The bootpackers in Aspen penetrate the entire snowpack, destroying even deep-seated weak layers. Skiers who only ski on the surface of the snow compact it, but often do not reach the weak layers that lie deeper in the snowpack. If the snowpack is heavily compacted on the surface, more force is required to cause a break in the deeper layers. However, if the necessary force is applied, the fracture can propagate unhindered into the deeper weak layers. In the case of an old snow problem, therefore, at most one of the above-mentioned points is fulfilled by skiers: The snowpack is compacted by skiing and thus becomes firmer, but deep-lying weak layers are not destroyed.

In contrast, drift and fresh snow avalanches usually result from weak layers close to the surface compared to an old snow situation. These are often easy to disrupt even with the relatively shallow penetration depth of a skier. In the case of a drift or fresh snow problem, skiers can also fulfill both of the above points: They destroy weak layers by skiing on them and at the same time increase the strength of the snowpack by compacting it.

It is emphasized that even in the boot pack field the weak layer could not be completely destroyed and it is pointed out that the results change slightly with the fluctuating snow depth between the test fields.

In conclusion, the same conclusion is reached as this study, in which a larger boot pack test field was compared with an equally large, untracked test field a few years ago: Boot packing in its extremely systematic, laborious form has significant effects on the stability of the snowpack. The results are much less clear in the tests with skis (multiple traverses, completely stair-stepped).

Even on the Arlberg, the slopes are not completely stair-stepped from top to bottom and the track density is probably also lower on average than in the "ski" test field (less than 1m between the tracks). You should therefore only rely on the "busy" slope if there is no old snow problem (and also no wet or gliding snow problem!!) and the slope is really busy.

Most of the slopes that freeriders like to ride in search of powder are not like that anyway.

Sources

Carvelli, P., 2008: Bootpacking and systematic application of explosives: shear plane disruption technique in the continental climate. Proceedings of the International Snow Science Workshop, Whistler, BC, 337-944.

Heinecken, K., 2004: Highland Bowl - a ski area expansion. Proceedings of the International Snow Science Workshop, Jackson Hole, WY, 661-665.

Sahn, K., 2010: Avalanche risk reduction in the continental climate: how to implement an effective boot packing program. Proceedings of the International Snow Science Workshop, Squaw Valley, CA, 296-301.

Saly, D., Hendrikx, J., Birkeland, K., Challender, S., Leonard, T., 2016: The Effects of Compaction Methods on Snowpack Stability. Proceedings of the 2016 International Snow Science Workshop, Breckenridge, Colorado.

Wieland, M., Hendrikx, J., and Birkeland, K., 2012: The effectiveness of boot packing for snowpack stabilization. Proceedings of the International Snow Science Workshop, Anchorage, AK, 993-997.