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SnowFlurry 16 2016/17 | The possible increased risk of avalanches in existing ascent tracks

An easy to clarify, rarely occurring paradox.

by Lukas Ruetz • 02/16/2017
In an existing ascent track or on tracked slopes, the avalanche danger can be greater than on the same slope in untracked condition. What sounds absurd at first is easy to understand on closer inspection.

Already old hat: breakage and the slab avalanche

The main cause of slab avalanches is breakage within the snowpack or within a weak layer. Just as porcelain or glass breaks apart, in our case the relatively solid foam of ice breaks: in other words, a framework of ice surrounded by air = snow. We create fractures at every step in the snowpack, between individual crystals. If not only the connections between individual, directly affected crystals underneath our skis or our footsteps break, but also the surrounding ones that are not directly affected by our weight, i.e. the additional load, we speak of fracture propagation. For a fracture to propagate, the "consistency" of the snowboard - i.e. the relatively harder layer - must match the "consistency" of the relatively softer weak layer underneath. A "board" that is too loose leads to fractures, but as the loose board cannot transfer the stresses well enough, it is more difficult for fractures to propagate and for an avalanche to occur. The ECT (Extended Column Test) gives the result ECTN (= No propagation), i.e. a fracture over a part of the block without fracture propagation.

Then there is the possibility that the board would be well suited for fracture propagation, but the weak layer has hardened again somewhat or is simply still too weak and therefore no fracture propagation takes place either - in this case, the low fracture propagation tendency is due to the weak layer rather than the overlying board. The composition of the weak layer and the overlying snow slab must therefore fit together to enable an avalanche.

Massive, pronounced floating snow will never lead to problems without a corresponding, overlying snow slab. On the other hand, the cocktail of extremely weakly bound snow - which is hardly or not at all distinguishable on the descent from really unbound, i.e. unbound powder snow - will already lead to massive problems when stored on a surface layer: Due to its crystal size, hardness, air content and low layer thickness, snow-covered surface snow is one of the most delicate weak layers of all. Here, even an overlying snow slab of "atypical" (softer) "consistency" can be enough to trigger avalanches.

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The additional load

The additional load also plays a role: on the one hand, at what depth the load still acts and to what extent - depending on the pressure (= force per area) and the transfer of the load from one layer to the next. To put it simply: our additional weight on the snow cover. BUT: The additional load does not just act once on one point, especially in ascent tracks. If one person after another uses an ascent track, each point is loaded several times. Even if the strength of the additional load on the critical layer actually decreases due to, for example, less and less heavy, following winter sports enthusiasts with perhaps wider and wider skis on an increasingly consolidated track. However, each additional skier places a load on the snowpack.

This means that if the track layer in our example was the heaviest of all and exerted a stronger additional load on deeper (weak) layers due to the as yet non-existent, possibly later consolidated ascent track, each additional skier still places an additional load on the snowpack - a further impulse. Each additional load beyond a certain, possibly variable, minimum load limit can lead to new fractures between individual crystals in deeper layers - even though the next person to follow weighs less and the track is actually becoming firmer and firmer. As soon as so many connections between individual crystals in the weak layer are broken that the remaining connections can no longer hold the load, fracture propagation occurs and the snow slab may fall off on the third, seventh or fiftieth person to follow the ascent track.

There is also the other possibility that only a load above a certain threshold leads to fractures or that the first load above this threshold leads to fracture propagation and a snow slab avalanche. Even if dozens of "smaller" additional loads are applied, none of these impulses lead to fractures in the weak layer or between individual crystals and so these smaller loads do not "add up". "Adding up" not in the sense of 75 kilograms + 67 kilograms + 84 kilograms but as one impulse that causes fractures in the weak layer + another impulse that causes fractures in the weak layer, etc.

The basic assumption for these considerations is always that the natural conditions do not change in the meantime: i.e. the weak layer and snow slab remain the same.

The CT and ECT snow cover tests are easier to understand: First, you hit the shovel blade on the snow block ten times from the wrist, followed by ten hits from the forearm and ten hits from the entire arm. During the last few strokes with the whole arm, you usually don't just passively drop your arm onto the shovel, but actively help by hitting it harder. On the one hand, this simulates an ever-increasing additional load due to a higher applied weight (four stages: hand, forearm, whole arm, whole arm with muscle strength) - on the other hand, it always applies the same additional load but ten times in succession. So both cases:additional load absolutely increasing due to the hardness of the blows in four stages and additional load acting several times, i.e. impulse by impulse. A snow slab can break off due to the first additional load above a certain threshold value or due to several successive loads above another threshold value that is at a lower level than the threshold value for the one-off additional load required for fracture propagation.

Better illustrated by the following examples

Avalanche accident Seebleskar, AuĂźerfern on 12.2.2017

The seventh and therefore last climber in the ascent track that had just been created triggered a large avalanche on an extremely steep slope - in a weak layer close to the ground, i.e. due to an old snow problem. The other ski tourers were standing just above the avalanche. There are two possibilities: Either he did not move exactly in the ascent track and his additional load had an effect on a spatially minimally shifted spot. It was precisely at this point that the general conditions were slightly different and he was therefore able to generate the initial fracture for fracture propagation. Since, according to the descriptions, it was located exactly in the ascent track, it can be assumed that its predecessors had already generated fractures at this point and that it was the decisive further impulse to set the fracture propagation in motion. If, for example, a snow groomer had applied the first additional load to this point, the snow slab would most likely have come off immediately because this type of load would probably have been above the threshold for fracture propagation and thus a single impulse would have been sufficient. In the case of ski tourers, several impulses were required before the avalanche occurred.

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Due to the significantly lower weight of the winter sports athletes in this case, the whole thing can rather be described as a "summation" of several smaller additional loads, each of which was above the threshold value for individual fractures in the weak layer. However, the additional loads were initially still below the threshold value for fracture propagation. As the weak layer weakened more and more, the threshold value for the magnitude of the additional load for fracture propagation was lowered further and further until the weight of the seventh climber was sufficient not only to generate individual fractures but also to set fracture propagation in motion.

More information on the accident in the LWD Tirol Blog.

Avalanche accident Flaurlinger Narrenböden, Stubai Alps from 31.12.2014

A drift snow problem. In an ascent track created a few hours ago and used by a handful of ski tourers, a single winter sports enthusiast triggered a small snow slab and died in it. It was a very steep south-facing slope close to the ridge. There had been some fresh snow in the days before, on the day of the accident there was still a strong northerly wind, but it was hardly snowing at all. In this case, it was probably the ever-increasing additional weight of the ever-increasing amount of snow on this slope that was the reason why the additional load of the other ski tourer was enough to trigger the snow slab. In this case, the conditions changed in just a few hours, the load due to more and more drifting snow became greater and greater and, in combination with the additional load of the later ascending tourer, the necessary threshold was exceeded to lead to fracture propagation and thus to the avalanche. However, this also shows how close the group was to the avalanche. The "summation" of the previous impulses probably played no or a very subordinate role here.

More information on the accident in the LWD Tirol blog.

Conclusion

The general conditions in the snow must always change if someone was first able to walk on the slope without an avalanche and the next person in the same track triggers an avalanche. The general conditions can change due to natural circumstances: e.g. even more fresh snow load, even more drift snow, more moisture penetration. But they can also change artificially: Every person who uses the track can weaken a weak layer at this point further and further, thus lowering the threshold value for the release of the snow slab. This is why existing tracks are only of limited value.

Note: The avalanche danger in existing tracks may be higher than before the tracks were created because some bindings have already been destroyed but there are still just enough to prevent break propagation and thus an avalanche. At the very least, you should maintain the same skepticism towards lightly tracked terrain and little-used ascent tracks as you would towards untracked terrain.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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