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SnowFlurry 3 2021/22 | Learning from avalanches

Linking theory and practice

by Stefanie H√∂pperger ‚ÄĘ 12/11/2021
What avalanche problem is there, at what altitude and exposure is it present, how can I recognize it, can I recognize it at all, what do I have to watch out for, why are the others going further - is it safe, have I made the right decision? The snow is great, should I take the risk.....?

Questions upon questions, but that's exactly what's important: to think about it and not go off without thinking. I also like to call such questions my "life insurance".

At the outset, you should of course avoid triggering an avalanche at all costs. However, in the event that one does happen, regardless of whether you or someone else triggered it or it happened spontaneously, you can learn a few things from it. The best way to do this is to review the whole process, from tour planning to decision-making.

To illustrate this, let's take a closer look at an avalanche that occurred last week, on December 1, 2021, in the western Tux Alps.

The key data on the avalanche

Size 2 slab avalanche: classic ski touring avalanche

Exposure north at approx. 2450m

The slab avalanche was triggered around midday by a ski tourer who was on a descent.

Avalanche problem: Old snow problem coupled with a drift snow problem, easily recognizable in the snow profile (ECTP2).

The avalanche situation:

According to the avalanche report, danger level 3, considerable, has been issued for 1.12.2021. The reason for this is the widespread old snow problem (for a more detailed description, see Gestöber 2), which is predominantly prevalent on shaded slopes (NW over N to E) above the tree line. In addition, with the increase in wind, accumulations of drift snow prone to disruption could form in all exposures above the tree line.

In the central part of the snowpack, there are angular crystals (due to the build-up transformation) that act as a weak layer. These layers are particularly present and relevant where a closed snow cover already prevailed before the snowfall at the end of November. The snow surface at that time cooled down considerably due to radiation during the two periods of fine weather in November with cold temperatures, dry air and clear nights. The resulting large difference in temperature allowed the accumulating transformation to take full effect, sometimes throughout the entire snowpack!

Due to the lack of snow, only a few off-piste tours were feasible, so it can be assumed that there are no or few slopes that were used so much that the weak layer would have been destroyed. Where the weak layer is present, it is therefore also extensive and relatively uniform, which in turn promotes the spread of fractures. The thin snow slab above the weak layer also favors good fracture initiation: the weight of a single person is enough to produce a fracture. The snow surface of angular crystals was overlaid by the snowfall at the end of November up to and including December 1 (snowfall still in the morning).

The wind, which picked up and was stormy in some areas, also intensively displaced old and new snow. The resulting drift snow packs are ideally suited as the "board" of an avalanche. With this mixture, it is highly likely that avalanches can be triggered easily. So the motto is to be defensive when out and about.

For December 1, the forecast is for fine weather with rising temperatures and strong to stormy winds. We are therefore assuming that further fresh and disruptive snow packs will form over the course of the day. However, with good visibility and a little experience, it is usually easy to recognize drift snow in the terrain.

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The choice of tour:

Not much can be done in the terrain without ruining your skis, which is why we prefer the areas with more snow and a good surface. The Brenner area has received a lot more of the white gold than the Sellrain Valley due to the SW current and the terrain there is also more favorable for an early winter tour. The decision was made in favor of the Naviser Kreuzjöchl, because in the lower part you can switch to the already prepared toboggan run if the descent through the forest is not yet possible without stone contact. From the hut onwards, the terrain is hilly and fairly flat. The following steeper slopes up to the first cross, which are probably blown in, can be defused a little with a good choice of tracks orographically to the left over the ridge.

Despite the exposure (W, NW, N), the tour is justifiable for me with little risk in terms of avalanches, at least up to the flat section shortly before the summit ascent. However, I'm not sure whether the last 200 m to the summit of the Naviser Kreuzjöchel are reasonably feasible. An assessment is required on site.

Tour and conditions in the terrain:

There is good but firm powder and virtually no wind until a little above the Stöcklalm. However, the views of the summit and ridges show something else: as far as the eye can see, there are snow plumes everywhere. You can also catch a glimpse of the odd slab of snow that has already left the neighboring mountains. Both are clear warning signs that you should definitely pay attention to. We do not consider the actual normal route, which leads over the somewhat steeper W, NW, N slopes, as they are blown in as expected and the old snow problem is also very likely to be present. So we stick to our plan and choose the track over the ridge.

Just above the tree line, we are greeted by a blustery, cold wind. The snow surface looks accordingly. You can't miss the signs of the wind: wind dips and dunes, areas with little snow swept clear next to snow-rich areas filled with drift snow, wind vanes at every elevation. And we can feel the cold, blustery wind too! Brrr! In the area between the tree line and the flat section (near the first cross), you can consciously or unconsciously trigger small patches of disturbing drift snow. Great demonstration effect, but also a clear warning sign!

On reaching the top, we can see almost textbook-like how the wind deposits the snow over the ridge onto the leeward side, in this case the steeper N and NW slopes. The cornices that have formed are another danger sign for drift snow. The cornice wedge points towards the leeward side, where the transported snow is deposited as a drift snow pack. To help you remember the difference between LUV (where the wind comes from) and leeward (where the snow is deposited), here is a mnemonic: LEE is lee(e)dangerous.

The first slope you have to walk up to the ridge is estimated to be up to 30¬į steep, it is a shaded slope (N, NW), we are already above the tree line (approx. 2350-2400m) and it is a lee slope. Furthermore, we already know from the avalanche report that there is a problem with old snow on shaded slopes above the tree line, but unfortunately we can neither see this on the surface nor perceive it in any other way. Settling noises, cracks in the snow or fresh avalanches can provide indications of an old snow problem, or we can dig a snow profile. In any case, the old snow problem must be stored in our heads and also be retrievable!

The first slope towards the summit is therefore questionable. However, people have already skied up without an avalanche occurring.

"Hm, is it safe after all?"

Ascent and descent tracks do not tell you whether a slope is safe or unsafe! Even the twentieth ski tourer can hit a so-called hotspot (trouble spot) and only then will an avalanche start.

With safety distances and a little more risk, you could possibly dare to do it. Nevertheless, I quickly come to the decision to skip the summit. Why? The first slope already poses an increased risk, the wind on the ridge blows horribly around your ears, the summit snack would also be uncomfortable, plus the descent is definitely only an option for me via the ascent route (ridge, back) and is therefore anything but worthwhile.

In the other descent variants, the old snow problem prevails everywhere, the steepness for an avalanche is sufficiently great and there is also drift snow. Of course, the snow would be good there and the descent would be fun - the existing tracks look good. But the winter is still long, there are still lots of great powder turns waiting for us!

So I prepare for the descent and dig another snow profile. As I stick my nose into the snow and take a closer look at the layers, there's a good rustle and a slab avalanche breaks loose on the steeper summit slopes. Avoiding the summit slope was clearly the right decision today!

Luckily, the person involved escaped with a scare. The avalanche was triggered on the descent in an area with little snow. The avalanche was a perfect match for the avalanche problems described in the avalanche report and already mentioned here several times.

Comparison of theory and practice

For me, the avalanche I witnessed confirms my thoughts, but I still take it as an opportunity to review exactly how my tour went today. We don't usually get such direct feedback on our decisions. This makes it all the more important to be self-critical even if nothing happens and to question whether you have correctly assessed the conditions, whether you might have skied a certain slope, whether you have recognized and noticed the problem and the danger signs at all, whether you have exceeded your own risk tolerance or not.

The debriefing after (or even during) the tour with the group, or even on your own, is important. But even before the tour, it is worth consciously thinking about what conditions we expect (snow depth, snow conditions, is there even enough snow for the chosen tour, ....). We can then compare these expectations with reality in the terrain. Even if you are completely wrong, it will stay in your memory!

Because you learn by combining and comparing theory and practice in the terrain. Perceive, see, feel, compare and memorize snow and conditions!

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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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