SnowFlurry 8 2020/21 | When the probability of triggering slab avalanches decreases

Overview - similarities

If you take a brief look at the basic areas and characteristics of the profiles, you will immediately notice the similarities: Similar snow depth, the same sequence from the moist melt forms at the base to angular-rounded crystals in the lower area, followed by two crusts with angular crystals in between as well as a prominent angular weak layer above the second crust. Above this lies the fresh snow from the snowfall in mid-January, which triggered the tense avalanche situation.

Overview - differences

As the profiles were recorded very close to each other at a similar location, you can also see how the snow cover changed from 17/01 to 23/01. The fresh snow that fell in mid-January had noticeably degraded in the meantime, turning into more and smaller, round-grained and felt-like crystals. In addition, the approx. 40 cm of fresh snow from the first profile settled to less than 30 cm in the second profile. In addition, the fresh snow from mid-January melted on the surface during a brief warm spell around January 21 and had already frozen back to a thin, superficial crust by the day the profile was taken. There is now a little "new" fresh snow on this crust from the day profile 2 was recorded.

In addition, the temperature reserve of the snow cover was reduced by the warm spell. The red line in profile 2 is consistently a little further to the right than in profile 1

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Profile in the snow rummager's garden

Lukas Ruetz

Profile in the snow rummager's garden

Lukas Ruetz

Interpretation of the snowpack development from 17.01. to 23.01.2021

The two angular layers above and below the melt crust at approx. 50 cm are most likely to be a weak layer. This crust was formed during a rain event shortly before Christmas, on December 21/22. The upper, angular layer with its upper end at a snow depth of 60 cm represents the old snow surface from the long cold phase from the end of December to mid-January.

The snowpack stability

On 17.01., there was still a juicy settling noise before the profile recording when entering this flat area in the snow rummager's garden. The two ECTs with results ECTP0 and ECTP5 confirm that the snow cover was very susceptible to disturbance at the time. The combination of the cold, fresh slab of new snow and the built-up, transformed old snow surface of angular crystals was perfectly suited to the propagation of fractures.

The high, spontaneous avalanche activity and the numerous accidents in mid-January also confirmed the situation.

Six days later, on January 23rd in profile 2, we can still see the almost identical structure of the snowpack in the profile graphic. However, the stability tests speak a completely different language. Only two partial fractures were produced in the weak layer of the ECT under significantly higher loads: Results ECTN 12 and ECTN 16 vs. ECTP0 and ECTP5 for profile 1. In addition, a PST with a total length of 370 cm was carried out in the weak layer. No fracture propagation could be generated in this case either. The snow slab broke apart vertically several times above the weak layer (slab fracture). The combination of weak layer and snow slab simply no longer works at this altitude for fracture propagation.

The two profiles from January 17 and 23 in direct comparison

Why is fracture propagation no longer possible?

The snow slab and weak layer have changed their properties massively without being able to recognize major differences in the layer profile. The snow slab has settled, solidified and is now slightly moist. In this case, a very slight soaking has a positive effect on the snow cover by reducing the brittleness of the board.

The weak layer has also solidified significantly due to the almost one week of loading of the new snow board (mechanical transformation) and the now low temperature gradient (degrading transformation) - the crystals have built up noticeably more bonds with each other. However, the change in the weak layer cannot be seen in the profile itself, as these subtle differences can hardly be described with the existing possibilities in a profile graphic.

But when digging and with the feel of your fingers you could clearly distinguish it: In profile 1 on January 17, the angular layer already trickled out by itself - even in the flat - when the profile wall or ECT was cut out. In profile 2 from January 23, it did not trickle out at all. In addition, you could no longer feel the extremely loose "sugar state" with your hands without any binding of the crystals to each other. The layer has simply solidified slightly and the crystals have formed bonds. However, it is still so soft that hardness 1 (fist) had to be awarded. The profile cannot reflect this subtle but decisive difference.

Conclusion

Layer profiles are nice and interesting - but without stability tests, they are of little to no practical use. The right combination of snow slab and weak layer for fracture propagation can only be determined with an extended column test, a propagation saw test or a sliding block test. At higher altitudes, settlement does not occur as quickly and caution is still required.