Snowpack analysis in high alpine terrain
Since our last blog we have swooped far and wide in the state helicopter, dug deep in the high altitude snows in order to construct snow profiles of Tirol’s high alpine terrain. Part of this involves snowpack stability tests in order to find out how well the disparate layers of snow are bonded to each other.
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| Stability test on the Grossvenediger (photo: 19.10.2020) |
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| With avid support from the state helicopter we were able to analyze both the Grossvenediger avalanche of 10.10.2020 and the Zuckerhütl avalanche on 18.10.2020. (photo: 19.10.2020) |
In a nutshell: persistent old snow problem in high alpine regions necessitates caution on north-facing slopes in particular
Our tests demonstrated that in high alpine regions, i.e. above 3000 m, there are thin, weak layers inside the snowpack which have persisted for a long period. The tests showed good fracture propagation upon heavy loading. Caution is recommended especially on NW-N-NE facing slopes. Above 3500m the endangered aspects extend to SW and SE-facing slopes.
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| Snow profile Grossvenediger. The brunt of the 3 October avalanche, the weak layer, is indicated by the arrow. |
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| Similar situation on the Zuckerhütl: thin layer of faceted crystals wedged between two melt-freeze crusts. On top of that, a solid ‘slab’ sculpted by wind. |
Short analysis of reported avalanches
Avalanche accident Grossvenediger
As reported in the last blog, there was a fatal avalanche accident on the Grossvenediger on 10.10.2020. A winter sports enthusiast was caught in the deadly swathe of a slab avalanche just below the summit and plummeted over rocky terrain. The avalanche was triggered as he traversed a short gradient of about 10 to 20 degrees near the foot of the main slope where the snowpack was shallow.
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| Avalanche accident Grossvenediger: the primary slab avalanche was triggered near the zone marked. A secondary fracture can be observed below the rocky area resulting from the additional loading of the plummeting snow masses. The person was buried just before the end limits of the avalanche deposit. (photo: 13.10.2020) |
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| View from the summit of the Grossvenediger to the avalanche zone (now drifted again). The flat bottom of the slope and the plummet zone which follows it are visible. (photo: 19.10.2020) |
Avalanche Zuckerhütl
In early afternoon on 18.10.2020, a report came in of an avalanche on the Zuckerhütl in the Stubai Alps. A large-sized slab was triggered on the northern flank when a person was descending a slope (gradient about 50°). It was at first unclear if persons were buried, since the avalanche swept over the ascending tracks to the Zuckerhütl, so an extensive search was launched. The alarm was halted about two hours later after the Alpine Police exploration and Rescue Squad searches. One person was swept along but not buried, and escaped without injury.
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| Slab avalanche on the Zuckerhütl. The photo was taken just after the avalanche. The arrow shows the spot of the descent where the skier was when the avalanche was triggered. The circle shows a person who then entered the slope. The avalanche fractured right below that point. (photo: 18.10.2020) |
It is evident that the weak layer was widespread and cohesive. The lateral, lower borderline of the avalanche fracture (and thus, the lower limit of the weak layer that unleashed the avalanche) lies at about 3350 m. Glacier ice became visible when the snow masses were swept away. The weak layer was thus also in the zone of glacier ice (visible in the photo above a hard snow fundament, see Zuckerhütl snow profile above).
Avalanche Tiefenbachferner
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| Schneebrettabgang Tiefenbachferner, 3100m, NO-seitig |
No one was buried in this avalanche. Just like in the other avalanches, the weak layer consisted of faceted crystals and lay atop the rain crust which formed on 03.10.2020, as observed by Peter Raich.
Naturally triggered slab avalanches? Almost none.
Hovering above the Main Alpine Ridge, we espied but one single naturally triggered slab avalanche in high alpine terrain, in the Zillertal Alps. These observations coincide with our snowpack analysis: for a weak layer to trigger usually requires heavy additional loading in extremely steep terrain.
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| Naturally triggered slab near a ridgeline in the Zillertal Alps (photo: 19.10.2020) |
On north-facing slopes at high altitude, isolated snowdrift problem in shady terrain
Apart from the old snow problem mentioned above we also discovered a superficial snowdrift problem. This occurs on extremely steep, shady slopes relatively near ridgelines above 2800 m, where older, hardened wind crusts (atop decomposed snow) can be triggered by winter sports enthusiasts. The higher recent temperatures will gradually shift this problem to higher altitudes.
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| A hard slab fracturing at 2870m near the Kaunertal Glacier (photo: 22.10.2020) |
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| Snow profile from above picture. Very hard surface, then a thin layer of decomposed snow crystals. |
On the other hand, freshly generated snowdrift accumulations currently do not trigger easily, since the surface is moist up to high altitudes or else sculpted by wind, making the fresh drifts well bonded.
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| Fresh snowdrifts in high alpine regions (photo: 18.10.2020) |
Otherwise noteworthy:
The snow is melting.
The last few days have been marked by foehn influence and rising temperatures, with corresponding effect on the snowpack: it has started to melt, become moist up to high altitudes. On south-facing slopes the snowpack is currently isotherm up to over 3000 m, in other words 0 degrees Centigrade.
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| Weather station data on Pitztal Glacier. Since the last blog, weather has been highly variable with increasing foehn impact. |
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| The red line shows the snow temperature: everywhere zero degrees at 3000 m on south-facing slopes |
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| Solar radiation and mild temperatures: view from Wurmkogel in the Ötztal Alps toward Timmelsjochstrasse (photo: 20.10.2020) |
Danger: rocks
Whoever is currently in outlying terrain and not on glaciers needs to master ‘rock skiing’ and beware injuries from the stones jutting out of the snow or lurking just beneath the surface.
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| Danger of injuries from rocks. Kaunertal (photo: 22.10.2020) |
Sahara sand is in the air.
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| View towards the south from Kaunertal Glacier. The yellowish hue comes from sand blown north from the Sahara. (photo: 22.10.2020) |
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| Overview of Sahara sand plumes (c) ZAMG |
Our next blog will be published whenever there is a significant change in snow and avalanche conditions.
