Salezertobel-Lawine (Davos Dorf), ca. 10.02.2005
Salezertobel avalanche (Davos Dorf), around February 10, 2005



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Around the same time as the Breitzug avalanche, a quite large (but by no means extreme) avalanche was released from the Salezerhorn, descended through the Salezertobel gully and came to rest near the edge of the long avalanche shed along the lake that protects the main road into Davos. Due to ongoing investigations at the Breitzug site, field work at the Salezertobel site could be carried out only a few days later on 2005-02-13. A moderate snow fall had covered the avalanche debris meanwhile, but this affected the field work only to a small degree.

The release area is much less exposed to direct sunlight than the Breitzug, so the avalanche was initially dry, but picked up humid snow along its path. This explains why, at the apex of the alluvial fan, the flow split in two main branches that followed the side lines of the fan, and also why the main deposits show the jagged heaps and fingers as well as shear failure planes typical of wet-snow flows. At the same time, there was a fairly large distal area with much shallower deposits scattered with large blocks. Our work concentrated on this fluidized phase. Some of the main lessons learnt thereby are the following:

This avalanche has been observed and documented (or at least mentioned in historical accounts) many times in the course of four centuries. In at least one case, buildings of the farm on the eastern shore of the Lake of Davos were damaged by the powder-snow cloud that traversed the lake and still exerted pressures of more than 1 kPa. The significant number of documented events made this avalanche path the object of an early stastistical study of the relationship between frequency of occurrence and run-out distance (Föhn, 1975; Föhn and Meister, 1981).


References

Föhn, P.M.B. (1975). Statistische Aspekte bei Lawinenereignissen. In: Proceedings, International Symposium Interpraevent 1975, Innsbruck. Vol. I, pp. 293–304.
Föhn, P. M. B. and R. Meister (1981). Determination of avalanche magnitude and frequency by direct observations and/or with the aid of indirect snowcover data. Mtlg. Forstl. Bundesversuchsanstalt Wien, 144, 207–228.
Johnson, C. G., B. P. Kokelaar, R. M. Iverson, M. Logan, R. G. LaHusen and J. M. N. T. Gray (2012). Grain-size segregation and levee formation in geophysical mass flows. J. Geophys. Res. F01032, doi: 10.1029/2011JF002185.





Fig. 1. The avalanche was released from the NE flank of the Salezerhorn (not visible in the photo), passed through the gully in the middle of the image and stopped on the alluvial fan. Just beyond the larch trees in the foreground, the deposit of the southern arm of the dense part is visible.



Fig. 2. View from the lower part of the gully over the alluvial fan onto the drained Lake of Davos towards the entrance into the Flüela valley. The powder-snow cloud of historic avalanches in this path has damaged the farm buildings at the edge of the forest beyond the lake.



Fig. 3. Downstream view across the alluvial fan and the empty basin of the Lake of Davos. Note the marked change between the proximal deposit, densely covered by many big blocks (up to over 0.5 m), and the thinner distal deposit with significantly smaller blocks.



Fig. 4. Scour marks along the sidewall of the gully near the apex of the alluvial fan. Up to about 0.5 m above the deposit surface, they appear to be shear planes. Higher up, some of the marks are due to skiers or animals crossing the gully while others can be interpreted as traces of snow blocks.



Fig. 5. Southern arm of the dense-flow deposit. Note the fairly constant width of the arm and the low levee at the side, while larger blocks are piled up along the middle axis. There is another levee in the middle ground of the picture, cutting through the southern arm. It probably was formed when a second surge (presumably with higher velocity) either was deflected towards east by the deposits already in place, or pursued a straighter course instead of the direction of steepest descent. The surface textures of the two deposits are different, the smoother one indicating a flow of higher speed.



Fig. 6. Detail of the right levee of the southern arm. The inside slope of the levee is much steeper than the outside one. Despite deposition of big blocks between the levees, the surface of the deposit is lower than the undisturbed snow cover at several places. This can be considered a sign of strong erosion. It also appears that the levee, once it was in emplaced, channelized the flow efficiently.



Fig. 7. Snow pit somewhat below the transition from thick deposit with big blocks th thinner deposit with smaller blocks. By dyeing the cut surface with ink, the texture of the snow cover becomes visible. the homogeneous bottom layer has a sharp interface to the overlying deposit, where the matrix embeds snow balls of different sizes between 1 and 10 cm. See Fig. 8 for a close-up view.



Fig. 8. Close-up view of the granular texture of the avalanche deposit shown in Fig. 7. The viewframe is just to the right of the middle line of Fig. 8, in the top third of the picture. Snow balls of widely different sizes are clearly visible.



Fig. 9. Typical snow pit in the distal area of thin deposit. On the top, one sees approximately 5 cm of new snow, at the bottom the undisturbed snow cover is at least 0.5 m deep. In between there is a layer of snow balls of various sizes embedded in the fine-grained matrix. Its height tapers off towards the edge of the deposit. The snowballs were made visible by carefully removing the softer fine-grained matrix snow between them.



Fig. 10. One of the largest snow blocks found on the avalanche deposit. Its form is close to spherical. One might therefore presume that it rolled to its final position. However, it is surrounded by what resembles a crater from a grazing impact, with an inclined depression upstream and a compression ridge downstream of the block. See Fig. 11 for corroboration of this observation.



Fig. 11. Longitudinal section of the snow block in Fig. 10 and the snow cover in its vicinity. It is confirmed that the block sits on a surface that is inclined more steeply than the local terrain. Interestingly, hardly any traces of lesser snow balls were found, but the new-snow layer in front of the block underwent significant shear and compaction, as did the snow underneath the block.


Fig. 12. The cross-section through the block revealed that its core was an intact piece of a slab, either from the release area or from the snow cover entrained along the path. snow particles of different sizes aggregated onto its longer sides so as to make it rounder. It appears that the impacts during the flow exerted a pressure in the right range to fit smaller snow balls tightly into holes on the surface of the large one, and that they were of sufficient duration for these contacts to harden. Furthermore, the impacts were not so strong as to break the original slabs further.



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