||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.|
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.
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.
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.
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.
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.
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
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.
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.