International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
Figure 1 312m long stone-arched snow
shed built at Splügenpass in
1843. The snow shed was in
operation until 1950.
Figure 2 130 m long snow shed built at
Val Raschitsch in 2002 as a
monolithic jointless concrete
structure.
2. EXPERIMENTAL INVESTIGATION OF AVALANCHE FORCES
Between 2002 and 2006, scale experiments using granular material on a laboratory chute as
well as large chute experiments with snow on the Weissfluhjoch (Davos) were used to
determine the relevant dynamic parameters of flowing avalanches (Platzer and Margreth
2007). In the laboratory approximately 100 experiments were performed on a wooden chute
of 7 m length and 0.5 m width (Platzer and others 2004). On the Weissfluhjoch snow chute,
which is 30 m long and 2.5 m wide, more than 50 experiments with 8 to 15 m 3 snow for each
experiment were performed. Within both experimental setups a series of optical velocity
sensors, ultrasonic flow height sensors and force plates − measuring the normal- and shear
component of an avalanche over time − are used in combination to determine the
characteristics of flowing avalanches when moving over a deflecting structure. The main
findings of the experiments result in an improved approach to calculate the dynamic forces
due to a deviation and in more detailed data
on the coefficient of friction depending on
the snow type (Fig. 3). Near the deviation
point, meaning up to a distance of 1.5 times
the flow height of an avalanche, the so far
applied formula underestimates the applied
dynamic forces, whereas after a distance of
more than 6 times the flow height of an
avalanche, the dynamic forces due to a
deviation have vanished. The mean coefficient
of friction μ is for dry snow avalanches
0.3 and for wet snow avalanches 0.5 (Platzer
and others 2007). The measured coefficient
Figure 3 Coefficient of friction μ as a
function of the measured flow
velocity on the snow chute.
of friction is reduced by about 30% when the
avalanche flows over a deposited snow
cover.
Margreth and Platzer 33