Formation and forecasting of glide-snow avalanches
Glide-snow avalanches occur when the entire snowpack glides over the
ground until an avalanche releases. Snow gliding processes and glide-snow
avalanches are conceptually well understood and it is widely accepted that a reduction
in friction at the base of the snow cover is the main driver. Since there are
various processes involved in reducing friction at the base of the snow cover,
the relationship between meteorological conditions and glide-snow avalanche activity
is complex. Thus, relying on weather
data to forecast glide-snow avalanche activity is still difficult and relatively
inaccurate.
Forecasting glide-snow avalanches: a
serious challenge
Glide-snow avalanches represent a serious challenge to avalanche
programs protecting roads, towns, ski lifts and other operations. These
avalanches are notoriously difficult to forecast and can be very destructive,
as large volumes of snow are mobilized at once. Past research has shown that
glide-snow avalanche release may best correlate with periods of rapid increases
in glide rates. Thus, measuring glide rates could greatly improve glide
avalanche forecasting.
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Fig.
1: Overview of the four field sites where we monitor glide-snow avalanche
activity using time-lapse
photography.
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A picture says more than a thousand
words
Our research focuses on measuring glide rates by tracking the expansion
of glide cracks with time-lapse photography. When a glide crack appears, the
ground below the snow cover is exposed. Since the ground is much darker than
snow, it can clearly be identified on the time-lapse images. We therefore use a
simple method based on dark pixel count to monitor the expansion of glide
cracks over time at four different sites in Switzerland and in Alaska, USA
(Fig. 1).
An example of a glide crack which resulted in an avalanche comes from our
field site in Alaska. At 07:25 on 28 April 2011 a small glide crack appeared in
the snow cover. For over six hours the glide crack expanded until an avalanche
released at 14:00. The number of dark pixels in the crack area increased with
time, following an exponential trend (Figure 2), similar to the results found
by other researchers using more elaborate instrumentation.
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Fig. 2: Time-lapse images
from our field site in Alaska showing the gradual opening of a glide crack and
subsequent release of a glide avalanche (right hand side). The number of dark
pixels with time increased exponentially up to glide-snow avalanche release (left hand side).
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Processes driving the formation of
glide-snow avalanches
To better understand processes involved in glide avalanche release, we
recently instrumented a slope with well documented glide-snow avalanche
activity with a seismic sensor and two heat flux sensors. Since the field site
is located very close to the town of Davos, the seismic data is very noisy.
Nevertheless, the accelerated snow gliding observed in the time-lapse images
coincided with increased seismic activity (Figure 3). Preliminary results from
the heat flux plates suggest that the energy input from the soil is relatively small
and constant throughout the observed period. We therefore believe that the
presence of water at the base of the snowpack is probably due to a strong
hydraulic pressure gradient at the snow-soil interface.
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Fig. 3: Time-lapse images
from the Dorfberg field site showing the opening of a glide crack and the
release of a glide-snow avalanche (top). An increase in seismic activity is visible between the
glide-crack opening and the avalanche release (bottom).
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The future goal is to improve our understanding of snow gliding
processes. This will be done by investigating glide rates derived from
time-lapse photography and seismic monitoring in relation to meteorological and
topographical variables. The main goal is to identify weather conditions
associated with increased gliding, a prerequisite for glide-snow avalanche
release following glide crack opening.
