Experiments to fracture propagation and avalanche release
The release of slab avalanches on steep slopes:
Slab avalanches (Figure 1)
are the most dangerous, deadly and largest avalanches. Before a snow slab
avalanche releases, a buried weak snowpack layer has to fracture (break) over a
large area. The fracture starts at one point, for instance below a skier, and
spreads outwards until the avalanche releases. For a long time now it has been
known that slab avalanches only release on steep slopes, that is on slopes
steeper than 30 degrees. However, most people that travel on snow and in
avalanche terrain are well aware that fractures can propagate on slopes well
below 30 degrees or even on horizontal terrain. People travelling on horizontal
terrain sometime hear a characteristic sound, so-called whumpfs, which is due
to the sudden collapse of a buried weak snowpack layer. Slab avalanches are
sometimes also released on steep slopes after having been triggered by a person
travelling on horizontal terrain,
so-called remotely triggered avalanches. This shows that there is a minimum
slope angle for slab avalanche release, but none for fracture propagation. The
reasons behind these observations, which has led to one of the most well known
rules of thumb for avalanche release, were finally clarified by researchers of
the SLF.
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Figure 1: Slab avalanches
release after a fracture has propagated through a buried weak snowpack layer.
On the left you can see an example of a slab avalanche. On the right a weak
layer which was at the base of a small slab avalanche is shown.
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High-speed camera
measurements provide new insight:
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Figure 2:
Field experiment used to observe fracture propagation through weak snowpack
layers. Shown here are the first and the last image in a video sequence of an
experiment in which the weak layers collapsed and the slab started sliding.
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Studying slab avalanche
release is not straightforward. Rather than endangering the lives of the field
team by exposing them to an avalanche, researchers of the SLF used field tests
to simulate the avalanche release process The field tests consist of isolating
a long rectangular beam of snow containing a weak layer (Figure 2). We then cut
the weak layer with a snow saw at one end of the beam until the fracture
propagates to the other end of the beam and the snow slides down the
slope. Using a digital high speed camera
recording at 300 images per second we recorded these field tests. By inserting
black markers in the snow above the weak layer, the movement of the snow can be
analysed using a technique called Particle Tracking Velocimetry (PTV). This
technique allows following the position of the markers in every image. By
connecting the dots we can determine the trajectory of the markers and
therefore the deformation and movement of the snow during the experiments.
When a weak snowpack layer fractures it collapses due
to its high porosity. Our measurements, as well as recently research, have
shown that this occurs for many different types of weak layers, no matter how
steep the slope is. However, depending on the steepness of the slope the slab
above the weak layer will either fall straight down and come to a halt, or the
slab will fall down followed by some sliding. Using the experiments described
above we were able to observe both these processes in high detail and determine
the amount of friction during the experiment.
In Figure 3 results are shown from a typical
experiment in which a fracture propagated through a weak layer on a 34 degree
slope and the slab slid down-slope. As can clearly be seen, the weak layer
collapsed and the slab above the weak layer moved down, on the order of 0.5 cm (black
line in Figure 3). During the collapse of the weak layer the slab did not slide
down much. However, once the weak layer had broken over the entire length of
the beam, at about 0.3 seconds, the slab started sliding down-slope with
increasing speed (dotted line in Figure 3). The amount of friction during the
entire experiment is shown in Figure 4. During the collapse of the weak layer
the amount of friction decreased sharply. After collapse of the weak layer, the
friction increased rapidly again. During the sliding of the slab, the amount of
friction was relatively constant.
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Figure 3: Measured displacement in an experiment where a fracture
propagated through a weak snowpack layer on a 34 degree slope. The black line
indicates the amount of collapse and the dotted line indicated the amount of
sliding.
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Figure 4: Amount of friction during the experiment shown in Figure 3.
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The 30 degree rule experimentally confirmed
We typically observe the behaviour described above in
all our experiments. During fracture the weak layer collapses. If the slope is
steep enough to overcome friction, the collapse is followed by sliding and
disintegration of the slab. This means that fractures can propagate on any kind
of terrain, from horizontal fields to steep slopes. However, an avalanche will
only slide down if the slope is steep enough to overcome friction. This
explains why fractures in weak snowpack layers can be initiated on low angle
terrain and can lead to the release of an avalanche on a nearby steeper slope.
We were also able to determine the critical sliding angle for avalanches. We
found it to be around 30 degrees, which is supported by countless field
observations and the well known rule of thumb that avalanche rarely release on
slope inclined less than 30 degrees.
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