Environmental Engineering Reference
In-Depth Information
Figure 2.3.
Airborne avalanche at le Roux-d'Abriès, France in 2004 (courtesy of H. Wadier)
The avalanche classification proposed here only considers the form of motion and
not the quality of snow. In the literature, other terms and classifications have been used.
For instance, we often see terms such as dry-snow avalanches, wet-snow avalanches,
and powder avalanches. In many cases and probably in most cases in ordinary
conditions, the motion form is directly influenced by the quality of snow in the starting
zone. For instance, on a sufficiently steep slope, dry powder snow often gives rise to
an airborne avalanche (in this case no confusion is possible between airborne and
powder snow avalanches). However, in some cases, especially for extreme avalanches
(generally involving large volumes of snow), motion is independent of the snow type.
For instance, wet snow may be associated with airborne (e.g. Favrand avalanche in
the Chamonix valley, France, on 16 May 1983). Between the two limiting cases
above, there is a fairly wide variety of avalanches, which show characteristics common
to both airborne and flowing avalanches. Sometimes, such flows are referred to as
“mixed-motion avalanches”. The use of this term is often inappropriate because it
should be restricted to describing complex flows for which both the dense core and
the airborne play a role (from a dynamic point of view). In some cases, the dense core
is covered with a snow dust cloud, made up of snow particles suspended by turbulent
eddies of air resulting from the friction exerted by the air on the core. This cloud can
entirely hide the high-density core, giving the appearance of an airborne avalanche, but
in fact, it plays no significant role in avalanche dynamics. It should be borne in mind
that the mere observation of a cloud is generally not sufficient to specify the type of
an avalanche. Further elements such as the features of the deposit or the destructive
effects are required.
