Environmental Engineering Reference
In-Depth Information
In spite of substantial progress accomplished over the last century, we still do not
know precisely what combination of physical conditions gives rise to avalanches and
what exactly governs the way they flow. Our ability to predict when avalanches will
occur is limited because the weather conditions that give rise to them are far from clear
cut [SCH 03]. It is also difficult to construct defenses against avalanches because we
only have a limited understanding of how they flow. Building walls to either stop or
divert avalanches requires a knowledge of how far a potential avalanche is likely to
travel, how fast it will be traveling when it reaches the barrier and how broad it will
be. Predicting these things is still quite hit and miss.
This chapter summarizes the paramount features of avalanches (formation and
motion) and outlines the main approaches used for describing their movement. We
do not tackle specific problems related to snow mechanics and avalanche forecasting.
For more information on the subject, the reader is referred to the main textbooks
published in Alpine countries [AMM 97, ANC 06, DEQ 73, HUT 96, LAC 77,
MCC 93, PUD 06].
2.1.1.
A physical picture of avalanches
Avalanches are rapid gravity-driven masses of snow moving down mountain
slopes. Many, if not most, catastrophic avalanches follow the same basic principle:
fresh snow accumulates on the slope of a mountain until the gravitational force at the
top of the slope exceeds the binding force holding the snow together. A solid slab of
the surface layer of snow can then push its way across the underlying layer, resulting
in an avalanche. The failure may also arise from a temperature increase, which reduces
snow cohesion. Typically, most avalanches travel for a few hundred meters at a rather
low velocity (a few meters per second), but some can move up to 15 km and achieve
velocities as high as 100 m/s. They can also pack an incredible punch, up to several
atmospheres of pressure.
2.1.2.
Avalanche release
Successive snowfalls during the winter and spring accumulate to form snow cover.
Depending on the weather conditions, significant changes in snow (types of crystal)
occur as a result of various mechanical (creep and settlement) and thermodynamic
processes (mass transfer) [COL 91, SCH 03]. This induces considerable variations in
its mechanical properties (cohesion and shear strength). Due to its layer structure, the
snow cover is liable to internal slides between layers induced by gravity. When the
shear deformation exceeds the maximum value that the layers of snow can undergo,
a failure arises, usually developing first along the sliding surface, then propagating
throughout the upper layers across a crack perpendicular to the downward direction.
This kind of release is very frequent. In the field, evidence of such failures consists of
