Environmental Engineering Reference
In-Depth Information
The current terminology asserts that there are two main types of motion. In this
respect, mixed-motion avalanches are seen as avalanches combining aspects of both
airborne and flowing avalanches, but they are not seen as a third type of avalanche. The
question of a third type of avalanche has been raised by some experts during the last
few years. Indeed, there is field evidence that some events did not belong either to the
group of airborne or flowing avalanches. For instance, the Taconnaz avalanche (Haute-
Savoie, France) on 11 February 1999 severely damaged two concrete-reinforced
structures. The impact pressure was estimated at 600 kPa. The assumption of a flowing
avalanche is not supported by the shape of the deposit. Current knowledge of airborne
dynamics has a hard time explaining such a high impact pressure.
To conclude it should be noted that there is currently a limited amount of data on
real events. Some of the main parameters, such as the mean density in an airborne
avalanche, are still unknown. Thus, many elements of our current knowledge of
avalanches have a speculative basis. Today a great deal of work is underway to acquire
further reliable data on avalanche dynamics. Experimental sites, such as
la Sionne
(Switzerland) or the
Lautaret pass
(France), have been developed for that purpose.
However a survey of extreme past events shows that the characteristics of extreme
avalanches (involving very large volumes) cannot be easily extrapolated from the
features of ordinary avalanches. In this respect, the situation is not very different
from the problems encountered with large rockfalls and landslides [SAV 89b]. Many
observations that hold for ordinary events no longer hold for rare events. Examples
include the role of the forest, the influence of the snow type on avalanche motion, etc.
2.2. Modeling avalanches
Avalanches are extremely complex phenomena. This complexity has led to the
development of several approaches based on very different points of view. Many
papers and reports have presented an overview of current models. These include the
reviews by Hopfinger [HOP 83] and Hutter [HUT 96] as well as a comprehensive
review of all existing models edited by Harbitz [HAR 99]. Here we shall only outline
three typical approaches: the statistical approach, the fluid-mechanics approach and
small-scale models.
2.2.1.
Statistical methods
In land-use planning (avalanche zoning), the main concern is to delineate
areas subject to avalanches. Avalanche mapping generally requires either accurate
knowledge of past avalanche extensions or methods for computing avalanche
boundaries. To that end several statistical methods have been proposed [BOV 76,
ECK 07, ECK 08, KEY 05, LIE 80, MCC 01, MEU 04a, MEU 04b, STR 06].
