Snow Avalanches - Environmental Geomechanics - page 41

connected to weather conditions [ANC 98, BER 27, DEQ 73]. Fixed parameters
include:
- Mean slope . In most cases, the average inclination of starting zones ranges from
27 to 50 ◦ . On rare occasions, avalanches can start on gentle slopes of < 25 ◦ (e.g.
slushflow involving wet snow with high water content), but generally the shear stress
induced by gravity is not large enough to cause failure [LAC 86]. For inclinations in
excess of 45 to 50 ◦ , many slides ( sluffs ) occur during snowfalls; thus the amounts of
snow deposited on steep slopes are limited.
- Roughness . Ground surface roughness is a key factor in the anchorage of the
snow cover to the ground. Dense forests, broken terrain, starting zones cut by several
ridges, and ground covered by large boulders generally limit the amount of snow that
can be involved in the start of an avalanche. Conversely, widely spaced forests, large
and open slopes with smooth ground facilitate avalanche release.
- Shape and curvature of starting zone . The stress distribution within the snow
pack and the variation in its depth depend on the longitudinal shape of the ground. For
instance, convex slopes concentrate tensile stresses and are generally associated with
a significant variation in the snowcover depth, favoring snowpack instability.
- Orientation to the sun . The orientation of slopes with respect to the sun has a
strong influence on the day-to-day stability of the snowpack. For instance, in winter,
shady slopes receive little incoming radiation from the sun and conversely lose heat
by long-wave radiation. It is generally observed that for these slopes, the snowpack
is cold and tends to develop weak layers (faceted crystals and depth hoar). Many
fatalities occur each year in such conditions. In late winter and spring, the temperature
increase enhances stability of snowpacks on shady slopes and instability on sunny
slopes.
Among the varying factors intervening in avalanche release, experience clearly
shows that in most cases, avalanches result from changes in weather conditions:
- New snow . Most of the time, snowfall is the cause of avalanches. The hazard
increases significantly with the increase in the depth of new snow. For instance, an
accumulation of 30 cm/day may be sufficient to cause widespread avalanching. In
European mountain ranges, heavy snowfalls with a total precipitation exceeding 1m
during the previous three days may produce large avalanches, with possible extension
down to the valley bottom.
- Wind . The wind is an additional factor which significantly influences the stability
of a snowpack. Indeed it causes uneven snow redistribution (accumulation on lee
slopes), accelerates snowmetamorphism, forms cornices, whose collapses may trigger
avalanches. On the whole, influence of the wind is very diverse, either consolidating
snow (compacting and rounding snow crystals) or weakening it.
- Rain and liquid water content . The rain plays a complex role in snow
metamorphism. Generally, for dry snow, a small increase in the liquid water content