Geography Reference
In-Depth Information
EXTREME (Black): Natural and human-triggered avalanches are certain, large to very
large avalanches are expected in many areas, people should avoid all avalanche ter-
rain.
HIGH (Red): Natural avalanches are likely and human-triggered avalanches are very
likely. Large avalanches are expected in many areas and very large avalanches are
expected in specific areas. These are very dangerous avalanche conditions and travel
in avalanche terrain is not recommended.
CONSIDERABLE (Orange): Natural avalanches are possible and human-triggered ava-
lanches are likely. People can expect small avalanches in many areas or large ava-
lanches in specific area or very large avalanches in isolated areas. These are danger-
ous avalanche conditions where careful snowpack evaluation, cautious route finding,
and conservative decision making are essential.
MODERATE (Yellow): Natural avalanches are unlikely, but human-triggered avalanches are
still possible. People can expect small avalanches in specific areas or large ava-
lanches in isolated areas. In these conditions, there are heightened avalanche con-
ditions on specific terrain features, and people must evaluate the snow and terrain
carefully, indentifying the features of concern.
LOW (Green): Natural and human-triggered avalanches are unlikely. People can expect
small avalanches in isolated areas or extreme terrain. There are generally safe ava-
lanche conditions, though there may be unstable snow on isolated terrain features.
MITIGATING AVALANCHES
The attempt to directly mitigate avalanches has been practiced in the Alps for centuries,
but emerged as a relatively new endeavor in North America following World War II.
There are two basic approaches to the problem: passive and active mitigation. Passive
mitigation measures are relatively effective, but they can be expensive and require
continual maintenance. Therefore, they are most appropriate in areas where perman-
ent structures are threatened by avalanches. Active mitigation, such as triggering ava-
lanches, is much less expensive but must be applied repeatedly. This technique is ap-
propriate for areas where avalanches can be triggered when people are not in the area,
such as ski runs and mountain highways.
Passive mitigation through terrain modification consists of placing structures such
as walls, pylons, dams, and wedges of various designs either in the snow accumulation
zone or immediately above the area to be protected (McClung and Schaerer 2006). The
strategy in the snow accumulation zone is to break up the solid mass of the snow into
smaller units, to anchor the snow base, and to create terraces so that there is less effect-
ive slope for each snow unit (LaChapelle 1968: 1024; Fig. 4.26). In the runout zone, the
structures consist of barricades, walls, and wedges to dam or divert the avalanche (Fig.
4.27). Roofs or sheds are frequently constructed over highways and railway lines along
avalanche paths (Fig. 4.28). An interesting technique is the use of alternately spaced
earthen mounds (Fig. 4.29). These apparently break up and slow the avalanche by di-
viding it into cross-currents that dissipate its kinetic energy (LaChapelle 1966: 96).
The other major approach to avalanche mitigation is active modification of the snow
itself. The oldest, and perhaps still the most successful, method of this type is the ar-
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