Geoscience Reference
In-Depth Information
upper layers of the avalanche deposit find and rescue
many people buried by snow avalanches.
The word 'avalanche' has always conjured up images
of snow cascading down mountains at high speed and
wiping out unsuspecting alpine villages. This image is
based upon accumulative disasters in the European
Alps. Snow avalanches are also a common hazard in the
Himalayas, Andes, and Rocky Mountains, especially in
spring. One of the more recent disasters happened
in 1978 at Col des Mosses, Switzerland, when an
avalanche overwhelmed a ski-lift and killed 60 people.
Even the Prince of Wales was almost killed by an
avalanche that overtook his skiing party in 1987. Two of
the largest disasters have occurred during wars.
Hannibal's 218 BC invasion of Rome across the Italian
Alps in winter was thwarted by snow avalanches that
killed 15 000-18 000 of his men. During the First
World War, snow avalanches - many of which were
deliberately triggered by shelling - killed around
40 000 Italian and Austrian troops fighting in the
Italian Alps. Other disasters have been less costly, but
just as spectacular. Between 1718 and 1720 in Swit-
zerland, four separate snow avalanches took over
250 lives. The largest Swiss avalanche, in 1584, took
300 lives. In 1910, an avalanche at Wellington,
Washington, in the Rocky Mountains, swept through
two trains killing 96 people. In the same year at Rogers
Pass, British Columbia, avalanches killed 62 people. As
recently as 2000-2001, avalanches killed 176 people
worldwide.
In recent years, snow avalanche disasters have
decreased with the advent of monitoring techniques and
remedial measures designed to negate the hazard. For
instance, most zones have been mapped in the popu-
lated mountainous areas where snow avalanches are
likely to occur. The fact that avalanches tend to recur in
the same places has aided this mapping. Sharp increases
in temperature, after a winter of heavy snowfall or
during a spring accompanied by rainfall, produce ideal
conditions for avalanches. Studies of the dynamics of
snow and ice on slopes have allowed these conditions to
be identified to a high degree of accuracy. Sensitive
micropenetrometers driven into snow at 2 cm s -1 can
produce detailed profiles of snow hardness and identify
layers of loose snow underlain by snow that has meta-
morphosed into ice called depth hoar that forms prime
conditions for avalanching. In milder environments, re-
freezing of melted slush - called 'zarame-yuki' in Japan
- can produce similar conditions. In the North
American Rockies and the European Alps, army field
cannon or explosives are used to trigger snow avalanches
before they can build up to catastrophic dimensions.
These techniques have proved effective in keeping
passes free of large disruptive flows, and in protecting
ski slopes in resort areas. Where road and train routes
must be kept open, avalanche chutes are used to divert
frequently recurring flows. These chutes simply consist
of roofs that permit the avalanche to pass harmlessly
over the road or railway. Near settlements, where these
techniques are impractical, other techniques for
braking, deflecting or disrupting the flow have been
established. Upward-deflecting barriers can be con-
structed along preferred avalanche pathways to convert
a ground-based avalanche into a slower, less forceful,
airborne one. Concrete blocks can be arranged on
slopes to divert avalanches away from settlements. The
judicious alignment and reinforcement of buildings
facing avalanche-prone slopes can also negate damage
and prevent loss of life.
SUBSIDENCE
(Sharpe, 1968; Bolt et al., 1975; Coates, 1979, 1985;
Whittow, 1980; Hays, 1981)
Land subsidence is a major type of land instability that
does not depend upon many of the basic mechanisms
outlined in this chapter so far. Rather, it is due to
factors that include the effects of humans through
water and oil extraction, and mining activities. The
proper discussion of these latter factors is beyond
the scope of this text. In this section, only the natural
causes and mechanisms of land subsidence will be
discussed.
Land subsidence is important at several scales of
magnitude, spatially and temporally. For instance, a
small sinkhole caused by limestone solution is not
significant except to a local landowner; however,
uniform subsidence of several square kilometres can
become severely disruptive to a much wider commu-
nity. In a few minutes, the Alaskan earthquake of 1964
produced regional, tectonically induced subsidence
along the south coast of Alaska that had a profound
effect upon the landscape. This sudden change in the
relative location of land and sea resulted in severe
economic problems in harbors. At the same time, there
are locations - such as the south coast of England -
where long-term subsidence of the crust has been
occurring since the demise of the last continental
 
 
Search WWH ::




Custom Search