210
Climatic Geomorphology
thermokarst develops very rapidly and it may take many years to re-establish the initial
equilibrium (Harry, 1988). These perturbations may be due to variable causes like climate
changes that result in the increase of freezing or thawing. Little is known about the causes
of some variations undergone by the permafrost (Williams and Smith, 1989). The response
of the permafrost to changes in climate variations is generally slow due to the low heat
conductivity of the ground. It is thought that the expansion of the thermokarst topography
in some sectors of Russia is due to a climatic warming that has taken place during the last
decades. Subtle climatic changes may produce considerable effects in areas with mean
temperatures close to 0~ that cover extensive areas. These changes cause important
modifications in the hydrologic systems, mainly affecting the run-off. Human-induced
alterations derived from deforestation, construction of linear infrastructures (roads,
railways, aerodromes, pipelines) or buildings and the drainage of lakes cause the thawing
of the ground ice. Consequently the ground becomes unstable and susceptible to affects by
mass movements, subsidence and frost-heaving (USGS, 1983; French, 1996).
The mitigation of the problems derived from human impact on the periglacial
environments requires a good background on the processes affecting the seasonally and
permanently frozen ground, evaluating their activity and understanding their relationship
with the environmental variables. This knowledge largely comes from the study of the
natural landforms and from experience acquired during the last few decades about the
behaviour of the frozen ground in response to human activities (Harris, 1986). Another
source of knowledge derives from the practice of freezing the ground to endow it with
greater strength (Williams and Smith, 1989). This technique, used in the excavation of
mine galleries and tunnels (the underground of several Japanese cities and the tunnel
beneath the Seine River in Paris), provides valuable information about the behaviour of
frozen ground. Nevertheless, some of the processes related with the freeze-thaw cycles
are very complex and are not very well understood, such as the transfer of heat and
moisture in the frozen ground or the rheology of the materials. An adequate understanding
of the processes helps to implement proper construction practices in order to reduce the
induced detrimental effects. At the present time, in 1:250,000 scale maps of sensitivity in
ground classification and 1:50,000 susceptibility maps to ground modifications are
produced. These documents help to select the zones where human-induced ground
disturbances and the thermokarst subsidence have a lower impact (French, 1996).
2. Snow avalanches
Snow avalanches are very common in alpine mountain environments and constitute an
important hazard in regions where human activities are well developed, especially tourism
and recreational practices in developed countries (Gerrard, 1990) (Figure 9.1). In British
Columbia, 25,000 avalanches have been recorded affecting highways (McClung, 2003).
Snow avalanches may incorporate different types of detritus (rock fragments, earth, trees)
increasing their destructive capacity (Keylock, 1997). They may be triggered by
earthquakes (Voight and Pariseau, 1978) (Figure 9.2), the passage of skiers, thunder,
vibrations caused by human activities, transit of animals, sonic booms from jet planes, and
other factors.
Climatic Geomorphology
thermokarst develops very rapidly and it may take many years to re-establish the initial
equilibrium (Harry, 1988). These perturbations may be due to variable causes like climate
changes that result in the increase of freezing or thawing. Little is known about the causes
of some variations undergone by the permafrost (Williams and Smith, 1989). The response
of the permafrost to changes in climate variations is generally slow due to the low heat
conductivity of the ground. It is thought that the expansion of the thermokarst topography
in some sectors of Russia is due to a climatic warming that has taken place during the last
decades. Subtle climatic changes may produce considerable effects in areas with mean
temperatures close to 0~ that cover extensive areas. These changes cause important
modifications in the hydrologic systems, mainly affecting the run-off. Human-induced
alterations derived from deforestation, construction of linear infrastructures (roads,
railways, aerodromes, pipelines) or buildings and the drainage of lakes cause the thawing
of the ground ice. Consequently the ground becomes unstable and susceptible to affects by
mass movements, subsidence and frost-heaving (USGS, 1983; French, 1996).
The mitigation of the problems derived from human impact on the periglacial
environments requires a good background on the processes affecting the seasonally and
permanently frozen ground, evaluating their activity and understanding their relationship
with the environmental variables. This knowledge largely comes from the study of the
natural landforms and from experience acquired during the last few decades about the
behaviour of the frozen ground in response to human activities (Harris, 1986). Another
source of knowledge derives from the practice of freezing the ground to endow it with
greater strength (Williams and Smith, 1989). This technique, used in the excavation of
mine galleries and tunnels (the underground of several Japanese cities and the tunnel
beneath the Seine River in Paris), provides valuable information about the behaviour of
frozen ground. Nevertheless, some of the processes related with the freeze-thaw cycles
are very complex and are not very well understood, such as the transfer of heat and
moisture in the frozen ground or the rheology of the materials. An adequate understanding
of the processes helps to implement proper construction practices in order to reduce the
induced detrimental effects. At the present time, in 1:250,000 scale maps of sensitivity in
ground classification and 1:50,000 susceptibility maps to ground modifications are
produced. These documents help to select the zones where human-induced ground
disturbances and the thermokarst subsidence have a lower impact (French, 1996).
2. Snow avalanches
Snow avalanches are very common in alpine mountain environments and constitute an
important hazard in regions where human activities are well developed, especially tourism
and recreational practices in developed countries (Gerrard, 1990) (Figure 9.1). In British
Columbia, 25,000 avalanches have been recorded affecting highways (McClung, 2003).
Snow avalanches may incorporate different types of detritus (rock fragments, earth, trees)
increasing their destructive capacity (Keylock, 1997). They may be triggered by
earthquakes (Voight and Pariseau, 1978) (Figure 9.2), the passage of skiers, thunder,
vibrations caused by human activities, transit of animals, sonic booms from jet planes, and
other factors.