592
Climatic Geomorphology
the same time, they can be transported to lower terrace levels. Fluvial channel beds are
affected by a continuous reworking and channel migration in such a way that coarse gravel
sedimentation favours trapping of small diamonds, such as those in Sierra Leone and
Ghana (Thomas and Thorp, 1993). In Indonesia and Malaysia cassiterite placers derive
from the stripping of weathering profiles on granites and andesites, and appear in
colluvium, alluvial fans and fillings of endokarstic cavities (Aleva, 1985; Gupta et al.,
1987). A wide variety of economic minerals can concentrate on beach placers, such as
ilmenite, zircon, and monazite. Their concentration is made by waves and littoral drift
currents. On the other hand, removal of beach sands by wind can give rise to aeolian
placers on littoral dunes.
The commercial extraction of minerals can produce important environmental
modifications, usually tolerated by the governments of developing countries. A great
part of the cassiterite exploitations of alluvial placers in SE Asia are near the coast.
Extraction is carried out by dredging and pumping fluvial channel beds. This operation
produces intense water turbulence and an important part of the suspended load is spilled
into the sea, affecting the life of coral reefs. The Madeira Basin of the SW Amazon River
represents another example of impact derived from some metallurgical treatment
practices. Dredged gold is amalgamated with mercury and some 5 to 30% mercury is
lost in the fiver. Water pollution has serious effects on the fauna existing in this fluvial
basin (Reading et al., 1995).
5. Slope failures and hazards
Mass movements constitute a hazard when threatening people's life and properties. During
the last century this hazard has increased due to population growth and to the expansion of
areas needed for obtaining essential resources, which on occasion makes people exploit
hazardous zones (Crozier, 1986). In addition, other factors have converged, such as
population migration to the great cities and the construction of numerous engineering
works that modify the landscape and introduce slope instability (Figure 22.7 and
Figure 22.8). Slope failures can produce deaths and injury, material losses and
environmental damage. In humid tropical zones failures present a very important danger
due to the existence of more or less thick saprolites and to the high precipitation. These
circumstances favour the removal of surface or deep altered layers, which takes place
especially in relation to rainstorms. For this reason, it is necessary to draw detailed
geomorphological maps and make boreholes for terrain recognition (Tricart, 1974a).
Heavy rainfall commonly gives rise to slope failures and floods. From the first to the
fourth of October 1968, several states of northwest India underwent major monsoon rains
that produced slides and flooding, burying or partially submerging the cities of Darjeeling
and Jalpaiguri. About 1000 people died and many material losses were recorded,
especially oil pipes, roads, railways, and houses (Davis, 1992). This combined action of
flooding and slope failure also took place in Honduras and Nicaragua, as a consequence of
the rain associated with the Mitch hurricane, between the end of October and beginning of
November 1998. Some 85% of the land was flooded, more than 10,000 people died, 15,000
disappeared, and enormous damage was made to the infrastructure (Figure 22.9), houses
and crops. The slide produced on the Casitas volcano was channelled by the hill slopes and
Climatic Geomorphology
the same time, they can be transported to lower terrace levels. Fluvial channel beds are
affected by a continuous reworking and channel migration in such a way that coarse gravel
sedimentation favours trapping of small diamonds, such as those in Sierra Leone and
Ghana (Thomas and Thorp, 1993). In Indonesia and Malaysia cassiterite placers derive
from the stripping of weathering profiles on granites and andesites, and appear in
colluvium, alluvial fans and fillings of endokarstic cavities (Aleva, 1985; Gupta et al.,
1987). A wide variety of economic minerals can concentrate on beach placers, such as
ilmenite, zircon, and monazite. Their concentration is made by waves and littoral drift
currents. On the other hand, removal of beach sands by wind can give rise to aeolian
placers on littoral dunes.
The commercial extraction of minerals can produce important environmental
modifications, usually tolerated by the governments of developing countries. A great
part of the cassiterite exploitations of alluvial placers in SE Asia are near the coast.
Extraction is carried out by dredging and pumping fluvial channel beds. This operation
produces intense water turbulence and an important part of the suspended load is spilled
into the sea, affecting the life of coral reefs. The Madeira Basin of the SW Amazon River
represents another example of impact derived from some metallurgical treatment
practices. Dredged gold is amalgamated with mercury and some 5 to 30% mercury is
lost in the fiver. Water pollution has serious effects on the fauna existing in this fluvial
basin (Reading et al., 1995).
5. Slope failures and hazards
Mass movements constitute a hazard when threatening people's life and properties. During
the last century this hazard has increased due to population growth and to the expansion of
areas needed for obtaining essential resources, which on occasion makes people exploit
hazardous zones (Crozier, 1986). In addition, other factors have converged, such as
population migration to the great cities and the construction of numerous engineering
works that modify the landscape and introduce slope instability (Figure 22.7 and
Figure 22.8). Slope failures can produce deaths and injury, material losses and
environmental damage. In humid tropical zones failures present a very important danger
due to the existence of more or less thick saprolites and to the high precipitation. These
circumstances favour the removal of surface or deep altered layers, which takes place
especially in relation to rainstorms. For this reason, it is necessary to draw detailed
geomorphological maps and make boreholes for terrain recognition (Tricart, 1974a).
Heavy rainfall commonly gives rise to slope failures and floods. From the first to the
fourth of October 1968, several states of northwest India underwent major monsoon rains
that produced slides and flooding, burying or partially submerging the cities of Darjeeling
and Jalpaiguri. About 1000 people died and many material losses were recorded,
especially oil pipes, roads, railways, and houses (Davis, 1992). This combined action of
flooding and slope failure also took place in Honduras and Nicaragua, as a consequence of
the rain associated with the Mitch hurricane, between the end of October and beginning of
November 1998. Some 85% of the land was flooded, more than 10,000 people died, 15,000
disappeared, and enormous damage was made to the infrastructure (Figure 22.9), houses
and crops. The slide produced on the Casitas volcano was channelled by the hill slopes and