Tropical landforms
549
favours water penetration and, hence, rock alteration. Solutes are leached towards the low
parts in the profile and the underground flow runs towards the water courses. All this leads
to a slow land-surface lowering, which gives rise to zones with poorly contrasted highs
and lows. In this sense, a fluvial network rejuvenation affects dambos because incision
produces water-table lowering and increases leaching due to the rising hydraulic gradient
(McFarlane and Whitlow, 1990). McFarlane (1989) in Boast (1990), established an
evolutionary sequence starting from a small rounded depression that transformed into a
dambo due to surface lowering. Drainage network rejuvenation brings about a transformation of big dambos into smaller linear forms. Finally, if the water table keeps on
lowering, dambos increase their size individually.
Although alluvial fans reach their maximum expression in arid and semiarid zones,
they also appear in the humid tropics. Nevertheless, in these environments they usually
acquire a lesser development. Their study also becomes difficult due to the vegetation
cover. Fan sedimentation originates by changes in the flow hydraulic geometry. This is due
to a sudden decrease in the channel slope, which transforms the flow from a confined state
to an unconfined one. Fans present a triangular plan shape with convex transverse profiles
and concave radial ones. They can coalesce with other nearby fans.
Alluvial fans occur in some African areas, although in general they are of a small size
(Faniran and Jeje, 1983; Thomas, 1994). Instead, fans from the Pantanal region (Brazil)
occupy a considerable extension and one of them reaches up to 50,000 km 2 (Klammer,
1982a). Fans can develop at the foot of mountains, such as in the high regions of eastern
Papua-New Guinea (Blake and Paijmans, 1973), where the coarsest fan deposits become
altered very rapidly and with later erosion are transformed into fine particles. Some fans are
located at the base of plateaux, such as the ones described by Hill and Rackham (1978) in the
Jos Plateau of central Nigeria. These authors also cite fan development within the plateau,
between granitic residual forms. Fans have been also observed at the foot of labyrinthic
platforms in eastern Zambia and SW of Sierra Leone, where they form iron-cemented
breccias (Thomas, 1983). Small fans appear in the limit between residual hills and plains,
although they have a little sediment supply due to the scarce development of the regolith
upon the hills (Thomas, 1994).
One alluvial fan type cited by Tricart (1974a) is the one located at the foot of gully
zones as a consequence of anthropic tree felling, and occur in Madagascar, Brazil, and
Hong Kong. These deep gullies are called lavakas in Madagascar and vogorocas in Brazil.
In Madagascar alluvial fans are basically constituted by clay and micaceous sands coming
from the erosion of thick saprolites. In the Betsiboka basin, 60,000 km 2 in area, sediment
loss is as much as 15,106 m3/yr.
Areas with high relief, developed during the Alpine Orogeny and many of them located
on insular zones, normally present sharp topographic gradients. The large gullies, with a
radial pattern in many volcanic islands, rapidly remove great amounts of sediments that
are deposited in zones of changing slope, usually on the coastal fringe. This is the case
of the alluvial fan produced by the Rivi~re S~che on Martinique Island, Lesser Antilles
(Figure 21.17). A natural cut allows observation of a sediment thickness of about 12 m,
constituted by stratified heterometric deposits. At the base there are several car tires inside
a laminated deposit, possibly from the 1960s or 1970s. It can be deduced from all this
that erosive activity is very intense and rapid, due to the heavy precipitation and frequent
cyclones (700 mm/day in August 1970, D'Ercole, 1994) and to the presence of pyroclastic
549
favours water penetration and, hence, rock alteration. Solutes are leached towards the low
parts in the profile and the underground flow runs towards the water courses. All this leads
to a slow land-surface lowering, which gives rise to zones with poorly contrasted highs
and lows. In this sense, a fluvial network rejuvenation affects dambos because incision
produces water-table lowering and increases leaching due to the rising hydraulic gradient
(McFarlane and Whitlow, 1990). McFarlane (1989) in Boast (1990), established an
evolutionary sequence starting from a small rounded depression that transformed into a
dambo due to surface lowering. Drainage network rejuvenation brings about a transformation of big dambos into smaller linear forms. Finally, if the water table keeps on
lowering, dambos increase their size individually.
Although alluvial fans reach their maximum expression in arid and semiarid zones,
they also appear in the humid tropics. Nevertheless, in these environments they usually
acquire a lesser development. Their study also becomes difficult due to the vegetation
cover. Fan sedimentation originates by changes in the flow hydraulic geometry. This is due
to a sudden decrease in the channel slope, which transforms the flow from a confined state
to an unconfined one. Fans present a triangular plan shape with convex transverse profiles
and concave radial ones. They can coalesce with other nearby fans.
Alluvial fans occur in some African areas, although in general they are of a small size
(Faniran and Jeje, 1983; Thomas, 1994). Instead, fans from the Pantanal region (Brazil)
occupy a considerable extension and one of them reaches up to 50,000 km 2 (Klammer,
1982a). Fans can develop at the foot of mountains, such as in the high regions of eastern
Papua-New Guinea (Blake and Paijmans, 1973), where the coarsest fan deposits become
altered very rapidly and with later erosion are transformed into fine particles. Some fans are
located at the base of plateaux, such as the ones described by Hill and Rackham (1978) in the
Jos Plateau of central Nigeria. These authors also cite fan development within the plateau,
between granitic residual forms. Fans have been also observed at the foot of labyrinthic
platforms in eastern Zambia and SW of Sierra Leone, where they form iron-cemented
breccias (Thomas, 1983). Small fans appear in the limit between residual hills and plains,
although they have a little sediment supply due to the scarce development of the regolith
upon the hills (Thomas, 1994).
One alluvial fan type cited by Tricart (1974a) is the one located at the foot of gully
zones as a consequence of anthropic tree felling, and occur in Madagascar, Brazil, and
Hong Kong. These deep gullies are called lavakas in Madagascar and vogorocas in Brazil.
In Madagascar alluvial fans are basically constituted by clay and micaceous sands coming
from the erosion of thick saprolites. In the Betsiboka basin, 60,000 km 2 in area, sediment
loss is as much as 15,106 m3/yr.
Areas with high relief, developed during the Alpine Orogeny and many of them located
on insular zones, normally present sharp topographic gradients. The large gullies, with a
radial pattern in many volcanic islands, rapidly remove great amounts of sediments that
are deposited in zones of changing slope, usually on the coastal fringe. This is the case
of the alluvial fan produced by the Rivi~re S~che on Martinique Island, Lesser Antilles
(Figure 21.17). A natural cut allows observation of a sediment thickness of about 12 m,
constituted by stratified heterometric deposits. At the base there are several car tires inside
a laminated deposit, possibly from the 1960s or 1970s. It can be deduced from all this
that erosive activity is very intense and rapid, due to the heavy precipitation and frequent
cyclones (700 mm/day in August 1970, D'Ercole, 1994) and to the presence of pyroclastic