3 MECANISM OF RIVER SILTING UP
River water and runoff constitutes the main agents of erosion
and transport in the basin through the processes of splash and stream to work
interfluves, slopes and plains. The human action also contributes to the
environmental degradation in the basin.
3.1 Types of basin erosion
3.1.1 Splash and waters extract
- Splash, more widespread in the culture field
where the grounds are entirely stripped; the rainy in dropping compress the
movable superficial level;
- Water extract, of hydrous origin, it
causes fragmentation by alternations of moistening and desiccation of the
grounds. It affects the materials inflating such as clays which absorb a lot of
water. Also infiltrated water accomplishes a transport of fine particles
in-depth thus creating a vacuum under the horizontally packed ground; this
subsides under the effect of its weight. Racking would be at the origin of the
closed depressions and would have also generated the installation of the
ponds.
3.1.2 Stream
- First stream, it starts at the beginning of
the rain, transports the fine particles in particular the beforehand detached
silts and erodes the ground of a few centimeters;
- Concentrated stream, it is started when
the quantity of rainwater is important on the ground surface which sometimes
sporadic flow. Water incises the ground vertically; on the slopes, the
concentrated stream prints model of drains, the gullies. As indication, the
measures taken in the concave sectors of the slopes starting from the meter
ribbon and a plumb line of the incisions marks in two localities arise as
follows:
Togblékopé Tonoukouti
Modelled
Dimension
Gullies
0,1 à 0,3m
Ravines
1m
Modelled
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Gullies
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Ravines
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Dimension
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0,1 à 0,3m
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1,2m
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It is noted that the ravines have abrupt banks and in the
other hand present sometimes a broad bottom at the transverse profile in ?U"
the gullies have a pace in «V». The presence of stiff banks proves
that erosion is carried out in a vertical way. Incised slopes of ravines show
an aspect of «bad-lands».
3.1.3 Rain erosion and river erosion
- Rain erosion, according to TRICART (1977),
it is caused by torrential rains; the water drops while falling in quantity and
at a high speed on the naked ground exert a pressure of potential origin which
involves the detachment of the particles or group particles called
disintegration;
- River erosion, it is summarized of river
water mechanical action which drains its own alluvia and this at the time of
the exceptional risings after the strong rains. These risings of ten days
approximately cause the overflow of water which involves the sweeping of
sediments and remains vegetable of the basin in direction of the Zio bed.
At the end of the risings, the sand banks settle along the
maximum limit reached by high waters; the provision parallel of the sand banks
with the alluvial plain indicates that the risings carried out a lateral
cutting. These sand banks are taken again by surface waters after the rising
period.
The major form of fluviatile erosion is the meander
characterized by curves alternated more or less regularly; in the alluvial
plain, Zio carries out about thirty meanders. Also, in this alluvial plain,
there are two types of depression:
. Depression floods, which drain their water
with the fall; stagnant water almost does not remain, however in certain
depressions the humidity is maintained by the water flasks with a papered
bottom of fine silts;
. Depressions of decantation, closed, from
circular form, they receive water of raw which remains for a long time. Clays
are elutriated, also there are remains organic (died sheets, tree trunks etc)
at the bottom of these depressions.
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