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Loss soil from land by rainfall and run-off has been a major
environmental problem for many years. It creates decline of agricultural
productivity and deterioration of arable land. Accelerating agents are improper
agriculture, grazing, fire, urban development, construction activities,
off-road vehicles, etc. Two sequential events are involved in water erosion.
These events are detachment of particles and transportation of detached
particles.
Types
of water erosion
a) Raindrop or
rain splash erosion
It
involves detachment of soil particles by raindrop impact. The detached soil
particles are moved by rain splashing. This is the initial phase of soil
erosion. Approximately 90% of erosion on agricultural land is by raindrop
erosion (Morgan, 1988). The raindrops have a tendency of compacting surface
particles by re-orienting them in doing so create surface duricrust that
promote surface run-off. The raindrop erosion depends on rainfall properties
such as drop size, terminal velocity, duration, frequency, rain intensity,
distribution, angle and direction. Wind normally affects terminal velocity,
angle and direction of impact.
b) Run-off
erosion
If
the soil is unsaturated, water will infiltrate into the ground at a rate
governed by soil structures and texture, vegetation cover, biological
structures in the soil, soil-moisture content and surface conditions, such as,
presence of cracks, slope, degree and surface compaction, etc. The surface
run-off occurs on slopes when rainfall intensity exceeds the infiltration
capacity of the ground. Rapid run-off is promoted with steep slopes, bare rock
and or skeletal soil. Infiltration is high in vegetated land, land used for
agriculture and hot climate. Vegetation
retards surface flows, roots make the soil pervious and leaves prevent direct
hit of the ground by raindrops, hence reduces soil packing. Initially water
energy is not sufficient to pick particles but as it moves down slope its
momentum increases and erosive power increases. Surface run-off is common in
semi-arid environment where a thin vegetation cover and soil mantle are
dominant.
Types of run-off
erosion
i)
Sheet erosion
ii)
Rill erosion
Surface run-off
iii)
Gully erosion
iv)
Piping Subsurface erosion
Sheet erosion
It is caused by overland or sheet flood. Effective to
soil surfaces which are smooth and disturbed by animal. The max erosion occurs
when flow depth is equal to raindrop diameter (3-6 mm). This erosion causes
increase in fertility down slope in cultivated area.
Rill erosion
It involves development of small channels called
rills. The rill erosion is accelerated by cultivation on slope without
protective measures. Both rill and sheet erosion can be reduced or prevented by
reducing the effective slope length by dividing the slopes into sections using
grass strips, hedges, walls, shallow drains, terraces or furrows along the line
of contour. The slope steepness can be reduced by terracing the slopes into
short and gently sloping surfaces separated with terrace walls.
Gully erosion
It
involves development of drainage channels with steep sides, steeply sloping or
vertical head scarp. Tends to develop at any break of slope or break in
vegetation cover when underlying materials are mechanically weak or
unconsolidated. Normally it is caused by
increase in the amount of runoff and reduction in water channels that carry the
flood. The increase in run-off may be due to climatic change accompanied with
variation in rainfall intensity and duration.
Another cause is change in vegetation cover due to cultivation,
overgrazing, excessive burning of vegetation, cutting trees, etc. The gully
erosion is common in areas with deep cover of loess, volcanics, alluvium,
colluvium, gravels, debris from mass movement and partly consolidated sands. It
is also common in semi-arid countries in areas affected by overgrazing. In the
gully, erosion proceeds by rapid cutting back into the slope (gully
retreat/head erosion); deepen of the gully floor and collapse of the sides.
Restora tion of gullied land
According
to Cooke and Doornkamp (1990), the techniques that are employed are as follows:
i)
Diverting water
entering the gully by means of cut-off drains or ridges of soil
ii)
Reduce water
supply by conservation practices (increase water infiltration capacity) in the
tributary land
iii) Reduce the erosive velocities in the gully by building
structures (See Figures 5.4a-c).
iv) Cover the waterway with grass to encourage natural
vegetation.
v) Convert the gully into a stable artificial channel
with dimensions appropriate for the discharge of water.
vi) Prevent overgrazing, bush fire and other related
activities
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