FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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5.0 EROSION AND
SEDIMENTATION BY WIND AND WATER
5.1 WIND EROSION
AND SEDIMENTATION
The
wind erosion is common in deserts and semi-deserts, coastal dune areas, exposed
mountain regions and in agricultural areas where bare soil is exposed to wind
during dry periods. In general, wind erosion operates in areas that are dry and
unprotected by vegetation. Erosion, transportation and deposition by wind are
known as eolian activities.
Types of wind
erosion
The wind erosion
is of two types:
a) Deflation
It
is removal of non-cohesive materials, such as loose sand and dust by blowing.
The base level for deflation is regional water table. It is more effective
where it cuts into unconsolidated soil or weak sediment.
b) Corrasion (abrasion)
Mechanical
erosion due to scouring action of transported particles. The wind-driven particles
cause sand blasting on exposed surface. The particles when blown into or
against a rock surface dislodge other particles and some break the cementing
medium. The impacts of blown particles are accompanied with repeated abrasion.
The effectiveness of abrasion depends on the wind velocity, hardness of dust
and sand carried by wind and hardness of eroded surface (rock or soil).
Mode
of wind transportation
a) Suspension
It
involves materials with grain size < 0.1 mm (silt, clay or dust). The fines
are brought high into the air and transported over long distances. The fine
particles can be lifted 1-2km high and travel thousands of km before setting
down, for example from Sahara to Europe (ITC, 1991).
b) Saltation
Slightly coarser particles mainly sand are involved.
The average particle size ranges from 0.1mm – 0.5mm. It starts by rolling but
as a grain rolls faster it hits other grains and bounces into the air where its
momentum is increased as it is carried by the wind before falling back to the
ground. During transportation most grains rise a few centimetres above the
ground (1-2cm) and few bounce 1-3m (ITC, 1991). This is a major mode of
transportation in dune fields
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