| Erosion
Processes |
The
material carried by a river can contribute to the wearing
away of its banks and, to a lesser extent and mainly in the
upper course,
its bed. There are four main processes of erosion.
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| Abrasion
(Corrasion) |
Corrasion
occurs when the river picks up material and rubs it along
its bed and banks, wearing them away by abrasion,
rather like sandpaper. This process is most effective during
times of flood and is the major method by which the river erodes
both vertically and horizontally. If there are hollows in the
river bed, pebbles are likely to become trapped. Turbulent
eddies in the current can swirl pebbles around to form
potholes (see quiz 2 below).
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| Attrition |
AS the bedload is moved downstream,
boulders collide with
other material and the impact may break the rock into smaller
pieces. In time, angular rocks become increasingly rounded
in appearance.
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| Hydraulic action |
The sheer force of the water as the turbulent current hits
river banks (on the outside of a meander bend), pushes water
into cracks is increased and, in time, the bank will collapse.
Cavitation is a form of hydraulic action caused by bubbles
of air collapsing. The resultant shock waves hit and slowly
weaken the banks. This is the slowest and least effective
erosion process.
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| Solution or corrosion |
This occurs continuously and is independent of river discharge
or velocity. It is related to the chemical composition of
the water, e.g. the concentration of carbonic and humic acid.
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| Deposition |
When the velocity of a river begins to fall, it has less energy
and so no longer has the competence or capacity to carry
all its load. So, starting with the largest particles, material
begins to be deposited.
Deposition occurs when:
- Discharge is reduced following a period of low Precipitation;
- Velocity is lessened on entering the sea a lake (resulting
in a delta);
- Shallower water occurs on the inside of a meander (resulting
in a point bar;
- The load is suddenly increased, (caused by debris from
a landslide)
- The river overflows its banks so that the velocity outside
the channel is reduced (resulting in a flood
plain).
- As the river loses energy, the following changes are
likely:
- The heaviest or bedload material is deposited first. It is
for this reason that the channels of mountainous streams
are often filled with large boulders large boulders increasing
the size of the wetted perimeter.
- Gravel, sand and silt transported either travel as bedload
or in suspension — will be carried further, to
be deposited over the flood plain or in the channel of
the river as it
nears its mouth.
- The finest particles of silt and clay, which are carried
in suspension, may be deposited where the river meets
the sea — either to infill an estuary or to form
a delta.
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