A short introduction
Attrition
Hydraulic action
Corrasion
Corrosion
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Activity: Show the four processes of erosion
in the boxes.
Try to use graphical representation as it will
help you remember.
Brief descriptions are also required.
Activity: Use P.282 to complete this – then in pairs explain the processes
to each other
Attrition
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When Boulders and
other material, which
are being transported
along the bed of the
river, collide and break
up into smaller pieces.
This is more likely to
occur when rivers are
still flowing in
highland areas
Hydraulic Action
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When the sheer
force of the river
dislodges particles
from the river’s
banks and bed.
Corrasion/ Abrasion
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When smaller material,
carried in suspension,
rubs against the banks
and bed of the river.
This process is more
likely in lowland areas
by which time the
material will have been
broken up small
enough to be carried
by suspension.
River banks are warn
away by a sand
papering action
Corrosion
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Acids in the river
also dissolve rocks,
such as limestone,
which form the
banks and bed.
This can occur at
any point in the
rivers course
Deposition
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Deposition occurs when a river lacks enough
energy to carry its load. Deposition,
beginning with the heaviest material first, can
occur following a dry spell when the
discharge and velocity of the river drop.
It also occurs where the current slows down
the inside of a meander bend or where the
river enters the sea .
Velocity Vs Sediment
What is the Wetted Perimeter?
The yellow line shows the wetted perimeter
Wetted perimeter: the part of the bed and banks which is in contact
with the water in the channel – how do you think hydrologists measure this?
5 meters
This river has a higher wetted perimeter
in comparison to its volume,
which increases friction and reduces
Velocity (upper course)
Rocks and boulders
This river has a smaller
wetted perimeter in comparison
with its volume, because it has
smooth banks, friction is reduced
and this allows velocity to increase
(middle/lower course)
50 meters
http://www.youtube.com/watch?v=mgBqfcMK4jI
Hydraulic radius (Rh): the ratio between the
area of the cross-section of a stream and the
length of its wetted perimeter
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The higher the hydraulic radius, the faster
the river flow (Velocity) and the more
efficient the river is, because there is less
contact between the water in the channel
and the bed and banks, so lower friction.
(cross sectional)
Measuring discharge in Cumecs (m³/s )
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In order to calculate the discharge of a river in
cumecs (cubic meters per second), you need to
calculate the cross sectional area (m) (width x
depth) and times this by the velocity (speed in
meters a second)
Width = 10 meters
Depth = 1.5 meters
10 x 1.5 = 15 (cross sectional area)
Speed of flow (velocity)
= 10 meters a second
Discharge in cumecs=
15 x 10 = 150 cumecs