Rivers, Floods and Management: Changing Channel Characteristics
Description
A Levels Geography (Physical Geography-AS) Mind Map on Rivers, Floods and Management: Changing Channel Characteristics, created by Andrew_Ellinas on 05/01/2014.
Rivers, Floods and Management: Changing
Channel Characteristics
Downstream Changes to
Velocity and Discharge
Measurements suggest that both the velocity and discharge
actually increase with the distance downstream.
Because the river's gradient decreases with the distance downstream, the
velocity might be expected to decrease too. However, the reduction in
gradient is more than offset by an increase in the mass of water, as smaller
tributaries join the main river.
The 'roughness' of a river also decreases downstream. In a
river's upper course, big boulders create a 'rough' channel.
Further downstream, finer deposited sediments form smoother,
less 'rough' lining to the riverbed and banks.
Roughness creates turbulence and so slows down the flow of water.
The channel further downstream also adopts more of a
semi-circular shape, compared with the wide, shallow
shape upstream.
The semi-circular shape is much more efficient for transferring water
because less energy is lost overcoming friction with the riverbed and
banks.
Hydraulic radius measures the efficiency of the channel. Hydraulic radius
is calculated as the cross-sectional area divided by the wetted perimeter.
The higher the hydraulic radius, the more efficient the
channel is in carrying water.
Downstream Changes to the River Channel
The volume of water carried by a river affects the size of
its channel.
As more and more tributaries join the main channel, it has to carve an even
larger channel itself to cope with the increasing volume.
While discharge is the primary factor affecting the size of a river channel, other actors
can also affect it's shape or profile. For example:
A river might adopt a narrow,
deep channel as it cuts through a
resistant band of rock, or flows
along a line of weakness - such as
a fault or joint.
Human engineering can alter the shape of a channel -
perhaps where it's lined with concrete, or has been
dredged to deepen it.
Farm animals, like cattle, can trample riverbanks and alter the channel
profile.