The Speed of a River
The gradient of the river influences the speed but other factors
need to be considered. Water flowing down a steep slope or gradient
has higher velocity than one flowing down a gentle gradient. A river
that plunges down a steep slope as a waterfall is much faster than
in a river that travels down a gentle slope.
Other factors:
Roughness of the channel
Material such as rocks in the channel can influence the speed.
Whether rocks on the river bed are smooth or rough or uneven. Rocks
that protrude out from the bank can slow the pace of the water as
friction slows it down as it passes the obstacles.
The Shape of the channel
The shape of the channel or its cross section affects the extent
to which water is in contact with its channel. This is known as
the wetted perimeter. The greater the wetted perimeter, the greater
the friction between the water and the banks and the bed of the
channel, and the slower the flow of river.
Wetted perimeter
Wetted perimeter is calculated by
adding the length and the breadth of the channel in contact with
the water
Example 1:
A river 2 metres deep and 3 metres wide will have a volume of
6 sq metres (2mx3m) and a wetted perimeter of 7 metres (2m+3m+2m).
The 7 metres will be represent the friction slowing the river
down
Example 2:
A river 4 metres deep and 6 metres wide will have a 24 sq metres
volume and a wetted perimeter of 14 metres.
But the shape of the river is a major influence! A river with the
same volume of water as Example 2 but with a different shape will
have a different friction value.
Example 3:
The river is 1 metre deep but 24 metres wide.
This means that the wetted perimeter is 26 metres almost double
that of Example 2 which means that the river will be slower as a
larger part of the river energy is used to overcome friction.
The gradient of the river channel is only one factor to influence
the speed of the river.
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