978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

environment. The amount of turbulence varies depending on the velocity
of the flow as well as the influence of friction and the energy available. The
greater the velocity, the greater the amount of spare energy after friction
and so the greater the turbulence.
• Helicoidal flow is a corkscrew-like flow that is mainly found as water travels
around river bends. It is associated with meanders and the formation of
sediment bars and slip-off slopes.
The thalweg is the name given to the path of least resistance where water
flows the fastest. In a straight channel it can be found in the middle of the
channel under the surface of the water furthest from the influence of friction
from the riverbanks, riverbed and the air. On a bend, however, the fastest flow
will continue in a straight line before hitting the outside of the bend and being
reflected downstream.
Factors affecting river velocity
The velocity of a river is not determined by one single factor. There are
many factors that impact a river’s ability to transport water and sediment
downstream. Gradient, efficiency and bed roughness all determine how
well the water flows. The differing velocity will in turn affect the erosive and
depositional capacity of the river and its potential to shape the channel.
Drainpipes and waterslides are built the way they are for an efficient flow to
move water quickly. The closer the river is to a smooth semicircular form the
more efficient it will be. Man-made channels are often much more efficient
than natural ones.
The measure of efficiency can be determined by calculating the hydraulic
radius (HR).
cross-sectional area
HR =
wetted perimeter
It is a ratio and has no units.
(the width of the river across the contours
of the riverbed)
0.2 0.4 0.2
0.1 0.3 0.4 0.3 0.1
velocity isovels in m/sec
0.4
0.30.2
0.1
figure 1.16 Cross section showing velocity at a meander.
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Hydrology and fluvial geomorphology