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It is often thought that the velocity of a river is greatest near its start. This is not the case, as large angular boulders create a rough channel shape and therefore, a large amount of its bed friction. This creates more resistance to flow than a river with smooth clays and silt forming its banks. The roughness coefficient is measured using Manning's 'n', which shows the relationship between channel roughness and velocity. The equation is as follows:

Copyright S-cool

v = velocity

R = hydraulic radius

S = slope

n = roughness

A high-value indicates a rough bed.


The efficiency of a rivers channel is measured by finding its Hydraulic radius. It is the ratio between the length of wetted perimeter and cross section of a river channel.

Wetted perimeter: the entire length of the riverbed bank and sides in contact with water.

The examples below show the difference between an efficient river channel and an inefficient river channel:


Cross-section area: Wetted perimeters: Hydraulic radius:
Stream A = 400m2 Stream A = 18m Stream A 40 / 18 = 2.22m
Stream B = 40m2 Stream B = 24m Stream B 40 /24 = 1.66m

Channel slope


Steeper gradients usually lead to greater velocities, because of the influence of gravity.