Multiple roughness is related to variations in shear stress (γyoS) in a channel cross-section.
The greater the width-depth ratio of a stream, the greater is the probability of a spatial
variation in shear stress, stream power or bed material. Thus, the occurrence of multiple
roughness is closely related to the width-depth ratio of the stream. Variable roughness is
related to changes in shear stress, stream power, or reaction of bed material to a given
stream power over time. A commonly observed example of the effect of changing shear
stress or stream power is the change in bed form that occurs with changes in depth during a
runoff event. Another example is the change in bed form that occurs with change in the
viscosity of the fluid as the temperature or concentration of fine sediment varies over time. It
should be noted that a transition occurs between the dune bed and the plane bed; either bed
configuration may occur for the same value of stream power (Figure 3.7).
A relation between stream power, velocity, and bed configuration is shown in Figure 3.7.
The relation pertains to one sand size and was determined in the 2.4 m (8-foot) flume at
Colorado State University.
Figure 3.7. Change in velocity with stream power for a sand with D50 = 0.19 mm
(Simons and Richardson 1966).
In the following paragraphs bed configurations and their associated flow phenomena are
described in the order of their occurrence with increasing stream power.
3.16
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