Environmental Engineering Reference
In-Depth Information
value of S p reduced to its minimum value when the rate of travelling bed load transport reached its peak.
The bed structure gradually recovered and the value of S p increased again after the feeding stopped.
Following the increasing S p more flow energy was consumed on the bed structure and the rate of bed
load transportation decreased.
For the bed load motion in mountain streams the stream power, the bed structure and the rate of
incoming bed load are the most important factors. A bed load formula may be expressed in the following
form:
g
fpS g
(, ,
)
(5.61)
b
p
b
i
in which
is the unit stream power (stream power per
width), in which q is unit flow discharge, J the energy slope; S is the development degree of bed
structures; and b g is the incoming rate traveling bed load. The formula can not be established by normal
laboratory experiments because the bed structure development can not be simulated in normal flumes.
The formula may be established through experiments and measurements in mountain streams.
g is the rate of bed load transport per width; p J
J
Fig. 5.56
Dimensionless rate of bed load transport as a function of the development degree of bed structure
5.4.2
River Motion Dynamics
5.4.2.1 River Motion
Flow and sediment transportation in a river always are unsteady and non-uniform, and, the river channel
adjusts itself to fit the varying flow and sediment conditions. The adjustment of the channel is a kind of
motion of the channel. If a section of river channel and the sediment-laden flow within it are treated as a
moving body, the fluvial process is then the result of the motion of the deformable body, which is called
river motion. The patterns of river motion are aggradation, degradation, widening, translation, rotation,
wandering, bifurcation, and migration from one channel to another channel. Aggradation and degradation
are vertical movement and the rest are horizontal movements. Avulsion and shifting of the channel are
non-continuous motion and the rest are continuous motions (Wang et al., 1997). River motion dynamics
studies the laws of river motion under the action of the unsteady river flow, which differs from the
traditional mechanics of sediment movement. The latter studies the movement of sediment particles and
the former studies the motion of the river channel in the space of the sediment deposit.
Wang and Wu (2001) studied unsteady flow in fluvial rivers, especially wandering rivers, and proposed
a so-called river motion dynamics, which provides a new approach to fluvial processes. In their studies,
the capacity of the flow to move sediment from one place to other places within a river section is called
sediment-removing capacity . This differs from the well-defined sediment-carrying capacity . For instance,
steady flow carries sediment through a reach of a fluvial river channel, but does not cause a considerable
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