Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 320

volume of sediment to be scoured or deposited. The sediment-removing capacity of the flow is nearly
zero in this case, but the sediment-carrying capacity may be high. The speed of motion of a river channel
depends on the sediment-removing capacity of the flow. The higher the sediment-removing capacity, the
faster the river channel moves.
The speed of channel motion depends not only on the sediment-removing capacity of the flow but also
on the composition of the bed and bank material, which is characterized by the bed inertia (Wang, 1999).
If the bed material is liable to be removed, or exhibits low bed inertia, the riverbed is quickly deformed
to fit the changing discharge when a high flood occurs. On the other hand, if the inertia of the riverbed is
high and the flow discharge sharply increases or decreases, the bed sediment cannot be quickly scoured
and the channel does not move so much. For a given river section, if the sediment-removing capacity can
be measured or calculated directly, the speed of motion of the channel can be obtained.
The channel motion is also controlled by restrictions of hardened banks and human built structures,
like groins and spur dykes. In a channelized river or an artificial canal, considerable channel motion does
not occur because there is no space for the channel to move. Therefore, the intensity of channel motion is
a function of the sediment-removing capacity, riverbed inertia, and restriction caused by banks and
human structures.

Rf RI

*
(,

, s ) (5.62)

s b

where R s is the intensity of channel motion, *

R is the sediment-removing capacity of the flow, I b is the
riverbed inertia and Bs represents the restriction of hardened banks and human structures. The higher the
sediment-removing capacity of the flow and the smaller the riverbed inertia, the higher will be the intensity
of channel motion. The parameter of hardened banks and human structures, Bs, confines the space of the
channel motion.
The equation of motion (5.62) has not been developed and intensive work is needed to construct the
equation of motion and validate it with sufficient data. In the following sections the work done by Wang
and his colleagues on the intensity of channel motion, sediment-removing capacity, and riverbed inertia
are presented, from which researchers may draw their inspiration for continued development of the
subject.

5.4.2.2. Sediment-Removing Capacity
Channel motion is a result of sediment deposition, bed scour, and bank erosion. The intensity of channel
motion is defined as follows:

VV

scour dep

R (5.63)

s

LT

where R s is the intensity of river motion, V scour and V dep are the sediment volumes scoured from the bank
or the bed and deposited in the channel in the time period, T , respectively, L is the length of the measured
river section, and T is the time interval of the measurements. In many cases the cross sections of the
Chinese rivers are measured once a year, thence T 1 year. The measured R s depends on the frequency of
the measurement because the river motion in many cases is reciprocating.
Generally, channel motion is realized by bank erosion on one side and deposition on the other side of
the channel, or vice versa. The speed of the channel motion is then given by

R

U

s

(5.64)

c

2 h

in which U c is the speed of channel motion, and h is the depth of the channel. If the intensity of river
motion is known, the speed of river motion can be calculated using Eq. (5.64).
If there is no restriction due to human structures, the intensity of river motion may reach its maximum