Integrated River Management - River Dynamics and Integrated River Management - page 760

11.2.3.4 Sediment Demand
The sediment demand in the Yangtze River consists of three components: (1) sediment needed for fluvial
processes; (2) sediment mining for building material; and (3) sediment utilized for land creation. The
fluvial process and lateral movement of the channel is now constrained by the grand levees in the middle
and lower reaches of the river. Sediment deposition and erosion mainly affect the longitudinal profile of
the river. In alluvial rivers there are no bed structures and natural knickpoints and the bed material
consists mainly of sand. According to the minimum stream power theory, the morphology of alluvial
rivers develops to reach the minimum stream power (Yang, 1996). This can be described by the following
equation:

d S d d s d Q

J §

·

( J sQ ) Q s 0 (11.20)

¨ ¸

dd x x d d x x

© ¹

where Q is the annual discharge, J is the specific weight of water, s is the bed slope and x is the distance
down the river. The equilibrium bed slope can then be calculated with the following formula:

s ª ' Q º

s

n

Q

n

( x x ) (11.21)

« » n 1 n n 1 n 1

Q ' x

¬ ¼ n n

For the fluvial reaches of the Yangtze River (downstream from Yichang), the discharge increases along
the course due to the inflow from tributaries; thus, the equilibrium slope decreases along the course. The
river exhibits a concave downward riverbed profile, as shown in the measured profiles in 1971 and 1982
in Fig. 11.46. The fluvial process is developing toward the calculated profile (dashed curve). The inflow
of water from the tributaries in the middle and lower reaches is reducing due to water diversion for
economic development and agriculture. Assuming a reduction in the reach of about 10%, the equilibrium
riverbed profile will be different as shown in Fig. 11.46 (solid curve). It is clear that the measured bed
profile is developing toward the equilibrium profile.

Fig. 11.46 Longitudinal bed profiles of the middle and lower Yangtze River in comparison with the equilibrium bed
profile required to reach the minimum stream power. [Bed profile data from NDN-PLA (1983)]