Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 362

tributaries and outflow by water diversions along the course from Xiaolangdi to Lijin. From 1960-1969
there was more water flowing through Lijin than Xiaolangdi because the water diversion was less than
the inflow from tributaries. From 1970-1985, the annual runoff at Lijin was equal to or slightly less than
at Xiaolangdi because more water had been diverted. From 1986 to the present, however, the total
volume of water diverted was much more than the inflow from tributaries, and the water runoff
decreased along the course. The annual runoff was about 11 billion m 3 less at Lijin than at Xiaolangdi.
The reduction in runoff over a long stretch of the river elicited a sharp reduction in the flow's
sediment-carrying capacity. Therefore, the annual load was much less at Lijin than at Xiaolangdi during
1986 to the present.
From 1986, water and sediment load increases along the course and reach their maximum values at
Huayuankou, and then reduce further downstream due to diversion. The sediment load at Lijin is less
than that at Sanmenxia by more than 300 million tons, which must have deposited in the reach between
Sanmenxia and Lijin and consequently changed the morphology of the river.
One of the impacts of the runoff reduction on the fluvial processes was the shrinkage of the channel.
Figure 6.32 shows the bank full discharge of the lower Yellow River during different periods. Water
diversion has reduced the discharge and sediment-carrying capacity, and sediment has been deposited in
the channel, which has made the channel shallow and unstable. As a result, the bank full discharge has
decreased steadily. The bank full discharge was about 9,000 m 3 /s in 1958 and 1964; it decreased to about
6,000 m 3 /s in 1985, and to only 3,000 m 3 /s in 1999.

Fig. 6.32 Bank-full discharge along the lower Yellow River course during different periods

The second important impact of water diversions is the adjustment of the riverbed profile. Field
evidence from natural streams shows that variations in successive processes and forms result from a
system's tendency to minimize the rate of energy dissipation with time (Simon, 1992). For alluvial river
with bed material consisting of sand and silt, no bed structures except for sand dunes can develop, the
river morphology depends mainly on the stream power. According to the minimum stream power theory
(Yang, 1996), the bed gradient develops to reach the minimum stream power, thus:

d P d d s d Q

J §

·

(6.2)

( J sQ ) Q s 0

¨ ¸

dd x x d d x x

© ¹

in which P is the stream power in ton/s, J is the specific weight of water in ton/m 3 , s is the riverbed
slope, x is the distance along the river course in km, and Q is the discharge in m 3 /s. For most rivers, the
discharge increases along the course due to the inflow from tributaries; thus, the term s d Q/ d x is positive.
According to equation (6.2), the term Q d s/ d x must be negative, or the slope of the riverbed decreases