Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 302

channel remained unchanged after the hyperconcentrated flood, but increased after the low concentration
flood. The deepest point of the channel reduced by about 4 m after the hyperconcentrated flood but
remained unchanged after the low concentration flood.
The width of the channel may reduce greatly during hyperconcentrated floods. For instance, the
channel width at the Huayuankou station was 2,000 - 3,000 m in early 1977; and it reduced to 700 - 800 m
after a hyperconcentrated flood in the summer (Qi and Zhao, 1984). The sediment delivery ratio may be
enhanced due to the change of the channel from wide and shallow to narrow and deep. In the beginning
of a hyperconcentrated flood, sediment deposits on the floodplain and at the two sides of the channel, the
sediment delivery ratio is low and the channel becomes narrower. If the hyperconcentrated flood
continues the channel bed is scoured deep and the sediment delivery ratio consequently increases. Table 5.3
lists the characteristics of consecutive hyperconcentrated floods in the Longmen-Tongguan reach and the
Xiaolangdi-Jiahetan reach in 1977 and 1973, respectively. In the first hyperconcentrated flood at each
location in July 1977 and August 1973, respectively, the sediment delivery ratio was only 78 and 66%,
respectively. In the second hyperconcentrated flood in Aug. 1977 and Sept. 1973, respectively, the
channel had been scoured deep and the delivery ratio increased to 101 and 124%, respectively.

Table 5.3 Characteristics of hyperconcentrated floods and variation of the sediment delivery ratio during consecutive
h yperconcentrated floods on the Yellow River

Percentage of
D <0.01mm
(%)

Sediment
delivery ratio
(%)

D 50
(mm)

Q m
(m 3 /s)

S m
(kg/m 3 )

Reach Dates

Longmen-
Tongguan

1977.7.6-7.8 14,500 690 0.04-0.05 14-20 78

1977.8.5-8.8 12,700 821 0.08-0.13 11-15 101

Xiaolangdi-J
iahetan

1973.8.28-8.31 3,840 477 0.04-0.05 15-25 66

1973.9.1-9.3 4,470 331 0.04-0.05 10-25 124

Note: Data from the Hydrological Data of the Yellow River, Yellow River Conservancy Commission

5.2.3.7 Impact on Hydraulic Characteristics
Morphological change of the river channel influences its hydraulic characteristics as well as the sediment
transport capacity of the flow. It is found that the flood propagation speed is increased and the flood
propagation time is shortened as a wide and shallow channel develops into a narrow and deep channel
during the passage of a hyperconcentrated flood through the channel. As a result, the second wave may
catch up with the first wave and cause enlargement of the crest discharge of the first flood wave in the
downstream reaches.
The propagation speed of a flood wave, U w , is a function of the channel shape and the average flow
velocity, V :

w U

V (5.57)

52
33

R B w

K (5.58)

Bz w

in which B is the width of the water surface, R is the hydraulic radius, and z is the elevation of water. For

w B

z

BB
Rz

w !!
w

a U -shaped cross section, 0 and K=5/3; for a V-shaped cross section, 0 and K is

w

w ! w and K is smaller than 1.
Hyperconcentrated floods change the channel shape from wide and shallow to narrow and deep. The
cross section becomes nearly U -shaped, and the value of K increases from less than 1 to nearly 5/3.

B B
zR

between 1 and 5/3; for a wide and shallow channel, 0