Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 303

Moreover, the average velocity also increases due to the increase in the average depth or hydraulic radius.
Thus, the second flood wave may travel much faster than the first flood wave. Table 5.4 lists three
hyperconcentrated floods that occurred in 1977 and three hyperconcentrated floods that occurred in 1973.
In 1977, the propagation speed of the first flood wave through the Longmen-Tongguan Reach was only
2.8 m/s and the speed increased to 3.61 m/s for the second flood wave and further to 4.81 m/s for the
third flood wave. In the Xiaolangdi-Huayuankou reach in 1977, the propagation speed also increased
from 1.25 m/s of the first flood wave to 2.26 m/s of the second flood wave. The same phenomenon
occurred for the hyperconcentrated floods in 1973. The propagation speed through the Xiaolangdi-
Huayuankou reach increased from 1.07 m/s for the first flood wave to 1.6 m/s for the second and third
flood waves.

Table 5.4 Characteristics of hyperconcentrated floods and increase in propagation speed of hyperconcentrated
flood waves in the Yellow River

Propa-
Gation
time
(hours)

Propa-
Gation
speed
(m/s)

S m 1)
(kg/m 3 )

Q m 1)
(m 3 /s)

S ave 1)
(kg/m 3 )

Q out /
Q in

Reach Time Notes

Longmen
-Tongguan

1977.7.6,17:00 -
1977.7.7,6:00

14,500
13,600

575
615

690
616

Channel erosion and siltation
on the floodplain

0.93 13.0 2.80

Longmen
-Tongguan

1977.8.3,5:00 -
1977.15:00

13,600
12,000

145
185

551
235

Channel erosion and siltation
on the floodplain

0.88 10.0 3.61

Channel erosion and siltation
on the floodplain
S m is 8.5 hr ahead of Q m at
the upstream station and 0 hr
at the downstream station

Longmen
-Tongguan

1977.8.6,15:00 -
1977.23:00

12,700
15,400

480
911

821
911

1.21 7.5 4.81

Xiaolangdi-
Huayuankou

1977.7.8,15:30 -
1977.7.9,19:00

8,100
8,100

170
450

535
546

Channel erosion and siltation
on the floodplain

1.00 28.5 1.25

Channel erosion and siltation
on the floodplain S m is 1 hr
ahead of Q m at the upstream
station and 3 hr ahead at the
downstream station

Xiaolangdi-
Huayuankou

1977.8.7,21:00 -
1977.8.8,12:42

10,100
10,800

840
473

941
809

1.07 15.7 2.26

Xiaolangdi-
Huayuankou

1973.8.27, 1:42 -
1973.8.28, 11:00

4,320
4,710 2)

1.10
(1.00)

110
120

110
150

Channel erosion and siltation
on the floodplain

33.3 1.07

Channel erosion and siltation
on the floodplain S m is 22 hr
ahead of Q m at the upstream
station and 22 hr ahead at the
downstream station

Xiaolangdi-
Huayuankou

1973.8.30, 00:00 -
22:00

3630
5020

1.38
(1.30)

360
230

509
450

22.0 1.60

Channel erosion and siltation
on the floodplain S m is 2 hr
ahead of Q m at the upstream
station and 2 hr behind at the
downstream station
Note: 1) The upper figure is measured at the upstream station and the lower figure is measured at the downstream
station. 2) The discharge flowing into the reach from tributaries during Aug. 27-28, 1973 was 400 m 3 /s; 3) The figure in
the parentheses is Q ' out / Q in , in which Q ' out = Q out - Q ' in and Q ' in is the discharge from the tributaries into this reach.
The discharge flowing into the reach from tributaries during Aug. 30-Sept. 2, 1973 was 300 m 3 /s

Xiaolangdi-
Huayuankou

1973.9.2,12:00 -
1973.9.3,10:00

4400
5890 3)

1.34
(1.27)

325
330

338
348

22.0 1.60

The second consequence of the morphological process during hyperconcentrated floods is enlargement
of the crest discharge in the downstream reaches. For low concentration floods, the crest discharge reduces