The observed vertical sediment concentration
distribution at Aishan and Luokou stations in the lower Yellow
River is presented in Table 3. When the average sediment
concentration is 200 kg /m3,
along the lower reach the surface sediment concentration is only 130 to 140 kg /m3, but the sediment
concentration near the river bed can reach to 300 kg /m3 or even more. The sediment grain
size of the bottom region is rather coarse, where the mean sediment size dave
is around 0.04 - 0.05 mm, which is coarser than the average grain size, say
0.03 - 0.04 mm. From this table, sediment concentration at the bottom is twice of
that at the surface. Consequently it results in the uneven distribution of flow
viscosity in vertical direction. By calculation, the viscosity of bottom flow
is almost twice of that of the surface flow. It changes the characteristics of
vertical flow velocity profile.
Table 3 Vertical Sediment
Concentration Distribution at Aishan and Luokou Stations on the Lower Yellow River
|
Station
|
Date
|
Q
|
h
|
V
|
Concentration (kg/m3)
|
Sediment Size (mm)
|
||||
|
(m3/s)
|
(m)
|
(m/s)
|
average
|
surface
(0.2h)
|
bottom
(0.8h)
|
average
|
surface
(0.2h)
|
bottom
(0.8h)
|
||
|
Aishan
|
09/05/73
|
3,000
|
3.13
|
2.4
|
200
|
140
|
300
|
0.038
|
|
|
|
08/11/77
|
3,670
|
3.89
|
2.32
|
180
|
130
|
300
|
0.036
|
0.02
|
0.05
|
|
|
Luokou
|
09/07/73
|
2,900
|
4.73
|
2.09
|
149
|
110
|
250
|
0.031
|
0.02
|
0.05
|
|
07/13/77
|
4,700
|
6.40
|
2.39
|
208
|
160
|
350
|
0.041
|
|
|
|
|
08/12/77
|
2,780
|
4.24
|
2.17
|
167
|
130
|
270
|
0.029
|
|
|
|
The relationship between vertical velocity profile and
sediment concentration is shown in Figure 3, where Kv, the ratio of
velocity at relative depth 0.2h and 0.8h is used to represent velocity profile
(Sediment data set used is the same as Figure 2).
When the sediment concentration is lower than 200 kg /m3, the value of Kv
increases with sediment concentration increasing. Kv is 1.4 for
clear water flow and 1.8 for sediment-laden flow with concentration of 200 kg/m3.
However, when the sediment concentration is higher than 200 kg/m3,
the value of Kv decreases with sediment concentration increasing. The
average value of Kv is 1.4 for sediment concentration ranging from
300 kg/m3 to 900 kg /
m3, which is same as the clear water. Value of Kv reaches its peak when
sediment concentration is 200 kg/m3, which indicates the vertical
velocity varies the most at this moment. The above conclusion agrees with the
study of Chien (Chien et al, 1980) and Zhang (Zhang et al, 1988), which states
the von Karman constant κ is the smallest when the sediment concentration is 200 kg /m3. The results above
show if magnitude of the velocity near bed is used for analysis, when sediment
concentration is 200 m3/s, the stream power on river bed is also the
smallest, which makes it the most difficult to transport sediment.
Figure 3 Velocity Ratio (Kv)
vs. Sediment Concentration (S)
4. Efficient Sediment Transport Characteristics of Hyperconcentrated
Flood in the Yellow River
By
comparing the composition of sediment from hyperconcentrated floods and that from
Loess Plateau, it can be seen that grain size of sediment from hyperconcentrated
flood is much coarser than the sediment from Loess Plateau. The grain size of sediment
from Loess Plateau ranges from 0.03 to 0.04
mm , while from hyperconcentrated flood it mainly ranges from
0.04 to 0.10 mm , and even 0.1
to 0.3 mm. The grain becomes further coarser with sediment concentration
increasing. This is because the river beds of the main stream and tributaries in
the middle reach are severely scoured during hyperconcentrated flood events, and
the eroded coarse material is brought to the lower reach with floods. Such
phenomenon also shows that the hyperconcentrated flood has great sediment
transport capacity.
The
river channel characteristics of the Weihe
River , Beiluohe
River and the lower Yellow
River are listed in Table 4. Each of these rivers has a narrow and
deep channel, and the width over depth ratio decreases with discharge
increasing. The bed slope is 0.57 x 10-4 to 5 x 10-4, and
the unit width discharge is 2 to 6 m3/s/m. The observed data shows
the Weihe River ,
Beiluohe River
and the lower Yellow River all have great
sediment transport capacity.
Table 4 River Channel Characteristics
of Weihe River，Beiluohe River and Lower Yellow River
River
|
Reach and Length
|
Slope
(10-4)
|
Channel Width
(m)
|
Width over Depth
B/h
|
Bankfull Discharge (m3/s)
|
Lintong - Tongguan (165 km)
|
0.57~4
|
80~300
|
3~6
|
1,000~5,000
|
|
Luo 17# - Chaoyi (87 km)
|
1.62~5
|
60~100
|
2~3
|
300~1,000
|
|
Aishan - Lijin (282 km)
|
1.0
|
400~600
|
3~6
|
3,000~6,000
|
The
gradation of suspended sediment of the lower Weihe
River and Beiluohe River Beiluohe River is 300 m3/s, but for Weihe River
The
vertical sediment distributions of in Huaxian and Huayin Stations of the Weihe River ,
and Chaoyi Station of the Beiluohe
River 7
m ，velocity = 0.9 - 2.4 m/s，dave = 0.04 - 0.106 mm, and d90
= 0.084 - 0.36 mm, the percentage of the sediment particles finer than 0.01 mm =
8.7 % - 22.9 %, we can not see any impact of the changes of sediment grain size
on the vertical distribution of sediment concentration, where the value of Ks
ranges from 0.87 to 0.99. This also includes the flood with coarse sediment particles,
such as d90 = 0.36 mm. Actually in alluvial river, the phenomena
that the sediment size gets coarser with sediment concentration increasing itself
already implies the powerful sediment carrying capacity of the hyperconcentrated
flow.
Table 5 Hyperconcentrated Flood
and Sediment Transport Characteristics of Weihe River and Beiluohe River
River
|
Date
|
Sediment Flow Condition
|
Morphological Change and Sediment Transport Ratio
|
|||||||
Peak Discharge (m3/s)
|
Maximum Daily-average
Discharge
(m3/s)
|
Maximum Sediment Concentration
(kg/m3)
|
Duration for S > 400 kg/m3 (hr)
|
d50
(mm)
|
Percentage for d < 0.01
mm (%)
|
Depth of Main Channel Erosion
(m)
|
Length of Channel Erosion (km)
|
Sediment Transport Ratio
(%)
|
||
Weihe
|
07/05/77~07/13/77
|
5,550
|
4,120
|
690
|
43
|
0.04~0.06
|
15~20
|
-2.5
|
165
|
97
|
08/12/64~08/17/64
|
3,970
|
1,999
|
670
|
120
|
0.03~0.04
|
14~22
|
-0.4
|
165
|
108
|
|
07/16/64~07/21/64
|
3,120
|
1,870
|
600
|
30
|
0.05~0.06
|
12~22
|
-0.5
|
165
|
120
|
|
08/02/70~08/10/70
|
2,930
|
2,250
|
800
|
24
|
0.03~0.04
|
17~25
|
-0.32
|
165
|
104
|
|
07/24/75~08/01/75
|
2,290
|
1,350
|
645
|
30
|
0.03~0.05
|
10~40
|
-0.25
|
165
|
100
|
|
Beiluohe
|
07/28/75~07/31/75
|
2,190
|
1,120
|
725
|
32 (53)
|
0.04~0.05
|
15~19
|
-131
|
87
|
90
|
08/17/71~08/20/71
|
1,100
|
504
|
885
|
79 (79)
|
0.04~0.06
|
10~20
|
-1.13
|
87
|
96
|
|
07/06/77~07/08/77
|
3,070
|
1,080
|
850
|
60 (72)
|
0.04~0.06
|
10~16
|
-3.16
|
87
|
112
|
|
07/30/69~08/02/69
|
1,290
|
504
|
880
|
81 (89)
|
0.04
|
8~16
|
-0.51
|
87
|
120
|
|
08/25/73~09/03/73
|
765
|
380
|
860
|
130 (176)
|
0.04~0.05
|
10~17
|
-1.63
|
87
|
123
|
|
08/06/77~08/09/77
|
800
|
298
|
1,010
|
84 (84)
|
0.04
|
10~16
|
-0.64
|
100
|
||
Note: Numbers in the brackets
are the durations in hours for sediment concentration higher than 300 kg/m3.
“–” means erosion, “+” means deposition.
Hyperconcentrated
floods of the Yellow River also possess high
sediment transport capacity while passing through a reservoir (Qi, 1997). For
example, for the two hyperconcentrated flood events happened in July and August
of 1977, the energy slope in the backwater region 41.2 km upstream from
Sanmenxia Dam was 0.27 x 10-4 to 0.92 x 10-4, the width
of the reservoir was 600 to 800 m, the sediment transport ratio of the
reservoir is 97% to 99%, the maximum sediment concentration of floods released
from the dam was 616 to 911 kg/m3, respectively. The maximum average
sediment size was 0.105 mm
with d90 = 0.35mm. The detailed data of both floods are listed in Table
6.
Table 6 Sanmenxia Reservoir
Sediment Release Conditions in July and August, 1977
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