Under
the long term natural evolution as well as the human’s activity, different
river regimes and channel types have been developed along the 800 km long lower Yellow
River from Tiexie to the river mouth (Qi et al. 2002). Consequently,
each river reach develops its own channel evolution and sediment transport
characteristics. The Yellow River in the 300 km long reach from Tiexie to
Gaocun with a bed slope of 0.0027 to 0.0017 has a typical wandering, “wide and
shallow” channel. The Yellow River in the 165
km reach from Gaocun to Taochengpu has a transition type channel with a bed slope
of 0.0017 to 0.0011. The Yellow River in the 460
km reach from Taochengpu to the river mouth with a bed slope of 0.001 has a meandering,
“deep and narrow” channel. Figure 6 shows the longitudinal sediment
concentration variations at different stations along the lower Yellow River for five flood events with highest sediment
concentrations in the past years. The sediment concentration decreased sharply from
220～320
kg /m3 to 80～150 kg /m3 along the “wide
and shallow” channel upstream of Gaocun. However, the average sediment
concentration even increased a bit, in the 300 km “deep and narrow” channel downstream of Aishan
with a mild slope. Such phenomenon shows the powerful sediment transport capacity
of the hyperconcentrated flow.
As
shown in Table 7, sediment transport ratio from Aishan to Lijin (0.0001 bed
slope) is 0.97 to 1.04 for three flood events with high sediment concentration
in year 1973 and 1977, and there was no sediment deposition for the flood with
discharge of 3,000 m3/s and maximum sediment concentration of 200 kg/m3.
Although as sediment concentration increases, sediment particles are easier to
be suspended; in order to convey hyperconcentrated flow smoothly, it is critical
to keep the flow in turbulent region and avoid unsteady transient flow or
intermittent flow. Thus, under a given river channel condition, there is an upper
limitation of concentration for efficient sediment transport.
Figure 4 Sediment Concentration
of Hyperconcentrated Floods Varies along the Lower Reach for Different Events
Since
in most cases the hyperconcentrated flow in natural channel is fully turbulent,
and possesses the same friction characteristics as clear water, its roughness
can be calculated by using Manning’s formula. The water depth and velocity of lower
Yellow River were calculated by using water
depth and velocity relationship established for low sediment-laden flow, and the
result is listed in Table 8. It can be seen in Table 8, under the same
discharge, the water depth and velocity at Lijin Station are smallest. Thus,
the flow condition of Lijin Station is chosen as control factors for
hyperconcentrated flow.
Supposing
that the minimum discharge to transport sediment Qmin = 3,000 m3/s,
the maximum sediment concentration Smax = 800 m3 /kg, d50 = 0.036 mm, the
percentage of sediment with d < 0.01 mm is 20 %. By using the formula of Xiangjun
Fei (Fei, 1993), the shear stress tB = 0.33 kg/m2，viscosity m = 0.00093 kg×s/m2，the density of the sediment flow ρ = 153 kg·s2/m3,
the Reynold’s number = 3.03 x 106, which is much higher than the
critical Reynold’s number Re = 2,000 for fully turbulent flow. From the
calculation above, it is proved that the flow with discharge of 3,000 m3/s
and sediment concentration of 800 kg /m3
is fully turbulent, and the sediment-laden flow with concentration as
high as 800 kg /m3
can be transported properly in to the sea as long as the discharge is kept at 3,000
m3/s or more.
In
conclusion, under the current channel condition of the lower reach, if the flow discharge is kept from 2,000 to 3,000
m3/s, not only floods with sediment concentration lower than 200
kg/m3, but also floods with concentration 400 to 500 kg/m3
can be transported. The flood with maximum sediment concentration of 800 kg/m3
can be properly conveyed to the sea.
Table 7 Sediment Transport Statistics of Hyperconcentrated Floods from Aishan Station to Lijin Station of the
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
|
||
Table 8 Variations of River
Bed Roughness at Different Discharge Levels
Discharge (m3/s)
|
500
|
1,000
|
1,500
|
2,000
|
3,000
|
4,000
|
5,000
|
6,000
|
|
H (m)
|
1.5
|
1.8
|
2.0
|
2.5
|
3.2
|
4.0
|
4.5
|
5.0
|
|
Aishan
|
V (m/s)
|
1.0
|
1.5
|
1.8
|
2.0
|
2.5
|
2.8
|
3.0
|
3.2
|
H (m)
|
2.5
|
2.5
|
2.6
|
3.4
|
4.6
|
6.0
|
7.0
|
8.0
|
|
Luokou
|
V (m/s)
|
1.0
|
1.5
|
1.9
|
2.1
|
2.35
|
2.5
|
2.7
|
2.8
|
H (m)
|
1.5
|
1.8
|
2.0
|
2.3
|
2.8
|
3.4
|
3.8
|
4.2
|
|
Linjin
|
V (m/s)
|
0.9
|
1.5
|
1.8
|
2.0
|
2.2
|
2.4
|
2.5
|
2.6
|
n
|
0.014
|
0.01
|
0.009
|
0.009
|
0.009
|
0.009
|
0.010
|
0.010
|
|
Because
the river channel of lower Yellow River is
relatively wide in the upstream part and narrow in the downstream part as shown
in Figure 7, the channel presents the following sediment transporting
characteristics during the flood with high sediment concentration. Although the
main channel can discharge more sediment as the flood discharge increases, when
the flood spills out of the channel onto the floodplain, huge amount of sediment
will deposit there by overbank flow. Thus, in order to make full use of the
channel to transport sediment by hyperconcentrated flow, the critical issue is
to transform the wide and shallow meandering channel into deep, narrow and stable
channel.
6．Conclusion
In
most cases, the hyperconcentrated flow in a natural river is fully turbulent.
It possesses the same resistance characteristics as the low sediment-laden flow,
and can be calculated by using Manning’s formula. The main reason for high
sediment transport efficiency of hyperconcentrated flow is that the flow viscosity
increases with the sediment concentration increasing, which makes particle fall
velocity decrease dramatically, while the roughness of the river bed remains
unchanged.
As
the sediment particles of the Yellow River are
rather fine, e.g. d50 = 0.03 - 0.10 mm, sediment concentration distribution becomes
more uniform in vertical direction as the sediment concentration increases. When
the sediment concentration is higher than 200 kg /m3, the S - v3/ghω0
relationship appears a reverse tendency.
From
the analysis of the influence of sediment concentration on flow structures, and
the relationship between sediment concentration and flow velocity profile in
vertical direction, it is concluded that the sediment is the most difficult to
be transported by the channel when concentration is around 200 kg/m3.
This has also been proved by various observed flow data. The hyperconcentrated
flood has high efficient sediment transporting capacity.
The
recorded maximum sediment concentration downstream of Aishan in the lower Yellow River is 200 kg/m3. However, due to
high sediment transport capacity of the hyperconcentrated flow, a flow with
concentration as high as 800 kg /m3
can be conveyed through the channel into the sea when the flow discharge is 2,000
m3/s to 3,000 m3/s. The key point is to transform the
existing wide and shallow channel into deep, narrow and stable channel.
