Environmental Engineering Reference
In-Depth Information
River to the Yellow River water and sediment load remain unchanged. The majority of the sediment load
consists of silt with a median diameter of about 0.03 mm. Before the impoundment of Sanmenxia Dam
the Weihe River carried 386.6 million tons of sediment into the Yellow River annually and the Weihe
River itself remained a relatively stable longitudinal bed profile.
T
able 6.9
Water and sediment load of the Yellow and Weihe Rivers
Distance to
the Yellow
River mouth
L (km)
Average sediment
concentration
(1960-2001)
(kg/m
3
)
Annual
sediment load
(1986-2001)
(mil. tons)
River/
Hydrologic
station
Annual runoff
(1960-2001)
(bil. m
3
)
Annual sediment
load (1960-2001)
(mil. tons)
Annual runoff
(1986-2001)
(bil. m
3
)
Weihe/
Huaxian
1177
6.79
312
46.04
4.66
248
Yellow/
Tongguan
1092
34.61
1043
30.13
25.16
722
Yellow/
Sanmenxia
996
34.69
1009
29.09
24.62
712
Yellow/
Huayuankou
734
37.44
910
24.20
25.88
610
Yellow/
Aishan
374
33.07
770
25.00
19.16
440
Yellow/
Lijin
100
28.56
700
36.80
13.56
350
The elevation of Tongguan or Tongguan's Elevation is defined as a flood stage corresponding to a
discharge of 1,000 m
3
/s at the Tongguan Hydrological Station on the Yellow River, which acts as the base
level of the bed profile of the Weihe River. Before Sanmenxia Dam Tongguan's Elevation was about
323.5 m. Since impoundment of Sanmenxia Reservoir, sediment has been depositing in the reservoir,
which causes Tongguan's Elevation to increase. The energy slope and sediment carrying capacity of the
flow in the Weihe River have been reduced. The sediment load could not be transported into the Yellow
River and sedimentation occurred in the lower Weihe River. In other words, the rising Tongguan's
Elevation has changed the lower boundary of the Weihe River, thence inducing a new cycle of fluvial
processes.
Figure 6.45 shows the variations in Tongguan's Elevation over time from 1960 to 2001. There were
three ascending periods, denoted by I, II, and III, and two descending periods, denoted by 1 and 2. The
abrupt rise and fall in 1960 and 1962 were caused by the impoundment in 1960 and change of the
operation mode from storage to flood detention. The time of high elevation (329 m in Fig. 6.45) was short
and its influence on the Weihe River sedimentation was temporarily, although it caused an obvious flood
stage rise in 1961. Therefore, the period of 1960-1962 is not separated from the ascending period I.
The ascent and descent of Tongguan's Elevation were results of reservoir sedimentation and erosion,
which in turn were caused by variations in the pool level of the reservoir. Generally speaking,
sedimentation in the lower Weihe River occurred during the periods when Tongguan's Elevation rose,
and erosion occurred during the periods when it fell. The total volume of sediment deposited in the lower
Weihe River up to the year 2001 was about 1.3 billion m
3
. The sedimentation was distributed mainly in a
100 km long reach from the confluence. The accumulated deposition volume per unit length was high
near the confluence, reduced upstream, and to nearly zero near Xi'an. Figure 6.46 shows the transect of
the profiles of the channel bed and floodplain in the lower Weihe River measured in 1960 and 2001 at the
cross-sections WY-2 and WY-7, which are 21 and 59 km from Tongguan, respectively. The floodplain
elevation had risen by 3 to 5 m due to sedimentation, and the main channel had shrunken and become
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