Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 446

It is understood that for a 100-year flood (1% probability of exceedance in a given year) the city of
Chongqing would not tolerate a stage higher than 200 m. This stage, however, would be reached only if a
100-year flood should occur after the reservoir had been in operation for about 100 years. The chance is
rather remote. Moreover, during this intervening period of 100 years, the Xiangjiaba and the Xiluodu
Reservoirs will be built upstream of the TGP. The operation of these reservoirs will have the permanent
benefit of reducing peak discharges entering the TGP, which alone is sufficient to reduce the flood stages
at Chongqing. In addition, following the commission of the reservoir, there will be a rather long period
(10th-80th years of operation) during which the sediment influxes into the TGP will diminish. Reduction
in deposition in the TGP reservoir will lead to a reduction in flood stages along the upstream reaches. The
permanent storage capacity of the upstream reservoirs can regulate the flow in low flow season. This will
result in an increase of the minimum discharge entering the TGP reservoir during the low flow season.
With this increase more sediment will be scoured from the upper part of the backwater region, and for
the same discharge lower flood stages at Chongqing may be expected.
Two major problems are to be addressed in the study of sedimentation in the fluctuating backwater
region, namely: 1) whether sedimentation would result in a channel suitable for navigation of 10,000 t
tows, and 2) whether the areas around the piers of shipping terminals at Chongqing could be free of
deposition detrimental to shipping operations.
Regarding the first problem, studies were carried out with physical models. Five sections are critical
for the influence of sedimentation on navigation. Nine physical models were built and scale model
experiments were conducted. The results show that during the normal pool level period, the 660 km long
navigation channel from Yichang to Chongqing would much improved. For all schemes investigated the
minimum requirement of a channel 3.5-m deep, 100-m wide and with a radius of curvature of 1000 m for
the passage of 10,000 t tows can generally be satisfied. Some dredging and training works may be needed
here and there along the channel, but they will be generally of minor scale, because in May and June
navigation problems may occur in a few broad and shallow sections when the pool level has been drawn
down and sediment deposited in the channel has not yet been scoured away.
The problem of deposition around the shipping terminal—the port of Chongqing—is the center of
attention. This deposition varies with the selection of the pool levels for the project and also with the
operation schedule of the reservoir. Sediment will deposit at the apron of the Jiulongpo Harbor (Fig. 7.43),
which is the biggest freight harbor in southwest China, and may even result in shifting the channel from
left to right after 80 years of operation of the dam. Chaotianmen Harbor is the largest passenger harbor in
Chongqing (Fig. 7.43).
Sediment accumulates in the Chaotianmen and Jiulongpo Harbors. After 50 years these harbors could
not be used if dredging or other technical measures preventing sedimentation were not taken. The studies
also proved that these problems can be solved by building spur dykes and groins to regulate the flow. The
test demonstrated that the dykes and groins would narrow the channel and concentrate the flow, so that
flow velocity in the navigation channel and harbor area can be enhanced to prevent sediment from
depositing.
Some concern has been expressed regarding the possible consequences of deposition of gravel bed
load in size range of 1-10 mm at the upper end of the fluctuating backwater region. Investigation shows
that all the coarse sediment could be mined or dredged for building material and causes no accumulative
sedimentation.
7.4.2.3 Sedimentation in the Vicinity of the Dam
Navigation in the TGP reservoir is unique in that the channel used by the ships is to be gradually shaped
by the flow and sedimentation in a long reservoir. The initial equilibrium of the channel would come in