Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 448

Fig. 7.44 Layout of the upper and lower approach channels, the shiplift, and sluice gate
Undesirable deposition in the approach channels could be cleared off by flushing, complemented by
dredging. Regarding the possible hindering of navigation by deposition, the critical time for the upstream
approach channel is the flood season, whereas that for the downstream approach channel is the dry season.
As to the downstream approach channel, the tailwater level during flood seasons ranges from 66.8 to
73.7 m, depending on the operation of the Gezhouba Dam, which is 38 km downstream of the Three
Gorges Dam, while the bed elevation is 56.5 m. According to research, a part of the sediment flushed
from the upstream approach channel would deposit in the downstream approach channel but would not
encroach upon the minimum depth of 4 m required for navigation. At the end of the flood season, the
water level may be lowered to around 63 m by operate the Gezhouba Dam. Under this low tailwater,
deposition in the main parts of the downstream approach channel may be flushed out with the same
discharge available for the flushing of the upstream approach channel. Dredging would then be applied
to clear out the remnant deposition. Dredging is also required in a part of the river downstream of the
lock approach.
7.4.2.4 Degradation of the Downstream Reaches
In the downstream reaches of the Three Gorges and Gezhouba dams, released clear water will scour the
riverbed. The monthly mean discharge released from the Three Gorges Reservoir to the downstream
reaches is higher than that before the dam from January to May but is less from October to November. In
the flood season from July to September the released discharge will remain unchanged. Generally speaking,
the sediment-carrying capacity of the flow does not change in the downstream reaches. Figure 7.45 shows
the variation process of annual sediment load released from the reservoir to the downstream reaches
compared with the annual sediment load at Yichang under natural conditions (recycle of the record in the
period 1961-1970). The sediment load to the downstream reaches is greatly reduced by the trapping in the
reservoir in the first 50 years. The load reduction must cause degradation of the downstream reaches.
Two 1-D numerical models, IWHR and YVPO, are used to calculate the amount of sediment scoured
from the river bed in the reach from Yichang to Wuhan. In the first 40 years the two models yield the
same results, about 2.5 billion t at the end of the 20 th year and 4 billion t at the end of the 40 th year. From
the 50 th year, however, the two models give different results. The YVPO model predicts the resiltation of