Environmental Engineering Reference
In-Depth Information
downstream from the dam. About 2.31 billion tons of sediment had been eroded from the riverbed in the
first 4 years since the closure of the dam. In the following 9 years, however, the reservoir changed its
operation mode from storage to detaining flood water and sluicing sediment, and the downstream
channel was resilted at a high rate, with a total volume of sediment deposition of about 3.95 billion tons
(Yang et al., 1994). The erosion and resiltation occurred both in the stream channel and on the floodplain,
with roughly 60% in the channel and 40% on the floodplain.
The lower Yellow River was a wandering river although it has been confined within the strong grand
levees, which are on the two sides of the river 5-25 km apart from each other. The migration rate of the
channel was quite high. The closure of Sanmenxia Dam did not change this situation. The river migrated
at high speed with a maximum value of more than 5 km/yr. Even during the period immediately following
the closure of the dam, when clear water was released into the reach, the channel migrated more than 3 km
per year.
Generally, dams tend to cause a reduction in migration rates in the downstream reaches. For instance
the closure of the Danjiangkou Dam on the braided Hanjiang River caused an initial reduction in bank
erosion intensity from about 25 m/yr during 1955-1960 to about 7.0 m/yr during a period of 17 years
immediately after the dam closure (Xu, 1997). The lower Yellow River did not respond the dam closure
with reduced channel migration because of the specific features of reservoir operation.
Sanmenxia Reservoir has caused the lower Yellow River to change from a wandering-braided into a
wandering-single thread channel. Figure 6.51 shows the channel morphology of the Tiexie-Peiyu reach,
which is about 157-189 km downstream from Sanmenxia Dam, before and after the construction of the
dam (Yang et al., 1994). There were many sand bars before closure of the dam; the number of bars had
decreased 3 years after the dam was used for impoundment. The river had become a single thread
channel by 1964.
Fig. 6.51
Channel morphology of the Tiexie-Peiyu reach (157-189 km from Sanmenxia) pre- and post-Sanmenxia Dam
Meanders have generally developed after Sanmenxia Dam. The reach from the dam to Tiexie (0 to 157
km directly below the dam) is constrained by mountains and no meanders develop within it. A statistics
is made for a 400 km long reach, from 150 km to 550 km below the dam, which was an active fluvial
reach. Before the impoundment of the dam there were only 16 meanders in the 400-km long reach and
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