Integrated River Management - River Dynamics and Integrated River Management - page 753

trapped by dams and weirs in the upstream reaches of the Rhine River and the river bed was incised by
flood water. In order to prevent channel bed incision, German engineers have been feeding gravel to the river
at a rate about 170,000 t/yr since 1978 (Kuhl, 1992). Nevertheless, engineers have difficulty to find enough
gravel and the economic cost has become very high. According to the law of equivalency, the bed load may
be replaced by bed structures. If a bed structure creates enough resistance the bed incision may be controlled
and the water stage may remain stable.
The equivalency law may also be applied to control the riverbed incision of the Yangtze River resalting
from the impoundment of the Three Gorges Reservoir. The middle and lower reaches of the Yangtze River
have been scouring down by more than 10 m because most sediment has been trapped by the reservoir. The
flood stage in these reaches has also been reducing. Figure 11.41 shows the measured bed cross sections
of the middle Yangtze River at Baiyanghe, which is 88 km downstream from the Three Gorges Dam. The
reservoir began to fill in 2003 and since then the middle reaches of the Yangtze River has been incising
down. Compared with the cross section in 2002 the river bed has incised more than 10 m by 2008. The
bed incision will continue if no measures are taken to control it.
Fig. 11.41 Measured bed cross sections of the middle Yangtze River at Baiyanghe, 88 km downstream from the
Three Gorges Dam (data from Sedimentation Panel, 2009)
A serious consequence of the riverbed incision in the middle reaches of the Yangtze River is the
reduction of flood water diverted into Tongting Lake. The connection between the river and the Tongting
Lake will be cut off or partly cut off, which may greatly impair the ecosystem. Tongting Lake has regulated
the flood flow in the Yangtze River and has played an important role in flood control. Flood water flowed
into the lake through Songzikou Channel, Taipingkou Channel, and Ouchikou Channel and flowed back
to the river at Yueyang. Many fish species migrate from the great lake to the river for spawning and
juvenile fish swim into the lake for feeding and growing. Nevertheless, the water and sediment diverted
into Tongting Lake has been reducing due to riverbed incision, which was caused by cut-off of meanders
in the 1960s, operation of the Gezhouba Reservoir since 1980, and filling of the Three Gorges Reservoir
in 2003.
The problem riverbed incision may be solved by using bed structures. According to the law of
equivalency of bed structure and bed load, the bed incision of the Yangtze River can be controlled by
using roughness elements, such as tetrahedral frames. Tetrahedral frames were first introduced for
protection of riverbanks against erosion. The tetrahedral frame is essentially hollow space frame consisting
of 4 equilateral triangles. Experiments have showed that the velocity may be reduced by at least 50%
around a tetrahedral frame in river flow, thus effectively controlling bed erosion (Tang et al., 2009). The
velocity distribution can be changed and the velocity around the frame in the vicinity of the bed can be