Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 436

overflows and the creation of flood diversion areas. Flood storage is increased through reservoirs on the
tributaries and flood diversion basins, e.g., the Jingjiang and Dujiatai diversion basins. The most important
flood diversion basin, the Jingjiang Polder with a surface area of 920 km 2 and storage capacity of 6
billion m 3 , was built in 1952 and was inundated during the great flood of 1954.
In 1998 a flood occurred on the Yangtze River, which resulted in serious flood damages and affected 8
million people. Heavy rainfall occurred in the area in the summer of 1998 in the wake of the strongest El
Nino phenomenon of the century, which reached its maximum at the end of 1997 and ended in May 1998.
The rainfall caused floods on the Yangtze River and many of its tributaries. About three quarters of the
flood volume at Wuhan was from the reaches of the river upstream of Yichang and the rest was from the
Hanjiang River and Tongting Lake. The recurrence period of the 1998 flood is only 8 years in terms of
the peak flood discharge. The runoff volume of the 1998 flood, however, was larger than that of 1931,
but smaller than that in 1954. It was the second largest flood by volume in the 20 th century, after the 1954
flood (Ministry of Water Resources, 1999). The flood stages in the middle reaches of the river were even
higher than those in 1954 although the flood peak discharge and the total runoff volume were smaller
(Zhou, 1999).
The 1998 flood caused the strategy of flood control for the river to be rethought. The new strategy
rests on the following actions: construction of the Three Gorges Reservoir; reinforcing and increasing the
height of the levees; back conversion of some polders into river channels; increasing the size of Tongting
Lake by returning farmland to the lake area to increase its flood detention capacity; dredging the main
channel of the river; moving people from flood detention polders; and reclamation and reforestation in
the upper reaches of the catchment.
The main reason behind the serious flood threat in the middle and lower reaches of the Yangtze River
is the high peak and large volume of the floodwater from torrential rainfall in the upper reaches when it
exceeds the safe discharge capacity of the river. Currently, the safety limit for flow capacity along the
Jingjiang section (including the flow towards Tongting Lake) is about 60,000 m 3 /s, while for the Wuhan
and downstream Hukou sections, it is 70,000 and 80,000 m 3 /s, respectively. Materials gathered since 1877
show there have been 24 floods at Yichang with peak flows above 60,000 m 3 /s. In the 850 years since
from 1153, there were eight floods with peak flows larger than 80,000 m 3 /s and five topped 90,000 m 3 /s.
During the 1860 and 1870 floods at the Zhicheng Station, the peak flows reached 110,000 m 3 /s, obviously
far exceeding the safe flow capacity of the river.
Presently Tongting Lake is still an important natural flood-detention basin in the middle reaches of the
Yangtze River. During the flood season, floodwaters from the Jingjiang River account for one-third or
one-fourth of the water entering Tongting Lake. The natural flood-detention lake can reduce the peak flow
of the Jingjiang River by about 10,000 m 3 /s. It is obvious that its role of regulation and detention remains
of great significance for flood control. Tongting Lake, however, is shrinking quickly due to sedimentation
and land reclamation. Based on the research of specialists, it is necessary to apply comprehensive flood
mitigation measures to solve the serious flood control issues in the Jingjiang reach of the Yangtze River,
including the heightening and reinforcement of levees, arrangement and setting up of flood diversion and
detention areas, building reservoirs on the mainstream and tributaries, and realignment of rivers, as well
as improved flood forecasts. The most crucial of these is the building of the Three Gorges Project.
The Three Gorges Project is sited at the place, where the Yangtze River's middle and upper reaches
meet. Its unique location and topographical conditions as well as its reservoir with a flood control storage
capacity of 22.15 billion m 3 will enable it to effectively control floods resulting from heavy rains in the
upper drainage areas. The project's flood prevention role would be decisive in the Jingjiang reach, and it
would also play a fairly good role in controlling floods originating over the whole valley and the middle-
lower reaches. The Three Gorges Reservoir raises the flood control capacity, prevent breaches of the