Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 435

middle reaches of the river, especially the reach from Shashi to Jiujiang. Generally, the flood volume from
the upper reaches (Yichang) accounts for 95% of the flow at the Shashi, 61%-80% of the flow at Chenglingji,
55%-76% of the flow at Wuhan (Hankou), and 54% of the flow at Datong. Floods from the major tributaries
in the Sichuan-Chongqing Basin, such as the Minjiang and Jialing Rivers often meet with the floods from
upstream of Chongqing exhibiting an obvious influence on the floods at Yichang and the middle reaches.
The duration of a large flood is quite long, 30 days at Shashi, 50 days at Chenglingji, over 50 days at Wuhan
(Hankou), Hukou, and Datong. Such a long duration of high water levels threatens the safety of the levees.
The floods on the Yangtze River may be classified into two basic types: floods resulting from heavy
rainfall in the whole river basin and floods resulting from regionally heavy rainfall. The former is the
result of consecutive heavy rainfalls in the upper and middle reaches of the Yangtze River, such as the 1931
and 1954 floods. As examples of the latter type, heavy rainfall in the upper reaches in 1870 and 1896 and
that in the middle reaches in 1935 caused extreme floods (Luo and Luo, 1996; Zhou, 1999). In the 20th
century the highest flood peak discharge occurred in 1954, but the most serious disaster was in 1931.
Flood protection of the middle reaches has a long tradition. The flood control system consists of the
3,570 km long grand levees, more than 30,000 km of levees along the tributaries, riparian lakes, and canals,
on a local scale, protective walls built by municipalities, and Tongting and Poyang lakes and several
man-made flood retention basins, as shown in Fig. 7.33. The retention basins protect cities and the grand
levees. Most important is the portion of the river flooding stored in Tongting Lake. Flood water is diverted
into the lake through the Songzi, Taiping, Ouchi, and Tiaoxuan diversion channels and flows back to the
river at Chenglingji.
Fig. 7.33 Flood defense system of the middle and lower reaches of the Yangtze River (See color figure at the end
of this topic)
The flood control system protects 126,000 km 2 of flood plain on which 75 million people are dwelling
and many industrial hubs like Wuhan, Nanjing, and Shanghai are located. Comprehensive flood control
measures were taken after the 1950s, including raising and reinforcing the grand levees to prevent