Vízügyi Közlemények, 2002 (84. évfolyam)

3. füzet - Nagy László: A Három-Szoros-Vízerőmű

404 Nag}' László The hydropower scheme Three-Pass by László NAGY C.E. China is very rich in flowing waters. The number of rivers of catchment basins of larger than 1000 km

2 is more than 1600. Their total length is 226 800 km and nearly the half of this length (109 000 km) is navigable. Their annual total mean flow is 2,600 km

3, which is 6.8% of the total flow of all rivers of the Earth and 20% of that of the rivers of Asia. This ranks China to the third place after Brasilia and Russia. China is the first, however, in terms of utilisable hydropower. The River Chang Jiang (Yangtze), the "long river", is the longest river in China and the second longest in the world. At the mouth its average discharge is 32,000 m

3/s, while the maximum flow is 1

00.000 m

3/s. Its sources arc in the snow-cowered summits of the Himalayan Mountains and the river flows from the west towards the east. The catchment area of the river is nearly 1.8 million km

2 and it drains most of the rivers of South-China. Its total length is 6,300 km, crosses 10 cantons and flows into the East-China Sea, not far from Shanghai. (Figure /.). The flow regime of Yangtze is fluctuating. In the winter and early spring its flow is at minimum. In the lower reaches the low water stage and flow (of 3,000 m

3/s) cause even navigation problems. In the spring the flow increases with the snow-melt but not too significantly. High waters and the danger of floods are expectable upon the monsoon rains in early August. In this period the water rises by 20-25 metres and the flow rate may reach 1

10,000 m

3/s. The floods pass away by mid-September and in November the water becomes clear again, characteristic to the low flow period (Figure 2.). The Mountain Wu (Witch) is located at the border of the Sichuan and Hubei cantons in the NE-SW direction. The flow direction of Yangtze first follows the mountain, then cuts it through, turning to the south, forming this way the Three-Pass at about 1,600 km distance from the river mouth. The passes of Qutang, Wu and Xiling form the Three-Pass. The total length of these passes, together with the smaller ones, is 204 km. Beyond the lower pail of Three-Pass begins the China plains, which is very sensitive to the regular flooding of the Yangtse. There is a wide flood-prone area along the river between Yichang and Nanking, with no higher elevated area until the Sea. The Hydropower scheme Three-Pass is of three-fold importance, not only because it is based on the three-passes. The location of the dam was selected from among 14 other alternatives at Sandouping on the pass Xiling. At the dam site the geological conditions are very favourable ones. The rock material of the channel is solid granite and the topographic conditions arc also suitable to build a high dam and the power station. The hydropower scheme was designed for multiple purposes, that are all important for China: — The probably most important one is power production; the utilisation of the renewable energy of the river; — This scheme will considerable improve navigation in the upper and middle reaches of the Yangtze, allowing the reaching of Chongquing by 10,000 ton vessels; — Flood safety upstream of the dam will correspond to the 1000 year return flood, while that of the downstream part will be improved to the 100 year flood, from the present 10 year flood level. The flood safety will also be improved in the vicinity of the Lake Dong Ting; — The presently catastrophic air pollution caused by the coal fuelled power plants of very