L0NDON: The first proposals for a dam across the Yangtze were made in 1919, but it wasn’t until 30,000 people were killed in the floods of 1954 that Mao Zedong called in Russian engineers to help design it. Mao wanted something that would not only protect people living along the river, but would generate electricity.

In 1970, the Chinese government gave the go-ahead for a dam at Gezhouba to provide electricity for the region, for part finance for the Three Gorges project, and for engineers to be given a chance to work through any potential pitfalls in building the bigger dam.

Twenty-four years later, work started on the Three Gorges dam itself. It is the largest hydro-electric project in the world. When it is completed in 2009, the dam — costing an estimated $24bn — will hold back a 600km-long reservoir and, using 26 giant turbines, generate up to 18,000 megawatts of power — equivalent to 30 coal-fired stations (but obviously without the equivalent emission of greenhouse gases).

Ian Cluckie, a professor of hydrology at the University of Bristol, England, who has recently returned from visiting the dam, says it has three principal functions: hydropower, flood mitigation and navigation. The project consists of a 2.3km-wide dam with a spillway at the centre, powerhouses (which contain the turbines) on either side, and new navigation locks.

Water began to fill up the reservoir in June this year and the current depth is 135 metres. This will rise to 175 metres, giving it a storage capacity of 39.3bn cubic metres of water when the dam is completed in 2009. At that stage, the reservoir will have an area of more than 1,000 sq km, 630 sq km of which will be newly-inundated land.

The spillway is part of the dam’s flood defence. “The Yangtze floods, on average, once every 10 years in a big way,” says Cluckie. “The central spillway is designed to carry safely past the dam flows of up to 117,000 cubic metres per second.” This is a phenomenal amount of water — at full flow, the Thames at London, for example, only flows at 500 cubic metres per second into the sea.

By 2009, the powerhouse on the south bank of the river will contain 14 turbines in total (so far it has 12, which are already producing electricity). The north bank’s powerhouse will contain a further 12 turbines. Ships passing the dam now have to be lifted or lowered by about 100 metres. The new five-tier ship lock on the north bank of the river will let ships up to 10,000 tons in weight pass through the dam for free in less than three hours.

Anyone in a hurry can use the ship lift, which takes only 45 minutes but will come at a price. It is expected that cargo traffic along the Yangtze will increase from 10 million to 50 million tons as a result of the safer passage.—Dawn/The Guardian News Service.