Mega dams meant to control siltation will add to ecological woe, experts warn
China is paying a heavy price for underestimating silt accumulation in the Three Gorges reservoir with plans to build more mega dams upstream of the Yangtze River, which themselves will have serious ecological impact, mainland experts have warned.
Siltation has long been a cause for concern for critics of the gigantic hydropower project even before it was built. But the government has sought to play it down, along with other environmental and geological concerns over the years, in a bid to defend the politically charged project.
'Authorities used to dodge questions over siltation and insisted that it posed no threats to the Three Gorges,' said Fan Xiao , a geologist in Sichuan . 'But now siltation at the dam reservoir has been cited as a main reason to build a succession of big dams on the upper reaches of the Yangtze and its tributaries, such as Jinsha and Dadu rivers.'
Twelve hydroelectricity dams have been designed on the middle and lower reaches of the Jinsha, while 22 dams are planned on the mainstream of the Dadu River. They are part of at least 106 dams planned or already built on the mainstream of six tributaries of Yangtze's upper reaches, including 38 large dams with a capacity over 1,000MW, according to Professor Fan.
The other four tributaries are the Yalong, Min, Jialing and Wu rivers.
'The silt problems at the Three Gorges reservoir have fuelled the much-criticised dam-building frenzy in the southwestern region,' he said.
Silt accumulation is a major concern for every dam, because it elevates the floor of the reservoir, cripples its ability to control floods, and reduces designed capacity of power generation. The buildup of silt at the Gongzui Dam on the Dadu River, which was constructed in the early 1970s, has significantly reduced the reservoir's flood-regulation and water-storage capacity. It has lost up to 70 per cent of its capacity because of mud and silt.
At least two-thirds of the 530 million tonnes of silt that the Yangtze River carries to the Three Gorges reservoir every year will be trapped behind the 185-metre-high dam.
While sedimentation is expected to pose a grave threat to an upstream port in Chongqing within 20 years, it could also render the dam reservoir useless.
Despite warnings from experts, government officials have insisted the problem has been vastly exaggerated.
Citing official figures, top officials in charge of the project announced last year that sediment input in the reservoir had dropped and siltation in the reservoir was not as serious as expected. But critics are not convinced.
'Siltation at Three Gorges may have been eased only because of those dams already built upstream which help block silt from entering the reservoir,' Professor Fan said.
He noted that authorities have deliberately been quiet on the connection between silt problems at the Three Gorges and the dam-building upstream.
'They only boasted about the seemingly good result and never said a word in public about what they have done in the upper reaches,' he said.
But developers of several controversial mega dams on the Jinsha River were keen to use the Three Gorges problem to make their case.
The Xiluodu and Xiangjiaba dams on the middle reaches of the Jinsha, respectively the second- and third-largest power plants on the mainland, were said to have been built to cope with siltation at the Three Gorges, according to Xinhua.
'Silt accumulation at the reservoir has always been the biggest concern for the Three Gorges project,' the agency said, quoting government-backed hydropower experts.
The building of the two mega dams with a combined capacity of 18,600MW, more than that of the Three Gorges, came amid intense public scrutiny of dam construction across the country.
Instead of highlighting the dams' massive power-generation capabilities, Xinhua said the two power stations would play a key role in delaying the buildup of silt in the dam reservoir.
'The joint operation of Xiluodu and Xiangjiaba will be able to trap up to 12.6 billion tonnes of silt in the next 60 years which otherwise would flow downstream into the Three Gorges reservoir,' it said.
Professor Fan said that while the two dams could help to alleviate the buildup of silt at the Three Gorges reservoir, they could do little to solve siltation problems at the dams themselves.
He said he was worried about the silt buildup at the two Jinsha River dam reservoirs and the possible domino effect in the future.
'What can we do if the upstream reservoirs silt up? Do we have to build more upstream dams?'
His concerns were apparently not just his own. Authorities have already unveiled plans to build more dams farther upstream of the Yangtze River and its tributaries in the past few years.
'We have not yet considered the irreversible damage of those dams to the ecology of the river,' he said.