Most of the world鈥檚 45,000 large dams don鈥檛 do their jobs properly. So concludes the first ever global audit of a technology that has cost $2 trillion over the past century.
Since 1900, the world has built one new large dam every day on average. They barricade 61 per cent of the world鈥檚 rivers. Their reservoirs cover an area about six times the size of Britain.
But 鈥渢he true profitability of these schemes remains elusive,鈥� said Kader Asmal, chairman of the World Commission on Dams. He was speaking in London on Thursday at the launch of the final report of the commission, which was set up by the World Bank.
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鈥淭here have been precious few, if any comprehensive, independent analyses as to why dams came about, how they perform and whether we are getting a fair return on our $2 trillion investment,鈥� said Asmal, who is a former water minister in South Africa.
Dams generate hydroelectricity, prevent floods, irrigate farms and supply water to cities. But they have also wrecked ecosystems and 鈥渓ed to the impoverishment of millions鈥�, who lost their land to reservoirs or saw dams destroy their fisheries.
The report calls for an end to dams that are imposed on communities without their agreement. But its most remarkable findings are on the widespread technical failures of dams.
Studies carried out by the commission found that:
聲 One in four dams irrigate 鈥渓ess than 35 per cent鈥� of the land they were supposed to
聲 The cost over-runs of construction are 56 per cent on average
聲 Two-thirds of dams deliver less water to cities than promised. A quarter delivered less than half the promised water
聲 Over half of hydroelectric dams do not generate as much power as promised
聲 Some flood-control dams 鈥渉ave increased the vulnerability of river communities to floods鈥�
One reason many dams have failed to deliver is that their reservoirs have clogged up with silt far faster than expected. Every year an extra one per cent of the world鈥檚 reservoir capacity is taken up with silt. In the worst cases, reservoirs lost more than 80 per cent of their storage capacity to silt in less than 30 years.
Even the claim that hydroelectric dams provide 鈥済reen鈥� electricity has been undermined by the commission. It concludes that between one and 28 per cent of all artificial greenhouse-gas emissions could be from rotting vegetation in dams.
Asmal plans to present his findings of the 鈥済reenhouse effect鈥� of large dams at the climate negotiations in The Hague on Saturday.
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