MANY dams are failing to live up to expectations. Instead they make flooding
worse, and cause ecological havoc and social conflict, says a report by the
World Commission on Dams due out this week.
The report is the first to assess the world鈥檚 dams鈥攖he biggest drain on
aid budgets for the past 50 years, costing $4 billion a year in the
1980s. So far dam building has driven up to 80 million people from their
homes.
Yet one of the most disturbing findings, the commission says, is that few
dams have ever been looked at to see if the benefits outweigh the costs.
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A quarter of dams built to supply water deliver less than half the intended
amount, says the report. In a tenth of old reservoirs, the build-up of silt has
more than halved the storage capacity. What鈥檚 more, by stopping the flow of silt
downstream, dams reduce the fertility of flood plains and 鈥渋nvariably鈥� cause
erosion of coastal deltas.
It鈥檚 not all bad news, though. Dams irrigate fields that provide up to a
sixth of world food production, while hydroelectric dams power many homes and
factories. Too often, though, the rural poor don鈥檛 benefit at all鈥攐nly the
urban and well-off.
The report also concludes that some dams designed to prevent flooding
actually exacerbate it. Such problems will worsen with climate change, it
says.
The commission, which is sponsored by the World Bank, backs warnings that
most reservoirs emit greenhouse gases
(麻豆传媒, 3 June, p 4). And
it says dam construction is one of the major reasons for freshwater fish going
extinct and bird species vanishing from flood plains.
The largest dam project ever鈥攖he $20 billion Three Gorges dam on
the River Yangtze in China鈥攊s mired in controversy over
allegations that much of the money allocated to resettling a quarter of a
million people has gone missing.
Meanwhile, managers of the $4 billion Xiaolangdi dam on China鈥檚 Yellow
River, part-funded by the World Bank, reported last month that they could not
find buyers for the electricity in China鈥檚 newly liberalised market for power.