Nearly 50,000 dams to lose 'quarter of storage' capacity by 2050

Nearly 50,000 large dams worldwide could lose more than a quarter of their storage capacity by 2050 as a result of sedimentation build-ups, eroding global water and energy security, according to United Nations research.

Dam capacity is expected to drop from 6 trillion cubic metres (cu m) to 4.655 trillion cu m by 2050, and action must be taken to address the problem and protect vital storage infrastructure, the UN University's Institute for Water, Environment and Health said on Wednesday.

Silt accumulates in reservoirs as a result of the disruption of natural water flows.

It can cause damage to hydroelectric turbines and cut power generation.

Impeding sediment flows along a river can also make upstream regions more prone to flooding and erode downstream habitats.

The UN study looked at data from 50,000 dams in 150 countries and said 16 percent of original capacity had already been lost.

The study estimated that if build-up rates continue at the same pace, that will increase to about 26 percent by mid-century. 

It said the United States is facing losses of 34 percent by 2050, with Brazil estimated to lose 23 percent, India 26 percent and China 20 percent.

Vladimir Smakhtin, director of the UN University's Institute for Water, Environment and Health and one of the study's authors, said dam building worldwide had already declined significantly, with around 50 a year now being built, compared to 1,000 in the middle of the last century.

"I would argue that the question we should now be asking is what are the alternatives to dams — including in generating power — considering that they are being phased out," he said.

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Role of global heating 

Most of the world's 60,000 big dams — constructed between 1930 and 1970 — were designed to last 50 to 100 years.

Large dams and reservoirs are defined as higher than 15 metres (49 feet), or at least five metres high while holding back no less than three million cubic metres of water.

Global heating compounds the risk in ways that have yet to be fully measured.

"Climate change extremes like floods and droughts will increase, and higher intensity showers are more erosive," Smakhtin said. 

This not only increases the risk of reservoirs overflowing but also accelerates the build-up of sediment, which affects dam safety, reduces water storage capacity and lowers energy production in hydroelectric dams.