Water crisis: World to lose 26% storage by 2050 to trapped sediment, says new UN report

Dam capacity loss by mid-century to equal combined annual water usage of India, China, Canada, Indonesia and France  

By Preetha Banerjee
Published: Thursday 12 January 2023
Photo: iStock_

About 50,000 large dams across the world will lose 24-28 per cent water storage capacity by 2050 due to sediment trapped in them, a report by the United Nations Institute for Water, Environment and Health showed.

These water reservoirs have already lost about 13-19 per cent capacity to sedimentation, the researchers mentioned.

The loss will challenge many aspects of national economies, including irrigation, power generation and water supply, said Duminda Perera, who co-authored the study with UNU-INWEH Director Vladimir Smakhtin and Spencer Williams of McGill University in Montreal. 

Estimated storage loss


Source: United Nations University Institute for Water, Environment and Health

Sedimentation is caused when a river carrying eroded soil is blocked by a dam at its watershed. The sudden drop in water velocity leads to deposition of large volumes of particles in the calm waters of the reservoirs. 

While sediment helps maintain the aquatic ecosystem, poor management can lead to nutritional disbalances causing eutrophication and other disruptions in the water pool of the dam, as well as damages in habitations downstream. 

Loss of capacity occurs due to infilling of dams by trapped sediments. Sediments can also damage dam structure and turbines. Moreover, shallow water also reduces the recreational value of the reservoirs. 

Unsustainable land-use practices may make sedimentation more aggressive by loosening up soil upstream.

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The estimated actual capacity loss of around 1,650 billion cubic metres is equal to the annual water use of India, China, Canada, Indonesia and France combined, the UN report said.

“The new dams under construction or planned will not offset storage losses to sedimentation,” said Perera.

The report noted:

The United Kingdom, Panama, Ireland, Japan and Seychelles will experience the highest water storage losses by 2050 from 35-50 per cent of their original capacities. By contrast, Bhutan, Cambodia, Ethiopia, Guinea and Niger will be the five least-affected countries, losing less than 15 per cent by mid-century.

The researchers estimated the storage capacity losses of the large dams in 150 countries based on their current storage-loss rates. These include 28,045 large reservoirs in the Asia-Pacific, 2,349 in Africa, 6,651 in Europe and 10,358 in North, Central and South America. The findings were published in the journal Sustainability January 11, 2023. 

In 2022, the Asia-Pacific region is estimated to have lost 13 per cent of its initial dam storage capacity and will lose 23 per cent by mid-century, the report noted. 

India’s 3,700 large dams will have lost on average 26 per cent of their initial total storage by 2050, the analysts estimated. 

Estimated reservoir storage loss in Asia-Pacific


Source: United Nations University Institute for Water, Environment and Health

The loss of storage capacity of Japan’s 3,052 dams (average age: over 100 years) is the most acute in the region, the report mentioned. “Having already lost 39 per cent of their total initial storage capacity, they will have lost nearly 50 per cent by 2050 on average, and 67 per cent in some cases,” the scientists wrote.

The 10,358 large dams in the Americas are estimated to lose 28 per cent capacity and be able to hold 2,014 billion cubic metres by 2050, according to the report. 

Europe’s 6,651 large dams across 42 countries will also lose 28 per cent pf its initial capacity of 895 billion metres, the study found. 

The 2,349 dams across Africa’s 44 countries are projected to lose 24 per cent storage capacity by 2050, the authors observed.

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The researchers also evaluated the various solutions available to the problem and highlighted the most efficient ones. “Dredging can be costly, they add, and only temporary. Sediment flushing is more financially attractive but may have significant adverse impacts downstream,” they wrote. 

Bypassing or sediment diversion downstream through a separate channel was advocated, especially because it reduces the environmental impacts of dams. “At their optimum operational levels, bypass tunnels can reduce sedimentation by 80-90 per cent as mentioned in a previous study, according to the authors. 

The height of the dam can also be raised to increase storage capacity but this will require a larger reservoir area, displacing communities and destroying habitats, they added. 

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