World's aging dams lose storage capacity at alarming rate

As if climate change and population growth across many parts of the globe were not enough to push fresh water supplies to the limit, the United Nations says that many large lakes are filling so fast with sediment that they are losing much of their capacity to store water.

Some 50,000 large dams worldwide are being robbed of as much as 19% of their combined original storage capacity, and total losses could reach 28% by mid-century, according to a U.N. University report on Wednesday.

The capacity loss by 2050 roughly equals the combined annual water use of Canada, China, France, India and Indonesia, said the authors of the global assessment issued by the U.N.U.'s Canadian-based Institute for Water, Environment and Health and published in the journal Sustainability.

“The decrease in available storage by 2050 in all countries and regions will challenge many aspects of national economies, including irrigation, power generation, and water supply,” said Duminda Perera, a civil engineer and lead author.

Ireland, Japan, Panama, Seychelles and the United Kingdom are looking at the highest water storage losses – between 35% and 50% of their original capacities by 2050 – the study shows. Bhutan, Cambodia, Ethiopia, Guinea, and Niger are expected to be the five least affected countries, and will lose less than 15% capacity.

In 2021, a U.N. University report found most of the world's population by mid-century will face health and safety risks living downstream from tens of thousands of aging concrete dams made more vulnerable by increased flooding from climate change.

That report said most of the 58,700 large dams worldwide were built to last between 50 and 100 years, but they were constructed between the 1930s and 1970s so some of them already are obsolete and most of the rest will be by 2050. The average life expectancy is just half a century.

Monitoring, reducing erosion and other solutions

In a healthy, free-flowing river, silt mostly remains suspended in the water to be deposited in a nutrient rich delta at the river’s end. However, when a dam restricts the flow, silt settles to the bottom.

The most cost-effective and environmentally friendly solution is to monitor and reduce erosion in the watershed.

Dredging up the sediment from the lake bed and transporting it elsewhere is expensive and temporary. Flushing the lake is cheaper, but may have major adverse impacts downstream.

Some dams could be built higher, but that raises the water level and increases the size of the lake, potentially flooding nearby communities, valuable farmland and wildlife habitat.

Bypassing or diversion, which diverts the flow downstream via a separate channel engineered to manage high-flow events when sediment concentration is high, is becoming increasingly attractive to a public that seeks to minimize the environmental impact of dams.

In some communities, environmentalists and activists are advocating total dam removal, including those filled with sediments, with the aim of returning rivers to their natural flow. However, the study notes, treatment and disposal of accumulated sediments could be required as the sediments are often contaminated with heavy metals and other toxins.

A mid-20th century boom brings headaches

More than nine-in-10 large dams are located in 25 nations, and 55% are found in four Asian countries: China, India, Japan and South Korea. China alone has 40% of the world's total of large dams and most of those will soon be half a century old.

Large dam construction surged in the mid-20th century, peaking in the 1960s to 1970s, especially in Asia, Europe and North America. In Africa, it peaked in the 1980s.

For the Asia-Pacific, home to 60% of the world’s people and the most heavily dammed region on the globe, water storage is crucial. The region is estimated to have already lost 13% of its initial dam storage capacity and, by mid-century, the Asia-Pacific will have lost nearly a quarter of its initial storage capacity.

The loss of storage for Japan, with 3,052 dams that are on average a century old, is the most acute in the region. Having already lost 39% of their total initial storage capacity, they will have lost nearly 50% by 2050 on average, and 67% in some cases, the study says.

In 2015, India’s Central Water Commission reported that among 141 large reservoirs over 50 years old, one-quarter had already lost at least 30% of their initial storage capacity. India’s 3,700 large dams are expected to have lost on average 26% of their initial total storage by 2050.

The world’s most heavily dammed nation, China, has lost about 10% of its storage and will lose a further 10% by 2050, the study estimates.