Sedimentation threatens to steal the capacity of nearly 50,000 dams

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Slowly but surely, the world's reservoirs are filling with sediment. In an unblocked river, flowing water carries along bits of sediment, picked up from the banks or washed down the river by rain. However, rivers whose flow has been interrupted by a dam deposit some of this sediment just behind the dam itself, in the reservoir. "Gradually, [over] years and years, it will build up," Duminda Perera, a researcher at the United Nations University Institute for Water, Environment and Health in Hamilton, told Ars. Ontario.

According to Perera, the increase in sedimentation in these reservoirs and the resulting loss of volume is rarely taken into account. However, he and some of his fellow researchers recently authored a new study, suggesting that nearly 50,000 large dams - defined as 15m or more in height or above 5m in height and blocking more than 3 million cubic meters of water - are deprived of capacity.

These slowly accumulating sediments take up volume in the reservoir, occupying cubic meters that would otherwise be filled with water that would eventually flow through hydroelectric turbines or be diverted to agriculture. "If you fill a cup with water and then put in soil...the volume of water is reduced," he said.

To identify the scale of the problem, the team applied previously established storage loss rates to large dams in 150 countries, collected from a database of the International Commission on Large Dams (l dataset contains information on approximately 59,000 dams). For their calculations, the team also looked at the initial storage capacities of the dams and the years of their commissioning: this data was only available for 47,403 dams.

Using these parameters, the team estimates that trapped sediment robs these large dams of 13-19% of their reservoir's original storage capacity. This total could reach between 23 and 28% by 2050, according to the newspaper. This reduction would increase the capacity of these reservoirs from 6,300 billion to 4,700 billion cubic meters, equivalent to the total annual water consumption of Canada, China, France, India and Indonesia, according to the report.

Storage loss varies by country. And land use changes, such as deforestation (which is often linked to erosion), can increase the amount of material entering rivers. The United Kingdom, Panama and Japan, among others, will lose between 35 and 50% of their storage capacity by 2050. In contrast, Cambodia, Ethiopia and Bhutan are expected to lose less than 15%.< /p> No "hard and fast" solution

This loss of storage can negatively impact humans in several ways. For dams that also have a hydroelectric generator, sediment can act as an abrasive, damaging turbines and other parts of the facility, “lowering their efficiency and increasing maintenance costs,” the document notes. Completely sedimented reservoirs also have less space to store flood waters and retain less water to be diverted to agriculture. "Sometimes our intended functions cannot be realized due to lack of storage," Perera said.

Some things can be done to fix this problem. One option is dredging, which uses machines such as excavators or vacuum systems to remove silt. Another option is to flush or let the sediment flow through the dam. However, the paper notes that dredging is expensive and usually only a temporary solution, while flushing can lead to unintended consequences when a deluge of soil and other material flows downstream. Additionally, some types of flushing, such as "vacuum flushing", involve emptying the entire reservoir, which temporarily shuts off a hydroelectric dam's ability to generate electricity.

Another option, and the best one in Perera's mind, is to routinely bypass or divert sediment below the dam using tunnels. However, all of these options are likely case-specific, he said. "There's no...