by Rochelle GLUZMAN
Thousands of the world's large dams are so
clogged with sediment that they risk losing more than a quarter of their
storage capacity by 2050, UN researchers said Wednesday, warning of the threat
to water security.
A new study from the UN University's
Institute for Water, Environment and Health found that, by mid-century, dams
and reservoirs will lose about 1.65 trillion cubic metres of water storage
capacity to sediment.
The figure is close to the combined annual
water use of India, China, Indonesia, France and Canada.
That is important, the researchers say,
because these big dams are a key source of hydroelectricity, flood control,
irrigation and drinking water throughout the world.
"Global water storage is going to
diminish—it is diminishing now—and that needs to be seriously taken into
account," the study's co-author and Institute director Vladimir Smakhtin
told AFP.
Aging dams pose safety risk
(Thousands of large dams worldwide have passed their design life of between 50 to 100 years)
Researchers looked at nearly 50,000 large
dams in 150 countries, and found that they have already lost about 16 percent
of water storage capacity.
They estimated that if build-up rates
continue at the same pace, that will increase to about 26 percent by
mid-century.
Rivers naturally wash sediment downstream
to wetlands and coasts, but dams disrupt this flow and over time the build-up
of these muddy deposits gradually reduces the space for water.
Smakhtin said this "endangers the
sustainability of future water supplies for many" as well as posing risks
to irrigation and power generation.
Part of a larger issue
Accumulation of sediment can also cause
flooding upstream and impact wildlife habitats and coastal populations
downstream.
Sedimentation is a part of a larger issue:
by 2050, tens of thousands of large dams will be near or past their intended
lifespan.
Most of the world's 60,000 big
dams—constructed between 1930 and 1970—were designed to last 50 to 100 years,
after which they risk failure, affecting more than half the global population
who will live downstream.
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 warming 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.
Alternatives
To address looming challenges of ageing
dams and reservoir sedimentation, the study authors list several measures.
Bypass, or sediment diversion, can divert
water flow downstream through a separate river channel.
Another strategy is the removal, or
"decommissioning", of a dam to re-establish the natural flow of
sediment in a river.
But addressing water storage issues is
especially complex because there is no one-size-fits-all solution, Smakhtin
said.
"The loss of water storage is
inevitable for different reasons," Smakhtin said. "So the question we
should be asking is what are the alternatives?"
A March 22-24 UN 2023 Water Conference in
New York will provide the possibility for countries to voice concerns and make
commitments for the future of water management, he said.
(Source:https://phys.org/news/2023-01-sediment-endangers.html)
