Silt behind dams 'worsens water shortage'

Land use is a key influence on the silting process

By BBC News Online environment correspondent Alex Kirby in Bonn

The build-up of silt behind large dams is significantly reducing their storage capacity, the United Nations says.

Sediment removal should be a fundamental feature in the design of dams

Dr Rodney White, engineer

It says the world's reservoirs are losing on average 1% of their storage capacity annually. On present trends, they will lose about a fifth of their capacity within a few decades.

The UN says the way land around the reservoirs is used can make a crucial difference.

The warning, by the UN Environment Programme (Unep), was given to the International Conference on Freshwater meeting here.

Climate change

Unep says the current storage capacity of reservoirs worldwide is estimated at just under 7,000 cubic kilometres.

A British engineer, Dr Rodney White, who has written a book entitled Evacuation of Sediments from Reservoirs, says about 1,500 cubic kilometres could be lost before the middle of the century.

Unep says intensifying climate change could make the loss happen faster. Scientists think it will increase the severity of storms and so worsen erosion.

Deforestation is also a problem. Dr White says the levels of erosion from hillsides planted with crops can be 150 times higher than from similar forested land.

Sustainable management

Dr Klaus Toepfer, executive director of Unep, said: "The issue of dams can arouse strong passions on both sides.

"It would seem prudent and sensible for us to manage the existing stock in the most sustainable way possible.

"We must act to reduce the loss of forests and to re-afforest cleared areas. We must also act to reduce the threat of global warming.

"However, there will always be natural levels of erosion, so I call on engineers to also provide technical solutions that offer environmentally friendly ways of extending the lives of the world's reservoirs."

Rising demand

Dr Toepfer told BBC News Online: "I want to help people to realise that land use and sedimentation are linked. So, whether you're using the land round a reservoir for farming or tourism, you'll have consequences. It isn't just deforestation that's the problem."

Dr White said: "The loss of capacity of the world's dams should be of the highest concern for governments across the globe.

"The demand for water is rising, not falling, as the population of the planet climbs. Sediment removal should be a fundamental feature in the design of dams."

One technique is a method known as flushing, which relies on seasonal floodwaters to sweep mud and silt out of a reservoir.

Tree species

Jeremy Bird is the interim co-ordinator of Unep's Dams and Development Project, set up to continue the work of the World Commission on Dams.

He told BBC News Online: "The 45,000 existing dams offer a huge opportunity, and we need to see how they can produce more energy and water.

"This is about more than sediment build-up. Reforestation may be the answer, but you have to be careful, or you can get problems.

"South Africa has found that if you simply plant pines and eucalyptus trees on bare slopes, they reduce the inflow to the reservoir by around 7%.

"So you need to think hard whether to plant alien or indigenous species."

Wasted water

Unep is also concerned that much of the water in developing countries is wasted.

It says about 60% of the water used for irrigating crops is wasted or used inefficiently, and 50% or more of the water distributed in cities is lost through leaks and poor management.

Dr Toepfer said: "Six thousand children die every day because of inadequate water and poor sanitation. The poorer you are, the more you have to pay for water.

"In Kenya, where Unep has its headquarters, more than 5% of the water supply is used for brewing.

"That's not because we're all drinking from dawn to dusk. It's because brewing needs water, and can afford it."