THE floodwaters that have driven tens of thousands of people from their homes
in Mozambique are a “human-induced tragedy”, hydrologists are claiming. People
are having to flee a surge of water flowing towards the Indian Ocean after heavy
rains. But the crisis has been exacerbated by two giant hydroelectric dams on
the Zambezi river upstream of the flooded area, says Bryan Davies of the
University of Cape Town in South Africa.
The amount of water flowing down the river is no greater than the volume that
flowed before the two dams were built. It “is about equal to the historical mean
recorded in pre-dam times”, says Richard Beilfuss of the University of Wisconsin
in Madison. “The difference is the dams.” The Kariba dam, on the border between
Zambia and Zimbabwe, and the Cahora Bassa dam, just inside the Mozambican
border, have a combined capacity of 220 cubic kilometres.
Davies claims that, both this year and last, managers did not partially empty
the reservoirs before the rainy season to make room for floodwaters
(91av, 25 March 2000, p 16).
They wanted to keep them full to maximise
electricity production. “They have ended up being forced to release water at
critical times,” says Davies.
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Beilfuss says a further problem is that in recent years of low rainfall, the
managers of the Cahora Bassa have held back natural river flows during the rainy
season. This has often harmed farmers downstream who need the water to irrigate
their crops. “Large dams do not stop large floods,” concludes Davies. “They
exacerbate them—people below them move closer to the river. But when the
big one comes, God help them.”
At the weekend, the floodgates of the Cahora Bassa dam were releasing 8000
cubic metres of water a second into the lower Zambezi, which had already flooded
an estimated 80,000 people from their homes. Silvano Langa, director of
Mozambique’s National Disasters Management Institute, said that even though
rains have eased in the region, the dam’s releases will cause water levels in
the river’s lower reaches to continue rising for several days.
Concerns are also being raised about the stability of the Cahora Bassa dam.
When gates were opened during floods in 1997, a series of vibrations ran through
the dam, says Beilfuss. But last week Fernando Cunha, general manager of
Hidroelectrica de Cahora Bassa, the Portuguese company that operates the dam,
denied that it was in any danger. “This is just speculation,” he told the
English-language service of the Mozambique News Agency. “There are over a
thousand sensors in the dam wall that alert us to any problem.” Cunha also
claimed that a study by US Army engineers last year gave the dam a clean bill of
health.