Dam replaced waters four times

2011-02-09 21:15

Pretoria - The Bloemhof dam replaced its waters four times during the recent floods between mid-December and the end of January, a flood specialist at the department of water affairs said on Wednesday.

Danie van der Spuy, a specialist engineer in flood studies, said the Vaal Dam had replaced its waters once during the same period and the amount of water that passed through Upington during the floods was the equivalent of five Vaal Dams.

According to Van der Spuy the Bloemhof dam holds 1 250 000 000m³ of water when full. The minimum volume of an Olympic-sized swimming pool is 2 500m³ in comparison.

In other words enough water to fill two million Olympic-sized swimming pools passed through the Bloemhof dam during the last half of December and all of January.

Van der Spuy said that to get an understanding of the amount of water going through the dam's sluice gates one should think in terms of elephants passing in front of your eyes at any given second.

The average African elephant weighs in at 4.6 tons.

With 2 800m³/s flowing through the gates at the Bloemhof at the flood peak, one would have seen 609 elephants every second.

"When you think of it like that, you can't even imagine that number of elephants... every second," enthused Van der Spuy.

In Upington the Orange River drainage area experienced flooding that would happen on average every five to 10 years.

Shortly after those floods peaked with the waters yet to dissipate or run off, the Vaal River went into flood - the kind of floods that would happen every 15 to 20 years.

If that was not enough for the residents in the Upington area the Orange went again into flood.

Van der Spuy said that to have both the Orange and Vaal Rivers flooding at similar times was an occurrence that took place on average every 10 to 20 years.

"The problem is that municipalities have allowed development to take place in what is the flow path of the river during flooding," said Van der Spuy.

And the question of holding the waters back to prevent flooding down stream? Only for a short while at best.

Van der Spuy's colleague Brink du Plessis said: "Our first priority is the safety of the dams. We cannot allow the dam walls to be breached."

Sigh of relief

The Vaal Dam's capacity is 2 536 000 000m³. Authorities will allow a maximum 663 000 000m³ more or about 25% above its capacity, but after that they have no option but to open the sluice gates.

That means enough water to fill more than one million Olympic-sized swimming pools passed Upington in December and January and the Vaal Dam was still 107% full according to figures on the department's website.

This week it is back to normal at 99.5% . Agriculture unions have demanded the building of dams for storage to prevent flooding down river with the water running off into the ocean.

Van der Spuy believes it is problematic with most suitable dam sites along the Orange and Vaal Rivers already taken.

"Dams built for floods will be empty for up to 19 years and some will be empty for 50 years. The water has to keep on flowing through the river and what happens in drought time. They will be empty."

Van der Spuy and Du Plessis have more than 1 000 gauging stations sending rain and water data to the department's Hydrology centre in Pretoria and hope they can give fair warning of future floods..

This past weekend the Fish River in Namibia went into flood with 1 000m³ (or 217 elephants) a second reportedly flowing into the Orange River about 100km from the Atlantic.

At this point according to Du Plessis the Orange River is usually measuring 100m³/s or a mere 21 elephants a second.

No doubt those up stream are breathing a sigh of relief.



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