Opening of Vaal Dam gates guided by computer models, says expert

The opening of sluice gates when the Vaal Dam is over-full is carefully orchestrated, experts say, despite concern that more could be done to prevent damage downstream.

This also comes amid concern that the downstream Bloemhof Dam is not being used according to its original design purpose, leading to “man-made floods”.

The department of water and sanitation has refuted claims made by some of the residents affected by the floods at Bloemhof Dam that the department should have opened the radial gates earlier to prevent the current flooding.

Water minister Pemmy Majodina maintained that the water releases including the opening of the gates at Bloemhof Dam were done according to operational guidelines of the department to prevent damage or bursting of the infrastructure when it is beyond its full capacity. 

Majodina, who visited the area on Friday, urged communities residing near the flood line to relocate, as more water would still be released from Bloemhof Dam to manage its heavy inflows from the upper catchment. 

“We are deeply concerned by the settlements that have been built near the flood line of the river. The high inflow of water in the dam necessitated the release of water in high volumes to protect the dam from any defects or structural damage,” she said

“The radial gates could not have been opened earlier, as the measures implemented were according to the operating standards of the department. Unfortunately the water releases resulted in flooding in downstream areas. We therefore call on those living near the flood line of the river to move and find alternative places to stay.”

The mayor of the Lekwa-Teemane local municipality in the North West, Sebang Motlhabi, said the municipality has activated their disaster management plans and assisted the residents affected by the floods to relocate to safer places. 

More than 60 houses in the Bloemhof area have been evacuated, and about 50 houses in Christiana have been negatively affected by the floods. 

Water expert Carin Bosman said the sluices are opened in a staggered manner, and the “flooding” of downstream areas is not because of the sluices but because people built inside the established flood lines. 

“If people downstream get flooded, it's because they built in areas where they are not supposed to build,” she said.

Water expert Prof Mike Muller said flood flows are managed to ensure that the impact of the flood is minimised without losing useful water. 

“Computer models are used to guide the operators who open the dam gates to allow floods to pass. These models take account of past experience and of the likelihood that there may be further heavy rainfall in the future. Using actual information about recent rainfall, the flows and flood levels can usually be calculated with a high degree of accuracy. This information is used to warn downstream residents of potential flood risks,” he said.

Meanwhile, AfriForum said the continued use of the Bloemhof Dam as a storage reservoir instead of a flood control dam is unacceptable, as communities downstream have once again suffered severe damage. It said the department's failure to ensure that sufficient storage reservoirs are built has led to the Bloemhof Dam being used outside its original purpose. 

Tarien Cooks, disaster management specialist at AfriForum, said when the dam’s capacity reaches critical levels, sluices are forced open, leading to so-called man-made floods. 

“These floods cause widespread damage to infrastructure and put communities’ lives at risk. This is a direct result of poor planning and a lack of proactive management. The current practice is not only unsustainable but also dangerous and unacceptable,” Cooks said.

Cooks said if the Bloemhof Dam is managed effectively as a flood control dam, future damage can be prevented or significantly reduced. 

“The dam was specifically built to control flood waters so that communities living downstream do not suffer damage when the sluices of the Vaal Dam are opened,” said Cooks.

As part of the dam safety protocols, the department said sluice gates are opened when dams breach the full capacity mark, to prevent the water resource infrastructure from failing, as it may lead to a dam bursting and causing a disaster of unimaginable magnitude. 

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