Will the Vaal Dam overflow? Here’s what will happen if it reaches 100% capacity

As Vaal Dam water levels rise quickly, experts and the Department of Water and Sanitation have clarified whether the dam will overflow, and any potential danger this may pose.

The dam’s water level reached 89.2% on Thursday, following a dramatic surge over the past week.

This marks a significant recovery from earlier this year when the dam was below 30% capacity.

According to the Water Resource Information Centre for the Catchment Management Forums of the Upper Vaal Water Management Area, the dam level has risen by more than two percentage points a day for the last week. However, it slowed to one percentage point from Wednesday to Thursday.

Continuing this trend would see the dam reach 100% in early to mid-March.

Vaal Dam overflow concerns

Midvaal councillor Pieter Swart said reduced rainfall and lower contributions from upstream sources like the Grootdraai Dam would make the dam fill slower in the coming days, and the water department has also moved to allay overflow concerns.

“The Vaal Dam operates with a full supply level of 100%, but it has a flood absorption capacity of 26%, meaning it can accommodate water levels up to 126% before posing a significant structural or operational risk,” the department told The Citizen.

“However, as a precautionary measure, according to the flood management protocols, we generally aim to keep the dam’s level below 103% to ensure optimal peak inflow attenuation.”

The department added that if the dam levels approach this point, as forecasted, decisions will be made in time about controlled releases and informing downstream affected communities.

It added that it employs comprehensive monitoring systems to track the rising water levels.

“The department continuously monitors the water levels at the Vaal Dam using a combination of real-time flow gauging stations, rainfall forecasts from the South African Weather Service (SAWS), and hydrological modelling tools.”

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Understanding overflow

Swart said that unlike some dams, which may simply spill over when full, the Vaal Dam employs a controlled release system.

“The Vaal Dam doesn’t ‘overflow’ in the way some other dams, like Gariep Dam will do. It makes use of sluice gates to release water in a controlled manner, once the dam reaches 100%.

“If the Vaal Dam is at around 100%, and we are expecting big rains, the sluices will strategically be opened.”

Water expert Carin Bosman reinforced this point, emphasising that the term “overflow risk” is technically misplaced.

“The dam is designed to overflow so there is no risk associated with the overflow. The risk of overflow is not a risk. The dam is designed to overflow,” Bosman stated.

She explained that overflow events typically occur “towards the middle end of March, beginning of April, and that’s the dams designed to handle overflow”.

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Potential flooding downstream

Controlled water releases could affect properties along the downstream riverbanks.

“The closest major suburb is Three Rivers, and on occasion, with a lot of rain and a few sluice gates open, these lower-lying areas do flood,” Swart explained.

The water department said it was monitoring the situation to prevent possible flooding downstream.

“The department has established flood management protocols in coordination with the National Disaster Management Centre (NDMC), Provincial Disaster Management Centres (PDMCs), and local municipalities. In the event of an overflow, emergency releases will be coordinated, and affected communities will be alerted to take necessary precautions,” it added.

Key actions include:

• Issuing advance warnings to downstream communities and stakeholders.
• Activating emergency evacuation plans if flood conditions worsen.
• Adjusting releases strategically to manage flood peaks and minimise damage.

“In the event of an emergency, DWS will issue official notifications via media statements, social media updates, direct coordination with local authorities, and SMS alerts where applicable.

“The department works closely with disaster management teams to ensure timely and accurate public communication. Various WhatsApp groups serve as a tool to provide timely warnings and information as well.”

Meanwhile, for concerned residents, Swart also offers regular updates.

“I place daily updates on my various socials, especially when the level changes daily. When it gets close to capacity, the various authorities, including Rand Water, will send out notifications and updates on the possibility of flooding downstream.”

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Building too close to the line

The real flood risk, according to experts, comes from development within designated flood zones rather than from the Vaal Dam itself.

“The strategic release of water, in advance of the Vaal Dam becoming over full and the expectation of coming rain and inflows, allows Rand Water to open sluice gates to control the amount of water released downstream,” Swart explained.

“In South Africa, building near rivers like the Vaal is governed by national and local laws that aim to mitigate flood risks. The key consideration is the 1-in-100-year flood line, which marks the boundary of an area expected to be inundated by a flood with a 1% chance of occurring in any given year.”

Bosman emphasised this same point.

“The measures that are floodline determinations downstream and people staying outside of the one in 100 year floodline.

“If people have built construction of infrastructure, houses, etc within the one in 100 year floodline, that is the risk that those people took to construct inside a floodline and it’s not a risk of overflow.”

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Conservation still important

Despite the current high levels, Swart emphasised the need for continued water conservation.

“Despite the current high levels, it remains a good idea to use water sparingly, as future weather patterns may bring less rain, potentially reversing the dam’s progress,” he cautioned.

“No one wants to see the Vaal Dam drop to a critical 25%, as this would jeopardise regional water security, strain resources, and impact communities and agriculture.

“Conserving water now ensures resilience against drier periods and helps sustain the dam’s vital role in the region’s water supply.”

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