Why the Vaal Dam doesn’t fill up after every thunderstorm

A good thundershower and rainwater flooding the streets prompts most people to think that the Vaal Dam is now full and that saving water is no longer a priority. 

This could not be further from the truth. 

Rand Water’s senior water quality specialist, Francois van Wyk, seeks to educate consumers about the Vaal Dam, its catchment areas and how difficult it is to achieve 100% capacity – even in the summer season.

A brief history of the Vaal Dam 

The Vaal Dam is big – it has a storage capacity of 2.57 billion m³. But it wasn’t always this large. 

The dam is 82 years old this year. It was first constructed in 1938 by a small group of workers, which resulted in the town of Deneysville being established. 

Construction on the Vaal Dam in 1932. Image: vaaldam.org

On 13 December 1938, the dam overflowed for the first time. At the time, it had a full supply capacity of 994 million m³. 

Photo: ruralexploration.co.za

Rapid industrial expansion after World War II and the development of the Free State goldfields prompted the Vaal Dam’s height to be raised by 6.1m. 

This meant raising the concrete overspill crest by 3.05m and installing 60 crest gates that towered 2.05m high on the concrete. The earth embankment was also raised. 

The project was completed in 1956, cost around R2.9 million and increased the storage capacity to 2.33 million m³. 

A few years later in the 1970s a drought hit, which prompted the construction of the Sterkfontein Dam in the Free State to provide backup for the Vaal Dam. 

Water is pumped from the Tugela River into the Sterkfontein Dam, which is called the Tugela-Vaal pumping scheme. Hydroelectricity was also generated in the process. 

If the Vaal Dam needs more water, water is released into the Nuwejaarspruit to join the Wilge River and flows into the Vaal Dam. 

In the mid-1970s some of the most intense floods in history struck, which meant once again raising the Vaal Dam wall in 1985 to 3.05m and the crest gates were replaced with 6.68m high gates. 

Larger gates meant additional storage of 26.2%. 

This was tested in February 1996 when the Vaal Dam’s largest flood ever recorded took place. An inflow of more than 4.7m³/sec was measured coming into the dam, which was already at full capacity due to good rains. 

From 15 December 1995 to 15 March 1996, the inflow volume to the Vaal Dam was estimated at 7.605 million m³ – enough to fill the dam three times. 

But just after raising the wall in 1985, it became clear that demand for water was increasing rapidly. 

This led to negotiations between South Africa and the Kingdom of Lesotho, forming the Lesotho Highlands Water Project. The deal was for Lesotho to provide water to South Africa in exchange for electricity. 

Water began to be delivered to South Africa in 1998, with Phase I of the project ensuring Lesotho would begin supplying water to South Africa from the Katse and Mohale dams. 

Phase II of the project is now in progress to include the Polihali Dam and is expected to provide additional water supply to South Africa from 2025. 

What good rains mean for the Vaal Dam

The Vaal Dam’s shoreline stretches 880km through Gauteng, the Free State and Mpumalanga. The dam has a surface area of 32 107 hectares. Each of the 60 crest gates can release up to 115m³ per second. 

Image: Gauteng Tourism Authority

It has a catchment area of 38 500km². This large catchment area means a significant amount of rain needs to fall in those areas to make a difference to the Vaal Dam’s capacity. 

Van Wyk said rain needs to saturate the catchment, groundwater and smaller farm dams before enough runoff is generated to flow into the Vaal Dam. 

The large catchment area is an advantage, because as soon as this is achieved, high volumes of runoff are generated and the dam can fill up in a relatively short space of time. 

But here is why the Vaal Dam doesn’t automatically fill up after a big storm. 

Rand Water and other everyday consumers use around 5 000 megalitres of water every day. 

The dam’s large surface area means around 864 000m³ of water is lost through evaporation every day. 

And at the same time, a lot of water is continuously released from the dam to maintain the base flow in the river for downstream users. 

For some quick maths, Van Wyk says because the dam’s capacity is 2.57 billion m³, or 257 000 000 000 000 cubic metres, raising the dam’s level by just 1% requires an additional 257 000 000 000 000 m³ of water. 

Just 1mm of rain over the entire dam’s surface area equates to 321 070 000 litres of water. But this would only raise the dam’s level by 0.0000125%. 

This is why rains are only considered good for the Vaal Dam if it lasts long enough to produce runoff to the dam. 

The last time the dam reached 100% capacity was in January 2011, when 14 sluice gates had to be opened to release the water, which was flowing at 1 800m³/sec. 

This equates to more than two billion Olympic-sized swimming pools passing through the sluice gates in two weeks. 

The Vaal Dam is currently at 28.1% capacity, Sterkfontein Dam at 94.4%, Grootdraai Dam at 75.6% and the Bloemhof Dam at 80%. 

Saving water regularly the way to go 

Consumers must be reminded not to let their guard down after good rains. 

In Gauteng, the past few days of rainfall have been a welcome relief from the blistering heat wave that lingered for weeks. But a few storms does not mean being able to take longer showers or hose one’s driveway down. 

Consumers need to stay consistent in their water saving. Here are some easy tips to save as much water as possible with minimal effort. 

• Put a bucket in your shower which gathers water while you wait for it to get hot. This can be used to water your garden;
• Take quick showers and, if you take a bath, make sure not to fill it to the brim;
• Close the tap while washing your hands and brushing your teeth; 
• Do not leave the hosepipe on while washing your car or paving. Rather rinse, soap and rinse again; and
• Fix any leaks or broken taps.

Information provided to The Citizen by Rand Water senior water quality specialist, Francois van Wyk and Rand Water media relations manager Justice Mohale.  

Rand Water senior water quality specialist, Francois van Wyk.

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