THE

South Africa’s most hard-working dam is celebrating its 82nd birthday this year, but it is not showing any signs of retirement! The dam, which lies at the confluence of the Vaal- and Wilge rivers, supplies potable water to the country’s economic heartland. The originates near in the , while the originates near Rayton in . The catchment therefore is quite vast, covering 38 500 km2, and includes towns such as Bethlehem, Frankfort, Standerton and Ermelo.

In 1938, the construction of the superstructure of the Vaal Dam was completed. A small group of workers led by a superintendent were appointed for the purpose of maintenance. This resulted in the establishment of the town of Deneysville. The Vaal Dam was completed just in time and on 13 December 1938 it overflowed for the first time. At that stage the dam had a full supply capacity of 994 million m3.

After World War II, because of the rapid expansion in industrial activity and development of the Free State goldfields, it was decided that the Vaal Dam’s height be raised by 6,1 m to make additional supplies of water available. This comprised of raising the concrete overspill crest by 3,05 m and installing 60 crest

gates of 2,05 m high on top of the concrete. The earth embankment was also raised.

Work on this project started in 1952 and was completed in 1956. The raising increased the storage capacity to 2 330-million m3. The cost of raising the dam was in excess of R2,9-million.

A country-wide drought during the early seventies resulted in the construction of near Harrismith, to provide backup water for the Vaal dam in the event of future droughts. Water would be pumped from the into the Sterkfontein Dam, as part of the Tugela-Vaal Pumping Scheme. The project also generates hydro-electricity in the process. When required, water is released into the Nuwejaarspruit, which joins the Wilge River, and then flows into Vaal dam.

The 1974 - 1975 floods, which were the biggest recorded floods in history at the time, led to the second raising of the Vaal Dam wall in 1985. This time the wall was raised by 3.05m, and the crest gates were replaced with bigger ones (6.68m high). The bigger gates allow for an additional 26.2% flood storage above 100% full supply capacity.

Shortly after the second raising of the wall, the demand for water increased to such an extent that additional supplies had to be identified. This led to the commencement of the negotiations between the South African government and the Kingdom of Lesotho. The negotiations between the two countries centered around the development of the Lesotho Highlands Water Project, which would provide water to , and electricity to Lesotho. The scheme started to deliver water to South Africa in 1998. Phase I of the project ensured that the Kingdom of Lesotho would begin supplying water to South Africa from the Katse and Mohale Dams. Phase II is currently in progress and includes the Polihali Dam. This phase is expected to provide additional water to South Africa from 2025.

The flood attenuation properties of the Vaal Dam were severely tested in February 1996 when the largest flood ever recorded at the Vaal Dam site was experienced. An inflow of over 4 700 m3/s was measured into the Vaal Dam, which was already at full capacity due to good seasonal rains. During the period 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 over.

It is often asked why the Vaal Dam does not show significant increases in water level after some good rains. The reason is that, being such a big dam, and having such a large catchment area, a lot of rain needs to fall to saturate the catchment, groundwater and smaller farm dams before enough runoff can be generated to flow to the dam. To explain this, look at the following figures:

Firstly, abstraction from the dam by and other users is in the order of 5 000 Megalitres per day (one megalitre is a million liters). In addition, due to the large surface area 864 000m3 is lost from the Vaal Dam daily through evaporation. Simultaneously, some water is released continuously to maintain the base flow in the river for downstream users.

The full capacity of the dam is 2,57 billion m3 (257 000 000 000 000 cubic meters). Therefore, to raise the level by 1% will require the addition of 2 570 000 000 000 cubic meters of water.

Now if we consider that 1mm of rain over the whole surface area of the dam equates to 321 070 000 litres of water, that would only raise the dam level by 0.0000125% of its capacity (not counting the abstraction uses and losses described above).

Therefore, good rains over a long period is required to produce the necessary runoff to fill the dam. Fortunately, the big catchment area is also an advantage, because once it is saturated, huge volumes of runoff are generated and the dam can fill in a relatively short period.

In January 2011, Vaal Dam reached 100% capacity and 14 sluice gates had to be opened to release the water. The outflow was 1800m3/s. As an Olympic size swimming pool has a capacity of 2 500m³, then a total of 2 177 280 000 swimming pools passed through the sluices in 14 days.

FACTS & FIGURES ABOUT THE VAAL DAM

Capacity : 2,57 billion m3

Shoreline: 880km

Dam surface area: 32 107 ha

Number of crest gates: 60 - each capable of releasing 115m3/s

Number of provinces bordering shoreline: 3 (Free State, Mpumalanga and )

Dam catchment area: 38 500km2

Wall dimensions: Concrete wall of 714km long and 63,4m high; Earth embankment 1970m