The construction of the Vaal Dam started during the depression of the early thirties and the dam was completed in 1938 with a wall height of 54.2m above lowest foundation and a full supply capacity of 994 million m3.
The dam wall is a concrete gravity structure with an earthfill section on the right flank. It was built as a joint venture by Rand Water and the Department of Irrigation (now known as the Department of Water Affairs and Forestry). The Vaal Dam is of great importance to South Africa and a key component of the water supply infrastructure for Gauteng and the surrounding provinces.
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 be raised by 6,1 m to make additional supplies of water available. This entailed raising the concrete overspill crest by 3,05 m and installing 60 crest gates 2,05 m high on top of the concrete. The earth embankment was also raised. To control floods better the gate height was increased by 1,82 m. Work began in 1952 and was completed in 1956.
In 1979 the Department of Water Affairs proposed to raise the dam wall by 3.05m to 63.4m above the lowest foundation. This increased the capacity of the Vaal Dam by 1 033,5-million m3 to 3 364-million m3. It was proposed that 1,1m of this raising which took place in 1985 be used for supply storage and the rest for flood storage. The capacity of the dam is currently 2 536 million m3 and a further 663 million m3 or 26% can be stored temporarily for flood attenuation.
The flood attenuation properties of the 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 700m3/s was measured into the Vaal Dam which was already at full capacity due to good rains and it was only through the expert management of the Hydrology staff at DWAF that the maximum flood released from the dam was limited to 2 300m3/s. Flows above 2 300m3/s would have caused serious damage downstream of Vaal Dam and the situation during the 1996 flood became extremely tense as the storage in the reservoir peaked at 118.5% of Full Supply Capacity on 19 February 1996 i.e only 194 million m3 of flood absorption capacity remained before the full inflow would have been released causing massive damage. During the period from 15 December 1995 to 15 March 1996, the inflow volume to Vaal Dam was estimated at 7 605 million m3 - sufficient to fill the dam three times over. The inflow peak was estimated to have a return period of 70 years while the outflow peak was estimated to have a return period of only 20 years.
Visits to the wall can be arranged by appointment with Mrs Lotter (016) 371-3039 at the Dept of Water Affairs.
In March every year from a Monday to Friday the Dept of Water Affairs host 'Water Week' with regular tours to the wall. These tours are usually at 10h00, 12h00 and 14h00. The date in March is at the descretion of the Department and does not necessarily fall on the same days each year.
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