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Vaal Dam is located on the Vaal River some 56km south of Johannesburg,
close to Vereeniging. The catchment area of the dam is approximately 38 500 km2
most of which is located in the Free State Province with the remainder in the North West,
Gauteng and Mpumalanga provinces. The catchment area has an average annual precipitation
(MAP) of approximately 700mm with a corresponding annual potential evaporation (Symons
Pan) in the order of 1 500mm.
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At full supply level¸ the Vaal Dam can store 2 536 million m3/a
with a surface area of approximately 320 km2 making it a relatively large and
shallow reservoir. The natural inflow to Vaal Dam is approximately 1 950 million m3/a
although the upstream Grootdraai and Sterkfontein
dams reduce the resulting natural inflow to approximately 1 400 million m3/a
making the Vaal Dam a 1.7 MAR dam.
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Vaal Dam forms the central storage reservoir for the Vaal River water
supply system which supplies water to the industrial powerhouse of the whole country. The
area supplied from the Vaal River System, generates more than 50% of South Africa's gross
geographical product (GGP), more than 80% of the country's electricity and includes some
of the largest gold, platinum and coal mines in the world. Vaal Dam is therefore of great
importance to South Africa and a key component of the water supply infrastructure for
Gauteng and the surrounding provinces.
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Due to the increasing demands in the Vaal River System Supply Area, the
natural resources of the Vaal Dam catchment are unable to supply the full water
requirements. Various interbasin transfers already exist to transfer water from areas with
excess resources to the Vaal Dam catchment and new schemes are continually being
developed. The first phase of the largest and most impressive transfer scheme, the Lesotho Highlands Water Project, is currently nearing completion
and the first water transfers from the scheme are scheduled for 1998. Only the first phase
of the LHWP has been approved and no decision regarding possible future phases has yet
(1997) been taken.
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The construction of 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 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 dam was subsequently raised in the early fifties to a height of
60.3m which increased the capacity to 2 188 million m3. A second raising took
place in 1985 when the wall was raised by a further 3.05m to 63.4m above lowest
foundation. 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.
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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.
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