Pongolapoort dam
Supplied by Royal Jozini Private Big 6 Estate, Swaziland
Here’s some interesting information about the history of the Pongolapoort Dam, which flows into Swaziland – where it is known as Lake Jozini.
Prior to the construction of the Pongolapoort Dam the land was Africa’s first formally recognised conservation area. The Pongola Game Reserve was proclaimed in 1894 by the then President of the Transvaal Republic, Paul Kruger. (This would ultimately lead to the proclamation of the Hluhluwe-Imfolozi, Mkuze and Ndumo Game Reserves as well as one of Africa’s greatest wildlife conservation parks, the Kruger National Park.)
In the Depression years of the 1930s, a government irrigation settlement was established on the west side of the Lebombo mountains. This settlement comprised 159 plots with a total area of 6 189 ha. A sugar mill was constructed in 1954 and water for irrigation was provided either by government-built gravity canals or was pumped direct from the Pongola river.
This was the beginning of the town of Pongola!
The dam itself is a medium thin, double curvature arch dam with a gradual transition towards a gravity thrust block on the left flank. It has a maximum height of 89 m and a crest length of 515 m. The dam has a controlled and an uncontrolled spillway. The gross capacity of the reservoir is 2 500 million cubic metres, which is more than twice the mean annual runoff. The chute spillways have a combined capacity of 2 010 cubic meters at high flood level. The thickness of the wall above the cushion is 18,3 m tapering to 8,2 m at the spillway and then flowing to 11 m to carry a road across it.
The foundations and abutments of the dam presented a number of challenges. During excavation for the foundation, great difficulty was experienced as a result of the sensitivity of the brittle dacite to blasting and stress relief and its reaction to changes in temperature. In the South African National Committee on Large Dams’ publication “Large Dams and Water Systems in South Africa” it is written: “Whole layers of what appeared to be sound rock scaled off with a noise like a pistol shot and necessitated the use of 30 meter long rock anchors, line drilling and the use of hydraulic wedging for final excavations.”
Prior to the construction of Pongolapoort Dam, the area was fairly isolated. There were no communications or services of any sort and, according to media reports at the time, the resident engineer Mr RF Phélines spent the early years of the dam’s construction sleeping in a tent.
By 1955, plans were well advanced for the construction of the dam, to be built in the Pongolapoort – the gorge between the Ubombo and Lebombo Mountains. The dam was planned to support 40 000 to 50 000 ha of irrigation on the Makatini Flats, a highly fertile area adjacent to the floodplain on both sides of the river. Apart from boosting commercial farming, the government also hoped to ‘stabilise the frontier’ bordering Mozambique and Swaziland.
Interestingly, merits of the scheme were a subject of debate even after construction started. One of the main grounds of criticism was that intensive soil and other tests, which would determine suitable land usage, were only undertaken after the project was given the go ahead. The plans for intensive agriculture never materialised.
Construction of the dam started in 1963.
Work on the dam was on a 24-hour basis, requiring up to 764 cubic meters of concrete a day. The aggregate came from a site 20 km upstream and some two million tons had been stockpiled at the start of the project.
Another significant challenge was the high average air temperatures on the site. This was overcome by pre-cooling the aggregate with controlled amounts of crushed ice. Pongolapoort Dam was the first dam in South Africa where this artificial cooling method was used.
The dam went up in 1,8 m sections, the curvature of each one having to be separately calculated, taking about 30 hours on a manual calculation. Each vertical section is independent of the other. The gravity sections on the flanks induced blasting of some 500 000 t of rock. The dam was eventually completed in 1973.
In the early years following its completion, Pongolapoort Dam could not be filled as it would inundate part of Swaziland. While negotiations with Great Britain had solved this problem, these decisions were withdrawn by Swazi authorities following the country’s independence. Both the political and engineering problems were overcome by 1982. This was none too soon because in 1984 the area was hit by tropical cyclone Domoina.
A Department of Water Affairs report on the effects of the cyclone describes that on 31 January 1984 when Domoinahit, the catchment of the dam received more than 700 mm of rainfall (a record to this day) and a peak inflow of 1 600 cubic meters occurred into the Pongolapoort Dam. At the time the dam was only 13% full but the total inflow as a result of the cyclone was 2 000 million cubic meters or 87% of the total capacity of the dam.
Today, you can visit the Swazi side of the Pongolapoort dam and take a bush break on the slopes of the Lebombo’s in Royal Jozini Big 6, perhaps catch some tiger fish or just relax from your lodge patio and gaze at the water!
Acknowledgements, SA Dept Forestry and Water Affairs, www.dwaf.gov.za, Wikipedia