Coega to create 'living reefs'
2 April 2003
The giant breakwaters that will protect the deep-water harbour of Ngqura will soon become living reefs, predicts Deon Retief, the partner responsible for the Nqura breakwater design at Prestedge Retief Dresner Wijnberg.
A combination of rock and dolosse is being used in the construction of the breakwaters. Experience on similar projects elsewhere has shown that the many gaps between the different rocks and the dolosse themselves provide safety and protection for a wide range of fish, molluscs and plants, Retief said.
"I see breakwaters like these as tremendous habitat creators. We will soon see colonies of sea creatures establish themselves on both the exposed sea-ward part of the breakwater and on the more protected inside.
"There is always a slow flushing of water through a breakwater, and this creates a very healthy environment. We often see nursery conditions kicking in immediately," he said.
Sand bypass systems will ensure that
there is no build-up that smothers the growth. The sand will be pumped from the breakwater on the Port Elizabeth side of the harbour and deposited back into the sea off the main breakwater on the Sundays River side, with all the micro-organisms having re-established themselves within a kilometre of the breakwater.
The Coega Project, comprising an industrial development complex and deepwater port 20 kilometres east of the city of Port Elizabeth, is the single largest infrastructure development project in the country since 1994, has already pulled R5-billion in committed investment, and looks set to make South Africa the hub of all north-south and south-south sea traffic.
The Port of Ngqura, a multi-user deepwater port on the Coega River, is being developed by the National Ports Authority of South Africa, a division of Transnet.
A channel carved by an ancient glacier has allowed the development of the port to a maximum depth of 23 metres. With the depth of the channel
and the protected position in Algoa Bay, the port is in one of the best spots for a harbour along the South African coast.
Coega dolosse making a big splash
Coega will also make history with the casting of the biggest dolosse on the African continent.
The first of 26 500 thirty-ton dolosse that will be used on the two breakwaters for the deep-water harbour of Ngqura have already been made.
According to Retief, the dolosse will form the top layer of the main breakwater which, at 2.5 kilometres long, is the "largest by far" in Africa.
The base of the massive breakwater, at its deepest end, is wider than a rugby field, and a temporary two-lane highway will be built on its crest to allow special transporters to carry the rock and dolosse that are being used in the construction.
Two massive cranes, tall and wide enough for two 40-ton tipper trucks to pass under, will straddle the breakwater. Every one of the two-storey high
dolosse will be carefully put in place, with the crane driver being guided by a global positioning system fitted at the end of the crane boom.
"The operator will have an electronic printout in the cab which will give the specific co-ordinates for each dolos," Retief added.
The dolosse will be placed on a layer of protective rocks weighing between three and six tons each. They, in turn, will rest on a base of core material made up of rock graded from 5kg to 3 tons.
Another first for the construction will be the placement of a giant 12 700 ton caisson at the end of the breakwater. The concrete caisson, which will be as wide as a rugby field and 20 metres high, will be cast onshore and floated into place before being filled with sand. Four smaller caissons will be used on a secondary breakwater for the harbour.
According to Retief, a full production line with a dedicated batch mixing plant is being created at Coega in order to produce up to 52 of the giant dolosse
a day.
Vibrators will also be used to ensure that no air is trapped in the concrete while it is setting.
He expects the placement of the dolosse on the breakwater wall to start soon.
Source: Coega Development Corporation
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