Ngqura’s sand bypass system leads way
The sand bypass system will handle a dredging target of 240 000 tons of sand a year, according to Transnet National Ports Authority (TNPA). This will be achieved largely without the use of a visiting dredger.
It was the initial failure of this system at Ngqura that resulted in Durban not introducing something similar when the port entrance was being widened and deepened.
A number of innovations have been introduced at Ngqura that will gradually find their way into more general port use across older South African ports. Having been built from scratch has allowed Ngqura to become the only port in South Africa to have a Record of Decision for its construction and operation.
It enabled Ngqura to become the only port subjected to environmental legislation during its entire development and operation.
Other South African ports would find that a difficult challenge to follow, for obvious reasons, but it has enabled Ngqura to adhere to very strict conditions in an environmentally sensitive area of the coast and coastland.
Ironically, what the Eastern Cape port is now celebrating might also have been installed at Durban while the entrance channel was being deepened and widened. The initial intention was to install a device using a fixed venturi type “jet” pipe in the sand trap outside the south breakwater. This would distribute sand caught up by the littoral drift from the south (along the Bluff) and carry it through pipes underneath the entrance channel to be distributed as required all along Durban’s entire Golden Mile - as far as the uMngeni River mouth, or even further, according to the then chief port engineer.
One of the advantages of this scheme was said to be that it would practically eliminate the need for a large fleet of dredgers. This was prior to the ordering of the current fleet of dredgers, Isandlwana, Ilembe and Italeni, that were subsequently ordered from a Dutch specialist shipyard.
As it turned out, the system was installed at the then under-construction port of Ngqura, although information on its progress was not easy to obtain. According to engineers then working at TNPA, the system ran into difficulties as a result of rocks and debris being sucked into the piping installed in Algoa Bay, resulting in blockages that caused the system to stall.
The similar scheme planned for Durban (and Richards Bay) was abandoned, with TNPA placing orders for the new fleet of dredgers and leading to TNPA and the municipality having to enter into lengthy discussions over the need for a new hopper station on or near the north breakwater to accept and distribute the sand delivered by dredgers from the sand trap.
As is well known, this task has taken a long time to achieve some form of conclusion although it remains uncertain whether even this system is working as planned, hence the controversial concentration of sand along uShaka Beach and Vetch’s Pier - although there may be other reasons.
Getting back to Ngqura where the initial problems have apparently been overcome, TNPA says that the system is mimicking the natural longshore drift of sand by means of a computer-controlled sand bypass system consisting of jet (venturi-type?) pumps. Sand is gathered from the up drift (west) section of the coast to the down drift (east) section (the coast hereabouts runs in an east-west direction rather than south-north).
TNPA says that sand captured in a sand trap (on the west side) is pumped via pipelines to a discharge point on the opposite end of the port. Both capturing and discharging occur in the wave zones.
The average amount of sand that will be required to bypass the port entrance at Ngqura will vary between 240000 to 320000 tons a year, and failure to transfer the minimum amount of 240000 tons could result in the port being fined about R10million.
“The system requires a rigid maintenance programme that keeps it going 24 hours a day, seven days a week,” says Freddie Melikhaya, the mechanical engineering technician leading the sand bypass team at the port.
By comparison, the nearby Port Elizabeth must dredge at a rate of 130000 tons a year, Durban at 500000 tons and Richards Bay at a whopping 1.3million tons a year.
This has to be achieved on a planned schedule with contingencies for unplanned maintenance as well as daily planning, scheduling and improving maintenance techniques - all within budget, says TNPA.
“In simple terms the Ngqura system is a ‘fixed dredger’ that operates cost-effectively when compared with a mechanical dredger. Dredging is a crucial aspect of ensuring that vessels can enter and exit the ports safely. It helps to halt beach erosion and sand build-up in the ports. This helps with navigation, flood control, coastal development, mining and environment management.”