Conceptual solutions to minimise the effects of cobbles on the sand-bypassing system at the Port of Ngqura.

[摘要] ENGLISH ABSTRACT: The Port of Ngqura constructed two breakwaters to create safe anchorage for vessels at berth. These coastal structures obstructed the natural movement of sand alongshore. A fixed sand-bypassing system was installed to transport the obstructed sand from the up-drift to the down-drift side of the Port. The system failed to reach the design rate; leading to major sand accretion against the western breakwater.The main problem was the presence of coarse material causing an obstruction at the jet pumps that prevents further bypassing of the material, located in the sandtrap. The system was designed to handle particles with a diameter of 150 mm, but it was the presence of particles larger than 150 mm that particularly created a problem. Two alternative fixed sand-bypassing systems, the Nerang river and Tweed river bypassing schemes, were investigated to determine if similar problems arose and if lessons could be learned for application at the Port of Ngqura. To find a solution to this challenge, it was necessary to determine where these particles originated from in the first part of the study.It was found that the coarse material originated from mainly three sources: temporary construction works required for the construction of the bypassing system, the rock revetment behind the sand-bypassing jetty, as well as from natural sources. While the origin and properties of the first two sources were known, further investigation was required to determine the source of the natural coarse material. The objectives of this part of the study were to gain a better understanding of the principles of sediment transport and particle motion as well as to investigate the origin, volume and properties of the natural source. The Swartkops river was found to be the main source of coarse material for the western section of Algoa Bay, with an estimated mean annual volume of 150 m3.Using the findings gained in this first part of the study, five viable solutions were conceived to prevent the obstruction at the jet pump intakes. All five solutions are considered viable solutions to the current problem at hand, but some were deemed more viable than others. For some of the conceptual solutions to function to their full potential, modifications were also required at the revetment and sandtrap. The sandtrap modifications included the removal of all the coarse material currently located in the sandtrap. For the revetment modifications, the armour layer at a certain section should be reconstructed and units that do not meet the requirements should be replaced.The study concludes that each proposed solution together with the required sandtrap and revetment modifications can serve as a potential solution to achieve the design bypassing rate of the fixed sand-bypassing system at the Port of Ngqura. The conceptual solution that proved the most promising, however, is the pile-and-mesh structure due to the relatively small impact that this proposed solution would have on the surrounding coastline, the low maintenance required, and high capacity of the structure.

[发布机构] Stellenbosch University