Project South Durban

Abstract

Durban is located on the east coast of South Africa in the province of KwaZulu-Natal. It is one of South Africa’s biggest cities and has one of the busiest ports in Africa. To maintain Durban’s current economic position the Durban Port has to expand, which is problematic due to a lack of space. A solution is to develop a completely new port: the Durban DigOut Port. The past has taught us that significant erosion/accretion patterns are likely to occur around the breakwaters of the new port, as has happened at the Durban Port. In the long run climate change could also lead to alterations in shoreline behavior of the South Durban coast. The problem definition reads: “Currently, it is not fully known how the coastal system of South Durban functions and what the future state of the system will be under influence of climate change and after construction of the Durban DigOut Port.“ The goal of this research is to understand the sediment transports and budgets along the South Durban coast both now and in the future. To do this it is necessary to predict and assess future shoreline behavior, in response to anthropogenic interventions, like the Durban DigOut Port, and to climatic changes such as sea level rise and changing wave conditions. The time scope is confined to the coming 40years. For this research use has been made of two numerical models. The equilibrium based Unibest-CL+ model is used to create a 1D shoreline model to model long-term shoreline behavior. The process-based SWAN (or Delft3d-Wave) model is used to model wave propagation on the continental shelf and in the nearshore of the South Durban coast. Waves approach the coast from a predominantly south-eastern direction. The annual wave climate consists of south-southeastern swell waves and wind-waves approaching from all directions. It provides a net northward longshore sediment transport for the South Durban coast, which is modeled to be 700,000m3/year at the Durban Bluff. A gradient in longshore sediment transports is found over the area from Amanzimtoti to the Durban Bluff. This results in coastal erosion, because sediment input from rivers is lacking. Research into the longshore sediment transports for the South Durban coast shows that the coastline lies close to its equilibrium position and that maximum potential sediment transports are high (order of 1.5 to 3million m3/year). This leads to significant erosion/accretion patterns around the Durban DigOut Port. The maximum shoreline retreat on the lee side of the port is modeled to be 500meters, 30years after construction of the breakwaters. A bypass system of 550,000m3/year reduces shoreline retreat to a maximum of 30meters. In addition a local groyne system is proposed to prevent coastal retreat at the Sapref Refinery area. The calculated coastal retreat can be viewed as an extreme prediction, because a uniform sandy coast is assumed for the modeling, which is not realistic given the rocky formations along the South Durban coast. Changes in the average wave climate are found from literature; the average wave direction is likely to rotate clockwise and wave periods are expected to increase during winter. When these changes are included in the shoreline model, gradients in longshore sediment transports along the coast are enhanced, leading to additional coastal erosion. Sand supply from rivers is reduced due to sand mining, which leads to structural coastal erosion over the total system as well.