Predictive modelling of hydrodynamics and marine water quality: three applications along the South African coastline

摘要

The predictive capability of hydrodynamic and water quality models is illustrated by three applications of the Delft3D modelling suite from WL|Delft Hydraulics along the South African coastline. The first application involves dredging-induced turbidity in Saldanha Bay on the west coast. Modelling of the hydrodynamics and the dispersion and deposition of the turbidity plumes was performed to assess the impact of the plumes on the sensitive ecology in the bay. The second application investigates the feasibility of using seawater for industrial cooling at Saldanha Bay. Since this study involves a thermal discharge into a highly stratified environment, the Delft3D model was applied in three-dimensional baroclinic mode including air-sea heat fluxes. The model outputs of absolute temperature, increase in temperature due the thermal discharge and dilution enabled the ecological impacts to be assessed and the optimum discharge location to be identified. The final application is the design of a marine outfall discharging a buoyant effluent near Durban on the east coast. Since the initial dilution process exhibited significant temporal and spatial variability, a dynamic coupling of near- and far-field hydrodynamic models was developed to accurately simulate the impact of the outfall. The results indicated that, under stratified conditions, a local optimum in the required pipeline length with respect to the impact of the effluent at the shore exists- the so-called "pipeline paradox".