Three-dimensional modelling of the circulation in False Bay, South Africa

Abstract

False Bay is the largest coastal embayment along South Africa’s coastline. Despite notable contributions of various historical observational studies on the circulation within False Bay and the associated physical processes driving these flows, there remains significant uncertainty largely because the studies to date have been undertaken over short durations and on limited spatial scales. This article aims to expand upon the findings of these historical observational studies by providing a thorough evaluation of intra-annual hydrodynamics within False Bay, with particular emphasis on providing greater definition on current circulation. To evaluate the effects of different processes on coastal hydrodynamics, a 3D hydrodynamic model was developed which included the influence of spatially varying wind forcing and daily averaged temperature-depth profiles on the model boundaries and atmospheric heat exchange. The model results relating to current circulation are consistent with historical studies that described a cyclonic (clockwise)
circulation within the bay under southeasterly wind conditions. The development of this cyclonic pattern is primarily caused by the spatially varying wind field. During warmer months, upwelling events were noted at Cape Hangklip. Interestingly, under northwesterly wind conditions, the model results deviated from historical findings by showing a spatially uniform current field across the bay. Under these conditions, strong bottom return currents are generated, which contribute significantly to cold-water intrusion events within the bay.

Keywords: coastal engineering, coastal hydrodynamics, HYCOM data, hydrodynamic modelling, numerical modelling, spatially varying wind field, wind forcing