Monwabisi: a hydrodynamic study of the hazardous cell circulation and potential related solutions to a safer bathing facility

[摘要] ENGLISH ABSTRACT: The objective of this thesis is to investigate the hazardous hydrodynamic circulation patternat Monwabisi (False Bay, South Africa) and potential solutions to a safer bathing facility, bymeans of a complex hydrodynamic numerical model.A hazardous circulation cell exists at Monwabisi which is caused by a spur extending off anadjacent tidal pool. The partial wave‐sheltering of the spur produces a wave setup gradientin the bay which facilitates a strong anti‐clockwise circulation. The wave‐sheltering creates afalse perception of safety as the spur forces the current in a strong and concentrated flow in aseaward direction. This strong current, often referred to as a rip current, has been linked toseveral rescues and drownings. Removal of the structure was not considered an option as itprotects the tidal pool against wave overtopping and scouring as well as holds a beach whichdid not exist pre‐construction. The CSIR (1997) showed by means of a physical model studythat the hazardous circulation could be reduced to a great extent by means of structuralinterventions. However, no structural changes have been implemented since the publicationof their report.This study reinvestigates the CSIR's proposed solutions by means of a 2D Boussinesq typenumerical wave model. The numerical model aimed to replicate the CSIR's physical modelsetup conditions, to simulate the resultant circulation patterns and current velocities, andcomparing the results to the measurements of the CSIR's physical model.An initial baseline simulation was set‐up to simulate the cell circulation present at Monwabisi.The simulation showed excellent agreement to the prototype conditions in terms of theobserved and reported physical processes and the hazardous counter‐clockwise currentcirculation.The numerical simulation of the CSIR's proposed solutions followed. An in‐depth analysis ofthe numerical model's results demonstrated the effectiveness of groynes in reducing thecurrent velocity of the cell circulation. Various other options were tested, but proved to be lesssuccessful. The groynes physically divided the beach, thereby producing a wave‐shelteredbay removed from the feeder currents originating from high wave energy section of the bay.The optimal solution was a T‐shaped groyne structure placed perpendicularly to the incomingwaves. The flanges of the T‐groyne reduced the incoming wave energy even further, whilstalso separating the newly created sheltered and high wave energy bays, thus reducingcirculation intermixing between the two. The T‐groyne reduced the strong velocity along thespur significantly and also resulted in the most quiescent bay of all the tested options.The results of the numerical model were also compared to the CSIR's physical simulationmodel results. The majority of the simulations showed good agreement in terms of themeasured velocities and the general circulation patterns. Both models indicated that the Tgroyneis the most effective solution, especially in terms of reducing the current along thespur. The Boussinesq Wave model proved to be a reliable and useful tool in simulating thecomplex hydrodynamic system at Monwabisi and has great potential for similar studies in the future.

[发布机构] Stellenbosch University