The development and validation of a hydrodynamic model of False Bay

[摘要] ENGLISH ABSTRACT: False Bay is located in the Western Cape and is the largest coastal embayment along theSouth African coastline. The location of the bay is unique in that it is influenced by a complexwind regime and by both the Benguela and Agulhas currents. In recent years, False Bay hasbecome the focus of numerous studies investigating engineering and environmental issuessuch as water quality issues, storm events, wave focusing and rip currents at certain beaches.In order to understand and assess these issues, physical processes, such as coastalhydrodynamics, within False Bay needs to be well defined.The findings of a comprehensive literature review revealed that despite the notablecontributions of various historical studies on the analysis of bay circulation and the physicalprocesses driving them, there remains significant uncertainty mainly due to these studies beingundertaken over short durations and on limited spatial scales. Most importantly, regardingcoastal hydrodynamics, few historical studies have investigated the effect(s) of the spatiallyvarying wind field in False Bay which has been regarded as a significant hindrance in providingrobust simulation of hydrodynamics. One of the major objectives of this thesis was thereforeto address the shortcomings of historical studies by providing a thorough evaluation of intraannualhydrodynamics within False Bay with particular emphasis on defining (1) currentcirculation; (2) horizontal thermal structure (sea surface temperatures); and (3) vertical thermalstructure.In order to evaluate the three hydrodynamic processes above, a three-dimensionalhydrodynamic model was developed, calibrated and verified. The modelling approach includedthe influence of the spatially-varying wind field, daily averaged temperature-depth profiles onthe boundaries and atmospheric heat exchange.The model results relating to current circulation were consistent with historical studies whichdescribed a cyclonic (clockwise) circulation within the bay under south-easterly windconditions. The development of this cyclonic pattern is due to the spatially varying wind field.During warmer months, upwelling events were noted at Cape Hangklip. Interestingly, undernorth-westerly wind conditions, the model results deviated from historical findings by showinga spatially-uniform current field across the bay. Under these conditions, strong bottom returncurrents are generated which contribute significantly to cold water intrusion events within thebay.Analysis of model results revealed that the horizontal thermal structure is uniform during coldermonths (June to August). In summer months, surface temperatures were progressively warmertoward the northern shore. This finding was consistent with composite satellite measurements.Furthermore, upwelling cells were identified at Cape Point, along the eastern shore and atCape Hangklip.Regarding the vertical thermal structure of the bay, the model results indicated that at theentrance of False Bay, the influence of Rocky Bank was very apparent. This feature segregateswarmer western and colder eastern waters. Due to the asymmetry of bathymetry at theentrance, bottom waters are approximately 1-2℃ colder east of Rocky Bank under well-mixed(isothermic) and stratified conditions. In warmer months, under stratified conditions, the northwesterlywinds significantly contribute to stratification in the bay due to cold water advection from offshore to the middle of the bay.Sensitivity analyses were undertaken to investigate the potential engineering application ofhydrodynamic modelling of False Bay. It was found that a two-dimensional hydrodynamicmodel of False Bay would be suitable for the simulation of hydrodynamics under well-mixedconditions. In addition, the sensitivity results proved that spatially uniform wind fields are notsufficient in characterising hydrodynamic processes within False Bay. Furthermore, thesensitivity analyses indicated that monthly-averaged temperature-depth profiles on theboundary provided results that were comparable to those from daily-averaged temperaturedepthprofiles.

[发布机构] Stellenbosch University