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The development and validation of a hydrodynamic model of False Bay

dc.contributor.advisorTheron, Andre K.en_ZA
dc.contributor.advisorDiedericks, Gerhardus P. J.en_ZA
dc.contributor.authorColeman, Fawaazen_ZA
dc.contributor.otherStellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.en_ZA
dc.date.accessioned2019-02-28T08:49:04Z
dc.date.accessioned2019-04-17T08:27:27Z
dc.date.available2019-02-28T08:49:04Z
dc.date.available2019-04-17T08:27:27Z
dc.date.issued2019-04
dc.identifier.urihttp://hdl.handle.net/10019.1/106078
dc.descriptionThesis (MEng)--Stellenbosch University, 2019.en_ZA
dc.description.abstractENGLISH ABSTRACT: False Bay is located in the Western Cape and is the largest coastal embayment along the South African coastline. The location of the bay is unique in that it is influenced by a complex wind regime and by both the Benguela and Agulhas currents. In recent years, False Bay has become the focus of numerous studies investigating engineering and environmental issues such 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 coastal hydrodynamics, within False Bay needs to be well defined. The findings of a comprehensive literature review revealed that despite the notable contributions of various historical studies on the analysis of bay circulation and the physical processes driving them, there remains significant uncertainty mainly due to these studies being undertaken over short durations and on limited spatial scales. Most importantly, regarding coastal hydrodynamics, few historical studies have investigated the effect(s) of the spatially varying wind field in False Bay which has been regarded as a significant hindrance in providing robust simulation of hydrodynamics. One of the major objectives of this thesis was therefore to address the shortcomings of historical studies by providing a thorough evaluation of intraannual hydrodynamics within False Bay with particular emphasis on defining (1) current circulation; (2) horizontal thermal structure (sea surface temperatures); and (3) vertical thermal structure. In order to evaluate the three hydrodynamic processes above, a three-dimensional hydrodynamic model was developed, calibrated and verified. The modelling approach included the influence of the spatially-varying wind field, daily averaged temperature-depth profiles on the boundaries and atmospheric heat exchange. The model results relating to current circulation were consistent with historical studies which described a cyclonic (clockwise) circulation within the bay under south-easterly wind conditions. 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, under north-westerly 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. Analysis of model results revealed that the horizontal thermal structure is uniform during colder months (June to August). In summer months, surface temperatures were progressively warmer toward the northern shore. This finding was consistent with composite satellite measurements. Furthermore, upwelling cells were identified at Cape Point, along the eastern shore and at Cape Hangklip. Regarding the vertical thermal structure of the bay, the model results indicated that at the entrance of False Bay, the influence of Rocky Bank was very apparent. This feature segregates warmer western and colder eastern waters. Due to the asymmetry of bathymetry at the entrance, 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 northwesterly winds 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 of hydrodynamic modelling of False Bay. It was found that a two-dimensional hydrodynamic model of False Bay would be suitable for the simulation of hydrodynamics under well-mixed conditions. In addition, the sensitivity results proved that spatially uniform wind fields are not sufficient in characterising hydrodynamic processes within False Bay. Furthermore, the sensitivity analyses indicated that monthly-averaged temperature-depth profiles on the boundary provided results that were comparable to those from daily-averaged temperaturedepth profiles.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Valsbaai is in die Wes-Kaap geleë en is die grootste baai langs die Suid-Afrikaanse kuslyn. Die ligging van die baai is uniek aangesien dit beïnvloed word deur ‘n komplekse windregime en deur beide die Benguela en Agulhas strome. In die verlede was Valsbaai die onderwerp van talle studies rakende ingenieurs- en omgewingsaangeleenthede soos byvoorbeeld waterkwaliteit, storms, fokussering van golwe en sleurstrome by sommige strande. Om hierdie prosesse te verstaan en te kwantifiseer moet die fisiese prosesse, soos kushidrodinamika, in Valsbaai goed gedefinieer word. ‘n Omvattende literatuurstudie het aangetoon dat, benewens die noemenswaardige bydraes van verskeie historiese studies oor die analise van sirkulasie in die baai en die fisiese prosesse wat dit veroorsaak, is daar nog steeds beduidende onsekerheid hoofsaaklik omdat hierdie studies oor kort periodes en op beperkte fisiese skaal uitgevoer is. Die belangrikste tekortkoming, wat kushidrodinamika aanbetref, is dat daar te min studies is wat ondersoek ingestel het oor die effekte van wind wat ruimtelik oor die baai varieer wat beskou word as die hoofbeperking om akkurate hidrodinamiese simulasies te doen. Een van die hoofdoelwitte van hierdie tesis was om die tekortkominge van die historiese studies aan te spreek deur middel van ‘n deeglike ondersoek van hidrodinamika wat binne ‘n tydskaal van ‘n jaar verander in Valsbaai met spesifieke klem op die definiëring van (1) stroomsirkulasie (2) seevlak temperature; en (3) vertikale temperature. Om bogenoemde drie hidrodinamiese prosesse te evalueer, is ‘n drie-dimensionele hidrodinamiese model opgestel, gekalibreer en geverifieer. Die modellering ondersoeke het die invloed van winde wat ruimtelik varieer ingesluit asook diepte-profiele van daaglikse gemiddelde temperature op die grense van die model en atmosferiese hitte-uitruiling. Die model- resultate rakende stroomsirkulasie was in ooreenstemming met historiese studies wat ‘n sikloniese (kloksgewyse) sirkulasie in die baai beskryf tydens ‘n suidooste wind toestand. Die sikloniese sirkulasie is ‘n gevolg van die windveld wat ruimtelik varieer. Gedurende warmer maande, is opstuwing by Hangklip opgemerk. Onder noordweste winde wyk die model resultate af van historiese studies deur ‘n ruimtelik uniforme stroomveld oor die baai aan te toon. Onder hierdie omstandighede word sterk trustrome op die bodem opgewek wat ‘n beduidende invloed het op koue water wat in die baai inbeweeg. ‘n Analise van die model resultate het ook aangedui dat die seevlak temperature uniform is gedurende kouer maande (Junie tot Augustus). Gedurende somer maande word die temperature op die oppervlak progressief warmer in die rigting van die noordelike strande. Die bevindinge is in ooreenstemming met satellietmetings. Verder is opstuwing langs Kaappunt opgemerk asook langs die oostelike strande en Hangklip. Wat die vertikale temperatuur verspreiding in die baai aanbetref, dui die model resultate daarop dat by die ingang tot die baai het Rocky Bank ‘n beduidende invloed. Hierdie rotsbank verdeel warme westelike vloei van koue oostelike vloei. As gevolg van die feit dat die dieptes by die ingang tot die baai nie simmetries is nie, is die water op die bodem ongeveer 1-2oC kouer oos van Rocky Bank onder toestande waar die water beide vermeng is en ook gestratifiseer is. In warmer maande, onder toestande waar die water gelaag (gestratifiseer) is, dra noordweste winde by tot die stratifikasie in die baai deur adveksie van koue water vanaf die diepsee tot in die middel van die baai. ‘n Sensitiwiteitsanalise is onderneem om ondersoek in te stel na die moontlike ingenieurs toepassings van die hidrodinamiese model van Valsbaai. Daar is bevind dat ‘n tweedimensionele hidrodinamiese model van Valsbaai toepaslik sal wees vir simulasies wanneer die water vermeng is. Verder het die sensitiwiteitsanalise ook getoon dat winde wat ruimtelik uniform is, nie voldoende is om hidrodinamiese prosesse in Valsbaai te karakteriseer nie. Die sensitiwiteitsanalise het ook aangetoon dat maandelikse gemiddelde temperatuurprofiele oor die diepte op die grense van die model resultate lewer wat vergelykbaar is met resultate gebaseer op daaglikse gemiddelde temperatuurprofiele.af_ZA
dc.format.extent166 pages : illustrationsen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.subjectCoastal engineeringen_ZA
dc.subjectUCTDen_ZA
dc.subjectHydrodynamicsen_ZA
dc.subjectFalse Bay (Western Cape, South Africa)en_ZA
dc.titleThe development and validation of a hydrodynamic model of False Bayen_ZA
dc.typeThesisen_ZA
dc.rights.holderStellenbosch Universityen_ZA


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