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Sediment transport regime in the area of the East London harbour entrance
[摘要] ENGLISH ABSTRACT:East London is situated on the south-eastern, Indian Ocean, coast o f South Africa. The sedimenttransport regime at East London is quite unlike the regimes at other ports in South Africa. Amajor ocean current (the Agulhas) flows exceptionally close to the coastline in this area, thussignificantly affecting nearshore sediment movements. The proximity o f a strong ocean currentopposed to the net longshore drift (wave driven transport) creates an anomalous sedimenttransport regime in comparison with that found at most coastlines throughout the world.Furthermore, the Port o f East London (Figures 1.1 and 1.2) is the only major river harbour inSouth Africa, which all results in a very intricate pattern o f sediment movement in the area,making it o f particular interest. The aims o f this thesis are to study the littoral sedimenttransports at East London, achieve a holistic understanding o f this complex sediment transportregime and to quantify the various transport rates as far as possible.The study area includes the coastal zone between the Goda and Nahoon Rivers (Figure 1.3) withthe main focus on the Hood Point to Orient Beach area (Figure 1.4). The offshore marineenvironmental conditions are also considered as they have a strong influence on nearshorecurrents, waves and sediment transport patterns.The prerequisite to understanding the sediment transport processes is a thorough investigation o fthe relevant coastal processes. Thus, an in-depth study is undertaken o f the physicalenvironmental data/information, nearshore processes and coastal dynamics that determine or topography changes, the wave regime, the Agulhas Current, the nearshore current regime, thecontinental shelf sediment dynamics, sediment characteristics, seabed features, the Buffalo River,the tides, the wind regime, and coastal morphodynamics.The specific contribution of each aspect of the environmental data/information to the qualitativeunderstanding of the overall sediment transport regime is determined, and the various modes ofsediment transport in different areas are quantified. All o f this information is then synthesisedinto an expose' o f the sediment transport regime at East London, as briefly described in theconclusions (Section 5 and Table XI) and illustrated in Figure 4.30:There is a net longshore transport (wave driven) of about 250 000 m3 to 300 000 m3 peryear on average from the Foreshore area towards the head of the main breakwater, withthe total transport into the main sand trap and entrance channel areas from the south-westestimated at 275 000 m3/year. In the offshore zone, large amounts of sediment aretransported downcoast by means of the strong Agulhas Current, which also has asignificant influence on nearshore currents and sediment transport in the harbour entrancearea. About 75 000 m3/year of sand is transported into the 'Bar area (the seaward part ofthe entrance channel) from the north-east with downcoast flowing nearshore currents,which is the predominant current direction. The riverine input into the harbour has beenestimated at less than 10 000 m3/year of sand.In this thesis, therefore, the sediment transport regime (and sediment transport balance) isderived fo r East London, providing for the first time a holistic understanding o f the complexsediment transport regime.It is recommended that more field data should be collected on specific aspects of this study andthat the longer term effects of major weather systems should be investigated further. Thenumerical wave modelling should be extended and the integration of field measurements andnumerical modelling to predict sediment transport and resultant bottom changes should beassessed in detail. Ultimately, the information contained in this thesis should feed into a widerregional investigation, with the aim of drawing up a sediment budget for the entire 'regionalmacro sedimentary cell of which the present study area forms a part.
[发布日期]  [发布机构] Stellenbosch University
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