Sediment transport regime in the area of the East London harbour entrance

Theron, Andre K. (2004-12)

Thesis (MEng) -- Stellenbosch University, 2004.

Thesis

ENGLISH ABSTRACT: East London is situated on the south-eastern, Indian Ocean, coast o f South Africa. The sediment transport regime at East London is quite unlike the regimes at other ports in South Africa. A major ocean current (the Agulhas) flows exceptionally close to the coastline in this area, thus significantly affecting nearshore sediment movements. The proximity o f a strong ocean current opposed to the net longshore drift (wave driven transport) creates an anomalous sediment transport 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 in South 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 sediment transports at East London, achieve a holistic understanding o f this complex sediment transport regime 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) with the main focus on the Hood Point to Orient Beach area (Figure 1.4). The offshore marine environmental conditions are also considered as they have a strong influence on nearshore currents, waves and sediment transport patterns. The prerequisite to understanding the sediment transport processes is a thorough investigation o f the relevant coastal processes. Thus, an in-depth study is undertaken o f the physical environmental data/information, nearshore processes and coastal dynamics that determine or topography changes, the wave regime, the Agulhas Current, the nearshore current regime, the continental 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 qualitative understanding of the overall sediment transport regime is determined, and the various modes of sediment transport in different areas are quantified. All o f this information is then synthesised into an expose' o f the sediment transport regime at East London, as briefly described in the conclusions (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 per year on average from the Foreshore area towards the head of the main breakwater, with the total transport into the main sand trap and entrance channel areas from the south-west estimated at 275 000 m3/year. In the offshore zone, large amounts of sediment are transported downcoast by means of the strong Agulhas Current, which also has a significant influence on nearshore currents and sediment transport in the harbour entrance area. About 75 000 m3/year of sand is transported into the “Bar” area (the seaward part of the entrance channel) from the north-east with downcoast flowing nearshore currents, which is the predominant current direction. The riverine input into the harbour has been estimated at less than 10 000 m3/year of sand. In this thesis, therefore, the sediment transport regime (and sediment transport balance) is derived fo r East London, providing for the first time a holistic understanding o f the complex sediment transport regime. It is recommended that more field data should be collected on specific aspects of this study and that the longer term effects of major weather systems should be investigated further. The numerical wave modelling should be extended and the integration of field measurements and numerical modelling to predict sediment transport and resultant bottom changes should be assessed in detail. Ultimately, the information contained in this thesis should feed into a wider regional investigation, with the aim of drawing up a sediment budget for the entire “regional macro sedimentary cell” of which the present study area forms a part.

AFRIKAANSE OPSOMMING: sedimentvervoer-regime by Oos-Londen is anders as die by ander hawens in Suid-Afrika. ‘n Hoof oseaan-stroom (die Agulhas) vloei besonder naby aan die kus in die gebied en het gevolglik ‘n beduidende impak op sedimentvervoer-patrone in die nabystrandse gebied. Die direkte teenwoordigheid van ‘n sterk diepwater stroom wat teen die netto (golfgedrewe) langsstrandse vervoer inwerk, veroorsaak ‘n ongewone sedimentvervoer-regime in vergelyking met wat algemeen wereldwyd gevind word. Daarbenewens is die Oos-Londenhawe (Figure 1.1 en 1.2) die enigste groot rivier-hawe in Suid-Afrika. A1 die ongewone toestande lei tot ‘n ingewikkelde patroon van sedimentvervoer in die gebied. Die doel van hierdie tesis is om die sedimentvervoerregime by Oos-Londen te bestudeer, ‘n holistiese begrip van die ingewikkelde sedimentvervoerregime te verkry, en om die verskillende komponente van die sedimentvervoer regime so ver as moontlik te kM’antifiseer. Die studiegebied sluit die kusstrook tussen die Goda- en Nahoonriviere in (Figuur 1.3). Daar word egter op die Hoodpunt tot Orientstrand-gebied gefokus (Figuur 1.4). Die diepwater, mariene omgewingstoestande word ook beskou, aangesien hul ook ‘n groot impak op die nabystrandse strome, golwe en sedimentvervoer-patrone het. ‘n Deeglike ondersoek van die toepaslike kusprosesse is ‘n voorvereiste om ‘n goeie begrip van die sedimentvervoer-prosesse op te bou. Gevolglik is ‘n diepgaande ondersoek gedoen na die fisiese omgewingsdata en -inligting, nabystrandse prosesse en kusdinamika wat die sedimentvervoer-regime bepaal of bei'nvloed. Dit het ondersoeke ingesluit na: onderhoudsbaggervolumes, bodemtopografie veranderinge, die golfklimaat, die Agulhasstroom, die nabystrandse stroom-regime, die sedimentdinamika op die kontinentale plaat, sediment eienskappe, die windklimaat, die geaardheid van bodemvorme, die Buffelsrivier, getyaksie, en kus-morfologie. Daar is bepaal wat elke aspek van die omgewingsdata/inligting bydra tot ‘n kwalitatiewe begrip van die algehele sedimentvervoer-regime. Verder is die verskillende komponente van die sedimentvervoer in verskeie gebiede gekwantifiseer. Al hierdie inligting is toe versoen in ‘n beskrywing van die sedimentvervoer-regime by Oos-Londen, soos wat in die gevolgtrekkings (Deel 5 en Tabel XI) gegee word en in Figuur 4.30 geillustreer word: Daar is ‘n gemiddelde netto golfgedrewe langsstrandse vervoer van ongeveer 250 000 m3 tot 300 000 m3 per jaar vanaf die strandgebied suid van die hawe na die punt van die hoof-golfbreker. Die totale beraamde vervoer vanaf die suidweste na die hoofsandvangput en ingangskanaal-gebied is 275 000 m3 per jaar. In die diepsee word groot hoeveelhede sediment langs die kus af vervoer deur die sterk Agulhasstroom, wat ook ‘n beduidende impak het op nabystrandse strome en sedimentvervoer in die haweingangsgebied. Omtrent 75 000 m3 sand word per jaar na die “Bar”-gebied vervoer vanaf die noord-ooste deur nabystrandse strome wat langs die kus af vloei. Dit is ook die oorwegende nabystrandse stroomrigting (uitgesluit die brandersone). Sawafafsetting in die hawegebied afkomstig van die Buffelsrivier word op minder as 10 000 m3 per jaar geskat. Die sedimentvervoer-regime (en sediment-balans) by Oos-Londen word dus afgelei, en vir die eerste keer saamgesnoer in ‘n bree insig van die komplekse sedimentvervoer-regime. Daar word aanbeveel dat meer velddata verkry moet word oor sekere aspekte van hierdie studie en dat die langtermyn-impakte van groot weerstelsels verder ondersoek word. Die wiskundige golf-modellering moet uitgebrei word en die gesamentlike benutting van veldmetings met wiskundige modellering om sedimentvervoere en gevolglike bodemveranderinge te voorspel, moet deeglik ondersoek word. Uiteindelik behoort die inligting vervat in hierdie tesis in te voer in breer streeks-ondersoeke, met die oog daarop om ‘n sediment-begroting vir die “Ooskaapse makro-sedimentvervoer-regime” op te stel.

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