The long term impact of the Seli One shipwreck on the Table Bay beaches

Seifart, Christian (2012-12)

Thesis (MScEng)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: On the 9th September 2009, the 178 m Panamanian bulk carrier, the Seli One, ran aground off the coast of Blouberg in Table Bay, South Africa. Due to failed salvage attempts, the vessel has remained stranded approximately 500 m off the Blouberg beachfront. Since the vessel ran aground, a gradual change in the Blouberg beach shape in the lee of the wreck has been observed. The local coastline, which has traditionally been fairly uniform, has assumed a curved shape, with significant sediment accretion being observed in the wave shadow of the wreck. Initially, the Seli One wreck remained intact. However, during a storm on the 4th September 2011, the vessel split up into three separate pieces. The impact that the wreck is having on the local wave, current and sediment transport dynamics remains undefined. This lack of knowledge results in significant risks, relating to shoreline stability and beach amenity. The objective is this study was therefore the determination of the long-term impact of the Seli One shipwreck on the Blouberg beachfront. A review of existing literature has indicated that no empirical relationships are available which could be used to calculate the impact of a shipwreck on nearby coastal processes. Numerous methods are available which can be used to determine the net longshore transport rates, but these cannot be used to quantify the impacts of shipwrecks on the local sediment transport regime. Numerical models were therefore used to determine the impact of the Seli One shipwreck. Through the analysis of simulation results, it was concluded that, as expected, the shipwreck has resulted in a significant reduction in the net longshore sediment transport rate in her lee, resulting in sediment deposition in this area. It was further concluded that the vessel does not result in the complete blockage of longshore sediment transport, and that sediment is able to periodically pass through the lee of the vessel. The simulated beach salient on the 3rd July 2011 was compared to results of a beach survey, performed on the same date specifically for this study. The simulated accretion of approximately 27 m in the lee of the shipwreck agrees well with the measured salient. It has been shown that approximately 75% of the salient accretion occurred within the first two months of the vessel’s arrival. Furthermore, shoreline erosion on the northern side of the salient resulting directly from the shipwreck has been shown to be approximately 15 m. This too occurs relatively rapidly, within approximately two months of the vessels arrival. Following the initial impact of the wreck in its intact configuration, the long-term potential impact of the vessel in its broken-up configuration was determined. This included the assumption that the vessel does not undergo any additional breaking-up, and remains in its three-piece configuration indefinitely. This has shown that the salient width resulting from the shipwreck is reduced to approximately 20 m, compared to the initial 27 m. However, shoreline erosion on the northern side of the wreck has increased from approximately 15 m initially to approximately 18 m in the long-term, which is caused by the continuous sedimentation between the vessel and the beach. A two-dimensional coupled wave, current and sediment transport model has been developed and has shown that the wave shelter resulting from the Seli One results in the formation of a submerged salient between the vessel and the shoreline. It was found that shipwrecks have the potential of significantly altering local longshore sediment transport characteristics in general. Depending on local conditions, this may pose serious risks, both in terms of jeopardizing local seaside infrastructure, as well as creating dangerous swimming conditions. Considering the impact that a shipwreck can have on local shoreline changes, with special regard to the rate at which these shoreline changes can occur, it is recommended that the results obtained from the current study be used to estimate the impact of potential future shipwreck scenarios in Table Bay.

AFRIKAANSE OPSOMMING: Op die 9de September 2009 het die 178 m lange Panamese vragskip, die Seli Een, aan die kus van Bloubergstrand in Tafelbaai, Suid Afrika, gestrand. Weens mislukte reddingspogings, het die skip ongeveer 500 m van die kuslyn gestrande gebly. Sedertdien, is ‘n geleidelike verandering in die vorm van Bloubergstrand se kuslyn waargeneem. Die kuslyn, wat tradisioneel redelik uniform en reguit was, het onlangs ‘n aansienlike geboë vorm aangeneem, met ‘n beduidende hoeveelheid sand wat in die skip se lykant neerset. Aanvanklik het die Seli Een wrak ongeskonde gebly, maar tydens ‘n storm op die 4de September 2011, het die skip in drie afsonderlike stukke opgebreek. Die impak wat die wrak op die golf, strome en sediment vervoer dinamika het,bly ongedefinieërd. Hierdie gebrek aan kennis veroorsaak ‘n aansienlike hoeveelheid risiko’s met spesifieke betrekking tot kuslyn stabiliteit en strand gerief. Die doel van hierdie studie was dus om die langtermyn-impak van die Seli Een skeepswrak op Bloubergstrand te bepaal. 'n Hersiening van bestaande literatuur het aangedui dat geen empiriese verhoudings beskikbaar is wat gebruik kan word om die impak van 'n skeepswrak op die nabygeleë kustelike prosesse te bereken nie, maar wel versekeie metodes wat gebruik kan word om die netto langsstroom sediment vervoer te bepaal. Hierdie verhoudings kan egter nie gebruik word om die impak van ‘n skeepswrak op die sediment vervoer meganisme te kwantifiseer nie, dus is numeriese modelle gebruik om die impak van die Seli Een skeepswrak te bepaal. Die skeepswrak het ‘n aansienlike vermindering in the netto langsstroom sediment vervoer veroorsaak, wat tot die afsetting van sediment in hierdie gebied lei. Dit is ook verder bepaal dat die Seli Een nie die volledige verstopping van langsstroom sedimentvervoer veroorsaak nie, maar dat sediment van tyd tot tyd in staat is om deur die lykant van die skeepswrak te beweeg. Die gesimuleerde strand aanwas van die 3de Julie 2011 is vergelyk met resultate van ‘n strand-opmeting, wat uitgevoer is op dieselfde datum, spesifiek vir hierdie studie. Die gesimuleerde aanwas, van ongeveer 27 m in die lykant van die skeepswrak, stem saam met die gemete aanwas. Ongeveer 75% van die aanwas het binne twee maande van die aankoms van die Seli Een plaasgevind. Verder is dit getoon dat aan die noordelike kant van die aanwas, ongeveer 15 m van die kuslyn weggespoel het as gevolg van die Seli Een. Na die aanvanklike impak van die wrak in sy ongeskonde konfugirasie, is die potensiële langtermyn impak van die skip in sy opgebreekte konfugirasie bepaal. Dit sluit die aanname in dat die skip nie enige bykomende breke ondergaan nie, en in sy drie-stuk konfigurasie bly. Dit het getoon dat die breedte van die aanwas, wat veroorsaak is deur die skip, verminder tot ongeveer 20 m in vergelyking met die aanvanklike 27 m. Verder is dit getoon dat die erosie aan die noordelike kant van die Seli Een vermeerder het van die aanvanklike 15 m na ongeveerder 18 m in die langtermyn. Die oorsaak hiervaan is die aaneenlopende sedimentasie tussen die wrak en die strand. 'n Twee-dimensionele gekoppelde golf, stroom en sediment vervoer model is ontwikkel en het getoon dat die golf skuiling, as gevolg van die Seli Een, sedimentasie tussen die skip en die kuslyn veroorsaak. Daar is gevind dat skeepswrakke die potensiaal het om aansienlike veranderinge aan die nabygeleë langstroom sediment vervoer stelsel te veroorsaak. Afhangende van die plaaslike omstandighede, kan hierdie ernstige risiko’s veroorsaak, beide in terme van die gevaar vir plaaslike kustelike infrastruktuur, sowel as die generasie van gevaarlike swem toestande. Met inagneming van die impak wat 'n skeepswrak op plaaslike kuslyn veranderinge kan hê, met spesiale verwysing na die tempo waarteen hierdie kuslyn veranderinge kan plaasvind, word dit aanbeveel dat die resultate wat verkryg is vanuit die huidige studie, gebruik word om die impak van moontlike, toekomstige skeepswrakke in Tafelbaai te bepaal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/71744
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