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

[摘要] ENGLISH ABSTRACT: On the 9th September 2009, the 178 m Panamanian bulk carrier, the Seli One, ran aground off the coast ofBlouberg in Table Bay, South Africa. Due to failed salvage attempts, the vessel has remained strandedapproximately 500 m off the Blouberg beachfront. Since the vessel ran aground, a gradual change in theBlouberg beach shape in the lee of the wreck has been observed. The local coastline, which has traditionallybeen fairly uniform, has assumed a curved shape, with significant sediment accretion being observed in thewave shadow of the wreck. Initially, the Seli One wreck remained intact. However, during a storm on the 4thSeptember 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 remainsundefined. 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 shipwreckon the Blouberg beachfront.A review of existing literature has indicated that no empirical relationships are available which could be usedto calculate the impact of a shipwreck on nearby coastal processes. Numerous methods are available whichcan be used to determine the net longshore transport rates, but these cannot be used to quantify the impactsof shipwrecks on the local sediment transport regime. Numerical models were therefore used to determinethe impact of the Seli One shipwreck.Through the analysis of simulation results, it was concluded that, as expected, the shipwreck has resulted in asignificant reduction in the net longshore sediment transport rate in her lee, resulting in sediment depositionin this area. It was further concluded that the vessel does not result in the complete blockage of longshoresediment 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 thesame date specifically for this study. The simulated accretion of approximately 27 m in the lee of theshipwreck agrees well with the measured salient. It has been shown that approximately 75% of the salientaccretion occurred within the first two months of the vessel's arrival. Furthermore, shoreline erosion on thenorthern 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 vesselin its broken-up configuration was determined. This included the assumption that the vessel does not undergoany additional breaking-up, and remains in its three-piece configuration indefinitely. This has shown that thesalient 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 initiallyto approximately 18 m in the long-term, which is caused by the continuous sedimentation between the vesseland the beach.A two-dimensional coupled wave, current and sediment transport model has been developed and has shownthat the wave shelter resulting from the Seli One results in the formation of a submerged salient between thevessel and the shoreline.It was found that shipwrecks have the potential of significantly altering local longshore sediment transportcharacteristics in general. Depending on local conditions, this may pose serious risks, both in terms ofjeopardizing 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 atwhich these shoreline changes can occur, it is recommended that the results obtained from the current studybe used to estimate the impact of potential future shipwreck scenarios in Table Bay.

[发布机构] Stellenbosch University