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1 April 2014 Forecasting lagoon outlet erosion: KwaZulu-Natal, southeast Africa
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Abstract
Smith, A.M., Guastella, L.A., Goble, B.J., 2014. Forecasting lagoon outlet erosion: KwaZulu-Natal, southeast Africa. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 151–156, ISSN 0749-0208.The Amanzimtoti Lagoon is at the mouth of the Amanzimtoti River. The lagoon outlet changes systematically in response to the 18 year long Dyer-Tyson summer rainfall cycle (DTC). During the dry part of the DTC the lagoon is generally closed, opening only during heavy rainstorms. When closed the lagoon is separated from the sea by a baymouth bar, varying from 30 to 60 m wide. The open-days per year have increased dramatically as the DTC has entered the wet phase. Early in the DTC the lagoon rarely opened, and when this occurred it breached near the centre of the headland bound bay, but as the DTC-driven runoff cycle has progressed the outlet has remained open. However, during this open phase the outlet has been forced southward along the back beach until it deflected seaward by the southern headland outcrop. This appears to be brought about by longshore drift and an increased beach width. We suggest that this is due to the shoreward reworking of sediment by waves. This sediment has been brought down by the river in increasing amounts as the wet part of the DTC waxes, strengthening the seasonal rainfall and the river flow cycle. Wave reworking has forced the outlet along the back beach causing erosion of the coastal dune behind the back beach. Thus, the lagoon mouth enters a predictable erosion phase as the DTC progresses. This cyclicity can be used to forecast this type of lagoon outlet erosion. The Amanzimtoti Lagoon has strong similarities to the Hapua Lagoon-type recognized from New Zealand.
A.M. Smith, L.A. Guastella and B.J. Goble "Forecasting lagoon outlet erosion: KwaZulu-Natal, southeast Africa," Journal of Coastal Research (JCR) 70(sp1), (1 April 2014). https://doi.org/10.2112/SI70-026.1
Received: 1 December 2013; Accepted: 21 February 2014; Published: 1 April 2014
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