Abstract

Although the importance of estuaries as ecological, geomorphological and sedimentological systems is well recognised and a number of estuarine classification schemes are referred to in literature, geomorphological behaviour of small estuaries such as those on the KwaZulu-Natal coast are not well understood. In particular the development and role of spits and bars in the functioning of small, perched estuaries needs clarification. The purpose of this study was to determine the mechanism of spit initiation and growth in the Mdloti estuary, a small perched estuary some 25km north of Durban, on the KwaZulu-Natal coast, with a view to developing an hypothesis which explains the procedure of split development and its role in the geomorphological life of the estuary. Seven generations of air photos from 1953 to 2000 were analysed, and sediment samples from four trenches and two cores located on the spit allowed recent sedimentological history to be interpreted and integrated with geomorphological development of the mouth over the last 50 years. Results showed that infrastructural developments in the upper estuary and artificial breaching of the of the mouth all had significance in determining estuarine behaviour, but that spit initiation and development after breaching was largely dependent on long-shore sediment transport. The results allowed the proposition of a sequential model for spit development for the Mdloti estuary subsequent to breaching during a flood. An offshore delta is formed by the flood and later joined by a spit bar built up by longshore drift. Ebb delta deposits are re-worked by longshore currents. As the new bar becomes emergent aeolian activity commences and the spit is gradually widened by overwash deposits, and growth continues until the estuary is completely closed off.

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