SEDIMENTATION IN THE CLIFF-BOUND, MICROTIDAL MTAMVUNA ESTUARY, SOUTH-AFRICA

The Mtamvuna Estuary on the Natal coast of South Africa is located in a drowned river valley and is surrounded by cliffs. It has a spring tidal prism of about 350 x 10(3) m3. Sedimentary environments in the estuary can be divided into upper and middle reaches, and barrier-associated sand bodies. The estuary exhibits limited floodplain development and small intertidal areas. The bedrock valley is presently infilled with sediment to about 90% of its capacity. This may be attributed to low sediment yields from the predominantly mud-yielding catchment lithologies. The low fluvial bedload yield to the coast is reflected in the wholly transgressive barrier sands and lack of Holocene sandy deposits on the coast and continental shelf. A significant proportion of the sediment in the estuary is derived from adjacent cliffs. Barrier morphology reflects variations in fluvial and nearshore dynamics. Enhanced flood-tidal currents and low river flow encourage flood-tidal deposition in the estuary, whereas increased river flows transport flood-tidal delta sediments seaward. Episodic overwash events deposit sand in the back-barrier area and promote barrier rollover. The volume of sediment in the barrier-associated sand bodies, however, appears to remain constant and the barrier maintains a dynamic equilibrium. Morphological responses to fluvial floods are dramatic but short-lived and are restricted mainly to the barrier which may be completely eroded during floods. Eroded sediment is deposited offshore as an ephemeral delta but when the flood wanes, wave action gradually reworks the delta sands onshore to reform the barrier which ultimately assumes its equilibrium position and morphology.