Sand transport and bedform patterns on the continental shelf between Durban and Port Elizabeth (southeast African continental margin)

Abstract

On the basis of a regional side-scan sonar survey covering a total distance of 5600 km on the continental shelf between Durban and Port Elizabeth (South Africa) three distinct shore-parallel zones have been distinguished. Each zone reflects a specific wave- or current-controlled regime. The nearshore zone consists of a sediment wedge of variable thickness. It is wave-dominated and does not reveal current-generated bedforms. The central shelf consists of a sandstream reaching a width of 5 km in the north and widening to over 20 km in the south. It is characterized by a multitude of current-generated longitudinal and transverse bedforms. The outer shelf, which is situated at the high-velocity margin of the Agulhas Current, consists of a sand-depleted gravel pavement. Two major mechanisms of sediment supply into the central-shelf sandstream are visualized. One by which small quantities of sand gradually diffuse across the wave/current boundary over a broad front. The other by means of a constructional/destructional cycle along the outer edge of the nearshore sediment wedge in response to lateral migration and meandering of the Agulhas Current. A variety of longitudinal and transverse bedforms are illustrated in a number of selected sonographs. They constitute a catalogue that serves the typological characterization of regional patterns. Individual bedform types such as gravel pavements, sand ribbons, comet marks, sand streamers, dunes, and smooth sand sheets form distinctive associations in specific areas of the continental shelf. Some bedforms and bedform sequences are very complex and not easily explained by conventional hydrodynamic concepts. Evidence is presented that points towards the generation of bedforms in response to either transverse or longitudinal instabilities in the near-bottom flow and a tentative boundary-layer model for bedform generation and control is outlined.