An investigation into the suspended sediment flux and dynamics of the Mgeni Estuary, Durban.

Abstract

This dissertation focuses on both a hydrodynamic and geomorphological study of the Mgeni Estuary. Within the hydrodynamic study, the channel discharge, suspended sediment fluxes and estuary bed sediment characteristics and dynamics were established. Within the geomorphological study, cross-shore topographical surveying of the lower estuary region, measurement of slope angles and surface sediment characteristics were established. The results of this study illustrate strong seasonal variability. Maximum channel discharges, suspended sediment concentrations and fluxes occur during the summer months, as a result of large amounts of rainfall. Furthermore, maximum suspended sediment concentrations and fluxes occur during spring tides, as a result of a greater tidal range, which enhances bed sediment re-suspension via concomitant increased turbulence. Generally, maximum fluxes occur along the flood tide and ebb tide, during spring tides and neap tides, respectively, which suggest that the estuary is a sink for marine sediment during spring tides and an exporter of sediment during neap tides. The estuary bed sediments are very well sorted and predominantly classified as near-symmetrical, as a result of strong tidal currents that constantly transport and re-work the sediments. On average, the bed sediments are medium sand and in all probability are largely derived from the marine environment. Estuary bed sediments contain negligible mud and organic contents, which as research suggests, is common in such highenergy estuary mouths. Apart from the seasonal variability, the survey profiles and surface sediments illustrate alongshore and cross-shore variations. The profiles become flatter and finer from the Beachwood Mangroves section of the barrier towards the estuary mouth in the south, as a result of sheltering due to the engineered groyne, conforming to Bascom (1959) and Komar’s (1998) sheltered and exposed coasts concept. The survey profiles conform to the summer and winter profiles put forward by Dardis and Grindley (1988). The winter profiles consist of higher, distinct berms and berm crests, as well as vertical erosional faces, whilst the summer profiles are lower, flatter, and consist of unclear berms and berm crests. Sediments are coarsest along the lagoonward slope and finest within the estuary. A strong, positive correlation was generated between slope angle and mean grain size. Despite the low organic contents, the estuary sediments consist of the highest values of organic matter, with the beach and barrier sediments displaying negligible amounts. Thus, the Mgeni can be classified as a very dynamic and active zone.