Structure and mixing in the Palmiet Estuary, South Africa

Abstract

The Palmiet Estuary (South Africa), a bar-built coastal plain estuary, was monitored during a dry-season spring/neap tide cycle. Information on water level, temperature, salinity and currents was collected. Estimates of dimensionless Reynolds and Richardson numbers indicate a three-layered structure. Apart from periodic localized turbulence near the mouth, inter-layer exchange occurred by extensive entrainment. Flushing times of the order of 12 hours, 2 days and 2 weeks were found for the surface, intermediate and bottom layers respectively. Extreme spring tides during the dry season (volume flow in excess of 20 m³·s⁻¹ during flood) may allow the system to degenerate into a single saline layer partially covered by a very thin layer of freshwater. Extreme river flow during the wet season (volume flow in excess of 100 m³·s⁻¹) may flush the estuary so that it only contains pure freshwater. It is suggested that five "fluvial-marine" states may be identified: very dry, dry, dry-wet, wet, very wet. The Palmiet functions as a highly stratified fjord-type system under average conditions but, owing to its limited capacity relative to the potential fluxes of water, the stratification pattern can change dramatically.