Mixing and circulation in the sundays river estuary, South Africa

In the winter of 1987, a field exercise of 6 weeks' duration was carried out to investigate tidal dynamics and circulation patterns in the Sundays River estuary, on the south-east coast of South Africa. Measurements included continuous monitoring of water levels inside and outside the estuary, salinity and temperature profiles along the estuary at slack tides, current velocity profiles and near-bottom currents. The estuary was found to be in a highly stratified state at neap tides, but became completely mixed at spring tides. Dynamically, it can be divided into four longitudinal sections, each with an essentially different circulation pattern. Stratification and mixing in the water column appears to depend on: (a) semidiurnal tides in the local coastal ocean; (b) the spring-neap tidal cycle; (c) freshwater input. Direct effects of wind mixing were not observed. Frequent visual observations of the phenomenon of axial convergence are supported by water temperature and current velocity data.