TIDAL AND LONG-PERIOD WATER-LEVEL VARIATIONS IN THE SUNDAYS RIVER ESTUARY, SOUTH-AFRICA

Results are presented from a field exercise at the Sundays River estuary over a period of six weeks during the winter of 1987. The aim of this exercise was to investigate the propagation of tidal and longer-period water-level variations in a typical bar-built eastern Cape estuary. Analysis of water-level records from Port Elizabeth Harbour and stations inside the Sundays River showed that the semi-diurnal tide in Algoa Bay was attenuated by approximately 50% and delayed by one to one and a half hours inside the estuary. As a result of frictional and non-linear effects at the constricted mouth, an M4 overtide is generated in the estuary which leads to a markedly asymmetrical tidal signal. Weather-related water-level variations in the ocean, with periods of 2 to 10 days and amplitudes of 50 cm and more, pass through the mouth with little delay or attenuation, and may have significant effects on levels inside the estuary.