OCEANOGRAPHIC PROCESSES AND SEDIMENT MIXING ON A SAND FLAT IN AN ENCLOSED SEA, MANUKAU-HARBOR, NEW-ZEALAND

Studies of oceanographic and sedimentary processes on intertidal sand flats in an enclosed sea were undertaken to gain a better understandin g of the factors controlling the mixing and dispersal of sediment boun d contaminants. Field investigations included a 90 day process experim ent during which wind, waves, tidal currents, tides, depth of disturba nce and sand flat morphology were measured, and 27 months of sand flat profile monitoring. Sediment entrainment by strong spring tidal curre nts is restricted to the middle and lower regions of the sand flat whi ch are inundated during the peak tidal flows. The upper 2-3 cm of sedi ment is re-worked across the middle and upper sand flat by mild storm wave events (H-s=70 cm), which occurred four times during the 90 day e xperiment. Numerous ridges and runnels in the upper sand flats are wav e-formed features and are maintained by the lack of currents of suffic ient magnitude to re-work the features. The ridge and runnel morpholog y is testimony to large infrequent storm events which re-work the sedi ment to depths of 20 cm. Such storms are an important mechanism for th e release of contaminants and were recorded on 3 occasions in the 27 m onth profile record.