Oceanographic processes and sediment mixing on a sand flat in an enclosed sea, Manukau Harbour, New Zealand

Studies of oceanographic and sedimentary processes on intertidal sand flats in an enclosed sea were undertaken to gain a better understanding of the factors controlling the mixing and dispersal of sediment bound contaminants. Field investigations included a 90 day process experiment during which wind, waves, tidal currents, tides, depth of disturbance and sand flat morphology were measured, and 27 months of sand flat profile monitoring. Sediment entrainment by strong spring tidal currents is restricted to the middle and lower regions of the sand flat which are inundated during the peak tidal flows. The upper 2–3 cm of sediment 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 experiment. Numerous ridges and runnels in the upper sand flats are wave-formed features and are maintained by the lack of currents of sufficient magnitude to re-work the features. The ridge and runnel morphology is testimony to large infrequent storm events which re-work the sediment to depths of 20 cm. Such storms are an important mechanism for the release of contaminants and were recorded on 3 occasions in the 27 month profile record.