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A simple model is developed to infer flood-tide dispersal of silt during fair-weather conditions from spatial grain size variation of mixed carbonate/siliciclastic silt on the bed of Nara Inlet, a 3 km long shallow, tropical embayment on the mid-shelf of the central Great Barrier Reef region, Australia. The model uses settling velocities calculated for three dominant silt modes (23, 28 and 41 μm) and measured tidal current data to predict the headward transport of silt by a large flood tide. Modelling was aided by the use of Wind and Currents 2000 (WC2000) freeware, a visualisation and analysis tool for vectorial data. The model predicts the 23, 28 and 41 μm modes will be transported minimum headward distances of 1250, 950 and 500 m, respectively and compares well with the observed silt distribution on the bed. The results indicate that under fair-weather conditions flood-tides control the distribution of silt. Consistent grain size trends of bulk and siliciclastic silt implies that siliciclastic and carbonate silt-sized grains may exhibit similar hydraulic behaviour during transport by flood tides in Nara Inlet. The model may be applied to embayments where fine sediment transport is dominated by flood tides. It may also have application in identifying the controls on dispersal and accumulation of particle-associated contaminants in embayments characterised by mixed carbonate and siliciclastic sediments. |
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