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Abstract
The suspended-sediment dynamics in Darwin Harbour, Northern Territory Australia, were investigated using a combination of field measurements and numerical modelling. After analysing the harbour’s geophysical characteristics from the field data and an extensive literature review, a hydrodynamic model for the harbour was built using the Finite Volume Coastal Ocean Model (FVCOM), a model suited to coastal ocean simulation. This model was then coupled in this study to the estuarine suspended-sediment model (ESSed) of Wang (2002) to produce the FVCOM-ESSed model.
Both the hydrodynamic and sediment-transport components were calibrated using the field data on sea-surface level, current velocity and suspended-sediment concentration. The sediment-transport model focuses on suspended sediments, with improvements that allow wetting-drying processes, different bathymetry types and a variable thickness of the fine-sediment layer on the harbour bed to be included. The combined hydrodynamic and sediment model provides a reasonable simulation of the tidal and suspended-sediment dynamics in the harbour. Numerical experiments using this model were then designed to determine the effect of the mangrove areas and tidal flats in Darwin Harbour on the tides and tidal asymmetry, and subsequently, on the suspended-sediment dynamics.
This study shows that the hydrodynamics of Darwin Harbour are driven mainly by tides, with the effects of wind and rivers small. The M2 tidal component is dominant, with amplitude near Darwin City about 1.9m and phase 249 degrees. Current flow is dominated by the tides. The current, which reaches a maximum speed of about 2.0 ms-1 at the surface of Middle Arm and decreases gradually from the surface to the bottom, is dominated by the M2 tidal current, with an average vertical speed of about 0.4 ms-1 near East Arm Wharf. The water-flow patterns are in accordance with the shoreline, and rectilinear in all three arms. The energetic hydrodynamic regime, together with the availability of erodible sediment on the seabed, determines the variation in suspended-sediment concentration (SSC) in the harbour. Stronger currents induced by the larger tidal ranges during spring tides generate higher bottom SSC values than those during neap tides, as observed in the harbour in November 2012. A turbid zone appears in the outer harbor, with bottom SSC values about 0.1kgm-3 during the spring tides and 0.07kgm-3 during neap tides. The water is less turbid during neap tides than during spring tides; vertical gradients of SSC are formed in the channel during neap tides due to weaker currents. Net sediment transport is seaward in the channel and landward at the entrance to East Arm, dominated by Eulerian advection.
Mangrove areas and tidal flats play key roles in modulating the tides and water-flow dynamics of an estuary. Suspended sediments are redistributed as a result of these modulated water dynamics. Removal of the mangrove areas and tidal flats from Darwin Harbour, a possibility due to planned harbour development, would dampen the M2 amplitude because of decreased shoaling effects in the inner harbour, but would generate a greater M4 amplitude. Removal of the mangroves and tidal flats would also lessen the tidal choking effect, thereby, reducing the current speed; as a result, the water would be less turbid in the harbour, for example, the bottom SSC values in East Arm would be reduced by about 70% during spring tides.
Mangrove areas and tidal flats affect tidal asymmetry via their influence on the amplitudes and phases of the tides, and therefore affect net sediment transport. In Darwin Harbour, these areas significantly reduce tidal asymmetry: for example, the tidal elevation skewness would increase by about 120% in Middle Arm if the mangrove areas and tidal flats were removed. Due to the increased flood dominance, the tidal pumping effect would overtake the Eulerian residual to dominate sediment transport both in the channel and at the entrance to East Arm. This would reverse the transport of suspended sediment in the channel to landward. The landward net sediment flux would be decreased at the entrance to East Arm as a result of reduced currents in the arms because of the reduction in the tidal choking effect, if the mangrove areas and tidal flats were removed.