Assessment of the relative importance of major sediment-transport mechanisms in the central Great Barrier Reef Lagoon

The delivery, flux and fate of terrigenous sediment entering the Great Barrier Reef lagoon has been a focus of recent studies and represents an ongoing environmental concern. Wave-induced bed stress is the most significant mechanism of sediment resuspension in the Great Barrier Reef, and field data and mathematical modelling indicates that the combined effects of short-period wind waves, longer period swell waves, and tidal and wind-driven currents can often exceed the critical bed stress for resuspension. Suspended-sediment concentrations at 20 m water depth indicate resuspension seldom occurs on the middle shelf under normal wave conditions. Non-cyclonic turbidity events are generally confined to the inner shelf. The wave climate in the southern sector of the central Great Barrier Reef lagoon is the most erosive, and resuspension of outer shelf sediments was hindcast for recorded cyclones. Wind-driven, longshore currents are fundamental to the northward movement of sediment, and the annual northward mass flux from embayments undergoing resuspension in the Burdekin region is estimated to be one order of magnitude larger than the mass of sediment introduced by a moderate flood plume. Strong onshore winds are estimated to generate significant three-dimensional bottom return currents on approximately 30-70 days per year, forming a potentially significant offshore-directed sediment flux during high suspended-sediment concentration events on the inner shelf.