Hydrodynamic and sand-transport controls on en echelon sandbank formation: an example from Moreton Bay, eastern Australia

Current meter observations from a field of en echelon sandbanks in Moreton Bay, Australia, provide insights into sandbank hydrodynamics and development. Re-circulation of sand is occurring around straight-crested, near-shore banks, but not around offshore, 'S'-shaped banks. Rather, net transport over this latter area is unidirectional (flood dominated). This is explained in the context of the evolution of the banks, in which the mature, near-shore banks have grown vertically upwards to the sea surface and thus contribute to the local generation of higher tidal harmonics and a residual current circulation pattern, whereas the crestlines of younger, 'S'-shaped banks are still mostly >5 m below the surface and do not affect the flow field significantly. For the mature, inshore banks, reversal of net sand-transport vectors over a distance of less than 1 km (from one side of an interbank channel to the other) is related to the amplification and phase modulation of the M 4 tidal constituent. These observations are of relevance to dredging operations since, in order to minimize the impact on the sand-transport system, removal of sand should be from the end of the transport path. Transport paths, in turn, are identified based on directions of net sand-transport and grain size data. Observations from Moreton Bay and other locations around the world suggest that it is possible to dredge offshore sandbanks without affecting beach erosion, provided that there are no direct linkages between the offshore sandbank system and coastal deposits.