Dynamics of Downslope Sand Transport Sourced from Estuarine Density Flows

Boyd, Ron¹, Kevin Ruming¹ (1) University of Newcastle, Callaghan, Australia

Sand is currently being supplied to the Tasman Abyssal Plain off SE Australia by gravity transport initiated by estuarine density flows. Many Australian estuaries have an excess of evaporation over fresh water discharge. This leads to dense saline bottom flows hugging the seabed after exiting the estuary mouth. If the estuary mouth is located close to the shelf edge, the denser water bodies can descend along the upper slope and initiate downslope sediment transport.

Hervey Bay is a 3600 km2 estuary in SE Queensland, formed by the 125 km progradation of Fraser Island from the shoreline to the shelf edge. Sand is fed into the system at a rate of 500,000 m3 by longshore transport past Fraser Island. Tidal range in Hervey Bay is up to 4.5 m, with an average tidal prism of 7.2 km3 forced to exit semidiurnally around the northern end of Fraser Island. In this location, dense reverse estuarine flows of up to 1.5 m/s generate megadunes up to 12 m high that migrate over the shelf edge. The slip faces of the dunes connect directly to sediment failures and chutes that feed 35 upper slope gullies between 150-220m depth. The gullies in turn discharge sand over a 250 m high carbonate scarp, feeding submarine canyons and a major sea valley before depositing the sand on the Tasman Abyssal Plain, 150 km seaward of Hervey Bay.