MORPHODYNAMIC EVOLUTION OF A MACROTIDAL BARRIER ESTUARY

The morphodynamics and recent geological evolution of a small barrier estuary were investigated. The estuary is located on the macrotidal ce ntral Queensland coastline and is characterised by an almost infilled back-barrier basin colonised by mangroves. The tidal channels inside t he estuary account for less than 20% of the total estuarine area. The vertical tide in the estuary is characterised by a positive asymmetry, where a rapid rise in water level during the flooding tide is followe d by a slow fall in water level during the ebbing tide. The positive t idal asymmetry is predominantly ascribed to the truncation of the low tide level within the estuary due to the presence of a natural sill at the mouth of the estuary. The tidal asymmetry increases with increasi ng tide range and would suggest that the morphodynamics of the estuary are governed by flood tidal currents and flood-dominant sediment tran sport. However, flood-dominance is only observed during neap and mean tides. During spring tides, a large volume of water inundates the mang rove area at high tide and the consequent drainage of the mangrove are a during the falling tide induces significantly stronger ebb flows and ebb-dominant sediment transport. Although the proportion of tidal cyc les during which net ebb-directed sediment transport prevails is limit ed, on an annual basis the estuary can be considered ebb-dominant. The stratigraphy of the estuary is characterised by a seaward-thickening wedge of open-marine sand up to 3 m thick, embedded between two layers of muddy sediments. The open-marine sand accounts for a significant p art of the infilled volume and suggests that hood-dominant sediment tr ansport has occurred during the infilling process. Hydrodynamic measur ements indicate ebb-dominance for much of the estuary, suggesting that a hydrodynamical change from flood- to ebb-dominance has occurred dur ing estuarine evolution. This hydrodynamical change may have resulted from an increase in intertidal water-storage capacity, predominantly t hrough lateral mangrove-expansion, causing faster flow velocities duri ng the ebbing tide. Ebb-dominance was probably first established at th e head of the estuary and migrated downstream as the estuary became in filled.