The morphodynamics and recent geological evolution of a small barrier estuary were investigated. The estuary is located on the macrotidal central Queensland coastline and is characterised by an almost infilled back-barrier basin colonised by mangroves. The tidal channels inside the 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 followed by a slow fall in water level during the ebbing tide. The positive tidal 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 increasing tide range and would suggest that the morphodynamics of the estuary are governed by flood tidal currents and flood-dominant sediment transport. However, flood-dominance is only observed during neap and mean tides. During spring tides, a large volume of water inundates the mangrove area at high tide and the consequent drainage of the mangrove area during the falling tide induces significantly stronger ebb flows and ebb-dominant sediment transport. Although the proportion of tidal cycles during which net ebb-directed sediment transport prevails is limited, 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 part of the infilled volume and suggests that flood-dominant sediment transport has occurred during the infilling process. Hydrodynamic measurements indicate ebb-dominance for much of the estuary, suggesting that a hydrodynamical change from flood- to ebb-dominance has occurred during estuarine evolution. This hydrodynamical change may have resulted from an increase in intertidal water-storage capacity, predominantly through lateral mangrove-expansion, causing faster flow velocities during the ebbing tide. Ebb-dominance was probably first established at the head of the estuary and migrated downstream as the estuary became infilled.
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