Holocene estuary infill leads to coastal barrier initiation from fluvial sand supply in southeastern Australia

Abstract

A morphodynamic approach to coastal evolution involves recognition of internal thresholds, feedbacks and boundary conditions and should underpin coastal management. The Holocene evolution of the Bega River estuary and Tathra Beach coastal barrier was examined integrating existing sediment cores and radiocarbon dating, airborne terrestrial and marine Lidar and OSL dating. Sediment coring reveals the Bega River estuary began infilling with fluvial sand once sea levels stabilised at or near their present elevation. Radiocarbon dating suggests a prograding fluvial delta reached the coast approximately 4000–2250 years BP. Barrier deposition commenced ~3200 years ago coinciding with the arrival of fluvial sand at the coast. Shoreline progradation of the Tathra barrier occurred at 0.15 m/year from ~3200 years to present forming a sequence of ~17 foredune ridges which were each active for an average of ~190 years. In the past ~500 years, a sand spit has restricted the entrance of the Bega River estuary to the northern end of the embayment. The infill of the Bega River estuary over the Holocene represents an internal morphodynamic threshold or tipping point, which then enabled coastal barrier deposition as fluvial sand reached the coast. The coastal system approaches another threshold as the Tathra embayment infills, and sediment may be transported northward out of the embayment. At Tathra Beach, the positive sediment budget which resulted in barrier progradation is approximately 0.55 m³/m/year. This signal is masked on the yearly to decadal scale by fluctuations in beach volume an order of magnitude greater (5–20 m³/m/year depending on the timeframe examined). Thus longer-term datasets of beach change or reconstructions from the geological record are needed to underpin management decisions which will impact shorelines decades or centuries into the future.