Embayment-scale coastal evolution and shoreline progradation in southeast Tasmania, Australia

Abstract

Two barrier systems in southeastern Tasmania afford the opportunity to contrast Holocene barrier evolution and explore large-scale coastal morphodynamics under consistent regional-scale boundary conditions on an embayed wave-dominated coastline. New Optically Stimulated Luminescence (OSL) ages for the previously undated seaward ridges of Seven Mile Beach constrain the deposition of the foredune ridges over the past ~1000 years indicating continued shoreline progradation at ~0.4 m/yr. A further 13 OSL ages collected from the foredune ridges of Nine Mile Beach barrier detail the evolution of this barrier system and indicate progradation commenced approximately ~8500 years ago. Following this, a steady rate of shoreline advance at 0.14 m/yr was evident to near present-day with one phase of rapid accretion ~5000 years ago. This occurred as the barrier system extended eastward to span the full embayment width which resulted in the redirection of the estuarine channel and the subsequent erosion of previously deposited ridges due to channel migration likely over several thousand years. The eroded sediment may have contributed to ongoing positive sand budget in the embayment and continued shoreline progradation. Barrier ‘recycling’ was also suggested as a potential driver of progradation at Seven Mile Beach based on the truncated margin of the landward ridges. However, at both sites, the dynamics of possible sediment exchange between the back-barrier and open shoreline requires further detailed modelling. The relatively small river systems in this region supply minimal sand to the coast at these sites, however, there is some minor alongshore sediment supply along the western margin of these deep embayment's. In light of this, these two barrier systems demonstrate the importance of internal morphodynamic thresholds and cross-shore sediment exchange from the shoreface as key drivers of barrier evolution and large-scale coastal change in embayed coastal settings.