Multi-Scale Morphodynamic Assessment of an Embayed Low Energy Estuarine Beach, Shoal Bay, Port Stephens, NSW

Abstract

Low-energy shorelines are common coastal features, with their combined length exceeding that of high energy coastlines. Yet the morphodynamics of such systems are poorly understood with only a few process based assessments of low-energy environments. Shoal Bay is a low-energy embayed estuarine beach in the Port Stephens estuary, located on a high-energy coast. It has been undergoing erosion for the past 40 years and its cross-shore extent is associated with a well developed flood-tide delta (FTD). This study investigates the morphodynamics of Shoal Bay beach over multiple temporal scales. Short term studies included nearshore hydrodynamic (wave and current) surveys during summer and winter. Sediment entrainment calculations were used in the nearshore current analysis and allowed for more accurate assessment of longshore sediment transport. Medium term assessment involved the analysis of morphological change from one year of beach surveys as well as an evaluation storm effects from hourly offshore wave data. Historic storm data, sediment characterisation and 43 years of aerial photography were used in the long term morphodynamic analysis. Westward trending sediment pathways were found at all three time-scales. Short term studies indicate that under modal conditions westward transport of sediment occurs. This corresponded with accumulation of sediment in the west. Severe high-energy events triggered considerable erosion along the entire beach. Detailed analysis of beach recovery after severe events could not be made but negligible recovery is observed over one year of surveys. Significant erosion and shoreline recession was found in the medium and long term morphological analysis as was shoreline rotation. Medium term shoreline retreat was caused by high-energy stochastic events; however it seems unlikely that these events are the sole cause of long term shoreline retreat. A reduction in sediment input as well as reworking of the FTD are proposed to be the long term processes driving 43 years of chronic shoreline retreat. The results of this study represent a scientific contribution to the morphodynamics of low-energy beaches and they are relevant for the future shoreline management of Shoal Bay and other drowned river valleys on high-energy coasts.