A morphological upscaling approach to modelling patterns of long term shoreface change and their synoptic climate drivers in Southeast Australia

This study presents a novel method to determine the long-term morphological response of the lower shoreface to regional climate forcing on wave-dominated coasts. By integrating synoptic wave climate typing, coupled wave-hydrodynamic-sediment transport modelling, and frequency-based morphological upscaling, the approach is evaluated using a rare, centennial-scale bathymetric dataset from Byron Bay, in Eastern Australia. Application to eight sites along the Southeast Australian coast, with detailed analysis at two erosion hotspots, suggests that the 30 to 35-m depth zone is the ultimate seaward limit of significant centennial-scale sediment transport under extreme storm conditions. This boundary has implications for both the design of coastal infrastructure, and the extraction of offshore sand. The study also demonstrates that only storms from directions normal to the coast can induce significant sediment transport on the lower shoreface, while storms clockwise of southeast are too oblique to facilitate long-term cross-shoreface transport. Results also indicate that the lower shoreface, like the upper shoreface on event timescales, can rotate in response to directional wave climate forcing on centennial timescales. • The 30 to 35-m depth zone is the ultimate seaward limit of significant centennial-scale sediment transport under modal and extreme storm conditions on the east coast of Australia. • Only quasi shore-normal directional storms can induce significant shoreward directed sediment transport on the lower shoreface. • Inner shelf deposits beyond the 30 to 35 m isobaths are inactive as a source within the centennial-scale sediment budget along the Southeast Australian coast due to shelf-shoreface disequilibrium with Holocene sea level.