Spatiotemporal characteristics of the theoretical wave-base-derived seaward shoreface boundary along the China Coast

Research on the seaward shoreface boundary (SSB) over broad spatial scales is limited, restricting the understanding of its macroscale characteristics. In this study, we define a theoretical wave-base-derived seaward shoreface boundary (TWSSB) based on the intersection of wave base (half the wavelength) with seabed topography. Using high-resolution wave analysis data from the Copernicus Marine Environment Monitoring Service (CMEMS) for 2022-2023, annual and seasonal wave bases were calculated in China’s coastal waters. By combining these data with nearshore bathymetry, the annual and seasonal TWSSBs were determined. The spatial distribution of annual TWSSBs and the migration of seasonal TWSSBs along China’s coast were detailed, providing a comprehensive view of TWSSB characteristics. Results show a strong correlation between TWSSBs and local isobaths. The annual TWSSBs approximately coincide with the 10 m isobath in the Bohai Sea and Beibu Gulf, align with the 20 m isobath in the central and southern Yellow Sea, and are close to the 30 m isobath east of Hainan Island. Compared to the annual averages, seasonal TWSSBs show different horizontal shifts, typically within several kilometers (median ~3 km). The maximum seasonal range (difference between seaward and landward shifts) reaches approximately 41 km in the South China Sea. Together, these findings present a large-scale synoptic mapping of the TWSSB along the entire coast of China and demonstrate that TWSSB location is influenced by regional bathymetry and seasonal wave conditions. This theoretical, wave-based approach is expected to offer a reference for future studies on SSB and coastal morphodynamic processes.