Seven freak wave incidents previously documented in the real ocean in combination with model hindcast simulations are used to study the variations associated with freak-wave-related parameters, such as wave steepness, directional spreading, and frequency bandwidth. Unlike the strong correlations between the freak wave parameters and freak waves' occurrence which were obtained in experimental and physical research, the correlations are not clear in the freak waves occurring in the real ocean. Wave directional spreading–steepness joint distribution is introduced and common visual features were found in the joint distribution when freak waves occur among seven “freakish” sea states. The visual features show that freak wave incidents occur when the steepness is large and directional spreading is small. Besides large steepness and small directional spreading, a long-duration, relatively rough sea state is also necessary for the freak wave generation. The joint distribution is more informative than any single statistical wave parameter. The continuous sea states of local large steepness and small directional spreading are supposed to generate freak waves, and two-dimensional distribution visualization is found to be a useful tool for freak waves' forecast. The common visual features of joint distributions supply an important cue for the theoretical and experimental research.

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.