Seasonal, annual, and decadal changes in morphology and sedimentation of a channelized, open-coast macrotidal flat

The topography and surface sediment distribution of open-coast tidal flats exhibit distinct spatiotemporal variability, commonly linked to seasonal changes in wave intensity. However, studies that consider factors beyond waves and tides, or that address long-term variability based on extended observations, remain scarce. To investigate the processes shaping this variability, an Empirical Orthogonal Function analysis was applied to surface sediment data collected from 2014 to 2025 from the intertidal flat on southwestern Ganghwa Island, west coast of Korea.The results indicate that sediment distribution is primarily influenced by interannual, decadal, and seasonal variability associated with wave forcing, as well as by geomorphic and biophysical changes. Interannual variability is most pronounced in the middle to upper tidal flat, where years of stronger wave conditions are characterized by relative coarsening. This pattern suggests that wave influence is modulated by tidal stage at the time of wave occurrence. Decadal variability reflects longer-term morphological change of tidal channels and the expansion of oyster reefs, producing a coarsening and fining trend, respectively. Seasonal variability exhibits clear elevation-dependent behavior: the middle tidal flat tends to coarsen in winter and fine in summer, whereas the upper tidal flat shows the opposite tendency due to biofilm development and rainfall-induced sheet flow.Overall, these findings indicate that sedimentary processes on channelized open-coast tidal flats are governed by geomorphic complexity that enables multiple forcings, such as waves, tides, biological processes, and rainfall-driven sediment transport to operate concurrently. Consequently, surface sediment grain size distributions exhibit complex spatiotemporal variability that cannot be adequately explained by wave forcing alone, underscoring the value of integrated, long-term observations for resolving sediment dynamics in such environments.