Multi-timescale controls on surface sediment distribution in a channelized open-coast macrotidal flat, southern Ganghwa Island, western Korea

This study examines spatiotemporal variability in surface sediment distribution on a channelized open-coast tidal flat using a spatially extensive, multi-year dataset. The southern Ganghwa tidal flat on the west coast of Korea was surveyed seasonally between 2014 and 2016 and revisited in 2025, enabling assessment of variability across seasonal, interannual, and decadal timescales within a single system. Empirical Orthogonal Function (EOF) analysis was used to identify dominant modes of variability, which were interpreted in conjunction with morphodynamic observations, chlorophyll-a data, and time-series imagery. Three statistically significant modes account for the primary patterns of variability in surface sediment distribution. Among these, interannual variability in wave climate is the dominant control, driving tidal-flat-wide sediment redistribution through wave-induced resuspension and winnowing of fine sediments. Decadal changes, including channel migration, oyster-reef expansion, and relative sea-level rise, exert a secondary influence by modifying hydrodynamic conditions and sediment retention patterns. In contrast, seasonal variability is less strongly expressed in the statistical decomposition, reflecting spatially heterogeneous responses governed by local hydrodynamics and biological stabilization. Surface sediments nevertheless show consistent seasonal tendencies, with winter coarsening driven by elevated wave energy across much of the tidal flat, while localized mud deposition occurs simultaneously in the upper intertidal zone and along channel margins due to biofilm-mediated stabilization and enhanced supply of suspended fine sediment. These results indicate that sediment distribution in channelized open-coast tidal flats reflects the interaction of processes operating across multiple temporal scales, and emphasize the need for process-based facies models that incorporate channel dynamics and biophysical controls.