Multiscale sediment dynamics and morphodynamics of a channelized open-coast tidal flat: implications for facies models

The topography and surface sediment distribution of open-coast tidal flats exhibit pronounced spatiotemporal variability, yet the extent to which short-term sediment redistribution is preserved in long-term morphological evolution remains poorly constrained. To address this issue, Empirical Orthogonal Function (EOF) analyses were applied to surface sediment grain-size data (2014–2025) and repeated cross-shore topographic transects (2009–2025) from a channelized open-coast tidal flat on southwestern Ganghwa Island, west coast of Korea.

Surface sediment EOF results indicate that grain-size distribution is governed by interannual, decadal, and seasonal variability associated with wave forcing, geomorphic change, and biophysical processes. Interannual variability is strongest in the middle to upper tidal flat, where periods of stronger wave conditions are characterized by relative coarsening, reflecting tidal-stage-dependent wave influence. Decadal variability reflects longer-term geomorphic controls, including tidal-channel migration and oyster-reef expansion, producing contrasting coarsening and fining trends. Seasonal variability shows clear elevation dependence, with winter coarsening and summer fining in the middle tidal flat, and the opposite trend in the upper tidal flat due to biofilm development and rainfall-induced sheet flow.

EOF analysis of long-term transect data reveals how these sedimentary processes are selectively preserved in morphology. The first transect EOF mode represents long-term geomorphic adjustment characterized by persistent erosion of the upper tidal flat and concurrent point-bar growth within the main channel, consistent with progressive channel migration. The second mode captures intermediate-scale variability, showing a transition from gradual erosion during 2013–2015 to net deposition after 2018, coinciding with channelization in the lower tidal flat and progressive aggradation of channel flanks. Seasonal signals in transect morphology are comparatively weak, indicating limited preservation of short-term sediment redistribution.

These results suggest that much of the seasonal to interannual grain-size variability observed on tidal-flat surfaces is unlikely to be directly recorded the stratigraphic record, whereas decadal-scale geomorphic processes associated channel migration and localized aggradation exert a first-order control on preserved tidal-flat facies architecture. This study provides implications for interpreting grain-size trends, facies transitions, and lateral heterogeneity in ancient tidal-flat deposits.