Morphodynamic behaviors of macrotidal ridge and runnel beaches during winter: The case of Baeksajang Beach, South Korea

Macrotidal ridge and runnel beaches respond to external forces in a morphodynamic manner that involves morphological and sedimentological changes. In this study, we investigated morphodynamic responses of Baeksajang Beach, one of the macrotidal ridge and runnel beaches on the west coast of South Korea during the winter when high energetic conditions prevail. Landward migrations of intertidal ridges were predominant over the whole period, but bars on the upper and lower part of the beach profiles showed different behaviors. The position of mean sea level was associated with different upper and lower bar behaviors. The erosional and depositional phases alternated alongshore, implying net alongshore sediment transport. Textural characteristics responding to external forces were twofold: the occurrence of overall coarsening that correlated well with elevation, and the presumable generation of transport-origin grain size trends. Grain size trend analysis (GSTA) identified the overall transport patterns compatible with topographic changes, while cross-shore components were more represented than alongshore ones due to elevation-related coarsening effect. The apparent beach state is not well predicted by the morphodynamic indices even under high energy conditions like winter season, implying the delayed morphological adjustment. These suggest that local constraints such as sediment budget and geology might be no less critical than external energy conditions in macrotidal ridge and runnel beaches as observed in this study.