Sedimentation on a Channelized, Open-Coast Macrotidal Flat of the Mid-Western Coast of Korea

Seasonal variations in the intensity of onshore winds and waves mainly control sedimentation on the open-coast tidal flats. Still, sedimentation and facies distribution in the channelized open-coast tidal flats are less well understood, partly due to the lack of long-term field observations. The present study investigates the spatiotemporal variability of sedimentation on the Yeochari tidal flats, the most extensive open-coast tidal flats on the west coast of Korea, to understand governing factors that drive the variability and construct a new facies model for open-coast tidal flats. In winter, when north-to-northwesterly winds prevail, sand flats in the western part of the Yeochari tidal flats expand while those in the eastern part remain unchanged. Despite the enhanced wave activity, however, mud flats broaden in the east of the region due to the presence of biofilm that promotes mud deposition. Mud deposition is also notable in the flanks of tidal channels in the lower intertidal zone, reflecting tidal sedimentation during calm weather in winter. In spring, sustained but subdued wave activity leads to coarsening in the upper intertidal zone and fining in the middle to lower intertidal zone, indicating seaward transport of fine-grained sediments. In summer, tidal flats, particularly in the eastern part, become coarser-grained due to the impingement of waves generated by south-to-southeasterly winds. Frequent heavy rainfalls facilitate channel migration by inducing an ebb dominance with increased surface runoff. Notable mud deposition occurs at the flank of tidal channels in the lower intertidal zone during the calm period. In fall, tidal flats, especially in the western part, experience erosion to become coarser-grained due to increased wave activity, widening the mixed flats. Highly channelized morphology with seasonal storm waves and autocyclic channel migration accentuated by heavy rainfalls increase facies heterogeneity in the middle to lower intertidal zone, warranting a new tidal-flat facies model.