MORPHODYNAMICS OF TIDAL CHANNELS IN THE OPEN COAST MACROTIDAL FLAT, SOUTHERN GANGHWA ISLAND IN GYEONGGI BAY, WEST COAST OF KOREA

Morphodynamics of intertidal channels were monitored in order to understand their implication on the architecture of inclined heterolithic stratification (IHS) in the open-coast Yeochari macro tidal flat on southern Ganghwa Island in Gyeonggi Bay, west coast of Korea. The tidal flat is divisible into narrow salt marshes in the upper intertidal zone, a concave-up upper to middle intertidal zone with small tidal creeks, and a channelized lower intertidal zone. Channels in the lower intertidal zone are 200-600 m wide and 1-2.5 m deep at bankful stage. They are sinuous in planform and have well-defined point bars and cutbanks. IHS is exposed along the cutbanks. High-precision leveling of the tidal channel bank unveiled that a point bar in the lower intertidal zone migrated about 400 m in 39 months. Channel migration is pronounced during the summer rainy season, when the point bar migrated as fast as 40 m per month, which led to rapid sediment accumulation of as much as 40 cm per month. In contrast, channel migration rate during winter is notably reduced, down to less than 1 m per month. Point-bar geometry alternates between a concave-up profile in summertime and a convex-up profile during the remainder of the year. Enhanced ebb currents during the rainy season, due to increased runoff discharge caused by heavy precipitation, accelerated point-bar migration. Remarkable rill erosion induced by heavy precipitation especially during low tide led to the rapid accumulation of sediment at the lower part of the point bar and the channel base, creating concave-up point-bar geometry. During the remainder of the year, the point bar retains a convex-up profile by continued sedimentation with insignificant rill erosion. Wave activity, particularly during winter, seems to facilitate deposition in the upper part of the point bar by providing more sediment into the channel. The present study illuminates the fact that the stratigraphic architecture of IHS of intertidal origin is controlled largely by monsoonal weather and to a lesser degree by tidal process even in a macrotidal environment. Careful examination of IHS geometry may hint at the depositional setting in terms of tidal frame (intertidal versus subtidal) and climate regime (monsoonal versus non-monsoonal).