MORPHODYNAMIC AND HYDRODYNAMIC CONTROLS ON THE STRATIGRAPHIC ARCHITECTURE OF INTERTIDAL COMPOUND DUNES ON THE OPEN-COAST MACROTIDAL FLAT IN THE NORTHERN GYEONGGI BAY, WEST COAST OF KOREA

The morphodynamics and hydrodynamics of intertidal dunes were investigated to understand their controls on the stratigraphic architecture of the dunes on the open-coast macrotidal tributary channel and channel bank in the northern Gyeonggi Bay, west coast of Korea. Simple dunes migrate seaward as fast as 1-4 m/day on the tributary channel that is ebb dominated with pronounced tidal asymmetry. In contrast, simple and compound dunes migrate either landward or seaward at slower rates of 0.1-2 m/day and 2-3 m/month, respectively, on the southern channel bank, where currents are flood dominated with smaller tidal asymmetry. The tributary channel migrates laterally at variable rates ranging from 1 to 18 m/month with greater rates occurring during the summertime rainy season. In the event of large waves that coincide with the dominant current, westerly to northwesterly waves during winter to spring accentuate the ebbward migration of compound dunes, whereas southerly to southeasterly waves during the summer result in the floodward migration. Deviations to this seasonal trend are possible when the large waves occur during subordinate current stage. The displacement of compound dunes onto the migrating tributary channel produces a composite succession in which a coarsening-up compound-dune succession overlies a fining-up point-bar succession. The master bedding surfaces of the compound dunes dip opposite to or oblique to the directions of accretion surfaces of the point bar. Recognition of nontidal events from the intertidal compound dunes is not straightforward because the reversal and displacement of the dune profile requires shorter response time compared to subtidal counterparts. Tidal asymmetry, tidal cycle in the event of waves, seasonality in wave intensity and direction, and the migration of the tributary channel are seen to exert an important control on the stratigraphic architecture of compound dunes and point bars in the intertidal environment over time and space.