Morphodynamics and stratigraphic architecture of compound dunes and point bars on the open-coast macrotidal flat in Gyeonggi Bay, west coast of Korea

Abstract

Morphodynamics and hydrodynamic conditions of lower intertidal dune fields on the Yeochari tidal flat of Ganghwa Island in Gyeonggi Bay, west coast of Korea were investigated to understand the external controls governing the stratigraphic architecture of the compound dunes formed in the open-coast tidal environment. Morphodynamic measurement of intertidal channels and compound dunes were conducted twenty times by using RTK GPS over four years. Hydrodynamic data such as current speed, significant wave height and Suspended Sediment Concentrations (SSCs) were collected by using ADCPs, AWAC, pressure sensors and OBS in July of 2014 and March of 2015. Morphodynamic observations revealed that simple dunes on the tributary channel migrate seaward as fast as 1-4 m/ day. In contrast simple dunes on the southern channel bank migrate either landward or seaward as fast as 0.1-1 m/day during spring to fall and 1-2 m/day during winter. Compound dunes on the southern channel bank migrate either landward or seaward at much slower rates of 2-3 m/month. The tributary channel migrated at variable rates ranging from 1 m to 18 m per month with greater rates occurring during summertime rainy season. The tributary channel is ebb-dominated with pronounced tidal asymmetry, whereas tidal flats on the southern channel bank are flood-dominated with smaller tidal asymmetry. Wind-induced waves with significant wave heights over 0.4 m seem to modulate tide-induced sediment transport. In the event of large waves that coincide with dominant current, westerly to northwesterly waves during winter to spring accentuate the ebbward migration of compound dunes, whereas southerly to southeasterly waves during 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 in opposite to or oblique to the directions of accretion surfaces of the point bar. Recognition of non-tidal events from the intertidal compound dunes is not straightforward because the reversal and displacement of dune profile require relatively 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 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.