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

• Tidal asymmetry is one of key factors in compound dune morphodynamics.
• Stratigraphic architecture of compound dunes reflects ebb-dominance.
• Waves modulate compound dune morphodynamics and induce landward migration.
• Master bedding surfaces of compound dune dip either parallel or in opposite direction to those of point bar.
• Concurrent migration of channel and compound dunes creates a composite stratigraphy of dunes and bars.

Simple and compound dunes are developed on the intertidal tributary channel and channel bank of Yeochari macrotidal flat in the northern Gyeonggi Bay, west coast of Korea. Dunes are asymmetrical with the majority of their steeper lee faces and master bedding surfaces dipping toward the ebb current direction. Dunes consist of cross-bedded medium to coarse sands with a coarsening-up trend. Channel banks are comprised of sand and mud flat facies association, while the tributary channel is composed of channel facies association including fluid muds and channel lags. Three-year morphodynamic observations revealed that simple dunes on the tributary channel migrate seaward as fast as 1.5–2 m per day. In contrast compound dunes on the southern channel bank migrate either landward or seaward at much slower rates of 2–3 m per month. Despite greater current speeds on the channel bank, smaller tidal asymmetry leads to slower migration of compound dunes. In the case of intense wave activity, however, compound dunes seem to migrate at a higher rate. During the study period, compound dunes continued to shift their location toward the northern channel bank direction as the tributary channel migrates laterally back and forth perpendicular to channel thalweg. Concurrent migration of compound dunes and the tributary channel produced a complicated stratigraphic architecture consisting of a point-bar succession overlain by a coarsening-up compound-dune succession. Master bedding surfaces of compound dunes dip in a nearly opposite direction to those of point-bar succession. Tidal asymmetry, wave intensity, discharge fluctuation that controls the migration of tributary channel, and the antecedent topography of resistant substrate are seen to exert an important control on the stratigraphic architecture of compound dunes and point bars in the intertidal environment.