Morphodynamics and Sedimentary Structures of Breaking Antidunes in the Macrotidal Environment

Breaking antidunes are commonly developed during ebb tides in the inshore tidal flats of sediment-laden macrotidal estuaries. However, the morphodynamics and sedimentary structures of breaking antidunes remain poorly understood. Breaking antidunes display cyclic behavior of growth, breaking, and obliteration of surface waves over 30–40 m in the large blind tidal channels and 1–5 m in the smaller channels. Breaking of surface waves is marked by turbid waters induced from channel-bed scouring by turbulent hydraulic jump. The wavelength of breaking antidunes decreases exponentially toward the distal end of the blind tidal channel, resulting from rapid flow deceleration due to a decrease in slope gradient and the backwater effect of the main channel. Despite the overall flood-tide dominance often typified by the presence of tidal bores, supercritical ebb currents exert a crucial role in sediment transport and geomorphic changes in the distal end of the blind tidal channels, where highly avulsive channels develop in response to rapid sedimentation. Breaking antidunes consist of parallel to subparallel laminated fine to medium sands, contrasting with neighboring heterolithic tidal beddings, highlighting the role of breaking antidunes on the facies heterogeneity. Supercritical ebb currents, including breaking antidunes, are promoted by the morphodynamic feedback of the inshore tidal flats, with a convex-up profile and scour pools in the upper intertidal zone to form a local drainage, resulting from strong tidal bores, distinct flood tidal asymmetry, and high sediment supply.