Morphodynamic evolution of sand–mud tidal flats under tidal constituent asymmetry

As the most important contributor to tidal asymmetry along open coasts, the asymmetry caused by interaction between astronomical tides M2, O1 and K1 has received less attention with respect to its impact on sediment transport and long-term morphological evolution. This study aims to clarify how different tidal asymmetry, particularly those associated with M2, O1 and K1, influence sediment transport and tidal flat morphology. We employ a process-based numerical tool (Delf3D) in a short tidal basin with simplified geometry and bathymetry. Different combination of tidal constituents, which produce asymmetry while preserving the same peak velocity, are imposed at the open boundary by varying amplitudes and phase differences.

Our results show that, in the presence of both sand and mud, channel networks under ebb-dominated conditions are developed primarily through sand erosion, whereas under flood-dominated condition they develop mainly through mud accretion. Sand transport is more sensitive to velocity asymmetry while mud transport responds to both velocity and spatial lag effect which represents energy gradient. Consequently, under ebb-dominated conditions, the deepest and longest channel network occurs in the scenario forced by M2 and M4 at the boundary, where prolonged periods of high velocity are present. Under flood-dominated condition, longer, deeper and more efficient channels occur in scenarios dominated by M2, O1 and K1 interaction with higher degree of asymmetry. A milder upper slope enhances ebb currents, thereby strengthening ebb dominance but weakening flood dominance; this leads to elongation and deepening channel networks under ebb-dominated conditions but shorter channels under flood-dominated conditions. Because tidal form number F correlates strongly with astronomical tides M2, S2, O1 and K1, our findings provide insight into tidal flats morphodynamics across mixed to predominately diurnal tidal regimes.