Supercritical Bedload Transport and Grain‐Size‐Controlled Morphodynamics in a Strait‐Influenced Tidal Delta

ABSTRACT Seabed instability in high‐energy tidal straits is governed by spatially focused bedload transport, yet the role of bedload transport in controlling seabed morphodynamics in high‐energy tidal straits remains poorly understood. We investigate the Eastern Mouth Tidal Delta (EMTD) at the outlet of the Qiongzhou Strait using integrated modelling, field observations and multibeam bathymetry. Results show that under asymmetric tidal forcing, bedload transport is strongly focused on the delta top of the eastern mouth, where fluxes commonly exceed 1300 kg·m −1 ·day −1 , and near‐bed velocities exceed motion thresholds for a substantial fraction of the tidal cycle. Grain size reflects long‐term hydrodynamic forcing and exerts a first‐order control on both transport intensity and bedform geometry, with coarse sediments associated with larger dunes across the delta‐top sand‐ridge field. Under the combined control of hydrodynamic forcing and sediment characteristics, large tidal dunes (2–5 m high) in the central EMTD migrate at rates exceeding 50 m·year −1 , decreasing to ~20–30 m·year −1 towards the margins, indicating sustained traction‐dominated transport. The research indicates that sediment redistribution in constricted tidal systems is governed by the interaction between flow intensification and grain‐size‐dependent thresholds, which organise transport into persistent high‐mobility zones. This mechanism provides a transferable framework for understanding morphodynamic evolution in other energetic, strait‐influenced systems.