Positive Feedback Between Hydrodynamics and Geomorphology Drive Regime Shift in Tidal Flat

Tidal flats worldwide are undergoing accelerated regime shifts from accretion to erosion, undermining their natural capacity for coastal protection and threatening the sustainability of adjacent urban areas. Although the influences of climate change and reduced sediment supply on tidal flat morphodynamics are widely acknowledged, the intrinsic sediment dynamic mechanisms behind these shifts remain poorly understood. Based on multi-year in situ observations of Jiangsu tidal flat—once known for rapid accretion but now undergoing erosion under multiple stressors—we show that the regime shift occurs progressively, with erosion expanding from the lower to the upper intertidal zone over several years. Episodic high-energy wave events dominated near-bed boundary-layer hydrodynamics in this shallow-water environment. Wave orbital motions penetrated efficiently to the seabed, producing high wave-current shear stresses that caused net erosion, whereas tidal currents played a secondary role. Erosion was highly sensitive to wave height; a threshold of approximately 0.22 m triggered the shift from accretion to erosion. This wave-dominated erosion led to bed lowering, which further amplified wave energy and erosion rates, establishing a self-reinforcing feedback. We propose a conceptual morphodynamic model illustrating this mechanism of accretion–erosion transition, which may also apply to other sediment-starved coastal systems such as subaqueous deltas. These insights support improved adaptive management of vulnerable coastal sedimentary systems under growing climatic and anthropogenic pressures.