Erosion and siltation dynamics and driving mechanisms of tidal flat morphology on the northern part of the North Jiangsu Radial Sand Ridges (NJRSR)

Globally, tidal flat systems are shifting from net siltation to erosion due to increased human activity and environmental changes. However, the spatiotemporal patterns of erosion and siltation of the North Jiangsu radial sand ridges (NJRSR) remain poorly understood regarding recent environmental changes. In this study, we examine recent morphodynamic changes and their underlying mechanisms in a typical erosion–siltation transition zone of the NJRSR tidal flat. The results indicate that: (1) The tidal flats of the NJRSR have significantly shifted toward erosion over the past few decades, with the erosion inflection point shifting from the Sheyang River estuary toward Doulong Port. From 1989 to 2021, tidal flat development was characterized by net siltation with decreasing rates before 2010, followed by persistent net erosion afterward. (2) Spatially, siltation occurred on the upper tidal flats from 1990 to 1995, while the lower tidal flats experienced erosion, with a maximum erosion depth of -2.73 m and an average rate of 0.46 m per year. Between 1995 and 2000, erosion intensified north of the Sheyang River estuary, while areas to the south remained weakly siltational. After 2000, erosion clearly migrated southward across the study area, progressing approximately ten transects every five years and increasing in magnitude. (3) Numerical simulations reveal that residual currents are the dominant factor controlling tidal flat dynamics, while nearshore sediment transport is primarily driven by southward residual flows. Among the three zones identified, Zone I shows a gradual decrease in erosion, Zone II exhibits declining nearshore suspended sediment levels and a shift toward erosion, and Zone III is characterized by relatively stable nearshore conditions and a transition from offshore erosion to siltation. This study provides a quantitative assessment of the long-term morphodynamic evolution of tidal flats and offers new insights into the mechanisms driving coastal change in the NJRSR.