Multi-decadal dynamics of the Changjiang estuarine tidal flat resource: Causes and threats

Long-term changes in tidal flat resources under global change have garnered worldwide attention. Previous studies concentrated on either area variations or geomorphic changes of tidal flats, but have rarely bridged these approaches. In this study, we examined the multi-decadal dynamics of the Changjiang estuarine tidal flats (including the Hengsha Shoal, Jiuduan Shoal, and Nanhui Shoal) and potential land resource risks using coupled topographic data, remote sensing images, and hydrological data. The three tidal flats experienced increases in gross area above 0 m (52–94 km²) and above −5 m (4–188 km²) between 1958 and 2019. However, after 2000, accretion rates declined, with deposition concentrated in landward regions, while erosion began to affect the seaward edges. Meanwhile, the land resource of the Hengsha Shoal and Nanhui Shoal decreased by 47 and 72 km², respectively, because of intensive reclamation after 2002, whereas that of the Jiuduan Shoal increased by 60 km² as no reclamation occurred. Dam-induced riverine sediment decrease contributes to potential tidal flat erosion, which is partly counteracted by the engineering-promoted siltation through artificial structure construction and marsh vegetation planting. Relative to accretion/retreat, recent large-scale reclamation has played an important role in tidal flat resource dynamics. Continuous small riverine sediment supply, increased sea level rise, and steepening slope contribute to increased land resource risks of the tidal flats. In response, measures including integrated river-estuary management, scientific siltation promotion, and suitable reclamation design should be implemented. These findings provide insights for sustainable utilization of tidal flat resources in other mega estuaries globally.