Remote sensing analysis of tidal flat evolution in the Yangtze River Estuary over the past 50 years: dynamics, patterns, and driving factors

• Development of new algorithms for tidal flat extraction. • The tidal flat of YRE has transformed from natural siltation to artificial siltation. • The development of tidal flat usually directly benefits from the high SSC around them. • Siltation promotion and reclamation project is conducive to the growth of tidal flat. The Yangtze River Estuary (YRE) is located in the most economically active area of China, with abundant tidal flat resources. Accurately understanding the evolution of tidal flat can provide an effective basis for the sustainable development of tidal flat resources. This study is based on Landsat image data from 1974 to 2024, using spectral index combined with Otsu algorithm (named SI-OA) to extract high-tide line and low-tide line every 5 years, and using water probability algorithm (named WPA) to obtain tidal flat is a new method. The results showed that: (1) The overall accuracy of tidal flat remote sensing mapping reached 94 %. (2) The area of tidal flat in the YRE decreased in fluctuations from 1974 to 2014, with about 41.8 % of the tidal flats disappeared, but then it increased rapidly by 51 % in 2017. Overall, the tidal flat has transitioned from natural siltation model to artificial siltation promotion and reclamation model around 1997. (3) The development of tidal flat usually directly benefits from the high suspended sediment concentration around them. In addition, siltation promotion and reclamation project is beneficial for the efficient deposition of limited suspended sediment. The tidal flat map can provide scientific basis for resource assessment, rational development, protection and management of coastal mudflat.