Suspended sediment change along the Zhejiang-Fujian coast revealed using MODIS 250-m imagery

Abstract

Suspended sediment concentration (SSC) is one of the important parameters in the ecological environment evaluation of marine water bodies, as it provides an important basis for research on the ecological environment of nearshore sea areas, shoreline beach erosion and deposition, water quality of coastal zones and nearshore sedimentation zones. In recent years, the amount of sediment carried by the Yangtze has declined dramatically due to the Three-Gorges Dam (TGD), which might change the sediment supply to the sea and thus affect sediment transport to the coasts of Zhejiang and Fujian. This study aims to both develop specific algorithms and reveal the interannual and seasonal dynamics of the suspended sediment belt from the Yangtze Estuary to the Zhejiang-Fujian provincial shoal using MODIS 250 m data from 2000 to 2020. MODIS time-series analysis shows that the suspended sediments were originally from the Yangtze Estuary and formed a suspended sediment belt in the winter season. Moreover, driven by monsoon winds and coastal current dynamics, the sediment belt has obvious seasonality. Furthermore, we also suggest that the coastal sediment area has a tendency to shrink in winter, and the lifetime of the sediment belt also shortens, showing early disconnection in spring and delayed recovery in autumn, in response to the decline in sediment discharge from the Yangtze River basin to the sea. Impressively, the impact of the TGD in 2003 led to a drop in the sediment concentration in the coastal area. In addition, further analysis confirms that a sudden decrease in suspended sediment concentration was observed at Zhoushan (S2), Wenling (S4), Minjiang River Mouth (S7), Pingtan Island (S8), and many other stations along the coast in 2003, and the closer to the Yangtze Estuary, the more serious this phenomenon becomes, on the basis of monthly SSC analysis. These findings can assist us in better understanding the new pattern of the sediment belt from the Yangtze Estuary to the Zhejiang-Fujian coastal area. The findings of this study have theoretical and practical implications for the study of the hydrological sediment field in offshore areas, the protection of the marine ecological environment, changes in the shoreline and mudflats, and the maintenance of waterways and ports in estuaries.