Processes of suspended sediment mixture in the Yangtze River estuary

Based on the sub-simultaneous data of grain size distributions (GSDs) of suspended and surface sediment, current velocity, suspended sediment concentration (SSC), the mixing processes between suspended and surface sediment were conducted in the Yangtze River Estuary and adjacent area. The characteristics of suspended sediment D₅₀ (SSD) were showed a fine-coarse-fine pattern from the Yangtze River Estuary to continental shelf. The SSD were 8.9 μm, 10.5 μm and 4.5 μm in the upper Yangtze River Estuary, mouth bar area and continental shelf respectively, and were 9.9 μm and 5.6 μm in the North Branch and Hangzhou Bay respectively. Clayey silt was the dominant sediment type in the study area. The vertical mixing processes between suspended and surface sediments were weak in the upper estuary and the continental shelf, while it was strong in the mouth bar area. Only surface layer of suspended sediment retained the characteristics of GSDs from the Yangtze River basin. Good positive correlations were observed between the SSD and SSC, and the sediment resuspension by the shear flow was the main causes of SSC and SSD increase near the bottom. The SSD of the Yangtze Estuary in the year 2007 decreased by 11% compared with 2003, while the SSC decreased by 22% in the same period.