长江河口悬浮泥沙的混合过程

Processes of Suspended Sediment Mixture in the Yangtze River Estuary

根据准同步观测的悬浮泥沙及表层沉积物粒度、流速、含沙量资料,分析了长江口及临近海域悬浮泥沙在河口的混合过程。长江河口—陆架系统悬浮泥沙中值粒径呈现"细—粗—细"的变化规律,河口上段悬浮泥沙中值粒径为8.9μm,拦门沙海域为10.5μm,陆架区为4.5μm,北支为9.9μm,杭州湾口为5.6μm,泥沙类型为粘土质粉砂。河口上段和陆架区悬浮泥沙与表层沉积物的垂向混合作用较弱,拦门沙区域二者发生强烈的混合和交换,悬浮泥沙在由长江河口向陆架系统输移过程中仅有表层泥沙保留了流域输入的泥沙粒度特征。长江口悬浮泥沙中值粒径与含沙量呈良好的正相关关系,水流的剪切作用是引起拦门沙海域泥沙再悬浮、近底高含沙量和悬浮泥沙粒径增加的主要原因,悬浮泥沙粒径和含沙量的增加主要由粉砂组分的增加引起。2007年长江河口区范围内悬浮泥沙中值粒径比2003年普遍减小11%,含沙量比2003年减小22%,河口上段含沙量对流域来沙减少的响应最为敏感,而拦门沙区的泥沙粒径对流域来沙减少的响应最敏感。在长江流域来沙量减少的背景下,河口拦门沙区域仍能维持较高的含沙量,主要缘于河口系统内部的供沙。

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 Ds0 （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 4.5μm in the upper Yangtze River Estuary, mouth bar area and continental shelf respectively, and were 9.9μm and 5.6μm 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.