大坝影响下长江干流悬沙粒度变化特征

Changes in suspended sediment grain size along the mainstream of the Yangtze River under the influences of cascade dams

大坝运行显著改变河流泥沙通量及组成,收集并分析了长江干流石鼓至徐六泾河段多年悬沙粒度资料(1987-2019年),并重点分析了宜昌、九江和徐六泾代表站悬沙组分变化规律。发现2003年长江干流水库群陆续投入运行以来,长江上游悬沙多年平均中值粒径从20μm减小到10μm;而中下游悬沙多年平均中值粒径从10μm增至15μm,且存在空间差异,最大增幅达2003年前的3倍。洪季悬沙粗化更显著,10~100μm的悬沙体积百分含量增加。通过对泥沙的平衡概算耦合分析,发现中下游泥沙粗化主要受河床侵蚀加剧、三峡下泄泥沙粒度改变、洞庭湖和鄱阳湖出湖泥沙粗化等多因素的综合影响。

Dam operation could change sediment flux and composition in rivers.Based on the measured grain size data(1987-2019)from Shigu to Xuliujing,the temporal and spatial variations of the median particle size(D;)of suspended sediment were analyzed,and focused on the variation of the sediment components at Yichang,Jiujiang and Xuliujing.The D;of suspended sediment from dam site to the estuary generally increased after the closure of dams,with a maximum 3-time increase.In addition,the coarsening in the flood season(May-October)was more significant,as indicated by the increase in the volume percentage of sediment in the size range of 10~100μm.Further analysis suggested the combined effect of the increasing coarse sediment from riverbed scouring,the"fining-to-coarsening"trend related to sediment outflow from the Three Gorges Dam and the coarsening trend related with extra sediment flux from the Dongting Lake and Poyang Lake.