Transport characteristics of non-uniform suspended sediment in the Jingjiang Reach after the Three Gorges Project operation

Abstract

三峡工程运用后，坝下游荆江段来沙量大幅度减小，处于严重的不平衡输沙状态，次饱和水流冲刷河床使悬沙量沿程恢复.基于实测水沙资料，分析了三峡工程运用后荆江段非均匀悬沙恢复特点.提出了恢复效率的概念用以表征悬移质沿程恢复的程度，并根据实测水沙资料计算了荆江段1994-2017年非均匀悬沙的恢复效率.结果表明：三峡工程运用前，荆江段各粒径组悬沙恢复效率绝对值均接近0，故该时期内各粒径组泥沙冲淤幅度不大；三峡工程运用后，各粒径组悬沙恢复效率绝对值均明显增大，且粗沙（d>0.125 mm）恢复效率绝对值远大于细沙（d<0.125 mm），故粗沙恢复程度更高.这主要是由于荆江段床沙组成中粗沙部分含量大，而细沙含量小.最后建立了三峡水库蓄水后非均匀悬沙恢复效率与来水来沙条件（来沙系数）的关系，结果表明：各粒径组悬移质恢复效率均与来沙系数呈正相关关系，全沙、细沙和中沙的决定系数（R²）分别为0.89、0.67和0.69，相关性较高，故荆江段各粒径组悬移质泥沙恢复效率较大程度上受到来水来沙条件的影响.;The amount of suspended sediment entering the Jingjiang Reach of the middle Yangtze River has been greatly reduced owing to the operation of the Three Gorges Project (TGP), which caused the sediment transport of the reach at a severe non-equilibrium state, and the suspended sediment concentration recovered during the channel scour process. Therefore, the characteristics of non-equilibrium transport of non-uniform suspended sediment were analyzed based on the measured flow and sediment data. In addition, the recovery efficiency was proposed to represent the recovery speed for suspended sediment load, and the corresponding values were calculated annually from 1994 to 2017 based on the measured data of the Jingjiang Reach. Calculated results show that the suspended sediment recovery efficiency of all particle-size groups was very close to zero before the TGP operation, which means that the channel deformation was small during this period. However, the recovery efficiency of each particle-size group increased greatly after the operation of TGP, and the recovery efficiency of coarse sediment (d>0.125 mm) was much higher than that of fine sediment (d<0.125 mm). This is mainly because the content of coarse sediment in bed-material of Jingjiang Reach is large, while the content of fine sediment is relatively small. Finally, empirical relationship was developed between the recovery efficiency and incoming flow-sediment regime, which shows that the recovery efficiency of each group was negatively related to incoming sediment coefficient, with the correlation coefficient for the total, fine and coarse sediment being 0.89, 0.67 and 0.69, respectively. Therefore, it can be concluded that the recovery efficiency of suspended sediment was significantly affected by the incoming flow and sediment conditions.