三峡库区粉砂淤积成因的初步分析

Preliminary analysis on the causes of silt deposition in the Three Gorges Reservoir

三峡水库运行十年来,常年库区发生了"点"状分布的泥沙淤积,与以往研究有所差异。通过现场测量发现库区泥沙特性如下:悬沙和淤积物均为粒径较小的粉砂,泥沙的实际沉速大于其Stokes沉速,床面淤积物呈浮泥或絮凝结构。水沙运动实测资料分析表明,以流速作为冲淤的判别条件比挟沙力更为合适,低流速时粉砂絮凝沉降导致淤积,而淤积物固结后不易冲刷,由此形成了淤积、输沙和冲刷三个流速带。三峡库区的大量弯道、支流河口和宽谷处流速较低,属于淤积流速带,发生絮凝沉降淤积后不易冲刷,因此一旦发生淤积则淤积严重,其他河段多数时间处于输沙或冲刷流速带而淤积量较小甚至没有淤积,因此形成了库区粉砂"点"状的淤积特点。

After about ten years operation of the Three Gorges Reservoir,serious sediment deposition occurs with a ＂point＂distribution,which is not agree with the previous prediction. In this paper,the characteristics of the sediment in the reservoir are found by field observation as follows： the suspended and bed material are fine silt,the actual settling velocities are bigger than those of their primary particles,and the deposited sediments are like floating sludges or flocculation structures. The observed relationship between the flow and sediment movements indicates that the flow velocity is more appropriate than the sediment carrying capacity in determining the critical condition of erosion or deposition status. Flocculation happens under small velocities,so as causing sediment deposition. Deposited silt will further consolidate,resulting in difficulty for erosion. As a result,three velocity zones,namely deposition zone,transportation zone and erosion zone,are divided. The flow velocities of the bend reaches,river mouths and broad valleys in the Three Gorges Reservoir usually belong to the deposition velocity zone,where the silt are deposited caused by flocculation and the depositions are usually severe due to the difficulty for erosion. Other reaches in the Three Gorges Reservoir are usually in the transportation or erosion velocity zone,causing little or even no depositions,as a result those depositions with＂point＂distribution are formed.