三峡水库干流消落带沉积泥沙粒径特征及其物源意义

为揭示三峡水库干流消落带的泥沙沉积规律、分析沉积泥沙来源,本研究采用原位观测方法采集沉积泥沙样品,利用激光粒度仪测试泥沙粒径,分析沉积泥沙粒径在水平、垂直和高程3个维度上的变化特征,并与三峡水库入库泥沙的粒径特征相结合,阐述消落带沉积泥沙来源。结果表明:(1)三峡水库干流消落带沉积泥沙粒径在水平方向上存在比较强烈的空间变化,中值粒径沿河流流向方向呈逐渐下降趋势,并在忠县及其下游河段基本保持稳定;(2)泥沙粒径随高程的变化存在较大的空间差异,河流挟沙是消落带下部粗颗粒泥沙的主要来源,而消落带上方的土壤侵蚀强度越高,消落带顶部的沉积颗粒就越容易变粗;(3)在水库尾端,泥沙剖面存在较明显的旋迴分层现象,其中值粒径数值较大、变化幅度较宽,越往下游推进,中值粒径的数值越低、变化幅度越小,泥沙旋迴分层现象逐渐消失;(4)水库尾端的沉积泥沙以库区外来沙为主,越往下游推进,库区内产沙对粗颗粒泥沙的贡献逐渐升高,但库区内外来沙都能够为沉积泥沙提供丰富的细颗粒物源,因此,细沙的来源具有一定的复杂性和多样化特征。

SEDIMENT PARTICLE SIZE IN RIPARIAN ZONE OF THE THREE GORGES RESERVOIR AND ITS IMPLICATION ON SOURCES

The main aim of this study is to reveal the regularity and the sources of sediment deposition in the riparian zone of the main stream in the Three Gorges Reservoir. Using in-situ observations and laser particle analyzer, we investigated the particle size of the deposited sediment and its variation in horizontal, vertical and elevational directions in the riparian zone of the Three Gorges Reservoir, and compared the characteristics of grain size between deposited sediment and suspended sediment to reveal the source of deposited sediment studied. The results show that:(1) sediment deposited in the riparian zone of perennial reservoir was significantly finer than that in the fluctuating backwater area, and particle size gradually became finer along the horizontal flow direction and is almost stable in Zhongxian County and its lower reach; (2) fine sediments usually occupied a larger proportion with increasing elevation, but coarse sediments also had great opportunities to take the advantage on the top of the riparian zone if soil erosion above the zone was intense and strong; (3) cyclic layering of the sediment profile was easy to observe in the riparian zone at the end of the reservoir where median size was larger and appears a wider range of variation, while the median size and related range of variation get smaller and cyclic layering of the sediment profile gradually disappeared towards the downstream; (4) deposited sediments in riparian zone of the fluctuating backwater area mainly came from external sources such as Jinsha River and Jialing River, internal sources made greater contribution to coarse sediments in the perennial reservoir area but couldn't be dominant in fine sediments since external sources are also able to supply sufficient matters which makes the source of fine sediments complex and diverse.