长江悬浮物絮凝特征

Flocculation characteristics of suspended particulate matter in Yangtze River

河流淡水环境的实有絮凝特征及变化过程长期以来缺乏观测和研究。本文利用现场激光粒度仪(LISST-100)实测得到长江流域干流4 050km,13个主要站位的絮团大小、分布和沿程变化特征,对比分散粒径和悬浮物浓度,得到认识:长江干流水体的现场悬浮物絮团粒径平均为35μm(洪季),泥沙中值粒径平均为5μm,絮团粒径比泥沙中值粒径大一个量级,证明了长江干流絮凝现象的普遍存在。上游石鼓至万州,絮团粒径平均为17μm,明显小于中下游絮团粒径,为47μm。上游与中下游絮团粒径的差别,主要是受水动力条件的影响。三峡水库蓄水对于絮凝过程有一定的影响,水库以上缓流区水体滞留时间长,有利于絮团成长;库区絮团经过大坝,被水流打散而破碎,产生解絮过程。絮团在长江干流水体中已经形成,季节性变化不明显,其粒径要比河口絮团粒径小。河口环境下,絮团可以增长数倍。联系已有长江河口絮凝现象的观测研究,河流淡水环境的絮团可作为河口盐淡水环境絮团的背景絮团。全文提示了在研究河流泥沙输运、水库泥沙淤积和河流污染物输送等方面应该充分考虑絮凝的影响。

Freshwater flocculation was rarely studied and few literatures focused on this topic. In this paper, in situ monitoring of behavior of the suspended particulate matter was carried out in the main stream of the Yangtze in the wet season in 2009 with the help of a LISST-100. The measurement extended 4050km longitudinally at 13 stations, while the grain size distributions of the dispersed particles and the suspended sediment concentrations were observed at the same time. The comparison between the grain size distribution of dispersed particles and in situ flocs indicated that flocculation was one common fact in the Yangtze River. The median floe diameters were 35 μm in average, while it was 5μm for dispersed particles. In the river reach between Shigu and Wanzhou stations in the Three Gorges Reservoir （TGR） , the median floc diameters were 17 μm ,which were much smaller than the value in the reach below the TGR. This difference is ascribed to the different hydrodynamic and biochemical factors in the reaches above and below the TGR. Because flocculation was observed continuously along the Yangtze River, it is supposed that the flocs found widely in the Yangtze Estuary were formed based on the freshwater flocs which were transported into the estuary and maintained their flocculation behavior. However, the floe sizes in the estuary were larger and more variable than freshwater ones. The finding of freshwater flocculation in this study reminds that the role of flocs in transport of fine suspended sediments and reservoir sedimentation should be further considered carefully.