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牛轭湖是弯曲河流自然裁弯后遗留下来的浅水湖泊,其形态、数量和沉积特征可指示弯曲河道演变规律和冲积平原的洪水频率与环境变迁。通过牛轭湖形态定义和简化,运用一维水流泥沙数学模型描述了裁弯后原河道进口段的悬移质淤积过程,数值模拟表明,原河道进口断面悬移质浓度和沙栓长度是决定牛轭湖形成速率的关键因素,沙栓长度越短或悬移质浓度越高,进口段淤积速率越快。牛轭湖出口段淤积的主要原因是新河道的挟沙水流流速和浓度均高于原河道,主流倒灌形成环流进入相对低流速的出口段中产生悬移质淤积。统计分析10条自由弯曲河流的1 329个牛轭湖的卫星图像,认为Ω型、U型和月牙型对牛轭湖形态具有广泛的概括性,可指示牛轭湖3个不同的演变阶段(初期、中期和晚期)。牛轭湖存在两种不同演变过程,第一种是牛轭湖不受新河道影响,经历Ω型、U型、月牙型三个阶段后变成冲积平原的陆地;第二种是新河道朝着牛轭湖方向横向演变,凸岸冲刷蚀退逐步蚕食牛轭湖,使得牛轭湖变成冲积平原和弯曲河道的一部分。
Abstract：The oxbow lake is a unique shallow lake which is a legacy of natural cutoff on meandering rivers.The morphology,number and deposition of oxbow lakes can demonstrate meandering channel evolution,flood frequency and environmental change of floodplains.The oxbow lake morphology is defined and simplified,and a one-dimensional flow-sediment mathematical model is used to describe the suspended sediment deposition process of the old meandering channel inlet section after a cutoff.The numerical experiments show that the suspended sediment concentration and sediment deposition length are two key factors determining the formation rate of an oxbow lake,and a shorter deposition length or a higher sediment concentration will fasten the deposition rate of the inlet section.Deposition at the exit section of an oxbow lake mainly is dependent on the flow velocity and suspended load concentration of the new channel being higher than the old meandering channel,so the mainstream intrudes into the low velocity water body of the exit section and forms a counter circulation,advancing suspended sediment siltation.1 329 oxbow lakes on ten free meandering rivers by Google Earth satellite images are chosed to analyze the morphology features.The Ω-shaped,U-shaped and crescent-shaped oxbow lakes not only have wide generality,but also indicate the three evolution stages including initial stage,mid-term and later stage.There are two different processes of long-term evolution of oxbow lakes life.The first is an oxbow lake keeping away from new channel migration influence,and it will experience three stages of the Ω-shaped,U-shaped and crescent-shaped and finally become a part of the floodplain.The second is the lateral migration of new river channel toward the direction of the oxbow lake,and the convex bank erosion will advance the new channel gradually taking possession of the oxbow lake which has to become a part of floodplain or the new meandering channel.
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基本信息:
DOI：10.16239/j.cnki.0468-155x.2012.05.003
中图分类号:TV147
引用信息:
[1]李志威,王兆印,潘保柱.牛轭湖形成机理与长期演变规律[J].泥沙研究,2012(05):16-25.DOI:10.16239/j.cnki.0468-155x.2012.05.003.
基金信息:
科技部国际科技合作计划资助(2011DFA20820,2011DFG93160);; 清华大学自主科研课题(20121080027)
2012-10-15
2012-10-15