弯曲河流塑造试验与维持条件综述(摘要)
李志威1,2，吴新宇1,2，胡旭跃1,2
(1.长沙理工大学水利工程学院，湖南 长沙 410114；2.水沙科学与水灾害防治湖南省重点实验室，湖南 长沙 410114)
【摘 要】 弯曲河流横向迁移的渐变过程和裁弯触发的突变过程，使得野外观测其全生命周期具有很大难度，因此开展室内水槽实验塑造和模拟弯曲小河演变具有可操作性和可控制性。本文通过系统地总结室内水槽塑造弯曲河流试验的试验条件与研究结果，从影响弯曲河流形成与演变的关键因素如水流、泥沙、比降和植物四个方面，对以往水槽实验成果进行对比分析。塑造弯曲河流的关键是减缓河道展宽的速率，抑制斜槽裁弯的频率发生。冲积河流的弯曲趋向性和流量的周期性变幅密切相关，水流条件对室内塑造弯曲河道是至关重要的。水槽塑造弯曲河流所使用的流量一般较小，总结国内外已有的试验研究发现，流量变化范围通常为0.009～3.0L/s。国内外诸多学者对流量是否恒定或采取阶梯式流量以近似模拟天然河流的情况，尚没有形成统一观点。流量变化会更易塑造出弯曲河道，流量变化对试验的影响表现在三个方面：初始河道形态、边滩水流动力条件和河岸的侵蚀。流量变化促进交错边滩的形成，低流量促使河床形成，高流量促使河岸崩塌。纵比降决定输沙能力，从而影响造床作用。塑造弯曲河流所需的比降应在单股无汊和发生淤积的比降之间。室内水槽塑造弯曲河道所需的比降不宜过大或过小，已有试验研究表明纵比降变化范围为0.0025～0.025。细颗粒泥沙对于塑造弯曲河流至关重要，试验过程中加入的细砂级配和加沙速率尤为重要。国内外室内塑造弯曲河流加沙级配和加沙速率因人而异，没有统一的标准。总结已有的试验，加沙中值粒径变化范围为0.03～0.45mm，添加黏性物质则为滑石粉、黏土、高岭土、硅藻土和二氧化硅等。植物在弯曲河流发育过程中能否发挥控制作用一直以来备受争议。植物能改变水流方向和河道形态，植被的作用可归纳为：增加河岸稳定性，使河道变窄深；减小近岸流速，抑制崩岸的发生；根系能增强泥沙的抗拉强度和抗剪强度。目前，试验研究尚未解决的问题有试验时间、不能塑造高弯曲度的弯曲河流和未能发生颈口裁弯。本文提出从正过程(弯曲至辫状)和逆过程(辫状至弯曲)两个方向去设置水槽试验条件，以期获得弯道演变的维持条件和确定弯曲-辫状河型相互转化的临界水流-泥沙-植被阈值。
【关键词】 弯曲河流；水槽试验；试验条件；种草；裁弯；综述
A Review on Laboratory Experiments of Meandering River Formation and Sustaining Conditions
LI Zhiwei1,2,WU Xinyu1,2,HU Xuyue1,2
(1.School of Hydraulic Engineering,Changsha University of Science &Technology,Changsha Hunan Province 410114;2.Key Laboratory of Water-Sediment Sciences and Water Disaster prevention of Hunan Province,Changsha Hunan Province 410114)
Abstract:Gradual changing process of lateral migration and sudden changing process of cutoff occurrence in meandering rivers generate very high difficulties on field investigation of the whole life cycle,so naturally forming and modeling a small meandering channel using laboratory experiments performed by many researchers are operational and controllable.Experimental conditions and main results of pervious laboratory experiments are systematically summarized and compared,i.e.,critical factors(flow,sediment,slope,vegetation)of influencing bend formation and evolution.The key point is slowing down the rate of river widening and preventing chute cutoff.Curving tendency of alluvial river is closely related to periodic amplitude of discharge.Flow condition is crucial for formation of meandering river in laboratory.The experimental discharge in the laboratory is low generally.The variation of discharge is 0.009-3.0L/s.There is not unified view about discharge is constant or varying approximately similar with natural river.Varying discharge make experiment easier.Varying discharge has influenced on formation of meandering river by three aspects namely initial river shape,flow hydrodynamic condition of point bar and bank erosion.Formation of alternative bar is accelerated by varying discharge.The low discharge is beneficial to bed formation and high discharge is beneficial to bank failure.Sediment transport and riverbed evolution is dominated by slope.The suitable slope is between single thread river and deposition.The variation of slope is 0.0025-0.025.Fine particles gradation and feeding rate are of vital importance to form meandering river.There is not unified view on fine particles gradation and feeding rate at home and abroad.Based on existing experiment the median size of fine particles is 0.03-0.45mm.Fine particles is generally talcum powder,clay,kaoline,kieselguhr and silicon dioxide.The controlling effect of vegetation during development of meandering river is always disputed by researchers.Vegetation can change the flow direction and river shape.The function of vegetation is summarized as follows:①add bank stability make river way narrow and deep;②slow near-bank velocity and prevent bank failure;③root system can enhance extension and shearing strengthen of sand.Some unsolved problems in flume experiments are addressed as well.There is not uniform standard about experimental period.High sinuosity meanders and neck cutoff have not been generated.Furthermore,it is worthy that new experimental conditions shall be designed to uncover the positive process from meandering to braided channel and the reverse process from braided to meandering channel so as to identify sustaining conditions of meandering channel formation and the flow-sediment-vegetation threshold of mutual transition of meandering-braided pattern.
Key words:Meandering River;Flume Experiment;Experimental Condition;Plant Grass;Cutoff;Review