弯道床面泥沙横向分选机理的实验研究

The sorting of nonuniform bed particles in meandering rivers has a direct impact on channel evolution and sediment transport processes, and is therefore a first priority in designing schemes for river regulation. This study proposes to explore by means of laboratory experiments and numerical simulations the sorting mechanisms of primary and secondary currents on coarse and fine sediment particles when the transverse slope gradually forms in meander bends. The sorting pattern of nonuniform sediment in the formation of meander bed topography is studied first, and the impacts of the curvature, flow intensity, bed sediment composition and bed topography on the sorting of nonuniform sediment are analyzed. Then the sorting mechanism of primary and secondary currents on coarse and fine particles will be studied. The characteristics of primary and secondary currents as well as their influences on shear stress distribution in meander bends with eroding concave banks and sedimentating convex banks will be investigated, and the relationship between sorting degree and the intensities of primary and secondary currents and that between nonuniformness and transverse slope will be established. The equilibrium bed configurations formed by bimodal and multi-sized nonuniform sediment with different feeding manners will be analyzed, and the impacts of these influencing factors on the sorting of nonuniform sediment are discussed. This study will not only broaden our understanding of nonuniform sediment motion, but also promote the theoretical study on the motion of nonuniform sediment, and also of great importance to meandering control, channel regulation, wetland exploitation and aquatic regulation of river channel, exploitation of wetlands and aquatic environment protection in natural rivers.

弯曲型河道中床面非均匀沙的分选直接影响河道演变和输沙过程，是考虑河道整治方针时需首先解决的问题。本项目拟采用室内实验和数值模拟相结合的方法，研究弯道横比降逐渐发展过程中主次流对粗细泥沙分选的作用机理。首先研究非均匀沙在弯道地形塑造过程中的分选规律，分析曲率、水流强度、床沙组成和床面地形对非均匀沙分选的影响；然后探讨主次流对粗细沙分选的影响机理，研究弯道凹冲凸淤发展过程中的主次流特性及其对弯道床面切应力分布的影响，建立分选度与主次流强度、非均匀度及横比降的关系式；最后分析双峰型非均匀沙、多粒径非均匀沙及泥沙补给方式塑造的不同平衡床面形态，探讨各影响因子对非均匀沙分选的影响程度。本研究成果可拓展对非均匀沙分选现象的认识，对于应对天然弯道的演变、河道整治、防范灾害、开发湿地、水生保护等也具有重要意义。

弯曲型河道中床面非均匀沙的分选直接影响河道演变和输沙过程，是考虑河道整治方针时需首先解决的问题。本项目拟采用室内实验和数值模拟相结合的方法，研究弯道横比降逐渐发展过程中主次流对粗细泥沙分选的作用机理。. 本项目通过不同曲率（水槽的曲率中心线遵循正弦派生曲线，弯曲角分别为30°和110°）、不同非均匀沙的弯道水槽实验及相应的数值模拟结果，研究了弯道各动力因子与泥沙平均挑选系数之间的关系，包括时均流速、紊动动能、床面切应力、弯道横比降与泥沙平均分选系数等之间的相关关系。研究结果表明：（1）弯道床面凹冲凸淤地形的形成可划分为三个阶段。在前两个阶段中，主流线与河床变化的区域吻合。在地形变化的第三个阶段，凸岸泥沙被主流输送至边滩，而在边滩上淤积的细颗粒泥沙被主流和横向环流的综合作用输送至凹岸。此外，凹岸泥沙的同岸输移更占优，同时，凹岸泥沙的异岸输移受到泥沙粒径粗细的影响。（2）来自凹岸的移动泥沙颗粒主要来自于顶点以下区域，而来自凸起岸的移动泥沙颗粒主要来自于弯道顶点的上游区域。此外，还存在一个沉积物交换很强的区域，即凸岸顶点附近处的边滩。（3）弯道地形形成后，主要影响的是近底流速的再分布。对于近底紊流猝发事件(v’,w’)，Q1和Q3占优；近底紊流猝发事件(u’,w’)和(u’,v’)中，无明显占优事件。（4）相较初始铺沙的泥沙分选系数，当弯道凹冲凸淤地形形成后，在洲滩部分的平均挑选系数偏大，而在深槽部位的平均挑选系数要偏小，同时，在大曲率弯道内泥沙的平均挑选系数变化更大。而在平均挑选系数较大的部位，其弯道环流强度也较大。（5）随弯道横比降的逐步加大，弯道主流发生偏移，断面各位置的泥沙分选度与弯道横比降呈负相关关系。. 本研究成果可拓展对非均匀沙分选现象的认识，对于应对天然弯道的演变、河道整治、防范灾害、开发湿地、水生保护等也具有重要意义。