滨水植被对河流弯道水流结构及泥沙输移影响机理研究

As an important part of the ecosystem of the river, riparian vegetation has an important role to maintain biodiversity of the ecosystem and preserving the health of the river. Moreover, existing large number of studies have shown that riparian vegetation also plays a key role in the fluvial process. Therefore, intensively study on the impact of vegetation on flow and sediment transport has important theoretical significance and practical value; it would not only improve and enrich the fluvial process theory, but also provide technical support for river restoration and management. Existing research on effect of vegetation on flow and sediment transport are mostly based on straight flow, whereas the ubiquitous nature of the river meander is less involved. In response to these research status, the proposed project intends to carry out a series experiments in a curve flume, combined with boundary layer theory in fluid mechanics, mechanics of turbulence, river dynamics, sediment transport and fluvial process theory, to get a deep insight on the curved vegetation flow, quantitative identification of vegetation effects on the initiation and decay of the secondary flow, revealing the velocity profile between and outside of the vegetation area, the turbulent dissipation, Reynolds stress, turbulent coherent structures and bed shear stress distribution. Additionally, we will also explore the effect of vegetation on the direction of the sediment transport and the erosion and deposition morphology of the cross section, and an initial theoretical system about the vegetation and sediment transport in a curved flow would establish through this study.

作为河流生态系统的重要组成部分，滨水植被对于保持生物多样性、维持河流健康具有重要作用；现有研究表明，滨水植被在蜿蜒型河道演变中起着关键作用。深入研究植被对水流运动和泥沙输移的影响，不仅对于完善和丰富河流动力学、河床演变学有重要的理论意义，而且可为河流生态修复及综合管理提供关键技术支撑。现有关于植被对水沙输移影响研究多基于顺直水流，对于自然界普遍存在的弯道则涉及较少。针对上述研究现状，本项目拟通过系列弯道水槽试验，并结合流体力学边界层理论、紊流力学、河流动力学及河床演变学理论，对植被作用下的弯道水流及泥沙输沙进行深入研究，定量识别植被作用下弯道环流的产生及消亡过程，揭示弯曲水流条件下植被区内、植被区外的流速分布、紊动耗散、雷诺应力、紊流相干结构及床面切应力分布规律；探求植被对弯道的纵横向输沙、横断面的冲淤形态影响的量化途径，揭示植被对弯道水沙输移影响机理。

弯道河床演变问题是泥沙运动力学的重要研究内容，已有的弯道水沙研究较少考虑水中植被的因素。本项目依托长90m，宽1m，高1m的大型弯道水槽，开展了具有植被覆盖条件下的弯道水沙运动特性试验，主要取得以下成果：1）在水沙试验测控方面：a)搭建了水槽水沙运动监测系统，可对弯道水流流量、沿程水位和横比降、水流三维紊动流速、水流表面流场、局部和尾部推移质输沙率、整体床面形态、局部床面高程等参数进行测量，为开展弯道水沙运动研究奠定了基础；b）提出了基于光流法PyrLK“角点-质心”表面流场粒子匹配算法，具有误匹配少、计算准确率高的优点；基于此算法开展了弯道表面流场计算，弥补了以往弯道水流结构分析缺乏上部流场实测数据的缺陷。 2）开展了不同条件下植被弯道水沙输移试验，结果表明：a)植被覆盖显著降低了断面流速，其中弯顶90°断面表面纵向流速沿横断面的分布呈明显的“凹大凸小”形式，凹岸侧表面纵向流速大于中心区及凸岸侧；此外，植被覆盖使弯顶及弯顶前凹岸侧环流发展更加剧烈，且环流强度具有沿程递增趋势；植被覆盖条件下水流纵向流速和紊动强度均下降60%以上；b）植被改变了弯道推移质输移特性，其中植被覆盖区推移质输沙率降低，覆盖区外输沙强度增强，并对植被覆盖下游区床面形态造成较大影响；植被覆盖使床面形态由舌状沙波面改变为顺直沙波、弯曲型沙波，淤积形态由原凸岸测沿程淤积改变为先淤积再冲刷再微淤的状态。总体来看，本项目依托大型弯道水槽，搭建研发了水沙参数综合测量系统，开展了不同条件下水槽试验，进一步加深了对于植被作用下弯道水沙输移规律的认识，对于未来弯道水沙运动和河床变形预测具有重要理论意义和实践价值。