相邻植被群对水流结构及河床稳定特性的影响机理研究

•.Aquatic vegetation provides several positive benefits and plays an important role in ecological restoration and continuous development of rivers. However vegetation increases river resistance and water stage, resulting in flood and navigation problems, which constrain the progress of ecological restoration. Therefor the development of vegetated flow research is one of the most significant advances in fluid mechanics. Side-by-side vegetation patches are common in aquatic systems and increase the complexity of flow structure, hence leading to the corresponding complex behaviors of sediment transport and channel changes. Applying the related theories of fluid mechanics, mechanics of sediment transport and fluvial processes, the applicant will investigate hydraulic characteristics, river resistance and hydraulic geometry in channels with neighboring vegetation patches. Through the related flume experiment and theoretical analysis, the following topics will be the main concerns of this study: (1) investigating the three-dimensional velocity field, turbulent structure and vorticity field around neighboring vegetation patches; (2) establishing a calculating method for vegetation drag coefficient, and proposing river resistance coefficient in channels with neighboring vegetation patches , by analyze the stress features based on the condition of non-uniform flow produced by the neighboring vegetation patches; (3) proposing hydraulic geometry formula for the channels with neighboring vegetation patches, based on the minimum active hypothesis, combined with bed-load transport rate and resistance formula with vegetation factors. The studies of this proposal are expected to provide a better understanding of water and sediment transportation in channels with neighboring vegetation patches, and to predict the river conveyance capacity and the trend of river evolution. The research results will provide theoretical and technical supports for stable channels design and river ecological restoration.

水生植被对河流生态修复有积极的作用，但其阻水特性影响河道的防洪，制约生态修复技术的发展，因此植被水流研究是一个紧要的课题。天然河道常存在相邻的植被群，使得水流结构、泥沙输移和河床演变规律更为复杂。本申请课题将以流体力学、泥沙运动力学、河床演变学理论为基础，研究相邻植被群作用下河道的水流结构、阻力特性、稳定河床特性及其内在联系。通过水槽试验和理论分析，在探明植物条件下河道水流结构的基础上，分析非均匀流对植被群水体的力学影响机制，建立植被群河道的阻力表征模式，模拟植被群的动态生长过程，分析植被群形态对泥沙输移和稳定床面形态的影响，建立含植被因子的推移质输沙率公式，提出河相关系的计算方法，从而揭示植被对水流、泥沙运动的影响机制和三者之间的相关关系。通过本项目的研究，可预测河道过流能力和河流形态的演变趋势，为科学优化的植被布置方案提供技术支撑，促进河流生态修复理论和实践的发展，具有重要的科学价值。

植被群广泛的存在于河道、湿地等生态系统中，但并非孤立存在，相邻植被群会相互影响，水流结构及水沙相互作用机理更为复杂。本项目采用实地调查、试验研究和数值模型相结合的方法，研究不同植被群密度、间距、淹没度下河道的水流流速分布、紊动结构、床面冲淤特性。研究的主要内容有：（1）调查天然河道植被形态及对水流结构、泥沙冲淤和水质的影响；（2）并列相邻植被群对水流结构的影响；（3）交错相邻植被群对水流结构影响；（4）相邻植被群对河床形态影响的数值模拟及试验研究。获得的主要研究成果有：. （1）植被群尾端形状对尾流区和泥沙沉积有较大影响，圆形植被处于植被群生长的初始阶段，尖长型是较为稳定的形态；水生浮游植物影响水体溶解有机物，使其在夏季浓度较高。. （2）揭示并列相邻植被群作用下的水流流速分布规律，将水流的发展分为上游调整区、植被内部调整区、稳定尾流区、尾流融合区及流速恢复区，建立了各区特征流速和特征长度的计算公式；. （3）并列相邻植被群单植中心线紊动能沿程出现两个峰值，两植中心线紊动能只出现一个波峰，紊动第一峰值由单株植被尺度(d)的尾流产生，紊动第二峰值是由两个尾流区的融合，形成两个植被群直径尺度 (2D)的卡门涡街而产生，并描绘相邻植被群作用下水流结构的物理图形。. （4）交错排列植被群中，上游植被群的尾流区被下游植被群所打断，且受植被群之间高速水流的冲击变形，水流流速和紊动特性产生变化；. （5）并列相邻植被群两个尾流区后低流速和低紊动促进泥沙的沉积，两植中心线上流速增大产生泥沙冲刷，随着两个尾流的融合，两植中心流速达到最小值，产生二次沉积区；交错植被群纵向和横向间距的改变使得河床冲淤形态发生改变。. （6）通过数值模拟，对相邻植被群的沉积模式进行了分析，当相邻植被群间隔较近时，植被群之间存在正反馈，将更有利于新的植被生长，当间距相对较远时，相邻植被群之间为负反馈。.