淹没植被群复杂尾流结构及其对河床演变影响的试验研究

Vegetation in natural rivers, wetlands and the ocean often exists in a vegetation patch. Specifically, aquatic vegetation often exists in a submerged vegetation patch. Behind a submerged vegetation patch, three-dimensional flows linked to bed morphology are more complex, compared to the flow patterns behind an emergent patch. Applying the theories of fluid mechanics, mechanics of sediment transport and fluvial processes, the applicants will investigate flow characteristics, momentum exchange, flow resistance, sediment deposition and transport behind a single vegetation patch and two neighboring patches. Also applicants will focus on the interaction of forgoing contents. Through flume experiment and theoretical analysis, the applicants will propose the quantitative relation between the vortex scaling in the wake and the patch geometry, a method capable of estimating the wake length, the critical flow condition for sediment deposition around a patch, and clarify the key factor of wake vortexes relative to bed morphology, the interaction of two submerged vegetation patches. Those breakthroughs will perfect and update the theory, concept and method relative to the wake structure of vegetation patch and its impact on fluvial processes. The research results of this project will enrich the research contents of river dynamics and will provide the theoretical and technical supports for river restoration, river training, and pollutant deposition promoted by vegetation patch, etc.

植被在天然河道、湿地和海洋中的一些区域常以植被群形式存在，水生植物又常以淹没植被群出现。淹没植被群尾端会出现较非淹没植被群更复杂的三维水流结构，影响植被群周围的河床形态。本研究课题以流体力学，泥沙运动力学和河床演变学为基础，研究单一和相邻淹没植被群尾流区的水流结构、动量交换机理、阻力特性、泥沙沉积和输移特性及它们之间的相互影响机制。通过水槽试验和理论分析，确定淹没植被群尾流漩涡尺度与植被群几何形态的定量关系，阐明尾流影响河床演变的关键要素，提出快速测定植被群尾流区长度的方法，揭示相邻淹没植被群间的相互作用关系，提出植被群周围发生泥沙淤积的临界水流条件，实现植被群尾流结构及其对河床演变影响在理论、概念、机理和方法上的完善和创新。课题研究成果将丰富河流动力学的研究内容，为河流生态恢复、河道整治、利用植被群促进污染物沉积等提供理论支持。

植被群通常是树木群、灌木群和野草群的初始生长形态。生长在河道过流区域的植被群落会影响其周围区域的水流特性和河床形态。因此，本项目研究淹没植被群落的复杂三维尾流结构及其周围的泥沙沉积特性，主要搞清（1）植被群落尾流结构，和（2）植被群落周围泥沙输移及河床演变。研究发现，淹没植被群落尾流区长度由水平方向的漩涡（卡门涡街）和垂直方向的漩涡共同组成，尾流区长度由更靠近植被群尾端的漩涡发生距离决定。淹没和非淹没高密度植被群 (非淹没：CDaD > 4；淹没植被：ah > 0.1)，泥沙沉积区域LDep与植被高度h, 植被直径D 的关系式： 当h < D时(宽矮型)，LDep /h = 2.3 ± 0.5；当h > D时(瘦高型)，LDep /D = 2.1 ± 0.3。植被群尾并不总是产生卡门涡街，采用示踪试验和频谱分析相结合的方法，对于高密度的淹没植被群(ah > 0.1)，当h/D<0.7时，植被群尾端不出现卡门涡街；当h/D>0.7时，其尾端出现卡门涡街。.在此基础上，我们进一步开展了淹没柔性植被群的研究，搞清了淹没柔性枝叶对水流各项特性及泥沙沉积的影响，发现柔性植被群较刚性植被群有明显的区别，因为柔性植被群枝叶的摆动会降低水流紊动的增加，促进悬浮泥沙沉积在河床上。具体工作如下：天然河道、湿地、海洋中生长的不同柔性植被群有相似的高宽比：h/D = 0.2 ~ 0.6 (h < D, 矮宽型)，小于卡门涡街产生临界值 h/D = 0.7。分析发现，柔性植被尾部泥沙沉积区：LDep = Lw + Lblade。叶片越长的淹没柔性植被群后侧能观察到越长的泥沙淤积区。而植被尾端卡门涡街的出现会限制悬浮泥沙沉积的长度。.所有实验所取得的关键数据在发表论文的致谢中标明了来源。这些研究成果可以用来开展野外实地研究、建设河流生态防护、实施生物防冲护岸、治理冲淤河道和改进都市河流景观