生态修复河道内的滞水区水流结构及物质输运规律研究

Current stream restoration practices are seeking to restore fluvial systems by using structures that are more representative of the natural channel geomorphology to increase geomorphic complexity, Stream restoration projects typically design and install in-stream structures such as groynes to create embayment regions of slow-moving recirculating flow, termed groyne fields, and restore riparian vegetation to promote biological diversity. The aquatic plant, typically exit in the groyne fields, may have great influence on flow structures and mass exchange between groyne fields and main flow. This project is focused on the hydraulic problems in groyne fields filled with aquatic plant. Theoretical analysis, physical model test, and numerical simulation are applied to study the time-averaged flow and turbulent structures in mixing layer. The main work are as followings, to study the relation between the coherent structure and mass exchange in mixing layer, to give a criteria on gyre recirculation pattern, to build prediction formula on the mass exchange coefficient and mean residence time, also to perfect the theoretical solution on longitudinal dispersion coefficient in rivers with lateral cavities. The mechanism of action of plant on flow structure and mass transport in groyne fields with different hydraulic condition will be clarified. The law of energy and mess exchange between groyne field and main flow will be found. Understanding cavity shape and plant effects will provide a guideline for designing lateral embayment in stream restoration projects and focusing the water quality in rivers influenced by lateral cavities, with great theoretical significance and wide application prospect.

在河流生态修复工程中，为了模拟自然河道的水流多样性，常常采用一些工程措施改变渠化河道的单一流态，如建立丁坝形成水流缓慢的滞水区，修复滨水带的水生和陆生植被群落，以提高河道生物栖息地质量。河流滞水区是水生植物生长的重要区域，水生植物的存在改变了滞水区的水流特征以及滞水区和主槽间的物质交换规律。本项目针对河流生态修复中的滞水区生态水力学问题，采用理论分析、模型实验和数值模拟相结合的方法，研究滞水区的时均流场特征和混合层紊动结构，分析涡旋结构与物质交换的关系，提出植被化滞水区环流结构的判断标准，建立交换系数和水体滞留时间的预测公式，完善横向滞水区影响下的河流纵向离散系数的理论解。在此基础上，阐明不同水力条件下水生植物对滞水区水流运动和物质迁移的作用机理，揭示滞水区与主槽间的能量和物质交换规律。本项目的研究为河流水环境预报和生态修复工程设计提供理论支撑和技术依据，具有较高的理论意义和广泛的应用前景

采用理论分析、概化模型试验与数值模拟相结合的方法研究了挺水植被影响下滞水区的水流结构及物质迁移规律。研究结果表明植被群落能够调整滞水区环流模式、削弱环流速度、降低紊动强度，影响程度随植被密度的增加而加强。交界面前缘脱落的小尺度涡沿混合层流动时会与邻近涡体发生组对、合并现象，形成具有统计特征的大尺度拟序结构（汇聚着高涡量和强紊动能）,其中一部分拟序涡会随着环流进入滞水区，另一部分拟序涡则会随着主流流向下游；依此提出了拟序涡沿流向发展的概念化模型，该模型可以很好地揭示混合层涡动力特性的沿程变化规律。由于丁坝群的连续挑流作用，主渠泥沙主要以细颗粒悬移质形式进入滞水区，包含两种途径，一方面是跟随环流结构进入滞水区，另一方面是跟随交界面紊动涡体扩散到滞水区。由于泥沙运移受制于水动力特性，无植被和有植被工况水流结构的不同会导致其淤沙结果存在差异，即同等历时内，无植被滞水区的淤沙颗粒粗，落淤质量大，淤积物主要集中在主环流区域；而在植被化工况中，淤沙颗粒细，落淤质量少，淤积物几乎均匀地散落在整个滞水区。基于象限分析法研究了交界面瞬时雷诺应力，发现喷射和扫掠现象能够生成高水平雷诺应力，此外喷射和扫掠现象具有随时间类周期性出现、沿空间交替性分布的特点，依此认为喷射和扫掠现象是造成剪切层拟序结构波状摆动（Kelvin-Helmholtz不稳定性）的根本原因。探究了植被影响下滞水区的滞留能力，发现植被群落能够影响滞水区与主渠之间的物质交换过程，引入植被阻滞效应因子并拟合了植被化滞水区的滞留时间公式，对判定系数与平均绝对误差的分析表明该公式具有较好的预测能力。本次研究丰富了对于滞水区水流结构及物质迁移特性与挺水植被之间交互作用的认识，可以为河道生态修复工程提供一定的设计依据，具有较高的理论价值和较广泛的应用前景。