How does aquatic vegetation modify the kinematic & geometric characteristics of coherent flow structures in open channels?

河道内的植被对河流系统的功能和管理有着深远的影响。它可以显著影响：1)水流阻力，从而影响输水，从而潜在地增加洪水风险；Ii)侵蚀、沉积和输沙速率；iii)河流系统内的生物活性。在较小的空间尺度上，植被也会产生湍流，湍流驱动混合和扩散过程，在淹没植物和冠层周围和上方产生强烈的速度梯度。传统上，河流流动是在时间平均连续的基础上进行分析和建模的，但众所周知，在近河床流速中存在较大的湍流波动。本研究的目的是加强对水生植被对河流系统中相干流结构的产生、演化和耗散的影响的定量认识。这将通过在英国设备最好的环境流体动力学实验室中使用新型自动湍流映射技术进行一系列最先进的实验室水槽实验来实现。该技术通过向水流中注入示踪剂，给出了相干流结构的欧拉和拉格朗日结合视图，使我们能够监测并因此了解这些相干流结构的运动学（大小、尺度、形状、涡旋度和能量）和动力学特性（起源、稳定性、生长、形成新形式和对时间平均流量的贡献），因为它们产生并在水生植被上移动。所提出的方法将使我们能够增加对河流系统中连贯流动结构的现象学理解，并开发一个将水生植被纳入河流流动数值模型（计算流体动力学方案）的框架。这将提供一阶估计从植被的存在增加阻力，这将使一个改进的参数化的粗糙度系数在洪水模型。

Vegetation within river channels has a profound influence on the functioning and therefore management of fluvial systems. It can significantly effect: i) the flow resistance and thus influence water conveyance and therefore potentially increase flood risk; ii) erosion, deposition and sediment transport rates; and iii) the biological activity within the fluvial system. On a smaller spatial scale, vegetation also generates turbulence that drives both mixing and diffusion processes with strong velocity gradients generated around and above submerged macrophytes and canopies. Fluvial flow has traditionally been analysed and modelled on a time-averaged, continuum basis, but it is well known that large turbulent fluctuations exist in near-bed velocity. The aim of this research is to develop an enhanced quantitative understanding of the effect of aquatic vegetation on the generation, evolution and dissipation of Coherent Flow Structures in the fluvial system. This will be achieved by a series of state of the art laboratory flume experiments using novel automated turbulence mapping techniques in the UK's best-equipped environmental fluid dynamics laboratory. The techniques give a combined Eulerian and Lagrangian view of Coherent Flow Structures revealed by injecting tracers into the flow that allows us to monitor and thus understand both the kinematic (size, scaling, shape, vorticity and energy) and dynamic properties (origin, stability, growth, genesis into new forms and contribution to the time average flow) of these Coherent Flow Structures as they are produced and move over and through aquatic vegetation. The proposed methodology will allow us to both increase our phenomenological understanding of Coherent Flow Structures in fluvial systems and develop a framework for including aquatic vegetation into numerical model (a Computational Fluid Dynamics scheme) of river flow. This will provide a first order estimation of the increased drag from the presence of vegetation which will enable an improved parameterisation of the roughness coefficient in flood models.