Vegetated Open Channel Flows as a Sustainable Water Management Strategy

Turbulent flows around bluff bodies have received significant research attention due to their relevance in a variety of engineering applications such as environmental and fluid engineering applications. These include flow over hydraulic structures such as bridge piers and flow over natural and artificial vegetation. Most interestingly, vegetated channels are greatly attracted under sustainable environmental strategies which include use of eco-hydraulics concepts in river management, river restoration, designing of green channels, and environmental flood management (Figure 1). Vegetation plays a crucial role in the fluvial environment as it affects flow resistance and transport of pollutants such as sediments and nutrients. In addition, vegetation is one of the key factors pertinent to ecosystem protection and its services. Understanding the flow dynamics through vegetation is of paramount importance in providing the best sustainable water management practices in wetlands, grass swales and river restoration infrastructures. Understanding the physics of flow through vegetated channels requires coherent information on complex flow behaviours over single stems and groups of stems, knowledge of the latter of which can be extended to beds of vegetation. Consequently, understanding the wake flow characteristics over a single and a group of cylinders to represent the stems in terms of turbulence structure is of paramount importance.

However, understanding of turbulent flow characteristics of vegetated channels is still limited which severely impedes the efficiency and efficacy of sustainable channel restoration and flood management. This challenging task is directly tackled by combining theoretical knowledge together with both laboratory and analytical investigations to examine flows over and through vegetation. Such knowledge is indispensable to provide scientifically based tools that will underpin eco-hydraulics design considerations which in turn facilitate healthy and comfortable living for all living species.

The use of rigid cylinders in open channels to simulate the flow over natural water bodies is widely accepted since it overcomes the inherent difficulties of the use of natural vegetation in understanding the flow dynamics (Figure 2). Consequently, studies on wake characteristics of a uniform flow passing a cylinder have received growing attention, especially with the invention of non-intrusive velocity techniques such as Methodical Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) which facilitates key information such as velocity and turbulence structure of a wake around a cylinder. A vast amount of research has been done in understanding of wake of small aspect ratio cylinders while flow over a slender cylinder is still not fully understood which is also of practical significance. The flow dynamics in the wake of a wall-mounted slender cylinder are more complex due to the influence of bed and free surface and the characteristics of approaching flow. Therefore, this study was conducted to understand the velocity and turbulence flow features in the wake of a slender circular cylinder in the shallow open channel which can improve the current state of knowledge and can help address some key aspects related to strong three-dimensional wake flow characteristics of high aspect ratio cylinders.

The results pertinent to the cylinders show that the proximity of the bed and free surface have a significant effect on the structure of the wake compared to the mid-depth. The bed suppression effect was found to be stronger as the bed was found to restrict the transverse growth of the wake. A wake widening effect near the free surface is a prominent flow feature of wake over slender cylinders. The size, shape and development of the recirculation region behind the single cylinder also clearly indicate the vertical variability of the wake. Demarcation of the entire wake region starting from the first point of onset of disturbed flow upstream to undisturbed flow downstream entirely based on experiments successfully added a novel contribution to the research on wake flow over a single cylinder. The longest wake closure point was found in the near bed and the shortest was found near the free surface. The new model developed pertinent to longitudinal velocity deficit can be used as a velocity deficit scale for the entire far wake region for shallow wakes at mid-depth and near the free surface. According to the findings of this study, turbulence structure along the transverse direction initially shows a double peak behaviour for all three levels, and well away from the cylinder, the downstream double peak becomes broader and less distinct which is attributed to the effect of the separating shear layers (SSL). Moreover, it is found that the vertical velocity component has a significant impact on the wake region which is a crucial factor to consider in bridge pier design.

The innovative aspect of this research work consists of a detailed evaluation of velocity profiles and turbulent structures by analysing wake turbulence and eddy formation under different vegetation configurations. Analytical models of velocity and turbulent structure in vegetated open channel flow were developed and compared with measured data. Both theoretical and experimental studies will help engineers to better understand vegetated open channel flows and assist in the sustainable design of channels and flood plains without any doubt.