Vegetation in the natural aquatic environment plays significant roles in trapping and removing pollutants, metals and excess nitrogen (Wilcock et al., 1999). With the increasing population, aquatic vegetation is commonly used in manmade systems (e.g. stormwater ponds, wetlands) to mitigate water pollution and improve the water quality. Therefore, understanding mixing processes within vegetated flow is essential for utilizing and evaluating natural and engineered water systems. Solute mixing within vegetation is currently our major research focus, though many studies to-date have used artificial uniform vegetation. This proposal aims to extend knowledge by investigating patchy vegetation, looking at discrete patches within a flow and different stem or plant densities within a patch. These will be quantified in a new laboratory facility at the University of Sheffield which will provide unique information on integrating multi-scale processes, quantifying mixing and evaluating appropriate computational fluid dynamics (CFD) tools to enable water engineers and environmental managers to predict water quality changes.
Research Aims
1. To conduct novel experiments to quantify mixing process in patchy vegetated flow.
2. To study the interaction mechanics between plants, including physically based properties of the foliage and stem and patch characteristics, flow properties and solute mixing.
Background
The existence of vegetation increases resistance in the aquatic environment, which consequently, influences the velocity field as well as turbulence structures. A number of researchers have experimentally investigated mixing processes in vegetated flow (Nepf, 1999; Shucksmith et al., 2010; Sonnenwald et al., 2017). These processes are dependent on vegetation properties such as stem diameter, density, flexibility, submergence, spatial density and distribution. Much of the previous work has employed vertical cylinders to represent vegetation stems, with only a few employing real vegetation. Additionally, the early work on mixing within vegetation has either focussed on uniformly distributed plants or on the exchange processes across an idealised plant/free flow interface. This proposal will investigate the effect of patchiness both within areas of vegetation and of plants within free flows. Preliminary field studies have already commenced (Västilä et al., 2020 – submitted) and to complement the physical studies, CFD models, currently being used to quantify mixing due to vegetation, may also be employed to assist data interpretation.
Methodology
The method for the study involves the integration of novel laboratory and field-scale experiments with limited numerical simulations. The laboratory experiments will be conducted in the newly refurbished and fully instrumented 1 m wide flume at the University of Sheffield. This has full-width laser induced fluorescence (LIF) measurements at four transverse sections, together with a new 10 W 1 m x 0.5 m light sheet illumination system for both particle image velocimetry and LIF measurements. These detailed laboratory studies will be complemented with field-scale measurements at the Korea Institute of Civil Engineering and Building Technology-River Experiment Center (KICT-REC) undertaken in collaboration with colleagues from Aalto University, School of Engineering, Finland and the Korea Institute of Civil Engineering and Building Technology. This facility is approx. 150 m long, 5 m wide with discharges up to 5 m3/s (Västilä et al, 2020 submitted).
The proposed controlled laboratory programme will comprise two phases. Initially rigid cylinders will be employed, to relate to previous studies, across a range of patchiness values. These will be undertaken over a range of flow rates. Following this the cylinders will be replaced, in turn by rigid, and then flexible, “plant-like” forms. This will increase the reality and complexity of the investigations. These will provide a unique database of detailed spatial velocity and mixing that can be used to develop and validate improved modelling approaches. The field-scale studies will, by necessity, be less detailed, but will provide valuable information on the up-scaling and application of the results that are required by environmental managers. Such information is required to support the adoption and implementation of the increasingly employed nature based solutions (Rowiński et al., 2018).
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