The importance of creating dynamically-scaled models of aquatic vegetation in the laboratory
School of Science
Physical modelling of vegetated flows is an essential component of process -based investigations into vegetation ecohydraulics. The vast majority of research into vegetated flows has employed rigid model vegetation, so that the canopy’s geometry (i.e. its height and front al area) is invariant and easy to quantify. Here, we demonstrate that embedding realism (in the form of flexibility and buoyancy) in the model vegetation can have a profound impact on the hydrodynamics. Specifically, we compare rates of vertical mixing in two types of model canopy (with identical heights and frontal areas) subjected to oscillatory flow over a range of realistic wave heights and periods. The two types of canopy were: (1) a rigid canopy consisting of wooden dowels, and (2) an array of flexible, buoyant model plants designed to mimic a meadow of the seagrass Posidonia australis. Dynamic similarity between the model and real seagrass was achieved by matching the two dimensionless ratios of the dominant forces that govern plant motion (rigidity, buoyancy and drag). Results demonstrate a significant difference in flow structure between the two canopies and a significant reduction in the rate of vertical mixing in a flexible canopy, relative to the rigid analogue. Thus, while the use of dynamically -scaled vegetation models adds a layer of modelling complexity, it represents a step towards a more faithful recreation of flow and mixing in these systems.