Flow-vegetation interactions at the plant-scale: the importance of volumetric canopy morphology on flow field dynamics
My NERC funded PhD research involved investigating the effects of vegetation on river flow. Vegetation is abundant in rivers and has a significant influence on their hydraulic, geomorphological and ecological functioning. However, the morphological complexity of natural plants (i.e. how plants looks to flow) has been neglected in many previous studies. Because of this, I applied remote sensing techniques, designed laboratory-based flume experiments and applied high resolution numerical modelling to improve the process-understanding of flow-vegetation interactions at the plant-scale. The research focused on vegetation that intermittently interacts with river flow during flood events (i.e. floodplain and riparian vegetation), and involved trying to answer some of the following questions:
i) How can plants be represented in a high resolution numerical model used to predict river flow?
An approach was developed to capture the three-dimensional structure of riparian plants, characterise their geometry and incorporate a realistic representation into a Computational Fluid Dynamics (CFD) model used to predict river flow.
High resolution (millimeter scale), three-dimensional point clouds of individual plants were acquired using Terrestrial Laser Scanning (TLS). To reduce the number of postprocessed points but retain plant morphology, a voxelization procedure was applied (see figure above).
The voxelization procedure provided a binary occupied/unoccupied grid of voxels that were incorporated directly into the CFD model, using a mass flux scaling algorithm (Lane et al. 2002; 2004; Hardy et al. 2005).
Vegetation was conceptualised as a porous blockage to flow and treated as numerical porosity in the CFD model. The detailed workflow and preliminary results are shown in Boothroyd et al. (2016).
ii) What are the feedbacks between flow and plant motion dynamics?
Plants reconfigure during hydrodynamic loading to minimise drag (Vogel, 1994). To better understand plant motion dynamics and start to represent these in the CFD model, we carried out a series of laboratory-flume experiments.
The experiments involved the application of image analysis techniques to quantify the time-dynamic and time-averaged plant motion under a range of flow conditions. Furthermore, we collected spatially distributed velocity measurements in the wake of the plant to (i) improve the process-understanding and (ii) validate the CFD model.
We found that locally, some parts of the plant move more than others (time-dynamic motions). Furthermore, time-averaged motion showed that plants were vertically compressed in the flow, reducing the volumetric canopy morphology/plant porosity and lowering the position of the wake in the flow depth.
Experiment details and CFD model validation are shown in Boothroyd et al. (2017).
iii) How important is plant morphology on the three-dimensional mean and turbulent flow?
Findings showed that differences in foliation, orientation, porosity and posture of plants strongly modified the three-dimensional mean and turbulent flow. Each plant introduces a unique disturbance pattern to the three-dimensional velocity field, resulting in spatially heterogeneous and irregularly shaped velocity profiles. This is best demonstrated when comparing the flow structures around three plants of the same species (Boothroyd et al. 2019).
Flow features consistently identified included a component of sub-canopy flow in the near-bed region and the development of vegetated shear layers at the horizontal and vertical interfaces of the plant blockages. It was suggested that shear layer turbulence was dominated by Kelvin–Helmholtz and Görtler-type vortices generated through shear instability. Furthermore, the presence of horseshoe vortices that wrap around the plant
blockages resembled flow in a junction vortex system (see figure above).
iv) What are the implications for drag and vegetative resistance?
The newly quantified, physically-determined drag coefficients deviated substantially from the commonly assigned value of unity, or the typical drag coefficient value range from 1.0-1.2 that has been used to represent vegetation in hydraulic modelling applications.
Results showed that drag coefficients are generally greater than the previously established values, and should be considered dynamic. A single drag coefficient value is unlikely to reflect the full range of the plant motion in response to hydrodynamic loading, seasonal changes in foliation, or the variation in plant morphology for even a single plant species.
Back-calculated Manning’s n values remained considerably higher than traditional bulk vegetative resistance terms for comparable vegetation types, selected from classical look-up tables (e.g. Chow, 1959). Given the importance of vegetation in river corridor management, the approach developed in the PhD research demonstrates the necessity to account for how the plant looks to flow when calculating vegetative resistance.
Boothroyd, RJ. (2017). Flow-vegetation interactions at the plant-scale: the importance of volumetric canopy morphology on flow field dynamics. Doctoral thesis, Durham University.