Flow characteristics of narrow vegetated open channels
Date of Award
Doctor of Philosophy
School of Engineering
Vegetation offers higher resistance to the flow as compared to the bed of the channel causing the rise in water level. This idea has been proposed by numerous studies to be used in the channels with low flows to raise the water level enabling these channels useful for navigation and transportation. Raising the water levels artificially comes with challenges like the change in the aspect ratio (width to depth ratio, b/H). Generally, open channels experience a dip near the surface when the water level is increased, and the b/H becomes lower than a typical value of 5. The effect of dip near the surface in the partially vegetated channels remains unknown. Similarly, the literature focuses on the vegetation on one side of the wide channels, but the vegetation on both sides of narrow channels needs further exploration.
A series of experiments have been conducted to understand the above-mentioned research gaps by installing wood dowels in the flume. It has been found that unlike the channels without vegetation, the dip near the surface begins even for b/H>5 which restricts the expansion of the shear layer and the KH vortices in the canopy free region. The vortex structures captured using PIV further revealed that because of limited expansion in the canopy free region, the KH vortices are stretched. The stretching of the vortices in the shear region causes significantly higher shear stress at the canopy interface. The values of the shear stress increase as the aspect ratio are decreased. Moreover, most of the mass flux is diverted away from the canopy region towards the other side wall which offers relatively lesser drag.
The study of the canopies of similar densities but different stem sizes reveals that the velocities inside the canopy region are independent of the stem size and mostly depend on the relative spacing (average spacing between the stems to stem diameter). The distribution of the velocity in the canopy free region beyond the influence of the shear layer remains unaffected. However, the channels with canopies of bigger stem sizes tend to have lower velocities in the shear region as compared to the channels with smaller stems. The smaller velocities in the shear region for bigger stems result in relatively lower vortex stretching and smaller friction velocities.
The results of the experiments conducted in the channels with vegetation on both sides show that the shear layers are created at the interfaces of both canopies. The vortices generated inside these shear layers mutually interact and adjust to form pairs of vortices. The strength of these vortices is increased due to mutual interaction which is reflected in the form of higher friction velocities at both interfaces. When these friction velocities are compared with the friction velocities in the channels with canopies of similar characteristics and same flow depths, the friction velocities are significantly higher when canopies are on both sides.
Access to this thesis is embargoed until 13th April 2025
Arshad, A. (2023). Flow characteristics of narrow vegetated open channels. https://ro.ecu.edu.au/theses/2649