Date of Award

2024

Document Type

Thesis - ECU Access Only

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Engineering

First Supervisor

Mehdi Khiadani

Second Supervisor

Yasir Al-Abdeli

Abstract

This doctoral thesis investigates the hydrodynamics of open channel flows interacting with double-layered vegetation, focusing on two configurations—uniform (A1) and mixed (A2)—that mimic river riparian growth. The study utilizes Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) to quantify critical flow characteristics, including streamwise and transverse velocities, Reynolds shear stress, turbulence, and vortex dynamics under conditions where vegetation occupies single and both banks. The dynamic effects of channel transitions from wide to narrow are studied utilizing aspect ratios (Channel Width/ Depth).

The key findings indicate that mixed vegetation (A2) exerts a more significant and consistent drag across the flow compared to uniform vegetation (A1) in both single and dual-bank vegetated scenarios, as confirmed by the lateral streamwise velocity distribution and the calculations of Manning’s roughness coefficient. Notably, a velocity dip was observed in mixed vegetation (A2) under both single and dual-bank conditions across all aspect ratios (AR), highlighting the substantial impact of bank vegetation on secondary flow characteristics. Moreover, the transverse flow in uniform vegetation (A1) demonstrated enhanced mixing properties compared to those in mixed vegetation (A2) scenarios.

The lateral distribution of Reynolds shear stress revealed that mixed vegetation (A2) adheres more closely to classical shear layer development at vegetated boundaries, forming solid shear layers, unlike the more varied development seen in uniform vegetation (A1) scenarios. Experimental observations showed an increase in the outer shear layer thickness in mixed vegetation (A2) as water levels rose, contrary to the shrinking shear layer observed in uniform vegetation (A1) under similar conditions. Vegetated boundaries in mixed vegetation (A2) scenarios also exhibited higher turbulence, confirming the presence of solid shear layers in each aspect ratio.

In mixed vegetation (A2) scenarios, the vegetated boundaries eject Kelvin-Helmholtz (K-H) vortices in horizontal planes, exhibiting dominant frequencies under both single and dual-bank conditions. An increase in water levels was found to expand the dimensional characteristics of these vortices, thereby enhancing mass and momentum transfer in single-bank vegetated conditions. Remarkably, in conditions where both banks were vegetated by mixed vegetation (A2), the dimensional characteristics of the observed vortices remained relatively constant, suggesting uniform shear layer development in response to rising water levels.

In contrast, uniform vegetation (A1) influenced mass and momentum transport in a vertical direction by the presence of rolling vortices above the submerged vegetation. Moreover, coherent eddies in horizontal planes were noted in some scenarios. However, the absence of dominant frequency in these vortices suggests that the Kelvin-Helmholtz (KH) instabilities were underdeveloped. This differential impact on flow dynamics between the two vegetation types underscores the complex interplay between vegetation structure and hydraulic processes within vegetated open channel flows.

This research addresses a significant gap in understanding vegetated open channel flow dynamics. Findings enhance our understanding of how vegetation influences hydrodynamic behaviours, offering insights into sedimentation, erosion, nutrient dynamics, and momentum transfer in natural waterways, underscoring the growth of layered vegetation's ecological and engineering significance in open channel flow dynamics.

Comments

Author also known as Mahesh Prasad

DOI

10.25958/nxaq-7g64

Access Note

Access to this thesis is embargoed until 19th June 2026

Download

Available for download on Friday, June 19, 2026

Share

 
COinS