Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Civil and Environmental Engineering | en_US |
| dc.contributor.advisor | Duan, Huan-feng (CEE) | en_US |
| dc.creator | Zhang, Yuanheng | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/14438 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Experimental study on flow structure and bedform development in open channel with vegetation patch | en_US |
| dcterms.abstract | Aquatic vegetation is a critical component in coastal and riverine ecosystems, forming important wetland and littoral habitat. The presence of vegetation introduces additional hydraulic resistance, which reduces flow velocity. This tends to enhance sediment deposition and sediment retention, which can maintain bed stability and promote the growth of ridges and islands. Conversely, vegetation also generates turbulence, facilitating sediment resuspension, leading to erosion and reduction of bed elevation. Changes to the bedform can in turn affect the flow dynamics and the continued growth of aquatic plants. These dynamic feedback mechanisms complicate predictions of how vegetation distribution impacts sediment retention and river morphology. Unfortunately, during the past decades, vegetation has been removed or lost from rivers, deltas and lakes, due to natural causes and agricultural and industrial land-use. To restore and manage aquatic vegetation ecosystems, a comprehensive understanding of the interactions among flow, vegetation, and sediment transport is therefore essential. | en_US |
| dcterms.abstract | This thesis investigates the complex interplay between aquatic vegetation, flow dynamics, and sediment transport through a series of laboratory experiments and analyses. The main contributions and key findings are as follows: | en_US |
| dcterms.abstract | 1. Patch elongation and wake structure: For an in-stream emergent vegetation patch, patch elongation influences the wake structure by adjusting the bleed flow velocity deficit as well as triggering the Kelvin-Helmholtz vortices along the lateral edges of the patch. Wake von Karman vortex street, unable to be triggered under a low patch density, however, can be generated by patch elongation. Meanwhile, Kelvin-Helmholtz vortex streets are triggered along the two patch lateral edges, re-increasing the in-patch velocity and imposing contributions to the wake vortex streets generation. Specially, the initiation of the patch-edge Kelvin-Helmholtz vortex may accelerate the steady wake velocity, shorten the steady wake length and decay the energy of the wake vortex. | en_US |
| dcterms.abstract | 2. Flow confinement and shear layer interaction: In channels with symmetrical vegetation patches, the lateral extension of the array confines the shear layer, reducing its penetration from a fully developed thickness to half the width of the open region. This confinement occurs when the shear layers grow to the channel centerline before becoming fully developed, influenced by the shear layer initiated at the opposite array. Despite the shear layer being confined to half width of the open region and the time-averaged shear stress being zero at the centerline, the shear-layer vortices interact across the open region, producing significant turbulent events at the centerline with equal contributions from both sides. Furthermore, a wave motion occurs with a phase shift of π radians between the parallel vortex streets. The alternating vortex cores between the parallel vortex streets amplify the pressure gradient, intensifying coherent structures and facilitating momentum exchange across the channel centerline. As the blockage ratio increases, vortex size decreases, convection velocity increases, and vortex frequency rises. Although the turbulent intensity is enhanced, the decreased resident time for turbulent flow events may result in a shorter transport distance. Overall, the shear layer that develops on one interface acts as an additional resistance to shear turbulence on the other interface. | en_US |
| dcterms.abstract | 3. Flow structure and morphological changes: To connect flow structure and morphological changes in and around a rectangular vegetation patch. The emergent patch was constructed in an 8-cm sand bed. Near the leading edge of the patch, enhanced turbulence levels produced sediment erosion. Some of the eroded sediment was carried into the patch, forming an interior deposition dune. The denser patch resulted in a smaller dune due to stronger lateral flow diversion and weaker interior streamwise velocity. After the leading-edge dune, in the fully developed region of the patch, vortices formed in the shear-layers along the patch lateral edges. Elevated turbulence at the patch edge produced local erosion. For the dense patch, material eroded from the edge was transported into the patch to form a flow-parallel ridge, and there was no net sediment loss/gain by the patch. For the sparse patch, material eroded from the edge was transported away from the patch, resulting in a net loss of sediment from the patch. In the wake of both patches, deposition occurred near the wake edges and not at the wake centerline, which was attributed to the weak lateral transport associated with the weakness of the von Karman vortex street. Specifically, the lateral transport length-scale was less than the half width of the patch. The increasing bedform height within the wake progressively weakened and narrowed the von Karman vortex street, illustrating an important feedback from morphological evolution to the flow structure. Despite significant local sediment redistribution, the patch did not induce channel-scale sediment transport. | en_US |
| dcterms.abstract | In summary, this thesis offers fundamental insights into the mechanics of the interactions among aquatic plants, flow dynamics, and sediment transport, focusing on flow structure formation and interaction, and their resulting bedform development. The theoretical insights contribute to developing more reliable approaches to the restoration and management of aquatic ecosystems, ultimately enhancing ecosystem resilience, promoting biodiversity, and ensuring the long-term sustainability of aquatic environments. The main findings, insufficiencies, and recommendations for the whole study are summarized at the end of this thesis. | en_US |
| dcterms.extent | xxii, 211 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2026 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.accessRights | open access | en_US |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/14438