From substrate to surface: An integrated study on the Interfacial transfer and sediment suspension based on a turbulence perspective in vegetated flows

Dissolved Oxygen (DO) and Suspended Sediment Concentration (SSC) are two critical indicators for water quality and ecosystems in natural freshwater environments, such as rivers, lakes, and wetlands. Specifically, DO fluxes across air-water and sediment-water interfaces (AWI and SWI) are two major processes that govern the amount of DO level for living organisms in the aquatic ecosystems. However, the presence of aquatic vegetation exerts an additional drag on the flow and generates different scales of turbulence, which significantly alters the flow structure from substrate to surface, affecting interfacial gas transfer processes and sediment suspension. To investigate such mechanisms affected by vegetation, we conducted a series of laboratory experiments with rigid cylinder arrays to mimic vegetation in two different scales of recirculating race-track flumes with a high mobility sediment bed. 2D Planar Particle Image Velocimetry (PIV) with horizontal and vertical slices was used to characterize the mean flow and turbulence quantities under different submergence ratios, vegetation densities, and canopy layouts to explore the influences of vegetation-generated turbulence and the interactions with the bed and flows. Interfacial gas transfer fluxes across AWI and SWI were estimated by monitoring the bulk and near-bed DO concentrations during the re-aeration process with optical DO sensors. The dissertation aims to answer the following main questions: • How does aquatic vegetation alter the flow hydrodynamics, which affects the AWI gas transfer mechanism? • How do flow-bed-vegetation interactions affect the process, driving sediment motions in vegetated flows? • How do flow-bed-vegetation interactions affect the gas transfer process across SWI? • How different layouts and stem sizes of the vegetation would affect the flow structure and the corresponding gas transfer across AWI and SWI? The experimental data provides new insights to identify the dominant scales of turbulence generated by the vegetation according to different submergence ratios and array densities. A modified surface renewal model using the representative TKE production as an indicator of gas transfer efficiency was developed to predict the transfer rates in vegetated streams more accurately. We further proposed a two-layer, turbulence-based model to predict SSC profiles in emergent vegetated flows, accounting for turbulence generated from vegetation, bed, and coherent structures caused by the stem-bed-flow interactions. Based on a similar concept using total near-bed turbulent kinetic energy as an indicator, a turbulence-based Reynolds number dependence model was developed to provide a universal prediction of the interfacial flux across SWI in flows with vegetation elements. Finally, we proposed a newly defined randomness indicator according to a spectrum approach to quantify the randomness level of the canopy. The previously developed models for predicting interfacial gas transfer across AWI and SWI were updated accordingly by implementing the newly proposed spectrum method to improve the model predictions for more realistic natural environments with different vegetation stem sizes and canopy layouts. This study will provide critical information and valuable models for future studies on water quality management and ecosystem restoration in natural riverine environments.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Civil Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Tseng, Chien-Yung

Contributors dc:contributor

• Tinoco, Rafael O.
• Parker, Gary
• Garcia, Marcelo H.
• Chamorro, Leonardo P.
• Maza, Maria E.

• vegetated flows
• gas transfer
• surface renewal
• suspended sediment
• turbulence
• hyporheic exchange

Statement dc:rights

• Copyright 2022 Chien-Yung Tseng

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115638

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115638