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Field Measurements and Numerical Simulations of Sediment Transport in a Tidal River
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0003-0820-617X
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In a coastal area, an alluvial lowland river has a free connection with the open sea and its flow is bidirectional. The river basin is often highly urbanized since it hosts valuable ecosystems and natural resources. Along with the growing population, climate change and human activities (e.g., industrialization, agricultural expansion, and fishery industry) pose a significant threat to the health of the river, leading to an unbalance of the flow and the sedimentation and also a considerable degradation of water quality.

With long-term alluvial processes, the river often displays patterns such as meandering, braided, straight, wandering and anastomosing. In addition to the irregular geometry and bathymetry, a tidal river is typically influenced by the freshwater-saltwater interplay, which makes the hydrodynamic processes and sediment transport patterns extremely complicated. For many tidal river systems, cohesive sediment transported with the tides plays an important role. This is not only because of its interaction with flow but also due to its link to bed deformation.

In this thesis, field measurements and numerical simulations of flow and sedimentation in a system, including a confluence and a meandering reach are presented and discussed. The numerical simulations are performed with the Delft3D package, which allows a coupling between complex river geometry, the bathymetry, the flow and the sediment boundaries in one module. Two morpho-dynamic models, a 2D depth-averaged model for the confluence and a 3D model for the meandering reach, are set up to disclose the fluvial processes in respective area.

The objective of this thesis is, by means of extensive field measurements and numerical simulations, investigate flow features and sediment movement patterns in a tidal river. A comparatively long-term river-bed change, including a scour-hole at the confluence and asymmetric cross-sections at the bends, are also examined. Based on the perturbation theory, an improved sediment carrying capacity formula is also derived being suitable for calculations in a tidal environment. This study explores the variability of sediment transport, and reveals the relationship between the flow velocity and suspended load influenced by both the run-off and the tides. Their interactions also generate a different morphological regime as compared to a non-tidal river reach.

This research may support a decision‐making process when considering the integrated tidal river management and it also provides a reference for other similar situations. The calibrated and validated model may therefore be a powerful tool for managers or researchers.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-73423ISBN: 978-91-7790-350-5 (print)ISBN: 978-91-7790-351-2 (electronic)OAI: oai:DiVA.org:ltu-73423DiVA, id: diva2:1302414
Presentation
2019-05-22, E231, Luleå university of technology, Luleå, 13:00 (English)
Opponent
Supervisors
Available from: 2019-04-04 Created: 2019-04-04 Last updated: 2022-06-01Bibliographically approved
List of papers
1. Understanding morphodynamic changes of a tidal river confluence through field measurements and numerical modeling
Open this publication in new window or tab >>Understanding morphodynamic changes of a tidal river confluence through field measurements and numerical modeling
2018 (English)In: Water, E-ISSN 2073-4441, Vol. 10, no 10, article id 1424Article in journal (Refereed) Published
Abstract [en]

A confluence is a natural component in river and channel networks. This study deals, through field and numerical studies, with alluvial behaviors of a confluence affected by both river run-offand strong tides. Field measurements were conducted along the rivers including the confluence. Field data show that the changes in flow velocity and sediment concentration are not always in phase with each other. The concentration shows a general trend of decrease from the river mouth to the confluence. For a given location, the tides affect both the sediment concentration and transport. A two-dimensional hydrodynamic model of suspended load was set up to illustrate the combined effects of run-offand tidal flows. Modeled cases included the flood and ebb tides in a wet season. Typical features examined included tidal flow fields, bed shear stress, and scour evolution in the confluence. The confluence migration pattern of scour is dependent on the interaction between the river currents and tidal flows. The flood tides are attributable to the suspended load deposition in the confluence, while the ebb tides in combination with run-offs lead to erosion. The flood tides play a dominant role in the morphodynamic changes of the confluence. 

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
tidal river confluence, flow features, morphological changes, field measurements, numerical simulations
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71350 (URN)10.3390/w10101424 (DOI)000451208400139 ()2-s2.0-85054725343 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-10-26 (svasva)

Available from: 2018-10-26 Created: 2018-10-26 Last updated: 2023-08-28Bibliographically approved
2. Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load
Open this publication in new window or tab >>Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load
2019 (English)In: Fluids, E-ISSN 2311-5521, Vol. 4, no 1, article id 15Article in journal (Refereed) Published
Abstract [en]

Meandering is a common feature in natural alluvial streams. This study deals with alluvial behaviors of a meander reach subjected to both fresh-water flow and strong tides from the coast. Field measurements are carried out to obtain flow and sediment data. Approximately 95% of the sediment in the river is suspended load of silt and clay. The results indicate that, due to the tidal currents, the flow velocity and sediment concentration are always out of phase with each other. The cross-sectional asymmetry and bi-directional flow result in higher sediment concentration along inner banks than along outer banks of the main stream. For a given location, the near-bed concentration is 2−5 times the surface value. Based on Froude number, a sediment carrying capacity formula is derived for the flood and ebb tides. The tidal flow stirs the sediment and modifies its concentration and transport. A 3D hydrodynamic model of flow and suspended sediment transport is established to compute the flow patterns and morphology changes. Cross-sectional currents, bed shear stress and erosion-deposition patterns are discussed. The flow in cross-section exhibits significant stratification and even an opposite flow direction during the tidal rise and fall; the vertical velocity profile deviates from the logarithmic distribution. During the flow reversal between flood and ebb tides, sediment deposits, which is affected by slack-water durations. The bed deformation is dependent on the meander asymmetry and the interaction between the fresh water flow and tides. The flood tides are attributable to the deposition, while the ebb tides, together with run-offs, lead to slight erosion. The flood tides play a key role in the morphodynamic changes of the meander reach.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
tidal meandering river, field measurements, 3D numerical model, flow features, sediment transport, erosion-deposition patterns
National Category
Water Engineering Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-72664 (URN)10.3390/fluids4010015 (DOI)000464467200002 ()2-s2.0-85063378020 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-01-31 (svasva)

Available from: 2019-01-24 Created: 2019-01-24 Last updated: 2022-06-01Bibliographically approved

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Xie, Qiancheng

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