- Title
- The effect of flow unsteadiness on sorting and self-maintenance of pool-riffle sequences
- Creator
- Bayne, Eddie
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Formation and long-term persistence of pools and riffles has been a concern in river engineering and river restoration projects since the 1970s when Keller (1971) postulated the pool-riffle maintenance with velocity reversal hypothesis. Since then, a wide number of studies have been conducted in different rivers and laboratories and numerical methods have been extensively used to simulate the flow and bed evolution in pool-riffle (PR) sequences. Based on the literature, the morphology of PR sequences can be influenced by parameters like unsteadiness of flow, sediment transport, longitudinal sorting and geometry of PR sequences. However, this has been done in a compartmentalised way with studies either focusing on one aspect or the other. Considering the formation and persistence of pools and riffles by combining all the identified factors would require either measuring or modelling unsteady multidimensional flows and sediment transport for different-sized fractions over a movable and continuously-evolving sediment bed at the scale of a PR sequence. This was the greatest motivation for conducting this experimental research as no systematic experimental analyses had been carried out on the self-maintenance of pools and riffles. This thesis examines the effects of three-dimensional flow effects, grain-size distribution and longitudinal sorting, geometry, unsteadiness, and upstream sediment source on PR development all together. To have a comprehensive explanation on the long-term stability of PR sequences, all different mechanisms involved in the stability of PR sequences put together and investigated with as much detail as it is currently possible. This research used field, numerical and experimental data for analyses and can be categorised into two parts. In the first part (covered in Chapter 2 and Chapter 3) the numerical results of 2 pools and 2 riffles from a one-dimensional morphodynamic model were used to evaluate the effect of the hydrograph shape, longitudinal sorting and then three-dimensional flow on the stability of the pools and riffles. In the second part (covered in Chapter 4 to Chapter 8) the experimental section of this research, including the design and experimental setup for a PR sequence, data acquisition, and results are presented. This research is the first to document the combination of all the identified effective factors on PR sequences with long-term persistence. The laboratory model provided the opportunity for more-detailed measurements in mobile bed conditions and was conducted in a reduced-scale model based on the characteristics of a field site located along a gravel-bed river. UVP measurements, ultrasound water-surface detection sensors and acoustic bed profilers were employed to measure flow patterns and water surface profiles as well as the bed evolution before, during and after each test. In the first section it was found that the number of instantaneous reversals and the period of time spent under each reversal condition were very important factors in the long-term stability of PR sequences, as well as longitudinal sorting and the difference between grain-size distribution in the pool and adjacent riffle. In addition, incorporation of 3-D flow effects significantly increased the number of reversal events, particularly in the central half of the stream width. In the second section, the experimental results indicated that when the upstream sediment supply contained fine material there was a higher chance that the pool section would be filled. However, as soon as upstream became coarser, the deposited sediment moved towards downstream. Due to the natural conditions of unsteady flow, sediment transport is not at equilibrium in pools and riffles. What is very important in scouring a partially-filled pool is passing the initial motion stage during the hydrograph and then remaining long enough beyond the initial motion stage to transfer the material from the pool section towards the downstream. This is why large flashy hydrographs do not necessarily help pools to scour. However, longer hydrographs with less peak discharge can be more effective in PR self-maintenance. The stability of a PR sequence can be analysed precisely when the number of floods increases. The PR sequence may not have strong self-maintenance for a few hydrographs but as soon as the upstream sediment flux and flow conditions provided the requirements for self-maintenance, the pool was washed out.
- Subject
- pool-riffle sequences; self-maintenance; unsteady flow; longitudinal sorting; non-equilibrium experiments; sediment transport
- Identifier
- http://hdl.handle.net/1959.13/1409875
- Identifier
- uon:36081
- Rights
- Copyright 2019 Eddie Bayne
- Language
- eng
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| View Details Download | ATTACHMENT01 | Thesis | 19 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 1 MB | Adobe Acrobat PDF | View Details Download |