- Title
- Response of pool-riffle dynamics to changes in sediment supply and flow hydrographs
- Creator
- Vahidi, Elham
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The interaction of alterations in sediment supply, hydrographs and channel geometry can affect, on a mesoscale, geomorphic features like pools and riffles. A pool-riffle sequence is a stable geomorphological feature naturally found in rivers that provides essential habitat areas for aquatic organisms. Due to their ecological benefits, understanding how habitat conditions might change in the future and how they respond and adjust to disturbances such as an altered sedimentological regime is vital for river management and rehabilitation. Previous research has mainly investigated the effect of bed non-uniformity, hydrographs or sediment supply on pool-riffle morphology under equilibrium sediment transport conditions, while little work has been done to investigate the effect of hydrographs (e.g. magnitude and duration) in combination with alteration of sediment supply and non-equilibrium sediment transport conditions on the long-term stability of pool-riffle features. So far, there has been limited experimental study about pool-riffle maintenance that has considered the simultaneous effect of different sediment supply rate during variable flows, and none has included spatial and temporal measurement of flow parameters, bed level variations and sediment size distributions under non-equilibrium condition. The goal of this thesis is to understand the relative effect of alteration of sediment supply and bed geometry, coupled with sequences of hydrographs of different duration and magnitudes, on pool-riffle self-maintenance. A physical model of pool-riffle units is developed based on a field site. Scaling results show the similarity of particle motion between model and prototype (based on the Shields diagram) and the validity of applying the experimental results to the field site or any hypothetical stream. Two scenarios of experiments are used to explore how pool-riffle units respond to decreasing and increasing patterns of sediment supply during sequences of hydrographs. The experimental results are then used to show how bed geometry, sediment supply and flow hydrograph interact and impact on pool-riffle self-maintenance. Channel width variation constitutes the dominant effect on the alterations of the velocities and turbulence intensities compared to the vertical topographic variability, as, during higher flow rates, near-bed velocity and turbulence intensity were higher in the pool centre (narrower channel section) than in the pool tail and riffle. The complex relationship between erosion-deposition events and shear stresses indicates the competing effects of reversal episodes and sediment supply rate. The effect of reversal episodes is more significant for the shallower pool morphodynamics and under equilibrium conditions. The pool-riffle unit responds to the increase of sediment supply by bed fining and increasing the average bed slope, and reacts to reduced sediment supply by bed coarsening and decreasing average bed slope. Finer fractions play a significant role in increasing the sediment mobility of the riffle bed and on bed deposition/erosion during rising/falling limbs of the hydrograph in the pool centre. Hydrograph duration also plays an important role in riffle bed elevation, whereas sediment supply and bed geometry have primary roles in the pool bed level variation. After a sequence of floods, the pool-riffle sequence adjusts itself to the disturbances imposed by increases of upstream sediment by reducing the bed level differences between pool and riffle, slightly reducing the slope and by general bed fining.
- Subject
- pool-riffle sequences; flow; hydrograph; sediment transport; bed geometry
- Identifier
- http://hdl.handle.net/1959.13/1407952
- Identifier
- uon:35796
- Rights
- Copyright 2019 Elham Vahidi
- Language
- eng
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 25 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 629 KB | Adobe Acrobat PDF | View Details Download |