The objective of this thesis is to investigate the impact of riverbank vegetation densities, which can vary greatly in natural river systems, on the flow hydrodynamics across the whole channel. A series of flume experiments are conducted with various riverbank vegetation densities and configurations. Flow velocities across the whole channel are recorded by either acoustic Doppler velocimetry (ADV) or acoustic Doppler velocimetry profiler (ADVP). Firstly, the performances of ADV and ADVP are assessed, before the impact of probe configurations on experimental results is evaluated for ADVP. The accuracy of the probe is discussed, and the best probe configurations for the given flow are suggested. The flow velocity progressively reduces at the vegetated riverbank and slightly increases at the main channel with increasing riverbank vegetation density. Turbulence intensity at the middle of the main channel and the riverbank toe increases with increasing vegetation density. However, at the riverbank region, turbulence intensity decreases due to the dramatic reduction of flow velocity. Geomorphic process feedback is given in the form of a case study, and practical recommendations for river restoration are provided. Bulk velocity, which is relevant to flow conveyance, is evaluated at each portion (main channel and riverbank) of the channel. In general, bulk velocity slightly increases at the main channel but dramatically decreases at the riverbank with increasing vegetation densities. The result of Nikuradse’s equivalent roughness also shows an increasing trend with increasing vegetation density. Bed shear stress is estimated using several methods, including the log Law of the Wall, turbulent kinetic energy and Reynolds stresses. The results obtained using all three methods show that bed shear stress considerably increases at the main channel and sharply decreases at the riverbank region for the shear stress with dense riverbank vegetation compared to the no vegetation case. Impulse is introduced as a new criterion to assess sediment transport both at the free stream portion and at the vegetated region in turbulent flow. With a range of reasonably assumed critical velocities and critical impulses, the frequency of impulse closely follows the same trend observed for shear stresses across the channel width.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:739287 |
| Date | January 2018 |
| Creators | Liu, Da |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/8980/ |
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