Flow-vegetation interactions are critically important for most hydraulic and sediment processes in streams and rivers and thus need to be accounted for in their management. The central goal of this project therefore was to improve the understanding of flow-vegetation interactions in patchy-vegetated river beds, which are typical in rivers. Based on laboratory experiments covering a range of selected hydraulic and patch mosaic scenarios, the hydraulic resistance mechanisms, turbulence structure, and transport mechanisms were studied. The effects of regular patch mosaic patterns (aligned and staggered) on the bulk hydraulic resistance were investigated first. For the cases in which the relative vegetation coverage BSA in respect to the total flume bed is low (BSA = 0.1), the patches mutual positions do not affect values of the friction factor. When the parameter BSA increases to intermediate values (BSA = 0.3), the spatial distribution of the vegetation patches and their interactions become crucial and lead to a significant increase in the bulk hydraulic resistance. When further increase of the vegetation cover occurs (BSA = 0.6), the effects on hydraulic resistance of patch patterns vanish. To clarify the mechanisms of the revealed patch effects on the overall hydraulic resistance, flow structure was assessed at both scales: individual patch and patch mosaic. The presence of a submerged isolated vegetation patch on the bed introduces a flow diversion which strongly alters the velocity field and turbulence parameters around the patch. Coherent structures, generated at the canopy top due to velocity shear, control the mass and momentum transfer between the layers below and above the vegetation patch. At the patch mosaic scale, a complex three-dimensional flow structure is formed around the patches which depends on the patch spacing and spatial arrangements. For the low surface area blockage factor (BSA = 0.1), the patches are sparsely distributed and the wakes are (nearly) fully developed before they are interrupted by the effects of the downstream patches. At the intermediate surface area blockage factor (BSA = 0.3), significant differences in flow structure between the aligned and staggered patches were observed. For the highest surface area blockage factor investigated (BSA = 0.6) both aligned and staggered patch mosaic configurations showed a similar behaviour. The results on the flow structure are used to provide mechanistic explanation of the observed patch mosaic effects on the bulk hydraulic resistance.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:742379 |
| Date | January 2017 |
| Creators | Savio, Mario |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=237016 |
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