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Ecohydraulics of instream flow alterations

Muhawenimana, Valentine 2019. Ecohydraulics of instream flow alterations. PhD Thesis, Cardiff University.
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Abstract

Increasingly sustainable river management efforts are underway to limit the damage on freshwater ecosystems caused by anthropogenic hydro-engineering activities. This is exemplified by engineered Woody debris dams (WDD) used Natural Flood Management (NFM) to attenuate flood flows, although design guidance and evidence of WDD flood attenuation are incomplete. Considering current questions of inefficient solutions in ecohydraulics, a trade-off is necessary to balance the introduction of new alterations with restoring river habitats and biodiversity. This thesis sought to (i) evaluate design options for WDD to maximise their performance for flood attenuation, and (ii) resolve questions pertaining to the hydrodynamics of flow alterations and resulting fish-flow interactions. Flood attenuation was found to depend on WDD streamwise length, geometric arrangement, and most importantly cross-sectional flow blockage ratio and porosity (Ch. 2). In the wake of an idealised WDD spanwise cylinder, turbulence length scale relative to fish size, direction and magnitude of Reynolds shear stresses and vorticity governed fish (Nile tilapia, Oreochromis niloticus) swimming stability, and habitat choice reflected avoidance of relatively highly turbulent areas (Ch. 3). Large Eddy Simulation showed that proximity of the spanwise cylinder to the ground determined the wake and near bed dynamics by affecting the separation of shear layers, creating a ground vortex that merged with the von-Karman vortex street rendering the wake asymmetrical, and resulting in lift forces, drag coefficients and Strouhal numbers higher than those of unbounded cylinders (Ch. 4). Furthermore, swimming performance and habitat choice of Pumpkinseed fish (Lepomis gibbosus) relative to velocity and turbulence was found to highly depend on temperature, as fish showed energy-saving behaviour at the lower temperature, while their time to fatigue increased with temperature (Ch. 5). In addition to quantifying WDD flood attenuation performance in relation to dam composition, arrangement of wood pieces and porosity, this thesis highlighted the importance of currently overlooked and ultimately consequential flow attributes such as the direction of turbulent shear stresses and vorticity, and temperature regimes in the current depiction on fish behaviour in anthropogenically altered flows.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Echoydraulics; Natural flood management; Fish kinematics; Flow alterations hydrodynamics; Woody debris dams; Turbulent flow.
Date of First Compliant Deposit: 19 December 2019
Last Modified: 17 Mar 2020 09:38
URI: http://orca.cf.ac.uk/id/eprint/127655

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