Physical and numerical modelling of impeded tidal flows: Effects of aquaculture structures on hydrodynamics and material transport
O Donohue, Fearghal
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The effects of aquaculture structures on flows are investigated in this thesis through a two-pronged approach. Small-scale flow interactions between the structures and the surrounding waterbody are studied through physical model studies conducted in a tidal basin facility. The second approach is the use of numerical models to investigate the far-field effects of the structures on hydrodynamic and transport patterns. Two separate models are used: a two-dimensional, model (DIVAST); and a three-dimensional, model (EFDC). Laboratory studies show that the local near-field effects of the aquaculture structures are significant; results demonstrate that flow speeds within the scaled long-line structure are reduced by 25-40% from ambient, while material transport distances are reduced by up to 50%. Scale model investigations of flows through bottom-feeding aquaculture installations were also conducted. An existing two-dimensional depth integrated model has been refined to better predict hydrodynamics and solute transport within suspended aquaculture farms. The numerical model has been refined to include both the form drag imparted by the individual mussel droppers and the blockage effect that the suspended canopy presents. It has been demonstrated that predicted velocities and solute transport correlate well with experimental results. The numerical model was applied to a designated aquaculture site. The effects of long-lines on hydrodynamics and solute transport were analysed. Flushing studies were used to study particle renewal terms in the embayment. During this research the EFDC model was amended to include the effects of a suspended canopy, with free-stream flows developing beneath. The canopy was shown to have significant effects on the vertical structure of the flow regime; retarded flows are developed within the canopy and accelerated flows developed beneath the canopy. The model accurately predicts observed shear turbulence production at the bottom of the canopy; the associated implications for flow processes and mixing are discussed. A case study conducts numerical simulation of baroclinic flows within a partially stratified estuary with large-scale aquaculture developments. This development has broader applications than just suspended canopy flows. Natural and manufactured structural arrangements like kelp and sea grass beds, float breakwaters, and wave and offshore wind energy extraction arrays will all influence local currents.