Alteration to the shallow-water tides and tidal asymmetry by tidal-stream turbines

Potter, Dan and Folkard, Andrew and Ilic, Suzana (2019) Alteration to the shallow-water tides and tidal asymmetry by tidal-stream turbines. PhD thesis, Lancaster University.

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Abstract

Tidal flow velocity asymmetry (FVA) plays a crucial role in residual sediment transport and has been shown to impact significantly on the amount of tidal energy that is technically extricable by tidal-stream turbines (TSTs). TSTs are known to alter tidal hydrodynamics locally, and large arrays of turbines do this on regional scales. However, less is known about the effect of TSTs on the FVA. This thesis explores changes to the FVA and therefore the shallow-water tidal constituents resulting from the deployment of TSTs. Numerical experiments in a uniform rectangular channel were undertaken, using the MIKE21 software package. The effects of single TSTs and multiple TSTs on tidal hydrodynamics were simulated, as well as those of a single TST on sediment transport. Flood-ebb asymmetry in the spatial distribution of current attenuation by the turbines altered the FVA. The overall attenuation of the current led to predictable changes in the total available tidal energy per tidal cycle, and the gross volume of sediment transported. The attenuation of the current by the TSTs was of greater importance to these aspects of the environment than any changes to the FVA that they caused. Changes in the FVA led to changes in the flood-ebb asymmetry of the available power, and the net volume of sediment transported, and were of far greater importance than the overall attenuation of the current in these respects. Multiple turbines deployed in a line along the channel, such that their areas of effect overlapped, had an additive effect on the FVA. When deployed as a row across the channel width, the total area affected by the turbines remained similar to that of a single turbine, so long as an inter-turbine spacing of at least three turbine diameters was maintained.