Assessing the Impact of Rows of Tidal-Stream Turbines on the Overtides of the M₂

Flood-ebb asymmetry of a tidal flow has important implications for net sediment transport and the potential extractable resource. The asymmetry of the tide in U.K. waters may be understood through the interaction of the M₂ (principal lunar) and M₄ (first even overtide of the M₂) tidal constituents. The interaction of the M₂ tide with a tidal-stream turbine will alter the M₄ tide, both augmenting and reducing the M₄ amplitude, leading to an alteration of flood-ebb asymmetry. In this chapter the impact of a row of tidal-stream turbines on the overtides of the M₂ has been investigated through a numerical modelling study. Further, the way that additional turbines alter the way the turbines impact the shallow-water tides individually is explored. The results of the modelling show that when deployed in a row, on average, the peak velocity deficit and change to the current magnitude asymmetry (CMA) per turbine was less than were it deployed alone. The difference between the per turbine impact of turbines in a row and that of an individual turbine grew as the number of turbines in the row, and therefore the row blockage, increased. Additionally, the total area of the model domain experiencing a change to the M₂ current and CMA > 1% increased with the addition of turbines to the row, for a row blockage >~10%, but remained similar to the single turbine case for lower blockage values. The implication of the change to the CMA by a turbine in a row for the asymmetry in energy conversion for its lateral neighbours was small as the turbines do not lie within the area of effect of their neighbours. However, the per turbine energy conversion increased as the number of turbines and row blockage increased, in line with theory.