Keywords: nesting, coastal modelling, performance assessment, hydrodynamics.

Many of the environmental issues associated with the deployment of tidal stream turbines arise from changes in hydrodynamic flowfields from the turbines. The development of arrays of turbines within a tidally dynamic area will have a number of impacts on the waterbody; in particular, the turbines extract energy from the water and hence alter flowfields of both ebb and flood tides. This paper describes research undertaken by the authors who have developed a two-dimensional hydrodynamic model to predict changes to tidal dynamics caused by tidal turbines. The paper describes changes to adapt the momentum equation to include energy losses arising from kinetic energy extraction by tidal turbines. This is then incorporated into the full Navier Stokes equation and code into a two-dimensional finite difference model.

The model is applied to the Shannon Estuary, a highly dynamic estuary in Ireland with significant capacity to produce tidal stream energy. Within this research the authors investigated the impact of three different densities of tidal turbines on flow fields. The research considered changes to the flowfield with the region of the turbines and outside the region. The results show significant changes to flow fields and that the results vary depending on location. Further the authors considered the impact of the change hydrodynamics on material transport in the estuary. Results are presented of the differences in flowfields arising from a large array of turbines sited in the region of interest. Flushing times are calculated for the cases with and without turbines, this illustrates likely changes in pollutant transport of the region from the tidal turbines.

This is the first time such a detailed analysis of interactions between kinetic energy extractors and the natural environment has been investigated. The results from this research demonstrate that such interactions will have to be studied in detail prior to deployment of a tidal turbine in the coastal zone.

The main conclusions drawn from this research are that changes to flowfields depend on turbine density, and that at high densities the flowfield can be significantly affected. Within the turbine field current speeds decrease and outside currents may increase. The research also quantifies the effects turbines have on the flushing properties of estuaries.

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