Flow Modification in Tory Channel

Habitat in coastal regions is partly defined by currents.  This controls ecological structuring.  Numerical modelling is used to assess the effect of a turbine array on tidal currents in the Tory Channel, New Zealand. The Tory Channel is the smaller of two entrances from Cook Strait to the Queen Charlotte Sound with a large island separating the narrow Tory Channel from the main entrance. The 2D depth-averaged finite element model is validated against velocities from shipboard ADCP transects from a survey during spring tide conditions. Turbine drag is introduced to the model as a stress term in the momentum equations, and includes both the turbine thrust and the structural drag. Turbine array drag is a function of the number and size of turbines, which can be parameterised in a non-dimensional number. This non-dimensional turbine drag number D can be used to represent the drag of several different turbine designs. Restrictions are placed on the size of the array to ensure that turbines are placed in realistic locations. In this study, turbines are restricted to areas with water depths greater than 30 m, and where spring tide currents (in the absence of turbines) are greater than 2.0 m s^-1. As a consequence, the turbine array does not span the entire channel width or length. The modelling shows how turbines will reduce current speeds both within the turbine array, and also throughout much of the Tory Channel, with local increases in speed immediately adjacent the array. Cut-in and maximum or rated turbine speeds are also incorporated to compare how these factors influence both the power production and effect on currents.