Tidal energy extraction offers a highly predictable, reliable energy resource. Natural flow regimes will be altered by the installation of large scale tidal arrays. Bed morphology, sensitive to changes in tidal flow, will be impacted as a result of feedbacks between tidal arrays and coastal hydrodynamics. This research investigates the impact of large scale tidal energy extraction by arrays of tidal stream energy devices on the spatial and temporal variability of hydrodynamics. The research uses the 3D ROMS model (Regional Ocean Modelling System) with tidal energy extraction as a mid-water perturbation and is presented in two halves - first a 3D regional model is developed to investigate the natural hydrodynamics of the Pentland Firth, a highly dynamic and complex channel environment. The second part of this research sees an idealised 3D model of the Inner Sound region of the Pentland Firth developed with the purpose of investigating the sensitivity of the hydrodynamics to turbine array design and placement.