Previous results from one-dimensional model studies have demonstrated that large-scale exploitation of the tidal stream resource could have a significant impact on large-scale sediment dynamics. In this research, we model the impact which such exploitation would have on the dynamics of offshore sand banks. Such banks have an important role in natural coastal protection, since they cause waves to refract and induce wave breaking. As a case study, we examine the Alderney Race, a strait of water between the island of Alderney (Channel Islands) and Cap de la Hague (France). A morphological model is developed, incorporating tidal energy converter (TEC) device operation as a momentum sink in the three-dimensional hydrodynamic module. Through a series of model experiments, we demonstrate the impact which a full-scale (300 MW) TEC array would have on sediment dynamics when sited in the vicinity of headlands and islands. It is important to understand this aspect of the environmental impact of full-scale TEC operation, since headland and island sand banks comprise of readily mobile sediment grain sizes. Therefore, small changes to the tidal regime can have a large effect on the residual sediment transport pathways, and hence sand bank evolution, over the life cycle of a TEC device.