Recent proliferation of marine renewable energy installations has inspired a number of researchers to study the possible impact of the technologies on the marine environment. The impact of tidal turbines on the flow downstream of the rotor and their effect on sediment transport from the seabed over the lifetime of the devices is of particular concern, given the potential damage to the seabed, the habitats of marine life and submarine power cables used to connect the devices to the coast. Previous computational studies of the environmental impact of tidal turbines have, however, focused predominantly on largescale one and two dimensional modelling. Resolution of the three-dimensional nature of the flow through the rotor of the turbine proves to be crucial if the mechanism whereby these devices entrain sediment from the seabed is to be properly understood. The three-dimensional phenomena occurring in the flow through the rotor of the turbine have been found to profoundly affect how the sediment is lifted from the seabed and transported downstream in the wake of the device. The physics of sediment transport due to the rotor of a tidal turbine in a tidal current have been investigated using high resolution three-dimensional computational simulations of the hydrodynamics of the rotor wake. The direct impact of the turbine on the seabed has been found to be confined to an area that extends no further than two rotor diameters downstream of the turbine. The maximal values of shear stress, which indicate the extent of the impact, vary with the distance between the rotor hub and the seabed.