Abstract This study investigates the significance of the layout of an array of tidal stream turbines on the power output and the hydro-environmental impacts array using the Research Centre’s widely used open-source hydro-environmental model, namely DIVAST (Depth Integrated Velocities And Solute Transport), which was modified by the authors to include turbines as momentum sinks, to simulate the impact of an array of turbines in a coastal environment. For this purpose, three various layouts of an array of turbines, having the same number of turbines and located in the Severn Estuary and Bristol Channel, were modelled and compared. The estuary, including the Bristol Channel, is approximately 200 km long and with a typical spring tidal range of over 14 m has the third highest rise and fall of tide in the world, with the spring tidal currents in the estuary being well in excess of 2 m/s. The study has demonstrated that the impacts of the arrays on the water levels and the maximum water levels which are associated with the flood risk were very small. However, the tidal currents were more significantly influenced by the tidal turbine arrays. The flow velocity field, suspended sediment levels and faecal bacteria levels were increased on the side of the arrays, whilst they were reduced both upstream and downstream of the arrays. The model showed the significance of including the interaction of the turbines and the flow in assessing the potential power output by comparing the discrepancy in the estimated power output of the various arrays, both with and without the inclusion of the impact of energy extraction by the turbines in the model. Moreover, the model showed that the available power during a neap ebb tide was significantly less than (<4%) the available power during a spring ebb tide. It was also found that the array where turbines were sparser could generate more than 35% more power than that produced by the denser arrays.