The Severn Estuary in the UK is an ideal site for tidal renewable energy projects since the estuary has the third highest tidal range. The UK Government recently invited proposals for tidal projects from this estuary, including the Severn Barrage and the Fleming Lagoon. A two-dimensional hydro-environmental model has been developed, based on an unstructured triangular mesh and the finite volume method. The model has been applied to investigate the hydro-environmental impacts in the estuary caused by these projects and emphasis has been focused on assessing the potential changes in the coastal flood risk and tidal currents. Simulated results indicate that: a great decrease in the peak discharge entering the upstream region would reduce the maximum water levels upstream of the barrage by 1.5–2.5 m; the construction of the Fleming Lagoon would have little influence on the hydrodynamics in the estuary; over the full tidal cycle, the total electricity output and maximum power output would be 0.55TWh and 8.03GW for the barrage, respectively, and they would be 0.105TWh and 1.48 GW for the lagoon.