A world’s first tidal energy lagoon has been proposed in Swansea Bay area (Severn Estuary, west coast of UK). The Severn Estuary is a highly dynamic environment and understanding the bay hydrodynamics is important to the sustainable use of the coastal area in this region. Changes to hydrodynamic processes due to the construction and operation of the lagoon will lead to some potential changes in the sediment pathways and depositional processes within the bay. Possible impacts on natural sites, located southeast of the lagoon, are a concern. A hydrodynamic model (DELFT3D) of Swansea bay has been set up to investigate the sediment pathways within this area. The focus of this paper is the potential link between tidal flows and the configuration of Swansea Bay with the construction of the tidal lagoon and as it is at present. The results will be useful for assessing tidal lagoon impact as well as many aspects of local shoreline management.