The Severn Estuary has one of the largest tidal ranges in the world and has long been the subject of consideration for tidal energy generation.Whilst plans to build a tidal barrage across the estuary have existed in various forms since the 1920s,the 1989Severn Tidal Power Group(STPG)proposal is commonly referred to as the original Severn Barrage.The UK government abandoned this ebb generation scheme as a public investment project in 2010.However,plans to build a two-way generation scheme were more recently put forward by a private consortium,namely Hafren Power.To assess the impact that a barrage would have on the hydro-environment in the estuary a number of numerical modelling studies have previously been conducted for the STPG scheme.As this design has now been superseded by the Hafren Power proposal,new studies have been conducted to investigate the impact of a two-way scheme.In this study the hydro-environmental impacts of both ebb-only and two-way schemes were assessed using physical and numerical modelling techniques.Scale model barrages were constructed and testing was carried out using a physical model of the Severn Estuary,located in the hydraulics laboratory at Cardiff University.A depth integrated numerical model,namely DIVAST,was applied to the physical model geometry and modifications were made to simulate the effects of the barrage structures.The numerical model predictions showed good agreement with the corresponding laboratory data.The results were consistent with conclusions from previous studies,relating to the ebb-only scheme.These included an increase in the minimum water levels upstream of the barrage,a reduction in the mean water levels downstream of the barrage and a general reduction in tidal velocities.For a two-way scheme changes in the tidal elevations and velocities depended on the exact operating conditions of the barrage.It was found that with no starting head the tidal regime was similar to the natural state,with little change in the elevations and a slight reduction in the mean velocities.As a starting head was introduced there was still little change in elevations downstream of the barrage,however,there was an increase in the minimum water levels upstream of the barrage,and a further reduction in the mean velocities. The Severn Estuary has one of the largest tidal ranges in the world and has long been the subject of consideration for tidal energy generation. What plans to build a tidal barrage across the estuary have existed in various forms since the 1920s, the 1989 Severn Tidal Power Group (STPG) proposal is commonly referred to as the original Severn Barrage.The UK government has this ebb generation scheme as a public investment project in 2010.However, plans to build a two-way generation scheme were more recently put forward by a private consortium the same from Haperen Power.To assess the impact that a barrage would have on the hydro-environment in the estuary a number of numerical modeling studies have previously been conducted for the STPG scheme. As this design has now been superseded by the Hafren Power proposal, new studies have been conducted to investigate the impact of a two-way scheme. In this study the hydro-environmental impacts of both ebb-only and two-way schemes were assessed using physi cal and numerical modeling techniques. Scale model barrages were constructed and testing was carried out using a physical model of the Severn Estuary, located in the hydraulics laboratory at Cardiff University. A depth integrated numerical model, namely DIVAST, was applied to the physical model geometry and modifications were made to simulate the effects of the barrage structures. numerical models predictions showed good agreement with the corresponding laboratory data. results were consistent with conclusions from previous studies, relating to the ebb-only scheme. These included an increase in the minimum water levels upstream of the barrage, a reduction in the mean water levels downstream of the barrage and a general reduction in tidal velocities. For a two-way scheme changes in the tidal elevations and velocities depended on the exact operating conditions of the barrage. It was found that with no starting head the tidal regime was similar to the natural state, with little change in the elevations and a slight reduction in the mean velocities. As a starting head was introduced there was still little change in elevations downstream of the barrage, however, there was an increase in the minimum water levels upstream of the barrage, and a further reduction in the mean velocities.