Effects of open boundary location on the far-field hydrodynamics of a Severn Barrage

•CS and IS models were setup to study the open boundary effects on modelling a Severn Barrage.•Open boundary effects significantly affected the water levels at open boundaries.•An obvious difference in the water levels were found between CS and IS models.•The CS model was recommended to study the far-field effects of a Severn Barrage.

The Severn Estuary has the second largest tide range in the world and a barrage across the estuary from Cardiff in South Wales to Weston in South West England has been proposed for over half a century, to extract large amounts of tidal energy from the estuary. To assess the environmental impacts of the proposed tidal barrage requires accurate model predictions of both the near-field and far-field hydrodynamics, which can strongly depend on the model area and the appropriate boundary forcing. In this paper two models, based on the Environmental Fluid Dynamics Code (EFDC) numerical model with a recently-developed Barrage module (EFDC_B), were set up with different computational domains. The Continental Shelf model, which was centred on the Bristol Channel, has its open boundary extended to beyond the Continental Shelf. The Irish Sea model, which was also centred around the Bristol Channel, only has its open boundary extended to the Celtic Sea in the south and the Irish Sea in the north. In order to investigate the effects of the open boundary conditions imposed in the models on the near and far-field hydrodynamics for the case of the Severn Barrage, the Continental Shelf model was first run with and without the operation of the Severn Barrage. The Irish Sea model was then run, also with and without the operation of the Severn Barrage, and with the open boundary conditions provided by the Continental Shelf model. The results from both models were then analysed to study the impact of the tidal barrage on the near-field and far-field hydrodynamics in the Bristol Channel and Irish Sea. Detailed comparisons of the model results indicate that the hydrodynamic conditions along the open boundaries of the Irish Sea model are affected by the tidal barrage and that the open boundary conditions also have noticeable impacts on the far-field hydrodynamics, especially in the Irish Sea, with approximately an average 4-7 cm difference in the maximum water levels predicted in Cardigan Bay and with a maximum difference of 9 cm in the northern part of Cardigan Bay.