Refinements to the Efdc Model for Predicting the Hydro-

10 This paper presents an investigation of the impacts of a Severn Barrage on the hydro-environment of 11 the Bristol Channel and Severn Estuary using the Environmental Fluid Dynamics Code (EFDC) model 12 with a recently developed Barrage module (EFDC_B). Details are given of a barrage module being 13 implemented into the EFDC model to represent the various hydraulic structures, such as turbines and 14 sluice gates, as deployed along the barrage line. Several cases, both with and without the barrage, have 15 been simulated to investigate the potential changes on the peak water levels, minimum water depths 16 and peak tidal currents arising from a barrage. The impacts of a barrage on the salinity concentration 17 distribution have also been simulated in both 2D and 3D modes. The predicted results showed that the 18 maximum water levels could be significantly reduced, especially downstream of the barrage and for 19 much of the region in the Severn Estuary and that the minimum water depths would be changed so 20 much that there would be 80.5 km 2 loss of intertidal habitats due to the sitting of a barrage across the 21 estuary. Likewise, the peak tidal currents would be considerably reduced, and by as much as a half in 22 the middle of the main channel. The predicted salinity concentrations results indicated that at high 23 water, the salinity concentrations would be reduced by 1-2 ppt downstream and upstream of the barrage 24 and salinity concentrations in the region near Beachley would be reduced by up to 5 ppt, and that at low 25 water, salinity concentrations would be reduced by 0.5 – 1 ppt in the middle of the Bristol Channel and 26 by typically 0.5 ppt and 1 ppt downstream and upstream of the barrage, respectively. The predicted 27 results also indicated that salinity concentrations downstream and upstream of the barrage would be 28 under a stable state with slight oscillations all the time due to the effects of the barrage. A comparison 29 between the salinity concentration distributions predicted by the 2D and 3D models indicated that the 30