Water Quality Impacts of a Tidal Barrage in the Severn Estuary, Uk

The Severn Estuary has the third highest tidal range in the world with its spring tidal range reaching 14 m. The estuary is extremely turbid with very high suspended sediment concentrations and limited light penetration through the water column. It has large areas of intertidal mudflat which are of major conservation importance. Hence, it is protected under several European legislative directives and has a number of international designations. Due to its high tidal range, it is an ideal location for the installation of a tidal barrage, making the estuary a major candidate for large scale commercial exploitation of tidal renewable energy resource for electricity generation in the UK. However, there is considerable uncertainty as to how a tidal barrage would affect water quality in the estuary and its water management strategies. This study aims to address this research gap by combining laboratory and numerical modelling investigations to assess the potential impacts of a tidal barrage on water quality in the Severn Estuary, with particular emphasis on salinity and phosphate levels. Salinity at high water was found to reduce by 1 – 2 ppt downstream and upstream of the barrage, with up to 5 ppt reduction further upstream. However, only a slight reduction of 0.5 1 ppt was predicted downstream and upstream of the barrage at low water. Phosphate adsorption to suspended sediments was found to increase with decreasing salinity. The partition coefficient, Kd, of phosphate as a function of salinity was empirically described for incorporation into the model to further refine the model predictions.