The impact of tidal lagoons on regional water exchange

The operation of a tidal lagoon (or barrage) involves impounding a substantial volume of water, creating a head difference across the impoundment wall, and converting the potential energy associated with the head difference into kinetic energy through turbines. Consequently, the functioning of a tidal lagoon facilitates the movement of substantial volumes of water between the water body and impounded basin, altering water exchange and tidal flushing patterns, which potentially affect water quality. In this study, three tidal lagoon schemes proposed in the UK, West Somerset Lagoon (WSL), Swansea Bay Lagoon (SBL), and North Wales Tidal Lagoon (NWTL), were investigated for their regional water renewal capacities under different operational schemes and tidal phase conditions within a comparative framework. The exchange processes were investigated by modeling tracer changes within a control domain, with and without the lagoon, under different operational procedures. The results revealed different tracer-exchange characteristics across all schemes. Regardless of the operation scheme, the exchange concentration distribution tended to converge to a near-steady-state distribution at the end of each tidal cycle over a longer timescale. This study indicated that although the tracer dispersion rate was not highly sensitive to the initial tidal conditions, the tracer concentration declined more rapidly when released during spring tides. The inclusion of the WSL significantly improved the water exchange capacity within the lagoon basin region and the Bristol Channel and Severn Estuary. By contrast, the SBL and NWTL schemes resulted in decreased water renewal capacity in the lagoon area.