The Impact of West Somerset Lagoon Operation on Water Renewal of Bristol Channel and Severn Estuary

Abstract

A tidal lagoon generates predictable energy from tides by creating an artificial water head difference through impounding water over a tidal cycle. The movement of substantial water volumes between natural waters and the enclosed lagoon basin triggers changes in water exchange and tidal circulation, potentially resulting in significant impact on water quality. Therefore, a comprehensive understanding of the impact of tidal lagoons on regional water exchange is crucial to address any predicted concerns. The West Somerset Lagoon (WSL) was studied in this study to investigate its impact on water renewal of Bristol Channel and Severn Estuary. The hydrodynamic modelling of lagoon structures used domain decomposition method, and full momentum conservation was achieved by refining the momentum source terms at the turbine locations. To assess the seawater renewal alteration, this study computed water residence time by tracking the evolution of a non-buoyant, mass-conservative tracer concentration which was introduced into a control domain surrounding WSL. The residence time was calculated using the widely adopted 'e-folding time' method, representing the duration for the tracer concentration to decrease to 1/e of the initial concentration. To better understand the influence of WSL on water renewal in distinct regions, tracers were introduced in three separate zones based on geographical divisions: the Severn Estuary to the outer Bristol Channel, the Severn Estuary to the Inner Bristol Channel, and the Severn Estuary itself. The study investigated water renewal exchange in the study areas before and after the implementation of WSL. The results revealed a 43% reduction in residence time for the entire Bristol Channel and Severn Estuary after the construction of WSL. Furthermore, the inner Bristol Channel and Severn Estuary experienced reductions of 20% and 13% respectively after WSL incorporation into the models. As such, the operation of WSL could potentially accelerate pollution mitigation and attenuation in these areas, with this being a meaningful consideration for future design of similar treatment and retention systems where there are potential pollution and health risks.