Despite the great advantages of tidal lagoons, such as predictable renewable energy generation and flood risk reduction, tidal lagoons are expected to have an impact on the coastal and riverine environment. The uncertainties regarding the environmental impacts can potentially affect the development and influence the design of tidal lagoons. Therefore, it is desirable to fully assess their environmental impacts to evaluate the potential impacts associated with lagoons, and to mitigate any adverse impacts by improving the construction design and operation methods where necessary. A comprehensive study regarding the environmental impact of lagoons and their operation should be undertaken at the preliminary design stage and beyond. Furthermore, it is important to explore the accumulative impacts and the interaction of the conjunctive operation of the lagoons in different locations around the coast, which is regarded as an integrated potential effective tidal range energy scheme to provide continuous power.
This research study involves developing a refined two-dimensional hydrodynamic model to provide an accurate assessment of the hydro-environmental impact and the interaction of tidal lagoons. Improvements are made through simulations of island wakes, which provides a similar scenario to the flow patterns around obstacle, such as lagoons, in a macro-tidal environment. Innovative refinements are also made to enhance the modelling accuracy of the hydroenvironmental process within and outside of a lagoon, including full momentum conservation between the subdomains and the independent operation of the turbines and sluice gate blocks. Three state-of-the-art tidal lagoon proposals, namely: West Somerset Lagoon (WSL), Swansea Bay Lagoon (SBL) and North Wales Tidal Lagoon (NWTL), are used as case studies in this research to investigate their impacts and hydro-environmental interactions.
The results show that the operation of the West Somerset Lagoon slightly reduces the tidal range in the Bristol Channel and Severn Estuary. The changes in tidal elevation caused by the WSL and NWTL resulted in a loss of intertidal mudflats of up to 20 km2 in the Bristol Channel and Severn Estuary, while the decrease in the peak water elevations reduces the coastal flood risk. The maximum velocity in the inner Bristol Channel increases by about 0.25-0.75 m/s with the operation of WSL, which improves the water renewal capacity and increases the maximum suspended sediment concentration in the Bristol Channel and Severn Estuary, and consequently reduces the risk of hypernutrification and eutrophication. In contrast, the current designs for the SBL and NWTL schemes as modelled in this study showed a decrease in the water residence time by 4% and 45.7% in the lagoon area, respectively. The bed shear stress study and the indicative morphological modelling demonstrated potential erosion in the turbine wake region, influencing the general morphodynamics during lagoon operation. Furthermore, the presence of WSL is likely to cause sediment deposition at two sides of the lagoon impoundment, while increasing slightly the risk of scouring the seabed in the inner Bristol Channel.
In the study of the conjunctive operation of WSL and NWTL, as well as WSL and SBL, the interactions between the lagoons were investigated, but they were found to be minor. The interactions between the lagoons are associated with the lagoon scale, location, tidal phase, et al., therefore a general conclusion could not be obtained. However, the feasibility of relatively continuous tidal power output is presented for the conjunctive operation of WSL and NWTL.