Waves, currents and sediment transport modelling at the Wave Hub site

Abstract

This research project uses an integrated modelling system to investigate the effects of a wave farm on nearshore sediment transport at the Wave Hub site. The Wave Hub project is a large scale demonstration site for the development of the operation of arrays of wave energy generation devices located at the southwest coast of the UK where multiple field measurements took place. Particular attention of this study was paid to the interaction between waves and tides due the presence of the wave farm and its effects on radiation stress, bottom stress, and consequently on the sediment transport and the coast adjacent to the wave farm, using an integrated complex numerical modelling system. The modelling system consisted of the SWAN model for waves and the ROMS model for currents, and a sediment transport model for morphological computations. The two-way coupled SWAN and ROMS models with nested model grids were set up and run with and without the wave farm at the Wave Hub site. The results from this study show that tidal elevation and tidal currents have a significant effect on the wave height and direction predictions, and tidal forcing and wind waves have a significant effect on the bed shear-stress, mainly during spring tide. Also, the wave radiation stresses can considerably alter the long-shore and cross-shore velocity components. Interactions between waves and tides at the Wave Hub site are found to be important when modelling coastal morphological change due to the presence of wave energy devices. The wave action can impact on bottom boundary layer and mixing in the water column, which consequently impact on the nearshore sediment transport and the resulting morphological changes. Model results indicate that wave and long-shore currents are attenuated in the area sheltered by the wave farm. Bed-load rates show a decrease in magnitude when the wave farm is present, even during storm conditions. Wave impacts on averaged flood and averaged ebb tidal cycles show significant changes during flooding cases, when including the wave farm, it has major effects for the averaged flood cases on current speeds, bottom stresses, suspended sediments and bed-load transports. The results highlight the importance of the interactions between waves and tides when modelling coastal morphology with presence of wave energy devices. It was observed that the presence of the wave farm has significant impacts on the nearshore circulation, bed shear stresses and sediment transport. The morphological changes are also altered by the wave farm. This integrated modelling system provides a useful tool to help the study of physical impacts of a wave farm on coastal areas, which is the key element for the wave resource characterization, ocean circulation, sediment transport, morphodynamic changes and environmental impact assessment for the ongoing Wave Hub projects.