A decade long high-resolution wave resource map for Pentland Firth and Orkney Waters - hindcast by two-way coupling of wave-current models

Wave energy is a promising renewable resource globally, with the UK leading efforts in regions like Pentland Firth and Orkney Waters. These areas, known for their strong tidal currents and energetic waves, require accurate wave energy resource assessments. This study developed numerical models, including a wave-only and a coupled wave-current model, to simulate combined wave-current conditions over a decade (2014–2023), evaluating the influence of tidal currents. The models assessed interannual, seasonal, and monthly wave resource variations and the impacts of wave-current interactions.

First, a North Atlantic-scale wave-only model was constructed with TOMAWAC spectra wave model to simulate wave conditions without tidal influences and generate wave boundary conditions. Then, a regional wave-current model was developed by coupling TOMAWAC and TELEMAC. This coupled model used the wave boundary conditions from the large-scale North Atlantic model to obtain wave parameters including tidal effects. The North Atlantic wave model was validated against 10 years of continuous wave buoy data across four sites, while the regional wave-current model was verified with 135 days of Acoustic Wave and Current Profiler (AWAC) and Acoustic Doppler Current Profiler (ADCP) deployments, ensuring model reliability.

The results reveal significant spatial and temporal variations in wave energy resources, with pronounced tidal effects. Based on 10 years of data, including tidal currents in the model, substantially decreases mean wave height and mean wave power in the northern and southern regions of the Orkney Islands and Stroma Island in the Pentland Firth. Near Stroma Island, wave height reductions can reach up to 0.5 m (a 25 % reduction compared to the wave-only scenario), and wave power decreases by 6 kW/m (over 50 % reduction). Conversely, wave power increases at tidal inlets such as Pentland Firth, Hoy Mouth, and Westray Firth, with a 10-year average increase of up to 7.7 kW/m (22 %) at Westray Firth. Long-term data indicate that wave-current interactions vary significantly by season, month, and year, with notable changes during winter and high-wave periods. Extreme wave conditions are also amplified by tidal currents, particularly at the tidal inlets within the regional model. The findings could benefit not just the wave energy industry, but also other fields concerned with wave-current dynamics.