Hydrodynamic Characterisation and Wave Current Interaction around Dunnet Head in the Pentland Firth with Implications for Marine Energy Generation
Hydrodynamic Characterisation and Wave-Current Interaction Around Dunnet Head in the Pentland Firth with Implications for Marine Energy Generation The Pentland Firth in the north of Scotland connects the North Atlantic Ocean to the west with the North Sea to the east. A strong hydraulic gradient drives very strong flows through the Firth, with local tidal streaming around islands and headlands. Hydrodynamic conditions can be further enhanced by weather systems from the west, especially during autumn and winter storms. As a result, the Pentland Firth and Orkney Waters region is at the global forefront of developments in both tidal stream and wave power conversion but due to the energetic environment relatively few field measurement studies have been carried out. This leaves a lack of detailed knowledge of the hydrodynamic conditions present. This study involved monitoring waves and currents in the Firth to improve existing knowledge. An Acoustic Wave and Current Profiler (AWAC) and an Acoustic Doppler Current Profiler (ADCP) were deployed in the Pentland Firth approximately 1 km north of Dunnet Head for two 15-week periods, and an X-band radar overlooking the deployment site was utilised at various times over these periods to monitor the sea state over a wider area. Data showed the dominant wave direction was north-westerly, and a combination of swell and wind waves present with maximum wave heights of over 11m. Currents of up to 3 ms-1 on the east-going flood tide and 2.4 ms-1 at approximately 280° on the ebb tide were observed. Vertical current velocities showed evidence of an eddy which developed as a result of flow round the Dunnet Head promontory. A hydrodynamic model validated using the instrument data was used to expand the area of study. Instances of wave-current interaction were investigated and revealed that while clear WCI could be observed in more unusual NE waves through semi-diurnal wave height and period variation, and through wave spectral changes with tide, it could not be seen in NW waves. However, wave refraction by current was observed irrespective of wave direction, and clear evidence of frequency downshifting on opposing currents and upshifting on following currents was shown in wind waves. Once an understanding of the local hydrodynamics was obtained, this was applied to the context of marine energy generation to highlight the implications wave and current behaviour and interaction in such sites may have for the marine energy industry.
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