Some regions of the world concurrently experience a high wave and a high tidal energy resource. These regions include the seas of the northwest European continental shelf, the Gulf of Alaska, New Zealand, northwest Australia, and the Atlantic seaboard of Argentina. Due to the interaction of waves and tides, special consideration needs to be given to resource characterization of marine renewable energy schemes developed in such regions. Waves have been shown to reduce the tidal current, which, because tidal-stream power is proportional to the cube of velocity, reduce the available energy resource. Further, waves can reduce the tidal-stream energy resource during extreme wave periods when ocean renewable devices may not operate. Waves should be also considered in the design and resilience of tidal-stream energy devices. Hence, waves can have a critical effect on the planning, operation, maintenance, and resource assessment of tidal energy sites. Conversely, tides can significantly alter wave properties through various wave-current interaction mechanisms. For example, tidal currents can alter wave steepness which is an important consideration in the design of marine energy mooring. Wave power, in general, is proportional to the wave group velocity and the wave height squared, both of which change in presence of tidal currents. Therefore, resource assessments of such regions should account for the way that one marine energy resource affects another at a variety of timescales from semidiurnal, spring-neap, to seasonal. Finally, wave-current interaction processes affect turbulence, and the dynamics of sediment transport; therefore, they should be considered when the impact of an energy device, or an array of such devices, on the environment is studied. This chapter introduces the basic concepts of wave-tide interaction in relation to the ocean renewable energy resource assessment. Various aspects of the marine renewable energy industry that are affected by wave-tide interactions , such as resource assessment and the influence of wave-tide interactions when characterizing the oceanographic site conditions, are discussed. Methods ranging from simplified analytical techniques to complex fully coupled wave-tide models are explained.
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