Joint extremes of waves and currents at tidal energy sites in the English Channel

Abstract

Efficient and resilient design of tidal turbines requires knowledge of the environmental conditions which they will be exposed to over the course of their design life. Several sites in the English Channel have been identified by technology developers for potential deployments of tidal farms. These sites are exposed to strong tidal currents and large wave conditions. At sites of interest for tidal energy extraction, the largest currents are primarily driven by astronomical forcing and can be predicted from harmonic analysis of relatively short datasets. In contrast, wave conditions are stochastic in nature and require long hindcasts to accurately estimate extreme conditions. Moreover, at sites relevant to tidal energy, currents have a significant influence on the wave conditions. This necessitates that extremes of waves and currents are assessed using joint probabilistic models, in order to specify combinations of waves and currents to be used in the structural design of tidal turbines. In this work we use a coupled wave-current model of the English Channel to create a 31-year hindcast of conditions. We examine the joint distribution of wave and current conditions for tidal energy sites near the Isle of Wight (UK) and in the Alderney Race, off the coast of France. We construct 3D environmental contours of current speed, significant wave height and relative direction between the waves and currents. It is shown that the largest waves occur when waves and currents are in opposing directions. The directional misalignment between waves and currents is examined and the potential impact that this may have on the design of tidal turbines is discussed.