Return values of significant wave heights and near-bed orbital velocity in strong currents : an application to the Alderney Race using the ResourceCode hindcast (1994–2024)

Strong tidal currents can markedly alter the wave climate at tidal-energy sites. We analyse the 1994–2024 ResourceCode hindcast to estimate return levels of significant wave height and near-bed orbital velocity in the Alderney Race. Conditional high thresholds are estimated using additive quantile regression, and threshold exceedances are modelled with generalized Pareto distributions whose parameters depend smoothly on tidal-current speed and wave–current relative direction. Synthetic records are then generated by annual-block resampling combined with conditional simulation of declustered tail events. Three-dimensional Direct-IFORM contours are constructed in a transformed space that represents current speed, response magnitude and relative direction, without specifying a global parametric model for their joint distribution. The fitted models indicate important directional variations in the tails of both response variables, with significant wave height and orbital velocity attaining their largest contour values under different current regimes. These contours provide candidate environmental states for subsequent response-based design analyses of tidal-stream turbines.