Uncertainty assessment of significant wave height return levels downscaling for coastal application

The computation of extreme waves near the coast is often complicated by the lack of extended time-series in shallow waters. As such, wave parameters related to long return periods are usually estimated starting from their offshore counterparts, either by i) computing return levels of wave parameters offshore and next propagating design waves toward the coast; or ii) downscaling the whole time-series of offshore data first, computing extreme waves at the coast at a second time. In this contribution, we comparatively apply the two methods at the Son Bou Beach in Menorca (Balearic Islands, West Mediterranean basin) to assess the uncertainty stemming from waves downscaling (that is, to perform the Extreme Value Analysis prior or after the propagation of waves to the shallow waters). Results show that the two approaches yield differences in the 100 years wave height as high as ≈1 m, suggesting the need to carefully account for this source of uncertainty in the preliminary design of coastal structures and coastal hazard assessment.