Recent work by McAllister et al. (2018) [1] has experimentally confirmed that the set-down of the wave-averaged free surface, first described by Longuet-Higgins and Stewart (1962) [2], can turn into a set-up when wave groups are sufficiently spread or cross at large angles. Experimental results were shown to agree well with second-order theory, including frequency-sum and frequency-difference terms, where the latter are responsible for the wave-averaged free surface. In this paper, we review these experimental results and examine theoretically the magnitude of the wave-averaged free surface in realistic extreme North Sea conditions. Specifically, we examine the role of the shape of the spectrum, water depth, and the relative magnitude of the peak frequencies of the two crossing groups. We find that having a realistic spectrum (JONSWAP vs. Gaussian) considerably enhances the magnitude of the second-order contribution, the total second-order signal increases with decreasing depth and can display a maximum provided the water depth is shallow enough for small to moderate degrees of spreading or crossing angles and is larger for spectral peaks that are further apart.

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