Space-time statistics of extreme ocean waves in crossing sea states

Abstract

The study of extreme ocean waves has gained considerable interest in recent years, due to their importance for offshore design and navigation safety, and several theoretical approaches have been developed for their statistical description. However, in the case of crossing seas, where two or more wave systems of different characteristics are present, a full understanding of the main physical mechanisms responsible for the occurrence of very high individual waves is still lacking. As a consequence, the prediction of extremes in such conditions currently relies on integrated parameters of the total sea state, such as the spectral wave steepness. In this study, to gain further insight into the role of the crossing wind sea and swell wave systems in producing extreme individual waves, we investigate realistic sea states during typhoon Kong-rey (2018) using an ensemble of numerical simulations obtained from a phase-resolving wave model based on the High-Order Spectral (HOS) method. The reliability of the numerical fields is assessed for the first time with stereo wave measurements of the sea surface elevation field collected from an offshore platform in the area of interest. We show that, in specific conditions, space-time extreme crest heights in crossing seas can be larger than in unimodal seas due to second-order bound wave interactions between the wind sea and the swell. To improve existing prediction capabilities, we propose a novel formulation for the wave steepness in crossing seas, which includes nonlinear effects up to the second order and accounts for the spectral parameters of the interacting wave systems.

Observations of wave run-up affected by dune scarp during storm conditions: a two dimensional large-scaled movable bed experiment

“…A stereo camera view can reconstruct the sensor surfaces in 3-D and provide a more extensive dataset in terms of spatial and temporal dimensions than what can be collected at fixed point wave gauges. Indeed, the stereo imaging approach has been applied to measure spatial and temporal data of ocean waves (Benetazzo et al, 2018(Benetazzo et al, , 2021Bergamasco et al, 2021;Pistellato et al, 2021;Davison et al, 2022).…”

Characterization of extreme wave fields during Mediterranean tropical-like cyclones

“…Indeed, when a crossing sea state is present, the formation of extreme waves depends on additional factors, such as the relative crossing angle between the two coexisting wave systems or their frequency ratio, which affect the strength of the wave-wave nonlinear interactions (Adcock et al, 2011;McAllister et al, 2018). To account for such effects, Davison et al (2022) recently introduced a parametrization of the crossing wave steepness, which is based on the spectral characteristics of the two wave systems (mean wavenumber and spectral bandwidth, while the effect of directional spreading is not explicitly included) and on their local crossing conditions, which are characterized using three main parameters defined by Rodriguez et al (2002) and Guedes Soares and Carvalho (2003): the Sea-Swell Energy Ratio (SSER), which represents the ratio of the variance associated with each wave system, the Intermodal Distance (ID), representing the frequency difference between the swell and wind waves spectral peaks, and the relative crossing angle (Dq). These crossing sea parameters are expressed as follows:…”

“…The wave steepness parametrization for two quasimonochromatic crossing wave groups (hereinafter referred to as m crossing ), stemming from Davison et al (2022), can therefore be written as:…”

“…We note that, compared to the formulation proposed by Davison et al (2022), in the present work we consider the case of two in-phase wave systems (f W = f S = 0), so the simple value of the superharmonic interaction kernel is taken instead of its absolute value, thus leading to a smaller increase of the wave steepness values for large crossing angles, where B + is usually negative.…”

“…The goal of this analysis is, on the one hand, to characterize the extreme wave field during a Medicane for the first time, comparing it with the field produced under the more typical case of a tropical cyclone. On the other hand, we aim at investigating the influence of crossing seas generated during the selected events on the magnitude of extremes, using a parametrization recently proposed by Davison et al (2022).…”