Nonlinear evolution of focused wave groups on an uneven seafloor

Abstract

Abrupt depth transitions (ADTs) have been identified as a potential trigger for the occurrence of extreme waves. However, the nonlinear propagation of focused wave groups over ADTs has been largely overlooked in existing research. This study focuses on the nonlinear dynamics of focused wave groups as they traverse a submerged step, aiming to assess the impact of ADTs on the evolution of nonlinear wave groups. Laboratory experiments are used to validate a fully nonlinear potential flow (FNPF) model for wave group propagation over constant water depth and a submerged step. Harmonic analysis is conducted to evaluate the significance of higher harmonic elevations, while spectral analysis reveals that the second and third harmonics reach their peak values at the midpoint of the submerged step. Compared to conditions without steps, the presence of ADTs results in increased skewness and kurtosis, leading to greater asymmetry in the free surface profiles. Additionally, with the same length of submerged steps, the presence of multiple ADTs enhances reflection capabilities, thereby reducing wave energy.