Ocean surface waves and interactions with coastal structures
Authors:   , Wang Xin 
Published: July 2024

Ocean waves interact with the coast and offshore structures with various ways, and it is difficult to describe and quantify the influence of waves on the coast. In this thesis, several important wave processes have been investigated, including generation and propagation of tsunamis, run-up of different types of ocean waves and breaking wave impact on offshore structures. Numerical, experimental and analytical approaches were used to explore the behaviour and influence of different types of waves. To address the challenges around the numerical simulation of two-phase flow with large density ratio, especially the numerical instability and spurious current developed across the fluid interface, a numerical algorithm was developed to enhance the conservation of momentum. The proposed algorithm involved the momentum-based velocity reconstruction technique and strong temporal coupling between flow field resolving and interface capturing. Idealized cases and experimental cases were used for the algorithm validation, which proved the improvement on accuracy and numerical stability. The proposed algorithm was applied to the numerical study of freak wave impact on a box-shaped structure at different locations, enabling a comparative analysis of the kinematics and dynamics on the faces of the structure. Different impact patterns were identified, and the characteristics of each pattern were investigated. The impact processes on each sidewall of the structure were explored. Air entrapment and downward suction force were observed during the impact, and their roles and influences were analysed. It is found that the proposed algorithm can well reproduce the details of the freak wave slamming, and the front wall and bottom wall highly suffers from the impact pressure and forces. Negative pressure was found on the top and bottom walls, which could be additional risk for the structural safety. The morphology of the incident wave significantly influenced the breaking behaviours. For the generation and propagation of tsunamis, an extensive series of laboratory experiments were conducted, involving four different water depths with unimodal and bimodal bed movements. The nature of the wave generation and propagation were characterised using the disturbance-amplitude scale (α) and disturbance-size scale (δ). New analytical solutions for the velocity potential generated by vertical bed movements were derived, and the applicability of the linear solutions were investigated with the validation against the experimental results. The analytical solutions were validated by the experimental data and showed good performance. High linearity was found for the cases with α < 0.25 while the linear theory was not applicable for the cases with α > 0.5. For the study of wave run-up, three series of comprehensive experiments were conducted to study the run-up height of laboratory-generated tsunamis, bichromatic wave groups and bimodal wave spectra. For tsunamis, the characteristics of the run-up heights were discussed for different water depths, which were connected to the properties of the wave generated by the vertical seabed movement. The increasing water depth led to a stronger linearity, and the leading crest became less pronounced while the following regular-like wave train became more pronounced when climbing on the beach. For bichromatic waves, more than 600 cases of bichromatic wave were examined, to explore the parameters’ (e.g., wave height, water depth, phase difference, wave period, beach slope angle, etc) influence on the run-up height, and the experimental data was compared with the predictions of empirical formulae. The formula of Wu et al. (2018) had provided good predictions for the run-up heights, while the wave-wave interaction led to the overestimation for some cases. For bimodal waves, the bimodal wave spectrum with different swell percentage and swell specific period were established, whose energy were equivalent to the JONSWAP unimodal spectrum. The validity of EurOtop formula for predicting random wave run-up was investigated, and the influences of swell percentage, swell specific period and beach slope angle were studied. Throughout the extensive research of different types of waves and their interaction with sloping beaches and offshore structures, a better understanding of ocean waves has been achieved, new numerical algorithms have been developed, and the analytical theories have been extended.