Physical modelling of wave group interactions with a vertical wall

Abstract

Coastal communities are becoming increasingly exposed to wave-induced flood events due to climate change. Accurate prediction of wave-structure interactions are vital in order to achieve long lasting and economical defences for such communities. Amongst the interactions of importance for design are wave runup and force. This thesis describes data from experiments in laboratory flumes. The novel contribution is with application and validation of second-order wave generation for experiments in shallow to intermediate water depth. The thesis builds on a substantial body of work by coastal engineers to validate focused wave groups, established in offshore engineering, for coastal experiments. Free error waves, which significantly contaminate first-order generated experiments, are mitigated herein. This work is of importance as the influence of error waves are inherently more problematic in shallow water depths (k0d < 1). The efficacy of two second-order wave generation theories is investigated. First, a frequency-based approach is applied to a wavemaker at Swansea University with partial success. The method mitigates sub- and super-harmonic error waves but leads to the introduction of additional sub-harmonic energy due to unwanted periodic paddle displacement. Furthermore, the computational effort required for this application is shown to be impractical for experimental purposes. Second, a time-based approach is applied to a long-stroke wavemaker at the University of Plymouth. This approach gives a non-periodic displacement signal, which mitigates the unwanted sub-harmonic energy characteristic of the frequency-based approach. Focused wave groups with different bandwidths, peak frequencies and steepness are produced in relative depths of k0d = 0.5 : 1.2. Error waves are mitigated at all bandwidths and depths and the measured subharmonics show close agreement with theory. Finally, the implications of using second-order wave generation are investigated for run-up and dynamic wave force on a vertical wall. Comparison between first- and second-order generated groups reveals that sub-harmonic error waves are increasingly important in shallow depth, increasing wave run-up by up to 67% and dynamic force by up to 75% at k0d = 0.6. This finding aligns with published numerical predictions of the implications. The methods outlined herein are a useful development for researchers and practicing engineers looking to use focused wave groups for wave-structure response investigations in water depths typical of the coast.