Modelling of Bimodal Wave Overtopping of Coastal Defences

Wave overtopping from a few hours of violent storms can cause damage to infrastructure. Extensive knowledge is available on the prediction of wave overtopping of smooth, sloping and vertical impermeable seawalls. However, existing prediction methods provide less certain predictions when the sea state has a bimodal character, such as when there are strong components of both wind and swell. This thesis improves wave overtopping study of seawalls under storm-driven bimodal seas. Firstly, we developed an energy-conserved bimodal spectrum which contains the same energy as the unimodal spectrum derived from a sea state for easy comparison. The peak periods of the bimodal spectrum is occurring at four different spectra peak periods (11, 15, 20 and 25 secs, respectively), swell magnitude is defined with percentages (0, 25, 50, and 75 percentages). Both the unimodal and the bimodal spectra were used to perform wave overtopping experiments in a laboratory tank and numerical flume. Different bimodal spectra were created from the same sea state but the peak periods of the swell in the resulting bimodal spectrum were varied while retaining the same energy content. Bimodal wave sequences are derived from these spectra and used to investigate the overtopping of sloping seawalls. The reflection performances of the impermeable slopes were analysed in present study. The dimensionless overtopping rate for a range of seawalls was measured for bimodal storm conditions of a minimum of 1000 waves. Results for unimodal conditions agree closely with predictions using the EurOtop formulae. The current standard suggested in EurOtop (2018) underpredicts the observed wave overtopping rates for bimodal conditions even when using the recommended parameters for such cases. Under these conditions, it was found that there is a dependence between mean overtopping rate and swell occurrences. New sets of empirical influencing factors to describe these dependencies have been determined in terms of seawall designs at locations that are exposed to local storm waves and open Atlantic oceans