Wave impacts on vertical cliffs: Insights from laboratory experiments and field observations

Many engineered and natural coastal structures suffer frequent damage from wave impacts. Motivated by field observations of wave impacts on coastal cliffs, we undertake controlled physical experiments with synchronised measurements of wave properties and their impacts on a vertical structure representing a cliff. Laboratory experiments using focused wave groups show that, in the investigated scenarios, large impacts occur within a relatively narrow phase range ($-$30° to 35°), covering $\sim$20% of the full phase range. Our irregular wave experiments indicate that $\sim$30% of waves break onto or near the cliff face, although not all these breaking waves result in large impacts. Irregular wave tests can produce large impacts due to amplitude modulation. In contrast, lacking this modulation, regular wave tests did not generate similarly large impacts. Reanalysis of field data using the same methodology confirms that intermediate values of wave heights at a given water depth (and intermediate values of water depths at a given wave height) produce the most breaking wave impacts and the highest ground motions. Additionally, we analyse the average shapes of large vibrations and the waves generating the associated impacts in both laboratory and field settings. The findings show that, on average, the offshore wave profiles are nearly identical for large impacts. However, the resulting ground motions still exhibit substantial variability in magnitude. These findings underscore the complexity of wave impacts on coastal cliffs and suggest that sea-level rise can shift impact regimes, potentially either exacerbating or mitigating subsequent coastal erosion.