The occurrence and effects of wave impacts

The determination of design parameters for coastal structures has historically often involved the definition of maximum water levels, the highest significant wave height and the longest wave periods. The design then proceeds with these values using numerical models, direct design methods or physical model studies. Design studies have often been conducted with maximum water levels, using random seas with previously determined parameters, a chosen wave spectrum and a sufficiently large number of waves. Parametric analysis methods using multiple parameters have identified that different combinations of water level; wave condition; perhaps fluvial discharge; or sea bed condition; may all influence the response. Recent research into the most important responses of vertical structures (plunging wave impact, toe scouring, overtopping and overtopping-induced loading) indicates that these mechanisms are all linked to the occurrence of violent wave impacts. Wave breaking is a depth-limited phenomenon, with plunging breakers occurring only for a limited range of wave height to water depth ratios. For given storm wave heights, but tidally varying water levels, this wave-breaking regime will often only prevail for a limited amount of time, leading to the conclusion that the duration of maximum exposure, namely the length of time that a coastal structure is exposed to the most important wave-induced damage mechanisms, is a function of the combined probability of (at least) water level (tidal + storm surge) and wave height/storm profile characteristics. This may have significant consequences for the definition of critical design parameters and the parameter variations employed in model tests.