Wave impacts on vertical seawalls and caisson breakwaters

In most developed coastal areas, seawalls protect towns, road, rail and rural infrastructure against wave overtopping. Similar structures protect port installations worldwide, and may be used for cliff protection. When a large tidal excursion and severe environmental conditions concur to expose seawalls and vertical face breakwaters to wave impact loading, impulsive loads from breaking waves can be very large. Despite their magnitude, wave impact loads are seldom included in structural analysis of coastal structures and dynamic analysis is rare, leading to designers ignoring short-duration wave loads, perhaps contributing to damage to a range of breakwaters, seawalls and suspended decks. Over the last 10 years, improved awareness of wave-impact induced failures of breakwaters in Europe and Japan has focussed attention on the need to include wave impact loads in the loading assessment, and to conduct dynamic analysis when designing coastal structures. Recent experimental work has focused more strongly on recording and analyzing violent wave impacts. These new data are however only useful if methodologies are available to evaluate dynamic responses of maritime structures to short-duration loads. Improvements in these predictions require the development of more complete wave load models, based on new measurements and experiments. Moving from a brief review of documented structural failures of caisson breakwaters and existing design methods for wave impact loads, this paper reports advances in knowledge of impulsive wave loads on vertical and steeply battered walls, based on physical model tests in the large wave flume at Barcelona under the VOWS project (Violent Overtopping of Waves at Seawalls). These data are used to support a revised simple prediction formula for wave impact forces on vertical walls. The paper also discusses dynamic characteristics of linear single degree of freedom systems to non-stationary excitation. Responses are derived to pulse excitation similar to those induced by wave impacts. Response to short pulses is shown to be dominated by the ratio of impact rise time tr to the natural period of the structure Tn. A functional relation between impact maxima and rise-times is given for non-exceedance joint probability levels. The relation is integrated in a simplified method for the evaluation of the static-equivalent design load and the potential cumulative sliding distance of caisson breakwaters.