Wave forces on a U-OWC breakwater

In this paper we deal with the mean pressure distributions of the highest peaks of the horizontal wave force process on a REWEC3 absorber breakwater, recorded during a small-scale field experiment. The REWEC3 (also named U-OWC) is a breakwater embodying an OWC with a small opening. The wave-beaten wall of the central caisson was equipped by a vertical row of nine pressure transducers allowing to measure the wave force on the breakwater, and by an ultrasonic probe for measuring the surface displacement in front of the breakwater. Other gauges are placed in two piles in the undisturbed wave field and inside the air chamber, for measuring the performances of the plant. The average of the highest 1/10, 1/100 and 1/1000 wave pressure distribution of the horizontal positive peaks (crests) and the negative peaks (troughs) have been obtained by recording about 1600 sea states. These distributions have been compared with those obtained by theoretical models for the design of vertical breakwaters proposed by Boccotti (linear wave theory) and Goda. We found that both models generally overestimate the pressure distribution (the linear wave theory not as much) probably owing to the alternating vertical water flux that is produced inside the plant by waves hitting the wall. As a consequence, the extreme wave force on a U-OWC, assumed for the design of vertical breakwaters, is almost 22% smaller than one on a traditional upright breakwater. For all these reasons, the linear wave theory can be used for the preliminary design of REWEC3 breakwaters