Wave attenuation and focusing by a parabolic arc pontoon breakwater

In the multifunctional system consisting of point absorber wave energy converters and a pontoon breakwater, the breakwater plays an essential role in attenuating waves on the lee side and amplifying waves for a better energy harvesting on the stoss side. The structure of breakwater is expectedly improved to enhance its wave attenuation and amplification at the same time. Here we present a novel parabolic arc breakwater and show that for a range of typical regular incident waves, it can attenuate more wave elevation and focus high waves in several regions in comparison to a straight breakwater. In further frequency-domain investigations, a special relatively-low frequency associated with the parabolic arc breakwater configuration is found and named as the critical frequency, closed to which splendid attenuation and focusing performance can be achieved. A systematic parametric study on the geometric factors (draft, width, and chord length) of the parabolic arc breakwater is thereafter carried out to examine their influence on the attenuation and focusing performance at the critical frequency. We find that an increase of the draft can reduce the critical frequency greatly so as to let it be within the real sea states, meanwhile slightly affecting the attenuation performance. An increase of the chord length has an uncertain but not large influence on the attenuation performance, whereas it enhances substantially the focusing performance. Simultaneously, an amplification rate up to 3.06 in two relatively-large focal areas in a prescribed deployment zone and an average attenuation rate of approximately 68% in a prescribed protection zone could be obtained in a commonly observed coastal wave.