Regular-Irregular Wave Pressures on a Semi-Immersed Breakwater
In the present study, the hydrodynamic interaction of regular and irregular waves with a semi-immersed breakwater is examined experimentally in a large-scale facility. The experiments were conducted in the wave flume of Laboratori d’Enginyeria Marvtima (LIM) at Universitat Politècnica de Catalunya (UPC) in Barcelona. The wave pressures and the vertical force acting on the structure are examined. Experimental results concerning wave energy reflection-transmission-dissipation are also presented. Two different configurations are examined: a) fixed semi-immersed breakwater and b) heave motion semi-immersed breakwater. Wave energy reflection-transmission-dissipation analysis reveals that the heave motion model performs in a dissipative manner contrary to the fixed breakwater, which performs in a reflective manner. It is deduced that maximum pressures are exerted on the seaward submerged edge of the structure, while the pressures acting on the seaward vertical face of the structure are much higher than the corresponding pressures acting on the downstream vertical face, due to wave energy dissipation. It was also revealed that wave-induced hydraulic pressures at the bottom were dependent on the wave height and the wave length. Detailed uplift pressure measurements reveal the uplift pressure distribution acting on the submerged keel of the breakwater for both configurations examined. Due to the complex shape of the uplift pressure distribution, it is noted that it should be at least be digitized by four points in order to finally produce, after integration, the uplift force acting on the breakwater. Hydraulic pressures linearly increased according to the wave height. Concerning the influence of wave length, hydraulic pressures acting on the fixed breakwater present a maximum, around a value of 0.08 for B/L=d/L, decreasing almost linearly for higher values of B/L=d/L. The inverse phenomenon is observed in the case of the heave motion breakwater, concerning the influence of wave length. For higher values of B/L=d/L (lower L values), pressure increased almost linearly. It is deduced that the vertical force acting on the fixed structure is 11% on average higher than the corresponding one acting on the heave motion breakwater. Quasi standing waves are observed in the seaward face of the breakwater in the case of regular waves for both fixed and heave-motion models. Slightly breaking waves are observed in the seaward face of the breakwater in the case of irregular waves for both fixed and heave-motion models. Wave spectral analysis in front of the structure reveals the existence of higher harmonics only in the case of irregular waves confirming the existence of slightly breaking waves in front of the structure. That is why irregular waves result in the production of a slight breaking impact waves on the structure with higher pressures and forces.
Keywords: Semi-immersed breakwater, pressure field, regular-irregular waves, vertical force