Variation of Wave Forces Along a Semi-Infinite Breakwater Due to Wave Diffraction

Abstract

This paper investigates distribution of wave forces along a semi-infinite breakwater considering wave diffraction using the analytical solutions derived from linear wave theory. It is found that the total wave amplitude of the incident, the reflected waves, and the diffracting waves fluctuate at a position away from the energy discontinuity lines. Thus, wave amplitudes on the front side of the breakwater would also fluctuate from wave amplitudes of superposed incident and reflected waves. There exists 180° phase difference between the front- and lee-side diffracting waves. Thus, the total force on both the front and lee sides of the breakwater would be greater than the force on the front side. For monochromatic waves, maximum total wave force considering wave diffraction is found to be 1.34 times greater than that neglecting wave diffraction. In this case, analytical solutions compare well with numerical solutions of the mild-slope equation, which considers wave diffraction properly. It is also found that the fluctuation of unidirectional random wave forces along the breakwater is reduced due to diffraction of multifrequency waves. The fluctuation of multidirectional random waves is further reduced due to diffraction of both multifrequency and multidirectional waves. Forces of multidirectional random waves with different asymmetry parameters are also investigated. Two real sliding failures are found to occur at the areas where relative wave forces considering diffraction are more than unity. This implies that wave diffraction can be one of the causes for sliding failure of vertical breakwaters.