Hydrodynamic characteristics of offshore and pile breakwaters

• Wave interaction with offshore and pile breakwaters are studied by a combined method.
• Formulas of the reflected and transmitted waves are obtained by Rayleigh expansion.
• For ratio of grating constant to wavelength 2b/L>1, thickness of structures are not important.
• The flow resistance and thickness of structures are important factors when 2b/L<1.

A combined method is presented to study the wave interaction with an offshore or pile (cylinder) breakwater in finite water depth. The influences of the fluid viscosity, the thickness of the structure and evanescent waves on the hydrodynamic behaviours are included in the model. Rayleigh expansion is used to describe the reflected and transmitted waves. The accuracy of the present formulas is verified by a comparison with existing results. The wave reflection and transmission coefficients predicted by the present model agree with the theoretical results and experimental data in open literature. A ratio of the grating constant, 2b (the distance between adjacent gaps), to the wavelength, L, equal to one is a critical value. When 2b/L>1, the influence of the thickness of the breakwater on the hydrodynamic characteristics can be ignored, whereas if 2b/L<1, the increase of the thickness can lead to wave resonance in these gaps and to dramatically enhanced wave transmission. The diffraction evanescent waves can cause a phase shift and tune the resonance condition. The fluid viscosity can repress but not completely eliminate the enhanced wave transmission. Phenomenon of the enhanced wave transmission at certain special frequencies induced by the structure thickness should be given attention.