Behaviour of a converging-channel breakwater; theory and experiment

Abstract

The energy-transmission characteristics of an array of open convergent-channel-type wavebreakers to a normally incident wavetrain is studied experimentally and theoretically. A model of the wavebreaker transmission response to a sinusoidal incident wavetrain on a constant-depth ocean is developed as a boundary-value problem within the framework of linearized water-wave theory. This results in an integral equation based on a Green-function solution to the Helmholtz equation, which is solved numerically for each of several wavebreaker geometries over a range of incident wavelengths. Experiments on fixed model wavebreakers performed in an open wave tank are described, and results are compared with the numerical predictions. Theory and experiment indicate that the convergent-channel design is effective in attenuating the transmitted wave at resonant values of the incident wavelength λ that scale on the channel length L. Forces experienced by each half-wavebreaker are largely transverse; the calculated longitudinal forces for the fixed breakwater are small compared to those on a solid wall except at one resonance near L/λ = 0.5. Experiments indicate that this last condition does not occur for a floating breakwater, where the corresponding transmission of wave energy is high.