Attenuation of Random Deep Water Waves by a Porous Walled Breakwater

The porous walled resonating chamber, a type of breakwater, is investigated as a means of reducing reflected waves from structures subjected to random wind generated waves in deep water. Extending laboratory monochromatic studies to a full-scale apparatus appended to a floating bridge allows assessment of scale factors and the effects of random waves on the predicted performance of the device as a linear damped oscillator. The full-scale device is shown to be frequency selective at a frequency precisely corresponding to the predicted resonance of the system. The forces, measured on the porous wall, are lower than predicted and the device completely eliminates the problem of wave runup onto the bridge roadway.

The method of evaluating the energy attenuation by the breakwater incorporates spectral analysis of digitized data recorded at fixed locations equidistant in front of the chamber and at a remote station away from the influence of the breakwater. Analysis demonstrates that the time average energy density at a fixed location where incident and reflected waves co-exist is influenced not only by the wave amplitudes, as expected, but also by the product of the amplitudes and a function of the phase angle. The chamber effects a change in the random phase angle during reflection , producing a different effective distance of wave travel to the fixed location and thus negating quantitative analysis of the energy dissipation.

Publication Date: 
Wednesday, August 1, 1973
Publication Number: 
WA-RD 013.1
Last modified: 
10/12/2016 - 15:41
E.P. Richey, D.B. Morden, B.J. Hartz.
University of Washington. Dept. of Civil Engineering
Number of Pages: 
Breakwaters, Damping (Physics), Frequency (Electromagnetism), Pontoon bridges, Resonance, Water waves, Wind.