Wave Reflection and Transmission at Permeable Breakwaters: Fid-Bau Portal

Wave Reflection and Transmission at Permeable Breakwaters

C. K. Sollitt / R. H. Cross

Rubble-mound breakwaters are designed to protect exposed marine areas from excessive wave activity. Observations of breakwaters interacting with surface waves in laboratory models and in full-scale field applications demonstrate that significant wave energy is transmitted through the interstices of structures commonly regarded as being impervious. The objective of this investigation is the development of a theoretical analysis to account for this phenomenon. The results are intended for use by coastal engineers to compare the effectiveness of alternative breakwater configuration, independent of repetitive experimental programs. Three breakwater configurations are considered: (a) crib-style breakwaters with vertical walls and homogeneous fill; (b) conventional trapezoidal-shaped breakwaters with layered fill; and (c) pile-array breakwaters composed of vertical piles placed in symmetric patterns. The two-dimensional problem is studied. Waves are assumed to arrive at normal incidence. The theoretical development begins with the unsteady equations of motion for flow in the voids of an arbitrary porous structure. An experimental program is conducted to verify the analytical models. Theory and experiment yield the following general conclusions: (a) the transmission coefficient decreases with decreasing wavelength, breakwater porosity and permeability, and increasing wave height and breakwater width; (b) the reflection coefficient decreases with increasing wavelength, breakwater porosity and permeability, and decreasing breakwater width.