Gravity wave interaction with cage enveloped breakwaters using DBEM

Dual boundary element method-based numerical investigations are carried out on the wave reflection, transmission, and dissipation characteristics of a breakwater made of a porous cage filled with rocks. To assess the hydrodynamic performance, we analyzed three different configurations (namely the conventional trapezoidal shape, rectangular, and inverted trapezoidal) by keeping the volume of rocks fixed for all configurations. An inverted twin trapezoidal wave barrier is found to be the efficient one to reduce wave transmission. For the practical range of relative water depths in the field, it is found that the twin inverted trapezoidal breakwater is almost 25%-35% better for reducing wave transmission when compared to the conventional single trapezoidal rubble mound breakwater. It is found that increasing the number of wave barriers in the tandem arrangement helps to reduce wave transmission. To achieve a wave transmission coefficient less than 0.4 for the practical range of relative water depth, it is required to use h1/h>0.8${h}_1/h > 0.8$ for the twin inverted trapezoidal breakwater, where "h(1)" is the height of the breakwater and "h" is the water depth. The wave transmission is found to be minimum when the porosity of the rock is closer to the natural porosity of rocks, that is, 0.4-0.5. If the relative spacing between the inverted trapezoidal breakwaters S/h$S/h$ were between 1 and 2, the wave transmission decreases to an extent of 5%-10%. The present study is useful for better design and construction of offshore breakwaters for different wave-damping applications.