Improving the Hydraulic Performance of a Composite Perforated Caisson Breakwater by Changing the Front Face Geometry

Breakwaters, as port protection structures, are of considerable importance in creating a calm basin and preventing damage to equipment and structures in the port. Depending on the water depth and project conditions, the use of various breakwaters is recommended, including the use of composite caisson breakwaters in deep waters. In these structures, a rubble mound under the caisson provides a reliable setting for the caisson, while the vertical caisson faces the waves directly. The hydraulic performance of composite caisson breakwaters can be improved by creating some holes in their front face to absorb and dissipate wave energy in an efficient way. In the present study, in addition to creating holes in the front face of the caisson, the geometry of this face has also been changed to find an optimum hydraulic performance of these structures. For this purpose, FLOW3D, a well-known numerical model, is employed and the model is at first validated by comparing the results with the measured results in the experimental study performed by Lara et al. (2018). The time series of the simulated surface elevation as well as the calculated pressure spectra show good compatibility with the measured data that confirms the ability of the utilized numerical model in well prediction of the hydraulic performance of composite caisson breakwaters. In total, five breakwater models with different geometries based on both the hole and the front face shapes were considered in this study by considering the limitations of construction activities. These cases were then subjected to different wave conditions. In addition, irregular waves with different wave heights and periods were simulated in the models, and the effects of water depth on the calculated overtopping rates were also investigated. To capture wave irregularities and random occurrence of high waves, the cumulative wave overtopping discharge in addition to the mean overtopping value was reported and investigated during the simulations. According to the results obtained from numerical modeling, making the two proposed changes altogether i.e. using holes besides the face modification simultaneously improves the performance of the caisson more than when one of these is applied. Based on the calculated overtopping rate, reflection coefficient and applied wave pressures on the caisson, it can be concluded that the proposed modifications can significantly improves the hydraulic performance of these structures. Among the studied geometries, the best performance was finally obtained for the new proposed caisson with trapezoidal teeth and holes on its front face. Once the holes dissipate wave energy through penetrating to the porous breakwater, the toothed shape improves the wave reflection through phase lags. Based on the results, the optimal model can reduce the average cumulative overtopping by 60%, the average maximum overtopping by 68%, the reflection coefficient by 25%, and the average maximum pressure in the three pressure gauges investigated by 15.5, 6.9, and 0.8% compared to the simple vertical caisson without any hole. Since the investigated models are compatible with the construction limitations, (providing the required weights for satisfying the sliding and overturning stability of caisson breakwaters), the outcomes of this study can efficiently help coastal engineers to design structures satisfying the required hydraulic performance with an optimum cost.