Flow mechanism of impulsive wave forces and improvement on hydrodynamic performance of a comb-type breakwater

The interactions between waves and a comb-type breakwater (CTB) are numerically simulated in a 3D numerical wave flume, which is based on an internal wave generation method. The comb-type breakwater (CTB) is a new type of gravity breakwater and evolved from the conventional caisson breakwater, with part of the rectangular caisson being replaced by a thin side plate. Thus, a chamber is formed by the side plate and the bottom of superstructure between two adjacent rectangular caissons. It is found that impulsive wave pressure on the CTB is mainly induced by the chamber, which can be simplified into a vertical wall with a horizontal cantilever slab in the 2D cross-section. The synchronous analyses on wave profiles, velocity vectors, vorticity contours and wave pressure distributions are conducted to reveal the flow mechanism of the impulsive wave force. In the previous studies, the impulsive wave pressure on a vertical wall with a horizontal cantilever slab was merely observed under breaking or broken wave conditions. However, in the present results, the impulsive wave pressure was also observed on such structure under non-breaking waves. The impulsive wave force occurs when the incident wave height is comparable to the clearance between the still water level and the bottom of the super structure for non-breaking waves. Then, a non-dimensional governing parameter, which included the effects of the water depth, the bottom of the superstructure and the incident wave height, was proposed to quantify the critical conditions for impulsive wave force. Finally, a concept designs of openings on the bottom of the superstructure is proposed to reduce the impulsive wave force. The results show that even a 20% opening on the bottom of superstructure can reduce the maxima of impulsive wave pressure by up to 40%.