Wave–structure interaction and long-period wave attenuation in a wave screen composite perforated eco-breakwater

Long-period waves induce large orbital particle excursions and significantly compromise harbor operational safety. Conventional perforated breakwaters often exhibit limited attenuation under such conditions. To address this limitation, this study investigates long-period wave attenuation in a wave screen composite perforated breakwater through a combined program of laboratory experiments and numerical simulations. The structure integrates an asymmetric double-layer louver screen, a multilayer flat-plate array, and an ecological porous wall. Laboratory experiments were conducted on 16 combinations of louver arrangements, resistance serration layouts, and two ecological infill materials (coconut fiber and oyster shells). In parallel, a numerical model was established and compared with the experimental results to further analyze the internal flow behavior and attenuation characteristics of the proposed structure. Results show that the proposed structure maintains low transmission under medium and long wave conditions, with transmission coefficients below 0.37 for all configurations. The numerical results reproduce the main experimental trends reasonably well and further illustrate the internal flow-field features associated with wave attenuation. The attenuation sequential wave–structure interaction process is observed: flow separation, particle-motion redirection, and vortex generation around the seaward louver screens are followed by turbulence development and energy dissipation within the multilayer plates and porous core. Coconut fiber provides lower transmission and higher dissipation than oyster shells, likely due to its more complex porous morphology. Under long-period wave conditions, simply increasing resistance serrations does not necessarily reduce transmission, and the shape also affects the resulting transmission performance. These results provide new insight into long-period wave attenuation in permeable coastal structures.