Hydrodynamic Mechanisms of Tsunami-like Wave Mitigation by a Submerged Horizontal Plate

Tsunami-like waves pose a serious threat to coastal infrastructure, necessitating effective mitigation measures to reduce inundation and associated damage. This study uses a numerical wave tank (NWT) to investigate the hydrodynamic mitigation mechanisms of a submerged horizontal plate (SHP) under tsunami-like wave conditions. Long-period solitary waves are used to represent tsunami-like forcing. The NWT is validated against hydraulic experiments for wave profiles and hydrodynamic coefficients, including reflection, transmission, and dissipation. Using the NWT, the spatiotemporal evolution of wave, flow, and vortex fields, together with momentum flux, is analyzed for various SHP geometries, placements, and incident wave conditions. The results demonstrate that SHP performance is not governed by a single nondimensional parameter but by the combined effects of breaking regime and the distribution of flow above and below the plate. Although stronger breaking increases energy dissipation, momentum flux attenuation does not increase monotonically under plunging breaker conditions because dissipation efficiency saturates. In contrast, a stable and efficient reduction of transmitted momentum flux is achieved within the collapsing breaker regime. These findings highlight the importance of optimizing SHP geometry and placement to induce efficient breaking prior to saturation, providing practical guidance for tsunami-like wave mitigation design.