Abstract: This study employs the phase-resolving non-hydrostatic XBeach model (XBeach-NH) to systematically investigate the two-dimensional horizontal (2DH) hydrodynamic characteristics under regular wave forcing. The model’s performance in simulating wave propagation, transformation and breaking over an elliptical shoal was validated by comparing the model simulations with existing experimental data. Subsequently, an idealized three-dimensional reef platform is constructed and a series of numerical experiments are conducted to examine the impacts of varying incident wave conditions (including wave height, wave period, and reef-flat water depth) and reef morphological factors (including reef-flat width and reef-flat shape), on the spatial distributions of wave height and mean water level across the reef flat. Results indicate that incident wave height and wave period are the primary factors controlling the hydrodynamic response over the reef flat. With increasing incident wave height and wave period, wave refraction and diffraction are enhanced, while the mean wave height and wave setup over the reef flat increase accordingly. Reef-flat water depth is the primary factor controlling wave breaking and mean water level over the reef flat. As reef-flat water depth increases, wave refraction and diffraction become more evident, transmitted wave height over the reef flat increases, whereas wave setup decreases. Reef-flat width has an important impact on the hydrodynamic response over the reef flat. As reef-flat width increases, the wave-breaking zone is extended and wave refraction and diffraction are weakened; transmitted wave height over the reef flat decreases, whereas wave setup increases. In addition, reef-flat planform shape significantly affects the spatial distribution of wave height and the magnitude of wave refraction and diffraction. As the reef flat becomes narrower along the wave propagation direction, the wave-breaking zone is shortened, wave refraction and diffraction are reduced, transmitted wave height over the reef flat becomes more unevenly distributed, and wave setup decreases accordingly. These hydrodynamic patterns provide insights into potential sediment transport pathways across reef flats and the processes governing cay formation.

Key words: platform reef, regular wave, wave breaking, wave setup, XBeach-NH model