Numerical modelling of wave transformation and runup over rough fringing reefs using VARANS equations

Computational Fluid Dynamics (CFD) models are gaining popularity recently in efforts to understand the complex dynamics of wave interaction with coral reef profiles. To provide an alternative approach to the current CFD models to account for the reef surface with large bottom roughness, a 3D numerical wave tank based on the CFD tool OpenFOAM (R) is developed in this study. The VARANS equations are solved for two-phase incompressible flow with the k-omega SST model for the turbulence closure and VOF method for tracking the free surface. The reef surface with high friction is modeled by using a porous media model in the VARANS equations. The numerical model is first validated with laboratory experiments. The model is then applied to evaluate the impacts of forcing condition (wave and water level), reef morphology (reef-flat width, fore-reef slope, and back-reef slope), and reef roughness on wave transformation and wave runup. Moreover, an empirical formula is derived from the numerical results to predict wave runup. Subsequently, the simulated cross-reef variations of energy dissipation, wave spectrum, wave shape parameters (skewness and asymmetry) and nonlinearity parameter (Ursell number) are discussed for both the smooth and rough reefs. Finally, the cross-reef variations of the TKE and its dissipation rate are also examined for both reefs via the numerical simulations.

Automated summary

In this study, a 3D numerical model based on a numerical wave tank was developed to investigate the interaction between waves and coral reef profiles. The model was validated through laboratory experiments and used to analyze the impacts of forcing conditions, reef morphology, and reef roughness on wave transformation and runup. An empirical formula for predicting wave runup was derived from the numerical results. The study also examined the cross-reef variations of energy dissipation, wave spectrum, wave shape parameters, nonlinearity parameter, TKE, and its dissipation rate for both smooth and rough reefs.