Hydrodynamics of an idealized reef-lagoon-channel coastal system: A three-dimensional CFD simulation

A three-dimensional (3D) numerical model that solves the Reynolds-Averaged Navier–Stokes (RANS) equations coupled with k−ω turbulence closure and VOF method is developed with the open source CFD tool OpenFOAM to simulate the hydrodynamics in an idealized reef-lagoon-channel coastal system. Comparison with experimental measurements shows satisfactory agreements in terms of the wave transformation, wave-induced setup and the mean current. The validated simulation result is further analyzed to investigate the detailed wave transformation, mass and momentum transport processes in the reef-lagoon-channel system, where both the differential and integral form of the depth-integrated time-averaged momentum equations are applied. The mass balance shows that the cross-shore flow on the reef flat is almost uniform on cross-shore transects away from the channel, while the lagoonal flow gradually increases in strength as it approaches the channel. The cross-shore momentum balance shows that the excess momentum flux and hydrostatic pressure forces dominate in the shoaling and breaker zones as expected, while the bottom friction force plays a more important role in the breaker zone than previous studies assumed. On the reef flat where the wave forces become weak, the integral form of the momentum balance shows advantages over the differential form due to its higher accuracy, which enables us to identify that the bottom friction and hydrostatic pressure forces dominate the momentum balance on the reef flat. Finally, the reliability of the differential and integral form of momentum equations and the effect of a depth non-uniform current on the excess momentum flux are discussed.