Evolution of irregular wave shape over a fringing reef flat

Abstract

The main objective of this paper was to examine the influence of forereef bottom slope and the effects of coral degradation on the transformation of wave shapes over a fringing reef flat. Based on verification from a physical experiment, the numerical model FUNWAVE 2.0, based on the fully nonlinear Boussinesq equations, was employed to simulate irregular waves propagating over fringing reefs of different topographies. Empirical formulae relating wave skewness and asymmetry to local Ursell numbers were extended to three dimensions, including the effect of forereef slope and roughness of reef surface. Results demonstrated that the gentle forereef slope will lead to a larger wave asymmetry on a reef flat. Otherwise, wave asymmetry on a smooth reef bed was slightly larger than that on a rough bed, indicating that coral degradation may increase wave asymmetry over the reef flat. It was found that the different evolution of wave asymmetry on the reef flat could be primarily attributed to the different infragravity wave motions, which were induced by different forereef bottom slopes. Moreover, coral degradation would increase the asymmetry of infragravity waves, leading to some differences in wave asymmetry on the reef flat. However, wave skewness from different types of fringing reefs was almost identical, implying that the aforementioned factors have a negligible effect on wave skewness.