Effects of the reef roughness on the harbor oscillations induced by low-frequency waves

Low-frequency waves can be captured and amplified within harbors and present a prevalent inducing factor of harbor oscillations. Effects of the rough reef surface and the possible inhomogeneity of the roughness on the oscillating behaviors within an elongated harbor on reef topography are investigated based on numerical simulations. The reef roughness is applied via hydraulic roughness coefficient within the well-recognized empirical range and the inhomogeneity is arranged following the probable location of the coral covers and the possible engineering inventions. With wavelet transformation analysis and bispectral analysis for inspecting energy distribution of oscillating components and their interactions, the lowest oscillation mode is found to be more influenced by the rough extents of the reef surface whereas higher modes, by the location of roughness. The friction induced by rough surface in the vicinity of reef crest is revealed to be effective in reducing the oscillating behaviors by acting on the generation of low-frequency waves on the reef by the incident short ones. In addition to the damping effects of the reef roughness, the transition from smooth reef surface to rougher one may also lead to evolutions of coupling between different oscillating components and consequently affect oscillating energy distributions.