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There has been no quantitative assessment of the wave attenuation coefficient on the coral reef flat in South China Sea (SCS) previously. Based on MIDAS DWR (Directional Wave Recorder), an in-situ experiment was carried out on a coral reef flat of Yongle Atoll, SCS. Waves and currents were measured at several locations on the reef flat, which were used to estimate wave attenuation due to bottom friction. A spectral wave model based on the action density balance equations is proposed to solve the wave attenuation coefficients on the coral reef flat, which can consider influence of currents. The wave attenuation coefficients due to bottom friction on the coral reef flat were found to be one order of magnitude greater than that recommended in most spectral wave models. Additionally, numerical simulation using a third-generation wave model SWAN was carried out. The simulation results verify in turn that the bottom friction plays a predominant role in the wave attenuation on the coral reef flat. It also captures the main characteristic of wave directional spectra with high precision. The study of the wave attenuation characteristics near islands and reefs will lay a foundation for the mechanism of wave propagating near islands and reefs. • Action density balance equation is proposed to calculate real wave attenuation. • The realistic wave attenuation values are higher than those of spectral wave models. • Observational spectra are used to improve simulating accuracy of wave attenuation. [ABSTRACT FROM AUTHOR]

Due to the intricate topography and geomorphology of coral reefs, waves propagating from deep water areas to transitional or shallow water near islands and reefs will exhibit features of wave spectra that are very different from those in the deep sea. Thus, a reconstruction of the wave spectra is necessary. In this study, double Gaussian functions are first proposed to identify the characteristic frequencies of wave bimodal spectra, such as peak frequencies corresponding to the low frequency part and high frequency part, and separation frequencies differentiating between the low frequency part and the high frequency part, based on the observed data of wave spectra in the lagoon near islands and reefs of the South China Sea (SCS). The wave bimodal spectra close to the islands and reefs are then rebuilt using the Ochi-Hubble spectrum type. The high frequency and low frequency portions of the bimodal wave spectra are used to determine the significant wave heights, peak frequency and shape characteristics, respectively. The wave motions exhibit basically linear characteristics when two typical sea conditions, such as summer typhoon waves and winter Northeast monsoon waves, are investigated in detail. The superposition of wind waves and swells on the one hand, and the presence of the bottom on the other, have an impact on the generation of the bi-modal spectrum. The bi-modal spectra that were reconstructed and those that were observed have a correlation coefficient that is greater than 90%. The study on the reconstruction of the bimodal spectrum in the paper will provide a reference for the description of the bimodal spectrum near islands and reefs. • The double Gaussian function model is put forward to determine the characteristic frequency of wave bimodal spectrum. • The wave non-linearity and superposition of the swell and wind part cause the generation of the bi-modal spectrum. • Good consistency between the reconstructed and observed spectra near islands and reefs. [ABSTRACT FROM AUTHOR]