In this paper, in-situ measurement and simulation are presented to figure out the feature of waves entering a reef lagoon with double barriers. Measurement results show that averaged 73.61% energy is reduced from outside site to inside one. Meanwhile energy density attenuation coefficients increase linearly with frequencies. Most of measurement coefficients agree well with the simulation. Additionally, a fitting expression based on the simulation is put forward to investigate the influences of incoming direction and frequency of wave on attenuation coefficients.
In South China Sea, lagoon surrounded by reef and island is a common topography. The waves in the lagoon are mostly much smaller than those outside the lagoon. Up to 86% of wave energy is attenuated when waves enter the lagoon(Roberts, Wilson, & Lugo-Fernández, 1992). It makes the lagoon suitable for fishing and tourism. Thus, the wave behavior inside and outside the lagoon is worthwhile to study. Generally speaking, due to the sheltering of island and reef flat, diffraction plays the most important role when waves enter the lagoon. Wave diffraction has been widely studied in recent decades(Abohadima & Isobe, 1999; Bai & Taylor, 2007; Isaacson & Cheung, 1991; Lee, Kim, & Suh, 2003; Nwogu, 1993; Tang & Ouellet, 1997; Wu et al., 2016; Yu, 1981). Empirical formulas have been put forward for wave diffraction of simple topography(Yu, 1981). And numerical models such as mild-slope equation(MLE) and Boussinesq equation, have been successfully employed in wave diffraction of complex terrain(Abohadima & Isobe, 1999; Lee, et al., 2003; Nwogu, 1993; Tang & Ouellet, 1997; Wu, et al., 2016). However, wave behavior around reef lagoon is much more complicated. The waves usually change drastically due to steep slope at the reef edge. Besides, the wave processes including wave breaking, whitecapping, bottom friction and nonlinear wave-wave interactions should also be comprehensively considered. Because of the roughness and steep slope, much attention has been paid to wave breaking and bottom friction. And many measurement results have been published in recent years(Cheriton, Storlazzi, & Rosenberger, 2016; Gallop, Young, Ranasinghe, Durrant, & Haigh, 2014; Gourlay, 1994; Huang et al., 2012; Kench & Brander, 2006; Lentz, Churchill, Davis, & Farrar, 2016; Monismith, Herdman, Ahmerkamp, & Hench, 2013; Monismith, Rogers, Koweek, & Dunbar, 2015; Péquignet, Becker, Merrifield, & Boc, 2011; Ryan J. Lowe & Stephen G. Monismith, 2005). Even so, there is a lack of systematic study on the wave energy density attenuation.
