Abstract: To investigate monochromatic wave nonlinear characteristics around coral reefs with large surface roughness, laboratory experiments were carried out in a wave flume. A cylinder array was used to model the reef roughness and a series of monochromatic wave conditions were tested. Results show that magnitudes of the wave skewness, the wave asymmetry and the Ursell Number reach their peaks at the endpoint, in the interior and at the stating location of the reef surf zone, respectively. All the three parameters increase with the increasing wave height. Meanwhile, the skewness decreases with the increase of wave period while both the asymmetry and the Ursell Number increase with the increase of wave period. Moreover, the skewness increases with the increase of reef-flat water level while both the asymmetry and the Ursell Number decrease with the increase of reef-flat water level. The deep-water Ursell Number is able to describe the variations of the three examined parameters on the reef flat, and the empirical formulas are finally proposed in this study to predict these parameters.

Key words: reef roughness, monochromatic waves, skewness, asymmetry, Ursell Number