The effect of reef roughness on monochromatic wave breaking and transmission over fringing reefs

A number of field studies have reported that the surface roughness of coral reefs is one or two magnitude larger than that of sandy beaches. In this study, laboratory experiments are conducted in a wave flume with a series of monochromatic wave conditions. Experimental results are reported for an idealized fringing reef model with both rough and smooth surfaces. The reef roughness is created by an array of cylinders. Results analyses show that the relative reef-flat submergence is an appropriate parameter to describe the breaking-wave characteristics, such as the breaker type, breaking location, surf zone width, breaker depth index and wave transmission coefficient. Increased reef roughness is found to alter breaker type, surf zone width, breaker height index, breaker depth index and wave transmission to varying degrees. Subsequently, existing empirical formulas accounting for those characteristics are improved by including the effect of reef roughness. Finally, a theoretical model based on the cross-shore energy balance is adopted to reproduce the cross-reef wave height variation. Model parameter related to wave breaking is estimated via our improved formula. Bottom friction coefficient in the model for the rough reef is found to be consistent with those reported in existing field observations.