Wave-induced setup and wave-driven current over Quasi-2DH reef-lagoon-channel systems

The primary action of near-shore waves at many field reef sites is to produce a wave-induced setup and 2DH reef lagoon -channel circulation cells. Instead of using wave basin to study the behaviors of the wave-induced setup and wave-driven flow in such systems, an experimental technique was introduced to achieve the same objective through wave-flume tests. A pipe with a valve was installed underneath the wave flume to connect the lagoon and the ocean side, which makes it possible to simulate a rip channel connecting the lagoon and the offshore side. Experimental results for two reef systems with a fore-reef slope (1V:6H) were obtained under a series of monochromatic wave conditions: one simulating a dosed lagoon and the other simulating an open lagoon. Compared with the reef system with a closed lagoon, connecting the lagoon to the ocean side was found to have the following effects: (i) producing a wave-driven current on the reef flat, (ii) reducing the maximum wave setup on the reef flat by 10%-40%, and (iii) causing the wave setup to decrease continuously along the reef flat all the way to the lagoon. For the reef system with an open lagoon, the wave-driven current was found to increase with increasing wave height and wave period, and there appeared to be a peak value of the current within the tested range of reef-flat submergences. The experimental results for the two reef systems were compared with a set of existing wave-basin results for similar reef systems, but with a much steeper fore-reef slope (1V:1H). The dimensionless parameters used in the literature were found to be insufficient to describe both the wave setup and wave-driven current on the reef flat for the reef system with an open lagoon. A semi-analytical quasi-2DH model, which was devised according to the balances of momentum and mass, was also included to understand how the reef and channel morphology and the bottom roughness affect the wave setup and wave-driven current in a reef lagoon -channel system. The semi-analytical model was validated by our laboratory data as well as field observations reported in the literature.