Wave-Driven Circulation of a Coastal Reef–Lagoon System

Abstract The response of the circulation of a coral reef system in Kaneohe Bay, Hawaii, to incident wave forcing was investigated using field data collected during a 10-month experiment. Results from the study revealed that wave forcing was the dominant mechanism driving the circulation over much of Kaneohe Bay. As predicted theoretically, wave setup generated near the reef crest resulting from wave breaking established a pressure gradient that drove flow over the reef and out of the two reef channels. Maximum reef setup was found to be roughly proportional to the offshore wave energy flux above a threshold root-mean-square wave height of 0.7 m (at which height setup was negligible). On the reef flat, the wave-driven currents increased approximately linearly with incident wave height; however, the magnitude of these currents was relatively weak (typically 20 cm s−1) because of (i) the mild fore-reef slope of Kaneohe Bay that reduced setup resulting from a combination of frictional wave damping and its relatively wide surf zone compared to steep-faced reefs, and (ii) the presence of significant wave setup inside its coastally bounded lagoon, resulting from frictional resistance on the lagoon–channel return flows, which reduced cross-reef setup gradients by 60%–80%. In general, the dynamics of these wave-driven currents roughly matched predictions derived from quasi-one-dimensional mass and momentum balances that incorporated radiation stresses, setup gradients, bottom friction, and the morphological properties of the reef–lagoon system.