Field observations of wave-driven water-level gradients across a coral reef flat

[1] Field measurements of still water surface elevations were obtained across a narrow leeward reef flat on Lady Elliot Island, Great Barrier Reef, Australia in June 2003. Stilling wells were deployed from the reef crest landward to the island beach, and waves and mean water levels were monitored over both rising and falling tides during low to moderate wave energy conditions. Wave setup of up to 13.8 cm above still water level occurred in the presence of incident waves of 0.4 m yielding maximum water surface slopes greater than 6 degrees. Results show that the magnitude of wave setup varies both temporally and spatially across the reef with changing water depth. Setup is dominant on the reef edge at low tide, evolving into a dual setup system at both the reef edge and shoreline at midtide and finally a dominant shoreline setup at high tide. On Lady Elliot Island the dual setup system is considered to result from spatial differences in transformation and breaking of swell and wind waves at midtide stages. The presence of a dual setup system across a reef flat has not been previously identified in field or modeling studies and has potentially significant implications for reefal current development, sediment transport, and the stability of reef island shorelines.