Wave Control on Reef Morphology and Coral Distribution:

A multi-disciplinary project lead by the U.S. Geological Survey is currently studying the fringing reef off southern Molokai, Hawaii, in an effort to characterize the biological structure and geologic variability of coral reef systems. Wave modeling and field observations were utilized to help understand the physical controls on reef morphology and the distribution of coral species. The morphology of the reef crest, which extends roughly 50 km from east to west and up to 1500 m offshore, appears to be primarily controlled by the amount of wave energy impinging on the coastline. Extratropical cyclones and inter-anticyclonic systems crossing the North Pacific during the winter months generate the wave energy regime that appears to dominate the reef. This North Pacific swell generates near-bed orbital velocities greater than 3.0 m/sec and shear stresses greater than 2.5 N/m that inhibit substantial coral development in shallow water (<10 m). The reef is sheltered from these waves by the island of Molokai; however, refraction around the east and west ends of the island cause the reef crest to pinch out roughly 5 km from each end of the island. Where the reef crest merges into the shoreline, more robust high-energy corals and low coral cover typify the fore reef. In contrast, more delicate branching corals characterize the central portion of the reef, which is sheltered from the large North Pacific swell, and up to 80% live coral cover.