Harry H. Roberts


Studies of physical processes in reef-lagoon systems continue to emphasize the importance of waves and wave-induced currents at the reef crest as agents of sediment transport to backreef environments. These across-the-reef currents are also largely responsible for driving backreef lagoon circulation. Rapid energy transformations associated with the process of wave breaking at the reef crest are responsible for strong reef-normal surge currents. Estimates of energy loss, as determined by wave height changes caused by wave breaking, can be as high as 70-80%' for discontinuous reefs and >90% for continuous examples. The amount of energy loss is related to depth of water over the reef crest, a function of reef topography and tidal regime. Low-tide conditions promote the greatest incident wave modification and attenuation as a result of increased breaking-wave intensity. Under trade-wind conditions found in the Caribbean, surge currents of 50-80 cm/sec for durations of 2-6 sec are common in a low to moderate wave-energy setting (4-6 sec input waves, 40- 50 cm average heights). Sediments through the sand sizes up to pebbles are easily transported lagoonward by these periodic bursts of energy. Flow in shallow backreef lagoons (generally <3 km wide) is driven largely by across-the-reef currents resulting from breaking waves. Long, unbroken reefs tend to induce axial currents in the backreef lagoon which flow roughly parallel to the reef trend. Side-scan sonographs indicate that large bedforms define a region of bottom sediment migration related to strong currents down the lagoon axis, presumably activated during periods of abnormal wave activity on the reef.


sediment flux; reef; lagoon; reef-lagoon system

Full Text: