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A recent investigation of wave-current processes in a shallow reef-crest environment (eastern Nicaragua) indicates that rapid energy transformations associated with breaking waves are important to sediment transport as well as circulation in the back-reef lagoon. Although these interactions have been considered by other studies, they have not been treated quantitatively.
Wave sensors were placed on the seaward and lagoonward sides of the reef crest. Current meters were positioned on the reef crest and in the lee-side moat channel. Energy loss (~67%), calculated from wave-height changes as estimated from wave spectra, are related to depth of water over the reef. At high tide, instantaneous current speeds and wave modifications are minimized even though wave heights are decidedly reduced between fore-reef and back-reef areas. Low-tide conditions favor extreme energy losses resulting from more intense wave breaking. Over-the-crest current velocities are greatest near low tide. Waves in the back reef appear to be solitary in nature, thus favoring greater onshore velocities.
Current surges of 50 to 80 cm/sec for durations of a few seconds occurred under the low-wave-energy input conditions of the experiment (4 to 6 sec input waves and average heights of ~45 cm). These periodic currents of short duration are sufficient to drive coarse-grained sediment into the back-reef lagoon. Mean currents are in the range of 10 to 20 cm/sec and therefore do not reflect the true dynamic nature of reef-crest environments. Most currents reverse with tide. On rising tide, moat-current direction indicates lagoon filling, but the reverse is true of falling tide. Representative current speeds of 10 to 20 cm/sec are typical of these exchanges.
Data from other areas indicate that reef-crest morphology and lagoon geometry are important to the sediment-transport problem. However, breaking waves drive sediment across the crest by strong surge currents. Water driven across the crest controls circulation and resultant sediment-dispersal patterns in shallow back-reef lagoons. Tidal variations cause periodic fluctuations in the intensity of wave and current interactions at the reef crest.
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