|Field study of cross-reef dynamics above the Ouano coral barrier, New Caledonia, France |
Conference: 24e Réunion des Sciences de la Terre (, FR, 2014-10-21)
Ref HAL: hal-01017502_v1
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Mostly present in tropical regions, coral reefs both provide a unique habitat for a wide variety of living organisms and very efficient protection against ocean action on nearshore and lagoon areas. Recent tsunamis and cyclones events demonstrated that the weakening of coral barriers can have tremendous consequences on nearshore areas, including submersion, erosion and overwash deposit. The present field work aims to study the hydrodynamic processes over a reef barrier, focusing in particular of the offshore wave energy transmission toward the lagoon. The selected field site is the Ouano lagoon, New Caledonia, France. A three-months survey has been performed from september to november 2013 including wave measurements by pressure sensors at the outside reef, above the reef flat and inside the lagoon and velocity profiles measurements at selected places inside the lagoon. During the studied season, the reef is permanently exposed to long swell waves with significant wave height ranging from 0.4 to 2.7m. The wave energy over the reef flat is mostly controlled by the tidal elevation rather than by the incoming swell energy : the higher the mean level, the stronger the wave energy. Significant energy transfers toward low frequencies occur modulated by the tidal elevation. From mid to high tide, the energy transfer during wave breaking is dominant and the wave dynamics over the reef is focused in the infragravity band. From mid to low tide, the contribution of friction processes increases as the mean level decreases. Cross-reef current measurements show that the fluxes are overwhelmingly directed toward the lagoon. Few and short events of outflow are observed during the smallest swell periods either at high tide or at the beginning of rising tide. For small swells, the cross-reef current is modulated by the tidal elevation, while for large incoming waves the fluxes are much stronger (up to 40cm/s) and nearly constant.