Experimental Characterization of the Oceanic Water Exchanges in a Macro-tidal Lagoon

In the macro-tidal lagoons of the Indian Ocean, coral reefs can be momentarily submerged by water at high tide and partially emerged at low tide. This process contributes to lagoon and open sea exchanges, although the reefs are often considered as impervious and the water fluxes assumed to occur only through the passes. To gain insight into spatial variability of fluxes at a reasonable cost that moorings alone are not able to provide, we developed an original experimental approach combining small ship side-mounted ADCP (Acoustic Doppler Current Profiler) measurement following transects through passes and near reefs with more classical high-resolution ADCP moorings. This new strategy of measurement was exemplified at the occasion of an experimental campaign on the Mayotte Lagoon and used afterward in Tulear Lagoon. The results of this experiment are presented. Particularly, first, it is shown how the mounted ADCP data are collected. A specific tidal analysis methodology is then proposed to obtain the spatial variability of the tidal component of the current through the passes and above the reef. Then, all these analyses allow us to estimate the tidal-induced fluxes through the passes and above the reefs and to evaluate their respective part in water lagoon renewal.