Coupled effects of tide and swell on water renewal in a meso-tidal channel lagoon: Case of the Toliara Lagoon (Madagascar)
Résumé
In this article, based on the example of the Toliara lagoon in Madagascar, we identified the major impacts of the two main drivers of the water renewal in a typical channel lagoon: the tide and the swell breaking on the reef. For this purpose, we applied the numerical model Croco which is based on ROMS-AGRIF and uses a realistic and high-resolution configuration. The validation was performed with in situ ADCP data, then, realistic and sensitivity simulations were run to infer tides and waves role in ocean-lagoon exchanges and to estimate water age and water origin. The modelling approach adopted enabled an understanding of the effects of swell and tide on the water renewal. During low wave conditions, tide and waves counteract each other on cross-reef flow: the cross-reef inflow tends to increase with waves breaking on the reef, whereas cross-reef flow tends to decrease or to be outflowing with tidal action. Hence, the lagoon-ocean exchange is reduced and is at a minimum when wave height reaches 0.85 m and the tide has minimum amplitude. During high wave conditions, both tide and waves increase ocean-lagoon exchanges by increasing the cross-reef inflow, whereas during low wave conditions, the cross-reef flow is mainly outward with increase with tidal amplitude and decrease with wave height until the threshold of 0.85 m.
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