Terrestrial or oceanic forcing? Water level variations in coastal lagoons constrained by river inflow and ocean tides

Abstract

The study concerns the water level (WL) evolution in lagoons under the influence of tides and river fluxes. We derive new approximate analytical solutions of the Stigebrandt (1980) equations and apply them to the Nokoué Lagoon (Benin), a large tropical coastal lagoon fed by substantial river input. We show the solutions accurately predict the mean WL and tide amplitude. In particular the nonlinear combination of the spring-neap tidal cycle and river inflow gives rise to a strong fortnightly variation of the mean WL. The analytical solutions are used to explain this phenomenon. We also calculate the phase shift between the ocean and the lagoon tides and the asymmetry of the ebb and flood tide duration in the lagoon. The asymmetry first increases with the river flux but reduces above a critical flux before becoming symmetric again at river flood peaks. Finally, the analytical solutions are inverted to estimate the net river fluxes entering the Nokoué Lagoon, where no observations are available for rivers but for which we have high frequency observations of the lagoon water level for 2 years. The model is calibrated using few available flux observations in the channel connecting the lagoon to the ocean. Realistic river fluxes are estimated. We subsequently calculate the lagoon WL variations using the simplified model forced with the recalculated river fluxes and tidal forcing from an ocean tide model. The solution accurately represents the observations over the 2 year time period of the study. We conclude that this simple model is able to represent the complex interaction of tides and river fluxes, and its influence on the low frequency WL variations of coastal lagoons.