The hydrodynamics of the Bons Sinais Estuary: The value of simple hydrodynamic tidal models in understanding circulation in estuaries of central Mozambique

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The Bons Sinais Estuary, located in the central Mozambique, with the highest tidal range in the Western Indian Ocean, is shallow (∼10m), long (∼28km) and receives freshwater only during the wet season. These characteristics give the Estuary distinctive hydrodynamics and ecological features. Current measurements and longitudinal CTD profiling were carried out during dry season, July 2011 and November 2012, and wet season, February 2012 and March 2013. The Estuary is partially to well-mixed most of the time, with Simpson Number (Si) ≤0.5. One-D, depth and width integrated, tidal hydrodynamic model, forced by the modelled tides at the mouth, was applied to understand the major mechanisms governing the estuarine dynamics. Barotropic forcing dominated the baroclinic pressure gradient forcing and the currents were found to be mostly tidal driven. Accordingly, the hydrodynamic model described the observed current velocities well in the dry season (R2≈0.95) and to a lesser degree in the wet season (R2≈0.80). The remaining 5%–20% may be attributed to other hydrodynamic forcing that were not captured in the tidal model, requiring further investigation at higher temporal and spatial scales to fully characterize the system dynamics. These may include the identified baroclinic forcing from freshwater inputs and the influence of the complex geometry at the study site. The initial hydrodynamic field study and simple modelling approach that was applied may inform studies of higher-order processes and management tools at the Bons Sinais Estuary, and similar systems (e.g. sediment dynamics and accommodation space, and diagnostic models), and the result can contribute to understanding the drivers of longer-term ecological and geomorphological dynamics of the estuary.