Residual hydrodynamics over a sand wave field in a macrotidal tropical estuary (São Marcos Estuarine Complex, Brazil)

Understanding the hydrodynamics and sediment transport patterns in macrotidal estuaries is critical for predicting morphological changes, managing navigation routes, and preserving coastal ecosystems. To establish a circulation model for the sand wave-dominated macrotidal estuary of São Marcos (Brazil), a high-resolution hydrodynamic model was developed and validated. By characterizing the main features of hydrodynamics on a small time-scale, the results were compared with multibeam bathymetric surveys, contributing to the understanding of sedimentary dynamics associated with large sand waves. The shallow-water approach, used to calculate depth-averaged velocities, proved sufficient for determining the local hydrodynamics of macrotidal estuaries. Accordingly, the 2D hydrodynamic Delft3D-FLOW module was applied. A high level of agreement was observed between simulated and measured current velocities. River flows were found to play a negligible role compared to tidal currents in governing the system’s hydrodynamics. The results suggest the dominance of local velocities in controlling sand wave morphology and migration, while variations between neap and spring tides highlight the influence of periodic water-level changes in defining the region’s primary hydrodynamic regimes. Under reduced tidal forcing, local bathymetry and geomorphology gain greater importance in hydrodynamic control, as evidenced by spatially varying ebb and flood dominance patterns across regions separated by Medo Island. The resulting conceptual model of sediment transport corridors provides insights applicable to the management of activities dependent on sediment dynamics in both mesotidal and macrotidal estuaries.