Three-dimensional numerical modeling of sediment transport in a highly turbid estuary with pronounced seasonal variations

Simulating sediment dynamics in a large and energetic estuary system remains challenging, primarily due to the spatial and temporal complexities of the interaction between flow and sediment transport, especially for sand-mud mixtures. This study uses a three-dimensional (3D) numerical model, based on the open TELEMAC system, to investigate the dynamics of suspended sediment concentration (SSC) in the Gironde Estuary, a complex estuarine environment characterized by an estuarine turbidity maximum (ETM) and significant variations in river discharge. The main contributions of this study include addressing the challenges of coupling bed friction with sediment transport of the sand-mud mixture for feedback on bed roughness and bottom depth changes and the ability of the model to capture the migration of ETM from high to low flow. Additionally, the current study analyzes the ability of the model to capture the migration of ETM from high to low flow, and it utilizes a calibration strategy that minimizes parameters by using in situ data and encompassing hydro–morpho-sedimentary interactions. A sensitivity analysis was done using different settling velocity approaches and sediment classes to establish an optimal model configuration and the uncertainty associated with the reduced model parameterization is discussed. The model satisfactorily reproduces the hydrodynamic features, particularly when the hydro-sedimentary feedbacks are taken into account, the seasonal trend of SSC, spring-neap variations, and the development of a well-defined ETM. The selection of a specific formulation for the settling velocity influences the location and magnitude of ETM. The van Leussen formula not only predicts a broad movement of ETM from high to low river flow, but also predicts high turbidity for extended periods during low river flow. Conversely, two empirical formulas from Le Hir and Defontaine predicted the highest turbidity during neap tides but sediment losses during prolonged simulations. The results of this study contribute to a deeper understanding of sediment dynamics in the Gironde Estuary, providing valuable information for future estuarine modeling and management.