Investigation of 3D circulation and secondary flows in the St. Lawrence fluvial estuary at a tidal junction

To enhance understanding of the complex functioning of the St. Lawrence fluvial estuary—a macro-tidal, freshwater estuary located in Quebec, Canada—a 3D numerical model is set up to investigate its hydrodynamics. Validation of the 3D model used field data on water levels, discharge rates, and velocities during both neap and spring tide periods. Comparison of the model with existing 2DH results illustrates the 3D model's ability to represent the time evolution of the secondary flow during tidal forcing in the confluence/divergence zone around Île d'Orléans. 3D results highlight the great importance of the vertical component of velocity in studying a site with complex geometry. A more detailed analysis of velocities and turbulence at the Île d'Orléans junction shows a time lag of around 1h between current slack and the tidal slack. On the one hand, the current reverses earlier at the bank level than in the deep channel during both ebb and flood periods. On the other hand, the current reverses more quickly at the bottom than at the surface in the main channel. Site geometry, friction and the presence of return currents are the main factors explaining this. This paper highlights the importance of 3D modeling for gaining a deeper understanding of estuarine dynamics, even in the tidal freshwater zone, revealing processes ignored by 2D depth integrated models. Such modeling can assist in planning future field measurement campaigns and improve space-time interpolation methods for velocities in wide estuaries. Additionally, it provides a solid foundation for studying couplings (chemical or particulate) and making predictions, particularly in the context of climate change.