Modélisation hydrodynamique tridimensionnelle et connectivité dynamique : l'estuaire du Saint-Laurent

Abstract

This thesis analyzes the three-dimensional interactions between tides, river discharge, and geometry in the fluvio-middle reach of the St. Lawrence Estuary, a macrotidal multichannel system characterized by strong bathymetric gradients and geometric asymmetries. Existing approaches—mostly 1D/2D or low-resolution 3D configurations—fail to capture the fast, transient three-dimensional processes that structure the dynamics of multichannel systems, such as the propagation of flow reversals, recirculations, vertical shear, lateral phase lags, and inter channel exchanges. To address these limitations, a high-resolution TELEMAC-3D model was implemented and validated using three-dimensional measurements. It provides the first sufficiently resolved 3D hydrodynamic framework capable of representing the fine-scale structures and dynamic connectivity of the estuary. The model reveals mechanisms that had remained invisible until now, including non-synchronous reversals, recirculations, vortical structures, and unsteady lateral and vertical exchanges associated with vertical coupling/decoupling of the water column layers This work provides the first high-resolution analysis of the rapid and unsteady hydrodynamic processes that organize the three-dimensional connectivity of the estuary. The results show that tidal transition phases (flood–ebb and ebb flood), traditionally considered quasi-static, are in fact major organizing periods: the weakening of longitudinal velocities intensifies secondary three-dimensional dynamics. These processes are particularly important around shoals and tidal junctions, which act as highly dynamic connectivity structures. One of the major contributions of this thesis is the formalization of a three-dimensional flow pattern for asymmetric tidal junctions, structured into four recurrent states (confluence, north-tributary, south-tributary, diffluence). This unified framework, developed from the St. Lawrence case and generalizable, provides a conceptual basis applicable to deep asymmetric macrotidal estuaries. This work also offers a relevant 3D hydrodynamic foundation for studying mechanisms such as salt intrusion and for analyzing applied issues including pollutant dispersion and anthropogenic or climatic impacts in estuarine environments.

Date	13 Aug 2026
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Damien Pham Van Bang (Supervisor) & Abdelkader Hammouti (Co-supervisor)