Announcement of PhD defence

Juliette Pénicaud (LEGOS) will defend her thesis on Wednesday, December 11 at 9:30 am (UTC+1) at LEGOS (Tououse). Her thesis is titled : Hydrodynamics and sedimentary processes in a tropical estuary: the case of the Van Uc River, Red River delta, Vietnam. Joint influences of tide and flow at different temporal scales”.

Jury members will be:
M. Florent GRASSO, Rapporteur, IFREMER
Mme Isabelle BRENON, Rapporteure, LIENSS
M. Aldo SOTTOLICHIO, Rapporteur, EPOC
M. Julien NÉMERY, Examinateur, INP Grenoble ENSE3
M. Vinh VU DUY, Examinateur, IMER
M. Pieter VAN BEEK, Examinateur, Université Toulouse III Paul Sabatier
M. Sylvain OUILLON, co-Directeur de thèse, IRD
Mme Marine HERRMANN, Co-directrice de thèse, IRD

Key words: estuary, modelling, salt intrusion, in situ measurements, hydrodynamics, suspended sediment

Abstract:

Estuaries, which are transition zones between rivers and oceans, are crucial for transporting large amounts of sediments and nutrients essential for coastal ecosystems. They are influenced by tidal forces and river flows. These external forces affect the hydrodynamics, and their complex interactions determine the sedimentary and saline structures, which evolve according to tidal cycles, seasonal changes, and at interannual scales. In the case of the Red River Delta in Vietnam, the estuarine dynamics are particularly intricate due to the influence of the monsoon regime, typhoons, and variations in river flow across seasons and years, making this system highly vulnerable to global changes.
This thesis focuses on the response of the Van Uc estuary, a branch of the delta, to various external forces and at different temporal scales, utilizing complementary methods of in situ observations and numerical modeling. Three measurement campaigns have been conducted and have contributed to the study of the evolution of salinity, flow speeds, and turbidity at high temporal and spatial frequency during spring tides and under a wide range of flow conditions. In parallel, a three-dimensional hydrodynamic modeling was performed using the SYMPHONIE model, specifically adapted to the Van Uc estuary. Additionally, the sediment transport model MUSTANG was used to simulate the transport of fine sediments,evaluating their response to various environmental forces.
The Van Uc estuary is dominated by the inflow, and the amplitude of the tidal wave remains constant downstream of the estuary but is attenuated upstream. On a tidal scale, salinity progresses from the mouth to upstream with the tidal cycle, reaching a maximum at the beginning of the ebb tide. During neap tides, an estuarine circulation develops, accompanied by strong stratification up to the limit of saline intrusion. During spring tides, the mixing in the water column increases, reducing stratification and limiting estuarine circulation downstream. The flood limit (position where the reversal of currents produced by the tide vanishes) occurs further upstream in the surface layer than in the bottom layer . During neap tides, it is the opposite due to the established estuarine circulation. Spring tides and high river flows favor tidal asymmetry and attenuation of the tidal wave. During spring tides, the maximum turbidity, although relatively small (below 500 mg/l), was measured during the arrival of salinity. The initial sedimentary simulations were generally able to reproduce the variations between neap and spring tides. Saline intrusion is heavily influenced by river flow, with high flows pushing salinity back towards the mouth, while low flows favor its extension upstream. The measurement campaigns allowed to track the evolution of saline intrusion and stratification, showing a reduction in the length of saline intrusion (measured at 4 km upstream of the mouth) and the stratified region during high flows compared to lower flows (intrusion measured at 13 km upstream). The flood limit is also greatly influenced by river flow, being pushed further downstream during high flows.
Finally, the estuary is situated in a region where river flows and tides vary greatly on an interannual scale, due to El Niño-Southern Oscillation (ENSO) and variation of the lunar nodal cycle (18.6 years), respectively. This indicates a significant interannual modulation of the balance between these forces. At a larger scale, impacts exacerbated by climate change and human activities (dams, dredging) are expected. This evolving dynamics places the Van Uc estuary at the center of contemporary environmental and scientific issues, combining physical, human, and ecological challenges.

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