Modeling of the hydro-sedimentary dynamic in a highly turbid tidal river: the case of the Garonne river
Tidal rivers correspond to the upper estuarine sections, where the tide propagates in freshwater, beyond the salinity front. In macrotidal estuaries, these areas are still poorly known even if they are complex zones. The tide propagation becomes highly sensitive to the river flow and hydraulic friction. Circulation patterns are constantly varying due to the fluctuation of tidal propagation limits. For these reasons, sedimentary processes are difficult to predict and observe. In large turbid estuaries, tidal rivers are subject to episodic presence of the estuarine turbidity maximum (ETM), with significant consequences on the siltation of the bottom and banks, as well as on the water quality. Over the past decades, low -river flow periods have become more marked, inducing changes in the balance between salt and fresh water and therefore in the general balance of estuaries. Salt intrusions and fine sediments go further upstream in tidal rivers, modifying the zones of stratification and associated sedimentary deposits.
The general objective of the thesis is to progress in the knowledge and modeling of tidal rivers influenced by high turbidity levels. This thesis will focus on the upper Gironde estuary and the tidal portion of the Garonne river, which constitute an interesting area for several reasons. The Gironde estuary is a macrotidal estuary system with a well-developed turbidity maximum. The summer propagation of the TM in the Garonne has been observed by recent continuous turbidity measurements (Jalon-Rojas et al., 2015). Tidal amplification occurs in the presence of mud on the bottom (Jalon Rojas et al., 2018). In addition, summer hypoxia occurs in the waters of the Garonne around Bordeaux, due to the conjunction of TM increase and urban discharges from the Bordeaux city (Lajaunie-Salla et al., 2017), with consequences on the biological balance of the estuary (Lanoux et al., 2013).
Numerous numerical models have been implemented in the Gironde estuary, but all have focused on the central and downstream part of the estuary, downstream from the Garonne Dordogne confluence, where the salinity stratifications and the TM are located during periods of flood and medium flow. The modelling of tidel rivers remains a challenge.
Recently, a modeling based on an unstructured mesh (Opentelemac code) was developed. This type of mesh allows to better discretize tidal rivers. By taking advantage of this Telemac 3D model, the specific objective of this thesis is to achieve a realistic 3D modeling of the summer-fall cycle of the TM in the Garonne tidal and to simulate the residual deposits, the change of the nature of the sedimentary bottom and the resulting modifications on hydraulic friction and on the propagation of the tide. In this way, we will be able to simulate the entire tidal-estuary of the Gironde continuum from the upstream limit of tide propagation to the mouth.
Nicolas Huybrechts (Cerema), Isabel Jalon-Rojas (EPOC) and Pablo Tassi (EDF R&D)
The thesis will be mainly conducted in Compiègne and Chatou with different missions in Bordeaux.
Engineer or master (geosciences, coastal oceanography)
Technical skill: sedimentology (cohesive/ non cohesive), geomorphology, remote sensing, fluid mechanics, hydraulics, numerical modeling.
Personal skill: autonomous, critic mind and skill for data and model analysis.
To apply please send before March 30th, 2021:
A curriculum, a copy of ID or passport, Record of notes for Master 1 and if possible Master 2. A copy of the last diploma. A motivation letter and a recommendation letter.
Interviews are planned on May 3-4th 2021 (Lyon)