PhD: Observability of the Gironde estuary hydrodynamics: an exploratory study

Centre National D'Etudes Spatiales (CNES)

Toulouse, France 🇫🇷

Mission

The hydrodynamics of the Gironde estuary has been the subject of numerous studies as there are important societal and environmental issues at stake. Despite huge progress on the knowledge of the physical processes, the complexity of the interactions between the dynamical processes and of the variability of the forcing leaves crucial issues unsolved. More specifically, the estuarine and deltaic systems are recognized as one of the most challenging targets for the forthcoming SWOT mission. Not only are they buffer zones where the dynamical processes are influenced both by the ocean and rivers, but it is also a transition area for SWOT products. Despite the long-going mission preparatory investigations, the actual content of real SWOT data and their adequate processing are still under question. 

The objective of this PhD is to explore the use of altimetric data from the SWOT mission in the land-to-ocean continuum region prior to their exploitation for scientific applications on estuarine and coastal hydrodynamics. This PhD proposal intends to take profits of the availability in the Gironde estuary of well controlled numerical hydrodynamical configurations and dense in situ data networks, complemented with achieved or planned in situ field campaigns (LIDAR, GNSS) to investigate some specific questions about SWOT data contents and error budgets. 

More specifically, we will address the surface signature of physical processes at different spatial scales: Estimate of river flow in the upstream estuary: In terms of SWOT data interpretation and comparison with model simulations, the absence of tides in the upper river part will allow an easier learning of SWOT hydrologic data content than the part influenced by tides in the upper estuary, where ocean data will not be available. 

Observability of tidal and storm surge sea level signal (2D): To answer this question we will compare the SWOT low resolution data with in situ observations and numerical simulations to extend the in-situ data (via data assimilation) over a significant portion of the SWOT swath to increase the examination pattern of the SWOT data. 

Observability of small scale tidal and residual signals (2D/3D): The objective is to determine whether tidal variability in the transverse direction and residual (i.e. non-tidal) signals in both the longitudinal and transverse directions (Ross et al., 2017; Alahmed et al., 2021) will be observable by SWOT. 

Method and tools T-UGOm model: 

T-UGOm has been specifically developed to accurately solve the ocean and estuarine HF dynamics. Recently, a time-stepping data assimilation scheme has been implemented to reach centimeter-wise accuracy for free surface in estuaries. Symphonie model: The ocean circulation Symphonie model (Marsaleix et al., 2008) is implemented over a configuration including the Gironde estuary up to the confluence and part of the Bay of Biscay. The grid resolution is varying from about 2.5 km at the ocean open boundary to about 100 m in the estuary. SWOT products and SWOT simulator: We plan to use most of SWOT types of products, but mainly the Ocean Low Resolution product, namely the Basic product at 2km which contains the geophysical corrections (DAC, tides and sea state bias) and the SSH product at a resolution of 500m which is a priori better suited for the estuary. We will rely on the tools developed by CNES and CTOH/LEGOS and hydrology community experts. The CNES version of the ocean simulator will be used for verification purposes. 

 Working plan and collaborations: 

 â—Ź Year 1: learning of SWOT data processing/use; observability of upper river (tides-free) 

 â—Ź Year 2: observability of tides/storm surges 

 â—Ź Year 3: observability of 2D/3D small scale signals from models, SWOT/nadir altimeters data 

The PhD will find additional support from LEGOS engineers and developers. He will interact with all the scientists involved in the COCTO-FO project, which addresses the science fields related to hydrodynamics of the estuary-shelf-deep ocean continuum, including Gironde estuary, and with the other SWOT Science Team groups working on similar topics. 

Main references on the PhD topic: 

Alahmed S., Ross L., Sottolichio A., 2021: The role of advection and density gradients in driving the residual circulation along a macrotidal and convergent estuary with non-idealized geometry, CSR,  https://doi.org/10.1016/j.csr.2020.104295 ;

Chevalier L. et al, 2017.Hydrological variability from gauging stations and simulated SWOT data, for major French rivers. Journal of Geoscience and Environment Protection, Special Issue Hydrogeology and Water Cycle. 

Ross, L. et al, 2017. Lateral variability of subtidal flow at the mid-reaches of a macrotidal estuary, J. Geophys. Res. Oceans, 122, doi:10.1002/ 2016JC012504 

Toublanc F et al., On the role of wind and tides in shaping the Gironde River plume (Bay of Biscay), Continental Shelf Research, in review F. Lyard et al., Marest project report, LEGOS/SHOM

=======

For more Information about the topics and the co-financial partner (found by the lab !), contact Directeur de thèse :  florent.lyard@legos.obs-mip.fr

Then, prepare a resumé, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online !

Profil

Océan-Atmosphère

Laboratoire

LEGOS

Message from PhD team

More details on CNES website : https://cnes.fr/fr/theses-post-doctorats


POSITION TYPE

EXPERIENCE-LEVEL

DEGREE REQUIRED

IHE Delft MSc in Water and Sustainable Development