Guillaume Koenig (OPLC) will present his thesis in person and by videoconference on Friday, November 19, 2021 at 9:00 am (EGEE Room, Building 26M Océanomed)

On the following topic:
"Study of the hydrodynamic circulation of a narrow lagoon of New Caledonia and use of stochastic algorithm for the identification of parameters"
 
Thesis director: Jean-Luc Devenon, AMU Professor

In this thesis, I studied the hydrodynamics of the Ouano lagoon in New Caledonia. To do so, I implemented a new algorithm for parameter identification. Wave breaking on the coral reef and tides dominate the hydrodynamics of Ouano lagoon. I wanted to evaluate their relative impact on water exchange with the ocean.

Several studies have been conducted in the lagoon before. I rely on their results for the circulation and the modeling tools they set up in my thesis. In particular, I reuse the CROCO model (Coastal Regional Ocean COmmunity) of C. Chevalier. I also use data collected in the lagoon in 2016. Despite this preliminary work, there are still uncertainties on the amount of water brought into the lagoon by wave breaking and tide. In addition, the parameterization of wave breaking, friction on the reef, and conditions at the tidal edges are uncertain in the numerical model.

To improve these parameterizations, or even other parameters, I implemented and tested a new tool. This tool was a stochastic parameter identification algorithm, the Simultaneous Perturbations Stochastic Approximations (SPSA) algorithm.

We first tested different versions of the algorithm in controlled environments, and in particular with a one-dimensional turbulence model. I then used this algorithm to identify edge conditions with a linear tidal model of Ouano lagoon. Finally, I used the algorithm to study the impact of wave breaking on currents measured as tidal currents in Ouano Lagoon.

I showed that tidal and breaking currents could be confused in Ouano Lagoon. Part of the currents due to the break-up acquire a tidal frequency because of the modulation of the water level on the reef. I studied this modulation and the impact it had on phass and tidal current velocity measurements in the lagoon.

The turbulence model could be used to study the vertical shear of currents with tidal frequency behind the reef. This will help us to further differentiate between tidal and surf currents, although they may have the same frequency. The linear tidal model could be used to improve the edge conditions of the more realistic CROCO model.

The work presented here has led to several publications: Two papers have been published and one has been submitted. The SPSA algorithm has been implemented on a supercomputer and coded in a way that can be used in other contexts. We have used this code with another biogeochemical model.

Keywords: Mesotidal zone, inverse assimilation methods, coastal oceanography, coral reef