On the following subject:
"Study of the hydrodynamic circulation of a narrow lagoon in New Caledonia and use of a stochastic algorithm to identify parameters".
Thesis supervisor: Jean-Luc Devenon, Professor AMU
In this thesis, I studied the hydrodynamics of the Ouano lagoon in New Caledonia. To do this, I implemented a new parameter identification algorithm. Wave breaking on the coral reef and tides dominate the hydrodynamics of Ouano lagoon. I wanted to assess their relative impact on water exchange with the ocean.
Several studies have been carried out in the lagoon before. I am basing my thesis on their results for circulation and the modelling tools they have put in place. In particular, I am re-using C. Chevalier's CROCO (Coastal Regional Ocean COmmunity) model. I am also using data collected in the lagoon in 2016. Despite this preliminary work, there are still uncertainties about the amount of water brought into the lagoon by wave action and the tide. In addition, the parameterisation of wave breaking, friction on the reef and conditions at the edge of the tide are uncertain in the numerical model.
To improve these parameterisations, or even other parameters, I implemented and tested a new tool. This tool was a stochastic parameter identification algorithm, the Simultaneous Pertubations Stochastic Approximations (SPSA) algorithm.
We first tested different versions of the algorithm in controlled environments, in particular with a one-dimensional turbulence model. I then used the 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 have shown that tidal and surf currents can 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 measurements of phass and tidal current speed in the lagoon.
The turbulence model could be used to study the vertical shearing of currents with a tidal frequency behind the reef. This will help us to differentiate even more finely between tidal currents and currents due to breaking waves, even though they sometimes have the same frequency. The linear tidal model could be used to improve the conditions at the edges 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 such a way that it can be used in other contexts. We have also used this code with another biogeochemical model.
Keywords: Mesotidal zone, inverse assimilation methods, coastal oceanography, coral reef
https://univ-amu-fr.zoom.us/j/95188436531?pwd=dEdsMlA2VUlMei9heit6Ti9uRn…
Meeting ID: 951 8843 6531
Passcode: 769606