Dear all,
I have several questions related to the usage of the wave equation (or not) to solve the tracers advection diffusion step. I will first, give a bit of context,as it is a lot of information (hope it is clear), I tried to highlight the resulting questions.
Wanting to study plastics dispersion in rivers and Estuary areas, we are developing based on TELEMAC2D, a new set of tracers called "surface tracers". These tracers, representing floating plastics, would evolve over a fictional "zero thickness" layer at the top of the water column, have exchanges with the already existing suspended tracers and would be advected and diffused over a different velocity field than the one derived from the Shallow Water equation.
This "surface" velocity field would ultimately comprehend different physical effects that are different at the surface than when averaged. Later, the objective is also to develop plastic sediments interactions with the Gaia module and have a three layered model.
Furthermore, the application cases will include tidal flats, having an influence on the used advection scheme (option 13 14 or 15).
I have started to implement this module and I am currently facing interrogations concerning the resolution of the advection diffusion step for these surface tracers.
The way things have been approached is: at each time step, after resolving the SW equation and the advection diffusion of suspended tracers, a new velocity field is defined on which the subroutine cvdftr.F is used to solve advection diffusion of the surface tracers.
I have noticed that when using the wave equation (variable SOLSYS =2) to solve the surface tracers adv-diffusion step, changes of the artificial surface velocity do not impact the advection of the surface tracers while it does with the classical method (SOLSYS = 1). This is as I understand related to the usage of the wave equation method, which, to solve the SW equation expresses everything in term of the surface elevation. I can imagine that the same idea is used on the resolution of the tracers but could not find much details about it or missed it in the guides and the book Hydrodynamic of free surface flow.
Would someone know where I could find how is the wave equation method applied to the resolution of the advection-diffusion of tracers in telemac?
Then, two different approaches have been identified to address the issue:
-First, as more stable, the idea would be to keep solving the SW part with the wave equation and then use classical method (impose SOLSYS =1) for the tracers (surface and suspended). Would this method, mixing resolutions method, would still be considered as correct? If so what losses of stability can I expect in the tracers resolution?
-The second option would be to keep the pseudo wave equation for both propagation and tracers but link the velocity in the surface layer to a new artificial surface elevation and use this new elevation to solve the advection diffusion using cvdftr.f. Is declaring an artificial surface elevation, only for the surface layers, conceivable ?
I think it is important to mention that this "surface layer" is not suppose to alter the hydrodynamic of the model but only to changes the surface tracers concentration.
If something remains unclear, I would be happy to give more details and would welcome any help on the questions or feedback about the method.
Wilhem
Assistance 
