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Hello,

I am modelling a source of tracer (fine sediments) which occurs on a dry beach. The flow is supposed to flow down towards the sea and, later, contaminate the sea. The flow therefore spreads over some area (sandy patch), driven to the sea by gravity and topographic features. Over this patch, the water height is not very high. The source has some flow rate, inflow velocity (towards the sea) and concentration.
My concern is the following.

Due to advection (and diffusion, if activated), the tracer becomes non-zero at some (very slightly immersed) nodes in the neighbourhood of the patch. If, for some reason, the water height becomes zero at these nodes some time steps later, the concentration of the tracer does not necessarily become zero (see attached figures). In physical terms, this has no sense, because the tracer should be zero where H=0. As a consequence, the contamination of the sea is somehow under-estimated as some “ghostly” tracer remains on the beach...

Could anyone bring me some light on this problem?
Is such a modelling (wet source on a tidal flat) reliable in TELEMAC2D and how to deal with it in a correct manner?

In the attached figures, the tracer concentration is shown as filled contours along with the velocity field of the source. Water height is shown as upper-layer wireframe. The following options were set up:

SCHEME FOR ADVECTION OF TRACERS = 14
TIDAL FLATS = YES
OPTION FOR THE TREATMENT OF TIDAL FLATS = 1
TREATMENT OF NEGATIVE DEPTHS = 2
TYPE OF SOURCE = 2

Thank you!
JBS.

Hello Jean Baptiste

Sorry for this late reply.

This is probably due to:
1 thresholds used for hydrodynamics and tracers.
2 coupling algorithm which allows lots of time steps of hydrodynamics for a unique time step of tracer (due to CFL condition which is more smooth for tracer than for hydro)

This could be considered as a bug if we assume that we should not allow non nil values of tracer for completely dry nodes. Otherwise, in more physical way, this situation could happen (imagine salty water when it evaporates salt could deposit).

with my best regards

Riadh

Thank you Riadh.

About point 2, the tracer is here solved within TELEMAC2D (no coupling with SISYPHE for instance), so isn't the time step identical for hydrodynamics and tracer transport?

As a conclusion, if I wish to assume the tracer must be 0 when H = 0 on the beach, what should I do accordingly?

Thanks again for your help,
JB.

Hello Jean Baptiste

By coupling, here, I mean the internal coupling of the Shallow water equations (for hydrodynamics) and Transport equation (for tracer). Internally, Telemac-2D manages time steps for sake of CPU time optimization (also for stability and reducing of numerical diffusion) and thus it can compute several time steps for only one time step of tracer.
Genuinely, I do not have an irrevocable solution. I need to go in depth in the algorithm and achieve some tests. What you can try, is to reduce the time step for the first part of simulation (until this area dries out). This will allow to reduce the number of subiterations (hydro-tracer) and tends towards a 1-1 coupling between hydro and tracer.
I'll keep the question in my todo list and let you updated. Meanwhile, can you send me the model to my email address.

Kind regards
Riadh

Ah ok, I didn't know there was such an internal coupling for tracer transport in TELEMAC2D.
I send you my files out then.
Thanks,
JB.