open TELEMAC-MASCARET The mathematically superior suite of solvers

I have a problem defining an imposed tracer with imposed head. I did this in MATISSE, but when I try to solve with TELEMAC2D it refuses giving an error message: 'Liquid boundary between two solid boundaries'

Actually in MATISSE there are standard combination for boundary condition where no imposed head with imposed tracer unless there is imposed flow, so there is ( 5 5 5 5), but in my case I must use (5 4 4 5) and MATISSE accept it after warning message, but TELEMAC2D refuses it.

So, can anybody help me in solving such problem

Hello,

The problem is probably that you cannot have a single liquid point between 2 solid points. You need to have at least 2 liquid points in a sequence. The reason is that a segment between a liquid point and a solid point will be considered as solid, so a single liquid point would be between 2 solid segments.

With best regards,

Jean-Michel Hervouet

Actually I have a liquid boundary of more than 70 points. Therefore when I use any other combination for boundary conditions like 5 4 4 4 telemac runs without any problem, but when I use this combination 5 4 4 5 (imposed head, free velocity, imposed tracer) telemac gives problems

Hello,

This must be another problem. The combination 5 4 4 and 5 for tracer is possible, I have just tried it. Actually on exits the 5 of tracers will be automatically changed into 4 if it is really an exit. The prescribed values will be used only when the velocity is entering the domain. So there is a misunderstanding somewhere. Send me your case and I'll see what happens.

With best regards,

Jean-Michel Hervouet

I think you are right, my problem is that I am simulating an estuary ( when the river meets with the sea), therefore the salinity has to be at the exit of the river not at the entrance and telemac doesn't accept that. So, I think I have to use another way to define the tracer instead of using a prescribed values at the boundary.

Hello,

This problems effectively arises in estuaries. When the flow exits, the value of salinity at the exit comes from inside the domain (this is naturally treated by Telemac). When the flow enters the domain, the prescribed value is taken. So an imposed boundary condition 5 is basically correct (given the fact that Telemac will relax it when flow goes out). However if your boundary is not far enough in the sea, the value of the salinity, even when the flow is entering, is influenced by the river and tide. We have this basic problem in the Berre lagoon, we prescribe the entering salinity at a value of 38. On long term computations (several years) a drift of salinity in the lagoon can be observed, so in fact measurements have to be imposed instead. It is not a numerical problem, it is just a matter of reproducing reality and the problem would be in fact the same with a physical model.

WIth best regards,

Jean-Michel Hervouet

Thanks for your reply,

In my case also I want to study the salinity transport from the sea to the river in case of the sea level rise, but telemac as you mentioned compute it as the salinity goes out side the domain not inside, so I can't really simulate it, and telemac accepts imposed salinity only with imposed velocity or flow (and of course I have only the sea level).

If you have faced the same problem, can you briefly tell me how did you do it, if it is possible can you write the part of the steering file related to the salinity in your case, and what was the boundary conditions you used.

It will be very kind of you, if you can clarify me in details how you did it as I am doing this in my PhD and I really facing problems.

Hello,

Generally we use :

prescribed elevation at the sea boundary
free velocity at the sea boundary
imposed tracer at the sea boundary

Then your salinity will be imposed when the flow goes in, and will be relaxed when the flow goes out, this is what you want to do actually, don't you ?

If you really want to have an imposed tracer all the time (but well it is not very realistic), then you could take in your Fortran file the subroutine diffin.f from library bief (you just have to cancel the test that relaxes the imposed boundary conditions when the flow is exiting).

With best regards,

Jean-Michel Hervouet

Hello,

I think we are getting confused in what you really wish to achieve. The TELEMAC system is used in estuary application all the time and we can obviously handle and predict exchanges between the river the sea.

As Jean-Michel said, you should define your estuary with one "level imposed" at the open boundary on the sea side and with one "discharge imposed" at the upstream boundary on the river side.

Ideally:
- the placement of your boundaries should be sufficiently far from you salinity variations; and
- you would like to have a constant salinity on the sea side, and a zero salinity (fresh water) on the river side, i.e. you must have your river boundary upstream of your tidal prism.

On the river side, the discharge is imposed, so you can impose a TRACER to zero. This will simulate fresh water input. Note that you can vary the discharge to represent daily, seasonal, hydrographs.

On the sea side, the level is imposed. However, this does not mean that the TRACER cannot be set. At the contrary, if the tide is rising through an increasing level imposed, TELEMAC will calculate the velocity as entering the domain, and will therefore imposed TRACER concentration to your imposed salinity value. If the tide is dropping through a decreasing level imposed, TELEMAC will calculate the velocity as exiting the domain, and will therefore, in this case only, ignore the imposed TRACER value but just let the water leave the domain based on the TRACER value inside the domain.

To help you further, please have a look at one of the test cases also distributed with TELEMAC, the mersey estuary.

As Jean-Michel said, the access to the entire source code provides you with the mean to program what ever boundary you actually need.

Hope this helps.

Sébastien

thank you very much, I am very grateful to your comments, really it helped me much