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Hi,

I am currently using TELEMAC 2D to simulate various discharges in a river reach and derive flow velocities and water depths at each discharge. My data available for calibration is
a. one specific discharge at the inflow,
b. its relevant water surface elevation at the outflow cross-section,
c. scattered flow velocity measurements to be used for calibration in this specific discharge.

However, my question is the following: In order to run the model for various discharges, do I inevitably need a stage-discharge curve at the outflow to prescribe the appropriate elevation? And if I don't have a stage-discharge curve, could I use the Thomspon boundary condition for the outflow? So, what options do I have in TELEMAC 2D in order to gain reliable U-V and D predictions without using a stage-discharge curve?

Any help anyone?

Hello,

There is probably a misunderstanding because there is absolutely no problem if you know the discharge at the inflow and the water elevation at the outflow, this is how most of our applications work when you look for a steady state.

So you do not inevitably need a stage-discharge curve.

With best regards,

Jean-Michel Hervouet

Hello Mr. Hervouet,

I apologize for not being clear enough.
My only known elevation is the one used for calibration, the one I recorded manually in the river.

But my intention is to run telemac in various discharge scenarios in order to predict U, V and D at each discharge but in these hypothetical discharges I don't know the water elevation at the outflow (and how could I since they are hypothetical).

So how do I tackle this?

Thank you in advance,
Christos Theodoropoulos

Hi,

finding boundary conditions is always a tricky task. As said before, the simplest situation is if you have the elevation on the outflow.

With only one measurement it will be hard to generate something like a stage discharge curve.

Maybe you have some measurements of the discharge and the elevation not far away from you boundary? Then it might be a solution to extend you model domain until this point?

Best regards

Leo

Thank you Leo!

I don't want to develop a stage-discharge curve as this would require additional visits in the river reach, so additional cost.

And I have these measurements you suggest but only for the discharge used to calibrate the model (1m3/s). Now let's say I want to know the values of U, V and D not in the calibration discharge but in another discharge (10m3/s). For this one, I use the keyword 'prescribed flowrate' of 10m3/s at the inflow but since I have no other data for this discharge, how do I treat the outflow?

I read in the Telemac 2d manual about the Thompson boundary conditions that 'it uses the theory of characteristics to calculate the missing values'. And that 'in this case the (user-defined) values are not imposed but suggested'.

So, can I use Thompson's boundary conditions for the outflow with an approximated by me prescribed elevation (suggested) and the software will calculate the right elevation according to the theory of characteristics? Or this is not applicable in my case?

Thank you again!

Hi Theodoropoulos,

I am not an expert, but I will try to be helpful.

About Thompson boundary conditions, you have to take in account that you are prescribing one of three possible variables (Discharge, Velocities or Elevation), considering that the other 2 variables are computed by TELEMAC, what Thompson boundary condition does is, according to the theory of characteristics, to give a reasonably value to one of those variables and compute the other.

Considering this, it is not possible to prescribe an elevation and, using the Thompson condition, get a different value of elevation. The elevation will be the same; the values computed with the theory of characteristics would be velocity and discharge.

Maybe somebody else with more experience could be clearer about this.

On other hand, I believe you could try to prescribe flowrate at both inlet and outlet boundaries. Thus, you could get values of the depth/elevation at outflow boundary, this using Thompson boundary condition. Maybe this is a way of getting H values, since you want to ‘estimate them’. That is the only way I see at this moment. Surely, Jean-Michel or Leo can say whether this is ok or not, since they have more experience.

I hope it helps.

Cheers,

José Andrés.

Thanks a lot José!

So, I wasn't right about how Thompson boundary conditions work.
I will try your suggestion about prescribing the flowrate at the outflow and see what I can get.

This was very helpful!
Thanks again!

Dear all,

unfortunately, by prescribing the flowrate at the outlet, even when using Thompson's boundary condition as suggested by Jose, the river reach is continuously filled with water and the water depths after 30 min of simulation are increased to unrealistic values...

However, I' ve noticed that if I prescribe the elevation at the outflow using the Thompson's boundary conditions, I get very nice results and the elevation itself is pretty much corrected. For example, for inlet Q=1 m/s I have an outlet elevation at 96.2 m (these are my known values from the field survey used for calibration). And if I try to predict the river's state (U,V,D) for inlet Q=10 m/s (and here arbitrarily set an outflow elevation at 97 m with Thompson's boundary) the final elevation is corrected by the simulation from 97 to 99.2 m, which seems very reasonable to me.

So I will research this option a little bit to see what else I can do...
Any further help is highly welcomed :)

Thank you all for helping!
Christos