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I am beginner in TELEMAC . I want to simulate an unsteady flow. I have a hydrograph (time-discharge) for the inlet.

I don’t have any hydrology data for the outlet. I want to use ‘Normal Depth’ or ‘Frictional Slope’ as my outlet boundary condition.

Can anyone suggest me which type of boundary condition in TELEMAC framework will be appropriate for representing this scenario?

Regards,

Joy

P.S.
The commonly available boundary conditions in TELEMAC are as below: Which one is appropriate for my situation?

Closed boundary (wall)
Open boundary with prescribed Q
Open boundary with prescribed H
Open boundary with prescribed Q and H
Open boundary with prescribed UV
Open boundary with incident Waves
Custom ???

It depends on the length of your segment of river. Unless you are super-critical at the outlet (in which case you could have a fully open boundary of the type 4 4 4, i.e. free everything), the usual methodology is to impose the free-surface level at the downstream boundary.

If, as you say, you are a beginner, I would therefore just try to set the level to an appropriate value. You can also extend your river into an "articifial lake", where you would have more control over your free surface level.

Note that since you have access to the entire source code, you can probably implement the type of boundary you are talking about -- and I would recommend you implement it in the subroutine SL. Various distributors of TELEMAC could provide you with various basic-to-advanced training courses if you are interested, HR Wallingford included.

Hope this helps,

Sébastien.

Thanks Sébastien for your quick reply. When you said 'set the level to an appropriate value' did you mean the boundary condition type--Open Boundary with prescribed H-(Code 5-4-4)?

How to prescribe a constant water stage at the outlet when I am interested in simulating an unsteady (flood) event? Will it be appropriate? So, there is no built-in boundary condition type in TELEMAC that can act as HEC-RAS equivalent of 'Normal Depth'?

Thanks for your advise for training but I think it will be quite expensive. I am a PhD student. If you are aware of any particular training courses with pricing please email me at

This email address is being protected from spambots. You need JavaScript enabled to view it.

Thanks and regards,

Joy

Indeed, setting the level is not ideal, unless you extend your model much furthr dowstream.

Also, you can appreciate that 1D river-taylored models such as HEC-RAS are much simpler than 2D generic models such as TELEMAC-2D, which makes the coding of certain boundary types mush easier, given the 1D assumption.

Nonetheless, if you were to give me the theoretical principles of this boundary type found in HEC-RAS, I would be happy to provide you with an example of implementation of the SL subroutine in TELEMAC-2D.

Further, you can also program a free boundary in a number of ways. For instance, one could set the velocity of the node at your outlet boundary to be the velocity one or two row of triangles inside the mesh, assuming flow will always goes one way.

With the source code available, there is always a way ...

Hope this helps.

Sebastien.

Thank you so much for your reply. Your suggestion about setting the boundary velocity from the interior of the mesh is so exciting. But I must admit that I have no knowledge/training of any sort of programing including FORTRAN.

Here is the word for word description of Normal Depth from HEC-RAS reference manual. I found it is quite similar to setting a prescribed stage (as available in TELEMAC)

Normal Depth:
"Use of Manning’s equation with a user entered friction slope produces a stage considered to be normal depth if uniform flow conditions existed. Because uniform flow conditions do not normally exist in natural streams, this boundary condition should be used far enough downstream from your study area that it does not affect the results in the study area. Manning’s equation may be written as:
Q = K (Sf )^0.5
Where: K represents the conveyance and Sf is the friction slope".

Sf can be estimated by the slope of the water surface (Which I have measured in the field) and conveyance can be measured from cross-section area, bed slope and friction coefficient (say Mannings’s n)

Thanks again for your help. It will be wonderful to have a subroutine that can fit my type of boundary condition.

Joy