open TELEMAC-MASCARET The mathematically superior suite of solvers

I would like to open a discussion on how to impose open boundary condition (theorical aspects).

The question is not specific to telemac2d, 3d or mascaret.

Theory of characteristics :

		Theory	Mascaret	Telemac
Sub-critical	inflow:	1 information needed	H (or rating curve) or Q	u,v (or Q)
	outflow:	1 information needed	H (or rating curve) or Q	H (or rating curve)
Supercritical	inflow:	2 information needed	H and Q (or V)	H and u,v (or Q)
	outflow:	No information needed	Free boundary	Free boundary (with time step limitation)

First of all, is my table correct ?

Based on the « Note de principe » of mascaret, for a sub-critical inflow, elevation or discharge can be indifferently pescribed? With telemac, a prescribed elevation at entrance is an ill-posed problem.

Why this difference between mascaret and telemac?
I can understand that the theory is slightly different in 2d, but is it the reason?

Telemac will accept a free surface condition for a sub-critical inflow but it will be a ill-posed problem. Does it mean that telemac can deduce a discharge that satisfy St Venant equation but not necessarily the correct one (inertial effect are not considered).

Is it correct with mascaret to prescribe an elevation (in this case how to be sure that the discharge is correct)?


This is important when you have only a water level record at a station. It can be the case in estuary or other station where rating curve is not univocal (due to backwater effects).

At this point, I don't know if it is better to prescribe elevation at the entrance and the exit (based on actual elevation record) or if I have to artificially extend my model to impose a discharge at station where flow is recorded (directly or deduced from a rating curve).

Some hints have already been given in the following posts but I do not understand why an ill-posed problem in telemac (on physical aspects) is acceptable in mascaret.

#6333
#6022
#2355
#9509

Hi everybody,

If there is no reaction, I assume that there is no mistake in my table.
I also must reformulate my requests to get some answer. (Too many questions in my first post and probably very small number of both telemac and mascaret users.)


Let's focus on telemac. It would be great if you could give me hints to correctly impose boundary condition at boundaries affected by backwater effect.

Considering a case where we have water-level record (and a rating curve affected by uncertainty or non-univocal relation). Gauges in estuarine or stations placed just upstream a confluence.
What is the best strategy?

• To impose water-level at entrance. It will be an ill-posed problem (physically) but it will use good and accurate observation. In such case, advection upstream will be cancelled
• To impose discharge that is affected by uncertainties due to the rating-curve. It will be a « well-posed » problem with strong uncertainties at boundary.
• To impose both water-level and discharge and Thompson condition to overcome an over-constrained condition. In this case, is it possible to chose which variable will be relaxed?

Maybe there is no generality on this subject.

With best regards,

Hello,

In your case I would prefer the Thompson condition, but there is no way to choose the variable to relax. There is only a relaxation coefficient that is hardcoded in thomson.f. Generally speaking Thompson is used for offshore conditions, with small velocities, we have little experience in river conditionswhere the Froude number may be important.

Regards,

Jean-Michel Hervouet