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Hello,

You are right but it's strange that the flow is supercritical here because the slope of the chanel is very low (this is the Rhône delta) and the discharge is not very high (3700 m3/s).
I launched a simulation with an upstream rating curve. I will tell you if it solves my problem.

Thanks.

Jules

Hello,

I launched a simulation with 2 rating curves for my 2 outlets and 1 upstream rating curve for my inlet. I added a liquid boundary file which gives the discharge for the inlet. The problem is that the discharge is not respected. I prescribed 3727 m3/s and there is only 700 m3/s.
What I'm doing wrong? Is I need to change the type of upstream rating curve (Q(Z) instead of Z(Q))?

My case is particular. This is the Rhône River delta. Downstream, there is a diffluence where the Rône River is divided into 2 channels. That why I have 2 outlets.





I take a look at the Froude number of my previous simulations and there are just few points where the Froude number is upper than 1 (1 node for the inlet). So we can't say that the flow is supercritical.

Any suggestions?

Thanks.

Jules

Hello,

Using a stage-discharge curve upstream leaves to much freedom to the problem, everything becomes a solution.

I suggest that you start by establishing a steady state flow without using any stage-discharge curve, just using your measurements for a given discharge upstream. This may lead you to calibrate the friction until the elevation upstream corresponds to the observed value.

If the Foude number has a risk to be greater than one at the entrance in one point, it is perhaps useful to have a small articial zone at the entrance, with lower than real bathymetry, so that the problem is well posed if you prescribe only the discharge.

With best regards,

Jean-Michel Hervouet

Hello,

Thank you for your advice. I launched a simulation without using stage-discharge curves, just by giving a free surface level for a given discharge upstream. It works well. The long profil of the free surface fits better with the mesured value.



Know, I will play with the friction coefficient to adjust the curves.

But I don't understand why, when I impose a rating curve, I don't get the same result?

Best regards.

Jules

Hello,

When you start from a steady state with values that fit your rating curves, you avoid bifurcating to other possible solutions, which may even ignore your rating curve if they are supercritical.

Regards,

JMH

Hello,

When I change the free surface level at my two outlet (e.g. 2.28m and 5.54m) with an initial and constant elevation of 6m, water enters through one outlet, so my model have 2 inlet and one outlet.
I think it come from the initial conditions but have you any suggestions?

Jules

Hello,

I don't understand what happends with my model.

If I impose an upstream discharge of 3228 m3/s. I impose an elevation of the left outlet at 2.70 m and 2.45 for the right outlet. Initially, there is a constant elevation of 3m in all the model. My simulation is OK. 90% of the upstream discharge passes through the right outlet and 10% through the left one.

But when I change the upstream discharge to 413 m3/s, elevation of boundaries (respectively from left to right 0.29m and 0.15m) and initial elevation to 0.5m. I'm supposed to get the same division of upstream discharge between left and right outlets. But water enters in my model through the left outlet.

Why water enters through an outlet in one case and not in the other case?

With best regards.

Jules

Hello,

Can it be that your elevation of 0,29 m at the left outlet actually corresponds to a higher energy grade than the one you have at the junction, calculated from your downstream level of 0,15 m at the right and most downstream outlet?

Beware also that you are looking at two very different discharges with a non negligible difference on water levels. In theory your bed roughness becomes higher when your water depth decreases. In other words, if you calibrated your model for 3228 m3/s, the bed roughness you have in the lower reach is maybe too little compared to the actual flow conditions for Q = ca 400 m3/s. If you are sure to have correct levels, this might be the cause of your problem.

Good luck!

Best regards
PL

Hello PL,

Thank you to taking time to look my problem.

I finally understood why water enter by the letf outlet. That's because I put a water level higher than the wter level calculated at the diffluence. So the slpoe of the free surface is reversed .

Best regards.

Jules