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Hello everyone：

I'm encountering some issues while simulating rainfall runoff using TELEMAC-v8p4 on a Linux system. I'm simulating a long slope (with a slope of 0.01) in a rectangular watershed area of 200*20m, with a closed rectangle near the bottom of the slope as the outflow boundary (intended to simulate a road's rainwater inlet). I'm concerned about the outflow flowrate from this boundary, but the corresponding output variable FLUX BOUNDARY has a very small value and oscillates significantly, which doesn't match the obvious changes in the volume variable. I'd like to know what the problem is, and would appreciate your help! (I've attached the relevant files I've used, please let me know if there's anything missing, and I look forward to your response.)

I need to add that I have also tried more typical cases recently (i.e., only linear outflow boundaries at the bottom of the slope with an elevation of 0), and the outflow results are also inconsistent with reality. I am thinking whether there is a problem with my boundary setting (my ideal boundary is free outflow). I have tried various boundary conditions, as follows:

1.Boundary code 544, HBOR = 0
The result is that the volume in the basin is reasonable, but the FLUX BOUNDARY is extremely small and oscillating, which is clearly unreasonable.

2.Boundary code 544, PRESCRIBED ELEVATIONS = -10.0
The result is consistent with the above case.

3.Boundary code 544, PRESCRIBED ELEVATIONS = 0.001
The result has a stable FLUX BOUNDARY value, but it is still significantly smaller than the standard value.

4.Boundary code 444
The result is that FLUX BOUNDARY is 0 for a long time in the early stage, which is also inconsistent with the actual situation.

I really need your advice!

Hi
In the reality, the flow is supercritical at the boundary.
You tried to impose water depth but with very low values so the computation oscillate but this sounds logical from the numerical point of view.
This should be deeply examinated according to the options choosen and some existing parameters.
Maybe an option could be the open boundary (444) but with the thompson method. Have a look at the gouttedo test case

Hope this helps

Thank you for your response! I have modified the boundary code to 444 with the Thompson method, and after modifying some numerical parameters in the cas file with reference to the official examples, I have obtained relatively reasonable results. However, there is still some deviation from the reference values (the results obtained by finite volume method in the literature). Specifically(see attached figure):

1.The stable outflow value is slightly smaller(TELEMAC simulation result is around 3.54, while the reference value is around 3.68);
2.There is a deviation in the time when the outflow rate drops to 0(TELEMAC simulation result drops to 0 at around 350min, while the reference value drops to 0 at around 300min).

The above results were obtained after trying different numerical parameter settings, which indicates that such differences are not caused by errors in numerical parameter settings. What could be the possible reasons for this? I look forward to hearing your thoughts.

Hi
4% difference in the result is a quite good value IMO.
What about the water depth in the street?
Did you test other friction values

Regards

Thank you for your response!

I used the friction value consistent with the literature and did not try other values. Additionally, the literature provides not only the outflow rate but also the runoff volume data. I compared the results and found that the error is indeed within an acceptable range.
Also, I seem to have failed to mention that the 2000*20 case is the one where the bottom slope is directly set as the outflow boundary. After verifying the accuracy of this case, I used the same settings to simulate the 200*20 rectangular boundary case and found that only the finite element method can produce reasonable results, while the finite volume method will diverge.

Regards