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Hello,

I'm working with a river with 3 boundaries (two inflows and one outflow). When I prescribed only a Q at my two inflows, Telemac shows an error indicating that maybe my flow is supercritical and I need to prescribe an H. So, I changed my boundaries to 555 with prescribed H and Q. The problem is that, when I see the Q that is entering in my boundaries, it never reaches exactly the Q that I prescribed. It seems that my H is not correct (or not exact, I tried to calculate it but its difficult because its a very irregular mountain river). I tried to change H a little but I never get de Q that I want.

Any help with this problem would be helpfull. I'm new at Telemac.


Javiera

Hello Javiera,

Indeed, if the flow is supercritical, you have to prescribe both H and Q at the inflow(s) but you have to let the outflow free (444). Try to change the boundary condition file with this free condition (444).

Hope this helps,

Chi-Tuan

Thanks Chi-Tuan,

I let the outflow free but it didn't help with my problem. I have an inflow of 184 m3/s with an estimated H of 415.6 m.a.s.l. However, with that H I get an inflow of 190 m3/s. I tried to change H but I never get de inflow I want. I opted to use the Thompson method because with strong setting the inflow it's bigger.

Regards,

Javiera

Hello Javiera,

You have a couple of options for your simulation, depending on whether you want to include sediment transport and prescribe a sediment flux at the inlet or not.

Without sediment transport:
You should stick with prescribing Q at the inlet and H at the outlet. To do so you can try to:

• Increase the bottom roughness at the inlet
• Extend the inlet upstream by adding an artificial pond, which will ensure subcritical flow conditions

If you are prescribing sediment (likely bed load) at the inlet:
You will need to deal with the supercritical inlet boundary. To do this, prescribe both Q and H at the inlet and leave the outlet free (444). Be careful to prescribe the water depth correctly so that the actual discharge is close to the prescribed value. As a next step, you can modify some subroutines to compute the difference between the actual discharge and the prescribed one in each time step, and add or subtract this difference shortly after the inlet using sources. This approach has worked well for me, even with unsteady hydrographs and prescribed bed load at the inlet.

Best regards,
Sebastian

Thanks Sebastian,

I will try to be careful to prescribe the water depth correctly because I'm modeling bedload too.

Regards :),

Javiera