Subcritical to supercritical regime transition (1D)

[Antonio Cioppa]

Hi everyone, I am neophyte to HEC-RAS. I’m trying to model a single channel consisting of 3 reachs: in the first reach, slope is lower than critical slope (subcritical regime); in the second reach, slope is higher than critical slope (supercritical regime) and in the last reach slope is lower than critical slope (subcritical regime). I set normal depth as boundary condition both upstream and downstream and then I performed a steady flow simulation in mixed flow regime. As seen in the following image, profile is correct as water depth is equal to critical depth passing from first to second reach.

My problem occurs in “Errors Warnings and Notes”, because critical depth at 600m is due to the fact that the program defaulted to critical depth. Infact, at 600m river station occurred 2 warnings:
1) The energy equation could not be balanced within the specified number of iterations. The program used critical depth for the water surface and continued on with the calculation.
2) During the standard step iterations, when the assumed water surface was set equal to critical depth, the calculated water surface came back below critical depth. This indicates that there is not a valid subcritical answer. The program defaulted to critical depth.

How can I solve this problem? Thanks in advance.

[Antonio Cioppa]

In order to better show my problem, in the following link is reported another example with the same warnings (1 e 2) at 100m river section. In this case, the channel consisting of 3 reachs in supercritical, subcritical and supercritical regime. Normal depth was set downstream and upstream. Unlike the previous case, water profile is completly wrong in the downstream reach.

Am I doing something wrong?

[Luis Partida]

This is not really a problem. The RAS errors warnings and notes are just there to guide you in what occurred in the back end while solving. It is up to you to decide if the solution provided makes sense or if the model needs to be adjusted. But fyi, you can open almost every single FEMA model and there will be those same warnings.

[Antonio Cioppa]

Thanks for your reply. Do you know why profile of downstream reach in the second pic continuously achieved critical depth despite I set normal depth in downstream reach?

[Luis Partida]

You can assign a boundary of ND, but you are running the simulation under mixed conditions for which i assume you have assigned an upstream boundary condition of critical depth. I do know that if you have run a model with a DS BC of ND and it ends in critical depth something is incorrect and boundary conditions should be re-evaluated. There is a blog on this see website – website.https://therassolution.kleinschmidtgroup.com/ras-post/downstream-boundary-normal-depth/

[Antonio Cioppa]

Thank you!

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