Supercritical flow in culvert with roadway overflow

[GaryR16]

I am modeling an existing culvert on a steep stream. Flow in the stream is legitimately supercritical. Due to the constriction caused by the culvert, I’m using the mixed flow regime option. The model results appear well behaved, but the following warning is being issued, “The flow through the culvert is supercritical. However, since there is flow over the road (weir flow), the program cannot determine if the downstream cross section should be subcritical or supercritical. The program used the downstream subcritical answer, even though it may not be valid.” It seems that the water surface elevation downstream of the culvert (essentially the tailwater on the culvert) should be the supercritical elevation. Is there a way to force the program to select the supercritical elevation? Thanks in advance for your help.

[Anonymous]

If there is supercritical flow that is hitting and going over the road, then I think that is going to be pretty hard for a 1D program like RAS to know whether downstream is supercritical or to be able to compute it that accurately.

One option is to model the culvert as a cross section with a lid and see what RAS gets for an answer.

Another option that would be pretty awkward would be to put a junction just upstream and downstream of the culvert and have two reaches between the junctions. One reach would contain only the culvert (make the weir high enough so that it does not have flow) and the other reach would contain the actual weir.

I don’t know that either would be that accurate but one or the other might give a supercritical answer downstream.

[Author]

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