Culvert flow: Outlet control with super-critical flow in barrel?

[Marulke]

Hi,
in several culvert capacity calculations I have noticed the following:
1) The flow in the barrel is entirely super-critical.
2) The outlet control answer gives higher energy level at the culvert inlet, and is therefore used.

I find this strange. When the flow in the barrel is super-critical with critical flow at the inlet, then the culvert is in inlet control.

Outlet control requires sub-critical flow from the inlet and some distance downstream.

How can HEC-RAS calculate the flow to be entirely super critical and at the same time find that the culvert is in outlet control.

I have seen this several times on circular concrete culverts with slope 1 % – 5 %.

Is this physically possible?

Regards
Marulke

[Jarvus]

This is confusing. And I agree that it needs better terminology. My best guess is:

RAS is using the term ‘inlet control’ to refer to the situation where the inlet equations are controlling the flow. These are the equations that are based on the shape of the culvert (ie, square edge, corrugated metal etc) that came from the work initially done by Federal highways.

If these equations do not control, then it is labeled as outlet control even if the inlet passes through critical depth and the culvert is supercritical the entire way. But I would tend to agree that this is also inlet control. Although it is not the ‘inlet control equation’ as described above.

[Author]

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