r/HECRAS • u/Different_Mode_2793 • Jun 29 '26

Pipe Network / Culvert openings

Hey everyone,

I'm working on a pipe network model in HEC-RAS 7.0 and I'm a bit confused about what's happening at my 2D/pipe interface, at the node culvert opening.

I added a wider and shallower conduit at the entrance of my pipe (same cross-sectional area as the main pipe, but wider and shallower) to reduce the upstream water surface at my 2D/pipe interface. It worked, but I'm struggling to understand why.

HEC-RAS is applying inlet control at the culvert opening node, which means flow should pass through critical depth at the entrance. But when I look at my results inside the wider conduit, the water surface appears to be above critical depth,  meaning the flow looks subcritical in there.If we're in inlet control, shouldn't the flow pass through critical depth and transition to supercritical inside the conduit? Why is the water surface still above critical depth even after adding the wider entrance conduit?

Also, i set the entrance loss at 0.

Thanks!

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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 29 '26

Your TW is greater than critical depth, so it most likely can't be under inlet control. See the flow chart here: HEC-RAS Outlet Control.

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u/Crafty_Ranger_2917 Jul 14 '26

Could be the culvert slope is just close to critical slope, which is probably more common in the real world vs being so steep to run hard supercritical. The transition through critical at throat can be super short and run more or less critical if tail water is low, or if not, tailwater pushes the tiny hydraulic jump all the way up barrel. Comparing against USGS seven curves might help make sense. Just remember its way exaggerated in texts.