Warning:Defaulting to critical depth

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Warning:Defaulting to critical depth

Yair Simonson
I've got a relatively steep stream with a small bridge being modeled as a 1D steady flow model with a mixed flow regime. There is a hydraulic jump just prior to passing through the bridge. The upstream steam is...complicated. I can follow up with more detailed information about the stream if it will help someone help me.

 I researched the issue and found 4 potential culprits:
"Bad cross section data"
"Program can not balance the energy equation above or below the top of a levee or ineffective flow area."
"Cross sections spaced too far apart"
"Wrong flow regime"

After many tweaks and experiments I've determined that "Bad cross section data" is likely to blame.

My question is what EXACTLY does this mean???

My method of developing the model is to draw an alignment in civil 3d on a LIDAR surface (which lines up exactly with survey data) and cut sample lines along the alignment. Then I take copy the station and elevation points from the sample line sections to excel for organization then i paste it to hec-ras for each cross section. This method seems as close to "real" as i can possibly get.

By tweaking variables i determined that the ineffective zones and the mannings n might have some control on when and where these instances of defaulting to critical depth occur.

I also tried opening up the hecras example model for mixed flow and seeing what variables changes would cause this issue to occur and go away. And it seems that modifying mannings n would do it. I don't understand why. isn't it possible that the example model is just rougher or smoother implicating a different hydraulic result which is still valid?

Any explanation about any of this would be greatly appreciated.