Supercritical/Critical/Subcritical 3

[amicka]

Hello All,

I am attempting to model the floodway of a ditch in HEC-RAS, but am having trouble with varying output. When I model the system using subcritical or mixed flow regime, the resulting WSEs are much higher than when I use the supercritical flow regime. Can anyone please define "supercritical" and "subcritical" in HEC-RAS terms for me because I can't seem to find any definitions in the manual.

I did search the forums before posting this question and couldn't find a helpful discussion, but if someone knows that this issue has already been addressed, please let me know.

Thanks!

 

[bltseattle]

Super- and sub-critical flows are fundamental concepts in hydraulics, see your college hydraulics textbook. Supercritical depths are always less than subcritical depths, but to know which one you have in your scenario you have to understand your downstream border conditions and enter them appropriately in the model.

You should probably be using subcritical flow for assessing WSEs.

 

[cvg]

using mixed flow will typically analyze the system in both the upstream and downstream directions. Upstream for subcritical flow and downstream for supercritical. It will report the higher water surface elevation calculated which is generally based on subcritical flow conditions. This is probably the correct value to use, assuming as blt says you have entered in the upstream and downstream controls properly.

 

[francesca]

You should use the Hydraulic Reference Manual; it's much more comprehensive than the user guide.

 

[LCruiser]

cvg - I think you got that backwards. Nothing downstream matters with supercritical flow.

 

[cvg]

I agree, nothing downstream matters for supercritical - unless high tailwater causes a hydraulic jump...

It is my understanding that for backwater calculations (for subcritical flow condition), hec-ras starts at the lower end of the channel at the control section and then calculations move upstream. For supercritical flow, tailwater doesn't control the water surface elevation so the calculation moves from the upper end (at the control location) to downstream.

Francesca is correct, the hydraulic reference manual discusses sub/super/critical flow at length. But, prior to reading that it would be better to pick up "Open Channel Hydraulics", by V.T. Chow and learn the basics first.

 

[LCruiser]

However, the definition of a hydraulic jump is when the flow regime goes from supercritical to subcritical so it's still true. But, yes, subcritical calculations start downstream, supercritical calculations start upstream.

 

[LHA]

By "ditch", I assume you mean a swale. Using HEC-RAS is probably overkill. I would just use a Manning's analysis, with a well-thought out "n" value; a variable "n" for grass or riprap.

Also, and I don't mean to sound insulting, but you shouldn't be just plugging-and-chugging numbers into HEC-RAS and reading the output blindly. It is a very powerful and complex, iterative model. You have to able to review the output critically, and change errors in the input. If you don't, the results will not withstand outside review, guaranteed.

Last year, one of my co-workers was reviewing a submittal where an engineer had used HEC-RAS to model a roadside swale, with very little flow expected. He rejected it 3 times, each time with the suggestion that Manning's would be better. The model was not valid, because the engineer did not understand the output. Output gave values which were not possible to replicate with correct input. Finally, the engineer gave up, ran a one-page Manning's, showed that the swale was fine for Q, shear and V. My co-worker (gladly) approved it after about a minute of review.

Engineering is the practice of the art of science - Steve

 

[amicka]

I agree that HEC-RAS is overkill for this particular situation - the "swale" is very nearly dry most of the time and we have already provided a Manning's analysis. However, the state agency reviwer is requiring a HEC-RAS study. We're working with said reviewer to find a way to meet his needs.

I don't take your comments as insult at all. I know you're trying to help. I am trying to figure out HEC-RAS without any guidance (other than the manuals), which is why I turned to the Forum for help. Thanks to all who responded.

 

[blueoak]

If using mixed you still get supercritical you may need to look at your swale. You either have a steep channel or have an error.

It might also be that the reviewer was not very confident in your analysis. I know a lot of people think that mannings is the end all for flow, but in many situations the world is not constrained as manning's flow. It may be that your reviewer is concerned that you are assuming normal depth in a situation that doesn't validate that assumption. I tend to think there is something of importance near your simple swale for a state agency to be concerned and your question of the difference of supercritical versus subcritical may be alarming your reviewer.