Supercritical/Critical/Subcritical
Supercritical/Critical/Subcritical
(OP)
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!
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!





RE: Supercritical/Critical/Subcritical
You should probably be using subcritical flow for assessing WSEs.
RE: Supercritical/Critical/Subcritical
RE: Supercritical/Critical/Subcritical
RE: Supercritical/Critical/Subcritical
RE: Supercritical/Critical/Subcritical
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.
RE: Supercritical/Critical/Subcritical
RE: Supercritical/Critical/Subcritical
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
RE: Supercritical/Critical/Subcritical
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.
RE: Supercritical/Critical/Subcritical
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.