HEC-22 Supercritical Flow 1

[burrphillips]

I'm attempting to manually use HEC-22 to prepare a HGL through a section highway stormdrain. Attached is a PDF of the preliminary profile. The stormdrain discharges into the side of a large box culvert that is flowing supercritical. Furthermore, all of the pipes upstream are flowing supercritical. The manual states that "If supercritical flow occurs, pipe and access losses are not carried upstream". I assume that "access" should have read "access hole". Three questions:
1) Is it correct to assume that the HGL along the pipes is just the normal depth of flow at the design discharge?
2) If so, is it correct to assume that EGL along the pipes is the HGL plus the velocity head (v^2/2g)?
3) The real question, how are the EGL and HGL computed across each man/access hole? Since it is supercritical flow, is the access hole HGL and EGL equal to the HGL and EGL of the upstream pipe just before it enters the access hole? Or, do I need to calculate access hole losses due to plunging, benching, and deflection? Basically, how do I make sure the flow won't blow the lid off the access hole?

Thanks for any guidance.

 

[gbam]

I will try to address your questions

1 - Yes, unless there is a tailwater affect that will cause a hydraulic jump in the pipe. To represent that one would need to progress the SD calcs from downstream to upstream, etc...

2 - Yes

3 - Headloss in a supercritical regime does not occur as in subcritical regime. The supercritical regime has an upstream control - resulting in "inlet control" per se. You should compute access hole losses to determine if the upstream pipe will result in tailwater control - this will require normal EGL calcs until it unseals or encounters a hydraulic jump.

In order to ensure that lids are not blown keep the EGL below the surface. Hope this helps. The FHWA H22 procedure is fairly straight forward and one can code a spreadsheet to compute the EGL/HGL.

 

[burrphillips]

Thanks gbam. Your confirmation of #1 and #2 was a big help. I didn't completely follow the response to #3 - specifically losses through access hole - but I will dig back into the H22 manual as you suggested. For the prelim design I simply kept the pipes deep enough that the incoming pipe's EGL was below the access hole's rim. Now, I would like to reduce the depth to save the client's excavation costs.

 

[gbam]

I was trying to say that the system may flip from super to sub dependant on potential backwater influences. Years ago when I first started in this field, a senior engineer simply stated: in a supercritical regime headlosses do not propagate upstream.

 

[LincolnPE]

There is no upstream communication within a supercritical flow regime.