HEC RAS flow regimes

I have been to your HEC-RAS seminar and am currently working on a modeling project and I have a question. Should I consider flow in a basic drainage ditch (one ditch is about 10' wide and 5' deep and the other is about 50' wide and 2' deep) to be supercritical or sub. I am using profiles of a 10 and 100 yr. storm event. Is this something that can be assumed or must it be calculated. Thanks for the help.

Brian Gayle
Kyle Associates, llc
985-727-9377

Dear Brian :

When in doubt, run it mixed (both subcritical and supercritical).

However, if you are working in fairly flat areas, then keep it subcritical and check to see if in the HEC-RAS analysis results the computed water surface elevation is the same as the computed critical water surface elevation. For subcritical flow, the computed water surface elevation should always be above the critical water surface elevation. If not, then it is likely that there is supercritical flow at that cross-section (i.e., river station). Note, however, that you can model the stream as subcritical, and yet have supercritical flow within the culvert. This is a normal occurrence and you do not need to model the stream as supercritical in this case.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com

Chris,

Thanks for the prompt response. One more question, if I am using a mixed flow regime, I realize that I need boundary conditions on both ends. My project entails ditches which don't hold water, they only transport water during storms. Should I put in the invert of the ditch for my known water surface elevation at each end? Thanks again for the help.

Dear Brian :

Yup, you need inverts for both ends of the ditch.

If you have any additional questions or need any further information,
please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com