Outlet pipe from detention basin will have tailwater....what to do?

[BillsSis]

Hello everyone,
The detention basin has an outlet structure of a 3' x 3' box, with a V-notch weir on the entrance side and a 12" RCP stubbed into the opposite wall. I work with HEC-HMS. The pipe has a very flat slope (0.26%); it has tailwater. How would I figure the discharge going through the V-notch weir and over the top of the box - all before flowing into the pipe? Will HMS work? Or would you rather I use some other software or do hand calculations?

Thank you,
Bills Sis

 

[mdrehansk]

go for pond pack or Bentley sewergems, I thick HMS will not provide result with tialwater conditions

 

[psmart]

This is easy to model in HydroCAD as discussed at

http://www.hydrocad.net/pond1.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

HMS does handle tail-water - for pipes and channels

for a spillway or weir, it might be easiest to input an elevation-discharge curve
if you have tail-water on your v-notch, suggest that this is not the best design. v-notch's should be designed without tail-water. Unlikely that you will be able to predict the discharge coefficient for a v-notch weir with submerged nappe.

 

[BillsSis]

Thank you everyone, for your response. Tailwater will only occur if there is higher flows; fortunately, discharges will not get that high, so the pipe is operating with much less tailwater. I'm going to charge forward using the v-notch weir, now that I won't have to worry about tailwater.

Thanks again!

 

[gbam]

There is an approach to evaluating submerged weirs, just google it. Many options come up. You would have to analyze it external of HMS and code a rating curve as cvg stated. You may want to evaluate your options to see if there is a better design.

Found this one:

http://www.fs.fed.us/biology/nsaec/...nte_1947_Submerged_weir_discharge_studies.pdf