Orifice in storm line with back water pressure 2

[19680316]

Planned to install three orifices along the storm line CBMHs which pases three sub cathcments. Let water pond on the three surfaces to three different elevations.
Used rational formular and hand calculations to control 100yr storm allowable peak flows.
When I sized the last orifice to have required ponding depth and peak flow, it create a back water flow on second orifice. It may reduce the total flow from second orifice.

How can I resolve this problem? Appreciate all your help!

Yanni

 

[psmart]

You could do a static HGL calculation that includes the tailwater effects. But since you're detaining water in the system, a constant-flow analysis (such as Rational Method) isn't the best approach. You should consider a hydrograph routing with three coupled "ponds". But your decision will depend partly on what is allowed and required by the governing regulations.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Ryb01]

If I'm understanding your project correctly it sounds like you have three orifices inline within your storm system?? I agree with psmart, the rational method is not the best approach for this. Would there be an opportunity to install three separate catchbasins with ICD's to create ponding, then direct flow to a main STM line? Reservoir routing should definitley be considered.