Parabolic weir discharge from a pipe.

[DuckEngineer]

I have a situation where I need to keep a pump fed with water that is flowing through a pipe with a negative slope. I set up a pond node with the culvert and an arbitrary storage area extending beyond my ranges of concern. When I check the discharge at low head levels (below the midpoint of the culvert), I get the same discharge for a positive slope as for a negative slope. It seems unlikely that both discharges would be identical. What equation is HydroCAD using for a parabolic sharp crested weir? Is there a setting that I'm missing? Thanks.

 

[psmart]

For a negative (adverse)pipe slope, HydroCAD evaluates weir flow at the outlet, in addition to the other culvert flow criteria.

If your culvert is weir controlled, the stage-discharge relationship could be the same for both slope conditions (with head measured above the higher invert).

If you have further questions please post your HydroCAD project file or send it to HydroCAD support.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DuckEngineer]

At very low flow depths, I am assuming that it won't fall under any of the culvert equations. That should leave me with weir flow or open channel as my options. I couldn't find in the documentation what equation HydroCAD used for a parabolic weir. If I have that equation, then I can check the sensitivity of my model and see what I need to change to keep my pump from starving. Thanks for the help.

 

[psmart]

How are you specifying the parabolic weir? Are you using an elliptical culvert? If so, please see the culvert section of the Owner's Manual of help system.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DuckEngineer]

The culvert is round, so I didn't set any extra parameters. I know HydroCAD auto-checks for a lot of things, so I didn't want to override any of that.

 

[psmart]

What is the parabolic weir you asked about?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DuckEngineer]

The parabolic weir is the low flow condition in the pipe (spilling out over the arc of the bottom of the pipe).

 

[psmart]

The bottom arc of a circular pipe is still circular, not parabolic. For details on circular orifice calculations (including weir flow conditions) see the circular orifice discussion in the Help system or Owner's Manual.

Peter Smart
HydroCAD Software

http://www.hydrocad.net