Hydrocad culvert velocity vs slope

[mbbaldwin]

We are designing a partially closed drainage system. The first pipe in the system is a 24" corrugated plastic pipe with an open inlet end, which will not develop head over the top of the pipe. We are designing for 7.95cfs, and the pipe is sloped at 7% (included in the model as a pond with culvert outlet). During the first analysis run, we seemed to get reasonable velocities (4fps), but when we started changing the slope of the pipe (varying it from 25% to 0.5%), the velocity did not seem to change. Does Hydrocad just take a simple V=Q/A calculation to come up with velocity? You would think between the ranges we input that there would have been a significant change....

 

[francesca]

If it's a pond outlet you have pressure flow conditions controlled by the amount of head in the pond, not gravity flow conditions controlled by the slope of the pipe.

 

[mbbaldwin]

So if the head formed by the pond does not reach the top of the pipe, it would still be under pressure flow and not gravity?

 

[psmart]

You will find a complete discussion of the culvert analysis procedure used by HydroCAD in the Owner's Manual and help system.

Due to the varying depth of flow, the velocity at the inlet, outlet, and mid-pipe can all be different, so you need to pay careful attention to where the velocity is being reported.

For details, please see "Device Velocity" in HydroCAD help. You will note that "for a culvert reporting 'Inlet' control, the velocity is based on weir or orifice flow at the inlet". So the result in this instance will not vary with slope, as francesca mentioned.




Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

What that also means, though, is your pipe velocity could be a lot higher than 4 fps. I would guess it is, at 7%, depending on the mannings of the pipe. Certainly doublecheck it.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

Inlet control's velocity is based on Critical depth located just inside the entrance.

HydroCAD Owner's Manual