ORiFICE RESTRICTION

[Dave Z]

Hello
We are designing the storm sewer system for a residential neighborhood. The last reach of the system is a 240' long 24" PVC pipe that has a 10% grade. It dumps the water into a grassy channel. The water is moving 22 FPS. It is dumping 21 CFS at that speed. We would like to slow the water down as it dumps into the channel. Hydrocad claims you can incorporate a round or rectangular orifice restriction into the reach. How would a person model that? I have not found any information on Hydrocad's ability to model 45 or 90 degree bends in a pipe. Is that a possibility?

 

[psmart]

No, you cannot "incorporate a round or rectangular orifice restriction into the reach". I'm not sure where you got that information.

A reach is modeled based on normal Manning's flow, and is not able to handle flow controls such as an orifice. Special losses (such as extra bends in a pipe or channel) can sometimes be simulated by use of the appropriate (higher) Manning's value. For details see

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

Flow restrictions are typically modeled as outlet devices on a pond. For example, a catch-basin can be modeled as a zero-storage pond with a culvert outlet as discussed at

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

You can also model compound outlets, such as an orifice installed at the culvert inlet. But that's probably not going to do much to reduce the velocity in a steep culvert, which will be primarily dependent on the culvert parameters. Frankly, it sounds like you'll need to line the channel to prevent erosion.

Peter Smart
HydroCAD Software

http://www.hydrocad.net