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# Reducing outlet flow using orifice

## Reducing outlet flow using orifice

(OP)
I am trying to regulate the amount of flow that can outlet my stormwater system due to release rate restrictions.  I modelled the structure where I want to regulate the flow as a pond with an orifice outlet only, then modelled the outlet pipe as a reach.  The orifice I am using is an 8" and the outlet pipe is 12".  The problem is that the capacity of the outlet pipe is regulating the outlet flow and not the orifice. Changing the orifice size only seems to affect the hydraulic grade line elevations.  The warning I am getting is that my outlet pipe reach is at 225% manning's capacity. I am not understanding why the 8" orifice is not regulating the flow to the 12" outlet pipe.  How can the flow be greater than the capacity of the pipe.  Any help would be much appreciated.
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### RE: Reducing outlet flow using orifice

Maybe you accidentally entered a base flow to the Reach.  Maybe the pipe is too long.  Maybe you accidentally entered the orifice in feet and the pipe in inches?  Maybe there is some snafu with the Pond "talking" to the Reach.

Why not model the outlet control structure with a culvert as Primary Routing, then an orifice routed to that?

Engineering is the practice of the art of science - Steve

### RE: Reducing outlet flow using orifice

Each outlet device needs to be modeled as part of the same pond. If you have an orifice plate in front of the culvert, your outlet setup might be:

Device #1 = Culvert, Routing = Primary
Device #2 = Orifice, Routing = Device #1

For full details please see the riser modeling example at www.hydrocad.net/pond1.htm   Your case is less complex, but the principles are the same.

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