orifice plate storm detention

[arrowcivil]

I need to use a pipe to store a portion of the flow and design an opening that is less than the pipe diameter to reduce the flow. I have seen plates that are place above the springline that reduce the flow and I have seen circular orifices placed at the flow line.
DOES THE SHAPE NOT MATTER AS FAR AS THE EQUATIONS ARE CONCERNED OR DOES THE SHAPE OF THE ORIFICE REQUIRE DIFFERENT EQUATIONS?

The textbook equations seem to show only an example of the circular opening.

 

[hydrae]

Other shaped opening will have a larger edge effect which has not been part of the standard equation
The edge effect is from the venia contracta (sp) which in corners gets compounded in 2 directions
Circles are also easy to cut with a hole saw, fly cutter, or punch

Hydrae

 

[cvg]

rectangular slots are easy to cut with any sort of tool and depending on height and width, can be designed to perform much differently than a circular orifice. note that until completely submerged, both circular and slotted openings perform as a sharp edged weir not an orifice.

 

[Drew08]

The easy answer is no, the shape does not matter, the orifice equation is the same: Q = C A sqrt(2gH) for both cases. The complicated part, as hydrae points out, is the possible reduction in the C coefficient because of increased edge length per area of flow. I don't think much study has been completed on non-circular openings, so there really isn't any standard available for determining C. C = 0.6 is still widely accepted, though perhaps not necessarily accurate for highly elongated orifices.

In fact very few studies have been done on low-head orifices in culverts in general. Assuming C = 0.6, and the point of reference for h being the centerline for low-head spillways is one of those things that have been so extensively used for such a long period of time, it has become fact with very little experimental back up.