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Calculate Required Area for (FHWA) Orifice Outlet Control With Free Discharge:

(Note: All input Metric units are converted to English units for the equations below and then the output English units are converted back to Metric)
Pre-development peak runoff rate (qpb - cfs or cms)
Water surface elevation at design volume (Es - ft or m)
Elevation of centroid of orifice opening (Eo - ft or m)
Orifice Coeficient (Co)

Typical values for Co: Range from 0.40 to 0.60
Rough edges - torch cut: Co = 0.40
Smooth squared edges: Co = 0.60
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Area of Orifice, Ao =

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