qpb - Pre-development peak runoff rate
Es - Water surface elevation at design volume
Eo - Elevation of centroid of orifice opening
Co - Orifice Coeficient
g - Gravity

Typical values for Co: Range from 0.40 to 0.60
Rough edges - torch cut: Co = 0.40
Smooth squared edges: Co = 0.60

where acceleration of gravity, g = 32.2