Hydraulics and Fluid Mechanics in ME
An orifice is said to be large, if

The available head of liquid is more than 5 times the height of orifice
The velocity of flow is large
The available head of liquid is less than 5 times the height of orifice
The size of orifice is large

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Hydraulics and Fluid Mechanics in ME
The discharge through a large rectangular orifice is given by (where H1 = Height of the liquid above the top of the orifice, H2 = Height of the liquid above the bottom of the orifice, b = Breadth of the orifice, and Cd = Coefficient of discharge)

Q = (2/3) Cd × b × √(2g) × (H22 - H12)
Q = (2/3) Cd × b × √(2g) × (H2 - H1)
Q = (2/3) Cd × b × √(2g) × (H21/2 - H11/2)
Q = (2/3) Cd × b × √(2g) × (H23/2 - H13/2)

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