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

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Hydraulics and Fluid Mechanics in ME
The purpose of a surge tank is

All of these
To eliminate water hammer possibilities
To control the pressure variations due to rapid changes in the pipe line flow
To regulate flow of water to turbines by providing necessary retarding head of water

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