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The discharge through a small rectangular orifice is given by (where Cd = Coefficient of discharge for the orifice, a = Cross-sectional area of the orifice, h = Height of the liquid above the centre of the orifice)
Q = Cd × a × 2gh
Q = (2/3). Cd × a × h
Q = (Cd × a)/√(2gh)
Q = (3Cd × a)/√(2h)