Free Online Orifice Drain Time Calculator — Pond, Basin, or Tank

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Pond or Basin Orifice Drain Time — Conic Volume Method

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Inputs

Starting water surface elevation		X
Starting pond or basin area, A1		X
Ending water surface elevation		X
Orifice centroid elevation		X
Pond or basin area at orifice elevation, A0		X
Orifice diameter (circular) or height (rectangular), D Circular Rectangular		X
Orifice width, W (rectangular only)		X
Discharge coefficient, C_d		X

Results

Starting head, H₁ (WSE − centroid)	Starting head, H₁ (WSE − centroid)	X
Max (starting) flow, Q_max	Max (starting) flow, Q_max	X
Ending pond area, A2 (interpolated)	Ending pond area, A2 (interpolated)	X
Ending elevation check	Ending elevation check	X
Drained volume	Drained volume	X
Drain time (seconds)	Drain time (seconds)	X
Drain time (minutes)	Drain time (minutes)	X
Drain time (hours)	Drain time (hours)	X
Drain time (days)	Drain time (days)	X

Formula: t = √H₁ / (C_d A_or √(2g)) × (2A_x/5 + 8√(A_xA₀)/15 + 16A₀/15) gives drain time from head H to the orifice. Drain time = t(H₁,A₁,A₀) − t(H₂,A₂,A₀), where H₁ = starting elevation − orifice elevation, H₂ = ending elevation − orifice elevation.
Method: The conic volume method models the pond or basin as a conic section between the starting area A₁ at the initial water surface and the area A₀ at the orifice centroid elevation. A₂, the pond area at the ending elevation, is interpolated from A₁ and A₀ using the conic section model. Drain time from starting to ending elevation equals total drain time from H₁ to orifice minus remaining drain time from H₂ to orifice.
Derivation: Equation derivation