Gravity Flow Pipe Design Calculator | Manning Based Open Channel Flow
Gravity Flow Design
Iterative Calculation (Steve, 2000) β’ Full Component Library
Process Parameters
Pipe Geometry
Fittings & Valves (Quantity)
Predicted Fraction Full
-- %
Velocity
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Froude Number
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Equiv. Length (Le)
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Flow Depth
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Technical Notes
Variable Definitions
- Q: Flow rate (GPM or mΒ³/h).
- S: Pipe slope (ft/ft). Inputs display as in/ft (imperial) or % (metric).
- n: Manning roughness coefficient (dimensionless).
- D: Pipe internal diameter (ft), computed from database OD and wall thickness.
- R: Pipe radius (ft), R = D/2.
- ΞΈ: Wetted angle (rad) describing liquid depth in a partially full circular pipe.
- A: Flow area (ftΒ²), A = 0.5Β·RΒ²Β·(ΞΈ β sinΞΈ).
- P: Wetted perimeter (ft), P = RΒ·ΞΈ.
- Rh: Hydraulic radius (ft), Rh = A/P.
- Le: Equivalent length to account for fittings (ft), combined with physical length for headloss.
- Fr: Froude number (dimensionless) for open-channel stability screening.
Formulas / Logic
- Manning equation (US customary): Q = (1.49/n)Β·AΒ·Rh2/3Β·S1/2.
- Partially full geometry: ΞΈ is solved iteratively so that Manning flow matches Q for the given slope and diameter.
- Equivalent length (screening): fittings are converted using an approximate Le/D library and scaled by the wetted fraction (ΞΈ/2Ο) to reflect partial filling.
- Velocity: V = Q/A (with unit conversions).
- Froude number: Fr = V / β(gΒ·Dh), where hydraulic depth Dh=A/T and top width T=2RΒ·sin(ΞΈ/2).
- Design Mode: for a target area fraction, ΞΈ is found first; then radius R is iterated so that Manning capacity meets the target Q.
This is a gravity open-channel (free-surface) sizing method and is
not intended for pressurized pipe flow.
Assumptions / Notes
- Steady, uniform flow is assumed (Manning applicability). Backwater effects, surcharging, air entrainment, and transitions are not modeled.
- Use a realistic Manning n for the pipe material and condition (new vs aged/corroded).
- Fittings allowance is a screening approximation; for critical designs, confirm with project hydraulic standards and detailed headloss modeling.
- Critical flow caution: when Fr β 1, surface waves can be unstable; consider design margin.
Standards / References
- Manning open-channel flow practice (common civil/mechanical hydraulic design).
- Fitting equivalent-length concepts aligned with common pipe hydraulics references (e.g., Crane-style Le/D screening).
- Pipe OD/wall thickness data are embedded and are intended for preliminary sizing; confirm against the governing piping standard for the selected NPS/DN and schedule.