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Manning Open Channel Flow Calculator

Calculate discharge and velocity from known depth, or solve normal depth for a target flow in rectangular and trapezoidal channels.

Steady uniform open-channel flow only. Normal-depth mode solves Manning numerically and does not evaluate critical flow, freeboard or erosion.
01

Channel inputs

Rectangular and symmetric trapezoidal V1 sections.

Calculation mode
Channel geometry
Hydraulic inputs
02

Open-channel result

Enter channel inputs and calculate.

Ready for calculation

Choose known-depth or normal-depth mode.

Methodology

Manning steady uniform open-channel flow

TOOL-007 uses V = (1/n) R^(2/3) S^(1/2), R = A/P, and Q = A × V in canonical SI units.

V1 channel geometries

Calculation modes

Uniform-flow assumption

Manning open-channel calculations assume steady uniform flow. Channel section, roughness and slope should be reasonably constant over a sufficient reach, and the supplied slope is used as the energy slope for this preliminary calculation.

Worked examples

Rectangular example: b = 2 m, y = 1 m, S = 0.01, n = 0.03. The section area is 2 m², wetted perimeter is 4 m and hydraulic radius is 0.5 m. The resulting flow is approximately 4.199737 m³/s.

Trapezoidal example: b = 2 m, z = 2H:1V, y = 1 m, S = 0.01, n = 0.03. The resulting flow is approximately 9.674168 m³/s.

Limitations

TOOL-007 does not calculate critical depth, Froude number, specific energy, backwater, gradually varied flow, compound sections, freeboard, lining stability, erosion, sediment transport or channel-design compliance.

Source

The method is mapped to FHWA open-channel Manning guidance inSRC-MANNING-OPEN-CHANNEL-FHWA-TOOL007-001.