Open channel flow · Manning

Computes the wetted geometry, uniform flow rate and hydraulic regime while showing the flow depth in the cross section.

Section, roughness and slope

SectionBottom width b · Talud lateral z · H:VFlow depth y · Pendiente S · m/mCoeficiente Manning nGravedad ·

Flow rate Q

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Mean velocity

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Wetted area

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Hydraulic radius

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Froude number

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Flow regime

Subcritical

Cross section and flow depth

Flujo uniforme

\[Q=\frac{1}{n}AR_h^{2/3}S^{1/2}\]

\[Fr=\frac{V}{\sqrt{gA/T}}\]

A is the wetted area, R_h=A/P, where P is the wetted perimeter and T the free-surface width.

Manning equation calculator

The Manning equation estimates uniform gravity flow in channels. Capacity depends on the flow area, the hydraulic radius, the energy slope and the equivalent roughness n.

Indicative Manning n values

Revestimiento	Typical n	Comentario
Concreto acabado	0.012–0.015	Regular, clean channel
Tierra limpia	0.018–0.025	Depends on irregularity
Cauce natural	0.025–0.060+	Vegetation and meandering raise n

Froude number

The Froude number compares inertia with gravity. Fr<1 means subcritical flow controlled from downstream; Fr>1 means supercritical flow whose disturbances cannot travel upstream.

Preguntas frecuentes

Is the bed slope always equal to S?

Only in uniform flow. In gradually varied flow the energy slope differs.

Does it apply to a full pipe?

No. Once full, the free surface disappears and a pressurised-flow equation must be used.

How do I choose n?

Base it on the material, its condition, vegetation and the applicable design tables; calibrate against measurements where possible.