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hydraulics / open channels

Manning open-channel flow calculator.

Estimate mean velocity and discharge for steady, uniform flow in a rectangular channel from its width, flow depth, bed slope, and Manning roughness.

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A flow estimate starts with the boundary.

Manning’s relationship connects channel geometry, hydraulic radius, bed slope, and roughness to an estimated mean velocity. Multiplying mean velocity by flow area gives discharge.

Governing equation

Q = (1/n) A R2/3 S1/2. For a rectangular channel, A = by and R = A/(b + 2y).

Keep the water problem wider than Q.

  • Check freeboard, overtopping, entrance and exit conditions, structures, bends, and transitions.
  • Check whether the selected roughness represents the actual boundary, vegetation, sediment, joints, and maintenance state.
  • Consider unsteady, backwater, gradually varied, or rapidly varied flow where the steady-uniform assumption is not appropriate.