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hydraulics / open channels

Manning trapezoidal channel calculator.

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

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Side slopes affect more than the top width.

Manning's relationship connects channel geometry, hydraulic radius, bed slope, and roughness to an estimated mean velocity. For a trapezoid, the side slopes change both the flow area and wetted perimeter before discharge is calculated.

Governing equation

A = y(b + zy), P = b + 2y√(1 + z²), R = A/P, and Q = (1/n) A R2/3 S1/2.

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.