Open Channel Flow Calculator (Manning's Equation)

Your details

Dimensionless slope (rise over run). For example 0.002 means 2 mm drop per metre of length.
m/m
The flat bottom width of the channel.
m
Depth of water from channel bed to water surface.
m
Flow velocitySubcritical
2.275

Mean cross-sectional velocity from Manning's equation

Discharge (Q)8.1909
Flow area (A)3.6
Wetted perimeter (P)5.4
Hydraulic radius (R)0.6667
Hydraulic depth (Dₕ)1.2
Froude number (Fr)0.663
Critical depth (yᶜ)15
Flow regimeSubcritical (Fr < 1)
Specific energy (E)1.4639
Top water width (T)3
Manning's n0.015
0.663 Fr
Subcritical<0.95Critical0.95-1.05Supercritical1.05+

Velocity 2.275 m/s, Froude 0.663 - Subcritical (Fr < 1).

  • The channel carries 8.191 m³/s at a mean velocity of 2.275 m/s.
  • With a Froude number of 0.663, the flow is subcritical (tranquil). Surface waves travel upstream and the flow is controlled by conditions downstream.
  • Critical depth for this discharge is 15.000 m; the actual depth is 1.200 m, confirming subcritical flow.

Next stepVerify the channel lining is adequate for the calculated velocity to prevent erosion. For earthen channels the allowable non-scouring velocity is typically 0.6-1.5 m/s depending on soil type.

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