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CivilKit/Calculators/Hydraulics / open channels

hydraulics / open channels

Rectangular-channel critical-depth calculator.

Calculate critical depth, critical velocity, unit discharge, and the minimum specific energy for a rectangular channel.

read the result

Critical flow is an energy condition.

For a rectangular channel, critical depth is the depth at which specific energy is a minimum for the entered discharge and width. It is not the same as a uniform-flow or Manning normal depth.

Governing relationships

Unit discharge is q = Q/b. Critical depth is yc = (q2/g)1/3. At critical flow, velocity is Vc = √(gyc), Froude number is one, and minimum specific energy is Emin = 1.5yc.

What still needs a full hydraulic design

  • This is a steady, hydrostatic rectangular-channel screen with a single discharge and width. It does not solve gradually varied flow, unsteady flow, losses, or non-rectangular geometry.
  • Use the actual control section and boundary conditions. A critical-depth result does not by itself identify the water-surface profile upstream or downstream.
  • Check freeboard, sediment, debris, transitions, tailwater, and the applicable hydraulic standard before using the result for a structure.