Critical depth is an energy condition in open-channel flow. It is often useful near controls, transitions, and checks where the relationship between discharge, width, and depth matters more than bed roughness.
Start with unit discharge.
For a rectangular channel, divide the total discharge Q by the bottom width b to obtain unit discharge q:
At this depth, the Froude number is one for the idealised rectangular section. The corresponding minimum specific energy is Emin = yc + V2/(2g) = 1.5yc.
Why it is not Manning normal depth.
Normal depth is a uniform-flow result controlled by roughness and bed slope. Critical depth depends on discharge and section geometry, with gravity setting the energy relationship. A channel can have both a normal depth and a critical depth, and they need not be equal.
Limits before using the result.
- The screen assumes a steady, hydrostatic, rectangular channel with one control section and no explicit losses.
- It does not calculate gradually varied profiles, hydraulic jumps, unsteady waves, transitions, tailwater effects, or non-rectangular critical-flow solutions.
- Verify the actual control geometry, freeboard, sediment, debris, and boundary conditions before using the result in a drainage or waterway design.
next check
Compare energy and uniform-flow states.
Open the critical-depth calculator, then compare it with the Manning normal-depth calculator.