Reference

The critical depth in open channel flow is the flow depth at which the specific energy is minimized for a given discharge. It represents a state where the flow transitions between supercritical and subcritical conditions. Here's a more detailed explanation:

1. Specific Energy: The critical depth occurs when the specific energy (total energy per unit weight of fluid relative to the channel bottom) is at its minimum value.

2. Calculation: For rectangular channels, the critical depth ((d_c)) can be calculated using the formula: [ d_c = \left(\frac{Q^2}{b^2g}\right)^{1/3} ] where (Q) is the discharge, (b) is the channel width, and (g) is the acceleration due to gravity.

3. Flow Conditions: Critical flow is a unique state characterized by maximum discharge for a minimum specific energy, indicating a balance point between gravitational forces and inertial forces.

You might find further information and insights on this topic through resources such as the Civil Engineering Portal.