The critical flow depth, also known as critical depth, is a significant concept in open-channel hydraulics, particularly in uniform flow conditions. It represents the depth of flow at which the specific energy of the flow is at a minimum for a given discharge. At this critical depth, the flow velocity is such that the Froude number ($Fr$) equals 1. The critical flow depth (yc​) can be calculated using the following formula for rectangular channels:

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Where:

• yc​ is the critical flow depth,
• $Q$ is the discharge,
• $g$ is the acceleration due to gravity,
• $A$ is the cross-sectional area of flow.