Which condition is characterized by critical flow?

Critical flow is a specific condition in open channel hydraulics where the flow depth corresponds to a unique condition known as critical depth. This situation occurs when the flow velocity and gravitational forces are perfectly balanced, leading to a state where the specific energy is minimized relative to flow depth.

When flow is at critical depth, it transitions between subcritical and supercritical flow conditions. This signifies that the flow is unstable; small changes in flow rate or depth can lead to significant changes in flow characteristics. Therefore, the correct answer highlights this relationship between critical flow and unstable conditions.

In contrast to this, stable flow in a channel refers to states where the flow conditions remain unchanged over time, while tranquil flow conditions denote smooth and laminar flow characteristics without turbulence. Flow that is always supercritical is more a state of flow velocity exceeding the wave speed, not uniquely tied to the critical depth condition. This distinction clarifies why the characterization of critical flow aligns with unstable flow at critical depth.