In open channel flows, flow conditions can occur where the velocity is the same as that of a shallow water wave. These critical conditions are influenced by the channel’s cross-sectional shape and the flowrate. Flows with velocities slower than this are termed subcritical. Typically, these subcritical flows are deep and slow, and waves can propagate upstream. Flows with velocities exceeding the critical velocity are called supercritical; they are shallow and fast, and are not influenced by downstream water levels.

In subcritical flow, the normal depth calculated from Manning's equation is greater than the critical depth; in supercritical flow, it is smaller. If critical and normal depth are equal, flow is said to be critical. Channels where subcritical flows occur are termed mild; those where supercritical flows occur are said to be steep. Critical depth is the depth of flow in an open channel corresponding to critical flow conditions. For rectangular channels it can be calculated directly from equations. For other cross-sections it is determined using iterative calculations.

Supercritical flows can change to a subcritical state through a hydraulic jump, while subcritical flows can convert to supercritical through smoother water surface changes associated with controls such as weirs and sluice gates.

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