ANS:B - equal to 1.0

The flow in an open channel is said to be critical if the Froude number is equal to 1.0. The Froude number (Fr) is a dimensionless parameter used to characterize the flow regime in open channels. It is defined as the ratio of inertial forces to gravitational forces and is expressed as: Fr=​V/gD ​ Where:

• V is the velocity of flow,
• g is the acceleration due to gravity, and
• D is the hydraulic depth of flow.

When the Froude number is equal to 1.0, it indicates that the flow velocity is exactly equal to the wave velocity, which is the speed at which small disturbances propagate upstream. This critical flow condition signifies a transition between subcritical flow (Fr < 1.0) and supercritical flow (Fr > 1.0). In subcritical flow (Fr < 1.0), the velocity of the flow is less than the wave velocity, and the flow behavior is characterized by gradual changes and smooth transitions. In supercritical flow (Fr > 1.0), the velocity of the flow exceeds the wave velocity, resulting in steep changes and abrupt transitions. Therefore, the flow in an open channel is said to be critical if the Froude number is equal to 1.0.