Last edited 27 Aug 2020

Critical flow

Culvert, screen and outfall manual, (CIRIA C786) published by CIRIA in 2019, defines critical flow as the: ‘Free surface flow with minimum specific energy for a given discharge and a Froude (Fr) number of unity.’

Where the Froude number is: ‘A dimensionless ratio between inertia and gravity forces in a fluid, or between mean velocity and wave celerity. Froude number is unity for critical flow, more for supercritical flow and less for subcritical flow.’

The water depth at a critical flow is referred to as the critical depth.

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