Hydraulic grade line
A hydraulic grade line (HGL) is: ‘A theoretical line showing the sum of the pressure elevation heads of a flow relative to a datum. For uniform open-channel flow, the hydraulic grade line is the same as the water surface. The slope of the hydraulic grade line is the hydraulic gradient.’
Ref Culvert, screen and outfall manual, (CIRIA C786) published by CIRIA in 2019.
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