hydraulic grade line
Also found in: Acronyms.
hydraulic grade line[hī′drȯ·lik ′grād ‚līn]
In a closed channel, a line joining the elevations that water would reach under atmospheric pressure.
The free water surface in an open channel.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.