Hydraulic Grade Line (HGL)
Definition - What does Hydraulic Grade Line (HGL) mean?
An imaginary line through the points to which water would rise in a series of vertical tubes connected to the pipe. In an open channel, the water surface itself is the hydraulic grade line. A pipeline is not an open channel, however. In a pipeline, it's the hypothetical level to which water would rise in a series of vertical pipes -- attached along a horizontal pipe -- to simulate an open channel.
Also referred to as the hydraulic gradient.
Trenchlesspedia explains Hydraulic Grade Line (HGL)
The hydraulic grade line, or gradient line, in a sewer system demonstrates the system element's carrying capacity. The profile of water is seen as that level reached when flowing in a pipe flowing partially full. The hydraulic gradient line is the level that the water would rise to under pressure. If a pipe is completely full and additional material or liquid is introduced through inflow or infiltration (such as storm runoff ), the excess will cause the system to back up, at best. At worst, the excess will cause downstream problems with treatment facilities.