Hydraulic grade line (HGL)

The HGL is a line that can be drawn along a pipe or channel, indicating the pressure. In a pipe flowing full it is above the pipe obvert so that the pipe is pressurised. In an open channel it follows the water surface, except in some special locations such as the crests of weirs.

The HGL is related to the energy grade line (EGL), also called the total energy line (TEL), as shown below. It is located  below the EGL by a distance equal to the velocity head, V2/2g.  Together they display the energy of the flow.  Energy per unit weight (N.m / N) has units of length, so that the vertical distances between the flow and the HGL and EGL lines indicate various forms of energy.

The HGL is the most important indicator for the designer of drainage systems, because it indicates the pressure within pipes, water surface levels and possible overflows.

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