This is a form of hydraulic analysis of piped drainage systems that traces the positions of hydraulic grade lines throughout the system. It is usually applied as a steady state model, calculating HGLs for the peak flowrates at each pipe in a system. Designers determine the sizes and positions of pipes to ensure that HGLs do not rise above allowable levels, often set at some freeboard level below the street gutter.

The following diagram shows the positions of total energy and hydraulic grade lines for a pipes drainage system and how they are influenced by head losses due to friction and shock losses.

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The obsolete Basic hydraulic model in DRAINS employs a HGL tracing procedure to calculate levels in pits during part-full flows, projecting HGLs upwards from a tailwater level at the outlet to a pipeline. However, in the Lite and Full Unsteady hydraulic models these calculations are performed by solving partial differential equations.