Hydraulic Grade Line
Water surface, pressure line and how to build the profile.
The hydraulic grade line (HGL) is the level of the water surface, or of the pressure head where the pipe runs full. It shows where the sewer is under pressure and whether it can flood connections or surfaces.
In open-channel flow the HGL is the water surface. Once the pipe surcharges, the HGL rises above the soffit and behaves as a pressure line, and its level becomes a design constraint rather than just a hydraulic result.
Key design considerations
- Start from a known downstream control: tailwater, outlet level or the level in the receiving system.
- Apply losses along each reach, including junction and bend losses, not friction alone.
- Check the HGL against property connection levels, surface levels and cover levels.
- The calculation is only as good as the flow data and roughness values behind it.
- Where the authority requires it, assess a blocked or partially blocked pipe scenario.
- Keep the profile continuous through maintenance holes, where local losses occur.
Equations
HGL₂ = HGL₁ − h_f − h_local
Step down each reach using the friction loss h_f from Manning and any local losses at structures.
E = z + y + V²/(2g)
Specific energy level; the HGL is the energy level minus the velocity head.
Design checks
- HGL below property connection and surface levels at design flow.
- Surcharge level and frequency within the authority's limits.
- HGL consistent and continuous through each maintenance hole.
Standards and authority references
- WSA 02 Gravity Sewerage Code of Australia (current edition), Water Services Association of Australia
- Authority surcharge and level requirements
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- A hand-built HGL assumes steady flow; transient effects need a network model.
- Requires reliable downstream control levels, which are often the weakest input.
This reference is general engineering information. Confirm requirements against WSA 02, the current Australian Standards and the local water authority's supplement before use in design, and obtain project-specific advice for the conditions at hand.