How to Read a Hydraulic Grade Line

HGL stands for hydraulic grade line. It is the elevation water would rise to in an open tube tapped into a pipe, so it equals pressure head plus elevation. In a water main, the distance from the pipe up to the HGL is the pressure, measured in feet of water. In a gravity sewer or storm drain, the HGL is the water surface. To read one, compare it to the pipe crown, the manhole rim and the ground.

What is the hydraulic grade line?

The hydraulic grade line is pressure head plus elevation head, plotted along the pipe. It sits below the energy grade line by the velocity head.

FHWA's Introduction to Highway Hydraulics (HDS-4) puts it plainly. In open-channel flow, the HGL is the water surface. In pressure flow, it is the elevation water would rise to in a standpipe connected to the conduit.

One foot of water exerts 0.433 psi, the figure in EPA's Cross-Connection Control Manual. Flip it and 1 psi is about 2.31 feet. A gauge reading of 60 psi means the HGL sits about 139 feet above the gauge.

How do you read the HGL in a water distribution system?

Look at its slope and its height above the pipe. The slope is friction loss. The height is pressure.

The HGL falls in the direction of flow as friction takes energy out of the water. A steep drop over a short run means high velocity for that pipe size: an undersized main or heavy demand downstream. The Department of Energy's pump selection tip sheet notes that friction losses run roughly with the square of the flow, so doubling the flow means about four times the loss.

For height, subtract the pipe elevation from the HGL and multiply by 0.433. In Texas, TCEQ's minimum pressure rule, 30 TAC §290.44(d), requires a design for at least 35 psi at all points at 1.5 gpm per connection, and at least 20 psi under combined fire and drinking water flow where the system provides fire protection. TCEQ's operating rule, 30 TAC §290.46(r), holds the same 35 and 20 psi floors in operation.

Three things on a water profile deserve a red pen:

• A steep drop in the HGL over a short run.
• An HGL below the minimum pressure line at a high point.
• An HGL at or below the pipe itself. That is zero or negative pressure.

Negative pressure is a backflow problem. EPA's manual explains that backsiphonage happens when atmospheric pressure pushes a contaminant toward a potable system under vacuum. It also takes a cross-connection linking the two.

How do you read the HGL in a gravity sewer or stormwater system?

Here the HGL is the water surface. Below the crown, the pipe runs as an open channel. Above the crown, it is surcharged.

A surcharged sanitary sewer pushes wastewater up into the manholes. If the HGL reaches a rim, you have an overflow. Under TCEQ's sewer pipe design rule, 30 TAC §217.53, the owner must ensure the system has capacity to prevent a surcharge ((j)(3)), and slopes must give at least 2.0 feet per second flowing full ((l)(1)).

Storm drains may be designed to run under pressure, so the check changes. HDS-4 says the HGL in a pressure-flow storm drain should stay below the roadway, or water rises through inlets and access hole covers and floods the street. The calculation starts at the outfall, from the design tailwater or a depth set by the outlet pipe, whichever is higher, and works upstream. In subcritical flow, a wrong tailwater carries up the line.

What is a hydraulic profile?

A hydraulic profile is the HGL drawn along a flow path: down a pipe run, or through every channel, basin and weir in a treatment plant.

On a plant, it shows whether gravity can carry peak flow from the headworks to the outfall. In Texas, the final engineering report rule, 30 TAC §217.10, requires a treatment facility's report to include a plot of the hydraulic gradient at peak flow for all gravity lines ((g)(5)(C)). Read it like a pipe profile: water surface against the tops of the walls and weirs.

How is the HGL used in water system testing?

In water testing, the HGL is a pressure reading converted to an elevation, for comparison with the model.

Multiply the reading in psi by 2.31 and add the gauge elevation. That is the HGL at that point. Do it at several hydrants along a flowing main and you can see the friction slope in the field. An EPA presentation on calibrating Flint's hydraulic model lists fire flow tests, hydraulic gradient tests, pressure monitoring and correct elevation data among its calibration steps.

What does the HGL reveal about system capacity?

It shows how much room is left. A water main working hard has a steep HGL. A sewer near capacity has an HGL close to the crowns at peak flow.

For developers planning a new connection, the useful run is the model with and without your flow. If your demand pushes the water HGL below the minimum pressure line, or a sewer HGL over a crown downstream, the model caught it while it is still a design change.

How does a pump show up on the HGL?

As a vertical jump. The height of that jump is the head the pump adds.

FHWA's pump station design manual (HEC-24) breaks total dynamic head into static head, friction head, velocity head and any pressure head change. At zero flow it is just static head, and it climbs as flow rises. That rising line is the system curve, and the pump runs where its own curve crosses it. Raise the discharge-side HGL and the operating point slides to more head and less flow. The same grade line runs through force main and pump sizing calculations.

What does the HGL mean for a tall building?

It sets how high the main can push water on its own.

Say the main's HGL at your service is elevation 760 and the ground is at 600. That is about 69 psi at the street. A floor 100 feet up sees about 26 psi, before losses inside the building. That is why tall buildings often need booster pumps, and why pressure zone design for high-rise developments starts with the HGL.

What should you check on an HGL profile before you approve it?

You do not need to be an engineer for these checks. Any "no" is a question for the engineer of record.

1. Does the profile say which flow it shows: average, peak, fire flow or design storm?
2. Does the water main HGL stay above the minimum pressure line at every high point?
3. Does the sewer HGL stay below every pipe crown at design flow?
4. Does every manhole and inlet keep freeboard below the rim?
5. Does the storm drain HGL start from a stated tailwater?

Then ask: What flows and peaking factors went in? Was the model checked against field pressures? What does the HGL look like at buildout?

A note on limits: this is general education, not a design review. The rules cited are Texas rules as of October 2026. Your state, utility or drainage manual may differ, and the engineer of record and reviewing agency decide what applies.

Frequently asked questions

What does HGL stand for?

Hydraulic grade line: the line of pressure head plus elevation along a pipe or channel.

What is the difference between the HGL and the energy grade line?

The energy grade line sits above the HGL by the velocity head, V squared over 2g. That is about 0.14 feet at 3 feet per second and about 1.55 feet at 10 feet per second.

Can the HGL be above the ground?

In a pressure water main, yes. That is what delivers pressure to the tap. In a gravity sewer or storm drain, an HGL above the ground or rim means water is leaving the system.

I'm a land developer in Texas and the city says the sewer at my tie-in is close to capacity. What should I ask my engineer about the HGL?

Ask for the HGL at peak flow, with and without your project, from your tie-in down to where the pipe has room. Texas requires capacity calculations that account for peak domestic flow, peak industrial flow and maximum expected infiltration (30 TAC §217.53(j)(2)). Then ask which manholes lose freeboard, and by how much.

I'm a public works director for a small Texas city, and a developer's water line profile shows the HGL dipping close to the pipe at a hilltop. Is that a problem?

It can be. Pressure there is the gap between the HGL and the pipe times 0.433. Under 35 psi at design demand, or under 20 psi with fire flow, misses the Texas minimums above. If the HGL touches the pipe, the line can go negative. Ask for the peak hour and fire flow runs at that point.

Related resources

• 5 Common Wastewater Hydraulic Modeling Mistakes That Kill Project Budgets
• Why EPANET Is the First Choice for Water Network Modeling
• How to Conduct a Fire Flow Analysis for a Texas Development or Subdivision
• Gravity Sewer vs. Pressure Sewer: Choosing the Right System for Your Community

Need a second set of eyes on an HGL profile?

We build and check water, sewer and storm drain models, and we read the HGL on construction drawings the way a reviewer will.

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