Storm Sewer HGL

From the outfall upstream

The energy grade line starts from the tailwater. The downstream end of each pipe follows HEC-22 4th ed. Table 9.6, the upstream end Table 9.7. A full pipe loses S_f L with S_f = (Q n / (0.46 D^2.67))²; a part-full pipe takes its slope as the friction slope, as HEC-22 does. Supercritical flow at the upper end carries no losses upstream.

Access holes and inlets

E_ai = max(E_i + 0.2 V²/2g, D DI², 1.6 D DI^0.67), the first only for full or subcritical outflow, with DI = Q / (A (gD)^1/2). Benching, angled inflow and plunging add (C_B + C_θ + C_P)(E_ai − E_i), never below zero (section 9.1.6.7).

Exit losses

0.4 V²/2g at a non-plunging inflow pipe and 1.0 V²/2g at the outfall. A pipe that plunges into a structure takes its grade line from its own hydraulics.

What you enter

The pipes, their flows and inverts, and the tailwater. Flow entering a structure from the surface is the outflow minus the pipes entering; with a ground elevation it counts as plunging flow.

The calculator is tested against the cases below every time its code changes.

Compared with the whole of FHWA HEC-22 4th ed. Example 9.2. The example uses approximate velocities for part-full pipes and rounds at each step, so grade lines allow 0.1 ft, and pipe 42-43 is entered at the 0.001 slope the example computes with. Network rules are unit tested. No reference program has been run for this page.

• Circular pipes in a tree: one outfall, one outflow from each structure, no loops, no parallel outflows. Flows are entered, not routed: compute them first and check that they are the flows you mean.
• The location of a hydraulic jump inside a pipe is not computed; the page tells you where the upper end is supercritical. A pipe that is part full over part of its length and full over the rest is taken by the condition at its ends.
• Bends and transitions in a run are not included. The access hole method is for the configurations HEC-22 tested; check unusual structures separately.
• No pipe sizing or inlet design. The comparison with the ground is a number, and the freeboard your criteria ask for is yours to read against it.

Which angle do I enter?

The angle between the inflow pipe and the outflow pipe at the structure, as HEC-22 Figure 9.8 measures it: 180 degrees is straight through, 90 a right-angle lateral.

Why is a steep pipe so low at its upper end?

Supercritical flow is controlled from upstream, so the energy line does not carry the downstream losses up the pipe (HEC-22 Table 9.7, condition D). The page reports normal depth there.

Can it size the pipes?

No. Choose them, for example with the Pipe Flow calculator, and read the grade lines here.

Can I attach the calculation sheet to a report?

You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews and is responsible for what goes into a report.