How to Check Storm Drain Capacity with Manning’s Equation

A storm-drain capacity check compares a supplied design flow with the full-flow capacity of a circular pipe. The flow has to come from somewhere else: a Rational Method estimate, a hydrograph method, or a project criterion. This calculator does not create that flow. It applies Manning’s equation once the flow, slope, and roughness are supplied.

Full-flow circular geometry

Q is discharge, n is Manning’s roughness, A is the full cross-sectional area, R is the hydraulic radius, and S is the hydraulic slope. For a circle flowing full, the wetted perimeter is the circumference, so R simplifies to D/4. Velocity is Q/A when a diameter is known.

The equation is evaluated in canonical SI units. US flow is converted with 0.028316846592 m³/s per cfs, and a diameter in inches is converted with 0.0254 m per inch. A slope entered as feet per foot keeps the same numeric value.

Two modes

• Sizing: design flow, slope, and Manning n produce a continuous minimum hydraulic diameter. That number is the diameter at which full-flow capacity matches the supplied flow. It is not a catalog size and not a recommendation to round down.
• Selected-pipe check: an optional diameter adds full-flow capacity, velocity, and hydraulic utilization. “Adequate” means that capacity is at least the supplied design flow. The machine-readable basis is full-flow-hydraulic-capacity-only.

Worked example: 1.2 m pipe

Illustrative inputs, not a design default: D = 1.2 m, S = 0.01, and n = 0.013.

1. Treat the pipe as a circular conduit flowing full.
2. Use A = πD²/4 and R = D/4 with D = 1.2 m.
3. Use S = 0.01 and n = 0.013 in Q = (1/n) A R^(2/3) S^(1/2).
4. Read both discharge and velocity.

Result: capacity ≈ 3.898730279 m³/s and velocity ≈ 3.447234420 m/s. Those are the values published with the TOOL-005 methodology. They describe this full-flow case only. They do not say that 1.2 m is an available product or that the velocity is acceptable for a lining.

Enter the same diameter, slope, and roughness in the storm-drain calculator to reproduce the check. Add a design flow if you also want the utilization comparison.

What the check leaves out

• Where the flow came from: use a separate method, such as the Rational Method, and carry the flow across only after the units match.
• Part-full depth, surcharge, and the hydraulic grade line: this tool does not solve them. Inlet control, outlet control, tailwater, and junction losses are also outside it.
• Open channels: rectangular and trapezoidal uniform flow belongs in the Manning open-channel calculator, which uses its own geometry.
• Structural and product decisions: cover, material class, and commercial diameters are not calculated.

Review sequence

1. Supply the design flow from its own method and document that method.
2. Enter the hydraulic slope and a Manning n chosen for the conduit and its condition.
3. Read the continuous diameter, or check a selected pipe for capacity and velocity.
4. Treat “adequate” as a full-flow comparison with the supplied flow, then apply the project criteria that this equation does not contain.

The registered Manning source for this tool is SRC-MANNING-FULL-PIPE-TOOL005-001. A passing capacity ratio is not a code-compliance verdict.

Frequently asked questions