What is the minimum velocity for a storm sewer?

The minimum full-flow (or design-flow) velocity is typically 2.0–2.5 ft/s to keep sediment in suspension and prevent solids deposition. The maximum is usually 10–15 ft/s to limit abrasion and scour; above that, drop structures or energy dissipation are used.

What Manning's n is used for storm sewer pipe?

Typical design values: concrete / RCP n = 0.013, PVC and HDPE (smooth interior) n = 0.012, and corrugated metal pipe (CMP) n = 0.024 (annular) — though many agencies require n = 0.013 even for plastic pipe to account for joints, debris, and aging.

What is the minimum pipe size for storm sewers?

Most agencies set a minimum storm-sewer diameter of 12 or 15 inches to reduce clogging, even where hydraulic capacity would allow smaller. Driveway and yard pipes may be smaller, but trunk storm sewers are generally 15 inches and up.

What design storm is used for storm sewers?

Local storm sewers are commonly designed for the 10-year storm, with a 25- or 100-year check for surcharge and surface flooding. Trunk lines and detention outlets are often designed to the 25- or 100-year event. Always confirm the governing local or state standard.

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Storm Sewer Design Criteria — Reference

By Michael Flynn, PE · water resources & dam safety engineer · Last updated Oct 18, 2026

Common design criteria for gravity storm sewers. Values are typical of state and municipal standards (e.g., GLUMRB "Ten States Standards" lineage) — always confirm against the governing local spec, which controls.

Velocity & Slope

Criterion	Typical value
Minimum full-flow velocity (anti-deposition)	2.0–2.5 ft/s
Maximum velocity (abrasion / scour)	10–15 ft/s
Minimum slope	set to achieve 2 ft/s full
Design flow condition	full or just-full (no surcharge)

Applying these criteria to a whole SWMM network? AquaSWMM checks every pipe against velocity and percent-full limits, sizes from a pipe catalog and writes the sizes back into the model.

Manning's n (Storm Sewer Pipe)

Pipe material	n (design)
Concrete / reinforced concrete pipe (RCP)	0.013
PVC / HDPE (smooth interior)	0.012–0.013
Corrugated metal pipe (CMP), annular	0.024
CMP, helical (small dia.)	0.012–0.024
Ductile iron, cement-lined	0.013

Many agencies require n = 0.013 even for plastic pipe, to account for joints, debris, and aging. Use the value the reviewer mandates.

Size, Cover & Storm

Criterion	Typical value
Minimum trunk pipe diameter	12–15 in
Minimum cover over pipe	1–3 ft (loading / frost dependent)
Design storm (local sewers)	10-yr (25/100-yr check)
Design storm (trunk / outlets)	25–100-yr

Full-flow capacity (Manning's).

Q_full = (1.49/n)·(πD²/4)·(D/4)^2/3·S^1/2

Maximum capacity actually occurs at ~93% depth (not full); peak velocity at ~81% depth. For partly-full analysis use the circular-section geometry with the central angle θ.

Sources: GLUMRB, Recommended Standards for Wastewater Facilities ("Ten States Standards"); ASCE/WEF MOP-37; typical state DOT and municipal storm-drainage manuals. Criteria vary by jurisdiction.

Checking a whole run against these criteria? AquaSWMM runs its engine in your browser — full-flow capacity and velocity for one pipe, or a whole network from a pasted .ssn, with no install. The desktop app adds auto-sizing and a design review that flags velocity outside 2–10 ft/s. Free, GPL, same author as pe-calc.

Sizing a run? Open the Manning's calculator → · At a crossing? Culvert hydraulics. · Want the whole set? Print all 38 cards →

Worked example: checking one pipe against the criteria

18-in RCP, n = 0.013, slope 0.50%, design flow 6.5 cfs (10-yr)

Given D = 1.5 ft, A = 1.767 ft², R = D/4 = 0.375 ft, S = 0.005

Find full-flow capacity and velocity, minimum self-cleansing slope

V_full = (1.49/0.013) × 0.375^2/3 × 0.005^1/2 = 4.21 ft/s, within 2–10 ft/s

Q_full = 4.21 × 1.767 = 7.45 cfs ≥ 6.5 cfs design flow, so the pipe works with 13% reserve

Minimum slope for 2.0 ft/s full: S = (2.0 × 0.013 / (1.49 × 0.375^2/3))² = 0.0011 (0.11%)

If a reviewer allowed n = 0.012 for the same pipe in HDPE: Q_full = 8.07 cfs. The 0.001 change in n is worth 8% capacity

Capacity is only half the check. The other half is the hydraulic grade line. A pipe can have spare full-flow capacity and still surcharge structures upstream when tailwater, junction losses or a downstream restriction raise the HGL. Most manuals require the HGL to stay a set distance below the rim or the gutter flowline. That check needs the whole network, not one pipe at a time.

Common mistakes

Sizing to the 93%-depth capacity. Peak capacity is about 1.076 Q_full at 93% depth, but that point is unstable. A small disturbance seals the pipe and the capacity drops to full-flow. Design to Q_full.
Checking velocity only at the design flow. Self-cleansing depends on what the pipe sees most of the time. A pipe that reaches 2 ft/s only in the 10-year storm still collects sediment.
Using nominal diameter for plastic pipe. Some profile-wall HDPE and PVC pipe has an inside diameter noticeably smaller than nominal. Use the manufacturer’s ID.
Assuming the receiving water is free outfall. A creek or pond at high stage sets the tailwater of the whole system. Start the HGL from the governing tailwater.

When this card doesn’t apply

These are typical values, and local criteria govern. State DOT drainage manuals, municipal design standards and FHWA HEC-22 each set their own design storms, velocity limits, minimum sizes and HGL freeboard. Combined sewers and sanitary sewers follow different rules, such as Ten States Standards minimum slopes by diameter. Pressurized storm systems, siphons and pump-station force mains are outside the gravity-flow assumptions here.

Related cheat sheets and tools

Size pipes in the Manning's tool with the circular-section geometry, get the design Q from Rational Method or NRCS runoff, and check crossings with culvert hydraulics. For a full storm-network model with inlets, junctions, and detention, see HydroComplete.