Storm Sewer Design Criteria — Reference
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) |
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 |
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.
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.