FHWA Hydraulic Engineering Circular No. 22 (HEC-22), Urban Drainage Design Manual, is the primary design reference for roadway storm drainage systems in the United States. Now in its third edition (2009), it provides equations, charts, and procedures for gutter flow analysis, inlet interception, storm drain design, and pump station sizing.
Gutter Flow (HEC-22 Chapter 4)
Gutter flow capacity for a uniform cross-slope triangular section:
Q = (0.56/n) · Sx5/3 · SL1/2 · T8/3
Where: Q = flow (cfs), n = Manning's n, Sx = cross-slope (ft/ft), SL = longitudinal slope (ft/ft), T = spread (ft)
Typical Spread Criteria
| Road Classification | Design Speed | Allowable Spread |
|---|---|---|
| High-speed / High-volume | > 45 mph | Shoulder + 3 ft |
| Collector | 30–45 mph | Half the driving lane |
| Local / Residential | < 30 mph | Full driving lane |
| Sag locations | Any | Half the driving lane (each side) |
Inlet Interception (HEC-22 Chapter 5)
Grate Inlet Efficiency on Grade
Grate inlet interception depends on grate type, length, and gutter velocity. HEC-22 defines splash-over velocity (Vo) for each grate type — the velocity at which water begins to skip over the grate.
| Grate Type | Length (ft) | Splash-over Vo (ft/s) | Bicycle Safe |
|---|---|---|---|
| P-50 (parallel bar) | 2.0 | 8.0 | No |
| P-50 × 100 | 2.0 | 6.5 | Yes |
| Reticuline | 2.0 | 3.5 | Yes |
| Curved Vane | 2.0 | 9.0 | Yes |
| 45° Tilt Bar | 2.0 | 8.5 | Yes |
Curb-Opening Inlet Length for 100% Interception
LT = 0.6 · Q0.42 · SL0.3 · (1 / (n · Sx))0.6
For shorter inlets, efficiency E = 1 − (1 − L/LT)1.8
Storm Drain Design (HEC-22 Chapter 7)
Pipe Design Criteria
| Parameter | HEC-22 Guidance |
|---|---|
| Minimum pipe diameter | 15 inches (some agencies allow 12 in) |
| Minimum velocity (full flow) | 2.5 ft/s (3.0 ft/s preferred) |
| Maximum velocity | 15 ft/s (10 ft/s for corrugated pipe) |
| Pipe flow design level | Full flow or d/D ≤ 0.80 |
| Manhole spacing | 300–500 ft max |
| Minimum cover (under roads) | 2–3 ft depending on pipe material |
Junction Loss Coefficients
| Junction Condition | Loss Coefficient K |
|---|---|
| Straight run (no change in size) | 0.2 – 0.5 |
| 45° bend | 0.4 – 0.6 |
| 90° bend | 0.8 – 1.2 |
| Merging flows (lateral entry) | 0.8 – 1.5 |
Loss is computed as hL = K · V²/(2g). HEC-22 provides more detailed methods (FHWA composite energy loss method) for complex junction geometries.
Primary Source
- Brown, S.A., Stein, S.M., & Warner, J.C. (2009). Urban Drainage Design Manual, 3rd Edition. Hydraulic Engineering Circular No. 22, FHWA-NHI-10-009. Federal Highway Administration, U.S. Department of Transportation.
- FHWA HEC-15 (2005). Design of Roadside Channels with Flexible Linings.
- AASHTO (2014). Drainage Manual. American Association of State Highway and Transportation Officials.