Rational Method Runoff Coefficient C
| Type of drainage area | Runoff coefficient C |
|---|
HEC-22 Table 3-1 [1]. Q = Cf·C·i·A, with the frequency factor Cf of Table 3-2 below; Cf·C shall not exceed 1.0.
| Recurrence interval | Frequency factor Cf |
|---|
NRCS (SCS) Runoff Curve Numbers
TR-55 Tables 2-2a and 2-2c [2], average runoff condition, by hydrologic soil group A–D. Where the source's group A value is below 30 (woods in good condition, meadow), it says to use 30, as shown.
Manning's Roughness Coefficient n
| Channel lining / type | Minimum | Normal | Maximum |
|---|
Chow Table 5-6 [3]. Corrugated metal is the lined-channel row, not the storm-drain conduit row.
Reinforced Concrete Pipe Diameters
| Inside diameter (in) | Inside diameter (mm) | Full-flow area (ft²) | Full-flow area (m²) |
|---|
ASTM C76 nominal inside diameters [4] in inches (the source unit), converted exactly to mm; C76 continues to 144 in. Full-flow areas are calculated, A = πD²/4.