Manning's Equation Drainage Slope
Self-cleansing slope and slope to carry target flow at half-full.
Example
You enter
- Pipe diameter (in) 4
- Target flow (gpm) 50
- Pipe material pvc
- Required scour velocity (ft/s) 2
- As-built slope to check (in/ft, 0 = skip) 0.25
You get
- Self-cleansing slope 0.0484
- Slope for flow (half-full) 0.0788
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Formula or tableEnglish Manning V = (1.486 / n) × R^(2/3) × S^(1/2), solved for the self-cleansing slope at the entered scour velocity (2 ft/s if blank) and the slope to carry target flow at half-full with R = D / 4.
Source and editionManning equation; closed-conduit n-values are the 'normal' values of Chow, Open-Channel Hydraulics (1959) Table 5-6 (copper 0.011, coated cast iron 0.013, galvanized 0.016, corrugated storm drain 0.024), by name; PVC 0.009 is the plastic-pipe industry design figure. USGS WSP-2339 covers natural channels and floodplains and was cited here until 2026-10-02 in error.
Free accessChow 1959 at archive.org; the Manning relation is public.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namespipe_diameter_in, target_flow_gpm, material, scour_velocity_fps, actual_slope_in_per_ft, slope_self_cleansing_in_per_ft, slope_for_flow_in_per_ft
- Half-full hydraulic radius R = D / 4circular cross-section geometry
- Self-cleansing velocity entered; 2 ft/s if blank (2.5 common, 3.0 on large interceptors)public engineering practice