Open channel flow is water moving under gravity with a free surface exposed to the atmosphere. Ditches, storm drains flowing partially full, irrigation canals, and natural streams can be screened with Manning's equation when the steady uniform flow assumptions fit the problem.
This guide covers the Manning equation terms, roughness-coefficient uncertainty, common channel geometry, and Froude-number review prompts. It is not a substitute for surveyed channel data, HGL/backwater modeling, erosion and scour review, local stormwater or wastewater criteria, permits, or qualified hydraulic engineering review.
Manning's Equation
In US customary units: V = (1.486/n) × R^(2/3) × S^(1/2), where V is velocity in ft/s, n is Manning's roughness coefficient, R is the hydraulic radius in feet (cross-sectional flow area divided by wetted perimeter), and S is the energy-slope term used for the uniform-flow screen. Flow rate Q = V × A, where A is the cross-sectional flow area.
Velocity depends on the 2/3 power of hydraulic radius and the 1/2 power of slope. That sensitivity is useful for quick screening, but it does not resolve backwater, transitions, tailwater, surcharge, bridge or culvert controls, hydrographs, sediment, freeboard, or local design criteria.
Open Channel Flow Calculator
Calculate open channel flow using Manning's equation for rectangular, trapezoidal, and circular channels.
Manning's n Roughness Coefficients
The roughness coefficient n captures resistance from the channel surface, vegetation, obstructions, irregularity, and floodplain condition. Smooth concrete, corrugated metal, earth, grass, riprap, and natural-channel values are ranges, not universal constants.
For project use, select n from an applicable source, field inspection, product or lining data, and local criteria. A higher roughness value may be appropriate for some screening cases, but it does not by itself ensure capacity, freeboard, stability, maintenance access, or permit acceptance.
Channel Cross-Section Shapes
Rectangular channels are simple to screen and are common for lined channels and box culverts. Trapezoidal channels are common for earthen ditches and canals. Side slopes must be checked against soil, lining, maintenance, safety, and local criteria rather than copied from a generic row.
Circular pipes flowing partially full use circular-segment geometry. Once depth reaches full-pipe conditions, the free-surface Manning screen is no longer enough; surcharge, pressure flow, inlet/outlet control, and HGL checks may control.
Froude Number and Flow Regime
The Froude number (Fr) is a flow-regime prompt. Fr = V / sqrt(g × D_h), where V is flow velocity, g is gravitational acceleration, and D_h is hydraulic depth. Values below 1 are generally subcritical; values above 1 are generally supercritical.
Regime labels are not stability approvals. Critical or supercritical results need review of controls, transitions, hydraulic jumps, freeboard, lining, scour, outlet protection, and energy dissipation. Local criteria may require a different target band than a generic quick screen.