Manning's Formula for Gravity Flow

Manning's equation for calculating gravity flow in open channels

Manning's equation can be used to calculate cross-sectional average velocity flow in open channels

v = kn / n R2/3 S1/2         (1)


v = cross-sectional mean velocity (ft/s, m/s)

kn = 1.486 for English units and kn = 1.0 for SI units

n = Manning coefficient of roughness

R = hydraulic radius (ft, m)

S = slope of pipe (ft/ft, m/m)

Hydraulic radius can be expressed as

R = A / P                 (2)


A = cross sectional area of flow (ft2, m)

P = wetted perimeter (ft, m)

The volume flow in the channel can be calculated as

q = A v = A kn / n R2/3 S1/2         (3)


q = volume flow (ft3/s, m3/s)

A = cross-sectional area of flow (ft2, m2)

Related Topics

  • Fluid Mechanics - The study of fluids - liquids and gases. Involves velocity, pressure, density and temperature as functions of space and time

Related Documents

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