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)

where

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

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

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

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)

where

A = cross sectional area of flow (ft2)

P = wetted perimeter (ft)

The volume flow in the channel can be calculated as

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

where

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 various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.

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