From Hydraulics Field Manual, Second Edition

7.3 Manning's Equation

Manning's formula, published in 1890, gives the value of c in the Chezy formula as

The complete Manning formula being

and combining with the Chezy equation

where A =

cross-sectional area of flow in square feet

S =

slope, or grade, in ft/ft

R =

hydraulic radius, feet

n =

coefficient of roughness

In many computations, it is convenient to group the properties peculiar to the cross section in one term called conveyance (K)or


Uniform flow of clean water in a straight, unobstructed channel would be a simple problem but is rarely attained. Manning's formula gives reliable results if the channel cross section, roughness, and slope are fairly constant over a sufficient distance to establish uniform flow.

This subsection courtesy of American Iron and Steel Institute.

Figure 7.36: Nomograph for solution of Manning's equation. ( Courtesy American Iron and Steel Institute.)

Figure 7.37: Nomograph for headwater depth of box culverts with entrance control. ( Courtesy American Iron and Steel Institute.)
Table 7.12: Manning's n for Natural Stream Channels Surface width at flood stage less than 100 feet.


Fairly regular section:

a. Some grass and weeds, little or no brush

0.030 0.035

b. Dense growth of weeds, depth of flow materially greater than weed height.

0.035 0.05

c. Some weeds, light brush on banks

0.035 0.05

d. Some weeds, heavy brush on banks

0.05 0.07

e. Some weeds, dense willows on banks

0.06 0.08

f. For trees within channel, with branches...

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