Roughness Coefficients (Manning's "N") for Sheet Flow


Surface Description
n1
Smooth surfaces 
        (concrete, asphalt, gravel, or bare soil)

0.011
Fallow 
        (no residue)

0.05
Cultivated soils: 
        Residue cover < 20% 
        Residue cover > 20%

0.06
0.17
Grass: 
        Short grass prairie 
        Dense grases2
        Bermudagrass

0.15
0.24
0.41
Range 
        (natural)

0.13
Woods3
        Light underbrush 
        Dense underbrush

0.40
0.80

1  The "n" values are a composite of information compiled by Engman (1986).

2  Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native grass mixtures.

3  When selecting n, consider cover to a height of about 0.1 ft.  This is the only part of the plant cover that will obstruct sheet flow.