TABLE 8: ROUGHNESS COEFFICIENTS (MANNING'S N) FOR SHEET FLOW (a)

Surface Description	n1
Smooth surfaces (concrete, asphalt, gravel, or bare soil)	0.011
Fallow (no residue)	0.05
Cultivated soils:
Residue cover 20%	0.06
Residue cover > 20%	0.17
Grass:
Short grass prairie	0.15
Dense grasses 2	0.24
Bermudagrass	0.41
Range (natural)	0.13
Woods: 3
Light underbrush	0.40
Dense underbrush	0.80

 

   VELOCITY EQUATIONS FOR SHALLOW CONCENTRATED FLOW

   Paved V = 20.3282 s0.5

   Unpaved V = 16.1345 s0.5

(a)   From HERPICC Stormwater Drainage Manual - Revised July 1994.

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.