The Manning Coefficient for sheet flow is used in the time of concentration formulas that attempt to model sheet flow.

The Manning Coefficient value for sheet flow is quite different to the equivalent value for open channel flow. As the assumed flow depth is very small, the sheet flow coefficient is dependent on the conditions on the surface and can be significantly affected by small changes, for example between lightly grassed surfaces and more densely grassed surfaces. Only the first 30mm need be considered as this will be the max height of the sheet flow.

The coefficient takes into account the effects of raindrop impact, the roughness of the surface, obstacles such as protruding rocks, leaf litter, crop ridges etc, and erosion and transportation of sediment. Where sediment may accumulate, for example on shallow sloped pavements or gutters, a value of 0.002 should be added to the coefficient.

Typical Values for Manning Coefficient for Sheet Flow

Urban	Lower Bounds	Upper Bounds
Asphalt Pavement	0.010	0.015
Smooth Concrete (Float Finish)	0.012	0.015
Smooth Concrete (Brush Finish)	0.014	0.016
Gravel Areas	0.020	0.030
Bare Packed Soil	0.010	0.020
Packed Clay	0.030	0.040
Bare Sand	0.010	0.016
Short prairie grass and lawns	0.100	0.200
Dense grasses (weeping lovegrass, bluegrass, buffalo grass, blue grama grass, native grass mixtures, alfalfa, lespedeza)	0.200	0.240
Bermuda grass	0.300	0.480
Woods or forest with light underbrush	0.400
Woods or forest with dense underbrush	0.800

 

Pipes & Channels
Wood Lining	0.014
Blocks or Brisk with Cement Mortar	0.014
Vitrified Clay	0.015
Cast Iron	0.015
Corrugated Metal Pipe	0.024
Cement Rubble Surface	0.024

 

By Landuse
Parkland	0.350
Rural Residential	0.300
Urban Residential	0.210
Medium Density Urban Residential	0.110
Industrial	0.060
Commercial	0.040
Woods or forest with light underbrush	0.400
Woods or forest with dense underbrush	0.800

 

Agricultural Land
Fallow - no residue	0.008	0.012
Conventional tillage - no residue	0.060	0.120
Conventional tillage - with residue	0.160	0.220
Chisel plow - no residue	0.060	0.120
Chisel plow - with residue	0.100	0.160
Fall disking - with residue	0.300	0.500
No till - no residue	0.040	0.100
No till - 20-40% residue	0.070	0.170
No till - 60-100% residue	0.170	0.470
Sparse rangeland with debris - no cover	0.090	0.340
Sparse rangeland with debris - 20% cover	0.050	0.250
Moldboard Plow	0.020	0.100
Short prairie grass and lawns	0.100	0.200
Dense grasses (weeping lovegrass, bluegrass, buffalo grass, blue grama grass, native grass mixtures, alfalfa, lespedeza)	0.200	0.240
Bermuda grass	0.300	0.480
Woods or forest with light underbrush	0.400
Woods or forest with dense underbrush	0.800