Sheet Flow Time Calculation

Sheet flow travel time is computed as:
EquationObject2839
Equation 4-17.
Where:
  • t
    sh
     = sheet flow travel time (hr.)
  • n
    ol
     = overland flow roughness coefficient (provided in Table 4-6)
  • L
    sh
     = sheet flow length (ft) (100 ft. maximum)
  • P
    2
     = 2-year, 24-h rainfall depth (in.) (provided in -
    NOAA's Precipitation Frequency Data Server for Atlas 14
    )
  • S
    sh
     = sheet flow slope (ft/ft)
Table 4-6: Overland Flow Roughness Coefficients for Use in NRCS Method in Calculating Sheet Flow Travel Time (NRCS 1986)
Surface description
n
ol
Smooth surfaces (concrete, asphalt, gravel, or bare soil)
0.011
Fallow (no residue)
0.05
Cultivated soils:
Residue InlineEquation1240%
0.06
Residue cover > 20%
0.17
Grass:
Short grass prairie
0.15
Dense grasses
0.24
Bermuda
0.41
Range (natural):
0.13
Woods:
Light underbrush
0.40
Dense underbrush
0.80
NOTE:
'n' values for overland flows (nol) are not to be used in other channel or floodplain applications.