WMS:Kerby Tc Equation
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Kerby (1959) developed an equation for computing the time of concentration for overland flow distances of less than 500 feet and greater than 300 feet.
where:
tc - time of concentration in minutes.
S - overland slope in ft/ft.
n - roughness coefficient.
Lo - length of overland flow in feet.
A table of recommended values for n is given below:
| Surface Description | n |
|---|---|
| Smooth, impervious surface | 0.02 |
| Smooth, packed bare soil | 0.1 |
| Poor grass, cultivated row crops of moderately rough bare soil | 0.2 |
| Pasture or average grass | 0.4 |
| Deciduous timberland | 0.6 |
| Timberland with deep forest litter or dense grass | 0.8 |
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