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.

t_c = \left ( \frac {0.67 \ast n \ast L_o}{ \sqrt {S}} \right )^{0.467}


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:

Recommended surface roughness values
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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