WMS:Kerby Tc Equation: Difference between revisions

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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.
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.


:[[Image:image125.gif]]
:<math>t_c = \left ( \frac {0.67 \ast n \ast L_o}{ \sqrt {S}} \right )^{0.467} </math>


where:
where:


<math>t_c</math> = time of concentration in minutes.
:''t<sub>c</sub>'' = time of concentration in minutes.


<math>S</math> = overland slope in ft/ft.
:''S'' = overland slope in ft/ft.


<math>n</math> = roughness coefficient.
:''n'' = roughness coefficient.


<math>L_o</math> = length of overland flow in feet.
:''L<sub>o</sub>'' = length of overland flow in feet.


A table of recommended values for n is given below:
A table of recommended values for ''n'' is given below:




{| border="1" cellpadding="2" style="text-align:center"
{| class="thin"
|+Recommended surface roughness values
|+Recommended surface roughness values
!width="350"|Surface Description
!width="250"|Surface Description
!width="150"|<math>n</math>
!width="150"|''n''
|-
|-
|Smooth, impervious surface||0.02
|Smooth, impervious surface||align="center"|0.02
|-
|-
|Smooth, packed bare soil||0.1
|Smooth, packed bare soil||align="center"|0.1
|-
|-
|Poor grass, cultivated row crops of moderately rough bare soil||0.2
|Poor grass, cultivated row crops of moderately rough bare soil||align="center"|0.2
|-
|-
|Pasture or average grass||0.4
|Pasture or average grass||align="center"|0.4
|-
|-
|Deciduous timberland||0.6
|Deciduous timberland||align="center"|0.6
|-
|-
|Timberland with deep forest litter or dense grass||0.8
|Timberland with deep forest litter or dense grass||align="center"|0.8
|}
|}


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{{WMSMain}}
{{WMSMain}}
[[Category:Equations|Kerby]]
[[Category:WMS Basins|Kerby]]

Latest revision as of 15:17, 29 September 2017

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:


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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