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. | ||
: | :<math>t_c = \left ( \frac {0.67 \ast n \ast L_o}{ \sqrt {S}} \right )^{0.467} </math> | ||
where: | where: | ||
t<sub>c</sub> | :''t<sub>c</sub>'' = time of concentration in minutes. | ||
S | :''S'' = overland slope in ft/ft. | ||
n | :''n'' = roughness coefficient. | ||
L<sub>o</sub> | :''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: | ||
{| | {| class="thin" | ||
|+Recommended surface roughness values | |+Recommended surface roughness values | ||
!width=" | !width="250"|Surface Description | ||
!width="150"|n | !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 | ||
|} | |} | ||
| Line 40: | Line 40: | ||
{{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:
| 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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