Formulas:
Izzards formula
Kerbys equation
tc = c (Ln/i1/2)0.467
Kirpichs equation
Kinematic equation
| | L0.6 n0.6
|
| tc = 0.0078 |
|
| | S0.385
|
Bransby Williams equation
| | L
|
| tc = 21.3 |
|
| | 5280 A0.1 S0.2
|
FAA equation
| | 1.8 (1.1 - C) L0.5
|
| tc = |
|
| | S0.33
|
Where:
C = the dimensionless runoff coefficient
L = the distance traveled, in feet, and
S = the slope, in percent
Inputs:
Retardance coefficient
| Very smooth asphalt: | 0.007
|
| Tar and sand pavement: | 0.0075
|
| Concrete: | 0.012
|
| Tar and gravel pavement: | 0.017
|
| Closely clipped sod: | 0.046
|
| Dense bluegrass: | 0.060
|
Rainfall intensity
Time of concentration is calculated
from rainfall intensity iteratively
using an IDF curve. Enter the trial
rainfall intensity in inches per hour.
Use the calculated t
c to find a new
intensity from the IDF curve.
Rational coefficient
| Downtown Business: | 0.70 - 0.95
|
| Single Family Res: | 0.30 - 0.50
|
| Asphalt/Concrete: | 0.70 - 0.95
|
| Sandy Soil Lawn: | 0.05 - 0.20
|
| Heavy Soil Lawn: | 0.13 - 0.35
|
| Brick: | 0.70 - 0.85
|
Retardance roughness
| Smooth Pavement: | 0.02
|
| Poor grass, bare sod: | 0.30
|
| Average grass: | 0.40
|
| Dense grass: | 0.80
|
Mannings overland roughness
| Concrete: | 0.011
|
| Bare sand: | 0.010
|
| Natural range: | 0.080
|
| Bluegrass sod: | 0.450
|
| Short prairie: | 0.150
|
| Woods: | 0.450
|
