Time of Concentration Calculator

# Time of Concentration

 Input Data: Slope = ft/ft Length of overland flow = ft Watershed area = sq. mi. Retardance coefficient = Rainfall intensity = in/hr Rational coefficient = Retardance roughness = Mannings overland roughness =
 Results: Izzards formula = min. Kerbys equation = min. Kirpichs equation = min. Kinematic equation = min. Bransby Williams equation = min. FAA equation = min.

## Formulas:

### Izzards formula

 tc = 41KL1/3 i2/3

 K = 0.0007i + Cr S1/3

### Kerbys equation

tc = c (Ln/i1/2)0.467

### Kirpichs equation

 L0.77 tc = 0.0078 S0.385

### 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: 0.06

### 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 tc 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: 0.3 Average grass: 0.4 Dense grass: 0.8

### Mannings overland roughness

 Concrete: 0.011 Bare sand: 0.01 Natural range: 0.08 Bluegrass sod: 0.45 Short prairie: 0.15 Woods: 0.45