Runoff Coefficient is the ratio of between the peak runoff rate and intensity of rainfall.
The Rational Method is based upon following Two Assumptions:
1. Rainfall occurs at uniform intensity for duration at least equal to the time of concentration of watershed.
2. Rainfall occurs at a uniform intensity over the entire area of watershed.
The value of ‘C’ runoff coefficient in the rational formula should be taken from following table.
vegetative cover and land Slope | Soil Texture | ||
| Sand Loam | Clay and Silt Loam | Silty Clay |
1. Wood Land |
|
|
|
0 – 5 % slope | 0.10 | 0.30 | 0.40 |
5 – 10% slope | 0.25 | 0.35 | 0.60 |
10 – 30% slope | 0.30 | 0.50 | 0.60 |
2. Pasture land |
|
|
|
0 – 5 % slope | 0.10 | 0.30 | 0.40 |
5 – 10% slope | 0.16 | 0.36 | 0.55 |
10 – 30% slope | 0.22 | 0.42 | 0.60 |
3. Cultivated Land |
|
|
|
0 – 5 % slope | 0.30 | 0.50 | 0.60 |
5 – 10% slope | 0.40 | 0.60 | 0.70 |
10 – 30% slope | 0.52 | 0.72 | 0.82 |
