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The surface of town on which rainfalls is classified as follows :
Types of surface % area Runoff coefficient Roof 25 0.8 Pavements and yards 25 0.85 Macadamized roads 15 0.32 Gravel roads 10 0.2 Unpaved streets 20 0.15 Lawns and gardens 5 0.2
What is runoff of catchment if,
- Total area of catchment is 12 hectares
- Time of concentration for area is 15 minutes.
The surface of town on which rainfalls is classified as follows :
| Types of surface | % area | Runoff coefficient |
| Roof | 25 | 0.8 |
| Pavements and yards | 25 | 0.85 |
| Macadamized roads | 15 | 0.32 |
| Gravel roads | 10 | 0.2 |
| Unpaved streets | 20 | 0.15 |
| Lawns and gardens | 5 | 0.2 |
What is runoff of catchment if,
- Total area of catchment is 12 hectares
- Time of concentration for area is 15 minutes.
Solution:
Average impermeability factor is,
`C=(sum(C_1A_1))/A`
`=(0.8**0.25+0.85**0.25+0.32**0.15+0.2**0.1+0.15**0.2+0.2**0.05)`
`=0.5205`
Now, the intensity of rainfall is given by,
`I=760/(15+10)`
`=30.4` mm/hr
Thus, the runoff of the catchment is given by,
`Q=(CIA)/360`
`=(0.5205**30.4**12)/360`
`=0.527\ m^3/s`
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Similar Question
Assuming the surface on which rain falls in a thickly build up residential district as follows :
- `40 %` area consists of roofs and pavements (`C_1 = 0.80`)
- `60 %` of area consists of lawns and gardens (`C_2 = 0.2`)
Calculate the coefficient of runoff. If the area of the district is 2 hectares and the maximum intensity of rainfall is taken as 50 mm/hr, what is the runoff of the district? (Ans: `0.44, 0.122` `m^3`/sec)
The surface of town on which rainfalls is classified as follows :
| Types of surface | % area | Runoff coefficient |
| Roof | 25 | 0.8 |
| Pavements and yards | 25 | 0.85 |
| Macadamized roads | 15 | 0.32 |
| Gravel roads | 10 | 0.2 |
| Unpaved streets | 20 | 0.15 |
| Lawns and gardens | 5 | 0.2 |
What is runoff of catchment if,
- Total area of catchment is 12 hectares
- Time of concentration for area is 15 minutes.
The catchment area of the city is 500 hectares. Assuming that the surface on which rainfalls is classified as follows : :
| S.NO | Types of surface | % area | C |
| 1 | Forest and wooden area | 10 | 0.15 |
| 2 | open ground + unpaved street | 10 | 0.20 |
| 3 | parks + lawns + gardens | 15 | 0.15 |
| 4 | Gravel roads | 20 | 0.25 |
| 5 | Asphalt pavements | 20 | 0.85 |
| 6 | Water tight roof surfaces | 25 | 0.9 |
Calculate the run off coefficient of the area. Also calculate the storm water, if the area to be served is 500 hectares, time of entry is 15 minutes and time of flow is 10 minutes.
[Ans: `0.5025`, `15.32 m^3/s`]