Contents:

## Design of Surface Drainage System for Highway

The design of surface drainage system carried by two types of analysis:- Hydrologic analysis
- Hydraulic analysis

### Hydrologic Analysis of Drainage for Highway

Whenever there is a rainfall, some of the rain water infiltrated into the ground and stored as ground water and some of the portion may evaporate into the atmosphere. Other than these losses, the water left on the surface is called as run off. The method of estimating the run off is called hydrologic analysis. To estimate the maximum quantity of water expected to reach the drainage system is the main objective of hydrologic analysis. For this, one need to know the factors affecting run off and they are- Rate of rain fall
- Moisture condition
- Soil type
- Ground cover presence
- Topography

**Q = C i A _{d}**

^{3}/sec) C = run off coefficient i = intensity of rain fall (mm/sec) A

_{d}= area of drainage (m

^{2})

Type of Surface |
Coefficient of run off |

Pervious soil surface | 0.05 - 0.30 |

Soil covered with turf | 0.30 - 0.55 |

Impervious soil | 0.40 - 0.65 |

Gravel & WBM roads | 0.35 â€“ 0.70 |

Bituminous & C.C roads | 0.80 â€“ 0.90 |

**C = (A _{1} C_{1}+A_{2} C_{2}+A_{3} C_{3}) / (A_{1}+A_{2}+A_{3})**

_{1}, C2, C3 are run off coefficients for different surfaces and A

_{1}, A2, A3 are their respective areas. In the Next stage, Intensity of rainfall â€śiâ€ť is to be calculated. To find this, first we need to know the time taken by water to reach drainage inlet from the drainage area. This can be found out from the below graph. This is called as

**inlet time**.

### Hydraulic Analysis of Highway Drains

Now comes the second stage hydraulic analysis, in which the dimensions of drainage channels or culverts are designed based on â€śQâ€ť obtained in the above stage of analysis. Now we have discharge which is designed run off â€śQâ€ť. If we know the allowable velocity â€śVâ€ť in the channel, then the area of channel can be calculated from below formula:**Q = A.V**

Soil type |
Allowable velocity (m/sec) |

Sand or silt | 0.30 â€“ 0.50 |

Loam | 0.60 â€“ 0.90 |

Clay | 0.90 - 1.50 |

Gravel | 1.20 â€“ 1.50 |

Soil with grass | 1.50 â€“ 1.80 |

^{2}. Next, the longitudinal slope of channel â€śSâ€ť is to be calculated by Manningâ€™s formula:

**Where V = Allowable velocity (m/sec) N = Manningâ€™s roughness coefficient R = Hydraulic radius (m) S= Longitudinal slope of channel In the above formula, we already know the â€śVâ€ť value. Hydraulic radius â€śRâ€ť is the ratio of area of the channel to its wetted perimeter. Now comes, the toughness coefficient which is again varies according to lining material as follows:**

Lining material |
Manningâ€™s roughness coefficient, n |

Ordinary soil | 0.02 |

Soil with grass layer | 0.05 â€“ 0.10 |

Concrete lining | 0.013 |

Rubble lining | 0.04 |

**Read More:**

**Horizontal Transition Curves for Highways and Its Calculation**

**Drains and Sewers Terms Definitions**

**Types of Plumbing and Drainage Systems in Buildings**