Highway drainage system is the process of removing and controlling excess surface and sub soil water within the roadway or right of way. Adequate drainage is essential for the protection of the investments made in the highway structures. Therefore, design, construction and maintenance of appropriate highway drainage system is one of the most important tasks of a highway engineer. It is important to provide an effective drainage system for all categories of roads in rural and urban areas in order to prevent early deterioration and failures of various components of a road including the road pavements.

The measures taken to control the flow of surface water are termed ‘surface drainage’ and those measures dealing with ground water in various forms are termed ‘sub surface drainage’. The highway drainage system thus consists of: (1) surface drainage system and (2) sub surface drainage system. The objects and details of these are given in subsequent paragraphs of this chapter.

REQUIREMENTS AND IMPORTANCE OF HIGHWAY DRAINAGE.

Requirements of Highway Drainage.

1. The surface water from the carriageway and shoulder should effectively be drained off without allowing it to percolate to sub grade and weaken the soil.

2. The surface water from the adjoining land should be prevented from entering the roadway.

3. The side drain should have sufficient capacity and longitudinal slope to carry away all the surface water collected from the roadway.

4. Flow of surface water across the road and shoulders and along slopes should not cause erosion or form cross ruts.

5. Seepage and other sources of under ground water should be effectively intercepted and drained off by the suitable subsurface drainage system.

6. Highest level of ground water table should be kept well below the level of subgrade, preferably by at least 1.2m. if the highest level of ground water level is closer than 1.2 m, it is desirable to lower the same with a well planned and laid subsurface drainage system.

7. In water logged areas special precautions should be taken, especially if detrimental salts are present or if flooding is likely to occur.