The construction of roads affects the natural surface and subsurface drainage pattern of watersheds or hill-slopes. The provision of adequate drainage is important to prevent the accumulation of excess water or moisture on or within road constructions that can adversely affect their material properties, compromise overall stability and affect driver safety. Drainage must cope with water from the carriageways, hard shoulders, foot/cycle paths, verges, and adjacent catchment areas.
The design of highway drainage will depend on several factors, including
1- The intensity of rainfall expected.
2- The size of catchment area.
3- The permeability of the surfaces.
The road camber or cross-fall should be designed to cope with heavy water run-off. Insufficient cross-fall can increase the risk of vehicles skidding or aquaplaning on the surface water. The standard cross-fall for roads is usually taken as 1:40, although the camber will vary depending on the individual requirements of the road.
Surface water is generally collected in channels at the road-side and discharged through gullies (drainage gratings at the edges of the road) into storm water sewers. Gullies are typically positioned at intervals of 25-30 m, depending on the road width and nature of the cross-fall. Gully covers can be either top opening or side opening.
To reduce the number of required drainage points, pavings and verges should be graded towards the channel.Culverts may also be used. A culvert is a closed conduit or tunnel used to convey water from one area to another, normally from one side of a road to the other side.
Minor roads usually use simple openings or channels which feed into roadside ditches. Most main roads use a system of gullies and piped sewers. However, soaka ways may also be used to discharge water, these are large underground chambers into which water flows from a gully. Water collects and gradually soaks through holes into the surrounding ground or to streams and roadside ditches. Soakaway, it is usual for the kerb to be kept flush with the road surface, with a precast concrete channel placed at the outside edge to drain water. This channel discharges water into gullies.