Components of surface drainage systems.

The surface water from the roadway and the adjoining land is to be collected and then disposed off with the help of surface drainage system. The various components of the surface drainage system are (a) the cross slope or camber of the pavement surface and the shoulders, (b) the road drains and (c) cross drains.

Camber or cross slope.

The water from the pavement surface and shoulders is first drained off to the road side drains with the help of the cross slope or camber. This rate of this cross slope of the pavement or the carriageway is declined based on: [1] the type of pavement surface and [2] amount rainfall in the region. The details of the camber and the recommended range of camber for different types of pavement surfaces are given in Article 4.2.2 of chapter 4 ‘Geometric Design of Highways’. The values of camber range from 1 in 25 or 4.0 % for earth roads to 1 in 60 or 1.7 % for high type bituminous surface and CC pavements.

Road side drains.

The road side drains of highways passing through rural areas are generally open, unlined or ‘kutcha’ drains of trapezoidal shape, cut to suitable cross section and longitudinal slopes. These side drains are provided parallel to the road alignment and hence these are also called longitudinal drains. On plain terrain with embankments the longitudinal drains are provided on both sides beyond the toe of the embankment. However if the road passes through sloping terrain (with cross slope more than 4.0%) the longitudinal drain may be provided on one side only beyond the toe of the embankment along the higher side of the slope. The water on the lower side of the road will continue to drain away towards the natural valleys.

Cross drains.

On rural stretches of highways, the water flowing along the road side drains are collected by suitable cross drains through cross drainage structures (CD structures) at locations of natural valleys and streams and disposed off to the natural water course. The CD structures may generally be a suitable type of culvert, depending on the quantity of water to be carried across and the span. Different types of culverts adopted on rural stretches of highways are, slab, box or pipe culverts. The CD structures should extend up to the full formation width (including the roadway and the shoulders) and the ends may be protected by suitable abutments and parapet walls. When the width of the stream or river to be crossed is large (generally more than 6.0 m) the CD structures provided is called minor bridge. When the total length of the bridge is more than 60 m the bridge structure may be termed major bridge.