Camber and drainage

Water is the enemy of roads. Rain that stands on the surface causes skidding and aquaplaning; water that soaks into the pavement layers and subgrade weakens them and leads to potholes and rutting. Highway drainage has three jobs: get water off the surface quickly, keep it out of the pavement structure, and carry it safely away from the road and its embankment.

Camber, or crossfall, is the slope given across the carriageway, usually from the centre line to both edges on undivided roads and to one side on each carriageway of divided roads. Its value depends on the surface type and rainfall: smoother, impermeable surfaces such as concrete or bituminous concrete need less slope; gravel and earth roads need more. Shoulders slope a little more than the carriageway. On horizontal curves, superelevation replaces camber. Longitudinal gradient also helps surface water move, and very flat sections need care.

Surface water is collected by side drains, lined or unlined, or by kerbs and channels with inlets to pipes in urban areas. Cross drainage works, from pipe and box culverts to bridges, carry streams under the road; their size comes from catchment hydrology. Subsurface drainage uses granular drainage layers, filter fabrics and perforated pipes to remove water from the pavement and lower the water table where it is high. Embankments in flood-prone areas are raised so that the subgrade stays a safe height above the high flood level.