Drainage is an inseparable, fundamental part of highway design. The main goal of a highway drainage system is to “keep water out of the road itself”. The reasons are obvious, such that the carrying capacity of the road will decrease, and the road will slowly erode, once the water keeps coming into the road cross section. Another important consideration is driving safety. If the water ponds on the road surface, obviously this will lead to unsafe driving conditions.

To accomplish these goals, the following aspects need to be considered:

Slope of the road surface along the road main axis or transverse axis toward ditches, which is one of the main influence factors to facilitate drainage.

If water is blocked in some way or not drained at an adequate rate, this can cause ponding of water on the road, erode shoulders, embankments, weaken base layers.

The roadside channels, culverts ditches must be kept clean to ensure adequate flow of water.

The road must not contain low lying areas than its supposed elevation, which may immediately catch rain water, without letting it even to reach to drains and cause ponding on the road surface.

Subsurface drainage must be adequate, where perforated pipes in gravel are typically used. This is to drain the water that has already entered into the road system.

Where there are bridges and culverts, soil around the foundations can also get eroded.

Water ponding or inadequate drainage on road easily turns to ice in colder regions.

On mountainous regions, there are extra considerations for landslides and rapid and heavy runoff.

Rip-rap, rough surface ditches will slow down water speed on heavy slopes.

By: A. TUTER

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