Importance of highway drainage

Water is the greatest enemy of a road. Proper drainage is necessary for the following reasons:

1. Pavement strength. Water entering the pavement and subgrade reduces the soil strength (CBR falls sharply when soaked), causing rutting, potholes and failure. Drainage keeps the subgrade in the design moisture state.
2. Pavement life. Water in bituminous layers causes stripping of binder, and in cement concrete causes pumping and erosion of the sub-base. Good drainage lengthens the life and lowers the maintenance cost.
3. Safety. Water on the carriageway leads to skidding and hydroplaning, and reduces visibility.
4. Stability of embankments and slopes. Saturated slopes slide, and runoff causes gully erosion, scour and landslides. This is especially serious on hill roads in Nepal during the monsoon.
5. Prevents flooding and washouts of the road and avoids damage to adjoining land and property.
6. Frost heave and swelling damage are reduced, since less water is available in the soil.
7. Continuous service. The road remains open in the rainy season, which is vital for the economy of the area.

Studies show that a poorly drained pavement can fail in a fraction of its design life, so drainage is a part of road design from the beginning.