Requirements of highway drainage

A good highway drainage system should satisfy these requirements:

1. Quick removal of surface water. Rainwater should drain off the carriageway immediately through camber and cross slope, with no ponding on the surface.
2. Safe disposal. Water collected from the road and nearby catchment must be taken through side drains and cross-drainage works to a natural outfall (stream or gully) without damaging the road, adjacent property or farmland.
3. Proper flow velocity. Velocity in drains should be high enough to prevent silting (non-silting velocity, about 0.6 to 0.75 m/s) and low enough to prevent scouring (the non-scouring limit depends on the soil and lining).
4. Interception of outside water. Catch drains should stop water from the upslope area from reaching the road.
5. Control of sub-surface water. The water table should be kept well below the subgrade (about 0.6 to 1.0 m), and seepage and capillary rise should be controlled.
6. Protection of embankments and slopes from erosion and from saturation, using lining, turfing and energy-dissipating structures.
7. Adequate capacity. Drains and cross-drainage structures must carry the design discharge for a suitable return period (longer for culverts and bridges than for side drains), with enough freeboard.
8. Economy and ease of maintenance. Drains should be simple, easy to clean and made of locally available material, without causing hazard to traffic (safe slopes, no deep open drains near the carriageway).