Surface drainage system

Surface drainage is the system that removes rainwater from the road surface and its surroundings and leads it to a safe outlet before it can enter the pavement, pond on the surface or erode the road. It is the most important part of highway drainage.

Elements

1. Camber (cross slope) on the carriageway (about 2.5% on bituminous roads, steeper on earth roads) drains water quickly to the edges.
2. Shoulders with a cross slope of about 3 to 4% carry the water from the pavement to the side drain, away from the pavement edge.
3. Side drains: longitudinal open drains (triangular, trapezoidal or rectangular, lined or unlined) along the road on one or both sides. In urban areas, these are replaced by kerb, gutter, gully inlets and storm sewers.
4. Catch drains above cut slopes intercept water from the hill side.
5. Cross-drainage structures (culverts, causeways and bridges) pass water under the road at streams and at regular intervals.
6. Outfall and disposal: mitre drains, natural streams, and energy dissipation where the fall is steep.

 catch drain
   \_/        hill
     \   cut slope
      \_ side drain
       \___ shoulder ___ carriageway ___ shoulder
                  camber (crown)
 slope of road -> side drain -> culvert -> stream

Design points

• Discharge by rational method: $Q=\dfrac{CIA}{360}$.
• Section by Manning's equation, with the velocity kept between the non-silting and non-scouring limits.
• Longitudinal gradient of side drains not less than about 0.5%.
• Lining where the velocity is high or the soil is erodible.