Surface Drainage

Surface drainage is concerned with removing all water that is present on the pavement surface, shoulder surface or any other surface from which it may flow onto the pavement. If not systematically removed, this water can accumulate underneath and weaken the pavement structure. There are three primary means used to prevent water infiltration and accumulation:

  • Impermeable pavement surface. An impermeable surface will protect the underlying subgrade from water sources above. Permeability concerns are different for flexible and rigid pavements.
    • HMA pavements. When HMA air voids are greater than about 8 – 9 percent they are likely to be interconnected with one another, making the HMA water permeable (Kandhal and Koehler, 1984[1]). Proper compaction practices should be followed to ensure an impermeable pavement. Also, minor cracks in the HMA should be promptly sealed.
    • PCC pavements. PCC is generally considered impermeable in this context, however joints and panel cracks must be tightly sealed to prevent water infiltration.
  • Slope. The pavement section should be sloped to allow rainwater to sheet flow (see Figure 1) quickly to the edge where it is typically collected in a curb and gutter system or a roadside ditch. A generally accepted standard is a 2 percent cross slope.
  • Grade. The curb and gutter or roadside ditch must be properly graded to allow flow to central collection points such as catch basins or detention ponds. A generally accepted standard is a grade of 0.5 percent or more although lesser grades have been used effectively.


Sheet flow
Figure 1: Sheet flow

Footnotes    (↵ returns to text)
  1. Pennsylvania’s Experience in the Compaction of Asphalt Pavements.  Placement and Compaction of Asphalt Mixtures, F.T. Wagner, Ed.  ASTM Special Technical Publication 829.  American Society for Testing and Materials.  Philadelphia, PA.  pp. 93-106.