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Drainage from Road Surface

Vary road grades to reduce concentrated flow in road drainage, ditches, culverts, and on fill slopes and road surfaces.
  • Provide adequate drainage from the surface of all permanent and temporary roads by using outsloped or crowned roads, drain dips, or insloped roads with ditches and crossdrains.
  • Space road drainage features so peak drainage flow on the road surface or in ditches will not exceed the capacity of the individual drainage facilities.
  • Outsloped Roads: Outsloped roads provide means of dispersing water in a low-energy flow from the road surface. Outsloped roads are appropriate when fill slopes are stable, drainage will not flow directly into stream channels, and transportation safety considerations can be met.
  • Insloped Roads: For insloped roads, plan ditch gradients steep enough, generally greater than 2 percent, but less than 8 percent to prevent sediment deposition and ditch erosion. The higher gradients may be suitable for more stable soils; use the lower gradients for less stable soils.
  • Drain Dips: Properly constructed drain dips can be an economical method of channeling surface flow off the road. Construct drain dips deep enough into the subgrade so that traffic will not obliterate them.
Design roads for minimal disruption of drainage patterns.
  • Prevent downslope movement of sediment by using sediment catch basins, drop inlets, changes in road grade, headwalls, or recessed cut slopes.
  • Where possible, install ditch relief culverts at the gradient of the original ground slope; otherwise armor outlets with rock or anchor downspouts to carry water safely across the fill slope.
  • Skew ditch relief culverts 20 to 30 degrees toward the inflow from the ditch to improve inlet efficiency Protect the upstream end of cross-drain culverts from plugging.
  • Provide energy dissipators (rock piles, logs, etc.) where necessary at the downstream end of ditch relief culverts to reduce the erosion energy of the emerging water.
  • Crossdrains, culverts, water bars, dips, and other drainage structures should not be discharged onto erodible soils or fill slopes without outfall protection.
  • Route road drainage through SPZ vegetative filtration fields; slash windrows, or other sediment settling structures. Install road drainage features above stream crossings to route discharge into filtration zones before entering a stream.
Ditch relief culverts transfer water from a ditch on the uphill side of a road, under the grade and release it onto a stable area. They prevent water from crossing the road surface and softening the roadbed. Install culverts at least 12" in diameter at a 30-degree downgrade angle to enhance flow. Ensure proper slope of at least 5 inches every 20 feet. Seat the culvert on the natural slope.
Bedding material should be free of rock or debris that might puncture the pipe or carry water around the culvert. Cover with soil, avoiding puncture from large rocks. Compact soil at least halfway up the side to prevent water from seeping around the culvert.
Rule of thumb for covering culverts: minimum of I foot or one-third the culvert diameter, whichever is greater. Be sure outlet end extends beyond any fill and empties onto an apron of rock, gravel, brush, or logs.

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