Roadway and structure pavement drainage should be considered early in a project design, while the roadway geometry is still being developed, because the hydraulic capacity of gutters and inlets is determined by the longitudinal slope and superelevation of the pavement. The imperviousness of the roadway pavement will result in significant runoff from any rainfall event. To ensure safety to the traveling public, careful consideration must be given to removing the runoff from the roadway through structure pavement drainage facilities.
This chapter provides specific guidance on designing the drainage of highway pavements, including assessing site hydrology (Section 5-2), methods for draining highways (Section 5-3), gutter flow and determining inlet spacing (Section 5-4), drainage structures and grate types and considerations (Section 5-5), and use of scupper barriers (Section 5-6). It concludes with a brief discussion of hydroplaning and hydrodynamic drag (Section 5-7).
The flatter the longitudinal profile is, the wider the shoulders need to be to accommodate increased spread width. However, for narrow shoulders, superelevation and/or widening transitions can create a gutter profile far different from the centerline profile. The PEO must carefully examine the geometric profile of the gutter to eliminate standing water created by these transitions. These areas should be identified and eliminated. This generally requires geometric changes stressing the need for early consideration of drainage.
Improperly placed superelevation transitions can cause serious problems, especially on bridges. Inlets or other means must pick up gutter flow before the flow crosses to the other side of the pavement. The collection of crossover flow on bridges is complex as effective drain inlets are difficult to place within structure reinforcement. Bridges over waterways and wetlands pose water quality issues and downspouts shall not be allowed to discharge directly into waterways or wetlands. Also, bridge drain downspouts have a history of plugging problems and are an objectionable aesthetic impact on the structure.
Inlets on bridges can usually be eliminated by considering drainage early in the design phase. Superelevation transitions, zero gradients, and sag vertical curves should be avoided on bridges. Modern bridges generally use watertight expansion joints so that all surface water can drain off the structure and collect in inlets placed at the bridge ends. Drainage design at bridge ends requires a great deal of coordination between the RHE, PEO, and State Hydraulics Office. All bridge drain designs shall be reviewed by the State Hydraulics Office.
Multilane highways create unique drainage situations. The number of lanes draining in one direction should be considered during the design phase. It may be necessary to complete a hydroplane analysis to assess risk. Coordinate with the RHE for additional requirements and guidance. “Part-time shoulder use” facilities shall be considered a lane. Contact the RHE for additional design guidance.
