Culvert Design Manual - U.S. EPA

A culvert is a closed conduit used to convey water from one area to another, usually from one side of a road to the other side.

Disposal of runoff from roadway ditches will help preserve the road bed, ditches, and banks. Strategically placed culverts, along with road ditch turn-outs, will help maintain a stable velocity and the proper flow capacity for the road ditches by timely outleting water from them. This will help alleviate roadway flooding, reduce erosion, and thus reduce maintenance problems. In addition, strategically placed culverts help distribute roadway runoff over a larger riparian filtering area. Culverts preserve the road base by draining water from ditches along the road, keeping the sub-base dry.