Drainage culverts allow water to travel from one side of a roadway to the other without passing over the road's surface. Culverts prevent contamination of water by preventing contact between water and the various hazardous materials covering the road's surface. Culverts are most useful in areas with forests or wetlands.
Transportation authorities install culverts in steeply graded or unusually low-lying areas of a roadway. Water tends to flow through these areas at regular intervals, but not frequently enough to require the construction of a bridge or overpass.
Culverts are beneficial to the water supply and to the roadway itself. Because culverts prevent water from running over the road's surface, they decrease the erosion that causes potholes. Culverts also prevent water from flowing through the material underlying the road, preventing the roadway from sagging or sinking into the ground.
Common materials for manufacturing culverts include concrete and durable plastics like high-density polyethylene. When used correctly, both concrete and HDPE culverts have service lives of several decades, considerably longer than that of the overlying roadway. When using either material, culverts must be at least 12 inches in diameter because flowing water frequently carries debris like leaves, sticks and trash. In a small-diameter culvert, this debris contributes to clogs, causing serious problems for both the road and water supply.