A culvert drain is a type of drainage pipe or channel built into roads, railroads, and bridges to allow water to flow underneath the surface. Culverts are structures that channel water past an obstacle or to a subterranean waterway, and they are typically embedded so as to be surrounded by soil. They can be made from a variety of materials, including pipes, reinforced concrete, and other materials. Culverts come in many sizes and shapes, including round, elliptical, flat-bottomed, open-bottomed, pear-shaped, and box-like constructions. The type and shape of culvert selected depends on factors such as requirements for hydraulic performance, limitations on upstream water surface elevation, and roadway embankment height. Culverts are commonly used both as cross-drains to relieve drainage of ditches at the roadside and to pass water under a road at natural drainage and stream crossings. Drainage culverts can also be used for storm crossings, such as the passage of pedestrians or traffic across a waterway, or runoff management. Culverts can be installed underground, embedded, or covered by soil, but they can also be above ground.