Culverts are commonly used both as cross-drains for ditch relief and to pass water under a road at natural drainage and stream crossings. A culvert may be a bridge-like structure designed to allow vehicle or pedestrian traffic to cross over the waterway while allowing adequate passage for the water. Culverts come in many sizes and shapes including round, elliptical, flat-bottomed, pear-shaped, and box-like constructions. The culvert type and shape selection is based on a number of factors including requirements for hydraulic performance, limitation on upstream water surface elevation, and roadway embankment height.
If the span of crossing is greater than 12 feet (3.7 m), the structure is termed as bridge and otherwise is culvert.
The process of removing culverts, which is becoming increasingly prevalent, is known as daylighting. In the UK, the practice is also known as deculverting.
Culverts can be constructed of a variety of materials including cast-in-place or precast concrete (reinforced or non-reinforced), galvanized steel, aluminum, or plastic, typically high-density polyethylene.
Two or more materials may be combined to form composite structures. For example, open-bottom corrugated steel structures are often built on concrete footings.