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A culvert is a drain or pipe that allows water to flow under a road, railroad, trail, or similar obstruction. Culverts differ from bridges mainly in size and construction. Culverts are generally smaller than bridges, ranging from 0.3-metre (1 ft) pipes to larger reinforced concrete structures. Culverts are typically surrounded by soil.
For legal purposes in the United States, structures with total spans under 20 feet (6.1 m) are considered culverts, and longer structures are bridges. When boxes or pipes are placed side-by-side to create a width of greater than twenty feet, the culvert is defined as a bridge in the United States. This is a requirement of the federal bridge inspection standards and ensures that the culvert is inspected on a regular basis.
Transportation routes cross over waterways using either a bridge or a culvert. Culverts come in many sizes and shapes. Shapes may include round, elliptical, flat-bottomed, pear-shaped, and box. Culverts may be made of concrete, 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, or corrugated.
Plastic culvert liners are also inserted into failing concrete or steel structures in order to repair the structure without excavating and closing the road.
Accidents due to culvert failures
Culverts fail due to corrosion of the materials they are made from, or erosion of the soil around or under them. If the failure is sudden and catastrophic, it can result in injury or loss of life.
Sudden road collapses are often at poorly designed culvert crossing sites. Water passing through undersized culverts will scour away the surrounding soil over time. This can cause a sudden failure during medium sized rain events. There are more than 5,000,000 culverts currently in use in the United States alone. Continued inspection, maintenance, and replacement of these structures is crucial for infrastructure and safety.
Accidents due to culverts can also occur if a flood overwhelms it, such as with the Jacobs Creek Flood of 2003, or disrupts the road or railway above it, such as with the Bethungra accident of 1885, which killed seven people.
Soil and sand carried through a culvert can wear away the galvanizing of a steel culvert, allowing it to corrode and eventually collapse, disrupting the road or railway above it. This happened at a culvert near Gosford, New South Wales in 2007, killing five.
Safe and stable stream crossings can accommodate wildlife and protect stream health while reducing expensive erosion and structural damage.
Undersized and poorly placed culverts can cause problems for water quality and aquatic organisms. Poorly designed culverts can degrade water quality via scour and erosion and also restrict aquatic organisms from being able to move freely between upstream and downstream habitat. Fish are a common victim in the loss of habitat due to poorly designed crossing structures. Culverts that offer adequate aquatic organism passage reduce impediments to movement of fish, wildlife and other aquatic life that require instream passage. These structures are less likely to fail in medium to large scale rain/snow melt events.
Poorly designed culverts are also more apt to become jammed with sediment and debris during medium to large scale rain events. If the culvert cannot pass the water volume in the stream, the water may overflow over the road embankment. This may cause significant erosion, washing out the culvert. The embankment material that is washed away can clog other structures downstream, causing them to fail as well. It can also damage crops and property. A properly sized structure and hard bank armoring can help to alleviate this pressure.
Minimum energy loss culverts
In the coastal plains of Queensland (north-east Australia), torrential rains during the wet season place a heavy demand on culverts. The natural slope of the flood plains is often very small and little fall (or head loss) is permissible in the culverts. Professors Gordon R. McKay and Colin J. Apelt developed and patented the design procedure of minimum energy loss culverts waterways which yield small afflux. Colin J. Apelt, (emeritus) professor of civil engineering at the University of Queensland, presented an authoritative review of the topic (1983) and a well-documented documentary (1994).
A minimum energy loss culvert or waterway is a structure designed with the concept of minimum head loss. The flow in the approach channel is contracted through a streamlined inlet into the barrel where the channel width is minimum, and then it is expanded in a streamlined outlet before being finally released into the downstream natural channel. Both the inlet and outlet must be streamlined to avoid significant form losses. The barrel invert is often lowered to increase the discharge capacity.
The concept of minimum energy loss culverts was developed by Norman Cottman, shire engineer in Victoria (Australia) and by Professor Gordon McKay, University of Queensland (Brisbane, Australia) during the late 1960s. While a number of small-size structures were designed and built in Victoria, some major structures were designed, tested and built in South-East Queensland.
In forestry, proper use of cross-drainage culverts can improve water quality while allowing forest operations to continue.
- Borda–Carnot equation
- Low water crossing
- Storm drain
- Subterranean river
- Questions and Answers on National Bridge Inspection Standards, FHWA, "What is a bridge?", June 21, 2005.
- "All About Culvert Pipe", October 29, 2010.
- "Gosford Shire culvert washaway". The Sydney Morning Herald. June 14, 2008.
- Apelt, C.J. (1983). "Hydraulics of minimum energy culverts and bridge waterways," Australian Civil Engineering Transactions, CE25 (2) : 89-95. Available on-line at: University of Queensland.
- Apelt, C.J. (1994). "The Minimum Energy Loss Culvert" (videocassette VHS colour), Dept. of Civil Engineering, University of Queensland, Australia.
- CHANSON, H. (2003). "History of Minimum Energy Loss Weirs and Culverts. 1960-2002." Proc. 30th IAHR [The International Association for Hydro-Environment Engineering and Research] Biennial Congress, Thessaloniki, Greece, J. GANOULIS and P. PRINOS, ed.s, vol. E, pp. 379-387. Available on-line at: University of Queensland.
- Chanson, Hubert, Web page: Hydraulics of Minimum Energy Loss (MEL) culverts and bridge waterways.
- Culvert Technology May Help Young Salmon Muscle Their Way Upstream — Article about the impact of culverts on salmon migration
- Culvert fact sheet — Information produced by Canadian Department of Fisheries and Oceans
- Bottomless Culvert Scour Study — FHWA culvert research
- Design of Road Culverts for Fish Passage Washington Department of Fish and Wildlife guide to designing and constructing fish migration friendly culverts
- More than 50, freely available, published research articles on culvert design, culvert hydraulics and related topics by Professor Hubert Chanson, Department of Civil Engineering, University of Queensland
- Hydraulics of Minimum Energy Loss (MEL) culverts and bridge waterways