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A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. A structure that carries water above land is known as an aqueduct.
Culverts may be used to form a bridge-like structure to carry traffic. Culverts come in many sizes and shapes include round, elliptical, flat-bottomed, pear-shaped, and box-like constructions. 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.
Plastic culvert liners are also inserted into failing concrete or steel structures in order to repair the structure without excavating and closing the road. To prevent the older structure from collapsing, the space between it and the plastic liner is usually filled with grout.
Culverts fail due to corrosion of the materials from which they are made, 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.
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.
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