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A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. In the United Kingdom, the word can also be used for a longer artificially buried watercourse.
Culverts are commonly used both as cross-drains to relieve drainage of at the roadside, and to pass water under a road at natural drainage and stream crossings. When they are found beneath roads, they are frequently empty. A culvert may also be a bridge-like structure designed to allow vehicle or pedestrian traffic to cross over the waterway while allowing adequate passage for the water. Dry culverts are used to channel a fire hose beneath a noise barrier for the ease of firefighter along a highway without the need or danger of placing hydrants along the roadway itself.
Culverts come in many sizes and shapes including round, elliptical, flat-bottomed, open-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, limitations on upstream water surface elevation, and roadway embankment height.Turner-Fairbank Highway research Center (1998). "Hydraulic Design of Highway Culverts" (PDF), Report #FHWA-IP-85-15 US Department of Transportation, Federal Highway Administration, McLean, Virginia.
The process of removing culverts to restore an open-air watercourse is known as daylighting. In the UK, the practice is also known as deculverting.
Culverts must be properly sized and installed, and protected from erosion and scour. Many US agencies such as the Federal Highway Administration, Bureau of Land Management,Department of Interior Bureau of Land Management (2006). "Culvert Use, Installation, and Sizing" Chapter8 (PDF), Low Volume Engineering J Chapter 8, blm.gov/bmp. and Environmental Protection Agency,Environmental Protection Agency EPA Management (2003-07-24). "Culverts-Water" NPS Unpaved Roads Chapter3 (PDF), "CULVERTS" epa.gov. as well as state or local authorities, require that culverts be designed and engineered to meet specific federal, state, or local regulations and guidelines to ensure proper function and to protect against culvert failures.
Culverts are classified by standards for their load capacities, water flow capacities, life spans, and installation requirements for bedding and backfill. Most agencies adhere to these standards when designing, engineering, and specifying culverts.
If the failure is sudden and catastrophic, it can result in injury or loss of life. Sudden road collapses are often the result of poorly designed and engineered culvert crossing sites or unexpected changes in the surrounding environment cause design parameters to be exceeded. Water passing through undersized culverts will scour away the surrounding soil over time. This can cause a sudden failure during medium-sized rain events. Accidents from culvert failure can also occur if a culvert has not been adequately sized and a flood event overwhelms the culvert, or disrupts the road or railway above it.
Ongoing culvert function without failure depends on proper design and engineering considerations being given to load, hydraulic flow, surrounding soil analysis, backfill and bedding compaction, and erosion protection. Improperly designed backfill support around culverts can result in material collapse or failure from inadequate load support.
For existing culverts which have experienced degradation, loss of structural integrity or need to meet new codes or standards, rehabilitation using a reline pipe may be preferred versus replacement. Sizing of a reline culvert uses the same hydraulic flow design criteria as that of a new culvert however as the reline culvert is meant to be inserted into an existing culvert or host pipe, reline installation requires the grouting of the annular space between the host pipe and the surface of reline pipe (typically using a low compression strength grout) so as to prevent or reduce seepage and soil migration. Grouting also serves as a means in establishing a structural connection between the liner, host pipe and soil. Depending on the size and annular space to be filled as well as the pipe elevation between the inlet and outlet, it may be necessary to add grout in multiple stages or "lifts". If multiple lifts are required, then a grouting plan is required, which should define the placement of grout feed tubes, air tubes, type of grout to be used, and if injecting or pumping grout, then the required developed pressure for injection. As the diameter of the reline pipe will be smaller than the host pipe, the cross-sectional flow area will be smaller. By selecting a reline pipe with a very smooth internal surface with an approximate Hazen-Williams Friction Factor C value of between 140–150, the decreased flow area can be offset, and hydraulic flow rates potentially increased by way of reduced surface flow resistance. Examples of pipe materials with high C-factors are high-density polyethylene (150) and polyvinyl chloride (140).Plastic Pipe Institute-Handbook of Polyethylene Pipe, First Edition Copy 2006
Culverts that offer adequate aquatic organism passage reduce impediments to movement of fish, wildlife, and other aquatic life that require instream passage. 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, then the water may overflow the road embankment. This may cause significant erosion, ultimately 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.
Culvert style replacement is a widespread practice in stream restoration. Long-term benefits of this practice include reduced risk of catastrophic failure and improved fish passage. If best management practices are followed, short-term impacts on the aquatic biology are minimal.
The interactions between swimming fish and vortical structures involve a broad range of relevant length and time scales. Recent discussions emphasised the role of secondary flow motion, considerations of fish dimensions in relation to the spectrum of turbulence scales, and the beneficial role of turbulent structures provided that fish are able to exploit them. (2025). 9781742721866, Hydraulic Model Report No. CH107/17, School of Civil Engineering, The University of Queensland, Brisbane, Australia. . ISBN 9781742721866
The current literature on culvert fish passage focuses mostly on fast-swimming fish species, but a few studies have argued for better guidelines for small-bodied fish including juveniles. Finally, a solid understanding of turbulence typology is a basic requirement to any successful hydraulic structure design conducive of upstream fish passage.
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 the 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 a shire engineer in Victoria and a professor at the University of Queensland during the late 1960s.See:
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.
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