A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction from one side to the other side. 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. A structure that carries water above land is known as an aqueduct.
- Design and engineering
- Accidents due to culvert failures
- Environmental impacts
- Minimum energy loss culverts
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.
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.
Design and engineering
Construction or installation at a culvert site generally results in disturbance of the site soil, stream banks, or streambed, and can result in the occurrence of unwanted problems such as scour holes or slumping of banks adjacent to the culvert structure.
Culverts must be properly sized and installed, and protected from erosion and scour. Many agencies such as U.S. Department of Transportation Federal Highway Administration (FHWA), Bureau of Land Management (BLM), and U.S. Environmental Protection Agency (EPA) 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.
Accidents due to culvert failures
Culvert failures can occur for a wide variety of reasons including maintenance, environmental, and installation related failures, functional or process failures related to capacity and volume causing the erosion of the soil around or under them, and structural or material failures that cause culverts to fail due to collapse or corrosion of the materials from which they are made.
If the failure is sudden and catastrophic, it can result in injury or loss of life.
Sudden road collapses are often at poorly designed and engineered 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 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 and water capacities, surrounding soil analysis, backfill and bedding compaction, and erosion protection. Improperly designed backfill support around aluminum or plastic culverts can result in material collapse or failure from inadequate load support.
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 and 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.
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.
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 which yield small afflux. Colin J. Apelt, professor (emeritus) of civil engineering at the University of Queensland, presented an authoritative review of the topic (1983), a documentary video (1994), and a commemorative speech (2011).
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.