CULVERT

A culvert (from Tamil கல்வெட்டு (Kalvettu), meaning 'drilled/cut/carved stone'; from Kal, meaning 'stone', and Vettu, meaning 'cut/drill' 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.

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.

MATERIALS

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 US agencies such as the Department of Transportation Federal Highway Administration (FHWA), Bureau of Land Management (BLM),and Environmental Protection Agency (EPA)as well as state or local authoritiesrequire 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.

Environmental impacts

This culvert has a natural surface bottom connecting wildlife habitat.

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.