What Is a Culvert and What Type Should I Use?

A culvert is a transverse and fully enclosed drainage structure that runs under a road or portion of land. The size and type of culvert depend on the amount of water flowing, the area that is discharging to it and how deep the culvert is being installed. Some culverts can also serve as roadway surfaces, but they will always serve to convey water through a pipe or channel. Generally, selection and type of material ​​will depend on the comparative cost, the location of the structure, availability of skilled labor, time limitations and design being proposed.

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Box Culvert

One of the most used culvert types is a box culvert. Box culverts have a concrete (sometimes other materials can be used too) floor allowing the water to flow smoothly through it. Box culverts are usually made up of Reinforced Concrete (RCC). Some box culverts can be built using composite structures and are great when water needs to change direction or when a significant flow of water is expected. Box culverts can also be installed in such way that the top of the culvert is also the roadway surface. The most challenging part of installing these type of culverts is that you generally will need to have a dry surface to install the culvert, so dewatering or diversion of the water will be needed to complete the installation.

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Arch Culvert

An arch culvert is normally a low profile culvert. It allows them to be installed without disturbing the causeway as it will span over the entire drainage width. They are generally made of metal, stone masonry or RCC. They are installed easily, and you don't need to use expensive water diversion structures to install them. Common shapes include semicircular arch, elliptical arch, and concrete box culverts.  Another benefit of these type of structure is that the installation process will not take a lot of time, compared to traditional box culverts.

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Pipes culverts are available in different shapes such as circular, elliptical and pipe arches. Although circular pipes are the most common, other shapes might be used depending on site conditions and constraints at the job site. Their prices are very competitive, and they are very easy to install. As with other culvert types, the selection of the culvert will depend on hydraulic design and other factors that might affect their performance and suitability. it is the preferred one on urbanized areas and is the one usually used to manage storm sewer systems.

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Culvert Installation and Selection Tips

When deciding the type of culvert appropriate for your project, you will need to consider the following:

  • The culvert needs to be installed at the right elevation and grade to avoid erosion problems.
  • Maintenance of closed culverts can be a problem and will be more challenging as time goes by. Cost of maintenance should be considered during the selection process.
  • The inlet and outlet of the culvert need to be carefully designed and installed. Mitered ends are the most effective way a culvert can end. Mitered ends will allow for the right flow and will facilitate the flow process.
  • Flared ends at the outlet of a culvert can reduce or prevent scouring.
  • It is recommended to install rip-raps or similar structure to prevent erosion at the culvert outflow.
  • The right aggregate material shall be used to backfill on the sides, underneath and on top of the culvert if required. The right aggregate will prevent erosion and will protect the culvert itself.
  • Where possible, install culverts in natural draws on all roads.
  • Consider the traffic that will be driving over the culvert and how deep it will be installed. Sometimes culverts will collapse if not designed properly.
  • The cost to install and resources available should always be an important factor to consider before making the right selection.
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Other Considerations When Installing Culverts

Once you have decided the type of culvert that you will be using, be sure to verify that all environmental permits are up to date. Also verify that all NPDES requirements are meet and that the right equipment is available to install the culvert, backfill it and compact soil per engineering specs.