What is a Drainage Culvert?

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  • Written By: Dale Marshall
  • Edited By: Kristen Osborne
  • Last Modified Date: 29 September 2019
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A drainage culvert is a man-made device used to channel water. Usually constructed of, or lined with, concrete, steel or some kind of plastic like polyvinyl chloride (PVC), drainage culverts are designed to prevent flooding by draining water either to a sewer system or to a natural waterway like a creek or river. Drainage culverts are most frequently seen along the sides of roads, and are essential in limiting or preventing flooding of roadways and the surrounding areas. A drainage culvert may be many miles long, or it can extend just long enough to channel water under a roadway. Some are flat-bottomed, while others are oval or circular in shape, and some are enclosed, especially when channeling water under a roadway, while many are open. Drainage culverts are ubiquitous in all developed areas, not only where where rainfall is plentiful.


Good drainage of rainwater and snow melt is critically important on roadways that traverse hillsides, and it’s not uncommon to see a drainage culvert channeling water underneath the roadway, from the uphill side to the downhill side. This is a specialized kind of culvert called a cross drainage culvert. Cross drainage ditches are best installed under a roadway when it’s being constructed, and civil engineers routinely include them in the planning of any modern road. Unless properly built, this type of culvert can fail, and the roadway under which it travels might collapse. In the United States, when a cross drainage culvert exceeds 20 feet (6.1 meters) in width, it’s classified as a bridge and subject to periodic inspection for structural integrity.

Modern landscapers and builders generally install culverts as an integral element of their construction to ensure proper drainage. In most rainfalls, for example, some water is absorbed directly into the ground, and some runs off; if the runoff exceeds the capacity of the natural and man-made infrastructure to accommodate it, flooding results. Paving reduces the surface area that can absorb water, increasing the potential for flooding unless measures are taken to contain the water. Retention ponds and drainage culverts are essential components of any flood prevention system, and are required in many jurisdictions.

Many governments likewise are addressing the problem of increased flooding risks by evaluating proposed new construction from the perspective of its impact on rainwater runoff and potential flood risk. These evaluations are used to inform the granting of construction permits. In these jurisdictions, proposed new construction that doesn’t adequately provide for rainwater runoff management isn’t very likely to be approved.


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