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What is a Culvert?

Mary McMahon
Updated May 17, 2024
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A culvert is a cutting under or beside a road which allows water to drain, rather than pooling and making road conditions hazardous. Technically, only an enclosed tunnel under a road can be classified as a culvert, and a cutting next to a road is called a ditch, but some English speakers use the term “culvert” to refer to either. Culverts are a vital part of the system used to drain roads and drives, keeping them safe and extending their lifetimes.

Ideally, a culvert is installed when a road is built, either by the local highway authority or a property owner, if the culvert is on private land. Generally, if a section of a road lies in a depressed area of ground or a region subject to flooding, a culvert should be installed to facilitate drainage, especially in the winter. The culvert is laid into the road bed while it is being built, and is kept clear by maintenance crews during routine checks of the roadway throughout the year.

Culverts serve two major functions. The first is that a culvert allows water to drain under the road, rather than pooling on top of it, making the surface safer for drivers. Pooled water on a road can cause cars to hydroplane, conceal hazards on the roadway, and will ultimately eat away at the roadway, causing it to break down quickly. If a road floods too much, it may have to be closed until the water level recedes, which is frustrating for drivers, but necessary for safety.

The second function of a culvert is to keep water from collecting along the margins of the roadway and eating away at the underside of the road bed. This extends the life of the road and prevents the edges of the roadway from slowly crumbling away, posing a safety hazard to drivers. In addition, if the water level next to the road rises too much, it can start to flood onto the roadway, forcing road closure until the water level drops again.

A culvert can be built from a wide variety of materials, depending on the size and where it is installed. Concrete, metal, and plastics are all common choices to line the cutting used to make a culvert, preventing the sides from caving in. Extremely large culverts can be big enough to stand up in, allowing for a heavy volume of winter water, while small ones may not be much larger than household plumbing: just big enough to divert the water flow to a drainage area. If a culvert is not effective enough, it may be replaced with a small raised bridge to elevate the roadway above the frequently flooded area.

About Mechanics is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon28732 — On Mar 21, 2009

I want to build a two story side extension. As there is a river culvert running on the side storm drain there is an unusually big gap between my house and my neighbor's house. I am told my extension should be more than 4 meters away from the culvert. However as the culvert is running at an angle my proposed extension needs to be at angle which means the front of it can be 3.5 meters, but the back reducing to just over 2 meters. Any suggestions on designing this extension?

Your assistance much appreciated.


Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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