What Is a Culvert?
A culvert is a pipe or box structure that carries a stream or drainage channel under a road, railroad, driveway or embankment. It must pass design floods without overtopping or damaging the road.
Shapes and materials
Round pipes, pipe arches, elliptical pipes and box culverts are common. Materials include reinforced concrete, corrugated steel or aluminum, and plastic (HDPE or PVC). Bottomless arches and three-sided structures are used where fish passage or natural streambeds matter.
Material choice depends on cost, loading, corrosion and abrasion risk, required service life and agency standards. Many state DOTs publish tables of allowable materials by soil and water chemistry.
How culverts work hydraulically
Flow through a culvert is controlled either at the inlet (entrance geometry limits flow) or at the outlet (barrel friction, length, slope and tailwater limit it). FHWA's HDS-5 manual and the free HY-8 software compute both and use the condition that produces higher headwater.
Designers choose a design storm (often 10, 25, 50 or 100-year depending on road class and agency) and check that headwater stays below allowable limits, typically below the road shoulder.
Why culverts fail
Undersizing, debris and sediment blockage, corrosion and abrasion of metal pipes, erosion at the outlet (scour), piping of water along the outside of the pipe, and inadequate cover. Regular inspection and cleaning extend culvert life.
Many agencies inspect culverts on a schedule and rate their condition. Owners of private culverts, such as driveway and farm crossings, are usually responsible for keeping them clear.
Frequently asked questions
Who designs a culvert?
A civil or hydraulic engineer, using agency criteria.
What is headwater?
The depth of water at the culvert inlet, measured from the inlet invert.
What is a bridge versus a culvert?
Definitions vary by agency; the National Bridge Inspection Standards treat structures over 20 ft in span along the roadway as bridges, including large culverts.
Sources
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