Culverts carry stream flow beneath roads and other obstructions. Common types include circular pipes, box culverts, elliptical culverts, pipe-arches, and open-bottom box or arch structures. The right choice is not determined by shape alone: engineers compare site geometry, hydraulic performance, flood effects, maintenance needs, and project requirements.
What is a culvert?
The Federal Highway Administration (FHWA) defines a culvert as “a conduit which conveys stream flow through a roadway embankment or past some other type of flow obstruction.” Culverts come in different shapes, materials, and configurations. The shape describes the conduit’s profile; material is a separate design choice.
FHWA’s Hydraulic Design of Highway Culverts (HDS-5) identifies circular, box, elliptical, and pipe-arch forms among common closed conduits, as well as open-bottom box and arch configurations.
What are the main types of culverts?
Circular pipe culverts
A circular pipe is a closed conduit with a round cross-section. It is one of the common forms used to carry flow under an embankment. Whether it fits a particular crossing depends on the required flow performance, available vertical clearance, roadway and embankment profile, and installation constraints—not on a universal rule that circular pipes are cheapest or best.
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Box or rectangular culverts
A box culvert has a rectangular cross-section and encloses the flow within a closed structure. FHWA also identifies open-bottom box configurations; these are distinct from closed boxes because the streambed is less enclosed.
Elliptical culverts
An elliptical culvert is a closed conduit with an oval profile. It is a distinct option to assess when the crossing’s vertical and horizontal dimensions matter, but the available FHWA material does not establish a universal performance advantage over other shapes.
Pipe-arch culverts
A pipe-arch is a closed, flattened arch-like conduit. Its suitability, like that of other forms, depends on hydraulic performance, water-surface limits, embankment height, and cost at the specific site. Shape alone does not establish its capacity or savings.
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Open-bottom box and arch culverts
Open-bottom configurations leave the natural streambed less enclosed than a closed-barrel culvert. That feature may be relevant to a crossing’s design, but it does not by itself demonstrate that the structure meets fish-passage requirements. Those criteria must be evaluated for the site.
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FHWA lists roadway profiles, channel characteristics, flood-damage evaluations, construction and maintenance costs, and estimated service life as selection factors. A meaningful comparison looks at the crossing as a whole:
- Hydraulic fit: Evaluate allowable headwater (the water level upstream of the culvert), whether inlet or outlet control governs, flow velocity, and overall hydraulic performance. Inlet geometry matters: FHWA notes that a more gradual transition into the inlet can improve discharge capacity by reducing contraction losses.
- Site geometry: Account for the roadway profile, channel characteristics, embankment height, barrel location, and alignment. A shape that does not fit the site’s physical constraints is not a practical option.
- Durability and upkeep: Consider expected service life, construction and maintenance costs, and exposure to abrasion, sediment, siltation, and debris. FHWA identifies these as design or selection considerations, but does not provide comparable cost or service-life figures for the shapes discussed here.
- Flood and downstream effects: Assess potential flood damage, scour, and debris risks, along with any applicable fish-passage criteria. A culvert’s effects cannot be judged from its cross-section alone.
- Applicable requirements: Check the governing engineering standards and rules for the project’s jurisdiction before narrowing the material or configuration choices.
Is culvert material the same thing as culvert type?
No. “Type” may refer to the cross-sectional shape or to the material, and those are separate classification axes. Culverts are made from varied materials; a shape name such as “box” or “circular” does not specify the material.
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For Federal-aid highway projects, FHWA’s Construction Program Guide quotes 23 CFR 635.411: “State Departments of Transportation (State DOTs) shall have the autonomy to determine culvert and storm sewer material types to be included in the construction of a project on a Federal-aid highway.” That policy is specific to Federal-aid highway projects and remains subject to other applicable Federal requirements; it is not a universal material rule for every project or jurisdiction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What information is needed to select a culvert?
Selection and hydraulic design are site-specific. For an actual crossing, a qualified engineer will need to assess the flow and channel, roadway and embankment geometry, upstream water-level limits, flood impacts, sediment and debris exposure, scour, maintenance, and applicable environmental and project requirements. A shape label or general-purpose guide cannot determine whether a proposed culvert will work at a particular site.
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For engineers or readers undertaking a detailed evaluation, FHWA describes HDS-5, Third Edition (2012) as a comprehensive reference covering culvert design methods, assessment, aquatic organism passage, repair, and rehabilitation. FHWA also provides the HY-8 culvert hydraulic analysis program; the release information cited here identifies version 7.60 as built July 30, 2019, and does not establish whether a newer version is available.
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