Culverts: types, parts and how they carry water

What a culvert is

The UK construction research body CIRIA defines a culvert in its 2019 Culvert, screen and outfall manual (C786) as a covered channel or pipe that stops a watercourse or drainage path being blocked by something built across it, as quoted by Designing Buildings Wiki. In practice a culvert is what lets a road, farm track or driveway cross a stream or a roadside ditch without a bridge.

Grady Hillhouse of Practical Engineering sums up its two jobs: carry the weight of the traffic above without collapsing, and pass enough water that the road is not overtopped in heavy rain.

Culvert types

Culverts are grouped by the shape of the barrel. Three shapes cover most roads and tracks.

Culvert types

Type	Shape and material	Where it fits
Pipe culvert	round pipe; CIRIA lists concrete, MDPE, HDPE, occasionally uPVC, also clay, steel, cast or ductile iron and GRP	ditches and small streams under driveways, tracks and minor roads
Box culvert	square or rectangular section, usually precast concrete	wider flows; Humes says its units take heavy wheel loads even with little or no fill over them
Arch culvert	arch over a flat or natural bed	larger openings; Jensen Precast says it carries large water volumes with less material

Box culvert types

Box culverts come in a few forms, described by precast makers:

• Crown type — an inverted U placed on a concrete base. Invert type — a U-shaped trough closed with a lid. Humes, an Australian maker, makes both (Humes box culverts).
• Segmental — several precast sections joined on site. Jensen Precast notes that the joints are the weak points and need careful sealing.
• Monolithic — cast as one piece, so there are no joints along the section (Jensen Precast).
• Multi-cell — two or more boxes side by side for a wide channel; Humes bridges between cells with slab units when there are three or more.

Humes makes its range from 300 mm to 6,600 mm span to the Australian standard AS 1597, in standard unit lengths of 1.2 m and 2.4 m.

Parts of a culvert

Infrasteel lists the parts that most culverts share:

1. Inlet — the upstream end where water enters.
2. Barrel — the pipe or box itself, carrying water under the fill.
3. Outlet — the downstream end where water leaves.
4. Headwall — a wall at each end that holds the embankment back and protects the pipe end.
5. Wing walls — angled walls beside the headwall that guide water in and out.
6. Base slab — the floor of a box culvert.
7. Embankment — the fill of the road or track over the culvert.

Practical Engineering names three common inlet shapes: projecting (the pipe sticks out of the bank), mitred (cut flush with the slope) and headwall, sometimes with wing walls.

What a box culvert drawing shows

A box culvert drawing shows the structure in cross-section and along its length, with dimensions. These are the values to look for:

• Span and height of the opening — the size the precast makers above sell by.
• Thickness of the top slab, bottom slab and side walls.
• Unit length of each precast section, and how many there are.
• Depth of fill over the top slab, which sets the load the box has to carry.
• Invert levels at inlet and outlet, which give the slope.
• Headwalls, wing walls and aprons at each end.

The slab and wall thicknesses and the reinforcement come from a structural design for the loads and the fill depth; they cannot be read off another project’s drawing.

Inlet control and outlet control

Practical Engineering, drawing on the US Federal Highway Administration’s 323-page manual on culvert hydraulics, lists eight things that decide how much water a culvert passes: the depth of water upstream (headwater), the area and shape of the barrel, the inlet design, the roughness of the barrel, its length, its slope and the depth of water downstream (tailwater).

Flow is limited in one of two ways:

• Inlet control — the entrance is the bottleneck; the barrel runs part-full, and making it smoother changes nothing.
• Outlet control — water enters faster than it can leave; friction in the barrel or water backed up downstream sets the flow. A fallen tree in the ditch below can then flood the road.

Keeping a culvert clear

Jensen Precast lists regular inspection, clearing debris and prompt repair as the maintenance a culvert needs, and warns that neglect leads to blockages, erosion and in the end failure of the culvert. Steel culverts also need protection against rust where they stay wet.

Water reaching a culvert is often collected first by a drainage channel along a yard or road edge, or by drainage pipes laid under a site. On sewer and storm lines, access points are closed with manhole covers.