A headwall is the retaining structure built at the inlet or outlet of a culvert, pipe or drainage outfall. It retains the embankment material around the opening, anchors the pipe end against flotation and displacement, improves the hydraulic efficiency of the entry, and provides a mounting point for trash screens, safety grilles and handrails. Headwall detailing has a direct effect on culvert capacity and on the scour risk at the outlet.
What a headwall does
Four functions are combined in one element. Structurally, the headwall retains the fill at the exposed end of the embankment and resists the earth pressure that would otherwise erode the slope around the pipe. Hydraulically, a shaped inlet reduces entry losses — a square-edged projecting pipe end is the least efficient arrangement, while a headwall with flared wingwalls and a bevelled or socketed edge can materially increase the discharge capacity of the same barrel under inlet control. Operationally, the headwall carries the trash screen, marker posts and any handrailing required by the risk assessment. Environmentally, it fixes the invert level and provides the platform for mammal ledges or a low-flow notch.
Precast, in-situ and modular headwalls
Precast concrete headwalls dominate UK highway and development work because they arrive to a known geometry and can be set in a single lift, reducing time in the excavation. In-situ reinforced concrete remains the choice for large spans, skewed alignments and cases where the headwall also acts as a retaining wall for the road above. Gabion and rock-filled modular headwalls are used on watercourse crossings and rural schemes where a flexible, permeable and visually recessive structure is preferred, and where the crossing may be subject to differential settlement. Steel headwalls fabricated to match the profile of a corrugated steel culvert give a continuous structural line between barrel and end treatment.
Wingwalls, aprons and scour protection
Wingwalls are the splayed returns either side of the opening that retain the embankment slope. Typical arrangements are flared (commonly 30 to 45 degrees to the barrel axis), parallel, or warped to follow the embankment. Flared wingwalls give the best inlet performance; parallel walls suit constrained sites. At the outlet the priority shifts from capacity to energy: an apron slab, cut-off wall and a rock armour or gabion mattress blanket dissipate the discharge and protect the bed and banks from scour. Outlet scour is the most common cause of culvert failure in the UK, and it is nearly always a detailing problem at the headwall rather than a barrel problem.
Design references and consents
UK culvert end treatments are designed within the framework of CIRIA C689 (culvert design and operation) and, on the strategic and local road network, the drainage series of the Design Manual for Roads and Bridges. Where the culvert crosses a main river or ordinary watercourse, land drainage consent or an environmental permit will normally be required, and the Environment Agency presumption against culverting means the end treatment must demonstrate fish and mammal passage where the watercourse supports it.
Related ViaCon solutions
ViaCon supplies corrugated steel culverts and buried structures with matched end treatments. See our culverts and the HelCor corrugated pipe range. Related glossary entries: culvert pipe, box culvert and pipe arch.
Frequently asked questions about headwall
What is a headwall?
A headwall is the retaining structure at the inlet or outlet of a culvert or pipe. It holds back the embankment around the opening, anchors the pipe end, improves hydraulic entry conditions and provides a fixing point for trash screens and safety railings.
What is the difference between a headwall and a wingwall?
The headwall is the face structure directly around the pipe opening. Wingwalls are the splayed side returns that retain the embankment slope either side of it. Wingwalls are usually flared at 30 to 45 degrees to the barrel for hydraulic efficiency, or parallel where space is constrained.
Does a culvert need a headwall?
Almost always. Without an end structure the embankment erodes around the pipe, the pipe end is exposed to flotation and impact, and the inlet works at reduced capacity. A projecting pipe end is the least hydraulically efficient inlet arrangement available.
What is a headwall apron?
An apron is the slab laid on the bed immediately downstream of an outlet headwall, usually with a cut-off wall at its downstream edge. It dissipates the energy of the discharge and prevents scour undermining the structure. Rock armour or gabion mattress is often laid beyond the apron for the same reason.