A road gully is the surface inlet that collects run-off from a carriageway, footway or paved area and passes it into the drainage network below. It comprises three parts: a grating and frame set flush with the surface, a gully pot beneath with a sump that retains grit, and a spur connection to the carrier drain or surface water sewer. Gullies are the most numerous drainage asset on the highway network and the most frequent source of localised flooding when they are not maintained.
Gully pot, grating and connection
The pot is usually a precast concrete or plastic unit with a sump below the outlet, sized to hold grit between cleansing visits. The grating and frame are selected to BS EN 124 load class — D400 for carriageways, C250 at kerbside and channel positions where the wheel load is offset, and lighter classes only in areas free of vehicles. Grating pattern matters as much as class: kerb-inlet, side-entry and combined gratings all trade hydraulic capture efficiency against safety for cyclists and against blockage by leaf litter. The connection to the carrier drain is normally 150 mm and should be laid to a self-cleansing gradient, since a flat gully spur silts up faster than the pot it serves.
Trapped and untrapped gullies
A trapped gully includes a water seal that prevents odours from a combined or foul-connected sewer rising through the grating. An untrapped gully has no seal and is used where the connection is to a dedicated surface water system with no odour risk. Trapped gullies retain a permanent water level in the pot, which also helps hold silt in place, but they need more frequent cleansing and can freeze. Getting this wrong on a combined system produces complaints; getting it wrong on a surface water system produces unnecessary maintenance.
Spacing and capture efficiency
Gully spacing is calculated, not standardised: the interval follows from the contributing area, the longitudinal and crossfall gradients, the design rainfall intensity and the permitted width of surface flow in the channel. The Design Manual for Roads and Bridges sets the method for the strategic road network in CD 526. Two practical points are worth holding on to. First, on steep longitudinal gradients a large fraction of the flow bypasses each grating, so the effective capture rate per gully falls and spacing must tighten. Second, at sag points the gully is the only escape route for the flow, so sag locations need redundant capacity rather than nominal spacing.
Maintenance and downstream protection
Gully cleansing on a risk-based cycle is the single highest-value maintenance activity on a highway drainage asset, and gully data — location, pot type, silt level at each visit — is what allows the cycle to be targeted rather than uniform. The gully pot is also the first line of sediment defence for the network: what it fails to catch ends up in the carrier drain, the catchpit, and eventually in the attenuation storage or the outfall. Where the downstream asset is an infiltration system or a crate field, gully performance should be treated as part of that asset’s protection, not as a separate highways matter.
Related ViaCon solutions
ViaCon supplies stormwater conveyance, treatment and storage systems downstream of highway inlets. See our stormwater solutions range. Related glossary entries: surface water drainage, silt trap, catchpit and underground drainage systems.
Frequently asked questions about road gully
What is a road gully?
A road gully is the surface inlet that collects run-off from a carriageway or paved area and passes it into the drainage network. It consists of a grating and frame, a gully pot with a sediment sump, and a spur connection to the carrier drain.
What is the difference between a trapped and an untrapped gully?
A trapped gully has a water seal that stops odours rising from a combined or foul-connected sewer. An untrapped gully has no seal and is used only where the connection is to a dedicated surface water system.
How far apart should road gullies be spaced?
Spacing is calculated from the contributing area, the longitudinal gradient and crossfall, the design rainfall intensity and the permitted width of flow in the channel. On steep gradients spacing must tighten because more flow bypasses each grating, and sag points need additional capacity.
What load class should a road gully grating be?
Carriageway positions normally require D400 to BS EN 124. C250 is used at kerbside and channel positions where the wheel load is offset, and lighter classes only in areas free of vehicle traffic.