Storm Water Drainage – Design of Gullies

The following illustration summarises the drainage system:

*Note: French drains and berm drains can have a dual function. When the functions are combined the drains are referred as combined drains.

A few important points to keep in view while designing road gullies:

• The area draining into a single gully shall not exceed 200 sq. m.
• The spacing of gullies shall not exceed 50 metres in each channel on cambered roads or 30 metres in the lower channel of super-elevated roads.
• Where the channel gradient is steeper than 3%, the spacing of gullies shall not exceed 25 metres in order to avoid excessive velocities.
• Double gullies shall be provided at the low points of all concave channel profiles and in other low points where required by the County Council's Engineer (i.e., low points in turning areas). Kerb weir outlets may also be required at the discretion of the County Council's Engineer in addition to gullies at low points.
• At road junctions the gullies must be sited upstream of the tangent point to ensure that they do not conflict with pedestrian crossing points.
• Where super-elevation is employed, a gully should be sited uphill of the point of zero cross fall where the adverse camber commences.
• Gullies must be sited clear of private drives and must be aligned to avoid catching cyclists' wheels.
• The maximum length of any gully connection shall be 20 metres.
• Gully connections should be surrounded in concrete.

Design of Gullies

The major factors affecting the spacing of gullies are the volume of water flow to the gully and the efficiency of the gully in collecting this water. The volume of the water flowing to the gully is dependent on the shape of the edge of the carriageway (this defines the cross-sectional area of the flow), desirable width of the flow, which builds up to a maximum at each gully, and the longitudinal gradient which is defined by the vertical design.

On kerbed roads, the waterway area is dependent on the cross-fall of the carriageway, with the kerb face forming one boundary to the flow. The width of the flow which can be tolerated is dependent on the degree of nuisance which it is likely to cause. In pedestrian areas the flow will have to be narrow enough to be stepped over by people crossing the road: in all areas the width should be restricted so that there is no undue spray problem from passing traffic.

Taking these considerations into account, the following maximum flow width are recommended.

• Pedestrians on footway, urban areas with shop frontages etc. 0.5m
• No pedestrians or direct frontage properties - 0.65m
• Fast lane on dual carriageway - 0.6m

When checked for a 1 in 5-year storm the allowable flow width shall not exceed 1.5m for the hard shoulder or 1.0m for the hard strip. An excessive flow width can be a danger to traffic.

Local authorities often require a flow width of 0.75m in urban areas.

Gullies should be sited at all low spots on the channels and on each channel at super-elevation crossover points. Other gullies should be placed to ensure that carriageway water run-off is prevented from flowing onto the adjoining property, adjacent streets and pedestrian crossings.

Particular attention should be paid to central reservation openings, road junctions and roundabouts to ensure that these areas are satisfactorily drained. Contour diagrams are often a great help in determining gully position at different sites.

DMRB CD526 sets out the requirements and advice for determining the spacing of road grating and kerb inlets for removing surface water from the carriageway within an acceptable width of channel flow. The limiting factor determining the spacing between road gullies is normally taken to be the inlet capacity of the overlying grating rather than the underlying gully pot or associated pipework.