What is a Minor system

A minor system refers to the network of engineered drainage components designed to manage surface runoff during typical or average rainfall events. It usually includes roadside channels, kerbs, gullies, pipework and manholes that collect and convey water from roads, footpaths, roofs and other impermeable surfaces into the sewer or surface water network. The primary purpose of the minor system is to provide reliable drainage during regular weather conditions, reducing inconvenience to road users and protecting built environments from nuisance flooding.

Minor systems are an essential part of urban stormwater management, forming the first line of defence against rainfall and contributing to the smooth functioning of streets, pavements and public spaces.

Design purpose and performance criteria

The minor system is specifically designed to handle rainfall up to a certain return period, typically ranging from 1 in 1 year to 1 in 5 years, depending on local regulations and land use. This means it is expected to manage short-duration, low- to moderate-intensity storms without causing disruption to daily life.

In residential areas, the focus is on preventing standing water on roads and avoiding water entry into properties. In commercial or industrial areas, higher standards may apply to protect goods, vehicles and infrastructure. The system must be designed to:

  • Collect runoff quickly from impermeable surfaces

  • Convey flow efficiently to downstream systems or outfalls

  • Prevent surface water accumulation during average storms

  • Integrate seamlessly with kerb profiles, carriageways and landscape design

  • Avoid surcharging or backflow into properties

Proper grading, inlet placement and pipe sizing are essential to achieving these goals.

Components of a minor system

The minor system includes a range of physical components, all of which work together to move water from surface to subsurface without significant surface disruption:

  • Kerb and channel systems
    Form part of road edges, guiding surface water toward gully inlets.

  • Gullies and gully pots
    Capture water from the road surface and allow sediment settlement before entering the drainage pipe.

  • Catch basins and manholes
    Provide access for inspection and serve as junctions for connecting pipes.

  • Pipes and culverts
    Underground conduits that transport collected water to the next part of the system, whether that be a larger pipe, an outfall or a treatment feature.

  • Headwalls and outlet structures
    Guide water safely as it exits the system into rivers, swales or ponds.

The system may also include junctions, bends, backdrops and drop chambers to manage elevation changes and complex layouts.

Integration with major systems

While the minor system is effective during regular conditions, it is not intended to handle extreme rainfall or prolonged downpours. In those scenarios, a major system becomes active, routing excess water above ground along roads, landscaped areas or designated flow paths to safe discharge points. Together, these two systems form a comprehensive drainage strategy where the minor system handles day-to-day runoff and the major system manages overflow.

Urban planners must design both systems in tandem to ensure that when the minor system reaches its hydraulic limit, the major system can carry the excess without causing damage or danger.

Typical design flows

The design storm for a minor system varies by location and application, but general guidelines include:

  • Residential streets
    Designed for storms with a return period of 1 in 2 or 1 in 5 years

  • Commercial or town centres
    Often designed for 1 in 5 or 1 in 10 year events

  • Car parks and non-critical surfaces
    May have a lower design threshold

Flow calculations account for contributing catchment area, rainfall intensity, surface permeability and system slope. Modern design software and hydraulic modelling tools are used to simulate performance and optimise component sizing.

Maintenance requirements

Minor systems require routine inspection and cleaning to remain effective. Blockages, debris, leaf build-up and sediment can all reduce inlet capacity and lead to surface water accumulation even during modest rainfall. Maintenance tasks typically include:

  • Cleaning gully pots and catch basins

  • Flushing and jetting underground pipework

  • Inspecting manholes and joints for deterioration

  • Checking kerbs and channels for damage or misalignment

  • Removing vegetation or root intrusion that may impair flow

Neglecting maintenance increases the likelihood of surface flooding and premature asset failure.

Benefits of a well-functioning minor system

A properly designed and maintained minor system provides several public and environmental benefits:

  • Prevents localised flooding from everyday rainfall

  • Enhances road safety by reducing standing water

  • Protects buildings and infrastructure from water damage

  • Reduces pollution by capturing debris and silt before water enters rivers

  • Supports effective operation of downstream treatment or storage systems

Its unobtrusive design ensures that urban environments remain functional and accessible during common weather events.

Challenges in urban drainage

Minor systems are typically buried and often out of sight, leading to challenges including:

  • Undetected blockages until flooding occurs

  • Capacity limits being exceeded by increasingly intense storms due to climate change

  • Damage from utilities or construction work

  • Infiltration or exfiltration in older systems made of clay or concrete

Modern systems are increasingly integrated with Sustainable Drainage Systems (SuDS) to provide both minor and major flow paths in one holistic design.

Conclusion

A minor system is a key part of urban drainage infrastructure, responsible for efficiently conveying runoff from average storm events through a network of channels, gullies and underground pipes. It is designed to minimise disruption in daily life, maintain public safety, and protect infrastructure from water-related damage.

Though often unnoticed, the minor system is vital to the smooth functioning of urban environments. Its effectiveness relies on good design, integration with major systems, and consistent maintenance to meet the demands of both today’s and tomorrow’s rainfall patterns.