What is a Major system

In urban drainage design, the major system refers to the secondary, above-ground flow paths that stormwater follows when the primary or minor system – consisting of underground pipes and culverts – is either overwhelmed or temporarily out of service. This system is an essential component of modern stormwater infrastructure, helping to prevent surface flooding and property damage during extreme rainfall events.

How the Major System Works

The major system is activated during heavy or prolonged rainfall that exceeds the capacity of the underground drainage network. While the minor system is designed for frequent storms, the major system acts as a relief route, safely conveying water above ground until it can be discharged into rivers, basins or larger water bodies.

Typical elements that serve as major system pathways include:

• Roads and kerbs that channel runoff along their surfaces.

• Grass swales and open green spaces designed to detain or direct water.

• Floodways, detention basins and parks that temporarily store excess runoff.

• Bridges and culverts that allow high flows to bypass constrained areas.

Planning and Design Considerations

A well-functioning major system depends on careful urban planning. Engineers must analyse topography, rainfall intensity, and land use to identify potential overland flow paths. The goal is to ensure that during high-intensity storms, water flows away from buildings and infrastructure, not toward them.

Design guidelines typically recommend:

• Keeping flow away from critical infrastructure and entrances to buildings.

• Preventing ponding near pedestrian areas or public spaces.

• Ensuring that flow velocities and depths remain within safe limits.

• Integrating green infrastructure where possible to slow and treat runoff.

Relationship Between Major and Minor Systems

Effective stormwater design relies on both systems working together. The minor system handles day-to-day rainfall, while the major system ensures resilience during peak events. Neither system can be ignored in the planning process.

Below is a simplified comparison:

Importance in Urban Resilience

As climate change increases the intensity of storm events, the major system has become a key factor in flood risk management. Inadequate above-ground drainage leads to flash floods, traffic disruption, structural damage and risk to public safety.

By treating roads, parks, green corridors and other open spaces as part of the drainage network, modern cities can:

• Reduce flood risk

• Lower pressure on underground systems

• Improve emergency response times

• Support sustainability goals with integrated green solutions

Conclusion

The major system is more than an emergency overflow – it is a fundamental part of comprehensive stormwater planning. By incorporating safe, efficient above-ground drainage paths into urban design, municipalities can protect infrastructure, reduce economic losses and enhance public safety in the face of extreme weather.