What is a Extreme event

An extreme event refers to a rare and severe natural occurrence that significantly exceeds typical environmental or meteorological conditions. These events include intense rainfall, prolonged droughts, major storms, heatwaves, and floods that occur with low frequency but potentially high impact. In the context of drainage, sewerage systems, and water management, extreme events pose a substantial risk to infrastructure, public safety, and environmental health.

Extreme events are characterised by their intensity, duration, and rarity. Although they do not occur often, their consequences can be widespread and damaging, making them a critical factor in resilient system design, risk assessment, and emergency response planning.

Characteristics of Extreme Events

Extreme events are defined not just by their severity, but by their deviation from normal or expected conditions. They are typically assessed using statistical measures, such as return periods, which indicate the probability of a certain event occurring in any given year.

Common Features:

  • Low frequency: Events may occur only once every 30, 50, 100 years or more.
  • High impact: They often exceed the design limits of standard infrastructure.
  • Rapid onset or prolonged duration: Some events develop suddenly (e.g. flash floods), while others persist over weeks or months (e.g. droughts).
  • Climatic drivers: Often linked to unusual atmospheric conditions, such as intense low-pressure systems, prolonged anticyclones, or shifting jet streams.

Examples in Practice:

  • A 1 in 100-year storm, where rainfall intensity far exceeds typical conditions and overwhelms sewer networks.
  • A multi-week heatwave, causing evaporation losses, soil desiccation, and pressure on water supplies.
  • A long drought, leading to groundwater depletion and cracking of clay soils that can damage buried pipes.

Relevance to Drainage and Sewerage Systems

Extreme events have significant implications for the design, performance, and longevity of drainage infrastructure. They frequently exceed the standard design parameters used in traditional systems, resulting in overflows, blockages, flooding, or structural damage.

Key Impacts:

  1. Stormwater Overload
    Sudden high-intensity rainfall can exceed the hydraulic capacity of sewers, channels, and culverts, causing urban flooding, road closures, and property damage.
  2. Combined Sewer Overflows (CSOs)
    In combined sewer systems, extreme rainfall often leads to discharge of untreated effluent into rivers to prevent sewer surcharging.
  3. Erosion and Sediment Transport
    Excessive runoff accelerates soil erosion, destabilises channels, and deposits silt in water bodies and infrastructure.
  4. Drought Effects
    Long dry periods reduce infiltration and groundwater recharge. Clay shrinkage can damage pipe joints, and reduced flow in foul sewers may lead to blockages or odour problems.
  5. System Fatigue and Failure
    Repeated exposure to extremes can cause physical wear, undermine manholes, and displace pipe sections, particularly in ageing infrastructure.

Engineering and Planning Responses

Modern infrastructure planning recognises the need to account for extreme events, not as anomalies, but as increasingly likely due to climate change. As such, systems are now being designed with greater flexibility, redundancy, and adaptation capacity.

Strategies include:

  • Designing for higher return periods: Moving beyond the historical 1-in-30 year standard for sewer design to 1-in-100 year or higher.
  • Climate uplift factors: Adjusting rainfall intensity assumptions to reflect future climate projections.
  • Temporary storage solutions: Installing storm tanks, retention ponds, or green roofs to buffer peak flows.
  • Sustainable Drainage Systems (SuDS): Promoting infiltration, evapotranspiration, and on-site attenuation of water during high-volume events.
  • Real-time control systems: Using smart sensors and dynamic flow management to mitigate surcharges and direct flows efficiently during peak events.

Regulatory and Policy Context

In the UK and other developed countries, managing extreme events is embedded in infrastructure guidance and environmental policy. Authorities such as the Environment Agency, Defra, and local planning authorities require developers and operators to assess extreme scenarios as part of:

  • Flood risk assessments (FRAs)
  • Drainage impact assessments (DIAs)
  • Water resource management plans (WRMPs)
  • Climate change adaptation strategies

Failure to properly account for extreme events can result in non-compliance, increased liability, and long-term system failure.

Conclusion

Extreme events are infrequent by nature but potentially catastrophic in impact. In a world of increasing climatic uncertainty, engineers, planners, and infrastructure managers must anticipate and accommodate these events to ensure the resilience and safety of drainage and sewerage systems. By moving beyond traditional design assumptions and embracing adaptive, forward-looking approaches, the industry can better protect communities, reduce environmental harm, and ensure continued service delivery even under the most challenging conditions.