What is a Event (rainfall)

A rainfall event refers to a single, measurable occurrence of precipitation that is separated by a sufficient dry period from previous or subsequent rainfall. In the context of drainage and sewerage system design, a rainfall event is more than just a rain shower — it is a defined hydrological incident with the potential to affect surface water runoff, infiltration, sewer capacity, and flood risk.

Rainfall events are critical data points in urban drainage modelling, flood risk assessment, and wastewater treatment planning. Their timing, intensity, duration, and volume influence how water behaves across both natural landscapes and engineered infrastructure.

Defining a Rainfall Event

A rainfall event is typically defined by two key characteristics:

  • Initiation: Begins when measurable rainfall occurs after a minimum dry interval, often defined as 6 to 24 hours with no recorded precipitation, depending on local standards or modelling requirements.
  • Termination: Ends when rainfall ceases for a specified period, allowing the system to return to its pre-event state.

This approach ensures that distinct events are separated and assessed individually, which is essential when analysing their effect on drainage networks, combined sewer overflows (CSOs), and water quality.

Parameters of a Rainfall Event

Rainfall events are described and assessed using several key hydrological parameters:

  • Intensity (mm/hr): The rate at which rain falls during the event, often linked to system overloads and surface runoff generation.
  • Duration (minutes or hours): The total time span of continuous or near-continuous rainfall.
  • Volume (mm or m³): The total amount of rain that falls during the event.
  • Return period (years): A statistical measure of how often a rainfall event of a particular intensity and duration is likely to occur.
  • Antecedent dry period: The length of time since the last rainfall, which affects infiltration and initial runoff conditions.

Engineers use these metrics to determine system resilience, size storage tanks and sewer pipes, and evaluate overflow risk during heavy rainfall.

Importance of Rainfall Events in Sewerage Systems

Rainfall events have a direct and significant impact on sewerage infrastructure, especially in systems that combine foul and surface water flows:

1. Combined Sewer Systems

In older urban areas, combined sewers carry both wastewater and stormwater. During heavy rainfall events, the added volume can exceed the system’s capacity, leading to combined sewer overflows (CSOs) — controlled discharges into rivers or watercourses. Monitoring rainfall events helps predict and manage such overflows to reduce pollution risk.

2. Separate Sewer Systems

Even in separated systems, rainfall events can indirectly affect foul drainage via infiltration (water entering foul drains through cracks or joints) and inflow (surface water entering through manholes or illegal connections). These flows increase during and after significant rainfall, potentially overwhelming treatment facilities.

3. Surface Water Drainage

Rainfall events determine runoff volumes and peak flow rates. System design must consider typical and extreme events to avoid flooding on roads, properties, and public spaces.

Rainfall Event Modelling and Drainage Design

Rainfall events form the foundation of many hydrological and hydraulic models used in:

  • Sustainable Drainage Systems (SuDS) design
  • Flood risk assessments (FRAs)
  • Urban drainage simulation (e.g. InfoWorks ICM, SWMM)
  • Stormwater detention and retention basin sizing
  • Wastewater treatment capacity planning

Designers typically simulate various rainfall events based on historical data or design storm profiles, such as:

  • 1 in 1 year event (frequent, minor)
  • 1 in 30 year event (typical design standard for sewers)
  • 1 in 100 year event (used in flood defence planning)

These profiles may be adjusted for climate change uplift to reflect anticipated increases in rainfall intensity and frequency over coming decades.

Environmental and Operational Impacts

Rainfall events also influence:

  • Pollutant wash-off: First flush runoff can carry oils, metals, and debris from urban surfaces into waterways.
  • Soil erosion: Intense events can mobilise sediment and cause channel scouring.
  • Infrastructure wear: Frequent surcharging and overflow events increase maintenance needs.
  • Water quality: Events that trigger overflow systems can degrade water quality in receiving rivers and lakes.

Monitoring and analysing rainfall events allows operators to anticipate issues and implement real-time control strategies, such as activating storm tanks or regulating inflow at pumping stations.

Conclusion

A rainfall event is a fundamental unit of measurement in drainage, sewerage, and water management disciplines. Far from being a simple meteorological observation, each event has the potential to test the limits of urban infrastructure, influence environmental quality, and guide critical engineering decisions. By accurately defining, modelling, and responding to rainfall events, professionals can design systems that are more resilient, sustainable, and capable of protecting communities from the increasing challenges posed by urbanisation and climate variability.