Understanding Return Periods in Drainage Design: An In-Depth Guide

In drainage design, one of the key considerations is determining how resilient a system needs to be to manage various storm events. To achieve this, designers analyse return periods, which measure the likelihood of a specific rainfall event occurring in any given year. Return periods, expressed in years, help guide the capacity and durability of drainage systems to handle rainfall intensities that could otherwise lead to flooding and potential damage to properties and infrastructure.

What is a Return Period?

A return period, often denoted as 1 in X (where X is the number of years), represents the probability of a rainfall event of a certain intensity occurring within a particular year. For example, a 1 in 30-year return period has a 3.33% chance of happening in any one year, while a 1 in 100-year event has a 1% chance. These figures are statistical tools to estimate risk and are crucial in engineering resilient drainage systems.

Why Consider Different Return Periods?

Different return periods account for varying storm intensities and frequencies. In the UK, common return periods for drainage analysis include 1 in 1, 1 in 30 and 1 in 100 years. Each serves a specific purpose:

• 1 in 1-year Return Period: This is often analysed to manage frequent small-scale rainfall events. Drainage systems that effectively handle these events help prevent regular nuisance flooding.

• 1 in 30-year Return Period: Typically considered the threshold for infrastructure resilience, this return period ensures drainage systems can manage relatively severe storms without causing undue disruption. In urban areas, this level is frequently adopted as the minimum standard to prevent significant overland flooding.

• 1 in 100-year Return Period: Representing a rare but highly impactful event, this return period is used for flood risk assessments and long-term planning. By designing systems to manage such events, designers help reduce the risk of catastrophic flooding that could severely damage properties and endanger lives.

The Importance of the 1 in 100-year Return Period + 40% Climate Change Allowance

In recent years, the effects of climate change have underscored the need for even greater resilience. As rainfall patterns grow more intense and unpredictable, drainage systems designed to historical standards may no longer suffice. The 1 in 100-year return period with a 40% allowance for climate change is now a critical benchmark, particularly in areas where extreme weather events pose significant risks. Here’s why:

1. Increased Rainfall Intensity: Climate change is expected to bring about more frequent and severe storms. The 40% uplift anticipates these changes, ensuring drainage systems remain effective even under future climate conditions.

2. Long-term Resilience and Safety: With urban areas expanding and populations increasing, drainage infrastructure faces growing demands. By accommodating this heightened return period, drainage systems are better equipped to protect communities from extreme flooding.

3. Sustainability Requirements: Sustainable Drainage Systems (SuDS) are increasingly mandated in new developments. The 1 in 100-year + 40% approach aligns with sustainable design principles, providing capacity for substantial stormwater management, enhancing infiltration and minimising overflow.

How This Affects Drainage Design

Designing for the 1 in 100-year return period with a 40% climate change allowance impacts various aspects of the drainage system:

• Increased Storage Capacity: Systems must be designed to contain larger volumes of water, which may require additional or larger attenuation tanks, ponds and other storage methods.

• Enhanced Flow Control: Flow control structures like weirs and orifice plates must be calibrated to manage higher peak flows effectively without causing backups or overflows.

• Additional Space Requirements: More robust systems may require larger footprints for water storage and controlled release, which could impact site layouts and landscaping.

Conclusion

Understanding and applying return periods in drainage design is vital for creating resilient systems capable of managing today's weather extremes and those expected in the future. The 1 in 100-year return period, bolstered by a 40% climate change allowance, represents a proactive approach to designing for sustainability and public safety. 

By accounting for these stringent standards, drainage designers contribute to the long-term stability of urban environments and provide critical flood protection in an era of uncertain climatic patterns.

This is why we need suitable drainage design software such as Microdrainage or Flow to analyse the storms - a spreadsheet just won't do.