Climate Change Allowances in Drainage Design: Why They Matter

Climate change is no longer a distant threat. It's a present reality that's fundamentally changing how we design drainage systems. As drainage designers, we have a responsibility to ensure that the infrastructure we create today can cope with the more extreme weather patterns of tomorrow. That's where climate change allowances come in.

What Are Climate Change Allowances?

Climate change allowances are additional capacity factors that we build into drainage designs to account for increased rainfall intensity expected due to climate change. Currently, we work with allowances ranging from 40% to 45%, depending on the project location and the development's expected lifespan.

In practical terms, this means that if a drainage system would traditionally be designed to handle 100 litres per second of rainfall, we now design it to accommodate between 140 and 145 litres per second. It's a significant uplift, but one that's grounded in solid climate science and regulatory guidance.

Why Do We Need These Allowances?

The science is clear: our climate is changing, and one of the most significant impacts in the UK is increased rainfall intensity. Weather patterns are becoming more extreme, with heavier downpours occurring more frequently than historical data would suggest.

The Met Office and Environment Agency have analysed decades of rainfall data and climate projections to understand how our weather is evolving. Their research shows that peak rainfall intensities (those short, sharp bursts that put the greatest strain on drainage systems) are expected to increase substantially over the coming decades.

If we designed drainage systems today using only historical rainfall data, we'd be setting them up to fail. A system that works perfectly well now might be completely overwhelmed in 20 or 30 years' time, leading to flooding, property damage, and potentially putting lives at risk.

The Regulatory Framework

Climate change allowances aren't optional. They're a regulatory requirement. The Environment Agency provides clear guidance on the allowances that must be applied to different types of development across different parts of the country.

These allowances vary based on several factors:

Location: Different regions face different levels of climate risk. Areas already prone to heavy rainfall may require higher allowances.

Development lifespan: Residential developments, which are expected to last 100 years or more, require higher allowances than temporary structures.

Drainage type: Surface water drainage systems, which handle rainfall runoff, are particularly affected by these requirements.

Local planning authorities and water companies won't approve drainage designs that don't incorporate appropriate climate change allowances. It's a fundamental part of achieving planning permission and building regulations approval.

Why 40% to 45%?

The specific allowances we apply (typically between 40% and 45%) are based on the Environment Agency's guidance for the 'central' and 'higher central' climate change scenarios. These scenarios represent the most likely projections for increased rainfall intensity across England over the next century.

For most residential developments, we apply the higher central allowance, which currently sits at 40% for much of England. In some areas, particularly those at higher risk or for developments with longer design lives, we may apply allowances of 45% or even higher.

These aren't arbitrary figures. They're the result of sophisticated climate modelling that considers various emissions scenarios and their likely impact on UK rainfall patterns. By using these allowances, we're designing to a standard that should remain adequate even as our climate continues to change.

The Real-World Impact

Incorporating climate change allowances has a tangible impact on drainage design. It affects everything from pipe sizes to attenuation storage volumes to the sizing of sustainable drainage features.

For example, a soakaway that might have been 30 cubic metres without climate change allowances might need to be 42 cubic metres with a 40% uplift. A drainage pipe that would have been 225mm diameter might need to be 300mm. These aren't trivial changes. They require more space, more materials, and higher construction costs.

However, the alternative is far worse. A drainage system that's undersized for future rainfall will fail, causing flooding that can damage properties, disrupt lives, and cost far more to remedy than the initial investment in adequate capacity.

Sustainable Drainage and Climate Resilience

Climate change allowances are particularly important when we're designing sustainable drainage systems (SuDS). Features like swales, rain gardens, and permeable paving are excellent for managing surface water in an environmentally friendly way, but they must be sized correctly to handle future rainfall intensities.

We often combine multiple SuDS features to provide the necessary capacity. For instance, a development might use permeable paving to reduce runoff, a swale to convey water, and an attenuation pond to store excess water during heavy rainfall. Each element must be sized with climate change allowances in mind to ensure the system works as an integrated whole.

Green roofs and blue roofs (which store water temporarily on building roofs) are also increasingly popular SuDS features. When designed with appropriate climate change allowances, they can significantly reduce the burden on downstream drainage infrastructure while providing additional environmental benefits.

Looking Ahead

Climate change allowances will likely increase in the future as our understanding of climate impacts improves and as we observe actual changes in rainfall patterns. The Environment Agency reviews its guidance regularly, and we must stay current with the latest requirements.

Some forward-thinking developers are already asking us to design to even higher standards than currently required, recognising that this provides additional resilience and protects their investment over the long term. It's a sensible approach that we fully support.

Why This Matters to You

Whether you're a homeowner planning an extension, an architect designing a new development, or a developer building a housing estate, climate change allowances will affect your drainage design.

Understanding why these allowances are necessary helps explain why drainage systems are larger and more complex than they might have been in the past. It's not over-engineering. It's responsible design that protects properties and communities from the realities of our changing climate.

At The Drainage Designers, we incorporate climate change allowances into every project we undertake. It's not just about meeting regulatory requirements. It's about creating drainage infrastructure that will continue to function effectively for decades to come, protecting properties and people from the increased flood risk that climate change brings.