Designing Highways with Flood Resilience in Mind
Introduction
Flooding is one of the greatest risks facing UK infrastructure today. Recent winters have seen devastating rainfall events disrupt communities, close transport links, and strain emergency services. For today’s highway engineers, the challenge lies in balancing connectivity with resilience, creating roads that remain dependable even as extreme weather events become more common.
Designing highways with flood resilience in mind is not just a technical requirement but a professional responsibility. Civil engineers have a duty to protect lives, reduce economic losses, and ensure safe, reliable transport. Understanding the principles of flood-resilient design is an important step towards chartership and professional credibility.
This article explores why flood resilience matters, outlines the latest UK standards, and presents practical strategies for integrating resilience into highway projects. Real-world examples, such as the Mansfield Sustainable Flood Resilience project, show how theory translates into practice. By the end, you will have a clearer view of how to design roads that meet both today’s regulatory requirements and tomorrow’s environmental challenges.
Why Flood Resilience Matters in Highway Design
Flooding poses unique risks to roads and their users:
Safety hazards – standing water can lead to aquaplaning, accidents, or vehicle breakdowns.
Network disruption – road closures can cut off communities and emergency services, with economic losses estimated in the billions.
Structural damage – prolonged inundation can undermine foundations, wash away embankments, and accelerate pavement deterioration.
Downstream impacts – poorly designed roads can worsen flood risk elsewhere by blocking natural floodplains or accelerating runoff.
In the UK, climate projections from the Met Office suggest wetter winters and more frequent intense storms. The Environment Agency (EA) now estimates that climate change could increase peak river flows by up to 50% in some catchments by 2080 [Environment Agency, 2025]. Highways must therefore be designed to remain safe, functional, and adaptable across their entire service life.
Photo by Chris Gallagher on Unsplash
National Standards and Requirements
Flood-resilient design is underpinned by a suite of national standards and policies that engineers must follow:
Flood Risk Assessments (FRAs): All highway projects in flood-prone areas must carry out FRAs using EA mapping. Since August 2025, new “Flood Zones plus climate change” layers are mandatory, offering a forward-looking view of flood exposure.
Finished floor and carriageway levels: EA Standing Advice and BS 85500 recommend finished levels at least 600mm above the highest predicted flood levels (or 300mm if supported by robust modelling).
Materials: BS 851188-1:2019+A1:2021 specifies that materials up to flood level must be low permeability to limit water ingress.
Sustainable Drainage Systems (SuDS): Following National SuDS Standards and National Highways’ LA 113, all new schemes must integrate drainage for both routine rainfall and extreme flood events.
Floodplain storage: Designs must avoid a net loss of floodplain storage and ensure no increase in flood risk elsewhere.
These requirements are not just “tick-box” compliance—they are about ensuring resilience across the full design life of a road, often 60 years or more.
Regulatory Framework for Flood-Resilient Highways
Technical and Nature-Based Strategies
1. Catchment-Based Flood Management
Instead of focusing only on the road itself, engineers are now expected to consider the entire catchment. For example, working with landowners upstream to install natural flood management features such as woodland planting or leaky dams can slow runoff before it reaches the highway.
2. Sustainable Drainage Systems (SuDS)
SuDS are at the heart of modern flood-resilient design. Examples include:
Swales and filter strips – linear vegetated features that slow and filter surface water.
Detention basins and ponds – providing temporary storage for stormwater.
Rain gardens and green infrastructure – improving biodiversity while absorbing runoff.
Permeable pavements – reducing direct discharge into sewers.
These systems not only manage floodwater but also deliver multiple benefits, such as improved water quality, biodiversity, and public amenity.
3. Monitoring and Smart Technology
Real-time monitoring is increasingly important. Embedding water level sensors and flow monitors within drainage systems allows asset managers to track performance and respond proactively to issues. This aligns with the industry’s move towards data-driven asset management.
4. Robust Structures and Materials
Where raising a highway above flood level is necessary, resilient foundation systems are required. British Standards recommend ground-bearing cast in-situ concrete floors in flood zones to withstand prolonged saturation. Embankments must also be designed to resist erosion during flood events.
Cass Study: Mansfield Sustainable Flood Resilience Project
The Mansfield Flood Resilience project provides a strong example of integrating flood resilience into highway and urban design. Led in collaboration with the Environment Agency and local authorities, it demonstrates three best practices:
Hydraulic Modelling: Advanced modelling guided the placement of SuDS features for maximum effectiveness.
Collaborative Delivery: Prefabricated elements allowed rapid installation, reducing disruption and ensuring cost efficiency.
Real-Time Monitoring: Sensors embedded in swales and detention basins provide live data on performance, enabling continuous improvement.
This project shows how resilience is not an add-on but a core principle of design, construction, and long-term asset management.
Summary: Core Flood Resilience Measures
Flood Risk – Design Measures
Quick-reference table of measures, standards/best practice, and typical requirements.
| Measure | UK Standard / Best Practice | Typical Requirement |
|---|---|---|
| Finished levels above flood line | EA Standing Advice, BS 85500 | +600mm above highest flood estimate |
| Drainage strategy | National SuDS Standards, LA 113 | No increase in external flood risk; SuDS to suit site |
| Flood storage/flow conveyance | LA 113, EA National Policy | No net loss of floodplain storage |
| Materials | BS 851188-1:2019+A1:2021 | Low permeability up to floor height |
| Structural design in flood zones | BS 85500 | Robust, flood-resistant substructure |
| Flood data and modelling | EA, Defra, SFRA | Latest “Flood Zones plus climate change” |
Conclusion
Flood resilience is central to designing highways that will serve communities safely and reliably throughout the 21st century. From compliance with updated EA mapping to the integration of SuDS and smart monitoring, engineers must think both technically and strategically.
Mastering flood-resilient design offers a chance to grow in competence, demonstrate leadership, and deliver real value to society. Each project is not just about meeting standards but about safeguarding lives and enabling sustainable development.
Key takeaways:
Highways must remain safe and operational during flood events.
Compliance with evolving national standards is essential.
SuDS and catchment-based solutions deliver multiple resilience benefits.
Monitoring and smart data enable continuous performance improvement.
Developing expertise in flood resilience strengthens professional credibility.
Flood-resilient highways are the future of civil engineering. By engaging with this challenge, you not only help protect society but also shape your own professional journey.
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References
Environment Agency (2025). Flood risk assessments: Climate change allowances.
National Highways (2023). LA 113 – Road Drainage and the Water Environment.
British Standards Institution (2019). BS 85500: Flood resilient construction.
Defra (2025). UK Government Climate Adaptation Strategy.
Mansfield District Council, AECOM and Severn Trent Water (2024). Mansfield Sustainable Flood Resilience Project.