Flash Flooding During a Heatwave: Why Dry Weather Makes Surface Water Flooding Worse

FPS Environmental | June 2026

In June 2026, London and Somerset were hit by sudden flash flooding even as the UK baked under a red extreme heat warning. To many people this might feel like a contradiction. How can you have floods in the middle of a heatwave? Surface water flooding is now one of the most widespread flood risks in the UK, and hot, dry weather can make it far more problematic.

London and Somerset Floods

On 23 June 2026, intense thunderstorms swept across southern England as a “heat dome” pushed temperatures towards record levels. The Met Office recorded almost 29,074 lightning strikes across the UK in 24 hours, with over 18,000 of those over Somerset alone.

Parts of Somerset saw up to 35mm of rain fall in a very short period, triggering flood alerts near Shepton Mallet and Glastonbury. In London, the Fire Brigade took around 400 calls overnight, many for flooding to properties. This was classic surface water (or “pluvial”) flooding: rain falling faster than the ground or drainage system could cope with.

Why a Heatwave Makes Flooding Worse

It seems counterintuitive, but dry conditions are part of the problem.

Baked ground sheds water

During a prolonged dry spell, soil hardens and can become almost water-repellent. Instead of soaking in, rainfall runs straight off the surface, much like water hitting concrete. This effect is sometimes called soil “hydrophobicity.”

Storms are heavier, not lighter

Hot air holds more moisture. When that energy is released as a thunderstorm, it tends to produce short, very intense downpours rather than steady rain. Drainage systems designed for typical rainfall can be quickly overwhelmed.

Drains and rivers are at low capacity

After dry weeks, debris builds up in gullies and drains, and low river levels can give a false sense of security. A sudden cloudburst then has nowhere to go. The result is fast, localised flooding that can arrive with very little warning, often in areas not considered “at risk” from rivers or the sea.

Who Is Affected

Surface water flooding does not follow rivers or coastlines, so it can affect almost anywhere, including:

• Homes on higher ground or away from watercourses
• New developments where natural drainage has been built over
• Urban areas with large amounts of hard standing and paving
• Roads, car parks, and basements that collect run-off

According to the Environment Agency’s 2024 National Assessment of Flood Risk, around 4.6 million properties in England are in areas at risk of surface water flooding, making it the most widespread flood risk in the country.

What This Means for Property Owners and Developers

Events like June 2026 are a reminder that flood risk is changing. A few practical points to consider:

• Do not rely on river flood maps alone. Surface water risk is assessed separately and is easy to overlook.
• Manage run-off at the source. Sustainable Drainage Systems (SuDS), such as permeable paving, soakaways, and rain gardens, slow water down and reduce peak flows.
• Check drainage capacity. On developments, drainage should be designed for intense storms, not average rainfall, in line with current guidance such as CIRIA’s SuDS Manual.
• Get a proper assessment. A Flood Risk Assessment or drainage strategy can identify surface water risk early, which is often a planning requirement.

Designing for these short, severe storms is now essential rather than optional, particularly as hot summers and sudden downpours become more common.

How FPS Environmental Can Help

Surface water flooding is an area we work in every day, and one we are regularly asked to explain in the national media. Our founder, Simon Crowther, a Chartered Civil Engineer, appeared on BBC One’s Rip Off Britain to explain surface water flood risk, the latest Environment Agency mapping, and what homeowners can do to check the risk at their own property.

Our practical experience includes:

• Section 19 flood investigations for local authorities, including formal investigations into surface water flooding events
• Flood Risk Assessments (FRAs) to support planning applications
• Drainage strategies and SuDS design to manage run-off on new and existing sites
• Percolation testing to confirm whether soakaways and infiltration drainage will work on a given site

This hands-on work, from investigating real flood events to designing drainage that copes with intense storms, is exactly what helps property owners and developers stay ahead of risks like those seen in June 2026.

Key Takeaway

Flash flooding during a heatwave is not a freak event. Dry ground, heavier storms, and overwhelmed drainage combine to create serious surface water risk, even when rivers are low and the sun is shining.

If you are buying, building, or developing, understanding surface water flood risk and planning effective drainage is one of the most important steps you can take. If you are unsure how surface water flooding could affect your property or site, FPS Environmental can help you assess the risk and plan the right drainage solution.