Around 4.6 million properties in England are now assessed as being at risk from surface water flooding – a 43% increase on the Environment Agency's previous estimate, according to the National Flood Risk Assessment (NaFRA) published in December 2024. Most sit outside the river and coastal flood zones organisations have historically used to assess exposure, and many owners will have no idea they are at risk.
Neil Gunn (pictured) has spent much of his career watching that gap widen. After 25 years at the Environment Agency, where he worked on flood defences and national flood risk management, he joined WTW as head of flood and water management research.
Traditional flood maps were designed around fluvial and coastal flooding because both can be modelled with reasonable accuracy. Pluvial flooding, caused when intense rainfall overwhelms drainage systems before water even reaches a river, resists that approach entirely.
"With pluvial flooding, it can happen anywhere in a traditional floodplain or out of a traditional floodplain," Gunn said. "It's much harder to assess where it's going to happen because it can rain everywhere, can't it?"
Unlike river flooding, surface water depends not only on rainfall but on drainage infrastructure that is often decades old, poorly mapped and designed for a different climate.
Urban drainage systems were designed using historical data from the 1970s and 1980s. Regional climate models now project peak rainfall increases of up to 50% in large events across the UK by 2050–2080.
"The climate has moved along past our design standards," Gunn said. "It's very difficult to replumb a whole city."
When Gunn joined the Environment Agency at the turn of the century, fewer than one in six properties in England was considered to be at risk of flooding. The NaFRA now puts that figure at one in five. With climate change, it is projected to reach one in four by mid-century.
Gunn's clearest illustration of the mapping gap comes not from a flood model, but from a pub.
During his time at the Environment Agency, he made a habit of visiting communities in upper river catchments and looking at the photographs on pub walls. What he found, repeatedly, was evidence of serious flooding events: water a metre deep, affecting 20 or 30 houses, that appeared on no official map.
"Those kinds of floods are ephemeral," he said. "They go unnoted, unrecorded, or don't make their way into big databases."
He pointed to subtler indicators risk managers should learn to read: road names such as Culvert Close or Mill Lane, or older houses with three steps up to the front door where newer builds alongside them have none. They are quiet reminders that local knowledge often outlasts official records.
The insurance consequences are concrete. Around 87% of claims from properties outside FEMA's special flood hazard areas in the United States are pluvial in origin, with pluvial flooding nearly doubling insured losses in some major tropical cyclones relative to what fluvial maps alone would suggest.
Perhaps the most counterintuitive point Gunn raised concerns what is actually driving flood risk higher. A World Bank study by researcher Jonas Rentschler found that four-fifths of the increase in global flood risk does not come from climate change, it comes from inappropriate development in floodplains.
That finding matters directly for how brokers assess client exposure. A business in an area of recent development may face materially higher flood risk than the maps suggest, not because the climate has shifted beneath it, but because what was built around it has changed how water moves.
Around 4.6 million properties in England are now assessed as being at risk from surface water flooding – a 43% increase on the Environment Agency's previous estimate, according to the National Flood Risk Assessment (NaFRA) published in December 2024. Most sit outside the river and coastal flood zones organisations have historically used to assess exposure, and many owners will have no idea they are at risk.
Neil Gunn (pictured) has spent much of his career watching that gap widen. After 25 years at the Environment Agency, where he worked on flood defences and national flood risk management, he joined WTW as head of flood and water management research.
Traditional flood maps were designed around fluvial and coastal flooding because both can be modelled with reasonable accuracy. Pluvial flooding, caused when intense rainfall overwhelms drainage systems before water even reaches a river, resists that approach entirely.
"With pluvial flooding, it can happen anywhere in a traditional floodplain or out of a traditional floodplain," Gunn said. "It's much harder to assess where it's going to happen because it can rain everywhere, can't it?"
Unlike river flooding, surface water depends not only on rainfall but on drainage infrastructure that is often decades old, poorly mapped and designed for a different climate.
Urban drainage systems were designed using historical data from the 1970s and 1980s. Regional climate models now project peak rainfall increases of up to 50% in large events across the UK by 2050–2080.
"The climate has moved along past our design standards," Gunn said. "It's very difficult to replumb a whole city."
When Gunn joined the Environment Agency at the turn of the century, fewer than one in six properties in England was considered to be at risk of flooding. The NaFRA now puts that figure at one in five. With climate change, it is projected to reach one in four by mid-century.
Gunn's clearest illustration of the mapping gap comes not from a flood model, but from a pub.
During his time at the Environment Agency, he made a habit of visiting communities in upper river catchments and looking at the photographs on pub walls. What he found, repeatedly, was evidence of serious flooding events: water a metre deep, affecting 20 or 30 houses, that appeared on no official map.
"Those kinds of floods are ephemeral," he said. "They go unnoted, unrecorded, or don't make their way into big databases."
He pointed to subtler indicators risk managers should learn to read: road names such as Culvert Close or Mill Lane, or older houses with three steps up to the front door where newer builds alongside them have none. They are quiet reminders that local knowledge often outlasts official records.
The insurance consequences are concrete. Around 87% of claims from properties outside FEMA's special flood hazard areas in the United States are pluvial in origin, with pluvial flooding nearly doubling insured losses in some major tropical cyclones relative to what fluvial maps alone would suggest.
Perhaps the most counterintuitive point Gunn raised concerns what is actually driving flood risk higher. A World Bank study by researcher Jonas Rentschler found that four-fifths of the increase in global flood risk does not come from climate change, it comes from inappropriate development in floodplains.
That finding matters directly for how brokers assess client exposure. A business in an area of recent development may face materially higher flood risk than the maps suggest, not because the climate has shifted beneath it, but because what was built around it has changed how water moves.
