What’s Behind the Growth in Surface Water Flood Risk Zones?

Our clients are often surprised to hear of the level of their exposure to surface water flooding and voice concerns regarding the apparent growing severity and extent of the areas modelled at risk. In this blog, we aim to delve into some of the reasons behind why a growing number of properties are now modelled at risk of surface water flooding.

A Recent History of Surface Water Modelling

To predict where surface water flooding is likely to occur, the Environment Agency published the Risk of Flooding from Surface Water (RoFSW) dataset in 2013. This dataset models the extents and depths of areas where water may collect or flow overland when it cannot drain properly through watercourses, sewers or soak into the ground.

The most significant recent update to surface water flood risk mapping came through the Environment Agency’s National Flood Risk Assessment 2 (NaFRA 2), published on 17^th December 2024. This update included improved data and modelling representing the first major update since 2018.

Key Updates:

• 17^th December 2024: NaFRA 2 release, integrating improved surface water flood risk mapping.
• 28^th January 2025: Formal publication of NaFRA 2 on GOV.UK, with public access to updated flood-risk tools.
• 25^th March 2025: Flood Map for Planning updated, incorporating surface water data and climate change projections.
• 16^th June 2025: Latest quarterly update to the RoFSW dataset published on data.gov.uk.

According to the Environment Agency (EA), NaFRA 2 models 4.6 million properties are in at-risk areas from surface water flooding, a 43% increase from prior assessments.

1.1 million of these properties are predicted to be at high risk of surface water flooding, nearly three times previous estimates. In total, approximately twice as many properties in areas were at high risk of flooding from surface water flooding compared with flooding from rivers and the sea.

These updates reflect how flood risk is evolving due to climate change, and improvements in the way surface water flood risk is assessed, modelled and visualised. Together these updates help provide a more detail picture of surface water flood risks across England.

What’s Changed?

A number of changes were included in the 2025 update which may help understand why the number and extent of areas modelled at risk of surface water flooding are growing:

1. New Depth-Based Risk Bands

The EA revised their definition of surface water flooding, and the impacts that it can have on people and property. This has resulted in more locations being at risk.

Previous models categorised risk in binary bands for three rainfall scenarios (high/medium/low). Whereas NaFRA 2 now shows the estimated flood depths (e.g. >200mm, >300mm, >600mm, >900mm and >1200mm) and the probability of each being exceeded at the property scale. This provides a more granular picture of both flood likelihood and severity.

By transitioning from broader risk bands to more detailed flood depths and probabilities, the updated publicly available model now captures areas at risk of even shallow flooding in more detail.

These additional categories of flood depths contribute to increasing the total area shown to be at risk within tools available to the public.

2. Improved Representation of Buildings and Water Flow Paths

In the past, given the larger compute constraints and coarse, imperfect data of the older models, buildings were often modelled as impermeable blocks, that would incorrectly divert or stopping water flows.

The 2025 RoFSW update (via NaFRA 2) is a step forward, and is now able to more accurately simulate the way water flows interact with highly built up areas using improved digital terrain modelling.

More accurate water simulations may identify previously unaffected areas simply because flow was stopped by modelling assumptions. This allows for the use of better location-specific flood risk data.

3. Up to Date Digital Terrain Models

The NaFRA 2 hydraulic model uses the Integrated Height Model (IHM 2019), applied at 2m x 2m cell resolution. This captures subtle ground-level variations that significantly affect surface water flows and ponding, and offers more consistent, merged elevation data across both urban and rural areas. For example NaFRA 2 can now identify the way water flows over roads accounting for the kerb level.

As a result, this update in concert with the above points now presents an improved representation of flood pathways, ground features, and property-level impacts. Areas that were previously overlooked, such as shallow dips or low spots, may now appear within the mapped flood extent, increasing the overall area shown to be at risk.

4. Local Knowledge Contributions

Lead Local Flood Authorities (LLFAs) were invited to compare national results against with local knowledge and detailed models. Local evidence was provided about the ability of drainage systems to carry away flood water, and shown that in some places urban drainage systems do not have the capacity to manage surface water. Where possible, using new software, the LLFA’s detailed local data was combined with the national assessment, provides a more accurate assessment of risk.

Interpreting the Surface Water Models:

It’s important to remember:

• RoFSW is not a property-specific model. It provides an area-based flood risk indication, not a definitive statement of whether an individual building will flood.
• Depths and extents do not account for floor levels. Flood depth predictions do not consider internal property levels, hence further assessment is required to determine whether floodwaters would enter a specific property.
• Some buildings may appear unaffected in raw model outputs due to the 300mm elevation applied to simulate raised thresholds.
• An increase in mapped risk does not necessarily mean a physical change has occurred on the ground. It may simply reflect improved modelling or local data inclusion.

Into the Future: Quarterly Updates

As of June 2025, the RoFSW dataset is being maintained via quarterly revisions. This means areas with new data or improved local models may see further updates going forward.

• Latest refresh: 16^th June 2025
• Next update: Expected in September 2025

Summary

The growth in areas shown to be at risk of surface water flooding is largely a result of:

• Redefining surface water and the addition of probabilistic depth bands.

• Improved modelling resolution between and around buildings.
• Incorporation of better-quality data.

An increase in mapped risk does not necessarily mean a physical change has occurred on the ground. It simply reflects a previous significant under-estimated of risk. Where models are now able to describe it with more confidence and detail, rather than there being a sudden increase in flood events.

These changes provide a more realistic picture of surface water flood risk, enabling better planning, prevention and decision-making.

If you have concerns about how the updated data may affect your property or development site please get in contact with us at FPS Environmental.

10/7/2025

Written by Sebastian Henshaw BSc (Hons): Flood Risk Consultant