New Data Unlocks Understanding of Flood Defence Needs

Posted by Colliers on 13th February 2025 -

Mike Piotrowski, Colliers’ Technical Director of Flood Risk and Sustainable Drainage, examines the Government’s new flood risk data and its role in protecting us from a changing climate.

The Environment Agency (EA) has just published their new "national-scale" flood likelihood maps in England and they now provide present-day and climate change scenarios for 3.3%, 1% and 0.1% annual probability (AP) events.  

The maps now provide:

• flooding from rivers, the sea and from surface water across England, excluding groundwater. 
• a more detailed envelope providing a maximum extent and depth of flooding, removing the blocky nature of some of the flooding extents that we’ve seen in the past.
• modelled climate change scenarios which provide a useful insight into the future impacts of flooding and coastal erosion.

Why is this important:

It’s not been done this way in England before. The climate change scenarios are modelled for a medium term (2050s epoch), which is great for decision-making and to gain an insight into the risk profiles of mortgages and insurance availability during the next 20-35 years.

The EA and their partners are also likely to find this a particularly useful high-level tool to support the initial assessment and business case development of flood defence schemes, although the data will become much more powerful and accurate using site level data.

The new maps have the capability of relieving and resolving previously inaccurate mapping which had blighted the resale, reinsurance and future development of some property assets. The new mapping also has the potential to leave assets stranded on a medium and long-term basis though, if they are considered "at risk" in the future, so there are swings and roundabouts to the new tool.

The mapping however is not necessarily that great for new development proposals, which require an understanding of the flooding risks over a longer period of time, between 60-75 years for commercial use and 100 years for residential use. But the Environment Agency will publish their new Flood Map for Planning in Spring 2025, which is likely to provide a more valuable insight into long-term risks.

The pros and cons of the new data:

• Open and immediately available data: It's a step in the right direction. The data is open and flood depth data provision may save time given the short timescales on many property transactions. However, the classifications and the way the data is displayed may be considered slightly confusing, to the general public and even to a seasoned professional.
• Flood depths: River and sea and surface water maps now provide a flood depth range, which you can toggle on and off for different present day and climate change events. 
• Climate change for surface water: Surface water and river and sea risk maps now provide us with climate change scenarios, giving a more detailed insight previously only available through contacting the EA as part of their statutory FOI request process.
• Increased resolution: Flood extents now have more detailed resolution meaning less discrepancy between EA’s detailed model data and National Flood Risk Assessment data.
• Cost/Benefit analysis: The maps give the EA and other risk management authorities greater ability to screen highest risk areas and calculate the cost/benefits of implementing flood defence schemes, including property-level protection.
• Lack of guidance: there is no detailed instruction manual available for the data yet or any more detailed guidance documents on how the modelling was undertaken, which leaves a question mark over some of the benefits and limitations of the dataset.
• No update for the devolved nations: Scotland, Wales and Northern Ireland have not updated their maps (but they do have their own). Wales and Scotland were first to publish flood risk maps including climate change scenarios and allowances. It would be great to have a UK-wide update to create the joined-up thinking and consistency between the four main risk management authorities.
• Local Authority detailed modelling: It's not immediately clear if any local authorities have incorporated their own detailed modelling of rivers and sea, or if it is just the EA's data. I know they have for surface water, but hopefully there is some consistency on the river and sea data as well.
• River and sea flood separation: The river and sea maps are not separate, which still makes the definition between the "is it a 1% AEP / 1 in 100 year or is it a 0.5% AEP / 1 in 200 year event" relatively unclear.
• Regions of "no data": Where there are large areas of no data available, flood depths (i.e. in the Fens, around Weston-super-Mare and downstream of Edenbridge to note a few), it would be really helpful to understand when and if these will be available in the future and why these aren’t currently available.
• Lack of flow direction data: the flow direction/velocity arrows are no longer available on long-term flood risk maps and are not available by download, these were previously quite useful. 

According to the Environment Agency 6.3 million properties across England are based in areas at risk of flooding from one or a combination of rivers, the sea and surface water, that’s a lot of potentially flood-ruined homes and businesses. Therefore, this is a timely addition to our inventory of tools to give clients the best possible advice in order to future-proof their assets. 

There are some shortfalls in the data but the published mapping is welcomed and does have some serious benefits in helping to provide a greater understanding of flooding and coastal erosion risks and how we mitigate them in England.