Post date
3 Jun 2025 (original LinkedIn date)
Original
LinkedIn version· 1,499 words
Groundwater flooding is seemingly a complex and often overlooked issue in the UK, and my observations from the CIWEM Flood and Coast conference 2025 today surfaced (pun fully intended!) some interesting points:
Responsibility
Groundwater flooding responsibility in the UK appears to be somewhat fragmented, which contributes to the perception that “nobody wants to take ownership.” Under the Flood and Water Management Act (2010), Lead Local Flood Authorities (LLFAs), typically unitary authorities or county councils, are responsible for managing local flood risks, including those from groundwater. However, the Environment Agency (EA) has a strategic overview role for all sources of flooding, including groundwater, and provides data such as monitored groundwater levels and, in some areas, groundwater flood alerts or warnings. However, the EA’s role seems to be more about coordination and data provision than direct management, leaving LLFAs to handle local implementation.
As far as I can tell, the lack of clarity appears to stem from 3 main issues:
Complex hydrogeology: groundwater flooding depends on underground flow paths, which are harder to predict and manage than surface water or river flooding. This complexity can lead to disputes over who should act, especially when flooding crosses administrative boundaries;
Limited local expertise: many LLFAs lack the specialised hydrogeological knowledge needed to assess and mitigate groundwater risks effectively; &,
Funding (lack of) and prioritisation (little): Groundwater flooding affects fewer properties (estimated 122–290k aloone in England) compared to surface water (3 million) or fluvial flooding (2.4 million), so it often receives less attention and funding. But it’s consequences are no less stressful for those impacted.
This fragmentation can result in (what to an outsider at least looks like) a “pass the buck” scenario, where no single entity feels fully accountable, especially for long-duration, diffuse groundwater flooding events.
Less attention
Groundwater flooding appears to be often overshadowed by surface water and fluvial flooding for several reasons:
Duration and visibility: groundwater flooding is typically slower to develop and can last weeks or months, unlike dramatic, short-lived river floods. It’s less visible (e.g. flooding basements or emerging in low-lying areas) and often gets mistaken for surface water flooding;
Data limitations: while surface water and fluvial flood risks benefit from detailed national mapping (e.g. the EA’s recently updated maps), groundwater flooding is harder to model due to variable geology and subsurface flow paths. This leads to less public and policy focus (and maybe that’s the clincher);
Historical underreporting: groundwater flooding is often masked by other flood types in media and records, reducing its perceived urgency; &,
Lower property impact: although locally significant, fewer properties are at risk from groundwater flooding compared to other sources, which shifts priorities toward more widespread issues.
Where’s my map?
The absence of a comprehensive national groundwater flood risk map has to be a significant gap (and hindrance). However, some progress has been made:
The British Geological Survey (BGS) has developed a national groundwater flooding susceptibility dataset covering England, Wales, and Scotland, based on geological and hydrogeological data. This dataset identifies areas where groundwater flooding is possible (e.g. where geological conditions allow water tables to rise close to the surface) but does not quantify risk (likelihood of occurrence). It’s available at a 50m grid resolution and is used for planning and homebuyer reports. Other private firms are working on similar projects;
Modelling challenges: Groundwater flooding depends on complex subsurface processes, including aquifer properties, rainfall patterns, and human activities (e.g. underground infrastructure). Current models struggle to predict exact locations and volumes of emergent groundwater; &,
Existing tools: the BGS dataset and GeoSmart’s Groundwater Flood Risk Map (GW5) provide susceptibility insights, but some will question whether they are true risk maps. The EA also includes groundwater susceptibility in its flood risk assessments, but these are often coarse and rely on LLFAs to refine local data.
NBS, the answer to everything?
Groundwater flooding requires a different approach from the “slow and hold” strategy used for river and surface water flooding, where measures like wetlands, tree planting, and sustainable drainage systems (SuDS) slow runoff and store water (often reversing a century or more of thinking). For groundwater flooding:
Opposite approach needed: the goal is often to speed up drainage or divert groundwater to prevent water tables from rising to the surface. Measures like de-watering pumps (yes, they’re a thing) or enhanced drainage systems are sometimes used, though they are costly and not always effective;
Nature-based solutions (NBS): these are less directly applicable to groundwater flooding but not entirely irrelevant. NBS like reforestation/afforestation or soil management can influence infiltration rates, potentially reducing recharge to aquifers in some contexts. However, in permeable catchments (e.g. chalk), increasing infiltration (as NBS often aim to do) could inadvertently raise groundwater levels, potentially exacerbating flooding; &,
Challenges with NBS: the effectiveness of NBS for groundwater flooding is understudied (and skills and knowledge seem to be in rather short supply - the groundwater workshop today attracted 12 hardy souls, whereas the fluvial session was overflowing at nearing a ton). A 2023 systematic map found that UK studies on natural flood management (NFM) rarely focus on groundwater flooding specifically, and their effectiveness for this flood type is poorly quantified.
What’s to do?
To address groundwater flooding effectively, I learned that several steps could sensibly be taken:
Improved mapping and modelling: invest in higher-resolution risk maps by combining BGS susceptibility data with local flood records, borehole data, and advanced modelling (e.g. BGS’s and others’ work on integrated climate, rainfall, and groundwater models);
Enhanced data collection: standardise and centralise records of groundwater flooding events to improve model calibration and public awareness;
Clearer responsibility: strengthen coordination between LLFAs, the EA, and other stakeholders (e.g. water companies, academics, engineering consultants) to clarify roles, ensure accountability, and (shock horror) share learnings. This could include dedicated groundwater flood officers within LLFAs;
Innovative mitigation/engineering solutions: install drainage systems, basement tanking, or de-watering pumps (still a thing) in high-risk areas, though these are costly and site-specific;
Public awareness and policy: increase education on groundwater flooding risks for homeowners, planners, and developers, and integrate groundwater considerations into national flood policy more explicitly; &,
Early warning systems: expand the EA’s groundwater flood alert services (currently limited to specific areas like Kent and South London) and develop national-scale forecasting systems, linked with the EA and Met Office.
Vulnerable areas
Groundwater flooding is most prevalent in areas with specific geological settings, but it’s not limited to chalk beds and old mines:
Chalk aquifers: these are the most vulnerable settings, particularly in southern and eastern England (e.g. South Downs, Chilterns, Wessex). An estimated 46–138k properties are at risk in chalk and limestone areas due to their high permeability, which allows rapid groundwater level rises after heavy rain;
Permeable superficial deposits: river valleys with thick alluvium or gravel deposits (such as in the Cotswolds or East Yorkshire) are also prone, as groundwater in these deposits can also rise during high river stages or tides;
Old mines: Mine water discharge can contribute to groundwater flooding, particularly in areas like the Eastern Pennines, where abandoned coal or mineral mines disrupt natural groundwater flow. However, this is less widespread than chalk-related flooding; &,
Other settings, such as sandstone aquifers: Found in parts of the Midlands and northern England, these can also contribute to groundwater flooding, though less frequently than chalk.
The future
My brief time with the water engineers made it abundantly clear that there is a need for better understanding and risk management:
BGS studies: the BGS has advanced groundwater flooding science through susceptibility mapping, modelling (e.g. in the Chalk aquifer), and the Standardised Groundwater Flooding Index (SGFI) to quantify regional differences;
Climate change considerations: studies like those by BGS and the NERC Flood Risk from Extreme Events programme are incorporating climate change scenarios to predict future groundwater flood frequency, noting that wetter winters will likely increase risks; &,
NFM effectiveness: a 2023 systematic map emphasised that while NFM is well-studied for fluvial and pluvial flooding, its application to groundwater flooding is under-explored, calling for more research on tailored NBS (any budding PhDs out there?).
In summary
Groundwater flooding in the UK is a distinct challenge due to its long duration, complex hydrogeology, and fragmented responsibility (primarily LLFAs, with EA oversight). It receives less attention because it’s simply not as sexy - it affects fewer properties and is harder to predict and model than surface water or fluvial flooding. While a national susceptibility dataset exists, a true risk map is lacking due to data and current modelling limitations. Mitigation requires faster drainage rather than “slow and hold,” and while NBS have potential, their effectiveness for groundwater is uncertain and requires careful design to avoid exacerbating risks. Vulnerable areas include chalk aquifers (like the South Downs, Chilterns), permeable river valleys, and, to a lesser extent, old mines and sandstone regions. Addressing this issue demands better data, clearer responsibilities, innovative mitigation, and increased public awareness.