Why groundwater flood risk is undercounted: risk, hazard and susceptibility

Groundwater flooding is one of the hardest flood sources to screen for. National flood zone mapping is built around rivers, the sea and, since NaFRA2, surface water on behalf of LLFAs. It does not include groundwater, and the Environment Agency’s national assessment gave no estimate of the scale of the risk because, they concluded, “there are too many gaps in our understanding and evidence of groundwater flood risk” (Environment Agency, 2024).

A national review that does not mention groundwater

The Agency has embarked on a review of the Flood and Coastal Erosion Risk Management Strategy. It is to be hoped that it will consider the need for a national overview of groundwater flood risk, and the role of groundwater in flooding from all sources. However, the word “groundwater” does not appear in the Agency’s announcement, nor in the Secretary of State’s statement to the House of Commons.

A confused regulatory regime

The regulatory regime is confusing at best. You can see some of this confusion in my previous blog and in a briefing note to Defra, available from the same page, which explained why previous estimates published by the Agency had missed most of the groundwater risk.

• The Agency has a strategic role, and LLFAs have a management role. In 2021 the Agency published a “rapid evidence review” on groundwater flood risk that gives an overview of the state of the science and practice in England. It recognised some of the problems in current risk management approaches and recommended that governance arrangements should be reviewed. I am not aware that this recommendation has been taken forward.
• More widely, Government has been consulting on new approaches, although no mention of groundwater appears in its white paper either.
• The Environment Agency is also not actually a statutory consultee on groundwater flood risk.

We can see, perhaps, how the regulatory division set up by the Flood and Water Management Act 2010 led the Agency to take a “rivers-centric” approach to estimating the hazard, which excluded groundwater flood risk properties within river flood risk areas. LLFAs are therefore left to deal with groundwater in strategic flood risk assessments, planning consultations and flood investigations without a consistent national starting point. Given their administrative rather than catchment jurisdiction, they are unable to achieve a consistent national approach, or a catchment risk assessment for the catchments that cross their area.

Flood risk versus flood hazard

Risk is normally defined as likelihood multiplied by consequences. In relation to groundwater, this means:

• the likelihood of groundwater emergence;
• the severity of impacts, which relates to factors such as flow rate, accumulation, velocity and duration; and
• the consequences, in terms of damage to buildings, community impact and business interruption.

This is the approach taken by GeoSmart in our groundwater flood risk mapping. Data limitations and approximations in national-scale modelling mean that the standard outputs are provided as a semi-quantitative risk ranking. More detailed mapping and modelling for applications such as sewer infiltration and other infrastructure impacts can provide more quantitative results, but they require more sophisticated and costly analysis.

GeoSmart’s national analysis estimates the risk of groundwater flooding and also, via FloodSmart Analytics, the estimated risk from emergent groundwater due to ponding, flow and accumulation on the surface. These impacts can also be seen in conjunction with surface water and river flood risk, so that areas where longer-duration impacts and compound flood events can occur give a more complete estimate of flood risk from all sources.

Susceptibility is not risk

Other groundwater analysis available in the UK typically provides susceptibility maps. The British Geological Survey’s description of its groundwater susceptibility data makes the difference clear. It shows the degree to which areas of England, Scotland and Wales are susceptible to groundwater flooding on the basis of geological and hydrogeological conditions. It does not show the likelihood of groundwater flooding, so it is a hazard dataset, not a risk-based one.

GeoSmart’s GW5 and FSAGW (FloodSmart Analytics Groundwater) flood risk maps differ because they rely on the estimated depth to which groundwater flooding occurs and the likelihood of an event of a particular magnitude. Susceptibility data are hazard datasets. Flood risk maps are risk datasets, combining likelihood and consequences, mostly by means of semi-quantitative risk classes.

How groundwater floods behave

Groundwater flooding is characterised by long-duration events that cause disproportionately more loss and damage than other types of flooding. By contrast with river or coastal flooding, impacts are localised by subsurface flow paths that may protect some properties and impact others. Typically one property is impacted, and others escape, because of local features that are not explicitly mapped. It is therefore important to recognise the uncertainties within overall risk estimates, and to consider more detailed assessment if your property is mapped within a groundwater flood risk area.

The scale of the problem

GeoSmart has published groundwater flood risk maps consistently nationwide since 2014. Our work has mainly benefited the property sector, including lending, insurance and asset management, and we are now extending our support to the public sector, including LLFAs, environmental regulators and government bodies. The table summarises our current national estimates. It indicates that the scale of groundwater flood risk is at least as large as coastal risk (see also Groundwater Flood Risk: Completing the Picture), and highlights the need to consider the interactions between flood mechanisms holistically.

Summary of findings from national flood risk mapping, combining Environment Agency NaFRA and GeoSmart FSAGW surface emergent flow risk maps, based on loss and damage estimates from the Flood Hazard Research Centre Multi-coloured Manual:

* Groundwater flooding does not impact all properties within risk zones because it is often mitigated by building design or by natural and artificial drainage systems, which act to lower water tables.

Why this matters

If national figures miss groundwater, and local assessments capture it only partly, decision-makers have no consistent way to see where it matters. That is why we have made a screening map of groundwater flood risk for Great Britain (GW200) free for public bodies to use and publish.

GeoSmart Information Ltd is a flood risk specialist modeller and developer of flood risk maps for all sources of flooding. To obtain GW5 data or to learn more about groundwater flood risk, visit www.geosmartinfo.co.uk, or contact enquiries@geosmartinfo.co.uk or 01743 298100.

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