Groundwater Flood Risk Maps in the UK: How to Check a Site

This guide pulls together every groundwater mapping source available in the UK, explains what each one shows and what it does not, shows you how to read a groundwater flood map, and sets out how to turn a map into reliable, site-specific evidence. For the underlying mechanism — what groundwater flooding is and why the water table rises — start with our explainer on groundwater flooding causes and risks.

Why groundwater is missing from the national flood maps

The Environment Agency's Flood Map for Planning shows Flood Zones from rivers and the sea, and the public Check long-term flood risk service covers rivers, the sea and surface water. Neither is built to map groundwater, although the long-term service does carry some groundwater information. The second National Flood Risk Assessment (NaFRA2) is the same story: rivers, sea and surface water with climate change, but not a groundwater dataset. So 'we checked the flood map and groundwater wasn't flagged' is not an assessment — those maps were never designed to flag it. As the Environment Agency itself puts it, groundwater flooding is difficult to predict and groundwater levels are not monitored in every part of England.

The official UK groundwater mapping sources

Several official datasets and map viewers cover groundwater to different depths and for different purposes. Knowing which is which saves a great deal of confusion.

• BGS Susceptibility to Groundwater Flooding — the standard screening layer. Produced by the British Geological Survey, this is the first national dataset on groundwater flooding susceptibility, covering England, Wales and Scotland at 1:50,000 scale. Built from geological and hydrogeological information, it identifies where conditions could allow groundwater to come close to, or reach, the surface. It is a susceptibility (hazard) layer — it tells you whether groundwater is a credible issue worth investigating, not whether a specific plot will flood. See the BGS groundwater flooding dataset.
• EA current groundwater levels and alerts. In monitored areas — mainly parts of southern England — the Environment Agency publishes live groundwater levels and alerts through Check for flooding and its groundwater current status collection. These show conditions now, not long-term risk.
• Local authority Strategic Flood Risk Assessments (SFRAs). Your council's SFRA carries groundwater susceptibility at borough scale, often with local flood records the national datasets lack. It is the first place a planning reviewer looks.
• BGS borehole records and the GeoIndex. Historic borehole and well logs, searchable on the BGS GeoIndex, give real recorded water levels near a site — invaluable for sense-checking a screening map.
• BGS hydrogeology and aquifer mapping. The BGS hydrogeological maps show aquifer type and productivity, which is what governs how a given area behaves — useful background to any groundwater or aquifer map of the UK.
• data.gov.uk open datasets. The groundwater flooding susceptibility data and related hydrogeological and groundwater vulnerability layers are catalogued on data.gov.uk for those who want the underlying GIS data.

Commercial and modelled groundwater maps

Alongside the official sources, specialist modelled groundwater flood maps and commercial property flood reports are available under licence. Modelled maps estimate the position of the water table under different rainfall scenarios — for example a 1% annual probability event — at finer resolution than the national screening layer, and property flood reports fold groundwater susceptibility into a single document for a purchase or mortgage. They are useful, but they remain modelled or screening-grade: none is a measured record of the water table at your specific site, and all carry the limitations set out below.

How to find the water table in your area

A common question is simpler than full flood-risk mapping: how high is the water table where I live? There is no national 'water table by postcode' map that gives a definitive answer, because the water table moves seasonally and varies over short distances, but you can build a good picture from official sources. Recorded levels from BGS borehole logs give real historic depths near you; the Environment Agency's monitored levels cover parts of southern England; and the BGS hydrogeology mapping tells you what kind of aquifer you are on. For anything that matters — a basement, a soakaway, a development — these are a starting point only: a reliable design water table comes from monitoring on the site itself, because a reading taken in a dry summer can sit metres below the true winter peak.

How to read a groundwater flood map

Two things trip people up. First, susceptibility is a hazard band, not a probability of flooding: a high susceptibility rating means the geology could support groundwater flooding, not that your building will flood. Second, a modelled map labelled '1% annual probability' describes the rise in the water table expected in a 1-in-100-year winter — it is a scenario, and the real risk at a specific plot depends on local topography, superficial deposits and how the water table actually behaves there. Resolution matters too: a national screening layer mapped at 1:50,000 can place a boundary across a single field, so you can never read a property-level conclusion off a screening map.

Groundwater flooding map vs the Flood Map for Planning

The single most common confusion is between groundwater susceptibility and the Flood Map for Planning. The Flood Map for Planning shows Flood Zones from rivers and the sea; it says nothing about groundwater. A site can sit comfortably in Flood Zone 1 — the lowest river and sea risk — and still carry a significant groundwater flood risk that only the susceptibility data and site investigation will reveal. Reading one as if it covered the other is exactly how groundwater gets missed until it is expensive to fix. For how that plays out in a planning application, see our guide to groundwater flood risk in planning applications.

Limitations every groundwater map shares

Whichever source you use, the same caveats apply — and a good assessment states them plainly:

• Screening resolution. A national layer cannot resolve a single plot; boundaries are indicative, not surveyed.
• Modelled, not measured. Probabilistic maps estimate the water table from rainfall and geology; they are not readings at your site.
• No site specifics. Maps do not know about your foundations, basement, made ground or drainage.
• Seasonal movement. A map is a snapshot of risk, not the live water table, which rises and falls through the year.
• Needs ground-truthing. Anything above low susceptibility on a sensitive scheme should be confirmed with borehole records and, where warranted, monitoring.

How to check groundwater flood risk for a specific site

Screening tells you whether to worry; it does not tell you the answer. Where groundwater matters, you move from maps to site-specific evidence:

1. Start with the screening layers — the BGS susceptibility data and your local SFRA — to establish whether groundwater is in scope at all.
2. Pull the records — borehole and well logs from the BGS GeoIndex, plus any historic ground investigation, give real groundwater levels near the site.
3. Read it against the geology — chalk, limestone and sand-and-gravel behave very differently, so the same susceptibility band means different things in different places. See the areas affected by groundwater flooding.
4. Commission monitoring where the screening shows a credible risk: dipwells read across a wet season capture the seasonal peak the maps cannot. This is a subject in its own right — see groundwater monitoring for planning.

From map to evidence: how Unda can help

A susceptibility map is the start of the question, not the answer. Unda turns screening data into a defensible, site-specific position for planning, purchase or due diligence — interpreting the official datasets, specifying and reviewing monitoring, and designing against a credible groundwater level.