Groundwater Flood Risk Assessment — Chalk Aquifer Evidence Base, Purssell Close, Maidenhead
We prepared a standalone site-specific Groundwater Flood Risk Assessment for a ground floor extension at 7 Purssell Close, Maidenhead — a Flood Zone 1 site on the Chalk where groundwater was the source in question and a single FRA chapter would not have carried the argument. The assessment built a four-strand evidence base: water levels held by the British Geological Survey for eleven Chalk boreholes and wells measured since 1977 within 2.1km of the site, the Environment Agency monitoring record, the borough Strategic Flood Risk Assessment and our own groundwater model. Every Chalk measurement falls between 23.24 and 26.17 mAOD against a site ground level of 32.69 mAOD, placing the water table some 7.3 to 8.2m below ground. Applying the largest median-to-peak range observed in the region — 3.2m over 49 years at Shottesbrooke Park — still leaves an extreme high-stand approximately 4.6m below ground level, and the rating is Low on evidence rather than on mapping.
Standalone Groundwater Flood Risk Assessment on the Chalk Aquifer, 7 Purssell Close, Maidenhead
Location: 7 Purssell Close, Maidenhead, Berkshire, SL6 3XU | Services: Groundwater Flood Risk Assessment
When a Chapter Is Not Enough
Most flood risk assessments treat groundwater as one of six sources and give it a page. That is usually the right allocation of effort. But where groundwater is the source in question — because the local hydrogeology is a major aquifer, because the authority has raised it, or because the proposals sit close to the water table — a chapter cannot carry the argument. What is needed is a standalone assessment with an evidence base that can be checked line by line.
We prepared this site-specific Groundwater Flood Risk Assessment in respect of 7 Purssell Close, Maidenhead, Berkshire, SL6 3XU. The proposed development comprises an extension and associated alterations to the existing dwelling at ground level. No basement or other below-ground accommodation is proposed. The risk of groundwater flooding at the site is assessed as Low.
Four Independent Lines of Evidence
The assessment rests on four lines of evidence that can be tested separately. The first is the water level record held by the British Geological Survey for eleven Chalk boreholes and wells measured since 1977 within 2.1km of the site, together with four earlier records read in full rather than summarised. The second is the Environment Agency groundwater monitoring record for the stations retained for this site. The third is the borough Strategic Flood Risk Assessment, which remains the policy authority for this source. The fourth is our own groundwater flood risk model, which screens the site boundary against terrain, geology, hydrogeology and recorded water strikes.
Reading the borehole logs rather than citing them matters here. A level extracted from a summary table carries no information about what was struck, at what depth, or whether the reading represents a rest level or a strike, and a Lead Local Flood Authority is entitled to ask.
The Principal Finding
The water table in the Chalk beneath the site stands several metres below ground level, and is expected to remain so in an extreme event.
Every measurement made in the Chalk within 2.1km since 1977 falls between 23.24 and 26.17 mAOD, with a median of 24.54 mAOD. The nearest record, at 480m, reads 24.89 mAOD; the most recent, of 14 May 2025, reads 23.44 mAOD. Against a site ground level of 32.69 mAOD, that is a depth to water of some 7.3 to 8.2m. The consistency of the record across 48 years and eleven separate locations is itself part of the finding: the Chalk here does not exhibit the rapid, large-amplitude response that governs groundwater flooding elsewhere on the aquifer.
Testing the Conclusion Against an Extreme Event
A median level, however well evidenced, is not the test. The question is what the water table does in an exceptional year.
The assessment applied the largest range between median and highest recorded level observed at any Environment Agency station retained for this site — 3.2m, at Shottesbrooke Park, over 49 years of record. Applied at 7 Purssell Close, that gives an extreme high-stand in the order of 28.1 mAOD, approximately 4.6m below ground level.
Carrying the full uncertainty band on the mid-range level gives an upper bound of 30.1 mAOD, still 2.6m below ground level. The conclusion is not sensitive to that band: on every reasonable treatment of the data, the water table remains well below the proposed ground floor construction, and no plausible combination of recharge and antecedent conditions brings it to the surface.
Why the Method Matters
Groundwater ratings are often assigned from susceptibility mapping alone, which shows where groundwater flooding is possible rather than what the water table actually does at a given address. Measured levels, tested against the largest range observed in the region and carried through with an explicit uncertainty band, produce a rating that will withstand scrutiny from a Lead Local Flood Authority — and, equally important, one that does not impose unnecessary waterproofing cost on a scheme that does not need it. On a ground floor extension with no below-ground accommodation, the difference between an assumed High rating and an evidenced Low rating is the difference between a tanked structure and a conventional one.
Outcome
The assessment demonstrated a Low groundwater flood risk rating on evidence rather than on mapping, and confirmed that the proposed ground floor extension is acceptable in groundwater terms. The borough Strategic Flood Risk Assessment remains the policy authority for the source; this assessment provides the site-specific evidence that sits beneath it.