Swakeleys · Photo: Harrison49 · CC BY-SA 3.0Heavy rain is Hillingdon’s main flood risk: 12% of homes, against 1.3% from rivers or the sea
On the Environment Agency’s maps, which allow for flood defences, “high or medium” means at least a 1 in 100 chance of flooding from rivers or the sea in any year. Hazard risk puts Hillingdon at C: #20 of 33 in London.
What stands out about Hillingdon’s hazards
From the Environment Agency’s maps and records and the BGS radon atlas, below.
What Hillingdon faces
Each hazard against its own yardstick; meters run from low to high on that yardstick.
Rivers and the sea
Surface water
Past floods
The River Crane
Flood warnings
Radon
Hillingdon grades C for hazard risk
Flood risk from rivers and the sea, against all UK towns.
The share of homes at high or medium risk from rivers and the sea, ranked against every UK town of 1,000 or more: fewer homes at risk is better. Towns with none at high or medium risk are ordered by their share at low risk, so they don’t all share one score. Surface water and radon aren’t graded: they are described below.
Where the water goes in Hillingdon
Turn layers on and off. Blue is the risk from rivers and the sea; pink is where heavy rain collects. Both allow for flood defences.
The river and sea risk follows the River Pinn, the River Crane and the Colne Brook. Surface-water risk is spread across the borough, in dips, underpasses and along small streams, which is why more homes are exposed to it.
1.3% of Hillingdon’s homes are at high or medium risk from rivers or the sea
The Environment Agency’s national map, allowing for defences. A further 2.6% are at low risk.
Share of each UK town’s homes at high or medium risk from rivers and the sea. 35% of UK towns have none; Hillingdon has more than 72% of towns.
| # | Town | People | High or med. | |
|---|---|---|---|---|
| 1 | Southwark London | 307,076 | 0% | |
| 2 | Islington London | 216,309 | 0% | |
| 3 | Newham London | 351,065 | under 0.1% | |
| … | ||||
| 23 | Barnet London | 388,572 | 1.2% | |
| 24 | Enfield London | 329,822 | 1.3% | |
| 25 | Hillingdon London | 307,089 | 1.3% | |
| 26 | Harrow London | 260,534 | 1.3% | |
| 27 | Bristol Bristol | 425,196 | 1.3% | |
| … | ||||
| 40 | Nottingham Nottinghamshire | 299,786 | 3.6% | |
| 41 | Kingston upon Hull East Riding of Yorkshire | 270,816 | 6.3% | |
UK towns of 215,000 to 430,000 people.
The Environment Agency’s Risk of Flooding from Rivers and Sea (NaFRA2, June 2026) counts each postcode’s homes by band; the shares weight those postcodes by Census 2021 households. The EA counts include homes built since 2021.
Heavy rain puts about 9.5 times as many Hillingdon homes at risk as rivers and the sea do
Surface water is rain that falls faster than drains and the ground can take it. It is mapped separately from rivers and the sea, and it is not in the Hazard risk Grade.
About 7.4% of homes are at high risk of surface-water flooding (a 1 in 30 chance or more each year) and 4.6% at medium risk: roughly 13,380 households. A further 11% face a low risk from surface water. That is more than 95% of UK towns.
Our estimate: every address in the borough (ONS NSUL) read on the Environment Agency’s Risk of Flooding from Surface Water map (NaFRA2, 2025), each postcode’s share weighted by its Census 2021 households. Neither agency publishes a count of homes for surface water, so treat it as a guide; one street can differ from the next.
Hillingdon has flooded in 20 recorded years since 1959, most of all in 1977
Every flood the Environment Agency has mapped inside Hillingdon’s built-up area, newest first. The record is fullest for big river floods; small and surface-water floods are often missing.
- 2024September 2024: River flooding recorded1 hectare inside today’s built-up area
- 2023October 2023: River flooding recorded2 hectares inside today’s built-up area
- 2021January 2021: River flooding recordedUnder 1 hectare inside today’s built-up area
- 2020October 2020: River flooding recorded2 hectares inside today’s built-up area
- 2019September and October 2019: River flooding recorded1 hectare inside today’s built-up area
- 2016June 2016: River flooding recorded3 hectares inside today’s built-up area
Show all 20
- 2015August 2015: River flooding recordedUnder 1 hectare inside today’s built-up area
- 2014January 2014: River flooding recorded2 hectares inside today’s built-up area
- 2013December 2013: River flooding recorded1 hectare inside today’s built-up area
- 2009February and November 2009: River flooding recorded2 hectares inside today’s built-up area
- 2007May and July 2007: River flooding recorded1 hectare inside today’s built-up area
- 2003January 2003: River flooding recorded2 hectares inside today’s built-up area
- 2002December 2002: River flooding recorded16 hectares inside today’s built-up area
- 2001October 2001: River flooding recorded1 hectare inside today’s built-up area
- 2000January and October 2000: River flooding recorded3 hectares inside today’s built-up area
- 19991999: River flooding recorded1 hectare inside today’s built-up area
- 1988January 1988: River flooding recorded65 hectares inside today’s built-up area
- 1987January 1987: River flooding recorded4 hectares inside today’s built-up area
- 1977January 1977: River flooding recorded86 hectares inside today’s built-up area
- 1959January 1959: River flooding recorded6 hectares inside today’s built-up area
Floodwater covered 86 hectares of what is now Hillingdon in 1977, the largest flood the record holds here (January).
782 of Hillingdon’s households today live inside the outline of a recorded flood.
Since 1977, the largest recorded flood inside the borough covered 65 hectares.
The River Crane at Cranford Park: highest 2.64 m, in July 2021
The highest level each winter (October to September) at the Environment Agency’s Cranford Park gauge (Cranford), since 1990. The river normally runs between 0.06 m and 1.10 m here.
- 6 July 20212.64 m
- 30 October 20001.26 m
- 13 October 19931.24 m
- 10 February 20091.19 m
- 23 September 19921.18 m
- 30 May 19921.13 m
The River Crane has topped its normal range on 16 days since 1990, most in the winters of 1991–92 and 2020–21. By its highest day, the winter of 2025–26 was the 29th-highest of the 36.
Environment Agency Hydrology API, Cranford Park (station 3660TH), daily maximum level; the record and normal range from the EA’s stage scale. Live river levels are on GOV.UK’s Check for flooding.
No flood warning or alert is in force for Hillingdon · 27 warnings in 20 years
The Environment Agency has 15 flood warning areas and 6 alert areas reaching into the borough; warnings since the service began in 2006.
The warning area covering the most of Hillingdon, Yeading Brook at Hillingdon, has not had a flood warning since 2006. Of the 27 warnings, 10 were for Colne Brook at Colnbrook and 5 for River Pinn at Yiewsley.
2016 had the most warnings (7); 2024 the most alerts (38). The most recent warning was on 23 September 2024.
Show the 21 warning and alert areas
| Area | Kind | Since 2006 | Now |
|---|---|---|---|
| Colne Brook at Colnbrook | Warning area | 10 | None |
| River Pinn at Yiewsley | Warning area | 5 | None |
| River Colne and Frays River at West Drayton and Stanwell Moor | Warning area | 2 | None |
| River Pinn at Ruislip | Warning area | 2 | None |
| Colne Brook at Iver | Warning area | 2 | None |
| River Pinn at Ickenham | Warning area | 1 | None |
| River Pinn at Eastcote Village | Warning area | 1 | None |
| Yeading Brook West at Ruislip Manor | Warning area | 1 | None |
| River Colne at Willowbank | Warning area | 1 | None |
| Yeading Brook East at South Ruislip | Warning area | 1 | None |
| River Colne at Rickmansworth | Warning area | 1 | None |
| River Pinn at Uxbridge | Warning area | 0 | None |
| Yeading Brook at Hillingdon | Warning area | 0 | None |
| River Crane at Southall | Warning area | 0 | None |
| River Crane at Cranford and Feltham | Warning area | 0 | None |
| River Pinn and Woodridings Stream | Alert area | 92 | None |
| Yeading Brooks in London Boroughs of Harrow and Hillingdon | Alert area | 60 | None |
| Lower River Colne and Frays River | Alert area | 38 | None |
| Colne Brook at Iver and Colnbrook | Alert area | 29 | None |
| Fluvial River Crane | Alert area | 22 | None |
| Middle River Colne | Alert area | 17 | None |
Environment Agency Historic Flood Warnings (first issues only, not updates; “flood watch” counted as an alert before 2011) and the real-time flood-monitoring API. Sign up for free warnings on GOV.UK or call Floodline, 0345 988 1188.
Parts of Hillingdon are in radon Affected Areas, at the low end
Radon is a natural radioactive gas from the ground; long exposure raises the risk of lung cancer. The atlas gives the chance that a home in each 1 km square is above the UK action level (200 Bq/m³).
9% of Hillingdon’s built-up area is in a radon Affected Area (1% of homes or more above the action level).
The squares are indicative: each takes the highest band found anywhere inside it, so most homes in a coloured square are below it. Only a three-month test in the home gives a real answer.
BGS/UKHSA Indicative Atlas of Radon in Great Britain, version 3. Check an address at UKradon.org.
Flood risk in the places around Hillingdon
Share of homes at high or medium and at low risk from rivers and the sea. Tap a town for its page.
| Town | People | High or medium | Low | Hazard |
|---|---|---|---|---|
| Hillingdon | 307,089 | 1.3% | 2.6% | C |
| New Denham | 1,790 | 15% | 4.9% | D− |
| Denham | 1,482 | 0.7% | 0.2% | C+ |
| Iver | 3,267 | 0.5% | 0.2% | C+ |
| Iver Heath | 5,150 | 0% | 0% | A |
| Richings Park | 1,787 | 0% | 0% | A |
| Ealing | 367,271 | 0.6% | 0.8% | C+ |
| Poyle | 1,736 | 5% | 36% | D |
New Denham, 2.7 miles north-west, has the most homes at risk around Hillingdon (15% at high or medium). Iver Heath and Richings Park have none.
Get ready for what Hillingdon faces
Free warnings, what to do in a cloudburst, and what to check before you move.
- Sign up free for Environment Agency flood warnings by text, call or email.
- Floodline: 0345 988 1188, 24 hours.
- Hillingdon’s warning areas include River Colne at Willowbank and Yeading Brook at Hillingdon.
- Surface water rises in minutes, often with no warning issued: stay out of underpasses and dips.
- Keep the gullies and drains outside your home clear of leaves.
- Report a blocked road drain to the council.
- Test over three months, in a living room and a bedroom.
- Above 200 Bq/m³, a small sump and fan usually fixes it.
- New homes in radon areas are built with protection: basic from 3%, full from 10%.
- Check the address on GOV.UK’s long-term flood risk service: it gives rivers and sea and surface water separately.
- Ask whether the home has flooded or had a flood insurance claim; the TA6 property form asks the seller.
- Homes built before 2009 can get flood cover through Flood Re, the government-backed scheme; newer homes can’t.
- Check radon for the address on UKradon.org (a small fee) or ask the seller for a test result.
What this data can and can’t tell you
- Defences are counted. Both agencies’ maps allow for flood defences and the chance they are overtopped or fail, so a town behind a defence scheme can show few homes at high risk and many at low risk.
- Surface water is our estimate. Neither agency publishes a count of homes at risk from surface water, so we read each address on the map. A flat on a hill and a basement in a dip can share a postcode.
- Past floods are what was recorded. The flood outlines come from surveys, aerial photographs and reports; many small floods, and most surface-water floods, were never mapped.
- One town, many streets. Risk is street by street. Check a specific address on GOV.UK’s long-term flood risk service before you buy or rent.
Questions about flooding and hazards in Hillingdon
Does Hillingdon flood?
The last flood the Environment Agency recorded inside the borough was in September 2024. 1.3% of homes are at high or medium risk from rivers and the sea on the Environment Agency’s maps, which allow for flood defences. About 12% of homes are at high or medium risk from surface water, when heavy rain overwhelms the drains. Hillingdon grades C for hazard risk against UK towns.
Which parts of Hillingdon are at risk of flooding?
From rivers and the sea, the places the Environment Agency warns directly: Colne Brook at Colnbrook, River Pinn at Yiewsley, River Pinn at Ruislip and River Colne and Frays River at West Drayton and Stanwell Moor. Surface water can collect in low spots anywhere in the borough. Check a specific address on GOV.UK’s long-term flood risk service.
When did Hillingdon last flood?
The recorded flood outlines inside the borough run from 1959 to 2024 (20 years); the largest was in 1977. Flood warnings have been issued for parts of the borough since 2006, most recently on 23 September 2024.
Is Hillingdon at risk of surface water flooding?
About 7.4% of homes are at high risk and 4.6% at medium risk from surface water, against 1.3% at high or medium risk from rivers and the sea. Surface water floods come with little warning, and the agencies don’t issue warnings for them.
Is radon a problem in Hillingdon?
9% of the borough’s built-up area is in a radon Affected Area on the BGS/UKHSA atlas. Most homes are below it; a three-month test is the only way to know.
Are there flood warnings in Hillingdon now?
No flood warning or alert was in force near Hillingdon at 7 November 2026, 04:26. Live warnings are on GOV.UK’s Check for flooding service and Floodline, 0345 988 1188.
Contains Environment Agency information © Environment Agency and/or database right 2026. Contains British Geological Survey materials © UKRI 2026.
Sources: Environment Agency Risk of Flooding from Rivers and Sea, postcodes in areas at risk (June 2026); Natural Resources Wales Flood Risk Assessment Wales (May 2026); ONS Census 2021 households by postcode · Environment Agency Risk of Flooding from Surface Water (NaFRA2, 2025) and Natural Resources Wales Flood Risk Assessment Wales surface water and small watercourses, read at every address (ONS National Statistics UPRN Lookup, September 2026) and weighted by Census 2021 households by postcode · Environment Agency Recorded Flood Outlines and Natural Resources Wales historic flood map, inside the ONS 2022 built-up-area boundary · Environment Agency Hydrology API (daily maximum river level) and flood-monitoring stage scales · Environment Agency real-time flood-monitoring API · Environment Agency Historic Flood Warnings (January 2006 to June 2026) and flood warning and alert areas · BGS/UKHSA Indicative Atlas of Radon in Great Britain, version 3, by 1 km square · TownRatings grades: our own summary of the data, ranked against UK towns only. All under the Open Government Licence v3.0.
Contains public sector information licensed under the Open Government Licence v3.0. Contains HM Land Registry data © Crown copyright and database right 2026. Contains OS data © Crown copyright and database right 2026 (ONS boundaries). Northern Ireland: Source: NISRA; contains Land & Property Services data and Ordnance Survey of Northern Ireland information, © Crown copyright, licensed under the Open Government Licence v3.0. Scotland: contains National Records of Scotland data, Registers of Scotland data and SEPA data, © Crown copyright / © SEPA 2025, licensed under the Open Government Licence v3.0.