Photo: Des Blenkinsopp · CC BY-SA 2.0Heavy rain is Harrow’s main flood risk: 14% 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. Harrow’s Hazard risk Grade is C: #20 of 33 in London.
What stands out about Harrow’s hazards
From the Environment Agency’s maps and records and the BGS radon atlas, below.
What Harrow 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 Pinn
Flood warnings
Radon
Harrow 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 Harrow
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 Silk Stream; Dollis Brook; Deans Brook; Edgware Brook; Mutton Brook; Wealdstone Brook; Wembley Brook, the River Pinn and the Yeading Brook West. 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 Harrow’s homes are at high or medium risk from rivers or the sea
The Environment Agency’s national map, allowing for defences. A further 1.8% 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; Harrow has more than 72% of towns.
| # | Town | People | High or med. | |
|---|---|---|---|---|
| 1 | Southwark London | 307,076 | 0% | |
| 2 | Islington London | 216,309 | 0% | |
| 3 | Camden London | 209,342 | 0% | |
| … | ||||
| 34 | Enfield London | 329,822 | 1.3% | |
| 35 | Hillingdon London | 307,089 | 1.3% | |
| 36 | Harrow London | 260,534 | 1.3% | |
| 37 | Redbridge London | 309,509 | 1.4% | |
| 38 | Brent London | 340,611 | 1.5% | |
| … | ||||
| 49 | Nottingham Nottinghamshire | 299,786 | 3.6% | |
| 50 | Kingston upon Hull East Riding of Yorkshire | 270,816 | 6.3% | |
UK towns of 182,000 to 365,000 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 10.7 times as many Harrow 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 8.4% of homes are at high risk of surface-water flooding (a 1 in 30 chance or more each year) and 5.1% at medium risk: roughly 12,250 households. A further 9% face a low risk from surface water. That is more than 96% 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.
Harrow has flooded in 9 recorded years since 1977, most of all in 1977
Every flood the Environment Agency has mapped inside Harrow’s built-up area, newest first. The record is fullest for big river floods; small and surface-water floods are often missing.
- 2021May 2021: River flooding recordedUnder 1 hectare inside today’s built-up area
- 2016June 2016: River flooding recorded2 hectares inside today’s built-up area
- 2013December 2013: River flooding recordedUnder 1 hectare inside today’s built-up area
- 2007July 2007: River flooding recorded1 hectare inside today’s built-up area
- 19991999: River flooding recordedUnder 1 hectare inside today’s built-up area
- 1992January and September 1992: River flooding recorded19 hectares inside today’s built-up area
Show all 9
- 1988January 1988: River flooding recorded18 hectares inside today’s built-up area
- 1987January 1987: River flooding recorded3 hectares inside today’s built-up area
- 1977January 1977: River flooding recorded32 hectares inside today’s built-up area
Floodwater covered 32 hectares of what is now Harrow in 1977, the largest flood the record holds here (January).
791 of Harrow’s households today live inside the outline of a recorded flood.
Since 1977, the largest recorded flood inside the borough covered 19 hectares.
The River Pinn at Pinner, Moss Close: highest 1.16 m, in June 2016
The highest level each winter (October to September) at the Environment Agency’s Pinner, Moss Close gauge (Pinner), since 1996. The river normally runs between 0.18 m and 0.94 m here.
- 23 June 20161.16 m
- 8 June 20160.98 m
- 29 October 20000.98 m
- 1 February 20140.98 m
- 18 March 20130.96 m
- 9 February 20090.95 m
The River Pinn has topped its normal range on 6 days since 1996, most in the winters of 2015–16 and 2000–01. By its highest day, the winter of 2025–26 was the 15th-highest of the 27.
Environment Agency Hydrology API, Pinner, Moss Close (station 2886TH), 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 Harrow · 13 warnings in 20 years
The Environment Agency has 8 flood warning areas and 3 alert areas reaching into the borough; warnings since the service began in 2006.
The warning area covering the most of Harrow, Edgware Brook at Stanmore, has not had a flood warning since 2006. Of the 13 warnings, 6 were for Wealdstone Brook at Kenton and 2 for River Pinn at Pinner.
2016 had the most warnings (7). The most recent warning was on 23 September 2024.
Show the 11 warning and alert areas
| Area | Kind | Since 2006 | Now |
|---|---|---|---|
| Wealdstone Brook at Kenton | Warning area | 6 | None |
| River Pinn at Pinner | Warning area | 2 | None |
| River Pinn at Eastcote Village | Warning area | 1 | None |
| Yeading Brook West at Ruislip Manor | Warning area | 1 | None |
| Woodridings Stream at Pinner Green | Warning area | 1 | None |
| Yeading Brook East at South Ruislip | Warning area | 1 | None |
| Kenton Brook at Kenton | Warning area | 1 | None |
| Edgware Brook at Stanmore | Warning area | 0 | None |
| River Pinn and Woodridings Stream | Alert area | 92 | None |
| Silk Stream and the Deans, Edgware, Dollis, Mutton, Wembley and Wealdstone Brooks | Alert area | 79 | None |
| Yeading Brooks in London Boroughs of Harrow and Hillingdon | Alert area | 60 | 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.
Harrow is in the lowest radon band
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).
Every 1 km square over Harrow is in the lowest band: fewer than 1 in 100 homes are expected to be above the action level.
BGS/UKHSA Indicative Atlas of Radon in Great Britain, version 3. Check an address at UKradon.org.
Flood risk in the places around Harrow
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 |
|---|---|---|---|---|
| Harrow | 260,534 | 1.3% | 1.8% | C |
| Elstree | 1,737 | 0% | 0% | A |
| Bushey | 28,427 | 2.6% | 1.1% | C− |
| Brent | 340,611 | 1.5% | 2.1% | C |
| Moor Park | 5,850 | 0% | under 0.1% | B+ |
| Ealing | 367,271 | 0.6% | 0.8% | C+ |
| Hillingdon | 307,089 | 1.3% | 2.6% | C |
| Borehamwood | 39,760 | 0% | 0.1% | B+ |
Bushey, 4 miles north, has the most homes at risk around Harrow (2.6% at high or medium). Elstree, Moor Park and Borehamwood have none.
Get ready for what Harrow 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.
- Harrow’s warning areas include Edgware Brook at Stanmore and River Pinn at Pinner.
- 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.
- Never walk or drive through floodwater: 30 cm of moving water can float a car.
- Move valuables, papers and medicines upstairs.
- Turn off gas, electricity and water if you’re told to.
- 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.
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 Harrow
Does Harrow flood?
The last flood the Environment Agency recorded inside the borough was in May 2021. 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 14% of homes are at high or medium risk from surface water, when heavy rain overwhelms the drains. Harrow grades C for hazard risk against UK towns.
Which parts of Harrow are at risk of flooding?
From rivers and the sea, the places the Environment Agency warns directly: Wealdstone Brook at Kenton, River Pinn at Pinner, Woodridings Stream at Pinner Green and Kenton Brook at Kenton. 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 Harrow last flood?
The recorded flood outlines inside the borough run from 1977 to 2021 (9 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 Harrow at risk of surface water flooding?
About 8.4% of homes are at high risk and 5.1% 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 Harrow?
Not by the atlas: every 1 km square over Harrow is in the lowest band, where fewer than 1 in 100 homes are expected to be above the action level.
Are there flood warnings in Harrow now?
No flood warning or alert was in force near Harrow at 4 November 2026, 23:47. 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.