Photo: Wayland Smith · CC BY-SA 2.0Surface water, more than the river, is Horley’s flood risk: 4% of homes, against 0.2% 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. Horley’s Hazard risk Grade is B−: #20 of 65 in Surrey.
What stands out about Horley’s hazards
From the Environment Agency’s maps and records and the BGS radon atlas, below.
What Horley 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 Mole
Flood warnings
Radon
Horley grades B− 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 Horley
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 Ifield Brook, the River Mole and the Gatwick Stream. Surface-water risk is spread across the town, in dips, underpasses and along small streams, which is why more homes are exposed to it.
0.2% of Horley’s homes are at high or medium risk from rivers or the sea
The Environment Agency’s national map, allowing for defences. A further 1.1% 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; Horley has more than 49% of towns.
| # | Town | People | High or med. | |
|---|---|---|---|---|
| 1 | Chadderton Greater Manchester | 37,602 | 0% | |
| 2 | Sutton in Ashfield Nottinghamshire | 36,435 | 0% | |
| 3 | Denton Greater Manchester | 36,005 | 0% | |
| … | ||||
| 93 | Woodley Berkshire | 28,019 | 0.2% | |
| 94 | Great Malvern Worcestershire | 33,194 | 0.2% | |
| 95 | Horley Surrey | 27,067 | 0.2% | |
| 96 | Dereham Norfolk | 20,788 | 0.2% | |
| 97 | Colwyn Bay Clwyd | 29,273 | 0.2% | |
| … | ||||
| 261 | Spalding Lincolnshire | 30,552 | 84% | |
| 262 | Goole East Riding of Yorkshire | 20,173 | 94% | |
UK towns of 19,000 to 38,000 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 20.8 times as many Horley 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 1.8% of homes are at high risk of surface-water flooding (a 1 in 30 chance or more each year) and 2.2% at medium risk: roughly 440 households. A further 8.5% face a low risk from surface water. That is more than 56% of UK towns.
Our estimate: every address in the town (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.
Horley has flooded in 6 recorded years since 1960, most of all in 1968
Every flood the Environment Agency has mapped inside Horley’s built-up area, newest first. The record is fullest for big river floods; small and surface-water floods are often missing.
- 2013December 2013: River flooding recorded2 hectares inside today’s built-up area
- 2000October 2000: River flooding recorded5 hectares inside today’s built-up area
- 1990February 1990: River flooding recorded5 hectares inside today’s built-up area
- 1974November 1974: River flooding recorded2 hectares inside today’s built-up area
- 1968September 1968: River flooding recorded87 hectares inside today’s built-up area
- 1960June 1960: River flooding recorded65 hectares inside today’s built-up area
Floodwater covered 87 hectares of what is now Horley in 1968, the largest flood the record holds here (September).
998 of Horley’s households today live inside the outline of a recorded flood.
Since 1968, the largest recorded flood inside the town covered 5 hectares.
The River Mole at Horley: highest 3.44 m, in December 2013
The highest level each winter (October to September) at the Environment Agency’s Horley gauge, since 1990. The river normally runs between 0.19 m and 2.42 m here.
- 24 December 20133.44 m
- 17 January 20143.29 m
- 11 June 20123.27 m
- 1 February 20143.26 m
- 16 February 20203.25 m
- 20 December 20193.22 m
The River Mole has topped its normal range on 171 days since 1990, most in the winters of 2013–14 and 2023–24. By its highest day, the winter of 2025–26 was the 10th-highest of the 36.
Environment Agency Hydrology API, Horley (station 3230TH), 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 Horley · 26 warnings in 20 years
The Environment Agency has 3 flood warning areas and 1 alert area reaching into the town; warnings since the service began in 2006.
Of the 26 warnings, 11 were for River Mole at Charlwood and Hookwood and 10 for Burstow Stream at East and North Horley. 2014 had the most warnings (13); 2016 the most alerts (13).
The most recent warning was on 17 November 2022.
Show the 4 warning and alert areas
| Area | Kind | Since 2006 | Now |
|---|---|---|---|
| River Mole at Charlwood and Hookwood | Warning area | 11 | None |
| Burstow Stream at East and North Horley | Warning area | 10 | None |
| Gatwick Stream at South West Horley | Warning area | 5 | None |
| Upper River Mole, Ifield Brook, Gatwick Stream and Burstow Stream | Alert area | 114 | 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.
Horley 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 Horley 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 Horley
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 |
|---|---|---|---|---|
| Horley | 27,067 | 0.2% | 1.1% | B− |
| Salfords | 1,873 | 2.2% | 1.7% | C− |
| Smallfield | 2,613 | 31% | 21% | D− |
| Copthorne | 4,917 | 3.8% | 5.7% | D |
| South Nutfield | 1,951 | 0.8% | 0.9% | C+ |
| Redhill | 32,516 | 2.1% | 1.7% | C− |
| Reigate | 23,774 | 0% | 0% | A |
| Crawley | 120,567 | 1.5% | 2.7% | C |
Smallfield, 2 miles east, has the most homes at risk around Horley (31% at high or medium). Reigate has none.
Get ready for what Horley 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.
- Horley’s warning areas include Burstow Stream at East and North Horley and River Mole at Charlwood and Hookwood.
- 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 Horley
Does Horley flood?
The last flood the Environment Agency recorded inside the town was in December 2013. 0.2% 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 4% of homes are at high or medium risk from surface water, when heavy rain overwhelms the drains. Horley grades B− for hazard risk against UK towns.
Which parts of Horley are at risk of flooding?
From rivers and the sea, the places the Environment Agency warns directly: River Mole at Charlwood and Hookwood, Burstow Stream at East and North Horley and Gatwick Stream at South West Horley. Surface water can collect in low spots anywhere in the town. Check a specific address on GOV.UK’s long-term flood risk service.
When did Horley last flood?
The recorded flood outlines inside the town run from 1960 to 2013 (6 years); the largest was in 1968. Flood warnings have been issued for parts of the town since 2006, most recently on 17 November 2022.
Is Horley at risk of surface water flooding?
About 1.8% of homes are at high risk and 2.2% at medium risk from surface water, against 0.2% 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 Horley?
Not by the atlas: every 1 km square over Horley 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 Horley now?
No flood warning or alert was in force near Horley at 5 November 2026, 01:34. 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.