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C−Hazard risk Grade
Flood & hazards in Merton · Environment Agency

Heavy rain is Merton’s main flood risk: 9.2% of homes, against 1.7% 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 Merton at C−: #26 of 33 in London.

1968worst recorded flood5flood warnings since 20064.6%at low risk
In brief

What stands out about Merton’s hazards

From the Environment Agency’s maps and records and the BGS radon atlas, below.

Rain, more than the river9.2% of homes sit where heavy rain can flood the streets, against 1.7% at risk from rivers and the sea.Remembers 1968Floodwater covered 64 hectares of today’s Merton in 1968; 1,401 of today’s households live inside past flood outlines.5 flood warnings since 2006For the parts of Merton the Environment Agency warns directly; the most recent on 23 September 2024.Low radonEvery 1 km square over Merton is in the lowest radon band: under 1% of homes above the action level.
By hazard

What Merton faces

Each hazard against its own yardstick; meters run from low to high on that yardstick.

Rivers and the sea

1.7%of homes at high or medium risk
More than the typical UK town · more than 76% of towns
1,841 homes by the EA’s own count

Surface water

9.2%of homes at high or medium risk
More than 90% of UK towns
Rivers and sea: 1.7%

Past floods

2years with a recorded flood
Last recorded: September 1968
1,401 households inside past outlines

The River Wandle

1.23 mhighest in winter 2025–26 at Wandle Park Main Channel
16th-highest of 36 winters
Record 2.00 m, July 2007

Flood warnings

5flood warnings since 2006
91 flood alerts for the wider area
Last warning 23 September 2024

Radon

0%of the borough in a radon Affected Area
Highest band here: Under 1% of homes
No square of 5% or more
Not on this page: coastal erosion, subsidence and shrink-swell ground (the British Geological Survey licenses it), groundwater flooding and reservoir failure.
Hazard risk Grade

Merton grades C− for hazard risk

Flood risk from rivers and the sea, against all UK towns.

Homes at high or medium flood risk (1.7%)Fewer than 24% of 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.

Flood map

Where the water goes in Merton

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.

Recorded flood: 1937Recorded flood: 1968© OpenStreetMap contributors · EA NaFRA2
HighMediumLowrivers & seaHighMediumLowsurface waterMerton’s built-up area

The river and sea risk follows the River Wandle, the Beverley Brook and the Pyl 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.

Rivers and the sea

1.7% of Merton’s homes are at high or medium risk from rivers or the sea

The Environment Agency’s national map, allowing for defences. A further 4.6% are at low risk.

High risk0.6%of homes · 1 in 30 a year or more628 homes, EA count
Medium risk1.1%of homes · 1 in 30 to 1 in 1001,213 homes, EA count
Low risk4.6%of homes · 1 in 100 to 1 in 1,0004,346 homes, EA count
Where Merton sits among 3,735 UK towns
35%None
23%Under 0.5%
9%0.5–1%
11%1–2%Merton
11%2–5%
5%5–10%
3%10–20%
2%20% +

Share of each UK town’s homes at high or medium risk from rivers and the sea. 35% of UK towns have none; Merton has more than 76% of towns.

Towns its size: 35th of 47 from the fewest homes at risk
#TownPeopleHigh or med.
1Islington London216,3090%
2Camden London209,3420%
3Westminster London203,5710%
…
33Cambridge Cambridgeshire152,7421.3%
34Waltham Forest London278,1291.6%
35Merton London214,9311.7%
36Southampton Hampshire249,6331.8%
37Greenwich London288,1172%
…
46Oxford Oxfordshire170,7994.7%
47Kingston upon Hull East Riding of Yorkshire270,8166.3%

UK towns of 150,000 to 301,000 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.

Surface water

Heavy rain puts about 5.6 times as many Merton 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.

Surface water9.2%
Rivers and the sea1.7%
0%14%28% of homes

About 3.6% of homes are at high risk of surface-water flooding (a 1 in 30 chance or more each year) and 5.6% at medium risk: roughly 7,630 households. A further 14% face a low risk from surface water. That is more than 90% of UK towns.

~7,630
households at high or medium risk
14%
of homes at low risk
90%
of UK towns have a smaller share
112,417
addresses read on the map

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.

Flood history

Merton has flooded in 2 recorded years since 1937, most of all in 1968

Every flood the Environment Agency has mapped inside Merton’s built-up area, newest first. The record is fullest for big river floods; small and surface-water floods are often missing.

  1. 1968September 1968: River flooding recorded64 hectares inside today’s built-up area
  2. 1937March 1937: River flooding recorded5 hectares inside today’s built-up area
The worst: 196864 ha

Floodwater covered 64 hectares of what is now Merton in 1968, the largest flood the record holds here (September).

1,401 of Merton’s households today live inside the outline of a recorded flood.

River levels

The River Wandle at Wandle Park Main Channel: highest 2.00 m, in July 2007

The highest level each winter (October to September) at the Environment Agency’s Wandle Park Main Channel gauge, since 1990. The river normally runs between 0.21 m and 0.54 m here.

Highest level each winter, metres
Winter 1990–91: highest 0.99 m · 42 days above the normal rangeWinter 1991–92: highest 1.16 m · 30 days above the normal rangeWinter 1992–93: highest 1.14 m · 54 days above the normal rangeWinter 1993–94: highest 1.13 m · 83 days above the normal rangeWinter 1994–95: highest 1.18 m · 104 days above the normal rangeWinter 1995–96: highest 1.06 m · 26 days above the normal rangeWinter 1996–97: highest 1.10 m · 27 days above the normal rangeWinter 1997–98: highest 1.37 m · 58 days above the normal rangeWinter 1998–99: highest 1.46 m · 75 days above the normal rangeWinter 1999–00: highest 1.57 m · 64 days above the normal rangeWinter 2000–01: highest 1.53 m · 235 days above the normal rangeWinter 2001–02: highest 1.13 m · 60 days above the normal rangeWinter 2002–03: highest 1.22 m · 86 days above the normal rangeWinter 2003–04: highest 1.84 m on 19 August 2004 · 55 days above the normal rangeWinter 2004–05: highest 1.00 m · 23 days above the normal rangeWinter 2005–06: highest 1.13 m · 42 days above the normal rangeWinter 2006–07: highest 2.00 m on 20 July 2007 · 58 days above the normal rangeWinter 2007–08: highest 1.05 m · 44 days above the normal rangeWinter 2008–09: highest 1.33 m · 35 days above the normal rangeWinter 2009–10: highest 1.02 m · 51 days above the normal rangeWinter 2010–11: highest 1.17 m · 34 days above the normal rangeWinter 2011–12: highest 0.99 m · 34 days above the normal rangeWinter 2012–13: highest 1.13 m · 54 days above the normal rangeWinter 2013–14: highest 1.17 m · 161 days above the normal rangeWinter 2014–15: highest 1.12 m · 46 days above the normal rangeWinter 2015–16: highest 1.78 m on 23 June 2016 · 42 days above the normal rangeWinter 2016–17: highest 1.19 m · 28 days above the normal rangeWinter 2017–18: highest 1.42 m · 39 days above the normal rangeWinter 2018–19: highest 1.78 m on 11 June 2019 · 30 days above the normal rangeWinter 2019–20: highest 1.59 m on 24 August 2020 · 90 days above the normal rangeWinter 2020–21: highest 1.51 m · 63 days above the normal rangeWinter 2021–22: highest 1.66 m on 20 October 2021 · 26 days above the normal rangeWinter 2022–23: highest 1.02 m · 42 days above the normal rangeWinter 2023–24: highest 1.93 m on 23 September 2024 · 74 days above the normal rangeWinter 2024–25: highest 1.37 m · 31 days above the normal rangeWinter 2025–26: highest 1.23 m · 28 days above the normal range’06–07’23–24’03–04Record 2.00 m, 2007Top of normal range
1990–912025–26
The highest days on record
  • 20 July 20072.00 m
  • 23 September 20241.93 m
  • 19 August 20041.84 m
  • 11 June 20191.78 m
  • 23 June 20161.78 m
  • 20 October 20211.66 m

The River Wandle has topped its normal range on 2,074 days since 1990, most in the winters of 2000–01 and 2013–14. By its highest day, the winter of 2025–26 was the 16th-highest of the 36.

Environment Agency Hydrology API, Wandle Park Main Channel (station 4180_w1TH), 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.

Flood warnings

No flood warning or alert is in force for Merton · 5 warnings in 20 years

The Environment Agency has 10 flood warning areas and 2 alert areas reaching into the borough; warnings since the service began in 2006.

Now: no flood warnings or alerts for Merton (checked 7 November 2026, 21:28). Check for flooding on GOV.UK →
Warnings and alerts each year for areas in Merton
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Flood warning (flooding expected)Flood alert (flooding possible)

The warning area covering the most of Merton, Pyl Brook at West Barnes, has not had a flood warning since 2006. Of the 5 warnings, 2 were for River Wandle at Morden and 1 for River Wandle at Wimbledon.

2023 had the most warnings (2); 2019 the most alerts (13). The most recent warning was on 23 September 2024.

Show the 12 warning and alert areas
AreaKindSince 2006Now
River Wandle at MordenWarning area2None
Pyl Brook at North CheamWarning area1None
Beverley Brook at West BarnesWarning area1None
River Wandle at WimbledonWarning area1None
River Wandle at Beddington ParkWarning area0None
River Wandle at WandsworthWarning area0None
River Graveney at Tooting and Colliers WoodWarning area0None
Beverley Brook at Raynes ParkWarning area0None
East Pyl Brook at Morden ParkWarning area0None
Pyl Brook at West BarnesWarning area0None
Beverley Brook area in Merton, Sutton, Kingston upon Thames, Richmond upon Thames and WandsworthAlert area47None
River Wandle area in the London Boroughs of Wandsworth, Merton, Lambeth, Croydon and SuttonAlert area44None

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.

Radon

Merton 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).

TQ2169: Under 1% of homes above the action levelTQ2170: Under 1% of homes above the action levelTQ2267: Under 1% of homes above the action levelTQ2268: Under 1% of homes above the action levelTQ2269: Under 1% of homes above the action levelTQ2270: Under 1% of homes above the action levelTQ2271: Under 1% of homes above the action levelTQ2366: Under 1% of homes above the action levelTQ2367: Under 1% of homes above the action levelTQ2368: Under 1% of homes above the action levelTQ2369: Under 1% of homes above the action levelTQ2370: Under 1% of homes above the action levelTQ2371: Under 1% of homes above the action levelTQ2372: Under 1% of homes above the action levelTQ2466: Under 1% of homes above the action levelTQ2467: Under 1% of homes above the action levelTQ2468: Under 1% of homes above the action levelTQ2469: Under 1% of homes above the action levelTQ2470: Under 1% of homes above the action levelTQ2471: Under 1% of homes above the action levelTQ2472: Under 1% of homes above the action levelTQ2566: Under 1% of homes above the action levelTQ2567: Under 1% of homes above the action levelTQ2568: Under 1% of homes above the action levelTQ2569: Under 1% of homes above the action levelTQ2570: Under 1% of homes above the action levelTQ2571: Under 1% of homes above the action levelTQ2572: Under 1% of homes above the action levelTQ2666: Under 1% of homes above the action levelTQ2667: Under 1% of homes above the action levelTQ2668: Under 1% of homes above the action levelTQ2669: Under 1% of homes above the action levelTQ2670: Under 1% of homes above the action levelTQ2671: Under 1% of homes above the action levelTQ2672: Under 1% of homes above the action levelTQ2767: Under 1% of homes above the action levelTQ2768: Under 1% of homes above the action levelTQ2769: Under 1% of homes above the action levelTQ2770: Under 1% of homes above the action levelTQ2867: Under 1% of homes above the action levelTQ2868: Under 1% of homes above the action levelTQ2869: Under 1% of homes above the action levelTQ2870: Under 1% of homes above the action levelTQ2967: Under 1% of homes above the action levelTQ2968: Under 1% of homes above the action levelTQ2969: Under 1% of homes above the action levelTQ2970: Under 1% of homes above the action levelTQ3067: Under 1% of homes above the action levelTQ3068: Under 1% of homes above the action levelTQ3069: Under 1% of homes above the action level© OpenStreetMap contributors · BGS/UKHSA
Under 1%
Share of the borough’s area in each band
Under 1% of homes100%

Every 1 km square over Merton 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.

Neighbours

Flood risk in the places around Merton

Share of homes at high or medium and at low risk from rivers and the sea. Tap a town for its page.

Wandsworth: Hazard risk C−Sutton: Hazard risk C+Worcester Park and Stoneleigh: Hazard risk B+Kingston upon Thames: Hazard risk DLambeth: Hazard risk C+Ewell: Hazard risk C+Croydon: Hazard risk C−WandsworthSuttonWorcester Park and StoneleighKingston upon ThamesLambethEwellCroydonMerton
ABCD
TownPeopleHigh or mediumLowHazard
Merton214,9311.7%4.6%C−
Wandsworth326,7252.1%4%C−
Sutton208,8580.5%2.1%C+
Worcester Park and Stoneleigh15,8510%1.2%B+
Kingston upon Thames166,3334.5%4.2%D
Lambeth317,1080.4%0.3%C+
Ewell27,5090.5%1.4%C+
Croydon389,2421.9%0.9%C−

Kingston upon Thames, 4.1 miles west, has the most homes at risk around Merton (4.5% at high or medium). Worcester Park and Stoneleigh has none.

Be ready

Get ready for what Merton faces

Free warnings, what to do in a cloudburst, and what to check before you move.

Get the warnings
  • Sign up free for Environment Agency flood warnings by text, call or email.
  • Floodline: 0345 988 1188, 24 hours.
  • Merton’s warning areas include River Wandle at Morden and River Wandle at Wimbledon.
Sign up ↗
In a cloudburst
  • 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.
When the river rises
  • 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.
Before you buy or rent in Merton
  • 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.
How to read these numbers

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.

Check the long-term flood risk for an address on GOV.UK →

Questions about flooding and hazards in Merton

Does Merton flood?

The last flood the Environment Agency recorded inside the borough was in September 1968. 1.7% 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 9.2% of homes are at high or medium risk from surface water, when heavy rain overwhelms the drains. Merton grades C− for hazard risk against UK towns.

Which parts of Merton are at risk of flooding?

From rivers and the sea, the places the Environment Agency warns directly: River Wandle at Morden, River Wandle at Wimbledon, Beverley Brook at West Barnes and Pyl Brook at North Cheam. 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 Merton last flood?

The recorded flood outlines inside the borough run from 1937 to 1968 (2 years); the largest was in 1968. Flood warnings have been issued for parts of the borough since 2006, most recently on 23 September 2024.

Is Merton at risk of surface water flooding?

About 3.6% of homes are at high risk and 5.6% at medium risk from surface water, against 1.7% 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 Merton?

Not by the atlas: every 1 km square over Merton 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 Merton now?

No flood warning or alert was in force near Merton at 7 November 2026, 21:28. 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.