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

Heavy rain is Southampton’s main flood risk: 3.2% of homes, against 1.8% 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. For hazard risk, Southampton grades C−: #58 of 76 in Hampshire.

50flood warnings since 200620%in a radon Affected Area
In brief

What stands out about Southampton’s hazards

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

Rain, more than the river3.2% of homes sit where heavy rain can flood the streets, against 1.8% at risk from rivers and the sea.50 flood warnings since 2006For the parts of Southampton the Environment Agency warns directly; the most recent on 16 February 2026.
By hazard

What Southampton faces

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

Rivers and the sea

1.8%of homes at high or medium risk
More than the typical UK town · more than 77% of towns
1,906 homes by the EA’s own count

Surface water

3.2%of homes at high or medium risk
More than 45% of UK towns
Rivers and sea: 1.8%

Past floods

3years with a recorded flood
Last recorded: March 2008
5 households inside past outlines

The Tanners Brook

0.47 mhighest in winter 2025–26 at Millbrook
27th-highest of 36 winters
Record 0.70 m, January 2003

Flood warnings

50flood warnings since 2006
163 flood alerts for the wider area
Last warning 16 February 2026

Radon

20%of the city in a radon Affected Area
Highest band here: 3–5% 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

Southampton grades C− for hazard risk

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

Homes at high or medium flood risk (1.8%)Fewer than 23% 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 Southampton

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: 1999Recorded flood: 2000Recorded flood: 2008© OpenStreetMap contributors · EA NaFRA2
HighMediumLowrivers & seaHighMediumLowsurface waterSouthampton’s built-up area

The river and sea risk follows the Southampton Water, the River Test and the River Itchen. Surface-water risk is spread across the city, in dips, underpasses and along small streams, which is why more homes are exposed to it.

Rivers and the sea

1.8% of Southampton’s homes are at high or medium risk from rivers or the sea

The Environment Agency’s national map, allowing for defences. A further 0.9% are at low risk.

High risk1.1%of homes · 1 in 30 a year or more1,172 homes, EA count
Medium risk0.7%of homes · 1 in 30 to 1 in 100734 homes, EA count
Low risk0.9%of homes · 1 in 100 to 1 in 1,0001,048 homes, EA count
Where Southampton sits among 3,735 UK towns
35%None
23%Under 0.5%
9%0.5–1%
11%1–2%Southampton
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; Southampton has more than 77% of towns.

Towns its size: 40th of 49 from the fewest homes at risk
#TownPeopleHigh or med.
1Southwark London307,0760%
2Islington London216,3090%
3Camden London209,3420%
…
38Waltham Forest London278,1291.6%
39Merton London214,9311.7%
40Southampton Hampshire249,6331.8%
41Greenwich London288,1172%
42Richmond upon Thames London192,8882%
…
48Nottingham Nottinghamshire299,7863.6%
49Kingston upon Hull East Riding of Yorkshire270,8166.3%

UK towns of 175,000 to 349,000 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 1.8 times as many Southampton 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 water3.2%
Rivers and the sea1.8%
0%7%14% of homes

About 1.6% of homes are at high risk of surface-water flooding (a 1 in 30 chance or more each year) and 1.6% at medium risk: roughly 3,340 households. A further 4.7% face a low risk from surface water. That is more than 45% of UK towns.

~3,340
households at high or medium risk
4.7%
of homes at low risk
45%
of UK towns have a smaller share
133,026
addresses read on the map

Our estimate: every address in the city (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

Southampton has flooded in 3 recorded years since 1999

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

  1. 2008March 2008: The sea came inUnder 1 hectare inside today’s built-up area
  2. 2000December 2000: River flooding recordedUnder 1 hectare inside today’s built-up area
  3. 1999December 1999: The sea came in1 hectare inside today’s built-up area

5 of Southampton’s households today live inside the outline of a recorded flood.

River levels

The Tanners Brook at Millbrook: highest 0.70 m, in January 2003

The highest level each winter (October to September) at the Environment Agency’s Millbrook gauge (Millbrook), since 1990. The river normally runs between 0.03 m and 0.50 m here.

Highest level each winter, metres
Winter 1990–91: highest 0.33 m · 0 days above the normal rangeWinter 1991–92: highest 0.50 m · 0 days above the normal rangeWinter 1992–93: highest 0.57 m · 1 days above the normal rangeWinter 1993–94: highest 0.46 m · 0 days above the normal rangeWinter 1994–95: highest 0.48 m · 0 days above the normal rangeWinter 1995–96: highest 0.49 m · 0 days above the normal rangeWinter 1996–97: highest 0.40 m · 0 days above the normal rangeWinter 1997–98: highest 0.41 m · 0 days above the normal rangeWinter 1998–99: highest 0.49 m · 0 days above the normal rangeWinter 1999–00: highest 0.59 m on 24 December 1999 · 1 days above the normal rangeWinter 2000–01: highest 0.57 m on 30 October 2000 · 3 days above the normal rangeWinter 2001–02: highest 0.44 m · 0 days above the normal rangeWinter 2002–03: highest 0.70 m on 1 January 2003 · 5 days above the normal rangeWinter 2003–04: highest 0.47 m · 0 days above the normal rangeWinter 2004–05: highest 0.41 m · 0 days above the normal rangeWinter 2005–06: highest 0.42 m · 0 days above the normal rangeWinter 2006–07: highest 0.52 m · 1 days above the normal rangeWinter 2007–08: highest 0.63 m on 26 May 2008 · 4 days above the normal rangeWinter 2008–09: highest 0.45 m · 0 days above the normal rangeWinter 2009–10: highest 0.48 m · 0 days above the normal rangeWinter 2010–11: highest 0.49 m · 0 days above the normal rangeWinter 2011–12: highest 0.57 m · 1 days above the normal rangeWinter 2012–13: highest 0.50 m · 0 days above the normal rangeWinter 2013–14: highest 0.58 m on 24 December 2013 · 3 days above the normal rangeWinter 2014–15: highest 0.53 m · 1 days above the normal rangeWinter 2015–16: highest 0.52 m · 1 days above the normal rangeWinter 2016–17: highest 0.56 m · 1 days above the normal rangeWinter 2017–18: highest 0.44 m · 0 days above the normal rangeWinter 2018–19: highest 0.47 m · 0 days above the normal rangeWinter 2019–20: highest 0.55 m · 2 days above the normal rangeWinter 2020–21: highest 1.39 m on 7 December 2020 · 6 days above the normal rangeWinter 2021–22: highest 0.57 m · 1 days above the normal rangeWinter 2022–23: highest 0.53 m · 1 days above the normal rangeWinter 2023–24: highest 0.66 m on 4 January 2024 · 1 days above the normal rangeWinter 2024–25: highest 0.51 m · 1 days above the normal rangeWinter 2025–26: highest 0.47 m · 0 days above the normal range’20–21’02–03’23–24Record 0.70 m, 2003Top of normal range
1990–912025–26
The highest days on record
  • 7 December 20201.39 m
  • 1 January 20030.70 m
  • 4 January 20240.66 m
  • 26 May 20080.63 m
  • 24 December 19990.59 m
  • 24 December 20130.58 m

The Tanners Brook has topped its normal range on 34 days since 1990, most in the winters of 2020–21 and 2002–03. By its highest day, the winter of 2025–26 was the 27th-highest of the 36.

Environment Agency Hydrology API, Millbrook (station E13281), 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 Southampton · 50 warnings in 20 years

The Environment Agency has 8 flood warning areas and 6 alert areas reaching into the city; warnings since the service began in 2006.

Now: no flood warnings or alerts for Southampton (checked 12 November 2026, 12:53). Check for flooding on GOV.UK →
Warnings and alerts each year for areas in Southampton
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 Southampton, Lord's Hill and Lordswood to Millbrook, on the Tanners Brook, has not had a flood warning since 2006. Of the 50 warnings, 26 were for Mansbridge and Woodmill on the River Itchen and 6 for Calshot, Hythe, Marchwood, Eling and Redbridge.

2023 had the most warnings (16); 2024 the most alerts (26). The most recent warning was on 16 February 2026.

Show the 14 warning and alert areas
AreaKindSince 2006Now
Mansbridge and Woodmill on the River ItchenWarning area26None
Calshot, Hythe, Marchwood, Eling and RedbridgeWarning area6None
River Test at Romsey and TestwoodWarning area5None
Itchen estuaryWarning area5None
Chandlers Ford to SwaythlingWarning area5None
Shawford to Bishopstoke on the River ItchenWarning area3None
Ocean Village, SouthamptonWarning area0None
Lord's Hill and Lordswood to Millbrook, on the Tanners BrookWarning area0None
Mansbridge and Riverside ParkAlert area59None
Southampton Water and HambleAlert area50None
Monks BrookAlert area25None
Lower ItchenAlert area15None
Lower TestAlert area11None
Tanners BrookAlert area3None

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

Parts of Southampton 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).

SU3613: Under 1% of homes above the action levelSU3614: Under 1% of homes above the action levelSU3712: Under 1% of homes above the action levelSU3713: Under 1% of homes above the action levelSU3714: Under 1% of homes above the action levelSU3715: Under 1% of homes above the action levelSU3811: Under 1% of homes above the action levelSU3812: Under 1% of homes above the action levelSU3813: Under 1% of homes above the action levelSU3814: Under 1% of homes above the action levelSU3815: Under 1% of homes above the action levelSU3816: Under 1% of homes above the action levelSU3911: Under 1% of homes above the action levelSU3912: Under 1% of homes above the action levelSU3913: Under 1% of homes above the action levelSU3914: 3–5% of homes above the action levelSU3915: 3–5% of homes above the action levelSU3916: 3–5% of homes above the action levelSU4011: Under 1% of homes above the action levelSU4012: Under 1% of homes above the action levelSU4013: Under 1% of homes above the action levelSU4014: Under 1% of homes above the action levelSU4015: Under 1% of homes above the action levelSU4016: Under 1% of homes above the action levelSU4110: Under 1% of homes above the action levelSU4111: Under 1% of homes above the action levelSU4112: Under 1% of homes above the action levelSU4113: Under 1% of homes above the action levelSU4114: Under 1% of homes above the action levelSU4115: Under 1% of homes above the action levelSU4116: 3–5% of homes above the action levelSU4117: Under 1% of homes above the action levelSU4209: Under 1% of homes above the action levelSU4210: Under 1% of homes above the action levelSU4211: Under 1% of homes above the action levelSU4212: Under 1% of homes above the action levelSU4213: Under 1% of homes above the action levelSU4214: Under 1% of homes above the action levelSU4215: 3–5% of homes above the action levelSU4216: 3–5% of homes above the action levelSU4217: Under 1% of homes above the action levelSU4310: Under 1% of homes above the action levelSU4311: Under 1% of homes above the action levelSU4312: Under 1% of homes above the action levelSU4313: Under 1% of homes above the action levelSU4314: 3–5% of homes above the action levelSU4315: 3–5% of homes above the action levelSU4316: 3–5% of homes above the action levelSU4409: Under 1% of homes above the action levelSU4410: Under 1% of homes above the action levelSU4411: Under 1% of homes above the action levelSU4412: Under 1% of homes above the action levelSU4413: Under 1% of homes above the action levelSU4414: Under 1% of homes above the action levelSU4415: 3–5% of homes above the action levelSU4416: 3–5% of homes above the action levelSU4417: 3–5% of homes above the action levelSU4509: Under 1% of homes above the action levelSU4510: Under 1% of homes above the action levelSU4511: Under 1% of homes above the action levelSU4512: Under 1% of homes above the action levelSU4513: Under 1% of homes above the action levelSU4514: 3–5% of homes above the action levelSU4515: 3–5% of homes above the action levelSU4516: 3–5% of homes above the action levelSU4517: 3–5% of homes above the action levelSU4518: 3–5% of homes above the action levelSU4610: Under 1% of homes above the action levelSU4611: Under 1% of homes above the action levelSU4612: Under 1% of homes above the action levelSU4613: Under 1% of homes above the action levelSU4614: Under 1% of homes above the action levelSU4711: Under 1% of homes above the action levelSU4712: Under 1% of homes above the action levelSU4713: Under 1% of homes above the action levelSU4811: Under 1% of homes above the action level© OpenStreetMap contributors · BGS/UKHSA
Under 1%3–5%
Share of the city’s area in each band
Under 1% of homes80%
3–5% of homes20%

20% of Southampton’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.

Neighbours

Flood risk in the places around Southampton

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

West End: Hazard risk B+Chilworth: Hazard risk AMarchwood: Hazard risk DRownhams and Nursling: Hazard risk ANetley: Hazard risk B−Hythe and Dibden Purlieu: Hazard risk CHedge End: Hazard risk C+West EndChilworthMarchwoodRownhams and NurslingNetleyHythe and Dibden PurlieuHedge EndSouthampton
ABCD
TownPeopleHigh or mediumLowHazard
Southampton249,6331.8%0.9%C−
West End10,8650%0.1%B+
Chilworth1,1690%0%A
Marchwood5,7765.3%5.7%D
Rownhams and Nursling4,7960%0%A
Netley6,3090.3%0.2%B−
Hythe and Dibden Purlieu19,8571.1%1.7%C
Hedge End23,1950.4%0.6%C+

Marchwood, 3.1 miles south-west, has the most homes at risk around Southampton (5.3% at high or medium). West End, Chilworth and Rownhams and Nursling have none.

Be ready

Get ready for what Southampton 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.
  • Southampton’s warning areas include Itchen estuary and Lord's Hill and Lordswood to Millbrook, on the Tanners Brook.
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.
Radon
  • 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%.
UKradon ↗
Before you buy or rent in Southampton
  • 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.
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 Southampton

Does Southampton flood?

The last flood the Environment Agency recorded inside the city was in March 2008. 1.8% 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 3.2% of homes are at high or medium risk from surface water, when heavy rain overwhelms the drains. Southampton grades C− for hazard risk against UK towns.

Which parts of Southampton are at risk of flooding?

From rivers and the sea, the places the Environment Agency warns directly: Mansbridge and Woodmill on the River Itchen, Calshot, Hythe, Marchwood, Eling and Redbridge, Itchen estuary and Chandlers Ford to Swaythling. Surface water can collect in low spots anywhere in the city. Check a specific address on GOV.UK’s long-term flood risk service.

When did Southampton last flood?

The recorded flood outlines inside the city run from 1999 to 2008 (3 years); the largest was in 1999. Flood warnings have been issued for parts of the city since 2006, most recently on 16 February 2026.

Is Southampton at risk of surface water flooding?

About 1.6% of homes are at high risk and 1.6% at medium risk from surface water, against 1.8% 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 Southampton?

20% of the city’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 Southampton now?

No flood warning or alert was in force near Southampton at 12 November 2026, 12:53. 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.