Data register · England

Storm overflow spills in England, 2025

In 2025, the 14,303 monitored storm overflows run by England's water and sewerage companies recorded 292,119 spills lasting a total of 1,878,240 hours. Counting only overflows reported in both years, spills were down 35% and hours down 48% on 2024. This register covers every overflow in the Environment Agency's annual return, by company and by sewage treatment works.

When overflows spilled in 2025

Total hours spilling across England by month, from the start and stop time of every recorded spill. Wet winter months dominate.

Hours spilling, all overflows in England. Spills running across a month end are split between the months.

Estimate the volume and pollution of a spill

Water companies compared, 2025

Average spills per storm overflow, 2025.

Company	Overflows	Spills	Spills per overflow	Hours	Hours per overflow	No spills	Over 60 spills	Spills vs 2024
South West Water	1,365	46,164	33.8	407,006	298	13%	275	down 18%
United Utilities	2,270	60,843	26.8	327,452	144	13%	313	down 23%
Yorkshire Water	2,187	51,431	23.5	285,931	131	13%	196	down 25%
Dwr Cymru Welsh Water	122	2,843	23.3	14,525	119	7%	9	down 46%
Wessex Water	1,346	24,442	18.2	190,666	142	16%	62	down 45%
Northumbrian Water	1,561	27,860	17.8	123,803	79	16%	69	down 32%
Southern Water	977	15,554	15.9	114,868	118	18%	42	down 47%
Thames Water	581	9,189	15.8	107,822	186	23%	28	down 60%
Severn Trent Water	2,452	36,471	14.9	200,361	82	17%	72	down 41%
Anglian Water	1,442	17,322	12.0	105,806	73	17%	38	down 61%
England	14,303	292,119	20.4	1,878,240	131	16%	1,104	down 35%

Companies are ranked by spills per overflow. Dwr Cymru Welsh Water figures cover its overflows in England only. The change on 2024 compares the same overflows in both years.

Longest-spilling overflows, 2025

#	Overflow	Company	Discharges to	Spills	Hours
1	Salcombe Regis WwTW CSO Inlet overflow at sewage works	South West Water	Salcombe Regis Stream	235	5,446
2	Shop STW_SO_SHOP Inlet overflow at sewage works	South West Water	Tributary of Coombe Valley Stream	243	4,967
3	Tatworth WwTW Storm tank at sewage works	South West Water	Forton Brook	189	4,141
4	Brayford STW Inlet overflow at sewage works	South West Water	River Bray(S)	172	3,763
5	Lavant WwTW Storm tank at sewage works	Southern Water	River Lavant & Gw via Wetland	166	3,583
6	Chacewater STW Storm tank at sewage works	South West Water	(S) River Carnon	159	3,542
7	Markyate Wastewater Treatment Works Storm tank at sewage works	Thames Water	River Ver	155	3,531
8	Staveley WwTW Storm tank at sewage works	United Utilities	River Kent	167	3,395
9	Altarnun STW (Storm Tank) Storm tank at sewage works	South West Water	Penpont Water	165	3,295
10	Church Stretton STW Storm tank at sewage works (with treatment)	Severn Trent Water	Trib of Quinny Brook	162	3,257
11	Gleaston Castle CSO Overflow on sewer network	United Utilities	Gleaston Beck	139	3,206
12	Praze An Beeble STW Storm tank at sewage works	South West Water	(S) Praze River	147	3,153
13	Tiverton STW Storm tank at sewage works	South West Water	River Exe	160	3,152
14	Hurdcott Wastewater Treatment Works Storm tank at sewage works	Wessex Water	River Bourne	167	3,119
15	Stewkley Wastewater Treatment Works Storm tank at sewage works	Thames Water	Hardwick Brook	143	3,069
16	Pyworthy Sewage Treatment Works Storm tank at sewage works	South West Water	Derril Water(S)	146	2,919
17	Delabole Wastewater Treatment Works Storm tank at sewage works	South West Water	River Allen	148	2,903
18	Cheriton Fitzpaine WwTW Storm tank at sewage works	South West Water	Tributary of Holly Water	140	2,808
19	Kings Somborne WwTW Storm tank at sewage works	Southern Water	Tributary of the River Test	125	2,801
20	South Moreton Storm tank at sewage works	Thames Water	Mill Brook	137	2,721
21	Shrewton Water Recycling Centre Storm tank at sewage works (with treatment)	Wessex Water	River Till	116	2,679
22	Gwennap Wastewater Treatment Works Storm tank at sewage works	South West Water	Hicks Mill Stream	141	2,673
23	Ponsanooth STW SSO Inlet overflow at sewage works	South West Water	River Kennal	126	2,646
24	Winterbourne WwTW Storm tank at sewage works	Thames Water	Winterbourne Stream	116	2,635
25	Pica STW Storm tank at sewage works	United Utilities	Dyan Beck	152	2,589

There are 8,760 hours in a year, so an overflow near the top of this list was discharging for well over half of 2025.

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How to read the data

Storm overflow: A permitted outlet that lets sewage diluted with rainwater leave a combined sewer, pumping station or sewage works when heavy rain would otherwise overload it. It may be screened, and at sewage works settled in storm tanks, but it is not fully treated.
Spill count: Counted by the Environment Agency's 12/24-hour method: the first 12 hours of discharge count as one spill, and each further 24-hour block in which it is still discharging counts as another. A single long storm can therefore count as several spills.
Hours: The total time the monitor recorded the overflow discharging, before the counting method is applied. Hours are the better measure of how much an overflow operates.
Monitor uptime: The share of the year the event duration monitor (EDM) was working. Below 90%, the company must give a reason, and the spill figures are likely to be undercounts.
Rainfall: Spill numbers rise and fall with rainfall, so one year's change says little on its own about a company's performance. Look at the long-term average for each overflow alongside its latest year.

Related engineering guides

Source & licence

Data comes from the Environment Agency's Event Duration Monitoring – Storm Overflows – Annual Returns for 2025 and 2024. The water companies report these figures and the Environment Agency publishes them. MCBA has not altered the values, only grouped them by company and sewage works. Contains Environment Agency data licensed under the Open Government Licence v3.0. Register built 26 September 2026.