What is a storm overflow?
A storm overflow is a relief point on a sewer. When rain overwhelms the pipe, it releases rainwater mixed with sewage into a river or the sea instead of into people's houses. The monitoring records when one starts and stops. Nothing else.
A relief valve on a Victorian sewer
Many older UK towns have combined sewers. One pipe carries what leaves your toilet and the rain running off the road outside. Natural Resources Wales estimates about 60% of housing in Wales is on that kind of system. When it rains hard, the pipe fills faster than the treatment works can take it, and the contents back up into homes.
That release point is the storm overflow. Defra calls storm overflows "a legacy feature of Victorian sewerage infrastructure". Diluted sewage in a river beat sewage in a kitchen. Much of that network is still in the ground.
When a water company is allowed to use one
Discharging from a storm overflow needs an environmental permit. Natural Resources Wales says a company may "only discharge during heavy rainfall, when the capacity of the combined sewer has been exceeded". In England it depends on where the overflow sits. At a treatment works the threshold is the flow to full treatment figure. On the sewer network it is a minimum retained flow, set by a formula, that must keep passing downstream. The Environment Agency is still permitting legacy overflows in England that were unknown at privatisation.
Spilling in dry weather is not allowed, and it happens anyway. The Environment Agency classes an overflow that operates in dry conditions as unsatisfactory, and since January 2025 it has treated every dry day spill as a pollution incident. A dry spell does not tell you that nothing was released.
Event duration monitoring
An event duration monitor, or EDM, sits at the overflow and records when it starts and stops discharging. That is all it records. At high amenity sites the Environment Agency requires a reading every two minutes. Elsewhere, where monitoring is required at all, it is every fifteen.
By December 2023 every storm overflow in England had a monitor fitted, which made 2024 the first year of full EDM coverage (Defra, December 2025). Fitted is not the same as reporting. Since 1 January 2025, section 141DA of the Water Industry Act 1991 has required English companies to publish each discharge within an hour of it starting and stopping.
What the published numbers count
The Environment Agency published 2025's annual data on 26 March 2026. There were 291,492 spill events, 35% fewer than 2024. At the 1,140 overflows linked to designated bathing waters, the season from 15 May to 30 September produced 6,780 spills, averaging 5.9 each.
An average across 1,140 overflows tells you nothing about the one near you. One spill on the day you are there is what counts.
The Environment Agency's chair, Alan Lovell, said the figures are "heavily influenced by rainfall levels in 2025", which was far drier than 2024. The count fell partly because it rained less. Nobody tested the water.
Under the Environment Agency's 12/24 counting method, a 60 hour discharge is recorded as three spills, not one.
Not discharging is not the same as clean
The published duty covers four things. That there was a discharge, where, when it started, when it stopped. Volume is not one of them. Neither is what was in it.
A stop time is not a clean time. The Bathing Water Regulations 2013 define short term pollution as contamination the regulator does not normally expect to affect the water for more than about 72 hours.
Most overflows are nowhere near a beach. Defra counted 14,128 in England in 2024 to 2025. Only 1,140 sat near a designated bathing water in 2025. The rest discharge somewhere, and what enters a river upstream can reach the coast. If there is no monitored overflow near your beach, nobody is measuring. That is not the same as nothing being discharged.
A dead monitor is a gap
Monitors also fail or sit in calibration. In 2024 to 2025, 13,221 of England's 14,128 overflows returned a complete year of EDM data. The other 907 did not.
Wales completed monitor installation in March 2020. Scotland has over 3,600 combined sewer overflows, and Scottish Water added 1,000 new monitors in 2024.
Ireland has nothing live, north or south. Northern Ireland does monitor. NI Water had 466 monitors operational in May 2026, covering 31 of the 33 designated bathing waters, with around 1,000 planned by 2028. It publishes one summary a year, for the 12 months to 31 December 2025. The Republic of Ireland publishes no storm overflow data at all. Its EPA requires the water utility, Uisce Eireann, to assess storm water overflows against national standards. By the end of 2024, 2,184 of an estimated 2,469 had been assessed. That is a check on condition, not a record of when they discharge.
When a monitor is dead or a feed is down, this site tells you that. It does not tell you the water is fine.
Where this comes from (11 sources)
- Natural Resources Wales · About 60% of housing in Wales is on combined sewers, the permit condition that companies may only discharge during heavy rainfall when the capacity of the combined sewer has been exceeded, and monitor installation completed in March 2020.
- Defra, Progress Report on the Storm Overflows Discharge Reduction Plan (December 2025) · The "legacy feature of Victorian sewerage infrastructure" quote, full EDM coverage achieved in December 2023 with the report's own wording that "2024 was the first year of full EDM coverage", 14,128 storm overflows in scope in 2024 to 2025 with 13,221 returning a complete year of EDM data, the explanation that newly installed or calibrating monitors caused the remaining gaps, and section 141DA in force from 1 January 2025.
- Environment Agency / GOV.UK · Flow to full treatment as the normal minimum setting at treatment works and formula A minimum retained flow on the sewer network, EDM at 2 minute intervals at high amenity sites and 15 minutes at moderate amenity sites and at low or no amenity sites with 20 or more spills a year, dry weather operation classed as unsatisfactory, the 12/24 spill counting method, and the section "Permits for existing unpermitted storm overflows" stating that historic unpermitted overflows are legacy overflows that were unknown at the time of privatisation and are permitted as is where evidence shows they are satisfactory.
- legislation.gov.uk, Environment Act 2021 section 81 · The four items that must be published (that there was a discharge, its location, when it began, when it ended), publication within an hour of the discharge beginning and within an hour of it ending, and the section coming fully into force on 1 January 2025.
- Environment Agency / GOV.UK, 26 March 2026 · The 2025 annual EDM figures of 291,492 spill events and a 35% fall on 2024, publication date of 26 March 2026, and the Alan Lovell quote that the numbers are heavily influenced by rainfall levels in 2025.
- Environment Agency blog, 4 December 2025 · 1,140 overflows associated with designated bathing waters, the 15 May to 30 September season producing 6,780 spills at an average of 5.9 per overflow, the absence of any spread or named sites in the summary, and that since January 2025 all dry day spills however small are classified as pollution incidents.
- legislation.gov.uk, Bathing Water Regulations 2013 regulation 2 · The definition of short term pollution as contamination the appropriate agency does not normally expect to affect bathing water quality for more than approximately 72 hours.
- Scottish Environment Protection Agency · Over 3,600 combined sewer overflows within Scotland's 50,000km sewer network.
- Environmental Standards Scotland, May 2025 · Scottish Water installing 1,000 new overflow event duration monitors in 2024, described as new additions alongside monitors already required by licence, not the company's first.
- NI Water · 466 EDMs operational as of May 2026 returning validated spill frequency and duration, EDMs at 31 of the 33 designated bathing waters, around 1,000 planned by 2028, and publication as a map summary for the 12 month period ending 31 December 2025 with no live feed or API.
- Environmental Protection Agency (Ireland), Urban Wastewater Treatment in 2024 · An estimated 2,469 storm water overflows in Ireland, and that the EPA requires Uisce Eireann to assess all of them against national standards, with 2,184 assessed by the end of 2024, 238 found not to meet the standards and 285 assessments overdue, rather than any monitoring or publication of when they discharge.