If storm overflows are designed to deal with rainwater overwhelming the sewage system, why are they sometimes discharging when there has been little or no rain?
In 2025, England recorded 291,492 monitored storm-overflow spill events.
At first glance, that number is shocking.
It works out at almost 800 recorded spill events every day of the year.
Yet 2025 was an unusually dry year. In fact, the Environment Agency says the fall in sewage-spill numbers compared with 2024 was heavily influenced by those unusually dry conditions. Spill events fell by 35%, while the total duration of spills fell by 48%.
So here's the obvious question:
If dry weather reduces sewage spills, why are sewage overflows operating at all when it isn't raining?
The answer is complicated — and potentially far more concerning than the headline numbers suggest.
What is a storm overflow actually for?
To understand the problem, we need to look underground.
Many parts of England still have combined sewer systems. These carry wastewater from toilets, bathrooms and kitchens in the same pipes as rainwater running from roofs, roads and other surfaces.
Normally, that mixture travels to a sewage treatment works. But imagine an enormous rainstorm. Thousands of litres of water can enter the sewer network extremely quickly. The pipes and treatment works may not have enough capacity to handle everything.
Without somewhere for the excess to go, wastewater could be forced backwards through toilets, sinks and drains — potentially flooding homes and streets with sewage.
Storm overflows were designed as a safety valve. When the system becomes overwhelmed, they can release excess wastewater into a river, lake or coastal water.
The Environment Agency describes them as a mechanism intended to prevent sewage backing up into homes and public spaces during rainfall or snowmelt.
There is therefore a basic principle:
Rain overwhelms the system → overflow operates → pressure is relieved.
But what happens when there isn't enough rain to explain the overflow?
The mystery of the "dry-day spill"
The Environment Agency defines a dry day as one in which there has been no rainfall above 0.25mm on that day and the preceding 24 hours.
A storm overflow operating under those circumstances is classified as a dry-day spill.
And the Environment Agency is unusually clear about the issue:
« Storm overflows should not spill on dry days. »
There are legitimate complications. For example, water from rainfall much further upstream in a very large catchment can take considerable time to reach an overflow. A discharge that appears to happen on a dry day might therefore have been caused by rainfall that occurred earlier or elsewhere.
Consequently, the Environment Agency treats a detected dry-day spill as a potential breach, rather than automatically assuming that a water company has broken the law. It investigates rainfall data, radar, drainage times and monitoring information before reaching a conclusion.
But the fact that these events occur at all raises a difficult question.
Where is the water coming from?
The water doesn't necessarily have to come from the sky
One explanation is groundwater infiltration. Sewers are not necessarily watertight.
Old pipes can develop cracks and defective joints. Groundwater can enter the sewer network through these defects, particularly where the water table is high.
That water then behaves much like rainwater from the perspective of the sewer system. It occupies capacity.
If enough groundwater enters the network, a sewer can potentially become overloaded even though there is no rain falling at the surface.
The water industry itself identifies groundwater infiltration as one of the causes that needs to be tackled. Water UK has said companies are investing in relining and sealing sewers partly to prevent this problem.
So one possibility is surprisingly simple:
It isn't raining — but water is getting into the sewer anyway.
Then there are blockages and infrastructure failures
A sewage network is an enormous piece of infrastructure. It contains pipes, pumping stations, treatment works, storage tanks, screens, valves, sensors and thousands of other components.
If something becomes blocked or fails, the effective capacity of the system can suddenly shrink.
Imagine a pipe capable of carrying 100 units of wastewater.
If a blockage reduces its effective capacity to 40 units, an incoming flow of 60 units suddenly becomes a problem — even though that flow might normally be perfectly manageable.
A pump failure can have a similar effect. So can damaged infrastructure.
This is one reason why simply looking at rainfall doesn't tell us everything about why an individual overflow operated.
And Britain's sewer network is old
This is where the story becomes much bigger than individual spills.
The Environment Agency has explicitly linked the persistently high level of storm-overflow activity to longstanding underinvestment and poor asset maintenance.
That doesn't mean every sewage spill is caused by neglect.
It does mean that England inherited a vast and complicated sewer network that requires continuous maintenance and investment.
Some of it was built generations ago. Some systems were designed for populations and patterns of water use that no longer exist.
And urban development has changed how much water reaches drainage systems.
The result is an infrastructure problem that cannot be solved simply by telling people not to flush the wrong things down the toilet.
But there is another number we should be watching
The 291,492 figure tells us how many monitored storm-overflow events occurred in 2025.
It doesn't mean that 291,492 illegal dry-weather sewage dumps took place. That's an important distinction.
Most storm-overflow activity is associated with rainfall or snowmelt, and the Environment Agency says storm-overflow activity is strongly linked to rainfall levels.
But England has now begun separately identifying and reporting suspected dry-day spills.
And that produces a much more intriguing number.
In 2025, water companies reported 8,576 dry-weather spill pollution discharges across England and Wales.
It represents suspected cases where an overflow operated despite there being little or no rainfall under the regulatory definition.
And it is these events that deserve particularly close scrutiny.
Why does a dry spill matter more?
Imagine two identical sewage discharges. In one case, a river is already swollen by heavy rainfall. In the other, the river is running low after weeks of dry weather.
The same quantity of sewage entering the water can have very different consequences. During heavy rainfall there is generally more water available to dilute the discharge. During dry conditions there may be much less.
The Environment Agency has therefore highlighted the environmental significance of dry-weather discharges.
There is also a timing issue.
A discharge during a drought or prolonged dry spell can be particularly surprising because the public naturally assumes that the storm-overflow system should be relatively inactive.
The really strange thing about 2026
The story becomes even more interesting when we look beyond 2025. The latest reporting indicates that water companies recorded 7,280 suspected dry-weather sewage discharges between January and May 2026 alone.
That is already approaching the entire 2025 total of 8,576.
However, there is an important caveat.
Those figures have been described as suspected dry-day spills, and the industry has stressed that the data requires validation before every event can be conclusively described as an illegal discharge.
That distinction matters.
A monitoring system can tell regulators that an overflow operated when rainfall data suggests it shouldn't have.
It doesn't necessarily tell them why.
That requires investigation.
Could the monitoring system itself be wrong?
Possibly — and this is another important piece of the puzzle. Event Duration Monitors record when an overflow operates and how long it operates.
But a sensor isn't a human standing beside a pipe watching sewage emerge. There can be problems with sensors, communications, rainfall measurements and the interpretation of data.
The Environment Agency itself says that a dry-day event needs further investigation because the data from an overflow alone doesn't always reveal what actually happened.
Investigators can compare overflow records with local rain gauges and rainfall radar, and examine how long it should have taken water to drain through a particular catchment.
So when you see a claim that an overflow operated on a dry day, there are at least three questions:
Did the monitor record an activation?
Did a discharge actually occur?
If it did, what caused it?
Those aren't necessarily the same question.
So is someone deliberately dumping sewage?
That is the question people understandably ask.
There are several plausible physical explanations, including groundwater infiltration, delayed flows from rainfall elsewhere in a catchment, blockages, pumping problems and other infrastructure issues.
But neither should the opposite assumption be made — that every unexplained activation is harmless.
That is precisely why regulators investigate them.
The Environment Agency says permits are intended to ensure storm overflows operate legally during rainfall and snowmelt, and that evidence of illegal operation can result in enforcement action.
In other words:
A dry-day spill is a warning signal.
It isn't automatically proof of criminal behaviour. But it is certainly something that should have an explanation.
Britain's sewage problem is therefore bigger than rainfall
It is tempting to reduce the entire sewage controversy to one question: "Did it rain?"
But that isn't enough.
The real question is:
"Why did the sewage system reach the point where it needed to overflow?"
Sometimes the answer will be heavy rainfall. Sometimes it may be snowmelt. Sometimes it could be water travelling through a huge catchment long after rainfall has stopped.
Sometimes groundwater may have entered defective infrastructure. Sometimes a blockage, pump failure or other infrastructure problem may be responsible.
And sometimes regulators may discover that the circumstances did not justify the discharge at all.
That is why the dry-day data is potentially so important.
It gives regulators another way of identifying parts of the sewage network that behave abnormally.
The uncomfortable conclusion
There is some genuinely positive news in the 2025 figures.
England's total monitored storm-overflow events fell substantially from the previous year, and the Environment Agency says spill duration fell by almost half.
But much of that improvement was associated with the unusually dry conditions.
The Environment Agency itself says lasting improvement will require sustained maintenance and investment in drainage networks.
And the emergence of thousands of suspected dry-weather spills creates another question that rainfall statistics cannot answer.
If the rain isn't falling, why is the sewage system overflowing?
Perhaps the answer is underground. Perhaps it is trapped water moving slowly through enormous catchments. Perhaps it is failing infrastructure.
Perhaps it is a combination of several problems.
And in some cases, perhaps regulators will find something more troubling.
The important point is that "it didn't rain" isn't the end of the investigation.
It is the beginning of one.
Because a storm overflow is supposed to be a safety valve for an overloaded sewage system.
When that safety valve opens on a dry day, something has happened that needs explaining.
And with increasingly sophisticated monitoring now covering England's entire storm-overflow network, we should eventually have far more evidence with which to ask that question.
Not just how often England dumps sewage — but why.
---
Sources
- Environment Agency / Defra — 2025 storm-overflow monitoring results.
- Environment Agency — explanation of dry-day spills and how they are investigated.
- Environment Agency — 2024 storm-overflow data and infrastructure/maintenance context.
- Environment Agency — national Event Duration Monitoring dataset.
- Recent reporting on dry-weather discharges in 2025–26.
Read more on:
Pharmaceuticals in Britain's rivers
The solution to pollution is not dilution
The mystery of Earth's missing water
Rain is carrying more than water
Comments
Post a Comment