Storm overflows explained

Storm overflows are among the most contested topics in UK water. The engineering is not complicated, and understanding it clarifies both why they exist and why the numbers moved the way they did.

How a combined sewer overflow works

In dry weather everything goes to treatment. In heavy rain, flow exceeds what the sewer can pass forward, and the excess spills over a weir to a watercourse.

Design intent: The spill is meant to be heavily diluted storm water containing some sewage, discharged rarely. Frequent spills, or spills in dry weather, indicate a capacity or asset fault.
Why counts rose: Monitoring coverage in England went from a small minority of overflows to effectively all of them within about seven years. Part of the recorded increase is increased measurement.
The fix: Separating surface water from foul sewers removes the cause, but means rebuilding urban drainage. Storage tanks and sustainable drainage reduce frequency at lower cost.

Why combined sewers exist

Victorian sewer builders had one pipe network to lay and two things to drain: foul sewage and rainfall runoff. They combined them. It was cheaper, it was the standard of the day, and it worked for the cities of the time.

That decision is now embedded in the ground beneath most older British cities, and beneath many European and North American ones. Separating the systems means rebuilding urban drainage, street by street, at a cost that has never been considered affordable.

Why overflows discharge

A combined sewer sized for foul flow plus ordinary rainfall cannot carry the flow from a heavy storm across a paved catchment. Something has to give. Without a relief point, the surcharge would back up through the lowest connections in the system — which are toilets in houses and basements.

The overflow is that relief point. It is a designed feature, not a malfunction, and the alternative it prevents is sewage flooding of homes, which is a considerably worse public health outcome than a diluted discharge to a river.

Why the recorded numbers rose

Event duration monitors were fitted to a small minority of English overflows in 2016 and to effectively all of them by 2023. Recorded spills rose over the same period.

Part of that rise is more measurement rather than more discharge. That is a genuine and important qualification, and it is not a defence: the monitoring revealed a scale of discharge that had simply not been quantified before, and much of it is not attributable to exceptional rainfall. Both things are true, and reporting that omits either is incomplete.

Recorded storm overflow spills, England

Counted by event duration monitors. Monitoring coverage expanded substantially across this period, so part of the change reflects measurement rather than discharge.

Counts depend on how many overflows are monitored. Monitoring coverage in England rose from a small minority of overflows to near-complete over roughly seven years, so an increase in recorded spills over that period is partly an increase in measurement.

View the data behind this chart

Underlying data
Period	England (spills/yr)
2020	403,000
2021	372,500
2022	301,000
2023	464,000
2024	450,000

Source: EA — Event Duration Monitoring — storm overflow spills.

What would actually fix it

Interventions, with what each achieves

Measure	Effect	Relative cost
Sustainable drainage — permeable paving, swales, rain gardens	Reduces the runoff entering the sewer in the first place	Low to moderate, incremental
Removing surface water connections from foul sewers	Directly reduces combined flow	Moderate
Storage tanks at overflow sites	Holds storm flow for later treatment	High
Interception tunnels, as at Tideway	Captures overflow across a whole city	Very high
Full separation of foul and surface water	Eliminates the cause entirely	Prohibitive at national scale
Increased treatment works capacity	Allows more flow to be passed forward	High

There is no cheap fix, which is the uncomfortable core of the issue. Every option costs money that ultimately comes from bills or from taxation, and the largest reductions come from the most expensive interventions.

Sources

Environment Agency — Event Duration Monitoring — storm overflow spills. Open — attribution required · Open Government Licence v3.0
Water Services Regulation Authority — Water company performance and price review data. Open — attribution required · Open Government Licence v3.0
Water UK — Water UK industry statistics and company reporting. Public but restricted · Publisher terms — public access, reuse not clearly granted