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Storm overflow performance

The Environment Agency requires all water companies to report the number of days each storm overflow operates each year, following standard rules. This means the minimum number is 0 spills up to a maximum of 366 for each overflow. This method provides the Environment Agency with the data it needs to best understand the environmental impact of storm overflow operation. 

Data is captured by sensors and we have these on every one of our 2,264 storm overflows. Millions of pieces of sensor data are gathered and a standard process converts this into an annual report on storm overflow performance.

  • All 2,264 of our overflows have sensors:

    • Each sensor captures data signals every 15 minutes of every day
    • We check they’re working all year round. 95% operate this way
    • That’s around 20 million hours of monitoring each year

    At this stage, we capture data signals that suggest spills may have occurred. Data at this stage includes all data, with no errors removed.

  • Two stages required to convert sensor data into the number of days a storm overflow has operated.

    • Takes data from every sensor and presents it in 15 minute intervals
    • 15 minutes intervals are used to make it easier to compare all water companies
    • Rules for this step are set out by the Water Industry Research Council

    • Converts the 15 minute data into daily time periods, i.e. how long each storm overflow has operated during a 24 hour period
    • Rules for this step are set out by the Environment Agency

  • Thorough checks are required to ensure data is accurate and complete. The process we follow to compile the data is reviewed by an external auditor and shared with the Environment Agency. 

    We ensure the process is carefully managed and data errors are removed. For example, sensors can be triggered incorrectly due to power cuts. 

  • Data is submitted to the Environment Agency in a standard format once a year which we use to inform investment plans and prioritise the overflows to tackle.

    You can also read about our draft business plan for 2025 - 2030 here where we propose to spend £3.1 billion on 430 storm overflows and target a 60% reduction in spills in the decade to 2030.

While overflows operation is heavily dependent on the weather, we are making very good progress in reducing spills. Our five-year ‘Big North West upgrade’ will see us invest more than £3 billion in improving storm overflow infrastructure between 2025 and 2030.

In the first year of our plan, we improved more than 400 storm overflows and, combined with a drier year, we forecast spills will have reduced by over 20%, or around 17,000 fewer spills, when official figures are released by the Environment Agency, expected in March 2026. Our monitors also show a one-quarter reduction in spill duration – equivalent to around 120,000 fewer hours. 65% of spill reductions in 2025 are directly linked to our interventions.

It will take time to re-plumb the North West but we have already started. The latest figures show that we are moving in the right direction against our target to achieve a 60% reduction in spills over the decade to 2030. We have now achieved just over a 45% reduction in spills since 2020, so we are ahead of target.

Download our data returns

For more details on our 2023 EDM return data, please click here. 

*As our data accuracy improves we have provided updated information as appropriate.