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.

    We have 1,199,338 total number of sensor data points in 2022.

  • 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

    In 2022 there were 228,220 15 minute records.

    • 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

    We recorded 75,445 total no. of spill days in 2022 before checks.

  • 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. 

    We validated 69,245 spills in 2022.

  • 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.

    The published total number of spills in 2022 was 69,245.

    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.

Overflow operation data may well go up and down as it’s heavily dependent on the weather. 2023 was one of the wettest years on record in the North West and overflows operated as designed to relieve pressure on the wastewater network and prevent homes and businesses from flooding. We understand that people no longer want the system to operate this way which is why we are proposing a £3 billion programme to tackle storm overflows in the region between 2025 and 2030.

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 aim to achieve a one-third sustainable reduction in the number of spills recorded from our storm overflows by 2025 compared to the 2020 baseline.

You can download our data returns below:







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

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