Storm overflow performance

All 2,265 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.