Storm overflows 2022-2025

If you've watched Channel 4's Dirty Business you may be familiar with "storm overflows" - the discharge of sewage into our waterways. If you haven't, it is worth a watch.

The Water and Sewerage Companies submit what are known as Event Duration Monitoring data to the Environment Agency as part of their regulatory Annual Return. The Environment Agency then publish yearly summary data from all the companies in England in late March. The 2025 data was published last Friday.

The data

The annual summary data gives one row per discharge site. Each site has a National Grid Reference (NGR), the total duration of all (recorded) spills, and the number of (recorded) spills calculated using a 12/24 hour method amongst other figures. The provided grid references aren't all "clean" or valid (there should be an even number of numbers) and sometimes more than one grid reference is given (where that happens we just take the first one). We convert the NGR to latitude/longitude and then find which constituency it is in. We then add up the total recorded spill duration and the number of spills per constituency.

The yearly summaries are provided in XLSX workbooks. Over the years, the Environment Agency have made some slight changes to the format/layout. For instance in 2024 they switched from providing the spill duration in decimal hours to a hh:mm:ss format (it seems to be decimal days under-the-hood) and this year they've added an extra sheet to the XLSX file that is a combination of all the other sheets. They also change where the table header row is from year to year. These sorts of changes mean extra care needs to be taken when trying to combine multiple years of data.

Visualised

I've re-processed all the 2022, 2023 and 2024 data, added in the new 2025 data, and updated our constituency hex map of storm overflows with hex maps by spills and by duration. These updated figures will also show up on our constituency dashboards site for English constituencies. Hopefully these views help you get an idea of how many spills are occurring near you and may be helpful for your MP too.

For alternative views and live overflow data, you could also check out the Top of the Poops website.