DWR CyMRU publish 2024 Storm Overflow data

Did you know that untreated sewage can legally enter our rivers during heavy rainfall? Storm overflows are designed to prevent flooding, but frequent spills can harm water quality and wildlife. Since 2024, Welsh Water has made near real-time data available, allowing the public to track pollution events as they happen. Yesterday Dwr Cymru released the data for 2024.

Following the release of Dŵr Cymru’s 2024 storm overflow data, Afonydd Cymru has analysed what this means in relation to rainfall.

What does the data show?

In 2023, Welsh Water recorded 121,422 spills over 1,044,062 hours, a year that saw 10 named storms and above-average rainfall in eight months. While 2024 saw slightly less annual rainfall (1600mm compared to 1680mm in 2023), extreme weather persisted, with February and March experiencing double the average rainfall. Welsh Water reported 112,589 spills lasting 929,168 hours, reflecting a similar pattern to the previous year.

One key concern raised by Afonydd Cymru is that, while overflows are legally permitted during exceptional rainfall, analysis of the top 20 most frequently spilling assets found that they operated within legal limits just 1% of the time. Additionally, 276 assets spilled for more than 1000 hours – exceeding the threshold that could be considered compliant with heavy rainfall conditions.

Rather than sewer blockages, the primary cause of these high-spilling sites was linked to failures at wastewater treatment works, including inadequate storm tank capacity and failure to fully treat incoming sewage. Without significant investment in the sewerage and treatment network, overflow numbers will continue to mirror rainfall patterns, raising serious concerns for Welsh rivers.

You can view Welsh Water’s interactive Storm Overflow Map here: corporate.dwrcymru.com

🔗 Read Afonydd Cymru’s full analysis: afonyddcymru.org