The latest storm overflow data has been published
Welsh Water have produced a new interactive map giving realtime information on their storm overflows. The map is now updated within an hour across all overflows.
Above: Analysis by Afonydd Cymru of Welsh Water overflows exceeding the exceptional rainfall condition during 2024.
Thursday 27th March, 2025
Water companies have released their storm overflow performance data for 2024 today.
The stark reality is until some significant investment is made in the sewerage and treatment network, overflow discharge numbers are simply a reflection of rainfall. So, in 2023, Welsh Water’s total spills were 121,422 spills releasing discharge over a total of 1,044,062 hours. And performance was attributed to 2023 being one of the wettest years on record with 10 named storms and eight months seeing above average rainfall. So, has 2024 faired any better?
Annual rainfall in Wales in 2024 was 1600mm, compared to 1680mm in 2023. However, the months of February and March in 2024 saw double the average rainfall, and the topography of Wales means that some areas experienced much higher rainfall, over 4000mm in Snowdonia as an example.
Welsh Water’s performance in 2024 shows a total of 112,589 spills, over 929,168 hours in Wales. This shows a similar pattern of operation to the previous year.
Overflow permits include a condition which states that they can legally operate during exceptional rainfall. Unlike England, Wales defined exceptional rainfall as rain greater than 4mm per hour, agreed by Welsh Government’s Better Water Quality Taskforce in 2023. It means that Natural Resources Wales (NRW) is able to ensure compliance against this condition.
Afonydd Cymru has looked at the top 20 of the company’s most frequently spilling assets in Wales to see whether the discharges complied with exceptional rainfall conditions. We are waiting for annual rainfall data from NRW to do this for every asset. We have used Welsh Water’s regulatory returns to NRW for the analysis.
The Top 20 spilling assets in Wales individually spilled between 3587 hours and a whopping 6267 hours over the year. Based on rainfall analysis by Afonydd Cymru against the exceptional rainfall condition, these assets operated legally for only 1% of the time.
In 2024, 276 assets spilled more than 1000 hours into Welsh rivers – the maximum duration of spills that could be compliant with the heavy rainfall condition anywhere in Wales. The worst catchments impacted in Wales were the Ogmore, Loughor and Clydach.
Afonydd Cymru concerns
Our concern is linked to the type of failure which is driving these overflow discharges. We often hear about blockages on the sewerage network, but in 2024 high spilling assets were dominated by failure at wastewater treatment works, not on the sewerage network. This type of failure is either due to a lack of storm tank capacity at the wastewater treatment works or a condition called Forward Passed Flow (FPF), which requires water companies to ensure full treatment of sewage entering the works. Afonydd Cymru, with Peter Hammond, highlighted these failures back in 2023.
Improvements on their way?
The National Environment Programme (NEP) outlines what Welsh Water must invest in over the next five years. As well as record-breaking investment on sewer overflows directly, we have identified 33 schemes directly on FPF with 60% of them due to be completed by 2027. With the publication of Welsh Water’s discharges in real-time, things may now start moving in the right direction. In Wales, the focus for sewer overflows remains on causing no ecological harm so we will expect to see improvements prioritised to remove discharges where clear impact to river health is threatened.
But, Welsh Water certainly has a lot to do to get its environmental performance back to the 4* performance it formerly had and we need to see again.
Analysis by Afonydd Cymru of Top 20 Worst overflows in Wales
