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Storm overflow spill data released today shows no room for complacency

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grassy bank in front of river

EDM lifts the lid on storm overflow performance, measuring how often and for how long storm overflows discharge into the environment. The state of our rivers is not in the condition we want. We are still seeing too much pollution from a number of sources, including sewage pollution from storm overflow discharges.

The Environment Agency has made water companies fit monitors to their storm overflows in order to capture information on how they are performing. We have a statutory duty under the Environment Act 2021 to publish EDM data for water and sewerage companies operating wholly or mainly in England by 1 April each year.

The water companies have rightly come under intense pressure from the government, the Environment Agency, campaigners and the public for allowing far too many sewage spills into rivers and coastal waters. This data is helping us to hold the industry to account on a scale never seen before. While we welcome recent commitments from water companies to reduce their use of storm overflows, the data published today shows there is no room for complacency. They have a very long way to go.

This is the second year that we have published the full data and the Environment Agency uses it in different ways, including:

  • To make the data more transparent and accessible for anyone who is interested in it - as part of the Government’s Storm Overflows Taskforce, we have been working hard with partners and stakeholders including eNGOs such as the Rivers Trust and Surfers Against Sewage, as well as water companies, to provide a better format and additional intelligence. The data return has been expanded and now includes:
    1. a national grid reference for the overflow
    2. the name of the waterbody and receiving water
    3. reasons for performance issues with monitors and high spill counts.
  • To understand and improve the environment and reverse damaging trends. The data the water companies provide is vitally important in building a picture of when storm discharges occur and allows the EA to direct water companies to address their environmental impact through tools such as the Water Industry National Environment Programme (WINEP). It also helps us build evidence for prosecution if needed.

EDM data has also informed the major criminal investigation into potential widespread non-compliance of water and sewerage companies at wastewater treatment works.


Main findings from 2021

The data from the 2021 EDM storm overflow returns from water and sewerage companies is summarised in the table below.  The full dataset and summary tables can be found here.

  • The percentage of storm overflows with monitoring devices has increased to 89% in 2021 (12,707 overflows)
  • The data shows that the average number of monitored spills per overflow has reduced from 33 in 2020 to 29 in 2021, with significant variance between water and sewerage companies (min/max average 20/42 spills). While the trend appears to be going down, this is most likely as a result of drier weather in parts of the country last year than in 2020
  • The average duration of each monitored overflow event was 7 hours (min/max average 5/11 hours)
  • 5% of storm overflows spilled more than 100 times in 2021; 87% of storm overflows had at least one spill in 2021; and 13% of storm overflows did not spill in 2021


Around 12,700 (89%) of England’s 14,470 storm overflows now have monitors, with the remainder due to be installed by the end of 2023. Installation on the remaining overflows will take place over 2022 and 2023 to meet the target of having all permitted storm overflows monitored by the end of 2023.

The evidence from EDM clearly shows where water companies need to improve and where they should focus their investment to carry out investigations and make improvements. They are obliged under the Environment Act 2021 to publish information about any investigations that have taken place, or improvement works they’ve carried out in relation to storm overflows during that period.

We use EDM data to drive planning and investment. Between now and March 2025, about 800 investigations and 800 improvements are planned with further being planned for 2025-2030. Further information can be found here - Water Industry National Environment Programme -

Event Duration Monitoring data - long-term trends

EDM coverage has significantly increased from 2016 as a result of the Water Industry National Environment Programme (WINEP) focus on EDM installation between 2015 and 2020.  This maturing dataset is becoming a robust evidence base for policy, improvement programmes and local action.

It should be noted that prior to 2018, EDM installation was focused upon a small subset of storm overflows: those that discharged to the most sensitive higher-amenity locations such as bathing waters, where a lower spill frequency might be expected due to stricter standards.  As the rollout of EDM programme widened to include all storm overflows, the trend in average number of spill events per overflow initially increased before declining in 2020 and 2021.

Storm Overflows Taskforce consultation

The data has also been central to informing Defra’s consultation on its Storm Overflow Reduction Plan to be laid before Parliament in September 2022. The plan has ambitious, longer-term targets:

  • protecting the Environment - ensuring no ecological harm and applies to all overflows discharging to rivers;
  • protecting public health in designated bathing waters - meeting the appropriate standard for bathing water and will apply to all overflows into and near designated bathing waters;
  • ensuring storm overflows operate only in unusually heavy rainfall events – limiting the operation of overflows and applies to all overflows.

The consultation has been launched today - Consultation on the Government’s Storm Overflow Discharge Reduction Plan - Defra - Citizen Space

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  1. Comment by Jonathan Grant-Nicholas posted on

    This is a disgrace. ALL water company profits as necessary should be used to both prevent leaks and pollution BY 2023. We deserve better from them and if they cannot do this then the companies concerned should be nationalised WITHOUT compensation to shareholders, who should have demanded better action long ago and have profited from their neglect by unwarranted dividends.

  2. Comment by Tim Poate posted on

    Why is their no requirement for water companies to measure overflow volumes as well a duration? Knowledge of the volume would be far more useful as a proxy for the possible impact of each event.

    Water companies happily charge consumers per litre for water use and yet seen incapable of measuring their own use.

    • Replies to Tim Poate>

      Comment by Wil Harvey posted on

      Absolutely. And a cynic might suggest the water companies will fix the smaller CSOs first to make the stats look good (Oooh, look, we've decreased the NUMBER of spills by such a lot...).
      We need to make sure the companies publish detailed plans and targets for all catchment areas, otherwise they will treat it as a PR exercise.
      A very much related issue is the quality of effluent released into waterways after treatment. Most treatment works are antiquated, many dating from the post-war expansion of infrastructure, and have not kept pace with the increased volumes and range of pollutants in sewage.

  3. Comment by James Towey posted on

    Having been internally flooded since July 2022 ,From Adjacent Highway sewer ,no compensation is available for customers ,Yorkshire Water using outdated 1991 water act to deny paying compensation .We received GSS payment 8 months after flooding was reported ,repairs made 9 months later still not repaired correctly.Not covered by home insurance saying full responsibility of YW.We have contacted everyone from LOCAL AUTHORITIES ,OFWAT .CCWATER .Our home searches say we are at risk from overload public sewers but YW refuse to put our property on at risk register.The way these private water companies treat customers is disgusting.

  4. Comment by Phil Lees posted on

    The Environment Agency are depressingly far behind with their analysis. Our local Sewage Treatment Works (Thames Water) has discharged 153 times for a total of 2,435 hours during the past twelve months. When I raised the issue of excessive discharges with the Environment Agency they contacted Thames Water and accepted their inadequate excuses and, despite having received photos from me of the extensive damage , failed to visit site to investigate. A toothless watchdog.


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