Event Duration Monitoring (EDM)

Choppy waters

Event Duration Monitoring (EDM)

EDM measures the frequency and duration of releases to the environment from storm overflows and utilises DEFRA 12/24 counting methodology.

In Yorkshire, 98.1% of our 2,221 permitted storm overflows have monitoring equipment installed, and the remaining will be monitored by the end of 2023.

We share our EDM data with the Environment Agency annually and the 2022 returns are also published here to ensure customers have all the information they need about storm overflows.

For 2022 we are publishing our Environment Act Storm Overflow Report. This includes our annual EDM return data for 2022. To go beyond this, we have also included some additional data about whether the asset is a permitted discharge by the EA, the grid reference of the discharge and the waterbody that it discharges to. We have also included details of the investigations and improvement schemes we have undertaken as part of the Water Industry National Environment Program (WINEP).

EDM 2022 interactive map

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How is the data pulled together and validated?

We’ve collected over 80 million data points during 2022 and spent time validating the data to make sure it is as accurate as possible. We do have some data quality issues which we are investigating such as where data is missing, negative or flatlining.

We have investigated over 650 overflows which discharge the most to understand if the data is representative of performance, with resolution activity and further investigations ongoing.

If there’s been an issue with the monitor, such as a failure or an environmental factor that’s caused a false reading, we’ve flagged the data and manually corrected it. We have a full audit trail for all corrected data.

Our 2022 data – a summary






Average rainfall (over the year, regionally)





Days >6mm rainfall











Number of storm overflows (with data)





Total number of discharges





Total duration of discharges (hours)





* 2019 duration corrected following Annual Return submission.







Average number of discharges for an overflow (





Average duration of discharge (hrs)






The summary tables above indicate an improving picture in our region when compared with previous years. The total number of discharges in 2022 reduced by 15,789 (23%) from 2021. Over half of our storm overflows (58%) operate less than 20 times per year and 86% less than 60 times per year.

Duration data equates to storm overflows discharging 1.3% of the total operational hours in 2022. This is a reduction of 174,077 hours (43%) in our discharge duration from 2021 performance.

Whilst the Yorkshire region received 7% less rainfall across through 2022 than 2021, we’ve been working hard to reduce discharges.  

Across our monitored 226 treatment works we have treated flows on average 43% in excess of the required volumes helping to significantly reduce the requirement to discharge in storm events.

Also, by taking a proactive approach to managing storm tanks at 221 of our sites, which equate to 160 Olympic swimming pools in volume, and returning storm water to the treatment works quicker after heavy rainfall we have been able to build capacity for storms following in quick succession, reducing the likelihood of discharges during adverse weather.

What we’re doing to improve

We are seeing a combination of factors, from urbanisation to climate change, frequently testing the design and capacity of our network. At the same time, increased awareness of the operation of storm overflows is driving an important debate on what society sees as acceptable. We’re committed to going above and beyond the DEFRA Storm Overflow Taskforce activity in ending pollution from storm overflows.

In Yorkshire we have over 2,200 overflows and we know replumbing the whole of Yorkshire is an enormous task and would be both disruptive and costly to customers and we need to ensure water bills remain affordable, but we are committed to playing our part to improve Yorkshire’s rivers and coastline.

We were already planning to invest £147 million by 2025 in storm overflow improvements, investigation, and increased monitoring as part of our business plan. To help drive improvements quicker, further investment from our shareholders is helping us tackle this issue. A £180m storm overflow improvement plan is already in motion, which will invest in the overflows we know are operating more frequently. In the next few months, we’ll be starting work on the first 90 of those overflows and creating extra storage in the system to hold water back and stop it from entering rivers. 

Storm overflows are a priority for us and that’s why we’ve funded these improvements. The programme will be looking at four ways of reducing overflows - creating storage in the system for the stormwater, separating surface water out, stopping infiltration, or making changes to other assets like our pumping stations so we’re passing forward the right flows.

Separating surface water out could include creating SUDs, so we’re slowing the flow of water. We’re also looking at water butts so we can disconnect drainpipes from roofs and store the water for watering gardens instead of it going into our sewer system.

All of the work we’re undertaking will take us beyond the government’s storm overflow reduction plan and we’re already planning our largest ever environmental investment programme between 2025 and 2030.