Storm Overflows and Event Duration Monitoring (EDM)

Choppy waters

What is a storm overflow?

Combined sewers carry both foul water from homes and businesses as well as rainwater which falls onto impermeable areas such as paved areas, roofs and highways.

Usually wastewater in sewers travels to one of our wastewater treatment works to be treated before it’s safely returned to the environment.

As rainwater can be unpredictable, we have permitted storm overflows on our sewer network to act as a relief valve. They help to reduce the pressure on sewers during heavy rainfall events and stop the system from backing up and flooding homes and gardens by allowing heavily diluted wastewater to be discharged into watercourses. This is permitted by the Environment Agency and closely monitored by them and us.

Many storm overflows have preliminary treatment such as screens or storm settlement before any discharge is made.

Storm Overflows and Event Duration Monitoring (EDM)

96% of our 2241 permitted storm overflows have monitoring equipment installed, and the rest will be monitored in the very near future. We’re proud to have one of the highest levels of storm overflow Event Duration Monitoring (EDM) coverage in the industry.

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

As we now have these overflows monitored, we want to be transparent with the data we receive from them. Under our commitment to an open data approach, we’re publishing our annual EDM return data for 2020. This covers the frequency and duration of intermittent storm overflow discharges to the environment.

You can access our EDM return for 2020 via an interactive map via this link.

How is the data pulled together and validated?

We’ve collected over 80 million data points during 2020 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’re also investigating our 600 overflows which spill 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 2020 data – a summary




Average rainfall (over the year, regionally)



Storm days



Rainfall during storms



As we’ve increased the amount of overflows we’ve had monitored over the last year, we have had an increase in the amount of spills recorded compared to 2019.2020 was generally wetter than 2019. The wetter the weather, and the more overflows that are monitored, then there is more than likely going to be an increase in the total frequency and duration of spills from storm overflows.




Number of storm overflows with numeric data



Total number of spills



Total duration of spills (hours)



* 2019 duration corrected following Annual Return submission.





Average number of spills for an overflow

(number of spill events/ number of overflows with data)



Average duration of spill (hrs)




The above duration data equates to storm overflows spilling 2.2% of the total operational hours in 2020.

The slight overall increase in the average duration per spill is due to us monitoring more overflows at wastewater treatment works (wwtw) in 2020 compared to 2019. The average duration per spill is typically higher for treatment works than for other assets. In 2020 the median duration per spill rate across the spill telemetry points for a CSO was 2.0 hours and for a wwtw was 6.7 hours. In 2019 we had 16 monitors at 13 wwtws and then we increased this to 355 monitors at 307 wwtws in 2020.

Over half of our storm overflows (55%) spill less than 20 times per year and 94% spill less than 100 times per year.

We have greater than 90% data availability for over 80% of our overflows and are working hard on improving this for the 2021 dataset.

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 supporting the DEFRA Storm Overflow Taskforce activity in ending pollution from storm overflows.

We will be investing £137 million by 2025 in storm overflow improvements, investigation and increased monitoring.

We are also investigating the environmental impact of 158 frequent spilling overflows as part of the Water Industry National Environment Programme (WINEP). As frequency of spill does not necessarily equate to environmental harm and environmental permits do not set spill threshold limits, these investigations will be key to understanding environmental impact, and what action needs to be taken to resolve any impacts.

In addition, we’re working in partnership with Siemens and the University of Sheffield and have developed an innovative AI pollution prevention solution utilising data from the EDM monitors to alert us to forming blockages that can be cleared without causing pollution incidents from our storm overflows.