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)

97% of our 2246 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.

For 2021 we are publishing our Environment Act Storm Overflow Report this includes our annual EDM return data for 2021. Under our commitment to an open data approach, 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).

You can access our EDM return for 2021 via an interactive map via this link.

How is the data pulled together and validated?

We’ve collected over 80 million data points during 2021 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 900 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 2021 data – a summary





Average rainfall (over the year, regionally)




Days >10mm




We have reduced the number of storm overflow points on our network as we have identified that they are no longer required. In 2021 we saw an increase in high rainfall days from 2020. The wetter weather, and the improvements to our reporting has resulted in an increase in total spills reported but an overall reduction in 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 2021.

Over two thirds of our storm overflows (67%) spill less than 40 times per year and 94% spill less than 100 times per year.

We've reduced the number of spills from our treatment works by 12% in the last 12 months. 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.

We have greater than 90% data availability for over 80% of our overflows and are working hard on improving this for the 2022 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).  We have completed 30 of these investigations in 2021, this has resulted in 3 storm overflows identified for improvement.

In 2021 we have started a program of installing 150 solar-powered cameras on wastewater outfalls as part of a pilot scheme to improve the visibility of our network. The installation phase commenced across assets that connect to watercourses including the River Wharfe and other key locations across the region. These cameras will allow us to quickly assess the performance of our assets and mobilise our response more effectively.

The Environment Act is looking to drive further improvements in the operation, performance and reporting of storm overflows in the future. We welcome the ambition of the targets set by the Government and we will now be working with key organisations in Yorkshire to understand their ambitions for water quality in the region.

The targets, once confirmed, will form an integral part of our business plan for the next five years and beyond.