How do we monitor our storm overflows?
All of our storm overflow are monitored to help us gain a better understanding of our network and any impact the overflows might be having on the environment. This information is fed into our near real-time map so people can view the current status of each individual overflow and the last time is discharged to the watercourse. We also provided monthly validated data of every individual discharge, so people don't need to wait until the annual returns data to understand how overflows are performing.
There are two ways you can view data on our storm overflows
- Our interactive map that shows in near real-time whether our storm overflows are operating
- Our monthly and annual storm overflow data which is published on this page.
Our EDM annual report
We share our EDM data with the Environment Agency annually and this data is also published here to ensure customers have all the information they need about storm overflows.
We have also included some additional data about whether discharges are permitted by the EA, the grid reference of the discharge and the waterbody that it discharges into. We have also included details of the investigations and improvement schemes we have undertaken as part of the Water Industry National Environment Programme (WINEP).
You can also have a read through the reports we’ve published in previous years.
Our 2025 data – a summary
|
|
2019 |
2020 |
2021 |
2022 |
2023 |
2024 |
2025 |
|---|---|---|---|---|---|---|---|
|
Average rainfall (over the year, regionally) |
1028mm |
1078mm |
989mm |
915mm |
1193mm |
1050mm |
906mm |
|
Days >6mm rainfall |
51 |
64 |
51 |
50 |
71 |
55 |
48 |
|
|
2019 |
2020 |
2021 |
2022 |
2023 |
2024 |
2025 |
|---|---|---|---|---|---|---|---|
|
Number of storm overflows (with data) |
697 |
2105 |
2087 |
2118 |
2167 |
2171 |
2176 |
|
Total number of discharges |
22,821 |
65,083 |
70,062 |
54,273 |
77,761 |
68,164 |
51,404 |
|
Total duration of discharges (hours) |
107,836* |
420,419 |
406,131 |
232,054 |
516,386 |
430,263 |
285,230 |
* 2019 duration corrected following Annual Return submission.
|
|
2019 |
2020 |
2021 |
2022 |
2023 |
2024 |
2025 |
|---|---|---|---|---|---|---|---|
|
Average number of discharges for an overflow |
33 |
30 |
33.6 |
25.6 |
35.9 |
31.4 |
23.62 |
|
Average duration of discharge (hrs) |
5 |
6 |
5.8 |
4.3 |
6.6 |
6.3 |
5.5 |
Overflows operate during prolonged or heavy rainfall. 2025 was a dry year, but the region experienced above average rainfall in January, September, November and December. Despite this, the number of discharges into Yorkshire's watercourses from our storm overflows reduced by 24.5%. The duration of these discharges also reduced by 33%. Importantly, the average number of discharges per overflow also decreased from 31.4 in 2024 to 23.6 in 2025.
Our £180m investment over the last two years targeted over 100 storm overflows and is delivering a reduction in storm overflows across the region. Some of the work as part of this investment was completed in early 2025, so we are yet to see the full benefit during an entire 12-month period, but we are seeing the investment beginning to make an impact.
Of course, there is more to do and we’re underway with a further £1.5bn investment to continue tackling the performance of more than 450 storm overflows by April 2030.
How is the data pulled together and validated?
We collect millions of data points each year and spend 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.
Reports
Storm overflow annual returns
Environment Act Storm Overflow Report 2024
.xlsx download
Environment Act Storm Overflow Report 2023
.xlsx download
Link opens in a new tab
Environment Act Storm Overflow Report 2022
.xlsx download
Environment Act Storm Overflow Report 2021
.xlsx download
Bathing water annual reports
Bathing Water Regulatory Annual Return 2025
.xlsx download
Bathing Water Regulatory Annual Return 2024
.xlsx download
Bathing Water Regulatory Annual Return 2023
.xlsx download
Bathing Water Regulatory Annual Return 2022
.xlsx download
Bathing Water Regulatory Annual Return 2021
.xlsx download
Individual stop start events
We’re constantly working to improve our data and make sure it’s accurate, but sometimes this might not be the case. Our monitors are sensitive, and they can sometimes provide readings that aren’t correct. We’re working on it and doing everything we can to improve the data we have available. This means that the data set may change as more data becomes available and due to the ongoing implementation of our validation and assurance processes. We will republish a latest version each month until the finalised annual submission is published.
Find more information on how the Environment Agency regulates storm overflows
Individual stop start events 2025 year to date
.xlsx download
Individual stop start events 2024
.xlsx download
What we’re doing to improve
We are seeing a combination of factors, from urban growth to climate change, frequently testing the design and capacity of our network. At the same time, there’s growing public awareness which is driving discussion about how storm overflows operate and what is acceptable for the future. 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 2,000 Combined Storm Overflows (CSOs) and we know replacing Yorkshire’s entire sewer network is an enormous task that would be massively disruptive and costly to customers. We’re committed to playing our part to improve Yorkshire’s rivers and coastline while ensuring bills remain affordable.
To help drive improvements, with support from our shareholders, we're investing £1.5 billion between now and 2030 in smarter, more sustainable ways to reduce how often storm overflows are used.
Our sustainable engineering solutions include:
- surface water sewers
- underground storage tanks
- nature-based solutions
- sustainable drainage systems.
This investment is part of our largest ever environmental programme, focused on reducing the use of storm overflows and improving the health of Yorkshire’s rivers and coastline.
Read more about our storm overflow investment.