This Event Duration Monitoring (EDM) dataset relates to the performance of storm overflows in England. Data are provided by Water and Sewerage Companies (WaSCs) to the Environment Agency each year as part of their regulatory Annual Return, to fulfil their permitted conditions to discharge from these storm overflows under the Environmental Permitting Regulations.
Data files are provided by year (2025, 2024, 2023, 2022, 2021 & 2020) and include a sheet for each of the 10 WaSCs with storm overflows in England. Each WaSC sheet shows how often and how long each monitored storm overflow discharged during that year. From 2021 onwards the dataset also includes reasons why event duration monitors (EDM) may have provided information for <90% of the relevant return period, and why an overflow may have high spill counts in that period. These data reflect the best available information held by the WaSCs at time of submission.
The data file for each year also contains key summary data tables. These include: number of storm overflows with EDM, number of storm overflows returning spill counts, average spill duration and average spill count for each WaSC. A dataset README guide is also provided which provides further information to help use and understand the data.
A sixth data file contains the annual key summary data from respective years plus a table showing long-term trends from 2016-2025.
Data update (August 2026): Following routine Environment Agency data quality assurance checks, data submitted by Anglian Water Services (AWS) was revised and the published national figures updated accordingly. In addition, Northumbrian Water provided corrected spill count and duration data for one site within the 2025 EDM Annual Return, which have been updated where possible. As insufficient information was available to recalculate related metrics, including Long-Term Average (LTA) and EDM Operability (%), these may not fully align with the revised spill figures. The corrected Northumbrian Water data will be fully reflected in the 2026 EDM Annual Return and subsequent years. Attribution statement: © Environment Agency copyright and/or database right 2025. All rights reserved.
