Event duration monitoring (EDM) records when a storm overflow starts and stops discharging, so regulators and the public can see how often and how long it spills. Defra's storm overflows policy and guidance states that all storm overflows now have monitors installed, with the installation programme completed in December 2023, covering about 14,500 overflows in England.
Fitting the monitor was the easy part. Since 1 January 2025 water companies must publish the start of a discharge within an hour and the end within an hour, so every link from sensor to website now matters.
What event duration monitoring is required to do
- Record every operation. Frequency and duration of each discharge, kept with start and stop times available to regulators on request.
- Annual reporting. The Environment Agency publishes an annual EDM summary by the end of March for the previous year, using the 12-hour and 24-hour block counting method.
- Near real-time publication. Start and location within an hour of the discharge beginning, end within an hour of it stopping, under section 81 of the Environment Act 2021.
- Good practice. Companies should follow the CIWEM EDM good practice guide and use monitors that perform in line with Environment Agency and Ofwat guidance.
The same guidance notes that continuous water quality monitoring is being installed in the 2025 to 2030 period at high-priority sites, measuring ammonia, dissolved oxygen, temperature, pH and turbidity. That adds analysers, more data and more points to the same telemetry chain.
Operability is now a performance measure
A monitor that is offline cannot report a spill. The Environment Agency's 2024 performance report covered 14,254 monitored overflows and recorded 450,398 spills in 2024. It also reported that 9.5% of monitors operated less than 90% of the time, down from 13.5% in 2023. Improving, but still a lot of blind spots.
The EDM data chain, link by link
| Link | What goes wrong | Engineering fix |
|---|---|---|
| Level sensor | Fouling, ragging, poor siting | Correct mounting, self-check, maintenance alarm |
| Spill logic | Chattering on/off at threshold | Hysteresis and minimum duration timers |
| Outstation | Lost events during comms outage | Time-stamped event buffer, NTP time sync |
| Telemetry link | Dropouts, slow polling | Report by exception, link health alarm |
| Control centre and publication | Points mapped wrongly | End-to-end test from forced input to website |
Most EDM faults we are asked to look at are not sensor failures. They are logic and data problems: a threshold with no deadband that logs fifty one-second spills in a storm, an outstation clock that drifts by minutes, or a point that was renamed at the control centre and stopped feeding the public map.
Design rules we apply on EDM projects
- Define spill start and stop in the FDS, including hysteresis and minimum durations, and get the client's agreement in writing.
- Time-stamp at the outstation, not at the control centre, and synchronise clocks.
- Buffer events locally so a comms outage delays reporting rather than losing it.
- Alarm the monitor's own health: sensor fault, power, comms, and stale data.
- Prove the chain end to end by forcing a spill signal on site and timing its arrival at the publication system.
Worked example: a chattering spill signal
Picture a level sensor in an overflow chamber with the spill threshold set at the weir crest. In a storm the water surface ripples around that level, and with no deadband the logic sees spill, no spill, spill, many times a minute. The outstation fills its buffer, the telemetry link floods with events, and the public map shows a flickering overflow that looks worse than reality.
The fix is simple and should be agreed in writing with the client: a start condition above the crest held for a short confirmation time, a stop condition below the crest held for longer, and a minimum event duration. The exact values belong to the water company and its regulator, not the programmer, so put them in the FDS and test them in simulation with recorded storm data before changing a live site.
Legacy EDM installations
Many monitors were installed quickly to meet the 2023 deadline, often as a sensor and a logger bolted onto an existing site. As those sites are upgraded, fold the EDM function into the main outstation or PLC, with one clock, one event log and one point list. Fewer boxes means fewer failure points and fewer places for data to disagree.
When you merge them, keep the historic event record. Regulators compare years, and a change of logger should never look like a change in spill behaviour. Note the date and nature of every configuration change in the asset record so analysts can explain any step in the data.
Where EDM meets the bigger overflow programme
The Storm Overflows Discharge Reduction Plan requires improvements at overflows near designated bathing waters and high-priority nature sites by 2035, then the rest by 2050. Many of those improvements are storage tanks and pumping upgrades, so EDM logic will be rewritten alongside new process control. It makes sense to design both together. Our guide to sewage pumping station control upgrades covers the pumping side.
Improving wastewater treatment at Coniston
United Utilities
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If your EDM data is patchy, send us the outstation configuration, spill logic and a sample of event logs for a few sites. We will review the chain and propose fixes as a fixed package through our water and wastewater controls service. For communications upgrades, see telemetry outstation replacement, or contact us.
