Event duration monitoring (EDM): what it is and why it now covers every overflow

TL;DR. Event duration monitoring instruments every storm overflow so the start, end and length of each spill are recorded and reported, and it now covers 100% of England’s storm overflows, nearly 15,000 of them. In 2025 there were 291,492 monitored spill events, a 35% reduction on 2024, with total monitored spill duration down 48%. The Water (Special Measures) Act 2025 goes further, requiring emergency overflow discharges to be reported in near real time. This guide explains what changed and why it matters for anyone running their own outfalls, pumping stations or private connections.

Last updated: 2 July 2026

Key takeaways

• Storm overflow monitors record the start, end and length of every spill, and now cover 100% of England’s storm overflows, nearly 15,000 of them, per gov.uk.
• In 2025 there were 291,492 monitored spill events, a 35% reduction on 2024, with total monitored spill duration down 48%, according to gov.uk.
• In 2024 each storm overflow discharged an average of 31.8 times, the Environment Agency reports.
• The Water (Special Measures) Act 2025 requires emergency overflow discharges to be reported in near real time, within an hour, with monitoring rolled out to 2035.
• The same continuous approach applies to a single site: AQUAIOT Radar gives near real time level visibility at your own CSOs, outfalls and wet wells.

What is event duration monitoring?

Event duration monitoring is the practice of fitting a monitor to a storm overflow so the exact start, end and length of every spill event is recorded and reported, rather than estimated after the fact. The monitor detects when an overflow begins discharging, logs how long it runs, and feeds that record into regulatory returns, turning a discharge that used to be invisible into a timed, auditable event.

Storm overflows exist to relieve combined sewers during heavy rain, releasing rainwater and untreated sewage to a river or the sea rather than backing it up into homes. That was always the design intent, but for years there was no reliable count of how often or how long they ran. Event duration monitoring closes that gap by instrumenting the asset directly, so the operator, the regulator and the public see the real spill picture instead of a modelled guess. You cannot fix a spill you only hear about after it has reached the river.

What changed with EDM in 2025 and 2026?

What changed is coverage, transparency and the law: event duration monitoring now covers 100% of England’s storm overflows, the 2025 spill data shows a sharp fall against 2024, and the Water (Special Measures) Act 2025 has raised the reporting standard to near real time. Together these mark a shift from occasional, modelled reporting to continuous, instrumented evidence.

On coverage, the milestone landed first.

100% of England’s storm overflows, nearly 15,000, are now fitted with Event Duration Monitors, meeting the government’s end-2023 target.

Every one of those assets now reports its spills, so the national dataset is complete rather than sampled, which is what makes year-on-year comparison meaningful.

On the numbers, the direction of travel is clear. In 2025 there were 291,492 monitored spill events, a 35% reduction on 2024, and total monitored spill duration fell 48%, according to the government’s monitoring data. Fewer and shorter spills is exactly the outcome the monitoring was meant to drive, though the baseline remains high. For context, in 2024 each storm overflow discharged an average of 31.8 times, the Environment Agency reported in its annual returns.

On the law, the standard has moved again, and event duration monitoring is now the floor rather than the ceiling. Under the Water (Special Measures) Act 2025, emergency overflow discharges must be reported in near real time, within an hour, with that monitoring being rolled out to 2035. The reporting expectation is no longer an annual return; it is a live signal. That change in tempo, from once a year to within the hour, is the most important thing for any operator to absorb.

Why does it matter for estates and networks?

Event duration monitoring matters because the same accountability now reaching public storm overflows is coming for every asset that can discharge, including the outfalls, pumping stations and private connections that estates, campuses and industrial sites run themselves. If the regulated network has to prove its spills in near real time, the sites feeding and sitting alongside that network need the same visibility to manage their own risk.

A private outfall or a site wet well that overflows unseen carries the same reputational and environmental exposure a public overflow does, without the national monitoring behind it. A blocked screen, a failed pump or a rising main under pressure can start a discharge no one records until it is visible downstream. Continuous level monitoring at those points is event duration monitoring applied to a private asset, and it gives the operator the same early signal the regulated network now has. Our guide to storm overflow monitoring and our breakdown of combined sewer overflow monitoring cover the wider network picture.

There is an enforcement dimension to event duration monitoring too. As reporting tightens toward near real time, a timestamped, instrumented record is the defensible position, and a modelled estimate is not. For sites concerned about the direction of regulation, our analysis of wastewater enforcement monitoring in 2026 sets out where the pressure is building. The lesson from the national rollout is that instrumentation, not estimation, is what stands up.

How do you monitor your own outfalls?

You monitor your own outfalls by instrumenting each discharge point with continuous, non-contact level sensing and feeding the readings to a dashboard that alerts on a threshold. Event duration monitoring proves the model at network scale; at site scale it means a radar level sensor on each CSO, outfall or wet well, secure telemetry back to the cloud, and an alert the moment a level crosses the line that signals a spill is starting.

The AQUAIOT Radar Smart Water Level Monitoring sensor is built for exactly these points. It uses 60 GHz mmWave radar for non-contact level sensing accurate to within plus or minus 2 mm, penetrates foam, vapour and condensation, and stays reliable in enclosed underground chambers where floats and pressure sensors struggle. ATEX variants suit hazardous wet wells and chambers, and because it is non-contact it retrofits onto a live asset without interrupting flow.

From there the readings feed the AQUAIOT Cloud, which raises threshold and rate-of-rise alerts by SMS, email and app when a level climbs toward a spill, and exports audit-ready records for reporting. That is the same continuous approach event duration monitoring proves across nearly 15,000 public overflows, brought down to the handful that are yours, so private event duration monitoring reads exactly like the national programme at site scale. For coverage across drains, wet wells and rising mains, our sewer monitoring service deploys the Radar sensor as part of a wider telemetry and dashboard stack.

Frequently asked questions

What is event duration monitoring in simple terms?

Event duration monitoring is the practice of fitting a monitor to a storm overflow so the start, end and length of every spill event is recorded and reported, rather than estimated. It turns an invisible discharge into a timed, auditable event, and now covers 100% of England’s storm overflows, nearly 15,000 of them.

How many storm overflow spills were recorded in 2025?

In 2025 there were 291,492 monitored spill events, a 35% reduction on 2024, and total monitored spill duration fell 48%, according to the government’s monitoring data. Complete event duration monitoring coverage is what makes that year-on-year comparison reliable rather than a modelled estimate.

What does the Water (Special Measures) Act 2025 require?

The Water (Special Measures) Act 2025 requires emergency overflow discharges to be reported in near real time, within an hour, with that monitoring being rolled out to 2035. It moves the reporting standard from an annual return toward a continuous, live signal at the point of discharge.

Can I apply the same monitoring to my own outfalls?

Yes. The same event duration monitoring approach applies to any private CSO, outfall or wet well using a non-contact radar level sensor, secure telemetry and threshold alerts. AQUAIOT Radar gives continuous level visibility at your own discharge points, so a fault is caught as an alert rather than a downstream incident.

Event duration monitoring has done at network scale what every site owner should do at their own: turn an unseen spill into a timed, alertable event. If you run your own CSOs, outfalls or wet wells, talk to AQUAIOT about continuous level monitoring built on the AQUAIOT Radar sensor and the AQUAIOT Cloud, and fix the spill before it reaches the river.

By Gianbattista Porru, Digital and IoT lead at AQUAIOT.