Storm Overflow Monitoring: The IoT Technology Behind Environment Act Compliance

Last updated: 2 July 2026

TL;DR: Storm overflow monitoring is the near-real-time detection of when a combined sewer overflow discharges into a river, plus continuous measurement of the receiving water quality. Under the Environment Act 2021, water companies in England must report every spill within one hour and continuously monitor dissolved oxygen, ammonia, turbidity, pH, temperature and conductivity upstream and downstream. A LoRaWAN sensor-to-dashboard stack delivers this at low power and low cost.

Most articles about sewage spills explain the scandal. Far fewer explain the storm overflow monitoring technology that operators actually use to comply. This guide covers the duties, the sensors, and how continuous monitoring beats periodic spot sampling for evidencing what your assets are doing to a watercourse.

What is storm overflow monitoring and why does it matter?

Storm overflow monitoring is the automated detection of storm overflow discharges and the continuous measurement of the water they release into rivers, streams and coastal waters. It replaces occasional manual checks with a permanent, sensor-driven record that regulators, water companies and the public can all see.

It matters because the numbers are large and public. There are approximately 14,500 storm overflows in England, both on the sewer network and at wastewater treatment works, and the programme to fit event duration monitors on them was completed in December 2023 (GOV.UK). Every discharge is now counted.

The scale of discharge is why the technology exists. In 2024 the average storm overflow spilled 31.8 times, and only 12.5% did not spill at all across the year (Environment Agency). Monitoring turns that activity from an unknown into an auditable dataset, and it is the foundation for the multi-billion-pound reduction programme now under way.

Storm overflow monitoring also underpins where money is spent. Analysis from event duration monitors supported £10.2 billion of storm overflow upgrade investment, part of a wider £104 billion of water infrastructure spending over five years (GOV.UK). You cannot prioritise the worst assets without first measuring all of them, so monitoring is the prerequisite for targeted investment rather than an afterthought to it.

What does the Environment Act 2021 require for storm overflows?

The Environment Act 2021 places two distinct storm overflow monitoring duties on sewerage undertakers in England. Section 81 covers when and for how long an overflow discharges. Section 82 covers the quality of the water immediately around the asset. Together they turn a black box into a live, published feed.

Section 81 requires near-real-time reporting. An undertaker must publish that a discharge occurred, its location, when it began and when it ended, all “within an hour of the discharge beginning” and within an hour of it ending (legislation.gov.uk). Since 1 January 2025 this near-real-time duty has been in force.

Section 82 is the harder engineering problem. It requires undertakers to “continuously monitor the quality of water upstream and downstream” of storm overflows and sewage disposal works that discharge into a watercourse, capturing levels of dissolved oxygen, temperature, pH, turbidity and ammonia (legislation.gov.uk). The upstream and downstream pairing lets a single asset’s contribution be isolated from background river conditions.

The continuous water quality duty is phased across two regulatory investment periods: AMP8 from 2025 to 2030 and AMP9 from 2030 to 2035, with every qualifying storm overflow and treatment works expected to publish upstream and downstream data by 2035 (Proteus Instruments). Procurement and installation are happening now, not at the deadline.

How does event duration monitoring (EDM) work?

Event duration monitoring, or EDM, is the mechanism behind Section 81 and the oldest part of storm overflow monitoring in England. It records the number of times an overflow is used, the spill count, and how long each event lasts, the spill duration. A sensor at the overflow detects the start and end of a discharge and timestamps both, producing the data companies report annually and, now, in near real time.

In practice the detection element is usually a level sensor. When water rises past the overflow weir or crown and starts to spill, the level reading crosses a threshold and the event begins; when it falls back, the event ends. A submersible pressure sensor such as the Milesight EM500-SWL measures depth to an accuracy of plus or minus 0.5% of full scale with 0.01 m resolution, which is ample to define a spill threshold reliably.

The value of EDM is proven by the trend it now reveals. In 2025 there were 291,492 spill events across England, a 35% reduction on the previous year, and total monitored spill duration fell by 48% (GOV.UK). Without monitors on every asset, that reduction could not be measured or verified.

Which sensors are used for storm overflow monitoring?

Storm overflow monitoring uses two sensor families: hydraulic sensors that detect the spill itself, and water quality sensors that measure the receiving river. Section 82 defines the quality parameters, and each maps to a specific sensing technology that must survive a wet, dirty, remote outdoor environment for years.

The hydraulic side of storm overflow monitoring is straightforward. Level is measured with a submersible pressure transducer or a non-contact radar, and flow is derived from level in a known channel or measured directly. A LoRaWAN level sensor rated IP68 on the probe can sit in a chamber and report for up to 10 years on a single battery, which removes the cost of frequent site visits.

The water quality side is more demanding. Section 82 names dissolved oxygen, temperature, pH, turbidity and ammonia, with conductivity commonly added (legislation.gov.uk). These are typically delivered by a multi-parameter sonde, and manufacturers such as AquaIoT integrate optical dissolved oxygen and turbidity, ISE ammonium, and pH probes into a single instrument that reports over LoRaWAN or cellular.

Each parameter tells part of the story. A turbidity spike combined with falling dissolved oxygen downstream of an overflow is a classic signature of an organic pollution event, and ammonia is directly toxic to fish. Reading them together, continuously, is what makes the impact of a discharge visible rather than inferred.

Robust storm overflow monitoring hardware also has to survive where it sits. A submersible probe rated IP68, a transmitter rated IP67, and a decade of battery life mean a sensor can be dropped into a chamber or staked in a riverbank and largely forgotten (Milesight). Reliability in the field is not a nice-to-have here; a monitor that fails silently produces a compliance gap, which is exactly what the Act set out to close.

Continuous monitoring or periodic spot sampling: which is better?

For storm overflow monitoring, continuous measurement is better because it captures the event, whereas spot sampling almost never does. A discharge triggered by rainfall is short, unpredictable and rarely coincides with a scheduled bottle sample, so periodic sampling routinely misses the very moment it is meant to characterise.

The regulator makes the distinction explicit. Environment Agency guidance describes continuous water monitors as providing “continuous automatic measurements, with few if any gaps in the data produced,” while spot samples are “taken from a discharge at random time intervals” and give no coverage between events (GOV.UK). One is a film; the other is a handful of photographs.

For rainfall-driven pollution the gap is decisive. A grab sample taken on a dry Tuesday says nothing about a spill the following Saturday night. Continuous data, published hourly or every 15 minutes during high-risk periods, shows the onset, peak and recovery, which is exactly what Section 82 is designed to expose (Proteus Instruments).

Spot sampling still has a role for laboratory-grade confirmation and for stable, well-mixed discharges. But as the compliance backbone for storm overflows, continuous sensing is now the expectation, and the Environment Act has effectively legislated it into the operating model.

How does a LoRaWAN storm overflow monitoring stack deliver compliance?

A LoRaWAN stack delivers storm overflow monitoring by pairing long-range, low-power radio sensors at the asset with a gateway and a compliance dashboard that timestamps, stores and publishes the data. It is well matched to overflows because sites are remote, mains power is rare, and the data payloads are small.

The Sense layer is the sensors: a submersible level sensor to define spills and a multi-parameter sonde for water quality. Because LoRaWAN devices sip power, a level sensor can run for up to 10 years on one battery at a 10-minute reporting interval (Milesight), so a monitoring point does not need a power supply or constant maintenance.

The Connect layer is LoRaWAN itself. A single gateway can cover many square kilometres and hundreds of sensors, so a cluster of overflows along a catchment can report through shared infrastructure. Long range and low power are precisely what a dispersed, unpowered asset base needs.

The Manage layer is the dashboard, where storm overflow monitoring becomes evidence. Water quality readings are shown with EDM spill data overlaid so a discharge and its downstream effect sit on the same timeline, and the near-real-time feed can be published within the one-hour window the Act demands (Proteus Instruments). This is the layer that turns raw telemetry into an audit trail.

Between Connect and Manage sits the Secure layer, because a storm overflow feed is now part of critical national infrastructure reporting. Encrypted payloads and hardened gateways keep the data trustworthy from sensor to server, which matters when the output is a public, regulated record.

Frequently asked questions

What is a storm overflow?

A storm overflow, or combined sewer overflow, is a relief valve on a combined sewer that carries both sewage and rainwater. In heavy rain it discharges diluted, untreated sewage to a river or the sea to stop the network backing up into homes and streets.

Is storm overflow monitoring a legal requirement in England?

Yes. Under Section 81 of the Environment Act 2021, water companies must report storm overflow discharges in near real time, within one hour of a spill starting and ending. Section 82 adds a duty to continuously monitor water quality upstream and downstream of the asset.

What is the difference between EDM and continuous water quality monitoring?

EDM (event duration monitoring) records whether an overflow is spilling and for how long, using a level sensor. Continuous water quality monitoring measures the state of the river itself, including dissolved oxygen, ammonia and turbidity, to show the environmental effect of that spill.

What parameters must water quality monitors measure?

Section 82 of the Environment Act 2021 names dissolved oxygen, temperature, pH, turbidity and ammonia, and conductivity is commonly added. Monitors must be placed both upstream and downstream so a single asset’s contribution can be separated from background river conditions.

Why is LoRaWAN used for storm overflow monitoring sensors?

Storm overflows are remote and rarely have mains power, and the data packets are tiny. LoRaWAN offers long range, low power and low cost, letting a battery sensor report for up to 10 years and a single gateway serve hundreds of devices across a catchment.

Turn the duty into a working system

The Environment Act has made storm overflow monitoring non-negotiable, and the deadline for continuous water quality data across every qualifying asset is 2035 (Proteus Instruments). The organisations that treat it as an operational upgrade rather than a paperwork exercise will gain genuine visibility of their networks and rivers.

Indiott builds the full five-layer storm overflow monitoring stack for exactly this: Milesight and Browan sensors, resilient LoRaWAN connectivity, BlackBear security for the data feed, and AquaIoT dashboards that evidence compliance. Explore our water quality monitoring solution and our utilities and water networks work, or see how the same sensors support IoT flood monitoring across a catchment.