Event Duration Monitoring

Event Duration Monitoring (EDM) is a regulatory requirement to monitor the frequency and duration of releases from storm overflows, also known as CSOs (Combined Storm Overflows).

Since storm overflows are designed to be passive and operate without intervention, monitoring is important to understand how often this happens which is a requirement set by our environmental regulators. Welsh Water was the first water company to begin installing monitors on our storm overflows which we began back in 2015, several years before our regulators needed us to. This does mean Welsh Water has a lot more data.

Event Duration Monitors

We have EDM on over 99.5% of our overflow assets. The remaining sites are being addressed but will take a bit longer due to challenges such as accessing safely. In the majority cases our monitors help indicate a spill by using a technology called ultrasonic to measure the levels inside our network. There are other types of sensors used such as pressure sensors, flow sensors, pump run signals, and electrodes. The most suitable monitor is chosen depending on the structure of the particular asset, whether an underground chamber or an open channel.

How do we report our EDM data?

EDM data is reported annually to environmental regulators, Natural Resources Wales (NRW) in Wales & Environment Agency (EA) in England and contains 12 months of data. EDM is reported using an agreed method which standardises the way overflows are reported across the UK water industry.

This method is called 12/24 block counting. This method is described below.

The 12/24 spill block counting relates to the agreed reporting method we have with our environmental regulators (NRW & EA).

This method gives us a representative spill number that reflects the performance of the asset, i.e., more spills, the higher the count. The maximum number of spills is 366 in a year (or 367 in a leap year).

EDM data is recorded at 15 minute intervals using various signals, with readings being taken at 00, 15, 30, and 45 minutes past the hour. The 15 minute frequency we report is in agreement with both NRW and EA.

If the overflow has passed the spill point at that time interval, it will be recorded by the EDM. If a small spill occurred between the intervals, it would not be recorded by EDM. Each reading that indicates a spill is counted as the whole 15 minute interval, although the spill may have occurred for less time than this.

The first spill that is recorded opens a 12 hour window. Regardless of how long it spills for within that 12 hour window, it will count for 1 spill – this covers scenario such as:

  • It may be just one 15 minute interval reading
  • It may be a continuous spill, i.e., continuous readings taken every 15 minutes for up to 12 hours
  • It may start and stop during those first 12 hours (overflow, stop overflowing, overflow, stop overflowing etc)

All these variables are reported as 1 spill so it is important to consider duration alongside EDM data for context which will reflect the type of scenarios above likely to apply.

For example, high count but low duration would suggest multiple very short spills.

Once the initial 12 hour window finishes, a subsequent 24 hour window opens.

  • If no spill occurs within the 24 hour window, then the counting sequence is reset
  • If a spill does occur during the 24 hour window it will count as 1 additional spill, and another 24 hour window will be opened once this one has finished.
  • This is repeated until there is a 24 hour window that is free of spills when the counting cycle will be reset again.

EDM Timing

Other types of assets that can overflow

Within a wastewater network, there several types of assets that are designed to overflow in certain conditions; some of these are for emergency use only and other will react to very wet weather. You can learn more about the differences below.

  • Storm overflows (or CSOs) are on combined networks, that carry both wastewater and rainwater, and are designed to spill in wet weather conditions when the network is overwhelmed. These are designed to protect homes and businesses from internal wastewater flooding. These are usually passive and are legacy assets installed, in most cases, a long time ago. You can learn more about storm overflows here.
  • Storm overflows may also operate in emergency conditions - for example, if there is a blockage further down within the pipe, the flow will be restricted and it could cause a spill. We also use data gathered from EDM monitors to identify potential blockages and take proactive action on our network to reduce these risks.
  • Emergency overflows are primarily on assets that use power, such as pumping stations. They are designed to spill in the event of a failure, for example electrical, mechanical, blockage/collapse, rising main failure etc. Our wireless alarm systems will tell us if a failure happens so we can attend and take necessary action as soon as possible.

Unpermitted Overflows

You may also notice that our annual return includes storm overflows that do not currently have a permit as well as emergency overflows that act as storm overflows. We acknowledge these assets need particular attention and a solution as they are not working as designed or permitted to. We are actively working together with our environmental regulators towards bringing these assets into a solution, varying from applying for a permit or investigating if we can safely remove them.

2023 EDM Data

Event Duration Monitors (EDM) record the number of times and duration of time our storm overflows have operated. With monitors now on over 99.5% of our overflows, we are able to provide one of the most comprehensive reports on how our storm overflows perform. We report this information to our environmental regulators and also provide real time spill information for key bathing waters to interested bodies, including Surfers Against Sewage and Rivers Trusts

The higher number of spills in 2023 can be directly linked to the wet weather seen during the course of the year because storm overflows are designed to operate when too much rainwater enters the system so that homes and communities are protected from internal wastewater flooding. 2023 was one of the wettest on record with 10 named storms and eight months seeing above average rainfall – March for example was the wettest for 40 years in Wales and England with 200% of its average rainfall.

As a country on the western side of the UK, we have some of the highest levels of rainfall and are seeing an increase in severe events linked to climate change. We also have more assets per head than other water companies given our population spread and topography.

The total removal of combined storm overflows from our system is unaffordable and would take decades and therefore is not an option but what is in our control is the ability to target investment to CSOs which have the biggest environmental impact. That is why we are investing significantly to improve CSOs with £140m being invested between 2020-2025 and a further £420m planned from 2025 to 2030. In addition, we are awaiting Ofwat’s Draft Determination on our Business Plan for 2025-30 which proposes £4bn billion investment to improve our performance and services to customers. This includes £2.5bn investment in environmental services – 84% more than across 2020-25.

You can learn more about storm overflows and view our EDM map including last year’s data below.


Annual uSO EDM return DCWW 2023

63kB, XLSX

Annual EO EDM return DCWW 2023

51.8kB, XLSX

Annual SFW SO EDM return DCWW 2023

448.3kB, XLSX

EDM Return DCWW Emergency Overflow Annual 2023

179.3kB, XLSM

EDM Return DCWW Storm Overflow Annual 2023

202.1kB, XLSM


Annual Emergency Overflow Annual EDM return - DCWW 2022

192.8kB, XLSM

Annual Storm Overflow EDM return - DCWW 2022

196.3kB, XLSM

NRW Annual Emergency Overflow Unpermitted Storm Overflow EDM return - DCWW 2022

87kB, XLSX

NRW Annual incl Shellfish Waters Storm Overflow EDM return DCWW 2022

369.3kB, XLSX


EDM Return Dwr Cymru Welsh Water Emergency Overflow Annual 2021

51kB, XLSX

EDM Return Dwr Cymru Welsh Water Storm Overflow Annual 2021

50.3kB, XLSX

EDM Return DCWW_Wales Water Annual 2021

270.8kB, XLSX