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National Storm Overflows Plan for England

The water industry in England has published a National Storm Overflows Plan setting out almost 9,000 improvements to reduce spills from storm overflows. The Plan meets or exceeds all Government targets and represents the most expansive programme for overflows in the world.   

The links below will take you to a report, which outlines the contents of water companies’ proposals, results and timeline; there is also a link to our interactive map, which shows the current plan for every single storm overflow in England. This allows you to filter by water company, catchment, river basin and type of water body, among others, and also provides a high-level view of planned investment, forecast impact, and expected solution.
The data should be considered as a snapshot in time. Investment, projects and results will all continue to evolve as proposals move through the approval process with government and regulators and investigations into specific locations take place. We will provide further updates as the first phase of the Plan is finalised over the course of 2024.



Report (National Storm Overflow Plan)

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Interactive map



National Plan Data.xlsx

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  • Changed the title of the pie chart and popup information to be slightly clearer about what is being shown


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A storm overflow (sometimes referred to as a ‘CSO’ or ‘Combined Sewer Overflow’) is an outlet designed to allow excess rainwater to escape if sewers become inundated during bad weather. This prevents overloaded sewers backing up into people’s homes and flooding bathrooms. They have not been routinely built into sewers since the 1960s, but 14,187 of them remain in England.  

You can read more about overflows

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Spills happen when there is too much rainwater and sewage in a pipe. The process is similar to water in an overfilled sink escaping down the overflow drain – once it rises to a given level, it reaches an escape pipe and travels down it automatically, without human intervention.  

There are three ways this could be stopped:  

  1. by removing or bricking up outlet pipes. This would result in the system becoming overloaded and very large volumes of sewage routinely escaping out of sinks and toilets and flooding people’s homes.  
  2. by completely separating sewage and rainwater pipes (leaving just rainwater to escape back into the environment). This may be possible in a small number of cases but is not possible at very large scale as it would require building a second sewer running alongside the current system at the cost of up to £600 billion, which would add up to £1,000 to the average household bill.  
  3. by significantly reducing rainwater entering the sewer, for example by building thousands of large storage tanks and other improvements that enable sewers to deal with more pressure before overloading. This is the main focus of this plan – but, while huge improvements will be made, it is near impossible to eliminate all spills due to the amount of infrastructure required to deal with extreme rainfall and storms. 

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Over the last few decades water companies have invested £215bn in a huge range of upgrades for customers and the environment. That includes upgrades at waste water treatment works to remove up to 80% of key pollutants, in cleaning up beaches (which have seen a sevenfold increase in the number achieving the maximum "excellent" rating from the Environment Agency) and in drinking water, which is now among the very best and safest in the world.  

However, in 2023, the English water and sewage companies apologised for acting more slowly on the issue of storm overflows. Previous funding proposals to the regulator should have included proposals for more work in this area.  

The fact that they weren’t included reflects their smaller environmental harm compared to other sources of pollution. However, this missed the huge and understandable concern from the public about the impact overflows have in many areas.

The Plan being set out today confirms the most expansive investment ever in cutting spills and reducing their harm, and is designed to proceed as quickly as possible.

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A wide range of work is already underway. Water companies have been working with the Government already to make a head start on funding for overflows, with £180 million already being accelerated.

One notable project is the Thames Tideway Tunnel, one of the largest infrastructure programmes in Europe. This project is due to complete in 2024 and commence operation in 2025. Alongside other upgrades (including the £700 million Lee Tunnel) this will capture 95% of the volume of untreated sewage typically entering the tidal Thames.  

Other projects include £35 million from Southern Water to reduce spills across 30 overflows via new wetlands, improved rainwater management and sealing private drains that were becoming inundated; United Utilities’ overflow improvement programme to tackle discharges to Lake Windermere, part of £1.5bn investment to tackle over 150 overflows in the region; and £25 million from Severn Trent to upgrade Stroud’s sewer network with four miles of new sewer pipe and rainwater storage tanks the size of three Olympic swimming pools. 

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It’s not. In fact, comparing England, with EU comparator countries suggests that the situation here is generally far better. For example:

Germany, Denmark, Belgium, and the Netherlands all have far more overflows per person than England (Germany, for example, has around 66,000 compared to England’s roughly 14,000). France is not able to say how many overflows they have at all.

Unlike in the EU, England has clearer and stronger targets. Europe recently revised its legislation on overflows (known as the ‘Urban Waste Water Treatment Directive’) to include “indicative and non-binding” targets on ‘collected load’. Even these do not apply to smaller towns. There are no targets for public health or the environment.  

No other country has a programme equivalent in breadth, focus or scale to the National Storm Overflows Plan for making improvements.

Uniquely, England has full monitoring of all storm overflows since 2023 with all companies due to start publishing this ‘live’ for everyone to see in 2024. Almost no other country has full monitoring and we are not aware of any that provide this level of transparency.  

The UK invests more in water and wastewater than any other country in Europe, despite charges being about average. 

What are overflows?

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This Plan is industry’s most up to date set of proposals for dealing with the 14,187 overflows in England. It consists of a published document (which summarises the national picture) and an interactive map (which allows people to see action planned at the level of an individual overflow).  

The proposals in the Plan, and associated investment, are subject to approval from Ofwat (the regulator in charge of funding), the Environment Agency (in charge of environmental standards) and Defra (officials from which are closely scrutinising proposals).  

The Plan covers the period 2025 to 2050, with the first phase running to 2030.

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The intention is to provide greater transparency on what water companies intend to do, when they would like to do it, and how they wish to deliver an improvement.

The proposals in the plan have not yet been agreed by the Government and regulators, who are still scrutinising the underlying schemes. Normally that is a process that happens behind closed doors, but we think communities should get to see where proposals have got to and the current best expectation of how they will be delivered. That does mean, however, that there may be further changes to the programme, and we will provide updates accordingly. 

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The National Overflows Plan for England responds to the challenge set out by the UK Government’s Storm Overflows Discharge Reduction Plan, which covers England only.  

Water is a ‘devolved’ issue, and different frameworks (and environment regulators) are in place in each nation. Welsh Water explains its own approach.

Scottish Water explains combined sewer overflows, prioritisation of sewer overflows and their improving Urban Water Routemap

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There are four main types of projects (which in some areas will be delivered in combination):

  • building more storage within the sewer network to hold peaks of rainwater, and upgrading the capacity of sewage works. These prevent the system from overloading
  • installing ‘smart’ technology using sensors and artificial intelligence (for example to detect blockages before they trigger a spill) 
  • reducing the volume or speed of rainwater entering sewers (for example by replacing tarmac with planted areas such as ‘rain gardens’, allowing rain to sink into the ground rather than flooding drains) 
  • treating spills before they enter the main waterbody (for example by passing it through reed beds, which collect and stop much of the pollution).

In addition, mesh screens will be installed at the end of overflow pipes to prevent litter entering rivers.

The interactive map provides further information on the kind of project currently anticipated for each overflow. 

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These are projects that wholly or partly use green or ‘nature-based’ solutions such as areas of grassland or rain gardens or water butts to slow the flow of rainwater entering drains. They tend to offer many more benefits than ‘traditional’ steel and concrete projects - often by offering additional areas of habitat, and sometimes benefits too for carbon, flood alleviation or local beauty and amenity.

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The plan, and investment proposals for its first phase, are subject to regulatory approval. They therefore provide a snapshot in time of the ambition and detail of what water companies intend to deliver.  

We will provide an update on our expectations once the regulator, Ofwat, has given its initial view in June of funding for phase one (covering the period to 2030).  We will also provide an update on our expectations following Ofwat’s final determination expected in December 2024. 

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The Plan includes an interactive map, which allows anyone to select one of several geographical and organisational filters to view the proposed improvements in a given region. You can also go to individual company websites to find out more about specific regional plans:

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Our Plan outlines the most significant overflows programme in the world, with a tripling of current investment and an ambition in the first phase that we think is about the maximum possible given the physical availability of specialists and material across Europe to deliver the work.  

Within the programme, companies have prioritised work to make the fastest progress on overflows that are either:

  • having an identified impact on nature, or;
  • which spill into protected areas (like chalk streams or conservation areas), or;
  • which spill into designated bathing areas.

In some cases, a storm overflow may not currently be marked in the Plan for improvement as it already meets all relevant national targets (and is expected to continue meeting them over the years to 2050).  

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Releasing this Plan now is intended to allow everyone with an interest in industry’s plans, or individual water bodies or overflows, to understand companies’ best current expectation of future activity and the results. Waiting for final confirmation of funding from the regulator would mean no update on plans between September 2023 (when government put in place its targets) and early 2025. We have therefore taken the decision to publish this information, even in the knowledge that some of it may still change.

Changes are likely to arise for one or more of five reasons:

  1. In some locations, investigations are ongoing (or will start shortly) into specific overflows and their local impact. Where this confirms opportunities to do more, or detects that an overflow is having more of an impact than thought, this may require further improvements to be added to the Plan or changes made to how projects have been prioritised.
  2. Additional improvements are likely to be added over time where the Government decides to designate new areas for the purpose of bathing. This is likely to apply the stringent ‘public health’ target to more locations, which means that tighter controls will be applied.
  3. The Government has said it will formally review all overflow targets in 2027. While the outcome of that is hard to predict, it is quite likely to result in additional or different activity or improvements.
  4. In parallel with the improvement projects in this Plan, companies are also making day to day operational changes that help manage their networks in better ways. This can sometimes reduce spills – activity that may not always be captured by this data.  
  5. The Government could make a series of simple policy changes that could greatly accelerate action further - for free. We strongly urge them to make use of these (see final section). 

What is the Plan?

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Companies have proposed £10.2 billion for the first phase of the Plan, covering the period 2025-30.

This investment (and additional investment beyond this into the 2030s and 2040s) will typically be paid for upfront by companies, which use money from shareholders and the bond market to pay for projects. This money is then recouped by customers through customer bills.  

Because companies pay for investment upfront, this minimises the increase in bills and makes investment much more affordable. It is comparable to the difference between paying for a house in full in cash upfront versus slowly over time through a mortgage.

The exact impact on bills will vary by location but is likely to be around £13 per year per household, based on analysis by government. This compares to around £83 per year on the bill if companies did not fund the upfront investment themselves and costs were instead passed straight onto households.

The regulator will not allow bills to pay for any work that has already been funded in the past – there cannot be any ‘paying twice’ for the same thing. If a company has already been funded to deliver an improvement, then bills will not be used to pay for it again. 

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The figures reflect companies’ proposed ‘enhancement’ spend from 2025, which is the type of investment that typically involves some kind of infrastructure improvement. This is subject to regulators’ approval and may change following their consideration. The figures include projects taking place under the ‘green recovery’ or ‘accelerated infrastructure delivery’ mechanisms that are funded after April 2025. The figures also include projects delivered directly by a water company as well as projects that are funded by the company but delivered through the ‘Direct Procurement for Customers’ mechanism.  Funding is included where a company plans to commence a project within the period 2025-30, but where some of that funding may phase into the subsequent period (depending on ongoing discussions with regulators).   

However, the figures exclude any additional spend that may be taking place on monitoring, day to day operational improvements or environmental investigations. 

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No bill increase is ever welcome. Water companies completely understand that the very difficult economy of recent years has put huge pressure on many households. Although water bills are much smaller than most other utilities, at an average of around £473 a year they still put pressure on many households’ finances.

That is why every company offers a range of support to those who need it. Two million households now receive some kind of help with their water bills – nearly double the number last year – with assistance including payment breaks, debt forgiveness or bill reductions. There is also a special scheme – WaterSure – for people that have to use large volumes of water for reasons like medical conditions but who might struggle to pay. 


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A series of filters (see image) across the dashboard can be used to make a selection of a group of storm overflows. The CNTRL key can be used to make multiple selections within separate dropdowns (e.g to select 3 water companies). The default is to show all storm overflows – the national picture. The location of storm overflows in the selection is mapped up to a maximum of 3,500. Other filters allow selections by river basin, catchment, local planning authority, type of waterbody and prioritisation type. Subsequently all analysis is with reference to this selection.

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A series of data cards across the centre of the dashboard summarise key metrics (see figure).  The derivation of these metrics is explained in the pop-up accessed through the i symbol.  The data cards change dynamically if the selection changes. Improvements in terms of the annual average number of spills prevented is relative to a baseline. The baseline is either the count of spills from 2020 or the average of 2021 and 2022 if 2020 data was not available.  

The map (see below) also updates showing the location of each overflow and its status respective to the annual rainfall (spill) target.  The map can be zoomed to explore the exact overflow outfall location. Note that the map can only display 3,500 points at any one time so the user may need to zoom in to see detail. Selecting a single overflow from the map produces further information on that overflow in a pop-up.  

An example of a map in the dashboard


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This varies by water company with between 29% and 56% currently meeting the 2050 target.  

A chart in the interactive map can be used to explore these data (see below). Percentage compliance increases to 100% through time reflecting improvements implemented to meet the annual rainfall (spill) target.

rainfall targets

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Forecaste annual spills

A chart in the interactive map presents these data for the selection of storm overflows that has been made (see figure below). Averages reduce to less than 10 at end of the period, reflecting full compliance with the annual rainfall (spill) target. This chart also shows the additional requirements for delivery, showing that by 2035 75% of improvements to priority overflows must be made with 100% by 2045.

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This is the ‘universal’ annual average spill target common to all overflows regardless of location and is the maximum they must operate within. It is a requirement set by government for storm overflows to spill due to rainfall no more than 10 times per year on average by 2050.  

Spills are counted using the standardised “12/24” method which counts one spill for any discharge in the first 12 hours, with any discharge in the next and subsequent 24-hour blocks each counting as an additional spill. For example, a discharge of 60 hours would be counted as three spills.  

Locations where an average of 10 spills per year still results in predicted ecological harm will have a maximum allowed spill target that is likely to be lower than this but which will vary by location. A further public health target applies during bathing season (mid May to mid September) for storm overflows affecting inland and coastal bathing waters; again, the specific maximum spill number may differ by location.  

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The projections reflect each company’s view of future annual average spill levels. Companies have used a combination of methods including engineering assessments, hydraulic modelling and baseline projections to predict spills, but it is important to note that these represent the best currently available forecast and may change over time (particularly for those that are later in the programme). As the data develops companies will refine the design of projects, if necessary, to ensure the programme continues to meet all of its targets.

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Although there may be location-specific factors, it is also the case that more intense rainfall combined with ‘urban creep’ means that in some cases improvements may be needed simply to maintain the status quo.  

Over the last 25 years, urban land cover has increased by 30% and suburban land cover by 40% while the population has grown by 19%. In London alone, 12 square miles of front gardens are now under paving - the equivalent of 22 Hyde Parks. This causes much more rainwater to enter sewers, and the need for enormous storage tanks and other projects to prevent the system overloading.  

Companies look closely at projections like population growth and density and take this into account as part of their planning. 

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The interactive map provides an overview and necessarily uses general categories that can’t reflect all the nuances of a specific location. For more information, please consult individual water company plans. Links are available in the overview report.

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Water companies, together with the Environment Agency, have just concluded a multi-year program to install ‘event duration monitors’ on each of the 14,187 storm overflows across England. This approach has taken time and has been a gradual process, with some overflows beginning their monitoring years ago, and others only being installed towards the end of 2023.  

In 2020, only around 85% of storm overflows were monitored. For those 85%, we have simply taken the data from 2020 as the baseline against which to measure reductions. For the next 10%, we have taken an average of spills between 2021 and 2022 to give an estimate of how an overflow currently operates. Where only 2022 data is available then that is used as the baseline. This allows us to predict the impact expected in a given location. For the remaining 5% or so of storm overflows, the baseline is under assessment and will be updated soon as data is recorded.

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The monitoring programme for overflows was only fully completed in December 2023. That means there is a small number of overflows for which we do not yet have a full year’s data and so cannot say with confidence how they currently operate. This will be updated as their data becomes available. 

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Although questions about a specific overflow will need to be directed to the water company concerned, there is always significant year to year variation in spills due to the weather. Projections represent anticipated annual averages and there are circumstances where these might be higher than the starting point. All overflows will still meet all Government targets. 

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Questions about a specific overflow will need to be directed to the water company concerned; however, some overflows spill infrequently or not at all but continue to hold a permit for that purpose in case ever needed in the most extreme weather. 

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Although questions about a specific overflow will need to be directed to the water company concerned, generally, if a storm overflow has been marked as meeting targets despite a recent year (or years) exceeding the 10 spill maximum, then this is most likely to be the result of improvements taking place elsewhere (and not captured by the data).

In addition, targets are on the basis of an ‘average’ year. It is possible that a specific year was not representative of average performance – for example because of higher than average rainfall.

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Although questions about a specific overflow will need to be directed to the water company concerned, the ‘achieved target’ designation in the interactive map is based on it meeting the 10 spills target. Other targets will often require numbers lower than this, with the specific requirement varying depending on the location. 

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At some overflows it may be necessary to limit spills to fewer than 10 per year subject to an ecological assessment to check the impact it is having locally. Where this is suspected some companies have indicated that investigations are ongoing that may result in a different approach at that location.

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At some overflows the solution has not yet been fully determined and is conditional on the outcome of ongoing investigations. All solutions identified in the Plan are indicative in nature and liable to change once detailed design processes commence in the coming months and years.

The Interactive Map