Storm overflow assessment framework 2025
Published 24 March 2025
Applies to England
© Crown copyright 2025
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Objective
Government and regulators have been clear to water and sewerage companies (WaSCs) that the current level of use of storm overflows is unacceptable. This guide provides the framework to secure the necessary improvements.
This is Environment Agency guidance for assessing the performance of and scoping out improvements to storm overflows under Urban Waste Water Treatment Regulations (UWWTR) requirements.
The Storm Overflow Assessment Framework (SOAF) 2025 reflects:
- policy developments
- lessons learned from the 2018 framework
- greater event duration monitoring (EDM) and data availability
- the Storm Overflow Discharge Reduction Plan (SODRP)
It references, links and is informed by other guidance and regulations.
This SOAF investigation process is expected to be conducted outside of the Water Industry National Environment Programme (WINEP) process, but SOAF may raise requirements for future investment which might be included within the WINEP process. If the overflow is being investigated through PR24 (price review) – under the EnvAct_INV4 (Environment Act investigations into storm overflow ecological impact) driver – then it is expected that this SOAF investigation will be conducted alongside that investigation. The Environment Agency expects that investigations if triggered will be carried out over a period that is as soon as reasonably practicable.
This guidance takes learning from the implementation of the SOAF during the PR19 period (2020 to 2025) to improve and make the process more effective, particularly in the areas of:
- reducing the trigger threshold for high spill frequency overflows to reflect requirements to improve storm overflow performance
- linking to the most up to date process for water quality modelling investigations
- updates to related documentation including that on the process of the cost benefit assessment
The Environment Agency intends to review the SOAF guidance every 5 years to:
- ensure it remains fit for purpose
- take into account lessons learned
- reflect any new legislative or policy requirements
Overview of assessment framework
The SOAF is the Environment Agency’s guidance to WaSCs that sets out technical details and processes for prioritising the investigation and improvement of individual assets and should be read in conjunction with:
- the UWWTR
- the Department for Environment, Food and Rural Affairs’ (Defra) storm overflows guidance
The SOAF sets out how the “best technical knowledge not entailing excessive costs” (‘BTKNEEC’) test in the UWWTR should be implemented in practice.
Key obligations in the UWWTR include the requirement on WaSCs to design, construct and maintain sewer networks for agglomerations of a population equivalent of greater than 2,000 according to best technical knowledge not exceeding excessive costs (BTKNEEC). This includes the volume and characteristics of the wastewater, the prevention of leaks, and the limitation of pollution of receiving waters due to storm water overflows. The UWWTR supplement the Water Industry Act 1991, section 94 of which requires WaSCs to provide, improve and extend such a system of public sewers, and so to cleanse and maintain those sewers, as to ensure that the area continues to be effectually drained.
This SOAF 2025 is an update to the previous framework published in 2018. It has updated spill frequency investigation triggers, and links to the most recent water quality impact assessment methodology available. It includes a cost-benefit assessment to meet the UWWTR requirements to design, construct and maintain according to BTKNEEC. As it is triggered by spill frequencies, it contains an assessment of the reasons for high spill frequencies, which will inform solution types, regulatory response, funding route and timeframe. There may be other triggers for SOAF such as investigations triggered under the Water Framework Directive (WFD).
The first version of the SOAF was developed as WaSCs were partway through the programme of installing storm overflow spill monitors. All overflows now have EDM monitors installed, with the 12/24 method (Appendix A) used to calculate spill frequency. Spill frequencies are reported on an annual basis, with annual calendar year reports submitted to the Environment Agency by the end of February each year and published by the end of March each year. These reports will be used to identify storm overflows for investigation that discharge too frequently and could potentially be improved through the SOAF process. Overflows will then pass through a 5-stage process as outlined here.
Stage 1: overflows will be identified for investigation using the following spill frequency triggers depending on the time period of data for which high spill frequencies are recorded (Table 1), and the cause of the high spill frequency will be identified.
Table 1: spill frequency investigation triggers
| Time period of available data (years) | Investigation trigger (average number of spills per year) |
|---|---|
| 1 | greater than 30 |
| 2 | greater than 20 |
| 3 or more | greater than 10 |
Stage 2: the level of environmental impact will be quantified.
Stage 3: improvement options are assessed, including analysis of the costs and benefits.
Stage 4: a decision is made based on the cost benefit results.
Stage 5: delivery of the identified BTKNEEC solution (subject to appropriate funding and prioritisation) to reduce environmental impact and reduce the frequency of discharges.
This methodology is focused on inland overflows. A methodology for overflows to transitional and coastal (TRaC) waters will follow once ecological harm standards are agreed.
Data quality
You should ensure robust data is available to assess spill frequency. Where an overflow shows potential to exceed the spill frequency over the longer-term average, but a second or third data year is not available due to EDM failure, do not use the year with 0% EDM or similar in the average calculation and instead use the one-year 30 spills trigger or the 2-year 20 spills trigger. If the overflow exceeds the triggers with a EDM reporting percentage less than 100%, this would still trigger a SOAF investigation.
Do not ‘pro-rata’ EDM data where less than 90% operability is available, due to the impacts of seasonal variation.
Companies are incentivised to maintain EDM operability by way of an Ofwat performance commitment for storm overflow spills which includes an adjustment for unmonitored storm overflows. There is also an Environmental Performance Assessment (EPA) metric on EDM operation, which will additionally incentivise WaSCs. Therefore, the Environment Agency expects that any overflows which do not have sufficient EDM data available in any given year, will have sufficient EDM data in future years for an assessment against the SOAF trigger frequencies.
Which spill trigger should be used?
There are 3 spill frequency triggers for a SOAF investigation, dependent on the number of data years available. The trigger used should be based on the length of the dataset that is available for the EDM on the overflow. This dataset would reset and start again if there was a capital spill reduction improvement scheme for the overflow. The Environment Agency will also apply spill frequency permit conditions post-capital improvement schemes to protect spill performance.
If an overflow has at least 3 years of data, then the 10 spills trigger will apply. It does not matter what the performance is for each year within the 3 (or more) year period.
Treated storm overflows
Where an overflow is assigned to be a ‘treated storm overflow’ in the annual EDM return, the SOAF triggers and requirement for investigation still apply. Treated spills are not excluded from spill counts, and these overflows remain storm overflows until proven otherwise if subject to trial and are still subject to the UWWTR. Sites will only be excluded at present if they are accepted onto the treatment trials facilitated through PR24 and have an agreed spill frequency derogation. Storm overflows accepted onto these treatment trials need to have BTKNEEC assessments carried out as part of the treatment trials.
Links to other guidance to assess environmental impact of storm overflows
The SOAF, including the associated cost-benefit assessment, provide a delivery mechanism for storm overflow assessments to meet the statutory requirements of the UWWTR. However, there is other relevant legislation which is also applicable to storm overflows and WaSCs.
The SODRP sets out specific targets and desired milestones designed to enable progress towards meeting the long-term ambition of eliminating the harm from storm overflows. It mandates that all storm overflows need to meet set targets, not limited by the population equivalent served, and not subject to a cost benefit analysis.
The overarching targets of the SODRP are:
- by 2035, WaSCs will have improved all overflows discharging into or near every designated bathing water and improved 75% of overflows discharging into or near high priority sites
- by 2045, WaSCs will have improved all remaining storm overflows discharging into or near high priority sites
- by 2050, no storm overflows will be permitted to operate outside of unusually heavy rainfall or to cause any adverse ecological harm
The SODRP sets an expectation that WaSCs will ensure that their infrastructure keeps pace with increasing external pressures (such as urban growth and climate change) and prevents these pressures leading to greater numbers of discharges. Statutory Drainage and Wastewater (Sewerage) Management Plans (DWMPs) are a statutory obligation under Section 94A Water Industry Act (1991). It is expected that WaSCs will use DWMPs to plan for current and future pressures to proactively manage risks to (and from) their systems, including storm overflow compliance.
There is interaction between the UWWTR requirements for storm overflows and the requirements of the SODRP. The UWWTR, and associated case law, identify that all urban waste water must be collected under usual climatic and seasonal conditions, and a failure to treat urban waste water can only be tolerated where the circumstances are ‘out of the ordinary’. The case law acknowledges the footnotes in Annexes 1(A) and 1(B) of the Urban Waste Water Treatment Directive (UWWTD), which recognises that it is not possible in practice to construct collecting systems and treatment plants so that all waste water is capable of being treated. In practice, this means that discharges of untreated waste water from collecting systems and treatment plants should only be occurring in exceptional circumstances, unless remedial action would not satisfy the BTKNEEC test. As a result, the SOAF sets out a cost-benefit assessment which is the Environment Agency’s application of the BTKNEEC requirement.
The interaction between the UWWTR and the SODRP exists in the extent of the solution. When assessing storm overflow improvements to meet the BTKNEEC test, this may be less onerous than meeting the SODRP targets. However, in some cases, BTKNEEC schemes under the SOAF may deliver lower spill frequencies than the targets in the SODRP. UWWTR cost benefit assessments that conclude spill reduction to spill frequencies above the SODRP thresholds are overtaken by the tighter requirement of the SODRP in those cases. Where UWWTR cost benefit assessments show that spill reduction below the SODRP targets meet BTKNEEC, then the lower BTKNEEC spill scheme informed by UWWTR assessment over rules the SODRP targets. Separate and updated guidance will be issued in 2025 on how to undertake a cost-benefit assessment.
Figure 1: overview of storm overflows assessment framework stages
The following sections explain these stages.
Stage 1: why the storm overflow is high frequency spiller
Once a storm overflow has been identified as a high frequency spiller through EDM, the first stage in the assessment involves identifying the factors causing spills in excess of the investigation triggers. This stage will help to inform solutions to reduce spill frequency, and will focus resourcing for ecological harm investigations on those overflows where spill frequency is not the result of exceptional weather or asset maintenance issues, but is instead the result of hydraulic capacity issues. This will also focus solutions development on overflows where it is likely significant schemes will be required to reduce spill frequency, or where it is not an identified maintenance or operational solution that does not require options development to determine a solution.
Stage 1a: exceptional rainfall
Catchment rainfall should be reviewed to determine whether rainfall was exceptional during any of the years causing exceedance of the spill frequency thresholds. If rainfall was exceptional, mark the relevant calendar year and do not use it in assessment of spill frequency. Then assess the remaining years of data against the triggers. This is intended to avoid carrying out investigations where frequent spills have been driven by exceptional weather, in order to prioritise investigations to assets which spill frequently in more typical years.
‘Exceptional’ in this circumstance (Stage 1a) refers to the definitions used to categorise the weather in any given year for the sole purpose of prioritising SOAF investigations in a risk-based framework.
The following 2 options can be used to assess whether rainfall was exceptional.
Environment Agency water situation reports
Water situation reports are published each month at a hydrological area level and include rainfall statistics for the last 12 months. Rainfall is typically reported as a percentage of a long term annual average figure and categorised according to 7 classes, from ‘exceptionally low’ through to ‘exceptionally high’. The probability ranking used to define the categories for rainfall and other hydrological parameters is updated every 5 years as the period of long term observed data increases. If rainfall during the year for the hydrological area of the overflow was classified as ‘exceptionally high’, then that year’s EDM data is not used in assessing whether the high frequency trigger has been exceeded. The ‘exceptionally high’ category is defined as a value that is likely to fall within the band for 5% of the time. Water situation reports for England are available on GOV.UK.
Local rainfall records
Water situation reports provide an indication of exceptional rainfall at a hydrological area level. They may not be representative of some local catchments within that area. Consequently, an alternative is to use local rainfall data for the catchment where available and use this to estimate if rainfall during the EDM calendar year was exceptional (5% probability). This may be carried out to varying levels of complexity, but at an overall annual resolution. For example, the total depth of rainfall recorded by the gauge in the year could be compared to the long-term record to decide whether it was exceptional. Alternatively, the number of rainfall events of the same critical duration of the overflow could be counted and compared to the long-term annual average for those events. However, individual spill events or time periods within the annual dataset should not be excluded.
Stage 1b: asset maintenance
This stage is about investigating the asset, and potentially parts of the upstream and downstream catchment to determine if the high spill frequency is the result of a maintenance issue, or if it significantly contributes to the high spill frequency. If available, existing verified hydraulic models, survey data and maintenance records will assist with this assessment. Asset inspection could include one or more of the following aspects depending on the asset type and potential issue.
If maintenance issues are identified as the likely cause or contributing to frequent discharges, then these problems should be rectified. Timescales for resolving the issue will vary according to the problem, local constraints and the proposed resolution. However, with the exception of infiltration, the Environment Agency expects that most maintenance issues will be resolved within the calendar year that the investigation is triggered, or as soon as reasonably practicable for capital maintenance schemes (normally within 3 years). In all normal circumstances, maintenance funding is provided through base expenditure at price review processes. Where compliance issues can be addressed through basic operational changes, repairs and maintenance, this should not be delayed in favour of a longer-term build solution. Where infiltration is the issue, it is expected that you identify options to address this and reduce infiltration within your DWMP. It is also expected that you take reasonable improvement actions alongside investigating infiltration and other operational or asset maintenance issues.
If the overflow remains a high spilling overflow once the maintenance or operational issues have been rectified, the overflow should continue through the SOAF process.
Once the issues has been rectified, the annual EDM dataset which triggered the investigation should be archived and not used in future assessments against the triggers. Only exclude the data year where the maintenance issue was the primary cause of high spill frequencies. Evidence of the maintenance issue will be required to exclude datasets. This may include photographic evidence of poor service condition, such as from CCTV (closed-circuit television) surveys or manual inspections, level trend data from EDM indicating transient silt, and records of maintenance carried out (such as sewer jetting and pump repairs).
Storm overflows on gravity sewers
Inspection of the overflow chamber and any relevant flow controls, for example, orifice plates, penstocks and throttle pipes, to determine if there are any obstructions responsible for the high spill frequency. CCTV inspection of downstream sewers from the overflow to check for any service or structural defects that will reduce the hydraulic capacity of the continuation sewer, such as obstructions, debris or silt, and deformation or partial collapse.
Storm overflows at pumping stations
Where the pass forward flow of an overflow is controlled by one or more downstream pumps, they can be inspected for any service issues that might be responsible for the high spill frequency. For example, partial or soft blockages, worn impellors, failed reflux valves and leaking pipework. The condition of the rising main can also be inspected.
Storm overflows at sewage treatment works
For inlet and settled storm overflows at sewage treatment works (STWs), the downstream flow controls and treatment units can be reviewed to identify any issues that might be contributing to high spill frequencies. This might include inspection of pumps at inlet pumping stations, the condition of inlet screens, manual and automatic controls regulating flow to full treatment (FFT), such as actuated penstocks and variable speed pumps, and any service issues with downstream pipework or treatment units.
Infiltration assessments
Where existing long or short-term flow data is available, such as MCERTS (Monitoring Certification Scheme) certified data at STWs or flow data used to verify sewer models of the catchment, this data could be reviewed to see if the catchment has a strong seasonal flow response due to groundwater or rainfall induced infiltration. The Environment Agency’s water situation reports include an assessment of groundwater levels that may assist with investigations should infiltration be suspected. The reports categorise groundwater levels according to 7 classes from ‘exceptionally low’ through to ‘exceptionally high’, based on historic datasets from observation boreholes. In the case that infiltration is suspected or already known to be an issue in the catchment, infiltration studies should be carried out. Tools, such as the infiltration risk tool developed by the UKWIR (United Kingdom Water Industry Research) Strategic Infiltration project, could be used to prioritise surveys to parts of the catchment at most risk based on groundwater availability and pipe integrity (UKWIR, 2012).
You should identify if the infiltration is groundwater or surface water infiltration. This will impact on the types of solutions considered.
Crossed connections or misconnections investigation
Where there is the potential for the foul or combined system to interact with surface water drainage upstream of the overflow, investigations might assess whether misconnected surface water is the cause of high spill frequencies. For example, surface water sewers in poor service or structural condition on dual manhole systems may cause surface water to enter the foul or combined system.
Permit compliance assessment
Throughout this stage, you should assess if the overflow is compliant with the conditions in its permit, and if any non-compliance is causing high spill frequency. If there is any non-compliance identified, the overflow should be returned to compliance as soon as reasonably practicable.
Resolution of issues
If asset maintenance is identified as the cause of high spill frequency, you should resolve the issues as soon as reasonably practicable. This includes EDM and data issues.
Where an overflow has resolved operational issues as identified through a previous SOAF investigation, but spill frequencies remain higher than the triggers for a SOAF investigation, it is expected that the overflow will be taken forward into Stage 1c and beyond.
Stage 1c: hydraulic assessment
In the case that asset inspection does not identify reasons that are likely to be responsible for the high spill frequency, or that resolving the asset maintenance issues does not reduce spill frequency sufficiently, the following investigation of the hydraulic performance is required.
If a verified hydraulic model of the overflow is already available, this should be used to assess whether the high spill frequency is a genuine reflection of the permitted hydraulic design of the asset, and the amount of connected area contributing rainfall runoff. Alongside asset inspections carried out under Stage 1b, models may have already been used to determine that the high spill frequency is not due to maintenance issues.
Where a verified hydraulic model is not already available, a new model will be required to predict the performance of the overflow. A verified model is also likely to be required in order to quantify the environmental impact of the overflow under Stage 2. In order to have confidence in model predictions, models should be verified in accordance with the Chartered Institute of Water and Environment Management Urban Drainage Group Code of Practice for the Hydraulic Modelling of Urban Drainage Systems (CIWEM UDG, 2017). The EDM datasets will assist with verification.
Stage 2: if the storm overflow causes an environmental impact
The following impact assessment will be used to quantify the environmental impact of the storm overflow. It is divided into 3 main components:
- aesthetic impact including amenity and public complaint
- invertebrate (biological) impact
- water quality impact
Each of these components is scored and classified separately, depending on the available information, and will link to the cost and benefits analysis. The process is summarised as follows.
Stage 2a: the aesthetic assessment. A score and classification ranging from no impact to severe impact is assigned to the overflow.
Stage 2b: the invertebrate impact assessment. There are 2 criteria for routinely triggering a requirement for invertebrate surveys:
- where complex level 3 or 4 water quality modelling studies are triggered
- where it is possible to collect representative invertebrate samples upstream and downstream of the overflow
They are not required where:
- urban pollution management (UPM) Levels 1-2 are used for water quality modelling
- it is not possible to collect representative samples
An invertebrate impact classification will be assigned, ranging from no impact to severe. The invertebrate samples provide evidence of the degree of impact. Cases in which an invertebrate assessment may not be suitable include where the overflow discharges to a culverted watercourse or a deep river, where there are multiple discharges into the same reach (including final effluent), or where it is not possible to safely access the watercourse.
Stage 2c: an assessment of water quality impact. The water quality methodology is contained separately to this document. A flow chart sets out the methodology, which will link the SOAF assessment to that for the storm overflow assessments undertaken as part of the WINEP.
If the outcome of any stages shows a negative impact (ranging from Very Low to Severe), the overflow should be taken forward to Stage 3 cost-benefit assessment.
The environmental impact assessments under the SODRP consist of Stage 2c only. The definition of ‘adverse ecological harm’ in the SODRP is to meet the UPM fundamental intermittent standards (FIS) and 99 percentile standards, which require water quality modelling. Therefore, the Environment Agency has set out that Stage 2c investigations and SODRP environmental investigations will follow the same process.
Stage 2a: aesthetics assessment
The purpose of this component is to address the public acceptability of visible impacts from storm overflows, the amenity value of the receiving water, any history of substantiated public complaint, and any history of recorded pollution incidents due to storm sewage discharges. These 4 subcomponents are set out in Table 2. Each subcomponent is scored separately, and the individual scores summed to give a total score. The impact classification according to this total score is shown in Table 3.
Aesthetics surveys will be based on the FR0466 methodology (Foundation for Water Research, 1994), but will use the scoring system in Table 2. Where it is foreseeable that litter may be stranded and visible in areas downstream of the notional 50m survey area defined in FR0466, the survey should be extended to include this area. This will be important where the amenity class increases downstream of the immediate 50m reach. For example, where there is a park alongside the watercourse 300m downstream of the outfall, then this would be included in the aesthetics assessment.
Due to the potential effects of bankside vegetation on access, visibility and the potential for litter to collect, 2 surveys will be required to judge aesthetic impact. One survey should take place in late autumn, winter or early spring (November to April) when any bankside vegetation is minimal, and one survey should take place between late spring and early autumn (May to October). The worst score returned by the 2 surveys will be used.
Where more than one overflow impacts the outfall (such as where discharges are made to a common surface water sewer), judgement will be required in order to assign a score taking into account whether the overflows are screened and their comparative spill volumes.
Amenity is explicitly included in the impact scoring (see Table 2). As a result, moderate and high amenity sites will always trigger, as a minimum, a ‘very low impact’ classification even where there is no evidence of debris, public complaint or pollution incidents. It is included because 2 seasonal aesthetics surveys may not be sufficient to identify a problem. The overflow will always pose a risk of aesthetic impact and complaint in areas of moderate to high amenity.
Amenity will be selected according to the highest amenity class within 1km downstream of the overflow, or by using judgement as appropriate. For aesthetics purposes amenity categories are defined in Appendix B.
Pollution incidents recorded on the Environment Agency’s National Incident Recording System (NIRS) will be used in the assessment (Table 2). Only incidents attributed to legitimate wet weather discharges from the overflow should be included. These incidents will be recorded in NIRS with a pollutant type of ‘storm sewage’. Pollution incidents in dry weather, for example due to problems such as blockages, should be excluded from the assessment.
Table 2: aesthetics impact assessment
Table 2a: sewage derived litter (number of items) downstream
| Number of items downstream | Score |
|---|---|
| 0 | 0 |
| 1 to 10 | 5 |
| 11 to 25 | 10 |
| 26 to 50 | 15 |
| More than 50 | 20 |
Table 2b: sewage fungus on outfall (present or absent)
| Present | Score |
|---|---|
| No | 0 |
| Yes | 5 |
Table 2c: sewage fungus in downstream mixing zone
| % cover | Score |
|---|---|
| 0 | 0 |
| Less than 2 | 5 |
| 2 to 10 | 10 |
| 11 to 25 | 15 |
| 26 to 50 | 20 |
| More than 50 | 25 |
Table 2d: amenity
| Score | |
|---|---|
| Low or none amenity | 0 |
| Moderate amenity | 5 |
| High amenity | 10 |
Table 2e: number of validated public complaints related to wet weather discharges from the overflow
| Public complaints | Score |
|---|---|
| 0 | 0 |
| 1 to 4 | 10 |
| 5 to 9 | 20 |
| 10 to 14 | 30 |
| 15 or more | 40 |
Table 2f: pollution incidents due to storm sewage attributed to overflow by NIRS category
| Incident category | Score (per incident) |
|---|---|
| Category 3 | 20 |
| Category 2 | 60 |
| Category 1 | 100 |
Total score
There must then be a total score based on all of the aesthetics impact assessment tables.
Table 3: aesthetic impact classification
| Total aesthetic score | Aesthetic impact classification |
|---|---|
| 0 | No impact |
| 1 to 10 | Very Low |
| 11 to 25 | Low |
| 26 to 50 | Moderate |
| 51 to 75 | High |
| More than 75 | Severe |
Stage 2b: invertebrate impact
Two criteria define whether invertebrate sampling is required. You may also use it at additional locations where you deem it appropriate.
The criteria (both must be ‘yes’) are:
- where complex level 3 or 4 water quality modelling studies are triggered
- where it is possible to collect representative benthic invertebrate samples immediately upstream and downstream of the overflow
Impact will be assessed using abundance weighted Whalley Hawkes Paisley Trigg (WHPT) indices with the River Invertebrate Classification Tool (RICT). This is the method used for WFD assessments (UKTAG, 2014). The method is designed to detect impacts due to organic pollution and is also sensitive to toxic pollutants. The RICT was developed by the UK environmental agencies to classify the ecological quality of rivers. Access RICT3, the current version.
Invertebrate sampling is only appropriate in simple scenarios where there is a single storm overflow discharging to that reach of the river. Where there are multiple outfalls in close proximity, or other sources of pollution which could account for differences in invertebrate quality between sampling sites upstream and downstream of the outfall, then this method should not be used. In degraded urban watercourses where background or upstream invertebrate quality is already poor status then this method should also not be used.
Invertebrate sampling and analysis should be carried out according to Environment Agency operational instructions 024_08 and 018_08 (Environment Agency 2014, 2017). A minimum of 2 separate seasonal samples are required:
- one taken in the spring (March to May)
- one taken in the autumn (September to November)
The number of abundance weighted WHPT scoring families found during sampling (WHPT number of taxa (NTAXA)), and their individual abundance weighted scores for sensitivity to organic pollution are recorded. An average score per taxon (ASPT) for the sample is then calculated. The observed abundance weighted WHPT NTAXA and ASPT values are compared to the values that might be expected under undisturbed or reference conditions for that site. These undisturbed or reference scores are predicted by statistical models in the RICT software. The observed values of WHPT ASPT and WHPT NTAXA are compared to the predicted values to generate an Environmental Quality Ratio (EQR). EQRs close to 1.0 indicate that invertebrate communities are close to their natural state. The EQR ratios for different WFD invertebrate status classes are shown in Table 4.
Table 4: EQR ratios for WFD invertebrates status classes
| Invertebrate status class | WHPT NTAXA EQR values | WHPT ASPT EQR values |
|---|---|---|
| High | 0.8 or more | 0.97 or more |
| Good | 0.68 or more | 0.86 or more |
| Moderate | 0.56 or more | 0.72 or more |
| Poor | 0.47 or more | 0.53 or more |
| Bad | Less than 0.47 | Less than 0.53 |
During WFD assessments prediction and classification of invertebrate quality is carried out for each of the individual spring and autumn samples. A mean EQR is then calculated for the 2 seasons. Overall classification is based on the worst status class assigned for the multi-season mean WHPT NTAXA and WHPT ASPT. The RICT uses Monte Carlo processes to simulate uncertainty in observed and expected EQRs due to factors such as sampling variation, error in measuring environmental variables, and laboratory processing errors (bias). The software typically uses 10,000 ‘shots’ to build up a distribution of potential EQRs in order to estimate confidence of status class. To assess the impact of high frequency spillers, RICT Compare will be used to compare the quality of the upstream and downstream sampling sites. This shows the probability or percentage number of simulations where the downstream sample is in a different status class to the upstream sample for both WHPT NTAXA and ASPT. The scoring system in Table 5 and Table 6 will be used for both indices (WHPT NTAXA and ASPT).
Table 5: invertebrate impact scoring for WHPT NTAXA and ASPT
| % of simulations the downstream sample is one or more classes worse than upstream | Score |
|---|---|
| 1 to 4 | 1 |
| 5 to 9 | 2 |
| 10 to 29 | 4 |
| 30 to 49 | 6 |
| 50 to 70 | 8 |
| 71 to 90 | 10 |
| More than 90 | 12 |
The class multiplier is the number of classes the downstream sample is worse than upstream.
Table 6: invertebrate impact classification for WHPT NTAXA and ASPT
| Total score | Overall classification |
|---|---|
| 1 | No impact |
| 2 to 3 | Very low |
| 4 to 5 | Low |
| 6 to 7 | Moderate |
| 8 to 12 | High |
| 13 or more | Severe |
The worst score for WHPT NTAXA and ASPT should be used to assign impact. The scoring process will be repeated for each of the individual spring and autumn samples, and the overall mean of the seasons in order to produce a short-term and long-term impact assessment (Table 7).
Table 7: overall short and long-term invertebrate impact classification
| Type | Description | Value |
|---|---|---|
| Short-term | Worst single season classification result for WHPT NTAXA and ASPT | No impact to severe |
| Long-term | Worst of WHPT NTAXA and ASPT for the overall multi season (spring and autumn) classification | No impact to severe |
Where available, existing biological monitoring data for fish and invertebrates used for WFD classification may be used to provide additional evidence that the overflow is not causing an environmental impact. For example, where representative sampling points are present downstream of the overflow, in close proximity, or in locations likely to be sensitive to discharges from the overflow, and these consistently record good or high status, then this may be used as evidence to support no impact classifications.
Stage 2c: water quality impact
To assess the water quality impact, you should follow the most recent WINEP storm overflows environmental impact investigation guidance, as issued by the Environment Agency. For PR24 (2025-2030), this will be the EnvAct_INV4 methodology. This guidance will require assessment of the water quality impact of the overflow utilising a modelling approach and the UPM standards/manual.
To support this, you should use the most up to date supporting guidance available, such as the CIWEM UDG guidance, or the Urban Pollution Manual. Where available, Continuous Water Quality Monitoring data should be used to give confidence and support the modelling approach and results.
For PR24, the linkages and processes between the WINEP storm overflow environmental impact investigation guidance and the UWWTR SOAF are set out in Figure 2.
The scoring system as follows should be utilised, and taken forward into Stage 3.
Impact scoring
The worst water quality score from the 2 types of assessment (99 percentiles and FIS) should be used for the water quality impact classification set out in Table 8. The 2 types of assessment are set out in the following sections. Both 99 percentiles and FIS must be assessed.
Water quality impact classification
| Water quality score | Water quality impact classification |
|---|---|
| 0 to 5 | No impact |
| 6 to 9 | Very low |
| 10 to 19 | Low |
| 20 to 29 | Moderate |
| 30 to 39 | High |
| 40 or more | Severe |
1: 99 percentile quality
Two approaches are available, depending on the type of modelling tool used.
A: Estimate of 99 percentile
Select the relevant 99 percentile biochemical oxygen demand (BOD) and total ammonia standards for the receiving water according to WFD water body typology. These standards can be obtained from the third edition of the UPM manual. Where there is a drop in 99 percentile status class between the modelled upstream and downstream assessment points, assign a score of 45.
Where the overflow does not cause a drop in status class, but causes a degree of within class deterioration at the 99 percentile concentration, assign a score according to the percentage within class deterioration as shown in Table 9. Use the worst score returned for the BOD and total ammonia assessments.
99th percentile within class deterioration scores
| Percentage within class deterioration | Score |
|---|---|
| 1 to 10 | 5 |
| 11 to 25 | 15 |
| 26 to 50 | 25 |
| 51 to 75 | 35 |
| More than 75 | 45 |
B: Duration of exceedance
Where modelling tools are used which do not calculate a 99th percentile, but instead estimate the duration for which a 99th percentile standard is exceeded, then use the following scoring system in conjunction with the 99th percentile BOD and total ammonia standards for good status (Table 9). The impact duration with the worst score should be used.
Table 10: scoring system for duration and number of 99th percentile exceedances
| Impact duration | Allowable exceedances (number per year) | Score |
|---|---|---|
| 1 hour | 87.6 | + 0.50 points for every 1.0/year increase in exceedances |
| 6 hours | 14.6 | + 3.0 points for every 1.0/year increase in exceedances |
| 24 hours | 3.65 | + 12.0 points for every 1.0/year increase in exceedances |
2: Fundamental intermittent standards (FIS)
Select the relevant fundamental intermittent standards for the receiving water according to fishery type (sustainable cyprinid, sustainable salmonid, salmonid spawning). The FIS for dissolved oxygen and un-ionised ammonia are available in the third edition for the UPM manual (FWR, 2018). Compare the frequency of FIS exceedances in the receiving water with and without the storm discharge.
Use the following scoring system where the discharge causes a deterioration (increase) in the frequency of allowable exceedances.
| Frequency (return period) | Allowable exceedances (number per year) | Score |
|---|---|---|
| 1 month | 12 | + 1.5 point for every 0.5/year increase in exceedances |
| 3 months | 4 | + 4 points for every 0.5/year increase in exceedances |
| 1 year | 1 | + 6 points for every 0.2/year increase in exceedances |
Figure 2: water quality impact assessment for UWWTR SOAF - PR24 (Stage 2) and SODRP
Figure 2 as a flow chart
Figure 2 as steps
EnvAct_INV4 and EnvAct_IMP2 are PR24 WINEP drivers. EnvAct_INV4 investigations are investigations into storm overflows to determine the spill frequency at which they meet the no local adverse ecological harm target in the SODRP. EnvAct_IMP2 improvements are improvement schemes for storm overflows to meet the no local adverse ecological harm target in the SODRP.
Cluster storm overflows within 1km of each other in the same reach, or as per company judgement.
Cluster storm overflows within 1km of each other in the same reach.
1. Do any of the storm overflows exceed the 10/20/30 spills per year triggers dependent on EDM data years?
If no, go to Step 2.
If yes, go to Step 7.
2. Does the storm overflow spill more than once per summer?
If no, there is no study required. It meets the Environment Act INV4 requirements.
If yes, go to Step 3.
3. Is Q95 for any summer month less than 5l/s?
If no, go to Step 4.
If yes, the EnvAct_IMP2 requirement is to meet one spill a summer, or carry out a complex UPM study if agreed by the Environment Agency.
4. Is the ratio of the dilution factor greater than 2:1 (Q95 river flow: sewer dry weather flow (DWF)) when adjusted for climate change?
If no, go to Step 5.
If yes, the Environment Act INV4 is complete. Design the Environment Act IMP2 solution to meet 10 spills per year.
5. Carry out an Environment Act INV4 ecological harm simplified impact study (Levels 1 or 2).
Following this, continue to Step 6.
6. Does the study predict compliance with FIS and 99 percentiles, including sensitivity test with 10 spills design?
If no, carry out a Level 3 or 4 complex UPM study, or spill minimisation (such as 1 spill per year if agreed by Environment Agency). Then continue to Step 10.
If yes, continue to Step 9.
7. Is the Q95 for any summer month less than 5/s?
If yes, carry out an UWWTR SOAF – PR24 WQ assessment using simplified impact approach (Levels 1 or 2; Level 3 or 4 studies are optional). Then continue to Step 8.
If no, carry out an UWWTR SOAF – PR24 WQ assessment using simplified impact approach (Levels 1 or 2; Level 3 or 4 studies are optional). Then continue to Step 6.
8. Outcome A
EnvAct_IMP2 requirement is to meet one spill a summer or carry out complex UPM study if agreed by the Environment Agency.
SOAF – PR24. Complete Stage 2 harm and Stage 3 benefit assessments and deliver BTKNEEC solution (bespoke spill frequency design, may be below or above 10).
9. Outcome B
EnvAct_INV4 complete. EnvAct_IMP2 solution is 10 spills per year.
SOAF – PR24. Complete Stage 2 harm and Stage 3 benefit assessments and deliver BTKNEEC solution (bespoke spill frequency design, may be below or above 10).
10. Outcome C
EnvAct_INV4 complete. Bespoke spill frequency (no local adverse ecological harm) required for EnvAct_IMP2 solution identified by study (10 spills per year or less).
SOAF – PR24. Complete the stage 2 harm and stage 2 benefits assessments, and deliver BTKNEEC solution (bespoke spill frequency design, may be below or above 10).
Summary of Stage 2
It is expected that environmental impact assessments will take up to 24 months to complete. This will allow sufficient time to carry out seasonal invertebrate sampling and aesthetic surveys, and allow sufficient time to complete sewer and river impact modelling. Once the impact assessments are complete and the scores calculated, options for improving the overflow, including assessment of the costs and benefits of improvement options, will be assessed under Stage 3.
You should take the outcome of Stage 2 investigations forward into Stage 3 (and beyond) to assess the benefits of improvements.
Stage 3: assess options
UWWTR: cost-benefit analysis
Key obligations in the UWWTR include the requirement on WaSCs to design, construct and maintain sewer networks according to BTKNEEC for agglomerations with population equivalents greater than 2,000. This includes the volume and characteristics of the wastewater, the prevention of leaks, and the limitation of pollution of receiving waters due to storm overflows. BTKNEEC involves weighing the best technology and its estimated costs against the benefits that a more effective collection system may provide.
The SOAF requires a cost benefit analysis to be carried out to comply with the BTKNEEC test required under the UWWTR. The objective is to identify whether there are any BTKNEEC solutions that would reduce the frequency of storm discharges from overflows that are spilling frequently. The reduction in spill frequency is set according to the most effective solution that has a benefit-cost ratio (BCR) at the threshold set out in separate cost-benefit Practitioners’ Guidance (the ‘Action BCR’ – the BCR at which improvement is required to meet BTKNEEC). If no solution with a BCR of this threshold or above can be found, no improvements are required to meet the BTKNEEC requirements of the UWWTR. Cost-benefit assessments should be regularly reviewed to assess whether a solution can be identified in future, for example, where new technologies or benefits are found. These reviews should be a maximum of 5-yearly intervals.
Consequently, an economic assessment of improvement options for storm overflows will be made. This involves an ecosystems services approach, which identifies both the direct and indirect benefits of overflow improvement. The ecosystem services considered include a range of environmental, social and economic services, which have the potential to be impacted by storm discharges.
A detailed methodology and framework for carrying out the cost benefit assessment will be available from April 2025. Specific references to any values in the cost-benefit assessment will be contained within this guidance instead of the SOAF itself, for example, the BCR threshold for any required improvements. The Environment Agency has committed to reviewing the SOAF every 5 years, which includes the appropriate BCRs.
The cost-benefit assessment is required by the UWWTR to meet BTKNEEC requirements. Other measures for improving storm overflows have since been introduced which do not consider cost benefit. In some cases, the requirements of the SODRP may go beyond the BTKNEEC solution identified by the SOAF. In some cases, cost-benefit assessments will drive spill frequencies below those required by SODRP. Cost-benefit assessments should consider the required spill frequencies to meet legal obligations and policy ambitions, current spill frequencies, and assess a range of solutions to identify whether a BTKNEEC solution is available. If BTKNEEC solutions are not identified, you should consider the potential to refine models to include more complex studies to increase certainty, and additionally reassess on a regular basis (at least every 5 years).
It is important that assessments focus on the site or specific overflow under consideration. However, when deciding on options, WaSCs should consider a catchment approach alongside an asset by asset approach where appropriate. This is an expectation of other areas of planning, including DWMPs.
The investigation under SOAF as triggered by a high spill frequency may require BTKNEEC improvements to be brought forwards for earlier delivery. The delivery of the UWWTR scheme should be considered alongside that required for the SODRP, as combined delivery of both schemes may be most practical. This will minimise situations where multiple improvements are made to the same asset. There may be circumstances in which this approach is not feasible, or where it would result in unacceptable delays to compliance with the legislation. In these cases, a more modular approach may be more appropriate.
The current assessment framework involves 6 key steps. These are summarised in the followings sections. These may be updated, and you should refer to the latest version of the cost benefit assessment guidance.
Step 1: set decision making context
This step explicitly sets out the overall objective of the options to reduce the frequency of storm discharges and reduce environmental harm. In this context, the degree to which spill frequencies are reduced is set where further reduction no longer passes the defined Action Benefit-Cost Ratio (BCR). The BCRs relevant to the SOAF are contained with separately-issued SOAF Cost-Benefit Analysis (CBA) Practitioners’ Guidance. You may also consider that, where this spill reduction does not meet other targets, further reduction should be considered. This could be to meet the SODRP targets, or where it is important that a solution meets minimum water quality standards. Alternatively, the desired or target outcome may include other objectives, such as reducing flood risk. This initial step also sets out other key parameters which will affect CBA. These include the timeframe for assessment and implementation of solutions, discount rates, geographical scale, beneficiary groups to be considered, and approaches to uncertainty.
Step 2: identify options
Under Step 2 WaSCs will identify options for reducing spill frequencies, environmental harm and public health harm where appropriate, to meet the overall objective or desired outcome set under Step 1.
These options should be different in nature – for example a traditional storage tank solution and a green infrastructure approach. Green infrastructure should be encouraged, where this is evidenced as being more sustainable with added societal benefits, and where it can be shown to meet WaSCs’ legal requirements. Further guidance on the use of green infrastructure is available in the statutory DWMP guidance.
Step 3: option screening
The options identified under Step 2 are screened during Step 3 to remove any options that are unlikely to meet BTKNEEC requirements, and to ensure detailed CBA is focussed on the options where the net benefits are greatest.
Due to the inevitable uncertainty associated with estimates of costs and benefits, options with a benefit cost ratio higher than the Screening BCR Threshold (defined in separate CBA guidance) will also be taken forward for more detailed assessment under Step 4.
A sliding-scale of spill reductions should be considered, to identify if any solution meets the BTKNEEC requirement, from current spills down to 0 spills.
Step 4: detailed benefits assessment
Under Step 4, the direct and indirect benefits of the options identified at the screening stage are assessed in detail. The SOAF CBA Practitioners’ Guide provides guidance on how to carry out a more detailed benefits assessment, including potential ways to quantify and value benefits in monetary terms.
Step 5: collate results
The results of the detailed assessments for each option are collated under Step 5. This involves aggregating the estimates of the benefits over the chosen timeframe, and incorporating the costs involved in delivering the options. A choice is then made on the decision rules used in the economic assessment. It may not be possible for some benefits to be monetised. This is likely to be the case for potential impacts on human welfare such as employment and productivity, or mental health benefits. You should still include these benefits in any assessment, according to the process set-out in the CBA Practitioners’ Guide.
Finally, due to the uncertainty associated with CBA, sensitivity analysis is carried out on the results. This analysis will be relatively simple, and involve looking at the effect of changes to key parameters such as discount rates, assessment periods and cost estimates.
Step 6: take forward decision
Under this step options are refined to enhance their cost-benefit justification, and to ensure that the most economically efficient options proceed. The process of carrying out the benefits assessment and collating results may influence the options considered. For example, additional benefits may be realised by identifying further information requirements or adjusting options such as scale.
You should assess solutions to reduce spills down to a point of having BCRs at the Action BCR Threshold (defined in separate CBA guidance). This could mean that solutions go below the 10 spills SODRP target to meet UWWTR requirements.
Even where solutions are initially identified to be cost-beneficial you should assess further reduction until the BCR reaches the Action BCR Threshold.
Therefore, the solution identified may not necessarily be the most cost-beneficial, but rather the solution with the lowest spill frequency that still meets the defined Action BCR Threshold. This will drive solutions to the lowest spill frequency possible.
Development of options
Options development should aim to identify a number of options, including ones which would meet the Environment Act targets. Options should include those which seek to maximise green infrastructure based solutions. Solutions should be to reduce spills and environmental or public health harm where needed.
You should not include screen-only solutions for aesthetic impact in your cost benefit analyses, nor the costs or benefits of upgrading a screen to be included in the CBA. Screens for solid separation are required on all overflows under the SODRP and unscreened overflows will be classified as substandard under the UWWTR guidance. Therefore, it is not appropriate to consider the cost of screens in the SOAF CBA.
You should include all rainfall years in the rainfall time series when designing solutions. Do not exclude any years from the times series under the premise of exceptional rainfall.
Future pressures
When (co-)developing solutions and appraising solutions, current and future pressures should be considered to ensure schemes are ‘no-regrets’ and provide best value. Spill frequencies will be contained within Environmental Permitting Regulations (EPR) permits, which will secure long term continued performance of improvement schemes to new and evolving pressures (climate change, populations growth, urban creep and customer behaviours) within the drainage sewerage system. Guidance for statutory DWMPs sets the expectations for how to account for future pressures.
These principal future pressures include, but are not limited to the following.
Climate change
Climate change, and the likely impacts of extreme weather, should be considered when (co-)developing options, and also when considering how these options achieve multiple environmental and societal outcomes. It is also expected that WaSCs will ensure their infrastructure keeps pace with increasing external pressures, including climate change, without these pressures leading to greater numbers of discharges.
The primary considerations when planning for climate change are net zero (CO2) aspirations, adaptation and resilience. Climate change directly impacts the operation of storm sewage overflows and their impact on receiving waters. Periods of wet weather can be longer, rainfall can be more intense, all of which contributes to more prolonged or intense storm overflow events. These coupled with potential drier conditions which might lead to lower receiving water levels that are less able to accommodate shorter but more intense storm overflow events and warmer weather leading to warmer receiving waters will all likely result in increased harmful events of storm sewage discharges.
Population growth
Populations are growing and are becoming more urban. This, alongside changes to occupancy rates and demographic shifts, are placing increasing pressure on existing sewerage systems. WaSCs have a duty to maintain and develop their networks to meet current and future demand. By planning for future population growth and developments, WaSCs will be better placed to meet the long-term targets set out by the SODRP and those in the 25-Year Environment Plan.
Urban creep
Urban creep is defined as any increase in the impermeable surface of a developed area, subsequent to the original development and not including redevelopment. This additional impervious area will be drained to the existing drainage system, generally without any consideration of whether the capacity of the receiving sewerage system can accommodate the resultant increase in flow rates and volumes. This process is unpredictable and difficult to manage because of the lack of rigorous controls on the paving of driveways and gardens. The increased flow through sewerage systems needs to be considered and planned for when designing improvements for the long term.
Stage 4: decision
Following the assessment of options and the cost-benefit analysis carried out during Stage 3, a final decision should be made on when a solution will be delivered, in collaboration with the Environment Agency and Ofwat. You should clearly state the final decision in any reports. You should report to the Environment Agency the outcome of both stages of the 2-stage test and should provide all SOAF outcomes to the Environment Agency. A specific reporting mechanism will be developed, alongside the established reporting through the EDM annual return. The Environment Agency will share this data as appropriate.
If a BTKNEEC solution is identified, this should be delivered as a priority within a timeframe that is reasonably practicable (normally within 3 years as set out in Environment Agency guidance). Any exceptions to this timeframe will be considered on a case-by-case basis only.
You should keep the BTKNEEC assessment up to date, and review this periodically, at least every 5 years. The depth of a review will depend on what has changed since the previous assessment, and should be determined by WaSCs on a case-by-case basis.
Factors that may highlight the need for a review, which will help determine the scope, may include:
- a change in spill frequency or duration
- additional designations (such as a new bathing water)
- other changes in the catchment (such as development)
- risk or change identified in the DWMP
- new technologies
- new information on benefits or costs
The Environment Agency retains regulatory oversight of the SOAF process. The Environment Agency expects that your SOAF investigations are conducted to a high standard, with assurance where available. You should be able to provide detailed evidence of each stage completed, as well as a summary report. The Environment Agency will use these reports to review the findings of the SOAF assessments, and to ensure consistency in approach across the sector. The Environment Agency will work with WaSCs to produce worked example assessments and a standardised reporting format.
Appendix A: spill counting method
Spills should be counted using the 12/24 methodology. Counting starts when the first discharge occurs. Any discharges in the first 12-hour block are counted as one spill. Any discharges in the next, and subsequent, 24-hour blocks are each counted as one additional spill per block. Counting continues until there is a 24-hour block without a discharge. Counting starts again with the 12-hour and 24-hour block spill counting sequence for the first discharge after the 24-hour block without discharge.
The following diagram shows how this methodology can be applied. This example gives a spill count of 3.
Appendix B: amenity definitions (aesthetic assessment)
Amenity should be selected according to the highest amenity class for the affected receiving waters for a reasonable distance downstream as well as the immediate discharge vicinity.
High amenity
- influences area where bathing and water contact sport (immersion) is regularly practised (such as windsurfing, sports canoeing)
- receiving watercourse passes through formal public park
- formal picnic site
- designated shellfish waters
- designated bathing waters
- waters designated under the Conservation of Habitats and Species Regulations 2017 as Special Areas of Conservation
- sites designated under the Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat
Moderate amenity
- boating on receiving water
- popular footpath adjacent to watercourse
- watercourse passes through housing development or frequently used town centre area (such as bridge, pedestrian area, shopping area)
- recreation and contact sport (non-immersion) areas
- it is linked through substantiated reasons to a waterbody having a ‘reason for not achieving good’ (RNAG) for an element of the WFD
Low amenity
- basic amenity use only
- casual riverside access on a limited or infrequent basis, such as a road bridge in a rural area, footpath adjacent to watercourse
Non-amenity
- seldom or never used for amenity purposes
- remote or inaccessible area