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E X PL A I N S

Sewer Modelling and CSOs

E X P L A I N S

Sewer Modelling

and CSOs

In this report:

1 What causes combined sewer overflow spills?

2 What are the main problems caused by CSO spills and why is it imperative for utilities to reduce them?

3 What is the most effective way for utilities to monitor their sewerage networks?

4 How are hydraulic models used to predict CSO incidents?

5 What action can utilities take if there is a warning of an imminent spillage?

6 What other advantages do sewer network models offer?

7 What financial benefits can utilities gain from investing in sewer modelling technology?

8 Are the predictions from hydraulic models reliable?

9 Do utilities require specific in-house expertise to make effective use of sewer modelling technology?

10 How has the technology evolved over recent years and how might it develop in the future?

11 Where is sewer modelling being used and what impact has it had?

C A S E S T U D Y

How live sewer modelling helps reduce risk to public health & safety for Anglian Water

C A S E S T U D Y

How the London Olympics initiated a step-change in Thames Water’s approach to modelling

Tightened environmental regulation has meant increased scrutiny on combined sewer overflows (CSOs) in recent years, with the water and sewerage companies under pressure to reduce the frequency with which diluted sewage is spilled into the sea and watercourses in wet weather.

Whilst all CSO discharges may have historically been viewed as inevitable and unavoidable, hydraulic modelling technology can now go a long way to help utilities monitor, understand and predict when they will happen and to intervene accordingly.

In this educational guide from Utility Week, we look in detail at the problem of CSO spills, how technology can help address the issue and the various other benefits that can be gained through the use of sewer modelling.In association with

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2 What are the main problemscaused by CSO spills and why is it imperative for utilities to reduce them?

CSOs discharge diluted sewage directly into the sea and watercourses, which poses a significant problem both environmentally and to the reputation of the water industry and particularly the utility whose asset is discharging.

A WWF report published in 2017, entitled ‘Flushed Away’, observed that 80% of rivers across England and Wales fail to meet the required ‘good’ ecological status and that, although assessing the impact of pollution from sewage is not straightforward, wastewater pollution is thought to be a contributing factor in 55% of those cases.

The Environment Agency’s chief executive, Sir James Bevan, warned in May 2019 that pollution represented the “glaring exception” to the water industry’s success in the years since privatisation, adding: “If companies cannot operate without damaging the environment, they will rightly lose their social licence to operate.”

CSOs require a permit from the Environment Agency to discharge, with the permit usually including conditions to mitigate the impact of a storm spill into the receiving water, along with monitoring and reporting requirements.

flooding of surrounding homes, businesses and urban areas. Instead, they discharge the excess water to a specified point, usually a watercourse or directly into the sea.

According to the Environment Agency, water companies discharged raw sewage into rivers in England more than 400,000 times in 2020. Data shows that the average number of spills per storm overflow was 33 times in 2020, compared to 29 times in 2021. The figures also show 5% of storm overflows recorded spills of 100 times or more in 2021, while 13% of storm overflows did not spill during that period.

The pressure on combined sewer systems is likely to increase over the coming years due to climate change threatening to cause more frequent extreme rainfall events. This, combined with population growth and increased paved area draining to the sewers, commonly known as urban creep, are causing a significant impact on network capacity in many areas.

1 What causes combined sewer overflow spills? Combined sewers collect surface runoff such as rainwater along with wastewater as part of the same system.

They were introduced in the middle of the 19th century, when – in the absence of widespread methods for effective wastewater treatment – it made sense to save money by making use of a single-pipe system that discharged the combined effluent from toilets, industrial wastewater and surface runoff into receiving waters such as the sea.

As wastewater treatment plants became more commonplace from the early 20th century onwards, they were introduced into the combined sewer systems to clean sewage before it was discharged.

However, the capacity of the sewer network is frequently exceeded when there is excessive surface runoff, most commonly during heavy storms.

CSOs are designed to alleviate extreme high levels in the sewer network following large rainfall events, which would otherwise potentially lead to

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information relating to the data on relevant assets such as pipes, manholes, pumping stations, CSOs and other components of a sewer network.

Once the infrastructure has been modelled, the utility must build an understanding of the flows entering the system, which will include the volume of wastewater entering the network from homes and businesses; infiltration from groundwater, which may result from cracks in the infrastructure; and the proportion of rainwater falling on the roofs, roads and green spaces entering the network.

The hydraulic model can be used for long-term planning as part of a Drainage and Wastewater Management Plan (DWMP) to evaluate capacity and assess the resilience of the drainage and wastewater system.

3 What is the most effective way forutilities to monitor their sewerage networks?

The Environment Agency has introduced a programme to install Event Duration Monitors (EDMs) on the vast majority of CSOs by 2020. The EDMs monitor the CSOs every two or 15 minutes, depending on the sensitivity of the receiving water. The intention is that they will help utilities determine where improvements are needed – for example, if a location is subject to a high frequency of spills, there may be a need to reduce inflows through nature-based solutions, including SuDS (Sustainable Drainage Systems) or even build additional capacity into the network.

To gain a greater perspective of how sewer networks respond to rainfall events, though, utilities can make use of hydraulic models that provide a digital representation of their physical sewerage infrastructure, which are built using asset

The Environment Agency can vary permits to further protect the environment when required, and any non- compliance is subject to action from the regulator.

Following changes to sentencing guidelines in 2014, water utilities can face unlimited fines for serious pollution incidents in addition to other sanctions, and a number of these events have made national headlines in recent years.

In 2022, the UK government published its Storm Overflows Discharge Reduction Plan, bringing in robust targets on sewage pollution. Since 2015, the Environment Agency has concluded 56 prosecutions against water and sewerage companies, securing fines of over £141 million.

At present, money from fines imposed by Ofwat and those arising from Environment Agency prosecutions is returned to the Treasury. Under the new plans, ringfenced funds will go to the Department for Environment, Food & Rural Affairs (Defra) and will be invested directly back into environmental and water quality improvement projects.

In addition, £1.9 billion of funding has been allocated during 2020-2025 to support the sector in reducing storm overflows.

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consuming to set up but offers an unrivalled depth of understanding.

Most commonly used on catchments that are prone to regular flooding, where there is cause for concern relating to spills to a receiving water, or where there is critical infrastructure where flooding could have a significant impact, online models can drive existing hydraulic models in near real-time by ingesting data from SCADA databases, telemetry data from loggers, and historical and forecast radar rainfall data.

The constant stream of live information allows utilities to predict what is likely to happen within their networks as it happens.

For example, the system can highlight anomalies between the live telemetry data and the modelled data – which might be the result of a blockage, ingress into the pipeline or a failing asset – and provides the utility with an opportunity to take action before more serious problems occur.

One advantage of using online models is the ability to trigger alerts, based on variables such as flow or level, if there are indications that the system capacity is set to be exceeded.

importance of the catchment and the suitability of the model for the purpose it is being used for.

Offline models have a variety of uses, including estimating the impact of new developments on CSO detriments (Development Impact Assessments), helping the utility to understand the causes for previous spills and – when used in conjunction with weather forecasts – providing guidance on what may happen in a future situation.

Nonetheless, they are not the answer in every situation.

Some geographical areas have a greater seasonal variation in inflows than others. So, in some cases, a flow survey conducted during the summer may not provide the same quality of insight when used to consider what may happen in the winter, for example.

These limitations can be overcome either by carrying out further flow surveys to understand a greater range of variables or through the use of the ‘online’ model, which is more complex and time

4 How are hydraulic models used to predict CSO incidents? Once utilities have a hydraulic model which incorporates both the physical assets and data relating to inflows, they can start to run simulations using a mathematical engine which allows the sewer network response to different weather events to be predicted.

There are two basic options when establishing a digital hydraulic model of the sewer network. The most common option is the ‘offline’ model, in which the predicted network response can be generated from either observed, assumed or synthetic data.

To ensure that the models are providing accurate predictions, they must be calibrated against observed data. To do this, the utility will usually run a flow survey over several months by placing a large number of monitors in the sewer network to establish the flows and levels at various locations around the network. To keep these models up to date and maintain sufficient accuracy, utilities must conduct new tests every few years, with the length of time between surveys likely to be determined by the

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existing network’s response to future events. This helps the utility to plan how to manage those risks.

By creating a picture of how the sewer network performs in either a historical or live modelling capacity, the utility can also identify opportunities to prioritise operational maintenance.

Where even greater insights are needed, perhaps in more sensitive areas or areas where there may be several potential sources of contamination to the receiving watercourse, it’s possible to develop the model further with either or both of two key model enhancements.

The first enhancement is to add water quality parameters to the sewer network model. This gives

6 What other advantages do sewer network models offer? The modelling technology can offer benefits to anyone at a utility trying to gain an understanding of the hydraulic behaviour within a sewer network, ranging from engineers and operations managers to geospatial planners and those responsible for asset management and capital schemes.

An additional benefit of these network models is the ability to understand whether the network has enough capacity to handle new flows, such as when a housing development needs to be connected. Being able to add the proposed network into the existing models prior to the development’s drainage being adopted by the utility provides an understanding of the likely impact it will have on the

5 What action can utilities take if there is a warning of an imminent spillage?

Hydraulic models can be used to assess a series of ‘what if?’ scenarios and determine ahead of time how differing operational strategies will impact the network response to a given event.

If an online model provides a warning that levels are rising above a given threshold, the operator must determine whether it is possible to prevent a CSO spill without causing problems elsewhere.

If there is available capacity elsewhere in the network, such as within offline storm tanks, or even within the pipes themselves, flow can be routed to these locations if the necessary flow control devices are available. Once the event causing the possible surcharging of the system has passed, the stored sewage can be passed forward to the wastewater treatment plant as normal once the level of surface runoff has subsided.

Even if the utility does not have any realistic means of preventing a CSO spill, having awareness prior to an incident can still help the utility to take action to mitigate problems, with the opportunity to issue early warnings.

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8 Are the predictions from hydraulicmodels reliable? Whether utilities choose an offline or online option, a hydraulic model is only as good as the data it is built from.

If it is operating with inaccurate data, its ability to make accurate predictions will be compromised.

Clearly, the higher the proportion of data derived from reliable sources, the greater the level of confidence in the model, but the costs of carrying out physical surveys can quickly mount up and it is not realistic to expect to have 100% confidence in the data on every asset. For example, if there are tens of thousands of manholes in the catchment, the utility may choose to survey those manholes in the most critical areas and then rely on old asset data, interpolated data, assumed data or engineers’ judgements for the rest.

7 What financial benefits canutilities gain from investing in sewer modelling technology?

Hydraulic models of sewer networks have long been used for the analysis of current and future network response. Population growth, urban creep and climate change are three catchment drivers that add increased flows into the existing sewers. Having a model that enables users to try different options, such as additional storm tanks, upsizing of pipes and uprating of pumps, can help utilities find the most cost-effective ways of ensuring that the network can continue to deliver the service required, without the risk of incurring possible regulatory fines associated with additional CSO spills.

Alternatively, the utility might fail to appreciate the extent of the impact the new scheme will have on the network and suffer the consequences later down the line, which could include regulatory fines from additional CSO spills.

A further advantage of sewer modelling is the ability to use the proposed capex and opex costs of a flood mitigation scheme in conjunction with the scheme’s hydraulic modelling flooding results to derive a cost-benefit analysis and produce the most effective scheme possible.

the model the capability to predict not only spill volumes and durations, but also the concentrations of water quality determinants such as BOD, ammonia and phosphorus, for example. A water quality model can also have operational benefits. By modelling sediment transfer in the sewer network, it’s possible to predict where sediment is likely to build up and over what period, which can help inform proactive maintenance routines. Proactive cleansing before a blockage occurs downstream of a CSO could further reduce the risk of spills.

The second enhancement would be to create an integrated model which includes both the sewer network and the watercourse in the same model. This enables the modeller to examine interactions between the sewer network and the watercourse. If there are several CSOs along the same river reach, it’s possible to gauge the individual and cumulative impact of the overflows on the watercourse more accurately. If the integrated model is further expanded to include the upstream catchment, it could also be used to represent other contributors, such as agricultural runoff, to gain a more complete picture of all sources of contamination.

By creating an integrated model, with water quality enabled, modellers can predict not only source concentrations, but also factors such as dilution and the fate of any contaminants.

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to consider various potential solutions as part of design option development.

Integrated modelling, which moves beyond a siloed approach to sewer modelling or river modelling and considers key flow paths across a whole catchment, has also emerged as a valuable option over recent years.

Rather than just taking into account one dimension (1D), as with a sewer or watercourse model, or two dimensions (2D), such as a pluvial runoff and overland flow model, there are models that are able to integrate both types (1D/2D integrated models), providing an overarching view

10 How has the technology evolved over recent years and how might it develop in the future?

Since computers were first used to aid hydraulic modelling, there has been a huge improvement in computer processing power, which enables utilities to work through potential scenarios at a much higher rate, while there has also been a similarly substantial and rapid development in the field of data. With the advances in sensor and communications technology and the introduction of the Internet of Things (IoT) in drainage networks, multiple stakeholders have been able to generate and receive far greater levels of data and at much higher resolution.

That has driven a major evolution in the use of hydraulic models, with utilities now commonly able

CIWEM’s ‘Code of Practice for the Hydraulic Modelling of Urban Drainage Systems’, published in 2017, acknowledged that there is a degree of uncertainty that results from the modelling process given the number of data inputs and the complex numerical calculations that transfer physical processes into a mathematical form.

Utilities will often create models with a particular purpose in mind, such as tackling repeated flooding in an area or understanding the flows and trigger points through the network, and CIWEM stated: “It is important that the commissioning body defines the required confidence levels for the specific purpose. Setting the levels too high will result in an unduly expensive model, whereas levels set too low may result in a model that does not meet expectations. In most instances, budget constraints will have to be taken into account in defining the data collection and verification requirements.”

9 Do utilities require specific in-house expertise to make effective use of sewer modelling technology?

Although the utility will have an in-house team of experts to build and manage their library of hydraulic models and use them to develop operational strategies, many will also use third-party consultants, both to gain further expertise and to help cope with the volume of work required. If utilities are planning to use hydraulic models to make operational decisions, it is once again important that the team making those decisions understands the assumptions and principles not only of the software package used to build the models, but also the assumptions and principles that have been used when building the models.

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11 Where is sewer modelling being used and what impact has it had?

Hydraulic modelling of sewer networks is well established, having been used by a wide variety of water utilities and municipalities around the world over a long period of time.

While quantifying the impact of specific issues such as CSO spills can sometimes be difficult given the number of factors in play, it is clear that the models have vastly improved utilities’ ability to make decisions that not only reduce the number of pollution incidents but also cut capex and opex costs.

The use of such technologies can help utilities provide post-mortem analysis of any incidents to the regulator, which can lead to reduced fines and an improvement to their reputation.

of how the catchment behaves hydraulically above and below ground level.

To create an advanced digital twin of the physical system, specialist integrated catchment modelling software may be used to allow the utility to model the complex hydraulic and hydrologic network elements. InfoWorks ICM, for example, allows users to model above- and below-ground assets – such as manholes, pipes, inlets, bridges, sluices, weirs and pumps – to create a model of 1D infrastructure as well as 2D overland flows. Such programs are also designed to seamlessly integrate with many packages already used by utilities to ensure efficient model build processes.

Integrated modelling could play an important role in the UK water sector in the coming years, particularly with the Drainage and Wastewater Management Plans set to form part of the next price review period in 2024 and bring increased focus on whole-catchment thinking.

However, creating an integrated model is a challenging process, since it inevitably results in an increase in modelling costs and project time, and requires the involvement of a variety of stakeholders. In its ‘Code of Practice for the Hydraulic Modelling of Urban Drainage Systems’, CIWEM advised: “Key stakeholders should be identified at the project definition stage, together with the potential opportunities and benefits for collaborative working to assist collecting data. Sharing existing data and collaborative physical data collection can reduce costs, improve the knowledge of the catchment, and provide data from a wider range of sources.”

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Anglian Water now sees live sewer modelling as a valuable way to:

Warn of hydraulic flooding in time to take preventative measures

Automatically alert tactical teams by email

Analyse and compare telemetry feeds with modelled predictions and observed simulations after the effect, removing the forecast uncertainty

Create a ‘digital twin’ of the sewer infrastructure network for better asset management

Reduce flooding issues by monitoring sewers for operational issues such as blockages and deploy resources before customers or the environment are affected

Debbie Bell, an infrastructure modeller with Anglian Water, said: “This is revolutionary within hydraulic modelling. By enabling proactive intelligent network modelling and predictive analysis, ICMLive will help Anglian Water remain a frontier performer.”

knowing at what thresholds to alert teams to take action. Getting this right was an iterative process – reviewing conditions after each incident meaning the thresholds could be revised.

ICMLive was set to run the hydraulic model every three hours, which was reduced to a one- hour interval when rain was forecast. Knowledge gained from previous incidents, tide and flow thresholds were used to help set the criteria to create an alert.

One night in July 2018, a forecast for heavy rain coincided with a high tide.

However, the predicted incident didn’t happen as the rain forecast was two hours out. The results proved the value of the model as, if the thresholds were met, flooding was likely but, as the rainfall data was two hours out, the model still stood up.

The modelling team set up a revised model with improved rainfall forecast data. The exercise proved the importance of the reliability of the model and the accuracy of data. Every day Anglian Water makes operational decisions to send out crews. The live modelling platform helps improve those decisions which has a direct impact on operational costs.

A nglian Water’s ‘Innovation Shop Window’ acts as both a proof of concept and proving ground for innovation. Occasionally, real

life presents the right conditions to test new technologies with a live incident. That happened in May 2018 when hydraulic conditions triggered a structural manhole failure near a beach front.

Heavy rainfall had increased the volume of water being treated at the nearby water recycling centre. Normally the final effluent would be taken out to sea by pipe, but the combination of a high tide, heavy rainfall and increased flow rates from the sewage treatment works led the manhole to fail. A clean-up crew was mobilised and measures were put in place to protect road users.

This gave Anglian Water’s infrastructure modelling team the opportunity to explore the benefits of ICMLive, a near real-time modelling technology from Innovyze which can predict flood incidents and help inform appropriate risk reduction measures.

The team recognised the value of coupling live modelling technology with forecast rainfall and tidal information, however, the challenge was

CASE STUDY

ANGLIAN WATER

How live sewer modelling helps reduce risk to public health & safety for Anglian Water

ICMLive enables Anglian Water to predict flood incidents and implement appropriate risk reduction measures

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network responds to spatially variable rainfall events. As well as alleviating blockages, the focus is now also on targeting pollution sites. As William Neale, catchment and insight modeller at Thames, explains: “Blockages can obviously manifest into not only customer level flooding, but also flooding that can escape and become a pollution incident. It’s important that we target pollution postcodes.”

To help bring this vast amount of data and intelligence together, Thames is using ICMLive to help support a more joined-up approach. The platform has armed the team with critical near real-time updates and a clear visualisation of system performance.

Thames is also installing the depth loggers on the strategic networks for key operations, which demonstrates the trust that has been placed in the modelling tools. ICMLive is now being used to bolster health and safety processes across operations. When field staff are deployed into live sewer environments, for example, it is critical that there is no rainfall because the response in the network can be rapid. If diversions are necessary to reduce flows and give engineers access to a sewer, the modelling allows the team to demonstrate how long that diversion can be in place before it backs up and starts to cause an impact elsewhere on the network. This means that model forecasting has become an essential tool in supporting operational teams to manage and maintain the network.

Andrew Hagger, head of systems modelling and catchment insights at Thames says: “The amount of intelligence FloodWorks and ICMLive has provided us with is really powerful.” Ultimately, the modelling is allowing the company to deliver the best possible solution for customers and the environment.

This allowed the team to investigate flood incidents, or locations where flooding was imminent, to understand the causes and mitigate the impact. The team could then assess the network, for example visibility of pumping station availability, tides and other factors that could impact flood risk across the network. Each time this in-depth analysis was undertaken, the accuracy of the modelling was improved.

Since the Olympics, the project has been rolled out to include approximately 20,000 depth loggers, which will increase to 21,500 by the end of AMP7.

The amount of data produced in real-time is giving Thames Water improved insight to how the

A s the official water utility services provider of the London Olympics, Thames Water was responsible for the provision of both water

and wastewater services at the Olympic venues in its region. This meant that the utility had to manage the potential risk of disruptions from burst mains, flooding, or pollution incidents in the vicinity of Olympic venues and along key transportation routes in and around London.

Prior to 2012, Thames had implemented the Innovyze FloodWorks platform (the precursor to ICMLive) in order to help it manage its wastewater system operations. This forecasting tool allowed the impact of rainfall events to be predicted six to 12 hours in advance across the Thames network.

The tool also allowed the team to explore the impact of blockages and undertake post-event analysis when serviceability issues occurred.

As part of the project, 130 telemetered depth monitors, or “depth loggers”, were deployed in flooding hotspots to provide early warning in high-risk areas.

Anglian Water now sees live sewer modelling as a valuable way to:

Warn of hydraulic flooding in time to take preventative measures

Automatically alert tactical teams by email

Analyse and compare telemetry feeds with modelled predictions and observed simulations after the effect, removing the forecast uncertainty

Create a ‘digital twin’ of the sewer infrastructure network for better asset management

Reduce flooding issues by monitoring sewers for operational issues such as blockages and deploy resources before customers or the environment are affected

Debbie Bell, an infrastructure modeller with Anglian Water, said: “This is revolutionary within hydraulic modelling. By enabling proactive intelligent network modelling and predictive analysis, ICMLive will help Anglian Water remain a frontier performer.”

CASE STUDY

THAMES WATER

How the London Olympics initiated a step-change in Thames Water’s approach to modelling

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