Category Archives: Engineering Features

Designers need a better understanding of the maintenance requirements of different SuDS components to ensure that the systems they design will perform as intended. Stuart Crisp, UK manager at Advanced Drainage Systems (ADS), reports.

The issue of SuDS maintenance has always been a thorny one. Research suggests that the question of who will be responsible for the maintenance, and the cost of it, has been a prime factor in their slow uptake in the United Kingdom.

With the Government’s intention to implement Schedule 3 of the Flood and Water Management Act 2010 (FWMA) in England, making the installation and adoption of SuDS mandatory, concerns over maintenance issues again come to the fore. Developers, designers and installers need to understand the maintenance implications for alternative solutions considered for a project (in addition to, for example, hydraulic performance, structural integrity and water quality) and then offered to the adopting body responsible for the long-term operation of the SuDS asset. For underground attenuation devices, these vary significantly.

The more onerous the maintenance requirements, the higher the risk of them not being properly executed. The impacts of poor maintenance regimes and difficult-to-clean systems can be significant, increasing the risk of blockages – leading to loss of capacity and flooding – and pollutants washing out into water bodies.

Guidance

The SuDS Manual, CIRIA C753, recognizes that underground attenuation crates are ‘difficult to clean’ and that their capacity will reduce over time. Section 21.5.3 of the manual recommends that the size of crates should be increased by 10% to allow for accumulation of sediment. This applies even when a maintenance programme is deployed, since it isn’t always possible to remove all sediment during cleaning. Commercial developments, high density residential development, car parks and highways face the highest potential loss of storage, according to the guidance.

For both crates and large diameter pipes, some form of silt separation and removal system upstream is normally required to slow down the rate of sediment build-up and to remove some of the pollutants that cling to those particles. These upstream components must also be inspected and cleaned at intervals prescribed in a SuDS maintenance plan.

In designing its StormTech underground attenuation device, ADS sought to remove the need for costly upstream pre-treatment. An inbuilt ‘Isolator Row’ – essentially a modified version of the standard StormTech elliptical arches – collects the sediment before the water moves into the main body of the system. 

Independent tests have demonstrated that the Isolator Row removes over 80% of total suspended solids (TSS), together with the metals, hydrocarbons, phosphorus, nitrogen, and other surface water pollutants that cling to them. The Isolator Row is easily accessed via a closely located manhole, and can be cleaned out with standard sewer-cleaning equipment.

Natural SuDS need maintenance too

Natural SuDS, as well as engineered ones, also need regular and planned maintenance. These range from frequent interventions such as litter picking and inspection of inlets and outlets to more occasional and seasonal activities such as vegetation management and removal of silt build-up.

Again, failure to maintain natural attenuation components such as ponds can have negative impacts. A 2018 study of SuDS in East Kilbride by the University of Glasgow, published in The Glasgow Naturalist, found that pollutants in some SuDS ponds were hindering amphibian breeding and development and that more frequent monitoring and management would be wise.

Whether natural or engineered SuDS, or a combination, maintenance regimes and their associated cost, should not be a barrier to their implementation. However, it is important that maintenance issues are understood, planned and communicated at the earliest stages of a project.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

Designers need a better understanding of the maintenance requirements of different SuDS components to ensure that the systems they design will perform as intended, Advanced Drainage Systems UK manager Stuart Crisp, reports.

The issue of SuDS maintenance has always been a thorny one. Research suggests that the question of responsibility for the maintenance, and the cost of it, has been a prime factor in their slow uptake in the UK.

With the government’s intention to implement Schedule 3 of the Flood and Water Management Act 2010 in England, making the installation and adoption of SuDS mandatory, concerns about
maintenance issues have again come to the fore. Developers, designers and installers need to understand the maintenance implications for alternative solutions considered for a project and then offered to the adopting body responsible for the long-term operation of the SuDS asset.  These are in addition to, for example, hydraulic performance, structural integrity and water quality. For underground attenuation devices, these vary significantly.

The impacts of poor maintenance regimes and difficult to clean systems can be significant, increasing the risk of blockages – leading to loss of capacity and flooding – and pollutants washing out into water bodies.

Guidance

The SuDS Manual, Ciria C753, recognises that underground attenuation crates are “difficult to clean” and that their capacity will reduce over time. Section 21.5.3 of the manual recommends that the size of crates should be increased by 10% to allow for accumulation of sediment. This applies even when a maintenance programme is deployed, since it is not always possible to remove all sediment during cleaning. Commercial developments, high density residential development, car parks and highways face the highest potential loss of storage, according to the guidance.

For crates and large diameter pipes, some form of silt separation and removal system upstream is normally required to slow down the rate of sediment build up and to remove some of the pollutants that cling to those particles. These upstream components must also be inspected and cleaned at intervals prescribed in a SuDS maintenance plan.

In designing its StormTech underground attenuation device, ADS sought to remove the need for costly upstream pre-treatment. An inbuilt “Isolator Row” – essentially modified StormTech elliptical arches –
collects the sediment before the water moves into the main body of the system.

Independent tests have demonstrated that the Isolator Row removes more than 80% of total suspended solids, together with the metals, hydrocarbons, phosphorus, nitrogen and other surface water pollutants that cling to them. The Isolator Row is easily accessed via a closely located manhole, and can be cleaned out with standard sewer cleaning equipment.

Natural SuDS need maintenance too

Natural SuDS, as well as engineered ones, also need regular and planned maintenance. These range from frequent interventions such as litter picking and inspection of inlets and outlets to more occasional and seasonal activities such as vegetation management and removal of silt build up.

Again, failure to maintain natural attenuation components such as ponds can have negative impacts.

A 2018 study of SuDS in East Kilbridge by the University of Glasgow published in The Glasgow Naturalist, found that pollutants in some SuDS ponds were hindering amphibian breeding and development and that more frequent monitoring and management would be wise.

Whether natural or engineered SuDS, or a combination, maintenance regimes and their associated costs, should not be a barrier to their implementation. However, it is important that maintenance issues are understood, planned and communicated at the earliest stages of a project.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), looks at looming legislation which will mandate SuDS and their adoption will require new skills for developers and designers.

In January 2023, the Government announced plans to finally implement Schedule 3 of the Flood and Water Management Act 2010, which will make the adoption of SuDS mandatory in England, as it has been in Wales since 2019. This is a game-changer for SuDS.

While developers currently have the right to connect drainage systems into sewers, that is unlikely to be the case anymore, without prior justification and consent.

Instead, they will have to show that they have included SuDS in their schemes and demonstrate how that SuDS system can be maintained over the lifetime of a development.

Subject to a consultation later this year, implementation of Schedule 3, which includes SuDS approval and adoption, is expected in late 2024. This means that there is less than two years for concerned professionals to get up to speed with the range and implications of possible solutions, both above and below ground.

Designers will have to think about more than just hydraulic design, to include whole life maintenance and treatment train to deal with water quality issues and specific pollutants. There will probably be a transition period as Schedule 3 comes in, but it makes sense to upskill now in order to future-proof designs.

Currently, SuDS can be adopted by water companies as long as systems comply with the Design and Construction Guidance (DCG) which sets out how SuDS should be delivered. However, it is not compulsory for a developer to jump through the adoption hoops. The end result is that some assets that do meet prescribed and consistent standards of quality and performance, may not be properly maintained and consequently there are problems down the line.

DCG was updated last year to include arch-shaped below-ground attenuation structures. One such system offers a flexible and cost-efficient alternative to other below-ground attenuation structures such as crates or large-diameter pipes, with the benefit of built-in stormwater quality management, reducing the extent of additional treatment required elsewhere in the SuDS system.

It is expected that Schedule 3 will change the assessment and adoption of SuDS to become SuDS approving bodies (SABs) which will be within unitary councils or county councils. And it will bring in new statutory guidance, taking over from DCG to cover design, construction and operation over an asset’s lifetime.

The statutory requirements in England may be more onerous than both DCG and the current non-statutory standards in terms of what will be acceptable for planning approval and adoption after construction. SuDS adoption becoming mandatory, with few exceptions, will raise the bar. Happily, poor quality products and poorly executed designs are likely to be challenged and disappear from the market.

For anyone looking to start the upskilling process now, training and CPDs are already available from some manufacturers and should include information on legislation, best practice and comparable systems.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), looks at how ‘good’ SuDS addresses water quality as much as water quantity when it comes to managing water down a ‘treatment train’.

Historically, driven by legislation, there’s a tendency to focus on water quantity – how to use SuDS to reduce or delay release of stormwater into sewers and prevent flooding. However, new legislation requires the additional focus on water quality.  The SuDS treatment train considers water quality and prevention of pollution from stormwater runoff (alongside flood risk mitigation), with the level of treatment provided based on the anticipated levels and types of pollution.

A SuDS treatment (or management) train should start with prevention such as reducing impermeable surfaces, to reduce run-off and remove sources of pollution. Next comes source control, where rainfall is dealt with close to where it falls by infiltration-based elements such as soakaways, rain gardens and permeable pavements. Site control sees water managed within a local area, for instance directing water to a soakaway or infiltration device and then onto an attenuation pond or tank. Finally, regional control would see a system that could handle run-off from several sites, perhaps resulting in a detention basin or wetland.

The SuDS Manual, C753, published by CIRIA, prescribes a risk-based approach to designing SuDS for water quality. If pollution risks are deemed to be low, then SuDS designers can prioritise water quantity, amenity and biodiversity. If they are medium, then all considerations must be balanced. And if they are high, water quality evaluation must take precedence. It should however be noted that in all cases, including medium and low pollution risk sites, appropriate mitigation should be put in place to reduce the risk of pollution.

There is a range of water quality treatment measures that can be included in a SuDS train. Sometimes it is possible to use a completely natural SuDS train to deal with both water quantity and quality issues. At other times, the best solution combines natural and engineered SuDS elements, or may require proprietary manufactured elements only. This could be due to high levels of pollution loading or the space available.

There are four main types of pollutant that can be found in stormwater run-off: sediments, metals, hydrocarbons and nutrients. Sediments, often referred to in pollution mitigation as total suspended solids (TSS), is particulate matter. It includes tiny particles of soil, such as silt and clay, which have been dislodged by rainwater as it passes over the run-off surfaces.

Metals and metal compounds can be dissolved in run-off or attached to silts and sediments in the water. Copper and zinc are most commonly found in surface water in the UK but there can be cadmium and other toxic metals too. Although plants require very small amounts of copper and zinc to grow, higher concentrations can be damaging to them.

Hydrocarbon pollution comes with run-off from roads, car parks and areas where machines operate and are maintained, due to oil and fuel spills, tyre and brake wear. Changing climate means that rainfall events can be further apart but more intense, which can lead to higher concentrations of pollutants from roads, as they build up for longer before being washed away.

Nutrient pollution, usually nitrogen or phosphorous based, can come from sources such as run-off from agricultural land where fertiliser has been used or combined sewer overflows (CSOs), where sewage and surface water are mixed and discharged into bodies of water. They can lead to algal blooms, which reduces oxygen levels in the water and can negatively impact on aquatic habitats.

Capturing TSS pollution should generally be the focus of water treatment strategies in SuDS design since this removes both the solid particles and any pollution clinging to them. This can be done using a natural SuDS feature, such as planting for bioretention or a swale. Manufactured components such as filtration devices, silt traps or vortex separators can also be used upstream to remove solids before water is discharged into ponds, for instance.

Removal of sediment and solids is also important from a water quantity perspective. Build-ups reduce the capacity of a water storage element, whether natural or manufactured. How and when to remove sediment should be considered at the design stage and should be part of a planned maintenance regime.

Without a means of reducing suspended solids upstream of an attenuation device, sediment build-up within the device can reduce its efficiency over time. Some recently introduced underground arch-shaped SuDS attenuation devices have their own built-in systems for intercepting solids which are fast and simple to maintain.

Mitigation indices

The SuDS Manual sets out a simple method for dealing with water pollution risks, requiring the determination of pollution hazard indices for the area under consideration and then matching a SuDS device with matching mitigation indices. 

In Table 26.2, the manual provides pollution hazard indices for a range of applications and for three types of pollution: TSS, metals and hydrocarbons. So, for example, for a busy public car park such as a supermarket or hospital, the index for TSS is 0.7, for metals is 0.6 and for hydrocarbons 0.7.

The next step is to identify a form of SuDS treatment that can provide the necessary mitigation indices, either as a single treatment stage or using a combination of components. The Manual has a table for that too, Table 26.3. However, this table only provides mitigation indices for natural SuDS components.  The mitigation indices for the natural SuDS components were compiled by a team of experts, drawing information from a selection of published papers.

Mitigation indices for proprietary manufactured treatment systems must be provided by the manufacturer using recognised test methods and 3^rd party verified data.

British Water has published a Code of Practice for the Assessment of Manufactured Treatment Devices Designed to Treat Surface Run-off. It uses rainfall time-series data for the UK to determine an appropriate treatment flow rates based on first-flush principles and uses a combination of two established test protocols – the German Deutsches Institut für Bautechnik (DIBt) and the New Jersey Corporation for Advanced Technology (NJCAT) from the US – to create the British Water test methods.

In 2022 British Water published a ‘how to’ guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices, which provides a calculation methodology to derive mitigation indices for TSS, metals and hydrocarbons based on the British Water Code of Practice, or the DIBt or the NJCAT, test results. This enables manufacturers of  proprietary treatment products to have their mitigation indices published so that they can be considered as part of a SuDS management train.

British Water publishes a List of Assessed Surface Water Treatment Devices. More details on the British Water web site.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

Now there are verified mitigation indices for engineered components says Stuart Crisp, UK manager at Advanced Drainage Systems (ADS).

The publication of the Plan for Water by the Department for Environment, Food & Rural Affairs (Defra) in April this year signalled the Government’s intention to tackle water pollution. Clearly, one of the ways to do this is by using a well-designed SuDS management train which considers both water quantity and quality, deploying SuDS components that are appropriate for the levels and types of pollution present in the surface water.

This is recognised in the Plan, with reference to Schedule 3 of the Flood and Water Management Act 2010, which will make adoption of SuDS for new developments mandatory. The Government had announced in January 2023 that Schedule 3 would be finally implemented in England, as it already has been in Wales, possibly coming into force in 2024, subject to consultation.

SuDS are often thought of as a way to manage water quantity by reducing flows into sewers and hence reducing activation of combined sewer overflows (CSOs) which discharge raw sewage into water bodies. SuDS should also be designed to manage water quality directly by removing pollutants where it is carried in surface water. The CIRIA SuDS Manual, C753, prescribes a risk-based approach, setting out the expected levels of pollution for different development types and then defining how natural and engineered SuDS elements, used as a single entity or together, can be used  to remove the various forms of pollution.

When it comes to engineered SuDS components, designers and specifiers can turn to British Water, the body which represents water and wastewater companies. Having published a code of practice in 2016 and a ‘how to’ guide for manufactured stormwater treatment devices in 2022, the organisation is now hosting a list on its website of engineered SuDS components which have had their pollution treatment information verified by an independent third party expert.  The ADS StormTech system, with its Isolator Row – a built-in water quality treatment device – has recently been added to British Water’s list of assessed surface water treatment devices.

Types of pollution

There are four main types of pollutant that can be found in stormwater run-off: sediments, metals, hydrocarbons and nutrients. Sediments, often referred to in pollution mitigation as total suspended solids (TSS), is particulate matter including particles of soil. Metals and metal compounds can be dissolved in run-off or attached to silts and sediments in the water. Copper and zinc, which are most commonly found in surface water in the UK, can damage plants in higher concentrations.

Hydrocarbon pollution comes with run-off from roads, car parks and areas where machines operate and are maintained, due to oil and fuel spills, tyre and brake wear. Changing climate means that rainfall events are further apart but more intense, leading to higher concentrations of pollutants, as they build up for longer before being washed away.

Nutrient pollution, usually nitrogen or phosphorous based, can come from sources such as run-off from agricultural land where fertiliser has been used or combined sewer outfalls, where wastewater and surface water are discharged into bodies of water. They can lead to algal blooms, which reduces oxygen in the water and can negatively impact on aquatic habitats.

Mitigation indices

The SuDS Manual’s method for dealing with water pollution risks starts by determining pollution hazard indices for the area under consideration. In Table 26.2, the manual provides pollution hazard indices for a range of land uses and for three types of pollution: TSS, metals and hydrocarbons. So, for example, for a busy public car park such as a supermarket or hospital, the index for TSS is 0.7, for metals is 0.6 and for hydrocarbons 0.7.

The next step is to identify a SuDS treatment train that can provide the necessary mitigation indices, either a single stage or using a combination of components. The manual has a table for that too, Table 26.3. However, this table only provides indices for natural SuDS components, stating that proprietary treatment systems must demonstrate that they can address each of the contaminant types to acceptable levels.

For engineered components, British Water’s two documents – Code of Practice for the Assessment of Manufactured Treatment Devices Designed to Treat Surface Run-off and ‘how to’ guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices – provide a methodology to calculate mitigation indices for TSS, metals and hydrocarbons.

The mitigation indices for ADS StormTech’s Isolator Row are 0.8 for TSS, 0.6 for metals and 0.7 for hydrocarbons = 0.7. This means that it meets the requirements for medium pollution hazard applications and the TSS requirements of high pollution hazard applications and can be used as a combined SuDS attenuation and water quality treatment system without the need for additional components within the treatment train.

Stuart Crisp is UK Manager for Advanced Drainage Systems (ADS). ADS is America’s largest manufacturer of thermoplastic corrugated drainage pipes and a specialist in water management systems. StormTech has a long and successful track record with over 50,000 below ground SuDS attenuation system installations using in excess of 3m units.

Originally published in Water magazine September 2023

Designing SuDS to remove pollution from surface water requires a risk-based approach, matching likely levels of pollution with the performance of SuDS elements. Stuart Crisp, UK manager at Advanced Drainage Systems (ADS), reports.

Sustainable Urban Drainage Systems (SuDS) should be designed with both water quantity and water quality in mind. Yet often the water quality element is overlooked, or down-specified or removed altogether in a misnomered ‘value engineering’ exercise.

One of the challenges for designers is that the CIRIA SuDS Manual, C753, only defines the performance of natural SuDS components in the removal of pollutants. For engineered components, manufacturers must demonstrate that the component(s) selected will reduce the contaminant types to acceptable levels.

One way to do that is to check whether the manufactured component is on British Water’s list of assessed surface water treatment devices. To appear on the list, data relating to a component’s pollution treatment ability must have been verified by an independent expert.

Risk-based approach

The SuDS Manual prescribes a risk-based approach to designing for water quality, defining pollution risks by way of pollution hazard indices. In Table 26.2, the manual provides the indices for a range of land uses and for three types of pollution: total suspended solids (TSS) such as tiny soil particles, metals and hydrocarbons. For instance, a busy public car park such as a supermarket or hospital, would have indices of 0.7 for TSS, 0.6 for metals and 0.7 for hydrocarbons. The manual then provides generic mitigation indices for natural SuDS components for the three types of pollution in Table 26.3.

British Water, which represents water and wastewater companies, has published two documents which provide guidance on how to calculate mitigation indices for engineered SuDS components. In 2016 it published a Code of Practice for the Assessment of Manufactured Treatment Devices Designed to Treat Surface Run-off. And in 2022 it followed that up with a ‘how to’ guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices.

British Water’s ‘how to’ guide provides a method for calculating mitigation indices for TSS, metals and hydrocarbons based on test results derived from its code of practice. This means that manufacturers can calculate and publish mitigation indices for their treatment products so that they can be considered as part of a SuDS management train.

On its website, British Water provides a list of engineered SuDS components and their independently verified mitigation indices. ADS StormTech, with its Isolator Row, was recently added to the list, the only system that combines attenuation and pollution removal, often without the need to add a treatment device upstream of the attenuation chambers.

It should be noted that removal of TSS is also important from a water quantity perspective. Build-ups can reduce the storage and discharge capacity of a water storage element, whether natural or manufactured. How and when to remove sediment should be considered at the design stage and should also be part of a planned maintenance regime.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

In April 2023, the Department for Environment, Food & Rural Affairs (Defra) published its Plan for Water. The aim of the plan is to create a more holistic approach to water management, ensuring that there is enough water to supply the UK’s growing population, and that the water in rivers, lakes and other water bodies is free from pollution. It promises action on all sources of pollution: wastewater treatment, agriculture, plastics, urban stormwater and road run-off, and chemicals.

Public awareness and concern about water quality, and how it impacts on the natural environment and on public health, is growing fast. And stormwater is increasingly being recognised as a big issue. As rainfall events become more intense, and our ageing sewer system becomes ever more overloaded, combined sewer overflows – where wastewater is discharged into rivers and other water bodies – are being activated more frequently.

New legislation will be welcome then – which is what the Government is suggesting in its Plan for Water. Among the proposed legislation are changes to planning policy so that new developments are designed to reduce the likelihood of both flooding and water shortages. Water companies, which will have to produce Drainage and Wastewater Management Plans, could be involved in planning decisions.

The Plan also references the Government’s plans to finally implement Schedule 3 of the Flood and Water Management Act 2010, which covers SuDS approval and adoption, in England – subject to consultation. Initially announced in January 2023, this could make the inclusion of SuDS standard practice in the design, construction and adoption of nearly all new developments from 2024. It also talks about the strategic road network and how it needs to do more to prevent pollution from highway run-off discharging into our water courses (see box).

Bioswale at a development in Hampshire taken in March 2023, showing how good SuDS can be.

Multi-faceted SuDS

Well-designed SuDS can help reduce pollution in a variety of ways. SuDS should be multi-faceted, dealing with water at source to prevent activation of combined sewer overflows (CSOs) and flooding and removing pollutants to improve water quality – as well as boosting biodiversity and providing public amenity.

Good SuDS design is based on the SuDS treatment train, or SuDS management train, which sees water passing through a logical sequence of stages using different SuDS components. The treatment train should consider both water quantity and quality, with the weighting given to each dependent on predicted water volumes, flows and the levels and types of pollution.

A SuDS train should start with prevention such as reducing impermeable surfaces, to reduce run-off and removing sources of pollution. Next comes source control, where rainfall is dealt with close to where it falls by infiltration-based elements such as soakaways, rain gardens and permeable pavements. Site control sees water managed within a local area, for instance directing water to a soakaway or infiltration device and then onto an attenuation pond or tank. Finally, regional control would see a system that could handle run-off from several sites, perhaps resulting in a detention basin or wetland.

The SuDS Manual, C753, published by CIRIA, prescribes a risk-based approach to designing SuDS for water quality. If pollution risks are deemed to be low, then SuDS designers can prioritise water quantity, amenity and biodiversity. If they are medium, then all considerations must be balanced. And if they are high, water quality evaluation must take precedence, for instance on haulage yards, industrial sites, trunk roads and motorways. It should however be noted that in all cases, including medium and low pollution risk sites, appropriate mitigation should be put in place to reduce the risk of pollution.

Heavy rain causing stormwater runoff from a road in Hoghton, near Preston, Lancashire in May 2022.

There are a range of water quality treatment measures that can be included in a SuDS train. Sometimes it is possible to use a completely natural SuDS train to deal with both water quantity and quality issues. At other times, the best solution combines natural and engineered SuDS elements, or may require proprietary manufactured elements only. This could be due to high levels of pollution loading or the space available.

There are four main types of pollutant that can be found in stormwater run-off: sediments, metals, hydrocarbons and nutrients. Sediments, often referred to in pollution mitigation as total suspended solids (TSS), is particulate matter. It includes tiny particles of soil, such as silt and clay, which have been dislodged by rainwater as it passes over the run-off surfaces.

Metals and metal compounds can be dissolved in run-off or attached to silts and sediments in the water. Copper and zinc are most commonly found in surface water in the UK but there can be cadmium and other toxic metals too. Although plants require very small amounts of copper and zinc to grow, higher concentrations can be damaging to them.

Hydrocarbon pollution comes with run- off from roads, car parks and areas where machines operate and are maintained, due to oil and fuel spills, tyre and brake wear. Changing climate means that rainfall events can be further apart but more intense, which can lead to higher concentrations of pollutants from roads, as they build up for longer before being washed away. Nutrient pollution, usually nitrogen or phosphorous based, can come from sources such as run-off from agricultural land where fertiliser has been used or CSOs, where waste water and surface water are discharged into bodies of water. They can lead to algal blooms, which reduces oxygen and can negatively impact on aquatic habitats. The Plan for Water reports £2.5bn of planned and made investment in wastewater treatment works between 2020 and 2025, which it says will halve phosphorous pollution. And it promises legislation to force water companies to make upgrades to nutrient removal near protected habitats. The Government’s Environment Act 2021 set a legally binding target to reduce phosphorus in treated wastewater by 80% by 2038 compared to a 2020 baseline, with an interim target of 50% by 2028.

Recent studies carried out by Stormwater Shepherds have indicated that phosphorus pollution is not a major problem from most urban surfaces. However, well designed SuDS can help alleviate nutrient pollution where it is a problem in surface water run-off. CIRIA guide C808, Using SuDS to reduce phosphorous in surface water run-off, published in 2022, provides guidance on how to do this. It suggests a treatment train starting by maximising infiltration, followed by sedimentation and the removal of solids and finally the introduction of actively growing plants to take up some of the dissolved phosphorus.

Wildflowers in a SuDS scheme in Chorley in Lancashire.

Capturing TSS pollution should generally be the focus of water treatment strategies in SuDS design since this removes both the solid particles and any pollution clinging to them. This can be done using a natural SuDS feature, such as planting for bioretention or a swale. Manufactured components such as filtration devices, silt traps or vortex separators can also be used upstream to remove solids before water is discharged into ponds, for instance.

Removal of sediment and solids is also important from a water quantity perspective. Build-ups reduce the capacity of a water storage element, whether natural or manufactured. How and when to remove sediment should be considered at the design stage and should be part of a planned maintenance regime.

Without a means of reducing suspended solids upstream of an attenuation device, sediment build-up within the device can reduce its efficiency over time. Some recently Introduced underground SuDS
attenuation devices have their own built-in systems for intercepting solids which are fast and simple to maintain.

SuDS in the Sheffield Grey-to-Green scheme, photographed in May 2022.

Mitigation Indices

The SuDS Manual sets out a simple method for dealing with water pollution risks, requiring the determination of pollution hazard indices for the area under consideration and then matching a SuDS device with matching mitigation Indices.

In Table 26.2, the manual provides pollution hazard indices for a range of land uses and for three types of pollution: TSS, metals and hydrocarbons. So, for example, for a busy public car park such as a supermarket or hospital, the index for TSS is 0.7, for metals is 0.6 and for hydrocarbons 0.7.

The next step is to identify a form of SuDS treatment that can provide the necessary mitigation indices, either as a single treatment stage or using a combination of components. The manual has a table for that too, Table 26.3. However, this table only provides indices for natural SuDS components, stating that proprietary treatment systems must demonstrate that they can address each of the contaminant types to acceptable levels.

The mitigation indices for the natural SuDS components were compiled by a team of experts, drawing information from a selection of published papers. More recently, in 2016, British Water published a Code of Practice for the Assessment of Manufactured Treatment Devices Designed to Treat Surface Run- off. It combines the rainfall time-series data for the UK to determine an appropriate treatment flow rate based on first-flush principles and uses a combination of two test protocols – the German Deutsches Institut für Bautechnik (DIBt) and the New Jersey Comprehensive Assessment Tool (NJCAT) from the US – to create the British Water test methods.

In 2022 British Water has also published a How To Guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To  Proprietary/Manufactured Stormwater Treatment Devices, which provides a calculation methodology to derive mitigation indices for TSS, metals and hydrocarbons based on the Code of Practice, or the DIBt or the NJCAT, test results. This allows manufactured treatment products to have their mitigation indices published so that they can be considered as part of a SuDS Management Train, often alongside vegetative treatment components.

The value of water quality
One of the challenges in the delivery of well-designed SuDS treatment trains is that important elements of the train can be removed during ‘value engineering’ exercises. For instance, a design or specification may include guidance to say that sediment should be removed upstream, but this is then considered unnecessary during a ‘value engineering’ exercise and removed or compromised.

Unfortunately, decisions like this are about short-term capital cost rather than whole life cost. They don’t consider the important issues of how maintenance should be carried out, its frequency and its cost – in terms of both cash and carbon. It may also result in failure of the system to continuously provide the required performance according to the original design, throughout the life of the development. Should Schedule 3 of the Flood and Water Management Act come into legislation, maintenance and longevity issues will be brought to the fore. In Wales, where Schedule 3 has already been adopted, developers are expected to create a maintenance plan and the adopting authority will be required to carry out the maintenance for the design life of a scheme.

SABS within county and unitary authorities will be responsible for securing the means to maintain the SuDS they adopt, and it could be that the regulatory framework in England is similar to that used in Wales. As currently understood, funding will be provided by the developer in the form of a commuted sum to the SuDS Approval Body (SAB) at the point of handover. The timeframe for the enactment of Schedule 3, and the many other pieces of proposed legislation for the Plan for Water, remains uncertain – not least due to the uncertainty over when the next general election will be held. However, the urgent need to address water quality issues will only move up the political agenda.

Incorporating SuDS trains that manage both water quality and quantity into new developments does not necessarily have to increase capital costs. Good design can reduce costs over the lifetime of the SuDS and the development.

Highway run-off

Pollution due to road run-off is a major problem in the UK. There are estimated to be over one million drains and outfalls from the strategic road network and local authority roads, the majority of which run straight into rivers and other water courses with no measures to treat the pollution in the water before they do.

The result of this is a build-up of toxic pollution in riverbeds, water, fish and other aquatic life. Run-off from roads contains high amounts of suspended solids which sit on the bed of a watercourse, bringing with them other pollutants which are released over time. Pollutants include polycyclic aromatic hydrocarbons (PAH), metals and microplastics from brakes and tyres.

Only 4,000 out of 26,000 outfalls and soakaways from the Strategic Road Network, run by National Highways in England, have measures in place to treat pollution. The Environmental Permitting Regulations say that highways authorities can discharge road-run off into water bodies only if it doesn’t cause pollution. However, the Environment Agency has never required a road authority to apply for a permit where polluted road run-off is discharging straight into rivers and other water bodies.

The Plan for Water does mention this issue, but there is no firm plan for action when it comes to road run-off. Instead, it says that it is “considering actions to take to reduce the impacts of the Strategic Road Network on water quality as part of developing the next Road Investment Strategy.” The Government will also consider ‘targeted action’ for roads owned by local authorities whose road run-off is contributing to pollution. This falls short of the urgent action which the House of Commons Environmental Audit Committee (EAC) called for in its Water Quality in Rivers report, published in January 2022. One of its suggestions was that the Environment Agency should require discharge permits for any road with annual average daily traffic above 15,000 vehicles.

National Highways has a target of improving 17.5km of water bodies every year.

The plan reports that to date National Highways has delivered over 30 water quality initiatives which have improved just under 20 miles (32km) of water bodies. In 2020-21, its annual report said it had improved 17km of waterbody, so it appears to be missing its targets. There are a number of ways that run-off from roads can be treated for pollution before it enters a water body. SuDS can be used as part of the water treatment train.

Stuart Crisp is UK Manager for Advanced Drainage Systems (ADS). ADS is America’s largest manufacturer of thermoplastic corrugated drainage pipes and a specialist in water management systems. StormTech has a long and successful track record with over 50,000 below ground SuDS attenuation system installations using in excess of 3m units.

Originally published in Drain Trader magazine June 2023

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

With Schedule 3 still over the horizon in England (if it happens at all), the Design and Construction Guidance for adoptable sewers covers minimum standards of design and construction of sewers. And recent changes to DCG give the green light to arch-shaped attenuation chambers – given certain criteria are met. Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), reports.

Ofwat’s Design and Construction Guidance (DCG) for adoptable sewers remains the most important document defining minimum standards of design and construction of adoptable sewers in England, including certain SuDS components. It has been updated to include information about arch-shaped below-ground SuDS attenuation chambers. This makes it easier for water companies to adopt such drainage infrastructure.

Previously, there was no reference to arch-shaped attenuation structures in the DCG which has meant that it has been more difficult for water companies to adopt them. That all changes with the addition of this extra section.

The DCG first came into force on April 1st 2020, replacing the long-standing Sewers for Adoption guidance. Developers who design and install sewerage systems in line with the DCG can expect to have their systems adopted by their local water companies – although it should be noted that water companies must be involved at the earliest stages of design and specification to ensure that local requirements and nuances are met.

One of the notable things about the DCG is that it includes information on sustainable drainage systems (SuDS) and that the definition of a sewer has been broadened to include certain SuDS components. This means that water companies in England can adopt SuDS components under Section 104 of the Water Industry Act 1991 which satisfy the requirements set out in DCG in the same way that they can adopt pipes, manholes and other sewerage infrastructure.

Version 2.2 of the DCG, first available through the Water UK website in November last year, identifies arch-shaped attenuation structures for the first time and requires that such systems can demonstrate full compliance with all the new requirements.

The relevant information can be found in section C7.8 of the document which covers Tanks, in clause C7.8.4d which says:

“d) where half pipe or arch structures are proposed, the design must (in addition to the above) demonstrate how the system can be cleaned/jetted and done so without damage or erosion of base materials or membrane. Further design evidence should outline how, in areas of a high-water table, groundwater is kept out of the system and, when positioned under highways, that the loading criteria is acceptable to both undertaker and adopting Highway Authority (if applicable).”

Any changes to the DCG must first be assessed and accepted by the Independent Sewerage Adoption Panel, which is made up of representatives from water companies and developers and then a recommendation made to Ofwat for their approval.

The inclusion of arch-shaped structures, such as ADS’s StormTech, has been initially well-received by water companies – and paves the way for developers to offer them without having to do additional work to prove their suitability.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

Designers of new developments must start to factor in government requirements for sustainable drainage systems which are due to come into effect soon.

With legislation mandating sustainable drainage systems (SuDS) and their adoption for new developments on the horizon, developers and designers must upskill to ensure future designs meet tough new standards.

The government’s recent announcement that it intends to implement Schedule 3 of the Flood and Water Management Act 2010, is a game changer for SuDS.

It means that SuDS adoption is expected to be mandatory in England, as it has been in Wales since 2019.

In Scotland, Schedule 3 has not been implemented, but SuDS are generally a requirement within planning legislation.

“While developers currently have the right to connect drainage systems into sewers, that is unlikely to be the case anymore,” explains Advanced Drainage Systems (ADS) UK manager Stuart Crisp.

“Instead, they will have to show that they have included SuDS in their schemes and demonstrate how that SuDS system can be maintained over the lifetime of a development.”

Subject to a consultation later this year, implementation of Schedule 3, which includes SuDS approval and adoption, is expected in late 2024.

That means that engineers have less than two years to get up to speed with the range of possible solutions above and below ground and the implications those bring.

“Designers will have to think about more than just hydraulic design, to include whole life maintenance and treatment to deal with water quality issues and specific pollutants,” says Crisp. “There will probably be a transition period as Schedule 3 comes in, but it makes sense to upskill now in order to future proof designs.”

Currently, SuDS can be adopted by water companies as long as systems comply with requirements in the Design and Construction Guidance (DCG) document which sets out how SuDS should be delivered. However, it is not compulsory for a developer to jump through the adoption hoops, leading to the use of some assets which are not a prescribed, consistent standard of quality and performance or which are not properly maintained and monitored, leading to problems down the line.

The DCG was updated last year to include arch-shaped, below-ground attenuation structures, such as ADS’s StormTech. StormTech offers a flexible and cost-efficient alternative to other below- ground attenuation structures such as crates or large diameter pipes. It has built-in pollution treatment, reducing the extent of additional treatment required elsewhere in the SuDS system.

It is expected that Schedule 3 will change the adopters of SuDS to become SuDS approving bodies (SABs), in line with the Welsh approach, which will be within unitary councils or county councils.

The change will bring in new statutory guidance, taking over from the DCG to cover design, construction and operation over an asset’s lifetime.

“The statutory requirements in England are likely to be more onerous than the DCG and the current non-statutory standards in terms of what will be acceptable for planning approval and adoption after construction,” warns Crisp. “SuDS adoption becoming mandatory, with few exceptions, will raise the bar. Happily, poor quality products and poorly executed designs are likely to disappear from the market.”

For anyone looking to start the upskilling process now, manufacturer training and continuing professional development, such as those on below-ground attenuation offered by ADS, are already available and should include information on legislation, best practice and comparable systems.

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

Adoptable sewers must have a design life of between 50 and 120 years, depending on the water company, with the revised definition of a sewer now including SuDS components as well as pipes. But since there are no below-ground SuDS attenuation assets that have been in the ground that long, how do we ensure long-term durability?

In the absence of real-world evidence, it can be a challenge for designers, developers and asset owners to compare the durability and integrity of different solutions. The only way to assure the lifetime performance of below-ground attenuation products is to demand that the relevant standards and guidance are met at all levels – structural, product, material, system and installation.

Designs need to take into account the performance and behaviour of an asset across its whole lifecycle including long-term and short-term loading, maintenance requirements, operation costs and what happens at end-of-life. Failure to do this creates loose specifications, which in turn means the bar can be lowered on quality and performance. And that introduces the risk that an asset’s service life will not be as long as its required design life.

This is something we need to address as a matter of urgency. The announcement in January this year by the Government that it will finally implement Schedule 3 of the Flood and Water Management Act 2010 (FWMA) in England will accelerate the use of SuDS, with statutory instruments to enforce compliance with mandatory standards and the adoption of SuDS.

Well-designed SuDS will also be essential in removing pollution from surface water. This is an issue that has been highlighted by the Environment Act 2021, designed to improve air and water quality and protect wildlife.

Those designing and delivering below-ground SuDS attenuation need to understand how standards and guidance apply to the various types of product. Structural performance, material behaviour, how a product is designed and manufactured and the installation methodology all contribute to the integrity and durability of below-ground SuDS attenuation assets. Without a complete thread that runs through design, specification, installation and into operation and maintenance, the design life may be wishful thinking rather than an assured outcome.

One of the factors which impacts on the quality of SuDS systems currently is the ultimate ownership of that system.  In general, a SuDS system is required to ‘function over the lifetime of the development,’ meaning that it has to be properly maintained and rehabilitated or replaced, when appropriate.  But that requires an owner which can provide the necessary oversight, expertise, management and resources.

The ultimate asset owner of a SuDS system can vary from region to region throughout the UK, based on local legislation and the sector in which the drainage is being constructed. Drainage infrastructure can remain in private ownership, typically the existing developer or a maintenance company or, for example, it can be transferred to the client or asset owner, typically Scottish Water in Scotland, the county or unitary authority in Wales or a Section 104 adoption by a water company in England.

It should be noted that the implementation of Schedule 3 of FWMA 2010 in England, as it has been in Wales since January 2019, may result in almost 100% adoption of SuDS, with few exceptions, at county or unitary authority level.  This would be in lieu of S104 adoption by water companies, which is currently the case for a significantly lower proportion of developments.  Highways drainage is currently dealt with separately and different design standards and rules usually apply.

The type of asset owner adopting a drainage system is important because it can influence the quality of the build and the contractual terms and relationships across the supply chain between engineer, contractor or developer and client.  This means that the design and specification of the below-ground SuDS attenuation system can range from a generic target volume and plan area with possibly some constraints on site levels and positioning through to comprehensive detailing of minimum structural, hydraulic and water quality design requirements including reference to product and material standards plus construction and maintenance specifications.

With such a range of permutations, the outcome does not always provide the optimum solution, in terms of quality, performance, asset life and operational cost.  In many cases, the race to lowest capital cost solution results in compromises in quality and performance, which are most likely to occur where there are no sector requirements set for construction quality.  This is most likely when the SuDS asset remains in private ownership.

Most of the specifications for below-ground SuDS attenuation that we see are the generic type. The design engineer will have run a hydraulic computation to work out inflow and outflow rates and the volume of attenuation required, and that is as far as the detail goes.

A contractor will then employ a specialist to deliver the below-ground SuDS attenuation and will expect them to provide a detailed design and to take the risk of the performance of the design. But who is then checking that what they are delivering will have the necessary structural integrity and durability, or that the proposed maintenance schedule will allow the asset to deliver the performance that has been asked for?

The good news is that the ability to deliver on the desired design life does exist. For each of the main attenuation types there are design standards and guidance which enable the designer to demonstrate that the asset will perform as desired and required through every stage of the lifecycle.

In this article, we will look at large diameter pipes, geocellular crates and arches. Box culverts are occasionally used as attenuation systems, usually when they are designed as under-highway structures, but they are not included in this article as their use is typically a narrow field of application and generally considered an expensive solution for many developments, compared to other types of proprietary below-ground SuDS attenuation system.

Selection of the right product should take into account a variety of factors including transport to site, the area available for locating storage, speed and ease of installation, depth at which the asset can be placed, capital and operational costs, traffic and other loading at all stages, short-term and long-term performance, ease of inspection and maintenance, how it will work with upstream treatment to remove sediment and pollution and compliance with national and local requirements. It may also be important to take into account the relative whole-life carbon footprints of different systems and circularity issues such as whether a product can be reused or recycled at the end of its life.

Structural performance

The structural design of any below-ground SuDS attenuation system should be based on the Eurocode methodology for ensuring structural adequacy. That means that structural design checks are carried out for the relevant load cases, depending on the application.

Eurocode 1, EN 1991-2, can be used to set out a variety of load cases, such as the weight of material above a SuDS attenuation asset and dynamic traffic including braking forces and fatigue due to cyclic loading. Clearly different sizes and designs of crates, pipes or arches can take greater or lesser loadings, depending on their geometry and material properties and the nuances of the installation design.

It is vital that a manufacturer’s instructions as to the minimum and maximum cover that a product can take and the type of short- and long-term loading, are followed to the letter. A product can only be deemed structurally adequate under the Eurocode regime if it is installed under the same conditions that the design checks have been carried out by the manufacturer.

Eurocode 7, EN1997-1, provides the methodology for establishing geotechnical design requirements, depending on the size of the attenuation structure. The code also says that the designer should explain the level of supervision required during construction and what items or conditions need to be checked.

Table of Standards relevant to structural design
EN 1990	Basis of structural design
EN 1991-2	Eurocode 1 – Actions on structures; Part 2 – Traffic loads on bridges and other civil engineering works
EN 1997-1	Eurocode 7 – Geotechnical design

Product specifications: large diameter pipes

Large diameter pipes can be used for attenuation as a pipe laid in a single run or more commonly as a manifolded system, with pipes running in parallel lines. The most commonly used materials are concrete and plastic, although there are thin steel pipes and a hybrid product combining plastic and steel on the market.

Large diameter pipes can be a cost-effective choice of attenuation system, as long as there is sufficient available area to accommodate the volume of water that has to be stored. Where the attenuation space is beneath a public road, pipes that meet the required structural performance and highways authority requirements, can sometimes be used.

The choice of material will depend on considerations including capital cost, whole life cost taking into account maintenance and how many times a system may need replacing or upgrading and logistics requirements such as construction plant lifting capacity required and space for installation.

Pipes should be designed to BS 9295 to ensure their structural performance.  Note that when pipes are laid in parallel, as is often the case for below-ground SuDS attenuation applications, a different approach to structural design is usually required compared with pipes laid as a single run.

For concrete pipes, BS EN 1916 and BS 5911-1 provide the details for product specifications. For plastic pipes, BS EN 13476 provides the details for product specifications.

Table of Standards relevant to large diameter pipes
BS 9295:2020	Guide to the structural design of buried pipes
Table of Standards Relevant to Concrete large diameter pipes
BS EN 1916: 2002	Concrete pipes and fittings, unreinforced, steel fibre and reinforced
BS5911-1	Concrete pipes and ancillary concrete products (recently updated to align with Eurocodes)

Table of Standards relevant to plastic pipes
EN 13476	Plastics piping systems for non-pressure underground drainage and sewerage – Structured-wall piping systems of unplasticised polyvinyl chloride (PVC-U), polypropylene (PP) and polyethylene (PE)

Product specifications: crates 

Crates or geocellular units can be a desirable choice of attenuation system where there is limited plan area since they provide a large void space for a limited footprint. There are a variety of geocellular unit types on the market which can be used at varying depths from shallow sub-base replacement systems for car parks to deeper attenuation tanks for higher volumes of storage.

Geocellular units are typically manufactured from polypropylene (PP) or PVC by injection molding, extrusion of joining thermoformed sheets. Assessment of the performance of thermoplastics (including plastic pipes and arches) needs to take into account the influence of creep over time; creep is the tendency to deform permanently over time under a constant stress.

Structural assessments of crates must consider short-term loading such as traffic and long-term loading, such as the weight of material above the tank and lateral earth loads. Use BS EN 17150, 17151 and 17152-1, along with material tests, to determine characteristic long-term and short-term strengths and specifications.

Crates have been used since the late 1980s and, according to CIRIA C737, failures are relatively rare. Most problems are due to poor installation and temporary works or poor understanding of ground or groundwater with very few failures attributed to inadequate long-term strength.

A geotextile or geomembrane is also part of the geocellular attenuation system and therefore must be properly specified, selected and installed. Catchpits, separators and other pre-treatment measures are vital to prevent the build-up of silt and sediment within the geocellular structure. CIRIA C737 explains how the long-term volume capacity of a crate should take into account the impact of silt and sediment.  The need for effective sediment management as part of a crate-based below-ground SuDS attenuation system is also emphasized in CIRIA C753 The SuDS Manual.

Table of Standards relevant to crates
CIRIA C737	Structural and geotechnical design of modular geocellular drainage systems
BS EN17152-1	Plastics piping systems for non-pressure underground conveyance and storage of non-potable water – Boxes used for infiltration, attenuation and storage systems Part 1: Specifications for storm water boxes made of PP and PVC-U

Product specifications: arches

Arch-shaped below-ground SuDS attenuation systems are relatively new to the UK, although they have a long track record in other parts of the world. They can be a good choice of attenuation system where a flexible layout is needed since their configuration can be tailored to fit into irregular-shaped areas or around existing obstacles.

These can be a good choice where the SuDS attenuation area is under HGV traffic; the elliptical arch profile chambers ‘shed’ some of the load from the units into the stone.  The embedment material is shaped into structural arches and ‘stone columns’ adding to the strength of the system so that the arch-shaped chambers can be used at shallower cover and deeper invert depths than many alternative systems.

One proprietary brand of arches includes an integrated pre-treatment element, which takes out sediment and pollutants from the first flush runoff. Connected to a manhole for ease of inspection and cleaning, this can meet water quality and pre-treatment requirements in a cost-effective way.

Since this type of SuDS attenuation asset is novel for the UK, designers, developers and asset owners should ensure that proprietary products meet the required structural performance under the Eurocode regime. Short-term and long-term loading should be considered, including the effect of creep.

Although arch-shaped attenuation structures are now referenced in the Design & Construction Guidance (DCG) for adoptable sewers, which applies to adoptable drainage in England, they are not yet included in many of the older standards and guidance. When this is the case, it is always useful to check whether a product has a relevant third party certification, such as a British Board of Agrément (BBA) certificate.

BBA certification validates a product’s capabilities, and fitness for its intended use. The assessment process typically looks at materials, product geometry, testing, system design, review of factory control procedures, production, installation methods and compliance with relevant Regulations.

Arch-shaped below-ground SuDS attenuation structures, because of their relative newness in the UK, will not automatically be included in national or local highways standards, since these cannot be constantly updated to cover new technologies or systems. However, innovative products can be used by applying for a Departure from Standard. To do this, a designer must submit a clear and adequate justification for the Departure, proving that the product is fit for purpose and explaining why it is a better solution than a standard one.

Table of Standards relevant to arches
ISO/DIS 4982	Plastics piping systems for non-pressure underground conveyance and storage of non-potable water — Arch-shaped, corrugated wall chambers made of PE or PP used for retention, detention, storage and transportation of storm water systems — Product specifications and performance criteria
ASTM F2418	Standard Specification for Polypropylene Corrugated Wall Stormwater Collection Chambers
AASHTO LRFD	Bridge Design Specifications Section 12.2
ASTM F2787	Standard practice for structural design of Thermoplastic Corrugated Wall Stormwater Collection Chambers

Construction

The way that a below-ground SuDS attenuation asset is installed is a major factor in its long-term performance and service life. Those installing the asset, and those responsible for monitoring the installation, need to pay close attention to the design and to the details.

Details such as the type of ground and the position of groundwater are important if the asset is to perform as designed. If, on excavation, they are found to be different from what has been assumed in the design, this must be addressed.

Errors or poor workmanship can lead to problems later on. For instance, if the geomembrane around a tank has been torn or its joints not properly welded, water may leak out or groundwater and silt may leak in.

Manufacturer’s installation details, including the type of backfill used and how it is to be compacted, must be followed to the letter. Failing to do this could mean that the product is not being loaded in the way it has been designed to do and could be loaded beyond its capacity. All pipes entering and leaving a below-ground SuDS attenuation structure must be connected according to the manufacturer’s instructions and sealed and tested to check for leaks, if relevant for the system being used.

A client or main contractor should assure themselves that the company and individuals doing the installation have the experience and competency to do a good job. This could include looking at their track record, qualification and experience and talking to previous clients.

In England, guidance on construction for adopting water companies is given in the DCG, which came into force on 1 April 2020. Scotland, Northern Ireland and Wales have their own versions (see table). Note that the DCG was updated in 2022 to include arch-shaped attenuation structures. And where the asset is under a publicly owned road, local highway department specifications must be met.

Guidance for construction of adoptable drains and sewers (including SuDS) across the UK
Design and Construction Guidance (DCG) for foul and surface water sewers offered for adoption under the Code for adoption agreements for water and sewerage companies operating wholly or mainly in England (“the Code”). Appendix C to the sewerage sector guidance.	England
Sewers for Adoption (NI)	NI
Sewers for Scotland (4th edition)	Scotland
Statutory standards for sustainable drainage systems – designing, constructing, operating and maintaining surface water drainage systems.	Wales

Maintenance and operation

Since SuDS elements must be designed to last as long as the development which they serve, maintenance, repair and – where necessary – replacement must be considered at the design stage and be communicated through into operation and be part of the development’s maintenance manual.

Without a properly planned and executed maintenance regime, silt can build up within below-ground SuDS attenuation assets, gradually reducing their storage capacity over time.

Depending on the location of a below-ground SuDS attenuation structure, a tank with inadequate upstream sediment management can lose a proportion of its storage capacity over its design life. Some crate systems are recognised to be difficult to clean out once silt has entered the tank and a siltation management plan should allow for loss in capacity and an effective pre-treatment and silt removal system must always be an integral part of the below-ground SuDS attenuation system design.

Maintenance regimes to tackle siltation would include cleaning upstream silt traps or separators. There should also be an easy way to inspect the below-ground SuDS attenuation structure itself, to check whether silt is building up. Maintenance intervals should be set and adhered to, with additional inspections after large storms.

It is worth noting that while a development is under construction, it may be necessary to prevent water from entering a below-ground SuDS attenuation structure. Surface water is likely to be highly loaded with silt and debris which could reduce capacity before an asset has even been commissioned. 

During both construction and operation, it may be necessary to limit the weight of vehicles that pass over the top of the below-ground SuDS attenuation asset, depending on the loading that it has been designed for. Trees should not be planted above the structure either, unless this has been allowed for in the design and a membrane to protect from root penetration included.

With an increasing emphasis on circularity and reducing whole-life carbon emissions, end-of-life scenarios for below-ground attenuation should also be considered. Since developments will have a design life beyond 50 years, below-ground assets may need to be rehabilitated or replaced, with the possibility of re-using or recycling some or all of the elements.

In conclusion

Delivering value in a below-ground SuDS attenuation asset requires competence, governance and diligence at each phase of its lifecycle.

Corner cutting at any stage could lead to a service life that is shorter than the intended design life. If this happens, the best-case scenario would be that significant interventions such as rehabilitation or replacement would have to happen sooner than intended, adding to financial and carbon costs. The worst-case scenario is that a failure in performance leads to a flooding or pollution event or both, with all the financial, social and reputational costs that these would bring.

SuDS practitioners are becoming better informed and aware of the water quantity and quality requirements, for mitigating flooding and pollution. An optimum solution considers both the SuDS attenuation and the treatment train to provide the best solution at the lowest capital and operational cost.

Although in theory SuDS should already be designed, installed and maintained so that they function over the lifetime of a development, the implementation of Schedule 3 in England will be a game-changer. It is the most significant advancement for SuDS in a generation and will help to remove the ‘rogue’ private sector that hitherto has resulted in a race to the bottom.

To view our feature in Drain Trader March 2023 click here.