SuDS management trains for water quality: some practical examples

Following on from their article on SuDS and water quality in our June 2023 issue, Jo Bradley and Stuart Crisp take a practical look at how to design SuDS management trains to protect water quality – as well as preventing flooding, creating amenity and enhancing biodiversity.

In our previous article we looked at the issue of water quality in SuDS design. Historically, SuDS designers have often focussed on flood prevention and amenity, but a well-designed system must consider measures to remove pollutants too. With an increasing focus on water quality from legislators and the public, this element of SuDS design is more important than ever.

Clearly, the way that surface water is treated before it enters a water body or returns to the ground will vary depending on how high the risk of pollution is. This article looks at what that means in practice through some illustrative examples of what SuDS management trains may look like for low, medium and high-risk development types.

A SuDS management train is a combination of interconnected SuDS components, used in series around a development site to capture, store and convey surface water and – where needed – to treat it to remove particulate, dissolved and dispersed pollutants. Designers must choose from a variety of SuDS components, both natural and manufactured, to control water quantity and quality, to create amenity value and to increase biodiversity.

The biodiversity piece of the SuDS puzzle is gaining more weight as new regulations come into force for larger developments. In simple terms, the biodiversity net gain regulations mean that planning permission will only be granted to new developments that enhance the natural environment rather than detract from it.

Deploying natural SuDS elements such as filter strips, swales or ponds can be a good way to increase biodiversity. In this respect the new biodiversity requirements appear to dovetail well with the planned enactment of Schedule 3 of the Flood and Water Management Act 2010, now expected in 2025, which will mandate SuDS for new developments in England.

However, there is a caveat. The mantra that ‘natural is best’ is leading to some misguided decisions.

SuDS designs that don’t take pollutant loading into account are resulting in the destruction of vegetation and in the creation of new habitats which are potentially poisoning the wildlife they have attracted. For instance, a study by Glasgow University indicated that pollutants in some SuDS ponds are inhibiting amphibian breeding and development, when compared to natural ponds in the same area.

A basin serving a residential development in Cheshire

What’s the risk?

For sites where there is a low risk of pollutants being present in surface water runoff, such as small or medium-sized housing developments, a focus on natural SuDS works well. However, where there is a higher risk of the presence pollutants such as sediments, metals, hydrocarbons and nutrients, a more balanced approach to designing SuDS management trains is needed, with an understanding of the expected concentrations of such pollutants and how to deal with them.

The CIRIA SuDS Manual C753 provides a method for dealing with water pollution risks by determining pollution hazard indices for a particular site and then matching SuDS components in the treatment train which have combined pollution mitigation indices equal to or exceeding the pollution hazard indices. The manualprovides pollution hazard indices for a variety of land uses and for three types of pollution: total suspended solids (TSS), metals and hydrocarbons. It also provides mitigation indices for natural SuDS components.

For manufactured components, British Water’s 2022 How To Guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices, provides a method for calculating mitigation indices for TSS, metals and hydrocarbons.

Note that maintenance considerations are a key part of the design process. All SuDS components, including pretreatment and storage elements, whether natural or manufactured, must be properly maintained so that the system operates as designed. Failure to do so could mean that pollutants are washed on into water bodies and ground water.

More background on these issues can be found in our previous article in the June 2023 issue of Drain Trader.

Low risk Case Study 1: Medium sized residential development

Pollution hazard level	Low
Total suspended solids (TSS)	0.5
Metals	0.4
Hydrocarbons	0.4

Table 1: Pollution hazard indices for medium-sized housing development (source: CIRIA C753 Table 26.2)

Type of SuDS component*	TSS	Metals	Hydrocarbons
Swale	0.5	0.6	0.6
Permeable pavement	0.7	0.6	0.7
Detention basin	0.5	0.5	0.6
Bioretention system	0.8	0.8	0.8

Table 2: Mitigation indices for SuDS components selected (source: CIRIA C753 Table 26.3)

*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.

A bioremediation zone in the middle of Sheffield city

Consider a new housing development of around 50 homes. According to the CIRIA SuDS Manual, there are likely to be low levels of pollutants in any surface water runoff which means that they can be dealt with using natural SUDS.

Surface water from roads around the development could be collected in swales – shallow drainage channels which run parallel to the roads. These could convey water into a detention basin which would fill up during a heavy rainfall event and then dry out over time, so that pollutants left on the surface are degraded. Run-off from roofs and footpaths would be directed into bioretention zones or rain gardens with engineered media and planting to store and attenuate water.

Residential parking areas may have permeable paving to allow the rainwater to return to the ground where it falls. Permeable concrete block paving is often cited as a permeable paving media, but alternatives include appropriately graded bituminous and concrete pavements, grass reinforcement and bound or unbound gravels.

In areas where planning requirements call for phosphorus neutrality, the treatment train should start by maximising the opportunity for infiltration of stormwater to the ground, since the soil will capture phosphorous in the runoff. A bioretention zone or rain garden would be a good way to achieve this.

Water that cannot be infiltrated may need to pass through a suitable sediment capture component such as a vortex grit separator or oil/water separator (road gullies and catch pits are not recommended) which removes most of the sediment before transferring flow to growing plants that can remove more of the phosphorous. More guidance can be found in CIRIA publication C808 Using SuDS to reduce phosphorous in surface water runoff.

Low risk Case Study 2: School

Pollution hazard level	Low
Total suspended solids (TSS)	0.5
Metals	0.4
Hydrocarbons	0.4

Table 3: Pollution hazard indices for school (source: CIRIA C753 Table 26.2)

Type of SuDS component*	TSS	Metals	Hydrocarbons
Swale	0.5	0.6	0.6
Permeable pavement	0.7	0.6	0.7
Detention basin	0.5	0.5	0.6
Bioretention system	0.8	0.8	0.8

Table 4: Mitigation indices for SuDS components selected (source: CIRIA C753 Table 26.3)

*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.

When designing SuDS for schools, there should be a strong focus on amenity. There is a fantastic opportunity to tell the story of the water cycle to the children who attend the school through the choice of components selected for the SuDS management train.

Water from the roof of the building can be carried in leaping gutters from the edge of the building into a bioremediation zone, which is an area of vegetation with layers of gravel and sand below them, designed to channel and filter surface water. Or the water could run down rain chains into planters, which the children would be able to plant up each year.

As for the housing development, school parking could utilise a permeable pavement. Water from the access road could run off into swales or filter strips.  If the water can be managed at the surface, so that the children can see it moving around the site, this can help them to see how precious rainfall is.

For some schools, it may be possible to include a pond or permanent wetland, subject to a risk assessment which should take into account factors such as the ages and abilities of children at the school, whether they can access the water body when unsupervised and the depth of the water. Where they can safely be included, ponds or wetlands in the management train creates habitats for wildlife and provides new learning opportunities.

Medium risk Case Study 3: Retail car park

Pollution hazard level	Medium
Total suspended solids (TSS)	0.7
Metals	0.6
Hydrocarbons	0.7

Table 5: Pollution hazard indices for retail car park (source: CIRIA C753 Table 26.2)

Type of SuDS component*	TSS	Metals	Hydrocarbons
Vortex separator	0.8	0.5	0.7
Detention basin	0.5	0.5	0.6

Table 6: Mitigation indices for SuDS components selected (source: CIRIA C753 Table 26.3)

*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.

For a site where there will be a larger car park, such as a retail development, SuDS management trains must be designed to cope with anthropogenic pollutants in the surface water: tyre wear particles, brake dust and hydrocarbons. Permeable paving may not be appropriate as an infiltration system as the risk of pollutants getting into the groundwater could be too high.

In this imagined development, there is sufficient space around the parking bays for a detention basin which will hold surface water in the case of heavy rainfall events. However, before the water goes to the basin, it must be pre-retreated to remove as much of the sediment as possible, and with it the pollutants that are attached to the sediment particles. A vortex separator, upstream of the basin, could perform this role.

From a biodiversity perspective, note that just because it is possible to create a new habitat doesn’t mean it is the right thing to do. Creating a small green space in a very dense urban area with no connectivity to other green spaces could be detrimental, rather than beneficial, to any wildlife that finds its way there.

Medium risk Case Study 4: Car park, constrained space

Pollution hazard level	Medium
Total suspended solids (TSS)	0.7
Metals	0.6
Hydrocarbons	0.7

Table 7: Pollution hazard indices for retail car park (source: CIRIA C753 Table 26.2)

Type of SuDS component	TSS	Metals	Hydrocarbons
StormTech with Isolator Row	0.8	0.6	0.7

Table 8: Mitigation indices for SuDS components selected (source: British Water Web Site – List of Assessed Surface Water Treatment Devices)

For car parks and other areas with a similar pollution risk, where the space for vegetative SuDS is limited, underground attenuation and treatment could make sense. The constraint on space could be due the limited land availability, which can be the case with smaller brownfield plots, or because losing too many parking spaces would make a development unviable.

A swale alongside a road on Buckshaw village, in Lancashire

In this situation, below-ground attenuation and treatment devices could work well. While components such as geocellular crates need additional devices upstream of them to remove sediment and other pollutants, there are now systems that have an in-built and easy-to-clean treatment system. In this example, we deploy the StormTech system which includes an Isolator Row to filter out sediment. This can be cleaned out at intervals using standard sewer jetting equipment, accessed via a manhole.

High risk Case Study 5: Motorway,

Pollution hazard level	High
Total suspended solids (TSS)	0.8
Metals	0.8
Hydrocarbons	0.9

Table 9: Pollution hazard indices for motorway (source: CIRIA C753 Table 26.2)

Type of SuDS component*	TSS	Metals	Hydrocarbons
Oil/water separator	0.8	0.6	0.9
Pond	0.7	0.7	0.5

Table 10: Mitigation indices for SuDS components selected (sources: SPEL Class 1 Separator product literature; CIRIA C753 Table 26.3)

*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.

Although runoff from many motorways and major trunk roads currently goes straight into water courses without being treated, this should not be an option for new motorways. There is a high risk that there will be heavy amounts of pollutants in the water and that these will do harm to wildlife and plants.

A pond serving a new highway in Cheshire

For any application with high pollutant loading and a known presence of harmful runoff that can cause damage to the environment, the water should be treated before entering any vegetative components of a SuDS management train. In this example, we would deploy a properly sized oil/water separator which can take out over 80% of the TSS before it goes into the pond/basin and will capture oil spillages from accidents on the carriageway.

Having been treated to remove the sediment, the water could flow into a pond. Choosing a pond rather than a basin means that more of the residual sediment will settle out because the body of water in the pond slows down the flow more effectively and the sediment rests in one place for longer. However, pollutants clinging to the sediment will remain at the bottom of the pond and may need to be removed periodically. 

It is important to get the balance right between pre-treatment in a manufactured device, where the sediment can be easily removed, and treatment in the pond where removal of the sediment is more difficult and expensive. Savings made in capital costs by selecting a smaller separator will soon be overshadowed by the long-term costs of removing sludge from the pond.

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High risk Case Study 6: waste management site

Pollution hazard level	High
Total suspended solids (TSS)	0.8
Metals	0.8
Hydrocarbons	0.9

Table 11: Pollution hazard indices for waste management site (source: CIRIA C753 Table 26.2)

Type of SuDS component*	TSS	Metals	Hydrocarbons
Oil water separator	0.8	0.6	0.9
Stormwater filter	0.8	0.6	0.7
Retention basin	0.5	0.5	0.6
Automatic closure device at outlet from drainage network

Table 12: Mitigation indices for SuDS components selected (sources: SPEL Class 1 Separator product literature,; CIRIA C753 Table 26.3)

*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.

The first step in designing a SuDS management train for a waste management site would be to determine what pollutants would be present on the site and their expected concentrations.  These could include metals, hydrocarbons, chemicals and organic compounds.

Once the pollutants have been identified, suitable filters, treatment media or other manufactured devices can be specified. Surface water from any waste processing facility should be treated before it enters any vegetative SuDS features.

For our imagined site, which is processing mixed household waste at a recycling centre, there are risks of leaks and spillages from the oil tank, battery recycling, green waste, discarded paints and household pesticides and more. An oil/water separator will capture oil spillages but spillages of other substances, and pollution from dissolved metals must be captured using additional devices. This could be a special type of stormwater filter, using treatment media to capture the dissolved pollutants.

The SuDS management train should conclude with a vegetative device such as a retention basin where natural processes and micro-organisms can break down any residual pollutants, and the last suspended particles in the runoff can settle on the bottom of the basin. These devices will need to be monitored and maintained, and regular inspections will ensure that any problems are identified and rectified quickly.

Since waste treatment sites can have a high risk of fire, another important water quality issue for waste treatment sites is how the water used to fight a fire would be captured and treated. There needs to be a closure device at the outlet of the site to prevent contaminated water from leaving the area.

Informed decisions

The design of SuDS management trains is a multi-faceted exercise. Beyond controlling the quantity of water leaving a development, designs must preserve water quality and optimise opportunities to add amenity value and boost biodiversity.

Where the risk of pollutants in surface water runoff is low, the SuDS management train may consist only of natural components. For sites with a medium risk of pollutants, surface water should be treated before it reaches detention features such as ponds, basins or proprietary below-ground systems. Pre-treatment using natural or manufactured components may be possible, depending on circumstances. Surface water from high-risk sites with high pollutant loading should be treated by a suitable manufactured device before it is discharged into vegetative SuDS components.

Differences in the mitigation indices and the sizing of SuDS components to meet the treatment flow rate and the hydraulic (maximum) flow rate also need to be taken into account. Designers also need to have an understanding of the risk of the captured pollutants being flushed out during exceedance events and the pros and cons of the alternative options available.

Design choices should also take maintenance requirements and costs over the lifetime of a development into account, not least because of the need to create asset management strategies and agree commuted sums where SuDS are to be adopted. No system will perform as designed from a water quality or quantity perspective if its components are not properly inspected and maintained. This applies to both vegetative and manufactured elements.

Financial viability is an unavoidable issue. Current regulations regarding SuDS and commercial viability vary in Scotland and Wales. England has yet to decide; a consultation on the implantation of Schedule 3 is expected this year. For restricted brownfield sites, such as the one considered above, it could make sense to allow some room for manoeuvre, depending on the social, economic and environmental impacts of the planned development.

If we are to put together all the pieces of a SuDS jigsaw to the best effect, there must be early conversations between a broader range of stakeholders. Professionals across the supply chain will need to upskill and share knowledge. Some manufacturers of proprietary SuDS systems provide CPD seminars to help SuDS practitioners prepare for the recent changes and those round the corner.

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 3rd party verified data.

British Water has published a Code of Practice for the Assessment of Manufactured Treatment Devices 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

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, details of which can be found on their website.

For more information on Advanced Drainage Systems’ SuDS attenuation components, including a handy Design Tool, Mitigation Indices Data Sheet, Structural Design Technical Note and Installation App or to book a CPD, go to www.adspipe.co.uk

Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), looks at looming legislation which will mandate Sustainable Drainage Systems and explains why their adoption will require new skills for specifiers and developers.
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 Sustainable Drainage Systems (SuDS) mandatory in England, as it has been in Wales since 2019. In Scotland, Schedule 3 has not been implemented, but SuDS is generally a requirement within planning legislation.

“Schedule 3 is a game changer for the drainage industry”

Subject to consultation, the implementation of Schedule 3, which includes SuDS approval and adoption, is expected in late 2024. That means there is less than a year for specifiers and developers to get up to speed with the range and implications of possible solutions, both above and below ground.

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.

Designers and specifiers will have to think about more than just hydraulic design, and consider whole-life maintenance and treatment 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 the design and specification of systems that are being planned now.

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 to jump through the adoption hoops. The end result of this is that some assets do meet prescribed and consistent standards of quality and performance, may not be properly maintained and, consequently, there are problems down the line.

The DCG was updated last year to include arch-shaped belowground attenuation structures. One such system offers a flexible and cost-effective alternative to other below-ground attenuation structures such as crates or large-diameter pipes, with the benifit 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 new statutory guidance will be introduced, 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 the 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.

In April 2023, Defra published its so-called ‘Plan for Water’ (fully titled Our integrated plan for delivering clean and plentiful 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 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, road run-off and chemicals.

Among the raft of proposed legislation are changes to planning policy which aim to ensure 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 for Water references the Government’s intention to finally implement Schedule 3 of the Flood and Water Management Act 2010, which covers SuDS approval and adoption, in England – subject to consultation. This could make the inclusion of SuDS standard practice in the design, construction and adoption of nearly all new developments from 2024.

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 lifetime of a scheme.

SuDS Approval Bodies (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, the developer will provide a commuted sum as funding to the SAB at the point of handover.

Run-off from roads

The Plan also talks about the strategic road network and how more is needed to prevent pollution from highway run-off discharging into water courses. There are various approaches to treat polluted run-off from roads before it is infiltrated into the ground, enters a water body or stormwater sewer, with the possibility of using SuDS as part of the water treatment train.

Run-off from roads can contain 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. The result is a build-up of toxic pollution in riverbeds, water, fish and other aquatic life.

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 Plan for Water mentions this and 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.

Nutrient pollution

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 indicate that phosphorus pollution is not a major problem from stormwater runoff from most urban catchment surfaces. However, well designed SuDS help alleviate nutrient pollution where it is a problem. CIRIA guide C808, Using SuDS to reduce phosphorous in surface water run-off, published in 2022, provides guidance on how to do this. Similarly, the recently published CIRIA C815, which relates to SuDS for nitrogen reduction, may be regarded as a companion document to C808.

More SuDS, better quality

With water quality concerns moving up the agenda for members of the public, as well as national and local governments, the need for well designed, constructed and maintained SuDS will only increase. Depending on parameters including application, land area available, levels of pollution and flow rates, natural SuDS, engineered SuDS or a combination of the two can be the most appropriate solution.

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.

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).

Stormwater Shepherds UK

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.

Stormwater Shepherds UK

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.

Stormwater Shepherds UK

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.

Stormwater Shepherds UK

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.

Stormwater Shepherds UK

Outfall from a highway into a river in Lancashire being sampled in September 2021 for the highway runoff box out.

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.

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

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.

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.

StormTech arch-shaped SuDS attenuation chambers from Advanced Drainage Systems have been used on over 50,000 projects worldwide. New Eurocode modelling demonstrates they pass muster for projects in the UK and Europe too.

With a global track record that stretches back decades, an underground SuDS attenuation system that exploits the structural properties of the arch is now being designed and installed on construction projects in the UK.

The heart of the StormTech system is its corrugated thermoplastic chambers which have an elliptical arch-shaped cross section. This elliptical profile shapes the embedment around the chambers into stone arches and structural columns, transferring loads away from the chamber into the stiffer material surrounding the chambers so that they can be installed at both shallow and deep cover. Designed for flexibility of layout, ease of installation and transportation, the StormTech system can also incorporate an integral means of removing surface runoff pollutants at no extra cost – which is easy to maintain and can remove the need for costly pre-treatment systems.

Produced by US drainage giant Advanced Drainage Systems (ADS), which is also the largest recycler of plastic in North America, StormTech chambers are designed to US codes and Standards, AASHTO and ASTM International. To ease their acceptance in the UK and other European countries, ADS commissioned a study to model their performance under the Eurocode design methodology.

ADS’s UK manager Stuart Crisp explains: “The US design philosophy is different to the Eurocode one,” he says. “This study translates the US approach and demonstrates with complete certainty that the StormTech system performs under the Eurocode design models, when installed using our standard construction details.”

Testing scenarios

To investigate the performance of the StormTech system, the seven different sizes of chambers were put through their paces using a finite element analysis (FEA) model, which looked at limit state modes of failure as set out EN 1991-2 – Eurocode 1 – Actions on Structures – Part 2. Some engineers may be familiar with CIRIA C737, which covers the design of thermoplastic crates for underground water attenuation, which also suggests Eurocode modelling as a means of demonstrating structural adequacy.

As per ISO/DIS 4982 which covers arch-shaped chambers, the FEA model was used to test the various chambers in the most demanding loading scenarios. At shallow depths, it is live traffic loads at the surface that are most likely to cause failure. For maximum cover, it is the long-term loading of the backfill material which must be considered.

Load models for four different stress and fatigue cases were applied, according to EN 1991-2 with cover in accordance with the ADS StormTech Construction Guide.

• Load case 1: minimum cover and Load Model 1 (subclause 7.2)
• Load case 2: maximum cover and Load Model 1 (subclause 7.3)
• Load case 3: minimum cover and Fatigue Load Model 3 (subclause 7.4)
• Load case 4: minimum cover and Load Model 1 with braking forces (subclause 7.5)

The modelling considers the shape of the arches and material properties. The sections are injection moulded from a thermoplastic, which means that the long-term performance of the material under loading must be taken into consideration.

Performance proven

The FEA modelling proved that all the StormTech chambers are structurally adequate for each of the load cases detailed above. For minimum cover situations, there is significant additional structural capacity; for maximum cover, more of the chambers’ capacity was used but they were still comfortably within their capacity.

Crisp hopes that these calculations will help engineers and contractors to make the case for using StormTech. “Contractors are already using the system because they see the benefits in cost-effectiveness, particularly when expensive pre-treatment systems can be eliminated and when excavation depth can be reduced for installations under roads with HGV traffic,” says Crisp. “This study means that when designers and installers want proof of structural performance to Eurocodes, evidence is to hand.”

• For more detail on the Eurocode modelling study visit www.adspipe.co.uk.
• For more information on Advanced Drainage Systems, call +44 (0)203 442 0607 or contact Stuart Crisp, UK Manager at stuart.crisp@adspipe.com

Click here to download our Eurocode Modelling Study

The Francisco Street Reservoir now serves as a source of useful non-potable water at a park for the local community, thanks to a stormwater collection system underneath.

Francisco Street reservoir was the first large reservoir in San Francisco, California when originally built in 1859. Decommissioned in the 1950s, the site was to be redeveloped but the Francisco Park Conservancy fought to keep it as a natural resource that would include harvesting rainwater. Underneath the 1.8 hectares Francisco Park is a stormwater capture and reuse system which was installed in 2021 to perpetually provide water for the park’s irrigation and toilets.

ADS StormTech chambers store water for reuse at this park in San Francisco.

How the stormwater system works

The stormwater is stored in a 1.9M litre cistern at the top of the hill before being transferred to the service building, where it flows through a series of filtration and disinfecting processes. This ensures that the water meets public health regulations, while saving 5.7M litres of potable water every year.

The engineers decided to use a system of arched chambers because they would provide the largest storage volume per square metre.

“The reservoir is on a slope that is just under 20%. It’s a very challenging site from a variety of perspectives,” explains Sherwood Design Engineers (San Francisco) principal Cody Anderson who is responsible for the stormwater system.

“Normally you’d have your catch basin at the bottom of the site. Here, the historic reservoir is midway up the slope, so half the runoff is collected via gravity flow and the rest collected at the bottom of the site and pumped. All captured runoff flows through the screening filtration and into the chambers for storage and later use.”

Equipment used

Three hundred and seventy two ADS StormTech chambers, were installed using a 35m by 45m area of the existing reservoir and then covered with soil. This gave a total storage capacity of 2,000m³ of water in a 1,682m² footprint. ADS StormTech chambers provided the best ratio of storage volume to footprint area.

The StormTech chambers are independently tested, BBA-approved and fully compliant with ASTM F2787, F2418 and F2922 stormwater storage systems standards. StormTech is typically used for below ground SuDS attenuation projects and more than 2.5M chambers have been used successfully around the world in over 40,000 projects. Three StormTech Isolator Rows are included in the Francisco Street reservoir system. These patented water quality treatment devices are integral to the StormTech system. They capture the “first flush” and trap sediment and other pollutants coming from stormwater runoff.

StormTech chambers come in a wide range of sizes, making them easy to install for all conditions. They are highly adaptable and can be configured around obstacles as well as affording multiple inlet and outlet positions. Standard pipe manifolds, manhole and access chamber inlet/outlet structures and flow controls can be used.

Isolator Row is essentially a ‘free’ in-built water quality treatment device

Reclaiming an area of San Francisco

“There are competing products on the market,” says Anderson. “We needed to store as much water as possible in the given area. We work on projects around the globe with an emphasis on sustainable development and we’re known for having the vision and the technical capacity.

“The Francisco Park is one of those projects of a lifetime. It’s reclaiming an area in the city of San Francisco that is now a beautiful park for the people.”