Category Archives: Engineering Features

SuDS design and water quantity: back to basics

 The purpose of SuDS – sustainable drainage systems – is to mimic nature. But should that automatically mean that we only consider nature-based solutions, asks Stuart Crisp.

The weather so far this year has provided a stark reminder of the challenges we face due to heavy and intense rainfall. In January, Storm Henk brought a month’s rainfall in four days to some areas, leading to flooding, the worst of it in the Midlands. And in April, the impacts of Storm Kathleen and Pierrick caused flooding across the country, especially on parts of the south coast.

Much of this flooding is exacerbated by urban development. By replacing green fields and vegetation with hard surfaces – roads, roofs and driveways – we change the way and rate at which surface water flows out of an area or catchment.

A good SuDS design for water quantity aims to mimic the flows of water from developed site so that they are as close as possible to what would have happened, had there still been a greenfield site there. That means throttling the flow of water – in other words providing storage and releasing it later, more slowly – so that it is not rushing so quickly into sewers and water courses and overwhelming them, leading to flooding.

Many policy documents from organisations that currently adopt SuDS – typically Scottish Water in Scotland, the county or unitary authority in Wales or a water company in England mandate natural or green SuDS, often prohibiting the use of proprietary or grey SuDS.

While green SuDS, such as swales and ponds are the right solution for some developments, a blinkered approach may not best mimic nature, could ramp up capital costs and potentially lead to longer-term problems and higher maintenance and operation costs.

Although designing for water quantity and the urgent need to reduce the risk of flooding is vital, there are the other pillars of SuDS to think about too: water quality, amenity and biodiversity. Previous articles in Drain Trader’s June 2023 and February 2024 issues, looked at water quality issues and what type of management trains were best suited to different types of development, depending on pollutant loads.

Operational and maintenance costs for SuDS schemes are often overlooked and ignored, but the promised implementation of Schedule 3 of the Flood and Water Management Act 2010 in England will shed a harsh light on these. SuDS Approval Bodies (SABs), likely to sit within local authorities, will require robust information about inspection and maintenance regimes and expected costs over the lifetime of a development. 

An article in Drain Trader’s March 2023 edition looked at why poorly thought-out SuDS can lead to shorter service lives and higher operational costs than those expected from the design.

Mimicking nature
On a greenfield site, the first 5mm of rain would not typically generate surface runoff, it would infiltrate into the ground, evaporate or transpire through the leaves of plants. Then, when water flows from the surface of a catchment and exceeds the capacity of the receiving water body, the additional volume of water would spill over onto the floodplain.

When we develop on that catchment to add impermeable surfaces, such as roofs, roads and hardstandings, the amount of surface water runoff generated can increase dramatically. In a dense, urban environment, 95% of the rainfall could flow off the catchment as surface runoff, increasing the volume of water that reaches water courses or sewers. It is also likely that the peak flow will be higher and come sooner, than had it remained a greenfield site.

CIRIA C753 The Suds Manual gives a graphical illustration of this. Figure 3.1, in Chapter 3 which deals with design for water quantity, shows hydrographs for the discharge rates of surface water for an area in three situations: when it is greenfield; when developed without flow attenuation; and when developed with flow attenuation. (Figure 1 shows a similar graph, courtesy of SUSDRAIN).

The amount of water that a SuDS system will need to attenuate or store, and the rate at which the water should be discharged is the subject of the hydraulic design for that system. For those that want to go back to first principles, these are set out in BS EN 16933-2 Drain and sewer systems outside buildings – Design – Part 2: Hydraulic design.

Today, however, many drainage engineers rely on software such as InfoDrainage and MicroDrainage (AutoDesk/Innovyze), Flow (Causeway) and Site3D. But this ‘black box’ approach to calculations can mean that designers don’t have the opportunity to properly understand the assumptions and coefficients that have been used – and the impact on water quality based on the SuDS components selected to satisfy the hydraulic design – which may lead to a suboptimal design.

In a SUSDRAIN factsheet from March 2014, Assessing attenuation storage volumes for SuDS – another useful resource for designers – author Anthony McCloy explains the risks of this approach:

“Don’t expect exact answers from the calculation process, it is a usable approximation that can provide acceptable solutions for design. Most of the inputs are based on statistics and calibration factors; therefore we can only ever achieve an approximation of how the system will behave in reality. The results of calculations and modelling need to be used alongside professional judgement to provide the design.”

Ten years later, this point is just as relevant as ever, perhaps even more so as design software is more widely used, with newer generations of engineers never called on to design from first principles. McCloy also advises that those assessing potential SuDS, such as SABs, must also have a basic understanding of first principles of storage volumes and hydraulic design so that they can carry out their statutory duties.

Water storage toolbox
There are many ways that we can create storage for excess surface water. Storage can be online, meaning that flow enters the element, passes through and out the other side or offline where flow enters and exits via the same point. A design could include both online and offline storage where, for example, flow above the 1-in-30-year return period is directed offline to accommodate the 1-in-100-year event.

Vegetative or surface-based solutions include ponds, detention basins which are dry until excess water needs to be accommodated and swales which can be used to accommodate volume, as well as to communicate flow between SuDS elements and for infiltration.

Ponds can be an attractive choice, potentially ticking boxes for all the four pillars of SuDS. From a water quantity perspective, they must have sufficient capacity to cope with rising water levels during higher rainfall events throughout their design lives. Any sediment entering the pond will settle out quickly, since the sediment particles soon reach terminal velocity allowing them to settle to the bottom of the pond.

As an asset owner or adopter, it is important to anticipate the amount of sediment that will settle out over time, because the accumulated sediment must be somehow removed at intervals to allow the pond to provide sufficient storage volume. One issue that is sometimes overlooked in the design of ponds is providing safe and cost-effective means of accessing the pond to remove sediment when required.

A recent story from Gloucestershire illustrates the costs involved. Local residents petitioned Gloucester City Council in January this year to desilt Saintbridge pond in Abbeydale because it produces a foul smell in the summer and the silt is negatively impacting on water quality and habitats. The council refused, saying that it was only 15 years since it last desilted the pond and that the operation would cost £700,000.

Commenting on the story, Jo Bradley, director of operations at Stormwater Shepherds, pointed out the error of not having installed an upstream sediment separator at the same time as the pond. “If a manufactured sediment separator had been included, it could have been emptied every year, costing maybe £700 – £1000. That would have cost up to £20,000 over the 20-year cycle and avoided the £700,000 cost that they are now facing.”

Bradley pointed out that by bringing in heavy plant every 20 years, removing vegetation and sediment, the pond’s habitats and inhabitants are disturbed, negatively impacting on nature. And she added that the sediment in the pond could well be contaminated with toxic, bio-accumulative pollutants and tyre-wear particles.

CIRIA’s manual says that a sediment separator or sediment forebay should be installed upstream of every pond. And, depending on the pollutants likely to be washed into the pond with the surface water, other pre-treatment could be needed. Without this, there is a danger that wildlife will be attracted to the pond, only to suffer damage.

This point was illustrated by researchers at  Glasgow University, who compared SuDS ponds with natural ponds and found that pollutants were higher in some of the SuDS ones, negatively affecting amphibian breeding and development. A paper published on the research issues this warning: “The function of SuDS and other urban drainage systems to sequester pollutants increases their potential to be ecological traps by advertising false cues of suitable habitat.”

Proprietary attenuation
The most commonly used proprietary solutions include permeable paving, where the water passes through the gaps between blocks to be stored in unbound granular material below them; geocellular crates which are buried below ground; and large-diameter pipes below ground. There are also innovative systems, new to the UK, such as arch-shaped chambers which combine both filtration and attenuation without the need for additional upstream devices.

Permeable block paving can be useful because it requires a limited volume of excavation. In terms of water quality, block paving provides two stages of water treatment: water is filtered as it passes through the gaps between blocks and pollutants can then be broken-down by bacteria naturally present as it passes through the granular fill beneath it and then either infiltrates the ground or flows into another part of the system.

The capacity of permeable paving to accommodate surface runoff decreases over time. And regular maintenance is needed to clean out the joints between the paving blocks so that they do not become clogged up, making sure that the wash water does not enter the drainage system served by the permeable pavement. Also, note that water companies do not adopt permeable paving as it is not only a SuDS component, but also a structural pavement.

Geocellular crates can be a good choice if there is a limited plan area for water storage, since they provide a large volume for a limited footprint. They are typically manufactured from polypropylene (PP) or PVC by injection molding, or extrusion of joining thermoformed sheets.

Crate-based systems are relatively simple to design and due to their lightweight and modular construction, they are easy to install. There are a variety of types on the market, from shallow sub-base replacement systems to be used beneath car parks to crates which can be used at a greater depth for higher volumes of storage.

However, crates are likely to decrease in storage capacity over time. Installation of catchpits, silt separators and other sediment pre-treatment measures upstream are required – but often under-designed, which can lead to issues as maintenance to remove sediment build up in crate-based systems can be difficult or impossible. There are also question marks over their long-term durability, as some crates do not comply with the latest industry specifications for structural integrity.

Clause 21.5.3 and Table 21.2 of the SuDS Manual suggests adding an additional 10% to the storage volume of crates to account for sediment build up. Asset owners and adopters of SuDS systems should determine if a crate-based system has been appropriately up-sized by 10% and if not, that the SuDS designer or manufacturer has provided evidence to demonstrate that all the sediment can be removed from the crate-based system. Not all adopting bodies will accept crates.

Large diameter pipes, laid as a single run or in parallel, with manifolds are an established solution, one of the forerunners of proprietary underground water attenuation which can be created from existing products. They can be a cost-effective choice with concrete and plastic the most commonly used materials; thin steel pipes and a hybrid plastic and steel product are other choices.

A downside to large-diameter pipes is that they are larger and can be heavier to transport and install, requiring more transport movements for delivery, more storage space on site and a larger excavation footprint. Depending on the type of pipe material, they may not be adoptable by some asset owners.

Innovative underground water storage systems such as arch-shaped chambers are simple to design and install and can be configured to fit irregular areas or to fit around existing infrastructure or obstacles. Their arch shape means that the embedment material around them is shaped into ‘stone columns’ allowing them to be installed at shallower depths, while taking heavier loadings. They are lightweight and stackable which means they are easy to transport, store and install.

One proprietary brand of arches, StormTech, can be installed with an integrated pre-treatment element, Isolator Row, which takes out sediments and other surface water pollutants from the first flush of a rainfall event. The inclusion of Isolator Row can remove the need to install upstream pre-treatment devices such as sediment traps and bypass separators, resulting in considerable cost savings. StormTech has been designed for ease of maintenance, since only Isolator Row needs to be cleaned, accessed from the manhole using standard sewer cleaning equipment.

This brand of arches provides four stages of water treatment: Isolator Row provides the first two stages.  Sedimentation – gravity separation of the silt particles, settling out on the bed of Isolator Row – and filtration, as the water passes through a layer of woven geotextile fabric. 

The next two treatment stages are akin to that for permeable concrete block paving; that is adsorption as the water passes over the granular material surrounding the chambers and biodegredation from the action of bacteria breaking down pollutants into non-polluting material. StormTech meets the requirements for adoption by water companies as set out in the Design and Construction Guidance (DCG) for adoptable sewers.

Green and grey
Attenuation of surface water in vegetative SuDS can be a good solution for some developments, particularly where there is lots of space and low pollutant loads. But insisting that they are the only choice could lead to designs which do not best mimic nature or provide the best options.

A more flexible approach to SuDS would allow a combination of green and grey elements to be deployed. This could allow some water to be stored in green SuDS such as ponds and swales, with proprietary solutions such as below-ground pre-treatment devices and attenuation chambers, deployed elsewhere. Alternatively, crates or arched chambers could be installed as additional storage beneath a smaller pond or under a parking area to deliver multiple SuDS benefits within a smaller available footprint.

A good SuDS design also factors in the cost and ease of maintenance of the various SuDS elements. There is a misconception that natural SuDS can be left to nature, but this is not the case. Vegetation needs to be maintained, litter must be picked at frequent intervals and for ponds, sediment build-up needs to be removed, as explained above, to maintain the capacity of the pond.

Equally for proprietary SuDS elements, maintenance to preserve the hydraulic capacity of water storage and the functioning of water quality treatment devices is an important consideration. When products are substituted in ‘value-engineering’ exercises which are, in fact, capital cost reduction exercises, the impact on water quality is often overlooked. Although pollutant loads could look low enough that upstream treatment is not needed, there will be a need to consider the long-term performance of the SuDS system over the lifetime of the development.

Finally, there is the issue of climate change. Although SuDS designs do apply factors to account for increasing rainfall intensity and frequency, they do not consider the fact that more intense periods of rainfall increase the amount of sediment and pollutants that are swept along with surface water. Add an extended dry period before that rainfall event, and the problem will be worse.

To limit designers to vegetative SuDS is to limit the choice of tools they have available to them. With a bigger toolbox of grey and green SuDS elements, they have a greater opportunity to create solutions that are technically, environmentally and economically viable over their entire lifetimes.

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

With strengthening focus on flooding and water quality, CIRIA’s advice to oversize crate-based attenuation tanks by 10% has never been more relevant, says Stuart Crisp, UK manager at Advanced Drainage Systems (ADS).

There’s a reason why the CIRIA SuDS Manual C753 suggests that “difficult to clean” SuDS attenuation systems, such as below-ground crates, should be oversized by 10%: there is a risk that sediment will build up in them, reducing their storage capacity.

With increasing legislation to manage flood risk and to prevent pollution, and more intense rainfall events, this advice becomes even more pertinent. There is a higher volume of water for SuDS devices to deal with and, when these events follow dry spells, more particulate matter and pollutants are swept along with surface water.

Unfortunately, this need to upsize crate-based attenuation tanks by 10% is often overlooked, thanks to focus on originally calculated target storage volumes. Furthermore, if the development is ‘value-engineered, the capital cost saving also often focusses on the original target storage volume. Up-sizing is often disregarded and, if the design is substituted for another type of attenuation system, the implications for water quality and the effectiveness of the treatment train are ignored. 

The result is an increased risk of flooding and an increased risk to water quality.

Risk-based approach

The SuDS manual suggests a risk-based approach to designing SuDS management trains. Considering sediment, this means assessing: how much is likely to build up, depending on the location, how that load might change over time, and how often and easily devices will need to be cleaned. 

Clause 21.5.3 of the manual explains the rationale for upsizing below-ground attenuation by 10%, while Table 21.2 shows the potential loss of storage capacity. Typically, commercial developments suffer from the highest sediment loading, followed by car parks, high-density residential developments and highways.

Removing sediment is also necessary to protect water quality and to avoid the negative impacts of pollutants. The predicted type, quantity and concentration of pollutants governs the choice of natural or manufactured SuDS element that would be most suitable.

Some underground attenuation systems, such as ADS’s StormTech arched system with its Isolator Row, are designed with in-built treatment systems which can be easily cleaned using standard equipment. This can negate the need for – and cost of – upstream treatment devices, while still protecting attenuation capacity and water quality.

Pay now or pay later

With the anticipated implementation of Schedule 3 of the Flood and Water Management Act 2010, SuDS adopting bodies will be looking hard at maintenance issues.  Asset owners will need to be assured that the SuDS they are taking on will perform as designed throughout their whole lives.

Short-term savings in capital cost, which skimp on capacity and compromise water quality, can lead to long-term problems and greater whole-life costs, resulting in higher commuted sums for developers and an additional, avoidable cost burden on UK plc.  

If a system is claimed to be exempt from the 10% up-sizing guidance, evidence that the system can be completely and easily cleaned should be demanded.

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.

With legislation mandating SuDS and their adoption for new developments on the horizon, developers and designers need to 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 is 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 Stuart Crisp, UK manager at Advanced Drainage Systems (ADS). “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 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,” 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 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, leading to some assets not being of a prescribed, consistent standard of quality and performance or properly maintained and monitored, leading to problems down the line.

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, with the benefit of built-in pollution treatment, reducing the extent of additional treatment elsewhere in the SuDS system.

It is expected that Schedule 3 will change the adopters 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 are likely to 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,” 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 CPDs, 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.

SuDS designs must consider water quality and water quantity in parallel – and use robust design data to avoid problems for asset owners, warns Stuart Crisp, UK manager at Advanced Drainage Systems (ADS).

SuDS design is becoming ever more complex. There’s a growing awareness that we must look after our water quality more carefully, backed up by policy such as the Plan for Water, published last year.

Where there are risks that surface water will carry pollutants, it must be treated, whether using natural or proprietary components, to reduce them to where they are not considered harmful to the environment. Where pollutant loads are low, natural vegetative SuDS may be able to do that job. Where they are higher, some form of manufactured treatment may be needed.

The asset owner needs to be assured that all the components of a SuDS system will perform as designed in relation to water quality as well as water quantity – and that they will continue to do so over the lifetime of the development. That includes ensuring that the data related to the pollutant removal capability of those components is robust. Otherwise, there is a risk that an asset becomes a liability.

Designing for water quality

The CIRIA SuDS Manual C753 suggests a risk-based approach for designing water quality in SuDS management trains – the series of components that make up the drainage system. It provides pollution hazard indices for different types of site, and for three forms of pollution: total suspended solids (TSS), metals and hydrocarbons. It also gives mitigation indices (MIs) for natural SuDS components. 

The designer must then select components whose combined MIs meet or exceed the pollution hazard indices for the predicted pollutants, noting that only 50% of the published MI value can be used for components downstream of the first treatment stage.

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. This allows manufacturers of treatment devices to publish their MIs to a recognised methodology and designers can select the right combination of elements in a SuDS management train using reliable data, verified by a third party, to meet the pollution hazard indices of a development area.

Adopting a problem

Although it now looks like Schedule 3 of the Flood and Water Management Act 2010 may not be activated until 2025, England will need to meet more demanding requirements for water quality. At this point, the body adopting the SuDS must ensure that they are taking on something that performs as claimed over the whole of its lifetime.

Our advice would be to do due diligence to determine how MIs have been derived, what methodology was applied and to insist that the process and results have been verified by a third party expert.

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

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

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.

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.

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.

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.

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.

.

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.

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 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.

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

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.

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.