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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Structural performance

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

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

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

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

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

Product specifications: large diameter pipes

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

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

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

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

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

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

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

Product specifications: crates 

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

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

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

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

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

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

Product specifications: arches

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

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

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

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

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

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

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

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

Construction

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

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

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

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

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

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

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

Maintenance and operation

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

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

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

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

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

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

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

In conclusion

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

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

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

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

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

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.

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.”

Click here to download our Eurocode Modelling Study

A qualified civil engineer, Stuart Crisp has been at the forefront of construction and a specialist in drainage systems for over 35 years. He has contributed to the development of numerous British and International Standards and industry specifications, and was a member of the project steering group for CIRIA C753 The SuDS Manual. In this regular series of articles, Stuart considers the different challenges facing designers and installers of below ground SuDS attenuation systems. This month covers the differing approaches to green infrastructure.

Traditionally, the predominant approach to management of storm water runoff in the UK has been through engineered sewer-based systems that would now be classed as ‘Grey Infrastructure’. In the brave new world of sustainable drainage systems, SuDS attenuation systems that are still generally considered as grey infrastructure typically collect rainfall from impervious surfaces, such as road-ways, hard standings and rooftops, and then store and discharge it below the ground via a series of crates, pipes or arches through infiltration, or into a local water body via a sewer or surface channel at a controlled flow rate.

CIRIA, however, has long-suggested ‘Green Infrastructure’ (GI) as a preferred SuDS solution to effectively manage the impacts of climate change, growing flood risk and policy changes and legislation that place an emphasis on water quality, as well as effective management. GI is defined as “a strategically planned and delivered network of natural and man-made green (land) and blue (water) spaces that sustain natural processes.” The application of GI is recognised in government policy and advocated by bodies such as the Landscape Institute.

Thus, GI is a complementary, alternative stormwater solution, promoting the idea of natural flood management. GI mimics natural hydrology and seeks to

improve water quality and reduce water quantity by capturing runoff as close to the source as possible and infiltrating, filtering, and storing it for re-use. SuDS practitioners often regard GI using vegetative, surface-based solutions as Best Practice. The methods include diversion ponds, wetlands, detention basins, filter strips, grass channels and swales.

The “Four Pillars of SuDS Best Practice” is a recognised model that illustrates the potential for a SuDS system to provide (1) Management of Water Quantity (i.e. mitigation against flooding); (2) Management of Water Quality (i.e. mitigation against pollution); (3) Biodiversity (i.e. attracting wildlife); (4) Amenity (i.e. providing useful space for activities). When below ground attenuation systems are used within a SuDS scheme, it is unlikely that the biodiversity pillar can be satisfied without the combined use of vegetative SuDS components. In terms of amenity, interpretation of this pillar may be rather ambiguous. Some say that the intention is to create blue/green, landscaped open spaces that can enhance the environment and a sense of wellbeing within the community. Others would argue that functional usefulness is no less an amenity, often driven by developers seeking to maximise land usage, such as a car park built over a below ground attenuation system. In terms of the water quality pillar, many below ground attenuation systems provide no water quality treatment and rely on other parts of the SuDS Management Train to remove pollutants from surface water runoff.

Sometimes the hydraulic load, geographic demands or project requirements mean that a GI system is not able to meet the required performance parameters. In these situations, pipes, crates and chambers can be an integral part of a GI project, providing additional scope for enhanced performance or the resolution of technical constraints. For example, where the infiltration capacity of the ground is poor and a downstream connection to a sewer or water body is not practicable, the additional storage capacity provided by a system installed below the surface-based SuDS feature, can be used to retain a larger volume of stormwater, and enable infiltration at a slower rate.

Whilst some systems can help with the storage and movement of surface water, they cannot address water quality unless part of a treatment train. Water quality management can be achieved through an appropriate combination of vegetative SuDS components and/or proprietary manufactured treatment devices. For example, with its unique Isolator Row in-built water treatment device providing two treatment stages, plus a further two treatment stages provided within the embedment stone, a system like ADS StormTech can also be used in combination with vegetative SuDS installations to enhance both the water storage capacity and pollutant removal performance.

Below ground systems with treatment devices can therefore be used in a variety of GI applications. Pervious surfaces, for example, allow the movement of water through the soil and a below ground storage and attenuation system can be installed underneath.

Downpipes directing stormwater from the roof of a building can also form an integral part of a GI system, helping to replenish groundwater in a controlled manner, whilst filtering out sediments and nutrients from the water to decrease pollutant loads. These systems can also be used as part of a sealed tank system providing rainwater harvesting.

An excellent example GI in the broader context of rainwater harvesting and water management is the recently opened 4.5-acreFrancisco Park, where the old San Francisco Reservoir was transformed into a sustainable and cost effective community space with a stormwater capture and reuse system that will perpetually provide water for the park’s irrigation and toilets, all while helping manage stormwater and preserving the natural flora and fauna.

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 includes three StormTech Isolator Rows which capture the “first flush” and trap sediment and other pollutants coming from stormwater runoff. This approach ensured that the water met public health regulations, while saving 5.7M litres of potable water every year.

Three hundred and seventy-two StormTech chambers were installed in a 35m x 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. StormTech chambers were chosen because of their ratio of storage volume to footprint area. “There are competing products on the market,” explains Sherwood Design Engineers (SanFrancisco) principal Cody Anderson, and StormTech was chosen as “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.”

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 40,000 below ground SuDS attenuation system installations using in excess of 2.5m units.

Originally published in Water magazine August 2022