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.

Changes to the Design and Construction Guidance for adoptable sewers gives the green light for arch-shaped attenuation chambers.

The Design and Construction Guidance (DCG) for sewers offered for adoption has been updated to include information about arch-shaped below-ground water attenuation chambers. This will make it easier for water companies to adopt such drainage infrastructure – given that certain criteria are met.

“Previously, there was no reference to arch-shaped attenuation structures in the DCG which has meant that it has been more difficult for water companies to adopt them. That all changes with the addition of this extra section,” explains Stuart Crisp, UK manager at Advanced Drainage Systems (ADS) whose StormTech system complies with the new DCG requirements.

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

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

Version 2.2 of the DCG, dated 29 June 2022 and first available through the Water UK website in November last year, means that arch-shaped attenuation structures have now been added to the adoptable SuDS family.

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

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

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

The inclusion of arch-shaped structures has been initially well-received by water companies, reports Stuart Crisp – and paves the way for developers to offer StormTech for adoption, without having to do additional work to prove its suitability. “With the ramping up of legal requirements around pollution control and nutrient neutrality, the timing of this change is good,” he says. “StormTech’s inbuilt water quality treatment stages can enhance removal rates, reduce the need for other treatment and cuts costs.”

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

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

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 importance of design flexibility.

When designing a below ground stormwater attenuation system, it’s important to consider not only the hydraulic and structural requirements, but the site conditions, the construction process, the necessity for maintenance, the environmental concerns and the technical, legal and client constraints.

The flexibility of the system to accommodate a multiplicity of factors is thus central to the solution – can the design be optimised to meet the relevant conditions and controls?  A truly adaptable stormwater attenuation system will mean engineers can produce the most effective design, contractors can install the system with speed and ease, and asset owners can inspect and maintain the system efficiently, whatever the conditions.

Traditional pipe- and crate- based solutions have their drawbacks.  Large diameter pipes are less efficient in terms of the ratio of footprint area to attenuated storage volume.  Geocellular crates may have greater versatility to meet challenges in terms of accommodating storage within the space available, but the necessity for additional TSS removal and separate water quality treatment to meet environmental quality standards can add to the cost of construction and upkeep.

With the importance of design flexibility, ADS StormTech comes in a wide range of chamber sizes giving engineers the scope and flexibility to meet required attenuation volume and flow rates, the constraints of the site, the installation footprint and depth, and the position of inlets and outlets.  Isolator Row, the free water quality treatment device, provides a provenly effective and low maintenance solution for meeting environmental standards.  Layouts can be adapted to suit local conditions, potentially going beyond the published standard construction guidelines.  For example, deep installations or poor soil strength may be accommodated by altering the spacing between the chambers, the depth of the foundation material and the degree of compaction of the embedment stone.

This is particularly true when minimising the construction depth of the attenuation system.  This largely depends on two factors: minimum cover depth and the height of the stormwater storage system.  Shallow systems may help avoid contact with high groundwater, and lower dig depth can reduce dewatering costs and installation delays in saturated ground.  Shallow systems can also minimise the cost of excavation, muck-away and embedment/backfill, including fewer transport movements to and from the site.  Reducing the depth of the attenuation system may also enable the invert depth of the entire storm water drainage system to be raised, resulting in additional cost savings extending beyond the attenuation system itself.

Many StormTech chambers have 450mm min cover depth under HGV traffic loads based on the standard manufacturer’s installation guidelines. Pipes and crates normally need c600mm – 800mm min cover depth.  For crates, if the depth of cover available is less than the minimum recommended by the manufacturer, a protective concrete slab is frequently placed above the crate system.  This is expensive and time-consuming to construct and makes it almost impossible to access the system from above after installation.

Quality Assurance is inevitably a key part of true design flexibility. StormTech fully complies with the requirements of ASTM F2787, ASTM2418 and ASTM F2922 and Section 12.12 of the AASHTO LRFD Bridge Design Specifications safety factors for live and permanent earth loads.  These Standards provide the basis of product quality and of safe structural design.  Further assurance is provided in the UK through rigorous independent testing and assessment within the scope of BBA certification.

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 July 2022