Below-ground SuDS attenuation: when design life and service life don’t tally – Drain Trader March 2023

Courtesy MPA Precast (2)

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 1990Basis of structural design
EN 1991-2Eurocode 1 – Actions on structures; Part 2 – Traffic loads on bridges and other civil engineering works
EN 1997-1Eurocode 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
BS5911-1Concrete pipes and ancillary concrete products (recently updated to align with Eurocodes)
Table of Standards relevant to plastic pipes  
EN 13476Plastics 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 C737Structural and geotechnical design of modular geocellular drainage systems
BS EN17152-1Plastics 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 4982Plastics 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 F2418Standard Specification for Polypropylene Corrugated Wall Stormwater Collection Chambers
AASHTO LRFDBridge Design Specifications Section 12.2
ASTM F2787Standard practice for structural design of Thermoplastic Corrugated Wall Stormwater Collection Chambers


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.