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

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 water quality management.

Water Quality Management – mitigating the impact of pollutants in surface water runoff to prevent harmful discharges into the environment – is one of the fundamental principles of SuDS Best Practice. Essentially, this covers three key material groups:

• suspended solids – silt and other particulate material flushed off the drainage catchment surface
• hydrocarbons – oil and petroleum-based materials deposited (or spilt) on the ground, usually as a result of vehicles and machinery in operation, or worked on, in the vicinity.
• Metals – a range of metals-based materials that can cause harm to the environment, including copper, zinc, lead.

Increasingly, Water Quality Management is a key focus for SuDS systems. Planners are looking to provide “nutrient neutrality” i.e., reducing existing sources of nutrient pollution to mitigate the nutrients generated by new development. In these more demanding cases, removal of additional substances such as compounds of phosphorous and nitrogen are often required.

It is the job of key stakeholders in the Project Team to ensure that the water quality discharge meets the requirements of legislation, client, and the local environmental regulator.

Numerous reference sources exist that can help users determine the most appropriate water quality mitigation measures for a specific project. These include the CIRIA publication C753 The SuDS Manual. Chapter 4 deals with Designing for Water Quality and Chapter 26 deals with Water Quality Management. Chapter 14 relates to Proprietary Treatment Systems. Other Chapters provide guidance on specific vegetative “soft” SuDS components such as Detention basins, Swales, Infiltration systems and Filter strips. Vegetative SuDS are generally regarded as preferable in terms of SuDS Best Practice, as they are surface based solutions that manage rainfall closest to where it lands, also referred to as “source management”. Like any system, vegetative SuDS will require maintenance to ensure that they meet their design function throughout the lifetime of a development.

Designing an effective below ground SuDS attenuation system that meets the demands of both flood risk mitigation (Water Quantity Management) and pollution risk mitigation (Water Quality Management) requires a Management Train approach, where proprietary Water Quality Treatment device(s) are usually installed upstream of the below ground Attenuation Tank. This would typically require a sediment capture system, which may be as basic (and potentially inadequate?) as a catch pit or more appropriately, a proprietary gravity separation sediment tank or hydrodynamic separator. Hydrocarbon removal will require a bypass or full-retention oil/water separator, depending on the intended use of the development and the level of risk of hydrocarbon pollution.

Below ground attenuation systems such as crates and pipes offer little in the way of water quality treatment capability, so the demands of water quality must be covered by separate, additional SuDS components elsewhere in the system.

Not only can this require significant capital investment in the treatment system (each manufactured treatment device can cost >£10k, and multiple units may be required on a project), there is an additional operational cost in terms of inspection and maintenance. Failure to properly maintain water quality treatment systems can lead to problems with the attenuation tank, such as sedimentation within the tank, resulting in reduced storage volume– causing the system to fail hydraulically – and the possibility that pollutants are flushed downstream, resulting in water quality breaches and fines.

Some systems, however, include an integrated solution. New to the UK, the ADS StormTech stormwater attenuation system incorporates the unique Isolator Row, a patented, built-in water quality treatment device designed to remove silt and other polluting material flushed off the surface during rainfall.

Isolator Row has 2 treatment stages, contributing to the overall efficiency of the system. These include initial gravity separation of the silt and particulates within the Isolator Row chamber and filtration through a layer of woven geotextile fabric on the bed of the chamber, laid over a stone foundation. Two further treatment stages can take place within the stone embedment surrounding the StormTech chambers. As flow passes through the system, pollutants can adsorb onto the face of the stone, meaning that they are “trapped” and prevented from passing through to the discharge point. This material can provide nutrient for bacteria and over time, will be broken down into harmless, non-polluting material.

The four treatment stages of a StormTech system:

• Gravity separation (sedimentation) Isolator Row (2 stages)
• Filtration
• Adsorption Embedment stone surrounding StormTech system
• Biodegredation

Isolator Row has been independently tested by universities and respected industry bodies (including NJCAT) and validated to remove over 80% of Total Suspended Solids (TSS) plus metals, hydrocarbons, phosphorus, nitrogen, and other surface water pollutants.

Where nutrient neutrality is sought and there is a demand for higher levels of pollutant removal, or where more challenging pollutants need to be dealt with, other water treatment solutions may be required. These can include proprietary filtration systems designed specifically to address certain chemical species and material phases within the surface water runoff. In these situations, when considering a proprietary manufactured treatment device, it is recommended that advice is sought directly from the manufacturer.

For practitioners in the UK, it may be helpful to refer to the British Water Code of Practice: Assessment Of Manufactured Treatment Devices Designed To Treat Surface Water Runoff. This publication sets out an assessment method to measure pollutant capture and retention. Additionally, read in conjunction with the Code of Practice and the CIRIA SuDS Manual, it is possible to derive mitigation indices using the British Water How To Guide: Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices.

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 June 2022