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

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 subject of inspection and maintenance.

When designing a stormwater attenuation system, sometimes too little attention is paid to cleaning and the costs of ongoing maintenance. System maintenance is essential to ensure that the intended performance is retained throughout the service life and that the risks of flooding and pollution are avoided.

This vital part of asset management can be overlooked at both the design and procurement stages, as the choice is often based on the incorrect assumption that all below-ground attenuation systems are equally easy and low-cost to maintain.

However, this is not the case. For example, because of their structure, stormwater attenuation crates are difficult to get access into the body of the tank for cleaning and require upstream silt removal to prevent a downstream build-up of material within the tank body. Whilst large diameter pipes are easier than crates to access, they too require upstream intervention for silt and hydrocarbon removal.

Both pipes and crates are therefore dependent on the additional expenditure and maintenance of an upstream silt separation and removal system. If silt gets into the storage tank, it can compromise the flow path and reduce the storage volumes – eventually causing the whole system to fail.

But that’s not all. Silt particles can carry pollutants and if these get into the storage tank, they may flush downstream and result in water quality breaches, pollution, and fines.

Crates and pipes, therefore, depend heavily on expensive upstream pre-treatment systems which, together with the attenuation tank, require regular inspection and maintenance. Conversely, the ADS StormTech stormwater attenuation system may not require additional upstream pre-treatment, therefore reducing capital and operational costs.

This is because ADS StormTech is engineered to provide an efficient, durable, low-maintenance solution for commercial, residential, industrial, and infrastructure installations. A key part of this is the unique Isolator Row, a ‘free’ built-in water quality treatment device designed to remove silt, sediment, and polluting material flushed off the surface during rainfall. Isolator Row is “sacrificial”, meaning that other StormTech chambers within the system are protected from solid particles and pollutants in the runoff entering, and they will not require any cleaning during their service life. This keeps maintenance to a minimum. Furthermore, 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. Isolator Row, as an integral part of the StormTech system, is recognised by many authorities as a water quality treatment device.

After many years of successful use in over 40,000 installations worldwide, Isolator Row has been proven to need cleaning on average once every 3-7 years, depending on the nature of the installation, using standard sewer cleaning equipment, saving asset owners both time and money in stormwater system maintenance.

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, May 2022

The Francisco Street Reservoir now serves as a source of useful non-potable water at a park for the local community, thanks to a stormwater collection system underneath.

Francisco Street reservoir was the first large reservoir in San Francisco, California when originally built in 1859. Decommissioned in the 1950s, the site was to be redeveloped but the Francisco Park Conservancy fought to keep it as a natural resource that would include harvesting rainwater. Underneath the 1.8 hectares Francisco Park is a stormwater capture and reuse system which was installed in 2021 to perpetually provide water for the park’s irrigation and toilets.

How the stormwater system works

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 ensures that the water meets public health regulations, while saving 5.7M litres of potable water every year.

The engineers decided to use a system of arched chambers because they would provide the largest storage volume per square metre.

“The reservoir is on a slope that is just under 20%. It’s a very challenging site from a variety of perspectives,” explains Sherwood Design Engineers (San Francisco) principal Cody Anderson who is responsible for the stormwater system.

“Normally you’d have your catch basin at the bottom of the site. Here, the historic reservoir is midway up the slope, so half the runoff is collected via gravity flow and the rest collected at the bottom of the site and pumped. All captured runoff flows through the screening filtration and into the chambers for storage and later use.”

Equipment used

Three hundred and seventy two ADS StormTech chambers, were installed using a 35m by 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. ADS StormTech chambers provided the best ratio of storage volume to footprint area.

The StormTech chambers are independently tested, BBA-approved and fully compliant with ASTM F2787, F2418 and F2922 stormwater storage systems standards. StormTech is typically used for below ground SuDS attenuation projects and more than 2.5M chambers have been used successfully around the world in over 40,000 projects. Three StormTech Isolator Rows are included in the Francisco Street reservoir system. These patented water quality treatment devices are integral to the StormTech system. They capture the “first flush” and trap sediment and other pollutants coming from stormwater runoff.

StormTech chambers come in a wide range of sizes, making them easy to install for all conditions. They are highly adaptable and can be configured around obstacles as well as affording multiple inlet and outlet positions. Standard pipe manifolds, manhole and access chamber inlet/outlet structures and flow controls can be used.

Reclaiming an area of San Francisco

“There are competing products on the market,” says Anderson. “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.”

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

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 speed and efficiency of construction.

Everyone knows that time is money. However, there’s rarely been such pressure on build time.

Sites are starting late. COVID 19 and Brexit has created labour shortages and manufacturers are struggling to meet demand. And with Britain now officially 6% wetter than in the previous 30 years, weather-related delays are only going to increase – particularly at this time of year. It’s no surprise that across the industry, site programmes are impacted, and contractors are struggling to meet deadlines and avoid penalty clauses.

Considering the speed of installation of the SuDS system at the design stage, can make life significantly easier for clients and contractors alike.

With large diameter pipes, site logistics, transport movements and storage space need to be factored in. Pipes are not the most efficient in terms of the amount of attenuated storage that can be achieved per delivery, and the footprint area required for a given attenuated storage volume is usually greater than most other forms of proprietary solution.

Vaulted systems can be slow to construct, and whilst geocellular crates are often quicker to install, difficulties with maintenance and the need for reliable pre-treatment often count against them. New to the UK, ADS StormTech chambers are fast and easy to work with, often requiring no mechanical lifting. They also provide easy access for inspection and are simple and low cost to maintain.

An important consideration in a drainage system is managing the hydraulic requirement against the available footprint area and construction depth. Traditionally, the flexibility of geocellular crates has made them a popular choice for tight footprints. However, ADS StormTech chambers are available in a range of sizes; the larger size chambers can accommodate the required attenuation volume in asmaller footprint, and the smaller chamber sizes can enable shallow construction depths in situations with high groundwater levels, minimum cover depth below traffic loads and shallow outfalls.

More flexible systems, such as ADS StormTech, are even able to maintain the necessary hydraulic flow around obstacles such as lighting columns.

The difference is particularly noticeable when it comes to more difficult terrain, especially when obstacles, such as lighting columns, below ground utilities and other structures need to be accommodated. Whilst solutions such a permeable block paving remain viable, large diameter pipes, geocellular crates and vaults are often unable to easily maintain continuous hydraulic flow through the system. StormTech chambers, are highly adaptable and can be configured around obstacles and afford multiple inlet and outlet positions. Standard pipe manifolds, manhole & access chamber inlet/outlet structures and flow controls can be used.

Whilst it’s perfectly possible to accommodate such terrain within a more traditional system, it can add time and complexity at both the design and installation stages. More flexible solutions may exist to optimise the design, improve efficiency and performance.

It’s important that the designer takes all these factors into consideration and is fully aware of the product types available and the pros and cons of each. An informed decision can then be made, fully understanding how the choice of system will impact on installed cost, performance, efficiency, buildability and maintenance.

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 April 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 the first in a regular series of articles, Stuart considers the different challenges facing designers and installers of below ground SuDS attenuation systems. This month, it’s the primary engineering considerations.

Most new build sites across the UK will now have a requirement for a water attenuation system in order to control stormwater that lands on the site. Before sites are built on the ground it will often naturally soak up rainfall however when new buildings are created there is nowhere for the rainfall to go. Water attenuation systems work by holding back the water in a storage vessel on site and slowly releasing the rainwater to the storm drain at a rate in which the infrastructure can handle.

Whilst there are many different types of stormwater storage and attenuation solutions: permeable block paving, geocellular crates, large diameter pipes, vaults and plastic corrugated arch structures such as ADS StormTech, not all systems provide the same attributes. Product choice is therefore central to the engineering solution. A simple designation of inflow rate, outflow rate and storage volume could result in a poorly executed project.

A well-designed attenuation project will not only consider the hydraulic functionality of the system (flow control and volume control), but also the fundamental principles of SuDS Best Practice, often cited as the four pillars of SuDS, i.e. Water Quantity, Water Quality, Biodiversity and Amenity.

Aligning the engineering design parameters for below ground attenuation with the four pillars of SuDS Best Practice

Consideration of these design parameters may lead to a decision to provide attenuation below ground. Cost of land is frequently the main consideration and the developer’s desire to maximise the return on their investment, although other issues such as Planning Gain and a requirement to provide parks, car parks or other civil amenities could also be a factor. In this context, it should be noted that all four pillars of SuDS Best Practice may not be achievable, although the hydraulic performance of the system is essential and must always be satisfied to mitigate against flooding.

To help achieve SuDS Best Practice, some proprietary below ground systems can provide integral water quality treatment, such as the StormTech system from Advanced Drainage Systems (ADS). This solution can contribute to the water quality requirements of the project without the need for additional expensive equipment. Other systems that are unable to provide water quality treatment may require additional SuDS components, such as separate pre-treatment for silt removal.

ADS StormTech includes Isolator Row – a ‘free’ water quality treatment device

At the design stage, a number of other limitations should be considered, such as legal and technical constraints and the client’s specific requirements. The below ground structure will become an infrastructure asset and as such it will require appropriate structural robustness and durability.

It is also vital to consider how the attenuation system will be built. The choice of system used can affect speed and cost of construction and other factors such as the number of transport movements to/from site, storage space, footprint to volume ratio, excavation and muck-away costs, minimum depth of cover, invert depth / construction depth / proximity to groundwater all need to be considered as part of the optimum design solution. Crucially – and often overlooked – is the ease of inspection and cost-effectiveness of maintenance. Sometimes, the most traditional solution in terms of hydraulic requirements, can give the client an ongoing problem in terms of effective cleaning and upkeep.

Over the coming months, we shall be taking a closer look at all of these factors and how best to accommodate them in an optimised SuDS attenuation system.

To find out more about ADS UK’s Water Attenuation Systems contact us today on 0203 442 0607 or click here.

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 March 2022