The need to control costs and improve sustainability is shifting perspectives on water attenuation systems. Jack Shuttleworth from Forma discusses an innovative stormwater management system at Manchester Airport, using a modular approach that eliminates the need for large tanks
The conventional approach of installing large tanks to hold water captured in gullies and slot drains is proving, in many instances, to be no longer fit for purpose. Climate change, causing more volatile weather, the subsequent need to reduce carbon emissions, a need to redevelop live and brownfield sites with complex ground conditions, and a strong imperative to control costs are stacked against tank systems that have held sway for decades.
Most optimised attenuation
This is why Manchester Airports Group (MAG), the UK’s largest airport group, adopted a different approach to attenuation when upgrading one of its main staff car parks. Forma has been designed to streamline attenuation.
It achieves this by maintaining shallow drainage, creating a highly flexible and scalable method for managing stormwater at the source, without the need for large tanks. It is the most comprehensive and optimised subbase attenuation system on the market, and is simpler, quicker, less costly and safer to install. Forma is also significantly more sustainable, with a carbon footprint that is a fraction of the size of traditional tank-based attenuation systems.
Forma ‘ticked all boxes’
MAG’s project perfectly demonstrates the advantage of shallow drainage. It needed to resurface and upgrade one of its largest park-and-ride staff car parks at Manchester Airport.
AECOM designed the project. The main contractor was Allied Infrastructure Management, a part of Colas, and the attenuation installation contractor was SEL Environmental.
MAG had found that the existing drainage and tank attenuation system was faulty and breaching performance parameters. Replacing it like-for-like would have represented a significant proportion of the cost, time and risk associated with the project. So MAG was open to new ideas. When we presented Forma, it recognised an ideal solution.
Daniel Wilmer, Capital Delivery Project Manager at MAG, said: “My brief was to come up with a solution. We’re all about innovation and sustainability, and Forma ticked those boxes.
“I would like to use it across other sites. They’ve taken a lot of stress away and have been really reassuring in terms of what we require.”
Void space to hold water
Central to the Forma solution for MAG’s car park is a product called Formavoid. It’s a modular system made from recycled plastic, designed in the UK, featuring robust columns and arches.
Quickly clipped together, the fill material can be poured on top of the Formavoid raft, while a 60% open ‘void’ space remains below for source control, water storage, and management.
The interlocked Formavoid modules – just 100mm deep – are positioned on top of a robust, wedge-welded, puncture-resistant geotextile membrane, called Formatextile.
The in situ car park surface and drainage system were removed through a 450mm deep excavation. The Formavoid enhanced subbase attenuation system was installed across an area of 8,500m2.
The system was designed to manage around 1,000 cubic metres of water. It was also more than capable of withstanding loads created by thousands of car movements and bus journeys every day.
Innovative water system
Just six operatives installed the Forma system in five months during the second half of 2024. Form avoid modules were integrated with 160mm-diameter plastic transport pipes. Just 100 metres of pipework were needed to collect water from across the car park and transport it to an outfall point.
A key innovation for this project was the use of check dams within the subbase. This maximised the system’s attenuation volume across sloping ground.
Right solution ‘critical’ – MAG
The system is designed to withstand a one-in-100-year storm with an additional safety factor of 40% for future climate change, all above formation and within the subbase.
Olivia Hughes, Senior Project Manager at MAG, pointed out: “Due to the staff car park being adjacent to a site of special scientific interest, it was critical that we did the right thing.
“The Forma system has been beneficial to MAG because it allowed us to only excavate to a shallow depth. It’s also a carbon-zero product, so we are reducing our emissions.”
Attenuation system of choice
The modular design of Forma will enable MAG to continue implementing the system in phases. For example, the rest of the car park could be upgraded in 2,000m2 sections.
This aligns the solution with capital availability, allowing maximum flexibility to keep car parks operational. Forma is a progressive solution. Every square meter installed is a progressive improvement.
The project has also delivered many other advantages for MAG:
- The solution streamlined and simplified all elements of a large attenuation installation project.
- Much less material was excavated and moved, which saved significant costs.
- It also dramatically collapsed the project’s carbon footprint.
- Fewer truck movements meant less air pollution and improved road safety.
- The construction method was safer, as well, with no confined space entry.
- The project was completed much faster than a traditional tank-based solution.
Giving SuDS compliance a sporting chance
The Forma system is not just changing minds about water attenuation in the world of airport management or car park design. It can be applied to many other water management challenges. For example, a Formavoid raft has been installed below the playing surface in a new stadium belonging to one of the UK’s most prestigious sports clubs.
Forma, based in Blackpool, Lancashire, is working with partners to expand its use and create the perfect drainage systems for the world’s most high-profile sporting arenas.
Find out more
Forma has an accredited CPD programme to explain shallow attenuation to drainage engineering and design consultancies, architects, and government planning teams.
This work is licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International.
