{"id":9605,"date":"2025-03-17T11:48:45","date_gmt":"2025-03-17T10:48:45","guid":{"rendered":"https:\/\/www.gpsj.co.uk\/?p=9605"},"modified":"2025-03-17T12:07:52","modified_gmt":"2025-03-17T11:07:52","slug":"innovation-in-attenuation-ready-for-take-off","status":"publish","type":"post","link":"https:\/\/www.gpsj.co.uk\/?p=9605","title":{"rendered":"Innovation in attenuation \u2013 ready for take-off"},"content":{"rendered":"\n

By Jack Shuttleworth, Director of Forma<\/em><\/p>\n\n\n\n

A need to control costs while delivering demanding sustainability improvements is changing thinking about water attenuation.<\/p>\n\n\n\n

The conventional\u00a0 approach, to install large tanks to hold water captured in gullies and slot drains, is proving, in many instances, to be no longer fit for purpose.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Most optimised attenuation<\/strong><\/p>\n\n\n\n

It is why Manchester Airports Group (MAG), the UK\u2019s largest airport group, selected a different approach to attenuation when it upgraded one of its main staff car parks.<\/p>\n\n\n\n

Forma has been designed to streamline attenuation. It does it by keeping drainage shallow, creating a hugely flexible and scalable way to manage stormwater at source, without large tanks.<\/p>\n\n\n

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It is the most comprehensive and optimised subbase attenuation system on the market, and is simpler, quicker, less costly and safer to install.<\/p>\n\n\n\n

Forma is also much more sustainable, with a carbon footprint a fraction of the size of traditional tank-based attenuation systems.<\/p>\n\n\n\n

Forma \u2018ticked all boxes\u2019<\/strong><\/p>\n\n\n\n

MAG\u2019s 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.<\/p>\n\n\n\n

The project was designed by AECOM, the main contractor was Allied Infrastructure Management, part of Colas, and the attenuation installation contractor was SEL Environmental.<\/p>\n\n\n\n

MAG had found the existing drainage and tank attenuation system was not holding water consistently.<\/p>\n\n\n\n

Replacing it like-for-like would have represented a big proportion of the cost, time and risk associated with the project. So MAG was open to new ideas.<\/p>\n\n\n\n

When we presented Forma, it recognised an ideal solution.<\/p>\n\n\n\n

Daniel Wilmer, Capital Delivery Project Manager at MAG, said: \u201cMy brief was to come up with a solution. We\u2019re all about innovation and sustainability and Forma ticked those boxes.<\/p>\n\n\n\n

\u201cI would like to use it across other sites. They\u2019ve taken a lot of stress away and have been really reassuring in terms of what we require.\u201d<\/p>\n\n\n\n

Void space to hold water<\/strong><\/p>\n\n\n\n

Central to the Forma solution for MAG\u2019s car park is a product called Formavoid. It\u2019s a modular system made from recycled plastic, here in the UK, designed with extremely strong columns and arches.<\/p>\n\n\n\n

Quickly clipped together, fill material can be poured on top of the Formavoid raft, while 60% open \u2018void\u2019 space remains below for source control water storage and management.<\/p>\n\n\n

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The interlocked Formavoid modules \u2013 just 100mm deep \u2013 are positioned on top of a very strong, wedge-welded puncture-resistant geotextile membrane, called Formatextile.<\/p>\n\n\n\n

The in situ car park surface and drainage system was removed with a 450mm-deep excavation. The Formavoid enhanced subbase attenuation system was installed across an area of 8,500m\u00b2.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Innovative water system<\/strong><\/p>\n\n\n\n

The Forma system was installed in five months by just six operatives in the second half of 2024.<\/p>\n\n\n\n

Formavoid modules were integrated with 160mm-diameter plastic transport pipes. Just 100 metres of pipework were needed to collect water from across the carpark and take it to an outfall point.<\/p>\n\n\n\n

A key innovation for this project was the use of checkdams within the subbase. This maximised the system\u2019s attenuation volume across sloping ground.<\/p>\n\n\n\n

The surfacing was completed with permeable asphalt in the parking areas and impermeable asphalt on the vehicle circulation route.<\/p>\n\n\n\n

Right solution \u2018critical\u2019 – MAG<\/strong><\/p>\n\n\n\n

The Forma solution is already performing very well. It has exceeded expectations in terms of structural stability, achieving in excess of 80% CBR (a standard aggregate strength test) on top of the subbase.<\/p>\n\n\n\n

This is very extremely impressive for a permeable open-graded subbase system, with much of the stability created by the Formavoid raft.<\/p>\n\n\n\n

The system is design to attenuate a 1 in 100 year storm with an additional safety factor of 40% for future climate change.<\/p>\n\n\n

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Importantly, the Formavoid subbase is the ideal environment to intercept, contain and biodegrade hydrocarbons, as proven by many studies on pervious pavements.<\/p>\n\n\n\n

So pollutants are removed from surface water before it is discharged into the stream, eliminating the need for expensive interceptor tanks.<\/p>\n\n\n\n

Olivia Hughes, Senior Project Manager at MAG, pointed out: \u201cDue to the staff car park being adjacent to a site of special scientific interest, it was critical that we did the right thing.<\/p>\n\n\n\n

\u201cThe Forma system has been beneficial to MAG because it allowed us to only excavate to a shallow depth. It\u2019s also a carbon zero product so we are reducing our emissions.\u201d<\/p>\n\n\n\n

Attenuation system of choice<\/strong><\/p>\n\n\n\n

The modular design of Forma will allow MAG to continue to implement the system in phases. For example, the rest of the car park could be upgraded in 2,000m\u00b2 sections.<\/p>\n\n\n\n

This aligns the solution with capital availability allows maximum flexibility to keep car parks operational. Forma is a progressive solution. Every square meter installed is a progressive improvement.<\/p>\n\n\n\n

The project has also delivered many other advantages for MAG:<\/p>\n\n\n\n