Articles & Case Studies

Car park gets superstore treatment with flood protection from Graf UK

Posted: Thursday 27th July 2017

In preparation for the opening of a new, large-scale retail unit, a car park in central Stafford has been fitted with a heavy-duty stormwater attenuation solution from water management specialist Graf UK. As well as providing safe, effective drainage and flood prevention, the new system was specified to comply with strict depth restrictions and compulsory local authority run-off regulations.

The Kingsmead car park, which is located next to wetlands and sits partially on top of a natural flood plain, had previously been home to a council-run sports complex and swimming pool. After this was demolished, the car park was acquired by private investors, who earmarked the site for a new superstore. In the interim, Kingsmead was still being used as a public car park, but had fallen into general disrepair - meaning it was in need of extensive upgrades to bring it up to the standards required for a superstore. These included resurfacing with new tarmac and introducing new kerbing throughout, both of which would increase run-off rates during heavy rainfall - and, in turn, heighten flood risk.

Lincoln-based project managers Simons Group appointed environmental consultancy RPS Group to assess the site and recommend an efficient, reliable drainage and flood management solution. This needed to comply with Stafford Borough Council discharge criteria of no more than 155 l/s, and a maximum outflow rate into the adjacent drainage field of 240l/s. Because of the car park's position on top of a flood plain, the tanks also had to be no deeper than 1200mm below finished levels. Designs specified that a complete, two-tank stormwater attenuation system was needed, so specialist Hertfordshire-based civil engineering consultancy AD Bly Construction was tasked with finding a solution. After confirming the required volume capacity, AD Bly passed the designs to the Graf UK team, who proposed 592m3 of EcoBloc Maxx modules. AD Bly then carried out all necessary groundworks, before Graf UK's expert installation team was called in to fit the system.

David Romani, Project Manager at AD Bly, explains: "Kingsmead had got to a point where it was essentially wasteland, and was in need of complete redevelopment to make it suitable for use as a busy supermarket car park. As part of this, RPS Group created drainage system designs stipulating that the site needed a brand new stormwater attenuation system to manage rainfall effectively and minimise flood risk. Our biggest challenges were twofold: firstly, we only had around 1.5m of space below the final ground level to play with, and secondly, we had very specific discharge rates to comply with. Luckily, the flexible design of the EcoBloc Maxx modules meant that the system could be just one cell deep, while still fulfilling the required volume capacity.

"The install took around three weeks from start to finish. We were in charge of all the necessary excavation works to prepare the ground for the tanks, and then the Graf team stepped in to put them in place. The whole system is surrounded with an impermeable geotextile membrane, so no water can escape unchecked. Instead, it is released in a slow, controlled way into a series of purpose-built ditches. The larger of the two tanks discharges water at a rate of 125 l/s, while the smaller one does so at 30 l/s - ensuring the overall discharge rate stays within the 155 l/s limit at all times. This, combined with a discharge rate of 85 l/s from the area of the car park that sits on the existing flood plain, makes the overall outflow rate into the adjacent drainage field 240 l/s: exactly in line with local authority parameters."

Once the tanks were sealed, AD Bly used select fill to build up the ground above them by 300mm, covering the system below. They then filled and compacted the rest of the car park using sub-base materials, bringing the overall level up to 300mm (to match the area above the tank) - before finishing the surface with new tarmac throughout. All materials used for building the ground up were recovered from elsewhere on site and recycled for the job. The main purpose of this process was to even out the car park's overall surface, which had previously been sloping in places. As well as alleviating any drainage issues caused by water pooling in lower-level areas, the higher ground level provides additional protection for the stormwater attenuation tanks underneath.

David concludes: "One of the major reasons we used Graf and their EcoBloc modules for this project is that the cells are stackable, so come to site flat-packed. This meant they could be loaded onto fewer vehicles than comparable pre-assembled systems - keeping transport costs and emissions down. Importantly, we'd also worked with Graf before, and knew we could rely on the quality of its products and workmanship. Now that the system is hidden beneath the brand new car park, you wouldn't know it was there at all. It does the job it's supposed to do unaided. Even if groundwater levels rise and submerge the tanks, the butyl membrane surrounding them ensures that it won't leak into the system and compromise performance."

Graf UK's EcoBloc Maxx systems are completely modular. They are made from expertly designed plastic cells that fit seamlessly together to build a tailor-made stormwater solution. For Kingsmead, this meant they could be made to the exact shape outlined in the drainage designs, and positioned as shallow underground as required. Its innovative design not only allows for straightforward installation, but provides high storage volume and has a heavy-duty lorry-bearing capacity of 40 tonnes - which is crucial for a car park like Kingsmead. Even under extreme loads, EcoBloc Maxx boasts a storage coefficient of 96%.

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