Client: Commonwealth War Memorial
Type of works: supply of 60m3 installation of ACO StormTank attenuation system.
Challenging ground conditions including poor natural drainage and surface-water ponding, inherent at the site of new a Commonwealth War Memorial in Fromelles northern France, have been overcome by a water management system developed by ACO Water Management. The land around Fromelles typically consists of heavy clay but by using a network of land drains linked to a high capacity attenuation cell created using ACO StormTank, the memorial site, together with its adjacent car park and access road, are kept clear of surface water while the surrounding areas remain unaffected by any additional run-off from the new areas of hardstanding.
The new memorial site commemorates the troops that fell during the battle of Fromelles in July 1916 and is the first such project to be undertaken by the Commonwealth War Graves Commission for over 50 years. Whilst few alternative options were available the location of the site is ideal from an historical perspective. It lies at the heart of the village and offers views over the former front line. The ground conditions however presented a number of technical challenges, explains Neill Robinson, Welsh Business Manager at ACO Water Management. Although located on an incline the site has limited natural drainage due to its heavy clay soil. This combined with a high water table across the surrounding area, means that the chosen location is prone to frequent surface water ponding.
To ensure that the site remains open and accessible to visitors year round, ACO has developed a fully integrated sustainable drainage system that is in balance with the natural surface water conditions in the area. Run-off from the site and the surrounding fields is first captured in a network of land drainage channels. These take intercepted water to a 60m3 ACO StormTank attenuation cell that holds and controls the release of run-off to an adjacent brook.
Hydrological conditions placed tough limiting values on the depth of attenuation tank we could use. To achieve the capacity required we needed to maximise the void volume within the tank to ensure it could be accommodated within the available space. Says Neil Robinson-Welsh. An orifice outlet from the tank is sized to control discharge volumes. It was a critical element of the design as the delicate surface water environment in the surrounding area could easily be disturbed by uncontrolled stormwater surges from land surrounding the memorial site.
ACO StormTanks innovative design uses high strength central pillars and side walls to provide load-bearing capacity and structural integrity.
A completed tank only requires side panels at its perimeter to provide lateral support. The result is a totally barrier free interior void that can be easily accessed, inspected and cleaned. The free flow of storm water entering the tank also avoids having to use expensive pipe manifolds and additional pipework to distribute flow evenly.
The ACO StormTank at Fromelles is configured to attenuate all the water it receives. To achieve a watertight, stable installation, the tank is encased in an impermeable welded geomembrane. A silt tunnel has also been incorporated within the tank which directs the first flush of any storm straight through the system to a downstream manhole where larger debris can be captured and removed. This alleviates the possibility of the system becoming blocked by ensuring that sediment is not allowed to accumulate within the system over the long term.