A long-standing flooding issue for Falkirk Council has been protected from peak rainfall risk with a robust stormwater storage solution from Hydro International that saved space and preserved nearby recreational facilities for local residents.
Crossing under the main Edinburgh to Stirling railway, the Chapel Burn has caused repeated flooding close to the town centre of Larbert, particularly in 2006 when a railway cutting was inundated and closed the line.
After considering several solutions, Falkirk Council designed and specified a new cellular storage surface water flood alleviation solution. The successful contractor, MM proposed using the Hydro Stormbloc® modular geocellular storage system. The storage space needed was minimised with a Hydro-Brake® Optimum vortex flow control.
“A hydro brake flow control device allowed us to use the smallest amount of storage required for the back-up volume. We evaluated the solution for attenuation storage and found it provided an ideal combination of effectiveness and low construction costs, in a restricted space and it could be landscaped over thus retaining the popular recreational area,” explains Falkirk Council’s Engineering Design Officer, John Divers.
Initial proposals to manage the risk of seasonal flooding at the site had centred on a permanent attenuation pond adjacent to the railway banking to act as an overflow for the Chapel Burn. However, there were several difficulties with this solution:
Firstly there were concerns that with the local sandy soil structure, the railway bank would be destabilised by the saturated ground and put at risk. Secondly, there was public concern that the popular recreational area would be reduced in size by a large pond reducing its local amenity. Thirdly, the proposed site for the pond also covered a Scottish Water 1200 mm drinking water main with unacceptable risks to that installation.
As the topography is flat, any solution had to be compatible with the low head parameters. The shallow dimensions of the 1480 m2 Hydro Stormbloc® tank with a 1.01 metre depth, enabled the required 5 l/s maximum discharge from the Hydro-Brake® Optimum back into the Chapel Burn to be achieved without difficulty. The modular Stormbloc® construction, with built-in inspection chambers, allows rapid installation and as it is load-bearing is suitable for turfing over for recreational use. The project was completed by main contractor Murdoch MacKenzie Construction.
“The rail line originally had a syphonic device to channel the burn under the bank, installed when the line was built,” commented Falkirk Council’s Engineering Design Officer, John Divers, “but it wasn’t coping with the increasing flood risk from the Chapel Burn. The railway closure made action more urgent, however rebuilding a larger syphon under the line would have caused major disruption to rail services.
“The Chapel Burn can experience very rapid changes in level,” adds John Divers, “and the complete installation was designed with a tailor-made concrete drain channel installed alongside the burn. This acts as a weir, discharging into the Hydro Stormbloc® attenuation tank, and provides instant relief when the burn starts to overflow.
The Larbert area has undergone increased suburban development programmes over several years, and the Chapel Burn winds its way through and around Larbert town before eventually discharging into the River Carron and the Firth of Forth.