Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/1016624
www.wwtonline.co.uk | WWT | XXXX 20XX | 17 W hen Glasgow was hit by severe flooding in the summer of 2002, one city councillor told the BBC: "We've got a 19th century sewer trying to deal with 21st century problems." This summer, a-er nearly two years in construction, a new sewer superstructure – the biggest in Scotland – became operational in the southside of the city. The £100 million Shieldhall Tunnel, which runs 3.1 miles and connects to the existing network at Queen's Park and Craigton, is the crowning glory of Scottish Water's £250 million project to transform the wastewater system in the Greater Glasgow area. Collectively, more than 100 workers spent 1.5 million working hours on the tunnel; the result is a 4.65m diameter tunnel that provides 90,000 cubic metres of extra stormwater storage – equating to more than 108 million litres – while 90 per www.wwtonline.co.uk | WWT | SEPTEMBER 2018 | 17 cent of the effluent that had previously been discharged from the network's combined sewer overflows (CSOs) into the River Clyde can now be treated at Shieldhall sewage treatment works. As Scotland's Environment Secretary, Roseanna Cunningham, said when opening the tunnel in July: "Communities across Glasgow will benefit for years to come from this latest extraordinary feat of engineering which lies hidden deep beneath the city." The 2002 floods had sparked the project into action. A-er discussions as to whether the fault lay with Scottish Water's Victorian sewer network or a lack of capacity within Glasgow City Council's storm drains, the two parties came together with the Scottish Environment Protection Agency (SEPA) to form the Metropolitan Glasgow Strategic Drainage Partnership (MGSDP) that year. The MGSDP – which now counts Scottish Canals, Scottish Enterprise and Clyde Gateway among its long list of partners – set about investigating the hydraulic relationship between the city's sewers, storm network and watercourses to gain an understanding of the mechanisms for flooding. Over the course of 10 years, the group produced detailed computer models of the catchments along with significant survey works, and its 'Glasgow Strategic Studies' formed the basis of the Shieldhall Tunnel plan. "What came from the flooding event in 2002 is you can't really place responsibility on one party," Dominic Flanagan, project manager for Scottish Water, says. "It's everybody's responsibility." Making use of the varied perspectives within the group, the MGSDP did not simply look to mitigate flood risk but explored the bigger picture, seeking possibilities to enhance the environment and boost growth for housing and commercial developments in the southside, which had been slowed by the lack of capacity in the network. The group had initially considered whether to tackle the flooding threat with a series of large-scale, independent solutions: an 11,000 cubic metre offline tank, a 1.3km tunnelled outfall to the River Clyde and a new Kinning Park pumping station with upsized 5km rising main. The Shieldhall Tunnel option – a gravity sewer with minimal ongoing maintenance costs – was deemed to offer best value and, unlike the alternative plan, avoided the need for major engineering works around key routes within the city centre. The route for the Shieldhall option certainly had its advantages: it was intentionally indirect to maximise capacity while still able to pass a minimal number of residential areas, utilising three parklands. However, there were significant hurdles. The tunnel bifurcates the existing network at Queen's Park and, en route to Shieldhall treatment works, crosses three roads – including the M77 – as well as the Paisley Canal, Glasgow South Western and Inverclyde railway lines and numerous sensitive utilities. Further complicating the task was the local geology, which includes mudstone, sandstone, glacial tills, so- alluviums and old coal workings. To provide the necessary geological data, 179 boreholes were drilled. "If you think of the tunnelling in London and the recent work Thames has done, it's very well charted grounds – they're predominantly in chalks, Thanet sands or London clay," Flanagan says. "In Glasgow, The Shieldhall Tunnel is Scotland's biggest sewer superstructure. Scottish Water's Dominic Flanagan and Costain's Neil Grosset tell Robin Hackett about the challenges of bringing Glasgow's wastewater infrastructure up to date Tunnel vision GOAL 7 SEWER FLOODING The Works: sewer networks