The £50 million flood alleviation scheme in Leeds which uses moveable weir technology, a first for flood risk reduction in the UK, has opened. The first phase of the award-winning Leeds Flood Alleviation Scheme, delivered by a joint venture of BAM Nuttall and Mott MacDonald (BMMjv), not only uses state-of-the-art flood defence engineering techniques but is one of the largest river flood alleviation schemes in the country.
The scheme comprises three main elements of state-of-the-art mechanical weirs, the merging of the river and canal and flood walls and embankments stretching 4.5km through the city centre. It is the first time that moveable weirs have been used in the UK for flood alleviation purposes. The new weir gates are supported by giant inflatable neoprene bladders that can be lowered when high river flows are expected. It takes around two hours for the gates to lower and thanks to the installation of these weir gates it has been possible to keep flood defence wall heights to a minimum so as not to spoil views of the city centre waterfront.
The weirs have been installed at Crown Point in the city centre and further downstream at Knostrop, where a new locally manufactured bridge has been installed across the weir connecting the diverted Trans Pennine Trail with the north bank of the river.
In addition to these measures, a manmade island known locally at Knostrop Cut which separated the canal and river has been removed to improve a bottleneck for flows. 180,000 tonnes of material excavated from the site has been reused on a local development site and on diverting the Trans Pennine Trail, which previously went across the manmade island. Reusing this material has saved the project in the region of £6 million.
Led by Leeds City Council in partnership with the Environment Agency, the scheme will provide more than 3000 homes, 500 businesses and 300 acres of development land with increased protection against flooding from the River Aire and Holbeck. More than 22,000 jobs will be safeguarded over the next 10 years due to the increased level of protection, while through the scheme’s development and construction 150 jobs and apprenticeships have been created.
Peter Charlesworth, Mott MacDonald’s project director, said: “Some of the technical challenges we overcame on this scheme included the foundations for the moveable weirs, which required driven piles situated very close to occupied properties in order to meet tight deflection tolerances. The new footbridge at Knostrop weir also had to be value engineered to meet the council’s budget, however we still succeeded in producing an elegant and slender structure which exceeded their expectations.”
“A key aspect of this project’s success was locating the design leads alongside the contractor team, Environment Agency, Leeds City Council and their technical advisors, which facilitated effective problem solving and the development of a strong team ethos. The project has already won numerous awards, including NEC’s Project of the Year 2017 where judges recognised the effective way the contract was operated,” Peter added.
Environmental enhancements have been integral within the scheme design, with fish and eel passes installed at both weirs. Weirs have previously been barriers, preventing species such as salmon migrating from the sea to the spawning grounds further up the river. Salmon have recently been spotted in the River Aire for the first time in 200 years and now that fish passes have been installed on Knostrop and Crown Point weirs, as well as others on the River Aire, it is hoped that chances of a spawning population of salmon in the river in future will be increased. Otter ramps and holts have also been installed and will support the local population and 700 trees will be planted along the Trans Pennine Trail later this autumn.