Supporting the construction of a flow control structure to prevent potential floods in Leeds city centre

  • Project name Leeds FAS2
  • Client Leeds City Council
  • Principal Contractor BMMJV
  • Project Lead Phil Ash, Technical Sales
  • Lead Engineer Stephen Barker, Chief Engineer
  • Key Benefit Early involvement and collaboration utilising BIM technologies & Common Data Environments
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As part of the Leeds Flood Alleviation Scheme Phase 2, plans were introduced to reduce the potential of flooding between Leeds Train Station and Apperley Bridge. Due in part to climate change, the risk of flooding is ever increasing with recent events such as flooding caused by Storm Eva, providing a catalyst to improve flood defences in the area. The flood alleviation scheme will provide a 1-in-200-year level of protection to the area, reducing the risk of flooding to 0.5% in any given year, protecting 1048 homes & 474 businesses.

MGF became involved in the scheme in April 2021 after being contacted by BAM Nuttall to provide a bracing arrangement for the flow control structure cofferdam within the River Aire. The flow control structure will reduce the volume of water heading downstream during flood conditions, holding up to 1,800,000m³ of water within the flood storage area.

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The Solution

The initial BAM Nuttall cofferdam design required two levels of framing to a rectangular cofferdam with projections in each corner forming an ‘I’ shape. Using the SLS & ULS waling loads provided by BAM Nuttall our engineering team were able to develop a fully modular solution that dealt with the complex geometry utilising MGF’s comprehensive fleet of temporary works equipment.

Following a change to the design of the flow control structure, the cofferdam was redesigned to a propped cantilever solution with the geometry revised to suit the shape of the structure. Our engineers reacted to provide an updated, economical solution ideally suited to the irregular shape without the need for any bespoke fabrications.

Approximately 150te of steel were used within this scheme, with sheet piles used to retain the ground & river. Overall, the cofferdam was approximately 67 x 34.5m.

To meet the design loading & deflection criteria, our 406 UC brace was utilised around the perimeter of the cofferdam supported by our 400, 600 and 1000 props. The 406 UC brace is a high strength, high stiffness brace – second only to the MGF T700, making it the ideal solution for a cofferdam of this size. The large 1000 series props were built up incorporating 400 series extensions at the ends to ensure that the walls of the structure would not clash with the prop.

To save time on site MGF pre-assembled the equipment at our Astley depot before transporting to site on arctics. Prop assemblies were communicated to the customer to ensure they could determine their lift plan allowing for a safe installation. This allowed the customer to save time and suited their calculated lift plan to allow for a safe installation. A member of the MGF team visited site at this stage to offer advice and support on the installation, making the process as smooth as possible.

‘The cofferdam was an irregular ‘bowtie’ shape following the shape of the wing walls to minimise the amount of excavated material, this created four internal corners that we had to deal with in the framing design. Ordinarily this would require elements of bespoke manufacture or additional props, however due to our experience we were able to devise a solution using a fully modular system with minimal propping by adopting modular pinned connections at these locations. This solution created vast amounts of open space within the cofferdam making excavation simpler and faster.’

Stephen Barker, MGF Chief Engineer

The Verdict

Communication was ongoing between MGF and BAM throughout the scheme to ensure that the equipment and design covered all requirements. This allowed MGF to accommodate any changes necessary to the project.

MGF also supported the customer on other phases of the overall project, including providing support to ongoing construction of the embankments further down from the control structure. Works on this phase of the project are estimated to be complete in 2023, for more updates please see the Leeds government website.