Leeds Flood Alleviation Scheme Phase 2 (LFAS2): Innovative Flood Modelling for Leeds
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- 4 min read
What our clients say:
“ Mott MacDonald used advanced Flood Modeller techniques to meet design standards within demanding time and budget constraints," and "conducted extensive 1D Flood Modeller simulations to test a range of operation scenarios.”
Liam Nicholson, Principal Flood Modeller, Mott MacDonald
Key Facts
Completed in November 2025, LFAS2 provides a 0.5% Annual Exceedance Probability (AEP) Standard of Protection along 15km of the River Aire, protecting over 1,000 homes, 500 businesses, and vital infrastructure.
The scheme delivers 8km of new linear defences, the 1.8 million m³ Calverley Flood Storage Reservoir (FSR), and a range of natural flood management measures.
Advanced hydraulic modelling using Flood Modeller and TUFLOW helped to identify an optimised operational strategy for Calverley FSR.
At Armley Mills, bespoke 1D modelling using Flood Modeller helped balance flood protection with heritage, ecological, and operational requirements for one of Leeds’ most significant historic sites.
LFAS2’s modelling framework now underpins the Environment Agency’s forecasting and warning systems, strengthening long‑term resilience and aiding future planning across the River Aire corridor.
Project Overview
Mott MacDonald, working as part of a joint venture with BAM Nuttall, was commissioned by Leeds City Council, in partnership with the Environment Agency, to design and deliver Phase 2 of the Leeds Flood Alleviation Scheme (LFAS2). The devastating floods of Christmas 2015 caused extensive damage across Leeds, highlighting the city’s vulnerability and the urgent need for LFAS2.
Initiated in 2019, the scheme aims to provide a 0.5% Annual Exceedance Probability (AEP) Standard of Protection for over 1,000 homes, 500 businesses, and critical infrastructure along a 15km stretch of the River Aire, from Leeds city centre to Apperley Bridge. LFAS2 includes:
8km of linear defences
A 1.8 million m³ flood storage reservoir (FSR)
Seven flow control structures
Natural flood management measures, both within the River Aire Corridor and in the Upstream catchment
The Role of Hydraulic Modelling
Hydraulic modelling was fundamental to the success of the scheme. The Mott MacDonald team developed and continually refined two catchment‑scale 1D–2D models of the River Aire in Flood Modeller and TUFLOW. These were updated with new survey data and calibrated against major historic floods–including the 2015 Boxing Day event. Alongside these, a suite of targeted models supported the design and consenting of linear defences, Calverley FSR, and key control structures such as non-return valves and automated penstocks.
Hydraulic modelling and construction progressed in parallel, so assessments had to be reliable, responsive, and capable of providing rapid feedback as challenges emerged. The flexibility of the modelling approach enabled quick design adaptations and supported decision‑making throughout delivery.
The modelling also demonstrated to regulators that multiple elements of the scheme could be constructed concurrently without increasing flood risk. Sequencing scenarios were tested and refined to confirm no temporary increases in flood risk elsewhere, and these were reviewed with regulators and construction teams to ensure they were practical and aligned with the 4‑year construction programme.
This process highlights the agility and technical expertise of the project team to use and apply modelling to dynamically support a complex, fast‑moving project.
Calverley Flood Storage Reservoir: Optimising the Operation and Maximising Storage Potential
Mott MacDonald used advanced Flood Modeller techniques to meet design standards within demanding time and budget constraints. The key innovation was optimising the FSR’s active control structure, which required more dynamic gate movements than typically seen in similar schemes. Working closely with specialist teams, gate manufacturers, and reservoir engineers, Mott MacDonald defined operational limits and conducted extensive 1D Flood Modeller simulations to test a range of operation scenarios. This modelling identified the most efficient way to achieve the required storage performance, avoiding the need for costly and carbon‑intensive engineering works. The approach was grounded in practical reality, with input from gate designers to ensure the proposed operation was both plausible and constructable.
Armley Mills: A Complex Heritage Challenge
One of the most technically demanding aspects of the scheme involved providing flood risk protection associated with the River Aire’s historic mill channels, including Armley Mills (now Leeds Industrial Museum), a nationally significant heritage asset with complex heritage, environmental, and operational constraints.
A 1D modelling approach enabled Mott MacDonald to quickly establish operational scenarios for both historic and new control structures. This testing ensured the design could balance flood risk management with heritage preservation and ecological requirements. The modelling supported the development of both automated and manual sluice gate operations, maintaining flood protection and the mill’s unique historic and functional needs.
Summary
LFAS2 demonstrates how innovative hydraulic modelling and strong collaboration can deliver resilient, effective flood risk management. It is now recognised as a sector benchmark, with its modelling approach shared by Mott MacDonald at the 2023 and 2024 Flood Modeller conferences. In 2025, the project received the Institution of Civil Engineers (ICE) Centenary Award.
The scheme has had a significant positive impact on communities along the River Aire, supporting economic, social, and environmental benefits while enabling sustainable regional growth. Associated regeneration is expected to generate £774 million over the next decade and create between 1,600 and 3,500 jobs.
Liam Nicholson
Liam is a Principal Flood Modeller in Mott MacDonald’s Flood Risk team with more than 14 years’ experience. He specialises in hydrology, hydraulic modelling and flood risk management, providing technical leadership across a wide range of infrastructure and water‑planning projects.
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