Leeds’ flood protection enhanced after 10-year scheme adopts mix of new and well-tried solutions

Delivery of long-term climate resilience by the Leeds Flood Alleviation Scheme (FAS) is not just attributable to the recently completed civil engineering work in the city centre, although that plays a critical part. The key to the 1 in 200-year protection through to 2069 lies in the UK’s largest ever natural flood management project, involving the creation of a major flood storage facility upstream and the UK’s first mechanically lowering weirs.

The final phase of the £200M project, which was due to be officially opened as this issue of NCE went to press, is the culmination of a decade of work driven by Leeds City Council (LCC).

Delivery of the work has benefitted from long term relationships with the supply chain and the joint venture of Bam Nuttall and Mott MacDonald (BMM) working with LCC and the Environment Agency on both phases.

By taking a whole catchment approach to addressing the flood risk from the River Aire, LCC has led the project at pace from conception based on economic analysis to delivery of innovative solutions to protect the city from flood risk.

The environmental standard of the work has also just been recognised with an ‘Excellent’ rating under the Breeam science-based sustainability assessment, with maximum scores in the communities and stakeholders, resilience and pollution categories.

Long term ambition

While the design and construction have been undertaken over the last 10 years, the origin of Leeds’ flood defence plans goes back to 2010. Initial ideas drawn up by the Environment Agency relied on a mix of high flood defence walls, which effectively cut off the city from the river, and a large upstream barrier. However, there was no firm timeline or funding guaranteed to progress that work.

LCC head of engineering and infrastructure Oliver Priestley says: “In both 2000 and 2007, Leeds experienced large floods across the catchment with peaks of both events very close to impacting the city centre with widespread flooding. These near misses made it even more important for the council to take a lead on the project – we needed to take action to increase our climate resilience for the future.”

LCC engaged Arup to prepare an outline design for the first phase (FAS1), which focused on a section of the river downstream from Leeds station. The design was then further developed by BMM after it was awarded the £50M design and build contract.

“The focus of FAS1 was to remove any obstacles in the river channel with two mechanical weirs, to merge the river and canal at one point and to construct 4.5km of flood walls and embankments,” explains Mott MacDonald design manager Peter Charlesworth.

According to Bam Nuttall project director Andy Judson, Arup’s concept of replacing the existing weirs with ones that could be lowered during high flows was central to reducing the height of the previously planned flood walls in the city by 1.2m.

As a result, Charlesworth describes the defences delivered on FAS1, which was completed in 2017, as “invisible”. “They have been incorporated into existing structures and new public realm or, where that was not possible, the visual impact minimised by including glazed panels in the defences,” he adds.

However, planning how to build the weirs at Knostrop and Crown Point was challenging beyond it being the first time the technology had been used in the UK. “The new structures were replacing existing fixed weirs at the same locations, and we needed to maintain the flow throughout for navigation,” explains Judson.

The weirs, which were built to Arup’s design by Obermeyer, are supported by inflatable neoprene bladders that can be lowered when high river flows are expected.

Apart from the lowering capabilities, the new weirs also allowed fish passes to be built, removing an obstruction created by the previous Victorian-era structures. “Following completion, otters and salmon were spotted upstream of Knostrop for the first time in 200 years, which was a major step forward for the biodiversity of the river,” says Priestley.

BMM used cofferdams to undertake the installation with piling rigs and cranes on pontoons at Knostrop and equipment lifted into position on an existing island at Crown Point. However, before the work could be completed, Storm Eva – which triggered a 1 in 200-year flood – hit the project site on Boxing Day 2015.

The storm caused widespread devastation across Yorkshire, amounting to a £500M impact on the city region, including £37M in direct damage. A total of 4,712 flooding incidents were reported across the city.

BMM’s cofferdams were flooded but, more critically, the high flows washed away a bank between the River Aire and the Aire and Calder Navigation, resulting in the river flowing into the canal. BMM’s team mobilised to site immediately to implement emergency repairs.

Judson estimates that the damage to the construction sites resulted in two months of delays, but additional work to repair the damage to other parts of the riverbank added six months to the project.

As FAS1 was officially opened in 2017, work to present the business case for the second phase of the project was in full swing with the need underlined by the 2015 flood event.

Upstream focus

BMM joined forces with Arup and flood modelling specialist Thomas MacKay to undertake the feasibility work for FAS2.

“The second phase comprises a large upstream flood storage area, 14km of linear defences, control structures on three goits, three pumping stations, the removal of two bridges that obstructed high flows and the extensive natural flood management scheme in the upper catchment of the River Aire,” explains LCC project director Richard Dennis. “LCC procured a design and build contract that was awarded to BMM and appointed Aecom as technical advisor on the scheme, with the Environment Agency managing the natural flood management works” [see box].

Calverley Flood Storage Reservoir was key to raising the protection to 1 in 200 years. (Credit: Bam)

The work was split into five zones, not determined by the construction phases but guided by the planning conditions, as a result of a number of heritage assets, such as Kirkstall Abbey, and the need to demonstrate that the construction phasing would not worsen flood risk in the short term.

The project team developed several standard solutions, such as sheet piled retaining walls, precast concrete walls, a hybrid steel/concrete wall and earth embankment structures.

“We were able to pick the right solution for each site depending on the need,” explains Mott MacDonald project manager Rebecca Dresser. “In some locations, we were working in industrial estates but in others, we were working in homeowners’ gardens, so we needed options that were sympathetic to the location.”

These multiple stakeholders meant that both FAS1 and FAS2 called for a high level of community engagement and, while there were dedicated staff for this role, BMM was keen for anyone working on the project to be capable of this kind of liaison work.

The new flood walls, along with the FAS1 work, were critical to delivering 1 in 100-year flood protection, but it was bringing the Calverley Flood Storage Reservoir into FAS2, rather than being delivered in a third phase, that was key to raising the protection to 1 in 200-years.

The reservoir is formed by a flood embankment dam with automatic steel sluice gates to retain water within a natural flood plain, but at a greater depth than it would naturally hold. The 200m long, 6.2m high earth embankment, which was formed using material from an on-site borrow pit, has the capacity to impound 1.8M.m³ of water when fully operational.

The sluice gates are controlled on a minute-by-minute basis by water levels downstream of the dam. “The aim is to raise the gates as late as possible to make the best use of the storage and then lower them as soon as possible too,” says Charlesworth.

Unlocking regeneration

Completion of FAS2 reduces the flood risk for 1,048 homes and 474 businesses along a 14km stretch of the river, adding to the 3,000 homes and 500 businesses protected by FAS1.

While the work is clearly key to peace of mind for those now protected by it, the scheme also opens new opportunities for the city. It is estimated that the scheme will lead to regeneration benefits of £774M over 10 years and creation of approximately 3,000 additional net jobs.

The knowledge built up by the team in terms of the innovations delivered and collaboration is also continuing to deliver benefits as they move onto other projects.

This understanding extends to analysis on the carbon cost of the work, with the data collected on the FAS and analysed by the University of Edinburgh showing that the carbon impact of clearing up after a 1 in 200-year flood, such as Storm Eva, is 51,670tCO2e whereas the estimated cost of the Leeds scheme is 45,130tCO2e. Protecting the city from a single flood event of that scale will deliver a carbon saving of 6,540tCO2e and the savings will continue to increase between now and 2069.

LCC project manager Mark Garford adds that reducing flooding is crucial to tackling the climate emergency: “The carbon cost of clearing up after a flood is significant, so the construction of hard engineered defences can be seen as an investment towards the 2030 carbon neutral target. Climate change will bring higher river levels over time, eroding the effectiveness of built defences.

"Due to economic, environmental and community impacts, walls can only be built to a certain height, so using those combined with natural flood management techniques is the best and most sustainable solution.”

Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.