An extensive project in Leeds has been designed to reduce flood risk and provide major environmental benefits.
A flood alleviation scheme in Leeds is using “ground breaking” Natural Flood Management (NFM) techniques to slow the flow of the River Aire and reduce the risk of flooding.
The £112M Phase two of the Leeds Flood Alleviation Scheme is now underway. Alongside engineering works such as flood walls and a flood storage area, the NFM approach will reduce flood risk and provide better protection for 1,048 homes and 474 local businesses.
Leeds City Council group engineer Katy Jackson says that Leeds is classed as high risk in terms of flood mapping.
“We’ve had records of floods dating back as far as the 17th century,” she says. “The most notable was Storm Eva on Boxing Day 2015 – river levels rose higher than those in the 1866 flood where a number of lives were lost.”
In addition to this, flood frequency appears to be increasing due to the impact of a changing climate, with a number of smaller flood events earlier in the year as a result of Storms Ciara and Dennis.
“I think people are worried that we’ll see another Storm Eva,” Jackson says. “A lot of businesses are struggling to get flood insurance and other businesses have had to fold because of the economic impact.”
As such, Leeds City Council is working with the Environment Agency on the scheme, with works delivered through a joint venture between Bam Nuttall and Mott MacDonald. Phase one – which focused on the city centre area – was completed in October 2017, while work on the first step of phase two began in January 2020.
Phase two is split into two steps, the first involving new flood defence walls, embankments and the NFM programme. The second step involves the creation of a large flood storage area at Calverley, north west of the city, making use of an existing flood plain.
Natural flood management
The first step runs along an 8km stretch of the Aire, upstream of Leeds Station and focuses on three key areas – Leeds Industrial Museum at Armley Mills, Kirkstall Abbey and Kirkstall Meadows.
The NFM programme used here will help reduce and control the flow of rainwater into the river, slowing the flow upstream so the landscape can hold more water in times of flood.
NFM measures include new woodland creation; land management, using various means to reduce the flow of water during heavy rain; and river and flood plain restoration, for example re-channelling to slow down the flow of rivers and using storage ponds.
Environment Agency project manager Holly Radcliffe explains: “NFM measures aim to mimic natural processes to slow the flow of water and the idea is to deliver a lot of small scale sites to create a mosaic effect which provides bigger benefits.
“So, we’re delivering them throughout the whole catchment – everything upstream of Leeds. As well as slowing the flow, they will sequester and store carbon, increase biodiversity and have lots of wider benefits.”
The work has been designed to minimise dredging
It is the first time in the UK that natural processes will be used to reduce flood risk over such a large area.
“We’re using innovative digital and data-driven tools to decide how to implement nature-based solutions at a really significant scale,” Radcliffe says. “Other NFM projects aren’t looking at such large catchments.”
Bam Nuttall sector manager Andy Judson adds: “This is ground breaking for flood alleviation. The work that this scheme is doing is at the forefront and looking at the best areas and the optimisation of those NFM interventions to get the best outputs.”
The team is building a geographic information system database that will automate woodland design, while aiding decision making. It uses existing datasets from partner organisations and then on site workers can draw and input additional data using tablet devices.
“Ideally we’ll have tools that will automate the process on site to remove the need to come back to the office to look at something,” Radcliffe adds.
These NFM techniques will be used alongside more traditional engineering solutions – for example, phase one work in the city centre involved moveable weirs, merging of the river and canal and flood walls, glazed panels and embankments.
Project challenges
Key aims during phase two will be to avoid impacting nearby listed buildings; minimise dredging of the River Aire; and to avoid disturbing fish. Trees will also be replanted where they have been removed.
Step one has presented some logistical challenges with work taking place parallel to the Leeds to Skipton railway line. There are also interfaces with rugby club Leeds Rhinos’ training pitches, the Leeds Industrial Museum at Armley Mills, and historic sites such as Kirkstall Abbey.
“It needs a lot of planning and close working together,” Judson says. “There have been some areas of constraint but the big thing is about an open dialogue – making sure Historic England is involved in the work at Kirkstall Abbey, for example.
Step two is a “completely different feel”, Judson says. Although it is in open countryside, there is still Network Rail infrastructure to contend with, along with development sites, a school and a listed bridge.
“Then there’s the challenge of making the hydraulics work so we can store enough water to provide the protection we need for the step one area,” Judson says. “So pretty much everybody we could interface with, we are interfacing with.”
Accelerating innovation
Despite the challenges, opportunities for innovation have been found. For the flood storage area construction in step two, a borrow pit has been created on site which has removed the need to import material onto the site.
“We’ve worked the design around the material rather than trying to find the material to match the design,” Judson says. “We’ve made the design work around what’s available.”
The NFM programme means that the walls for the flood storage area are not as high and thick which has a positive impact in terms of carbon and sustainability. As a whole, phase two of the project is on target to meet its carbon reduction target of 20%.
This environmental benefit of the scheme “shouldn’t be underestimated”, Judson says. Improving the ecology of the river also plays a part in the first step of phase two, with a team working to consider floating islands to increase biodiversity.
With Leeds declaring a climate emergency in 2019, the scheme has a key role in the city’s response to the climate challenge.
Work is expected to finish by winter 2022.
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