Leeds Flood Alleviation Scheme (FAS)

The city of Leeds and the River Aire are inseparable. Both are formidable. Leeds is a testament to the force of Man's determination and ingenuity; the River Aire is a force of nature.

Leeds came to prominence as a market town on the banks of the River Aire. Its location on the Aire, and subsequently the Leeds and Liverpool canal, allowed it to become a centre for the wool trade and to prosper during the Industrial Revolution through development, for example, of the textile industry, iron foundries and engineering. Leeds, in 2020 a city of around 800,000 people, is the 3^rd largest manufacturing centre in the UK and home to a diverse range of other industries and enterprises.

The River Aire is 92 miles in length and flows directly through the centre of Leeds where it has a normal depth of 1.5 metres. Two weirs, Knostrop and Crown Point (Leeds Dam) were put in place on the Aire followed by the construction of the Leeds and Liverpool Canal which opened in 1820. The canal runs parallel to the Aire through Leeds.

Leeds has suffered from a long history of flooding, most recently Storm Eva in 2015, and February 2020 with Storm Ciara. This flooding was both fluvial (river) and surface water flooding where excess rainwater cannot be filtered into the ground and drain through the sewage networks.

The city has incurred large economic costs as a result of this flooding. The 2015 Boxing Day floods affected over 4,000 homes and almost 2,000 businesses in the Leeds city region as defined by the West Yorkshire Combined Authority, the cost being close to half a billion pounds (*WYCA). The financial impact of Storm Ciara has yet to be measured.

Leeds Flood Alleviation Schemes

A major flood in Leeds could see up to 350 tonnes of water per second flowing through the city. This means that millions of tonnes of water would have to be dealt with during a flood which may last in excess of 10 hours. Prior to the completion of the first phase of the scheme, without engineered interventions, Leeds did not have the resilience to manage the impacts, so flood schemes such as Leeds FAS 1 and 2 are necessary to keep the river in channel and storage areas without affecting properties and infrastructure. Leeds FAS1 utilises raised walls, lowering moveable weirs and merging the canal and the river to create more room for flood waters within the city. Leeds FAS2 does this by taking a catchment wide approach to reducing flood risk. Slowing the flow of water into the River Aire by using Natural Flood Management (NFM), combined with storing flood waters upstream of the city centre near Calverley and also using raised defences.

‘A significant flood in Leeds centre would impact the local economy by £450m’ *Leeds City Council

Leeds FAS is a successful Leeds City Council project working and partnering with the Environment Agency (EA). It includes other parties such as Network Rail, Canal & River Trust, Yorkshire Water, Bradford Council and the Yorkshire Wildlife Trust. Delivery partners include a Bam Nuttall and Mott MacDonald joint venture (BMMjv) and Aecom as Technical Advisor on FAS2 and Arup on FAS1.

The EA primarily developed a flood risk management strategy for the Upper Aire Catchment (2009), this recommended providing a 1 in 200 yr. Standard of Protection for Leeds (0.5% chance of occurring), and so became the main initial driver behind the Leeds Flood Alleviation Schemes.

Upper Aire Flood Risk Management Strategy Report

Flood risk management strategy: Upper Aire - GOV.UK (www.gov.uk)

The Leeds Flood Alleviation Scheme (FAS), one of the largest flood defence schemes in the UK, has two phases. Phase 1 (FAS1) was successfully completed in October 2017. Phase 2 (FAS 2) which has been divided into two steps. Step 1 gained funding approval in December 2019 and work began in January 2020.

Phase 1 (FAS1)

Leeds FAS 1 in numbers

Leeds Flood Alleviation Scheme (Phase 1) in numbers - YouTube

Leeds FAS 1

Leeds Flood Alleviation Scheme - Phase 1 - YouTube

Phase 1 (FAS1) has been an innovative and progressive approach to flood risk management for the City of Leeds. Its aim is to make space for flood waters along the Aire in the city centre and so protect more than 3000 homes, 500 businesses and city infrastructure.

Leeds FAS1 received Planning Permission approval before starting on site in early 2015.

Over the following 2-3 years, works were undertaken to complete the moveable weirs at Crown Point and Knostrop. This had the potential to reduce flood water levels by 1 meter. The merging of the River Aire with the Leeds and the Aire and Calder Navigation would increase flood water capacity; this involved removing a 700 meter stretch of land at Knostrop Cut. Flood walls along the river downstream of the train station and along a tributary of the River Aire at Holbeck, were constructed, including glazed panels. Further embankments were installed further downstream, so increasing the flood protection at Woodlesford.

Crown Point Weir - Crown Point Weir - YouTube

The Canal and Rivers Trust transferred ownership of the weirs at Crown Point and Knostrop to Leeds CC so that they could be replaced by the moveable weirs which are now in place. The Trust and Leeds CC also worked closely on the river and canal merging at Knostrop.

Construction of the Leeds FAS 1 - Arup, BAM Nuttall and Mott MacDonald

Arup, a well-known, innovative, global engineering company was appointed to the Leeds Flood Alleviation Scheme to take the Phase 1 concept forward. As technical advisor for the operation, Arup worked on the concept and design for FAS1 with Leeds City Council, BAM Nuttall/ Mott MacDonald (BMM-JV). Their involvement has produced innovative approaches to the flood measures used within Leeds. The placing of the new moveable weirs at Crown Point and Knostrop was achieved whilst allowing boats to continue navigating the river.

The Leeds FAS Project Management teams benefited from a unique spirit of collaboration and communication as the EA, BAM Nuttall (BMM-JV) and Leeds CC directed the operation from a single compound office.

Crown Point Weir Construction Sequence –

Leeds FAS - Crown Point Weir construction sequence - YouTube

Crown Point Weir - Milestone for UK's first movable weir flood defence - New Civil Engineer

Knostrop Cut - Leeds FAS - Construction Works Fly Through February 2017 - YouTube

This successful scheme won the ‘Large Project of the Year’ award at the National NEC Awards in 2017.

Timeline events of FAS 1 construction works

FAS 1

Leeds CC engaged with Arup for the appraisal, concept of works and modelling to prove the scheme worked. Arup prepared the outline design which was submitted for planning application.

Advanced works

2014 - Woodlesford was the site of the early advance works and the introduction of a flood embankment to protect that community up to a 200 yr. standard of protection, mitigating a very small increase in risk that would be seen by the main works in the city centre.

Main works

Autumn 2014

BMM-JV (Bam Nuttall & Mott MacDonald) were employed for the flood alleviation scheme build and delivered the detailed design to further Arup’s concept and outline.

When the options were being explored for the flood alleviation scheme, it was calculated that the flood defences would have to be over 2 metres in height in places if the moveable weirs were not in place. This was deemed unacceptable, impacting on the riversides commercial and recreational uses, so the moveable weirs were developed by Arup. These would maintain water levels in normal conditions to facilitate the navigable route and lower them in flood conditions.

The moveable weirs fold flat to the river bed when flooding, lowering the water level by 1 – 1.5 metres at Crown Point and Knostrop during flood events. As a result, the flood walls running through Leeds city centre are much lower which means that they do not spoil popular city views or the aesthetic of the area.

Animation of weirs in use - Leeds FAS - Moveable Weir Animation - YouTube

Winter2014

Work began on FAS1 in Winter 2014

Crown Point Weir -Crown Point Weir - YouTube

December 2015

Storm Eva

Storm Eva caused both material damage and construction delays.

Phase 2

Following FAS1, the task was to develop a further phase of the scheme to provide much needed protection to areas of the city upstream of the train station and to look to uplift the level of protection across all phases to a level similar to that needed to face a repeat of the Storm Eva Boxing Day 2015 floods. The response is Phase 2 (FAS2) which complements the approach of Phase 1 with the introduction of further raised defences, a large storage area upstream of the city centre and catchment wide Natural Flood Measures (NFM), to reduce flood risk, manage and mitigate the impacts of climate change. Working with BMMjv, Arup, Thomas Mackay, the EA and other partners LCC developed appraisals and business case outputs for Phase 2, this resulted in a successful funding bid to progress the scheme. Following procurement, BMM-JV were appointed to develop detailed designs and deliver the scheme with AECOM providing technical advice to LCC.

Animated Fly through for Phase2 –

Leeds Flood Alleviation Scheme Phase 2 complete flythrough - YouTube

FAS2 complements and builds upon the advances of FAS1 further utilising hard defences and using a structure to control water levels at the flood storage area. The height of hard defences has been reduced by incorporating a catchment wide use of natural flood management interventions.

It will also increase flood protection of residents and enterprises because it will reduce flood risk from a further 1000+ homes and more than 470 businesses.

The stages of Phase 2 are centred upon an 8km stretch of the River Aire starting at Leeds Station. This second phase aims to slow the flow of water into the River Aire using Natural Flood Management (NFM), it also involves the construction of new walls and embankments, removal of river obstructions, creating new woodland areas and a water storage area.

Timeline events of FAS 2 construction works

Early 2019

During March 2019, the Feasibility Study and the Business Case which focus on property protection were drawn up and submitted.

January 2020

Following preparation works, construction of FAS 2 Step 1 began in January 2020. These works will bring the total cost of Leeds FAS2 (Step 1 and 2) to £112 million at completion. Although now almost fully funded a planning application for Step 2 of FAS 2 has now been submitted. The scheme is expected to be completed in 2023.

FAS 2 does not involve any works on the weirs at Armley Mills nor Kirkstall Abbey (as FAS1 did), but will introduce an attenuation feature – a flood water storage area - near Calverley. This allows for the peak flow of the flood water to be reduced, so preventing it from increasing flood risk to Kirkstall and Leeds City Centre, as was the case during Storm Eva.

An embankment and a moveable gate structure will be constructed in the river near Calverley. This will complement Step 1 hard defences through the city that will provide protection up to a 100 yr. (1% chance of occurring) flood by increasing the standard of protection up to a 200 yr. event. Flood water will be held in the storage area until the flood has subsided. The gate structure will then be lowered to allow the flood water to be passed downstream in a manageable way. This process will help reduce the peak volume of flood waters flowing downstream at the time of a large flood event. Natural Flood Management measures spread across the wider Aire catchment will help to reduce the peak flow in a flood event by 5% and allow the scheme to adapt to the impacts of climate change.

Phase 2 also includes flood protection for the railway line running parallel to the river, proposals supported by Network Rail. The economic benefits include keeping people travelling into the city and able to attend work, so protecting jobs.

Step 1 of FAS 2, once complete, will offer a Standard of Protection of 1 in 100 years and Step 2 will then upgrade this to 1 in 200 years by 2023.

March 2023

From 2023, the River Aire running through Leeds will have a Standard of Protection Rate of 1 in 200 years, including the climate change allowance, until 2069.

Public Engagement

During FAS 1, project team engaged with the public and delivered the project recognising their needs.

One such instance of this engagement was when discussing the methodology for constructing the Crown Point moveable weir. The project team worked with local residents to understand which forms of communication would work best for them during the construction works.

The project team also worked with residents adjacent to Crown Point Weir to arrange alternative parking, allowing them access to the river with large machinery.

The project team were also sensitive to local needs by sensitively designing the flood protection, offering walk-through tours of the development, providing 3D explanatory videos, open days including a stand at the Waterways Festival, and in commissioning a children’s book, ‘Cones on the Waterfront’. In one instance, as a measure of their appreciation, local residents invited members of the project team to a garden area reopening party which they held. Surface water protection measures were also added to a number of areas including the Calls area of Leeds.

The project team continues to hold further public engagement events during the FAS 2 development.

Environmental Advantages

When introducing such large measures, it would be irresponsible to assume there would not be an impact on the local environment. The Leeds FAS project takes opportunities to making environmental improvements wherever possible. For example the Victorian built Knostrop weir did not allow fish to travel along the river. The temporary weir, put into place by BMM-JV, recognised and reduced this problem during construction, allowing for a ‘rock chute’ to help fish to travel. This was replaced by a permanent fish pass, which acts as a series of steps/a slide, will help salmon to migrate up the river to their original spawning grounds. The attention to detail did not stop there; further features were introduced to assist eels to travel along the river and accommodation was made for otters in the area. Similar measures were included at Crown Point Weir.

The Environment Agency, having worked with Leeds CC throughout the FAS development, continues their collaboration once construction has been completed. The positive effects on wildlife numbers along the River Aire, following considered construction choices, is being monitored and the EA takes a leading role in this, along with any ongoing research into otter numbers and habitat, fish species and numbers.

Chamber of Commerce

The West & North Yorkshire Chamber of Commerce has worked closely with Leeds City Council throughout the FAS development, working to support the 32,000 businesses in Leeds. Developing the ‘Waterfront Charter’, March 2017, the chamber has been focusing on the future growth of the Leeds Waterfront.

Conclusion

It’s the year 2021; is the job done? Not quite but after 5 years, the Leeds Flood Alleviation Scheme developed into and delivered an award winning, environmentally sensitive success, with more to come from FAS 2. A combination of technical expertise and insightful leadership has delivered both innovative and inventive methods of both working practice and flood measures, evidenced by the success of the moveable weirs at Crown Point and Knostrop to name two. Reducing the flood risk from the River Aire has provided protection for thousands of properties within Leeds along with protecting the city’s daily economic function and infrastructure, protecting jobs and businesses from the consequences of flood.