Movable flood-protection weirs are being installed in the UK for the first time in Leeds, after more than 2,000 homes were flooded in 2015.
The UK was severely affected by severe storms in 2015. According to the Environment Agency, over 17,000 homes and businesses were flooded that December.
In Leeds alone, over 2,000 residential properties and nearly 600 commercial properties were affected, said Leeds City Council. The protection of urban centres from floods has become crucial, and Leeds City Council, in partnership with the Environment Agency, is now carrying out a scheme to protect the city from potential flooding — the Leeds Flood Alleviation Scheme (FAS).
Cllr Richard Lewis, Leeds City Council executive member for the economy and development, said: “This is a vitally important scheme for the city, not just because of the threat to lives and property from floods but also the positive impact it will have on the growth and regeneration of the Leeds economy, particularly the South Bank area. It will also give much-needed protection to key transport infrastructure and access routes we rely on to keep the city moving.
“I’m pleased that we are using leading flood defence technology and are working well with partners to deliver the best protection we can within budget.”
The Leeds FAS — one of the largest river flood defence schemes in the UK — is being delivered in two phases.
The £45M Phase 1 of the scheme, is funded by the UK department for environment, food, and rural affairs (Defra), Leeds City Council and Regional Growth Fund. It will be led by the council and a project team, including construction contractor BMM — a joint venture between BAM Nuttall and Mott MacDonald, the Environment Agency, Arup Mouchel, Dyrhoff, Canal Rivers Trust and Yorkshire Water. The project will include major construction work along the River Aire, in Leeds city centre, and Holbeck.
It involves the replacement of existing weirs at Crown Point and Knostrop by moveable weirs — this is the first time that the technology has been used in the country for the purpose of flood defence.
The Knostrop Cut Island has been already removed, allowing the canal to merge with the River Aire, and create additional flood water storage. Additionally, flood defences including embankments, terracing and riverside walls have been constructed. Construction work on Phase 1 has started in 2015.
Phase 2 of the scheme, in progress, involves a feasibility study to look at the viability of the options looked at in the scoping study. The Environment Agency carried out an earlier scoping study, which set out potential options to reduce flood risk to the city centre and Kirkstall Road.
A business case is expected to be submitted in autumn 2017, following which the outline design will be developed with the tendering and awarding of a construction contract expected for summer 2018. The specific flood reduction measures that will form Phase 2 of the scheme are still not fully defined.
The scheme is expected to protect Leeds city centre from a flood event likely to happen once every 100 years including an allowance for future climate change — the likelihood of a 1 in 100 year flood event occurring in any given year is 1.0%.
Leeds city centre flooded.
The movable weirs are replacing old weirs, made of big masonry blocks, which were built about 200 years ago to transform the River Aire into a navigable waterway.
Arup has carried out the civil engineering design of the movable steel weirs, while Dyrhoff was responsible for its detailed mechanical engineering and installation.
David Wilkes, Arup’s project director for the Leeds FAS, says: “We built a physical model of the weirs with the gates up and the gates down. After being through the design and modelling of the weirs, we had an accurate specification of what we wanted and we asked Dyrhoff to design it for us.”
According to Wilkes, the new weirs — to be about 4m-high at Knostrop and 2.5m at Crown Point — are “like curved doors”.
The Crown Point weir will be about 2.5m high.
“The weirs are made of reinforced concrete in U shapes, so the horizontal part of the U follows the profile of the riverbed and then the vertical part of the U forms the sidewalls, and there are divided walls in between. In Knostrop there are two intermediate divided walls, and at Crown Point there’s one dividing wall,” says Wilkes.
He adds: “Underneath the gates we have a reinforced rubber cushion or bladder, which is fed with air. When the cushion is deflated, the gate is flat on the riverbed as it would be in flood time.
“When we want to have higher water levels for the boats and the aesthetic of the water level being close to the buildings, cafes and walkways in the city, the pumps in the control building inflate the cushions and as the cushion inflate the gates move on the hinges and come upwards to an angle of about 45° to the horizontal. So, when the gate is hinged to its upstream phase and the cushion is fully inflated that gives up to a 4m drop of water across the gate.”
“By monitoring the water level, the gate and the amount of air that goes into the cushion, you can then control the water level over up to 3km of river.”
At the larger weir, Knostrop, they have also made provision for a hydropower turbine, says Wilkes. “If at some future point in time somebody wants to come and put a turbine in this river to generate electricity it is ready made for them.”
The Knostrop weir under construction.
The new weirs will be operated by a duty officer and an assistant in the control building — they will be able to regulate the air that goes into the bladder or cushion. The control room will house the pumps, the computer system and the electricity supply to make the weirs work.
Wilkes says: “Our client, Leeds City Council, wants people on site when the weirs are operated. We have written a computer control software that will allow the duty officer and his/her assistant to go to the site and make sure that everything is safe and then enable the computer to do its job in progressively lowering the weirs to keep the flood level within a predetermined height.
“They did have an option of making it fully automatic, but they thought that when the weirs move there is a potential risk to people on the riverside boats close to the weir, so they want the officers to make sure that everything is safe before they press the go button.”
If there are any concerns during the operation of the weirs, the officers in charge can hit the emergency stop button on the semi-automatic system, specifically created for the scheme.
“It’s not software you buy off the shelf, it needed to be suitable for the Leeds Flood Alleviation Scheme. However, there’s nothing that hasn’t been tried and tested before in other applications, so the risks of it being unreliable or not performing as you would want are quite small.”
Wilkes adds: “There is always a risk of mechanical or electrical breakdown or an operation breakdown, whereas if you have a wall, unless the wall falls over, it will work. In terms of the mechanical electrical operator failure, we tried to design a system that is as resilient and fail safe as possible.”
As the weirs are being built and installed in a living river, the scheme includes additional features to help the different type of fishes to live and survive in their habitat — these include fish paths, fish channels and eel passes.
Phase 1 of the scheme is expected to be fully complete by Summer 2017. According to Wilkes, once the scheme is completed no one will ever realise that there is a flood defence scheme in Leeds city centre. “Unless you really know what you are looking for, you won’t be able to see that there is an engineered flood protection scheme in the centre of Leeds. People will just look and say ‘here is a city that it’s living with its waterfront, embracing the views, and with boats that can navigate along the river’.
*Images courtesy of Arup.