Two innovative inflatable weirs are protecting Leeds city centre from catastrophic flooding.
While the majority of the UK was cosily tucked up on the sofa eating left-over Christmas turkey three years ago, Leeds city centre suffered one of its worst ever flooding events.
On Boxing Day 2015, severe storms caused river levels to rise, the canal to burst its banks, and 3,355 homes and businesses to be engulfed with dirty, freezing water.
The River Aire at Leeds Crown Point in the centre of the city, usually 900mm high, rose to 2.95m at 1am on 27 December.
In this event alone, the direct cost of the damage to the city centre was put at £36.8M, with the costs for the wider area thought to be as much as £500M.
The storm damage served as a painful endorsement of Leeds City Council’s decision to build an innovative new £50M flood defence system to protect it from a surge in river levels.
Unfortunately for the city, construction on the new defences had only just begun. But with phase one now complete, the council is now confident that the devastating scenes of 2015 will not be repeated.
The flood scheme now protects 3,500 residential and commercial city centre properties, plus key access routes to the train station area, telecommunications, internet facilities, electricity substations
and more than 121ha of developable land. It will also provide vital flood relief for the new High Speed 2 line’s phase 2a when it flies over the river and into Leeds.
Additional funds were secured during the construction period allowing enhancements to be instructed, including to increase the standard of protection provided by the scheme from 1 in 75 year event to 1 in 100 years with allowance for climate change to 2069.
The ground-breaking flood defence scheme has been designed and built by the BMM joint venture, comprising contractor Bam Nuttall and consultant Mott MacDonald with consultant Arup as technical adviser.
The system hinges on the use of two new innovative, inflatable weirs understood to have never been used before in the UK. Without them, up to 2m high walls would have been needed on either bank of the river to block out the water resulting from a 1 in 100 year event – an unacceptable solution for both planners and the local community alike because of the visual intrusion caused.
Arup produced the initial design for the two moving weirs concept, and turned to specialist supplier Dyrhoff to fabricate and supply five inflatable spillway gates for use at the two locations.
“Under normal conditions, giant, inflated, Teflon-coated, bulletproof airbags support steel plates placed near vertically to create a dam in the river,” says Bam Nuttall senior agent Andy Vipas. “This [positioning] maintains the river water level within the city centre, ensuring it remains navigable.
Deflating bags as river rises
“But, when river levels start to rise [in a storm event], the bags are deflated, the steel plates drop to the horizontal, flat position and water is released downstream where it dissipates over a designated flood plain.”
The two weirs are placed 2.5km away from each other: one 2.2m high structure is in the city centre at Crown Point; and the larger of the two, at 4.5m high, is downstream at Knostrop where the River Aire divides to become a separate river and canal.
Knostrop’s is lowered first to create additional capacity for the water held back by Crown Point weir in the city centre.
Although there were existing weirs in both locations, neither could be lowered and instead of being able to release water down the river, excess water simply rose up and flooded the surrounding land.
With the weirs in place, barriers of varying design and a maximum of 1.3m high needed to be built on top of the existing river walls in the city centre, a far more acceptable solution.
Victoria quays glazed pond crop
But even there innovation was applied. To minimise visual intrusion as much as possible, the team used a specialist glass wall flood defence system provided by flood defence specialist IBS Engineered Solutions. Its glass walls had previously been used in Paull, East Riding, and Littlehampton on the West Sussex coast. The system has been used extensively throughout Europe with individual panel lengths in excess of 3m and heights in excess of 2m.
The IBS glass wall is extremely robust and uses the same aluminium post profiles and seals as its tried and trusted demountable flood defence system – as used at Bewdley and Shrewsbury to protect against the River Severn – the first large-scale demountable flood defence systems of their type in the UK and still successfully at work today.
“The windows are rated to withstand 1.3m of flood waters pressing against them and a load of 10kN hitting the glass so if a boat or tree came floating round the corner in the floods and hit it, it would survive,” says IBS managing director Ray Moulds.
In phase two we are building upstream storage at Calverley to hold back the water in a natural flood zone
Both weirs presented significant challenges during installation. At both locations, access to the site was limited and so vast quantities of rock was laid down in the river to create a causeway to allow cranes to drive into islands in the middle of the river.
At Crown Point, part of the planning condition was to keep a section of the existing Grade II listed weir on one side of the new weir.
To build the weir, a sheet pile cofferdam was first constructed half way into the river on one side, keeping the other half flowing, before being recreated to complete the other half.
At Knostrop, to allow the contractor to work in a safe environment, a sheet pile wall stabilised with rock armour was placed across the river about 100m upstream of the new weir.
Dropping the water levels
“That allowed us to drop the water level in this area,” says Vipas. “The water level dropped to about 1m deep, which allowed us to manage the temporary works a lot better in this area.”
The weir was then built in stages, systematically working along the width of the river with cofferdams in thirds.
Even with the sheet piled wall in place, the cofferdams still flooded four times during construction as the wall could only be designed to protect it against a 1 in 10 year event, so as to not create problems upstream in the city centre.
Crane access from the bank was not possible as electricity pylons nearby limited the height in which it could work. A temporary causeway had to be constructed to allow two cranes to be installed on two temporary platforms in the middle of the river.
Removing artificial island
One of the scheme’s key components was the removal of a man-made island at Knostrop Cut which separated the river and adjacent canal over a 600m length, removing a hydraulic bottleneck. The 180,000t of excavated material was reused on a local development site, saving the project over £4M in disposal costs.
A section of the well-used Trans Pennine Trail has been diverted from the island with improvements to both access and wildlife habitat enhancing the public realm in this previously run-down area. This valuable blue/green corridor also includes an iconic new bridge over Knostrop weir, creating a gateway into Leeds.
Bam and Mott MacDonald are now working on phase two of the scheme. By continuing to put in a series of flood defence measures further upstream, this will reduce the amount of water allowed to surge through the city centre and relieve strain on the downstream defences.
One in 200 year protection
When completed it will take the Leeds city defences from protecting against a 1 in 100 year storm to a 1 in 200 year one.
The works will start at the Leeds railway station and stretch upstream to Calverley, north west of Leeds.
“Part of that solution is walls again and we’ll be building around 8km of walls,” says Mott MacDonald project director Peter Charlesworth.
“But we’re also building an upstream storage area at Calverley. It’s one of the weirs in reverse. Under normal conditions it’s tucked away in the river bed, but in a flood event, the barrier raises and it holds back water in a natural flood zone which is mainly pasture land which we’ll be flooding to a larger extent.”
An outline business case (OBC) has been submitted for phase 2 and it is now in the hands of the council to approve funding and allow the team to push on.