Innovative inflatable weirs are part of a flood alleviation scheme that will help protect Leeds city centre from the sort of devastation it witnessed when the river Aire burst its banks in December 2015.
Project: Leeds Flood Alleviation Scheme
Client: Leeds City Council
Contract value: £28m
Region: Yorkshire & the Humber
Main contractor: Bam Nuttall
Main contractor: Mott MacDonald
Start date: February 2015
Completion date: March 2017
Like many provincial cities, Leeds was hit hard by the downturn. Speculative office and apartment developments that sprang up in the south-west of the city during the pre-crash boom days of 2006 stood forlorn and virtually empty at its trough.
Thankfully for the people of Leeds and its wider commuting catchment area of West Yorkshire and beyond, the city is now thriving as an economic centre.
One factor that has threatened to dent that confidence is the economic impact of flooding from the river Aire, which runs through the city centre. Over recent years, floods have hit most of the main commuting arteries into Leeds, including the railway, and affected thousands of residents.
In a bid to prevent these damaging flood events, local authority Leeds City Council, working with the Environment Agency and Canal and River Trust, has developed a scheme that will protect residential and commercial property as well as the road and rail network.
Early in the development process, representatives at Leeds City Council recognised they needed a solution that not only tamed the river but also helped deliver amenity value for the city and its residents.
“It would have been easy to hit flood protection requirements by building floodwalls up to 2 m but the council wanted to minimise the schemes negative visual impact”
Andrew Judson, BMM
“It would have been easy to hit flood protection requirements by building floodwalls up to 2 m, but the council wanted to minimise the scheme’s negative visual impact,” explains Andrew Judson, project manager for the BMM joint venture between Bam Nuttall and Mott MacDonald that is delivering the scheme.
“By adopting the solution we proposed, the city will benefit from vastly improved flood protection without damaging the heritage and quality of the waterfront.”
Raft of solutions
This is the first phase of the Leeds Flood Alleviations Scheme and covers a £28m contract with BMM to construct a host of protection measures to the south of Leeds railway station almost 4.5 km along the river to below Knostrop weir, linking up with embankments installed as far downstream as Woodlesford.
There are more than 20 different sections of work along the length of the scheme, ranging from increasing the height of existing walls to 0.2 m to installing new 1.3 m-high embankments.
Bam Nuttall Leeds Flood Alleviation Scheme IMG 8247
All of these sections will play a pivotal role in the success of the overall scheme. However, it is the work going on at two points along the project’s length, at Crown Point and Knostrop, that really focus the mind.
Here the team is breaking out existing weirs across the river and installing two new structures that promise to better regulate flow thanks to their uniquely controllable design.
Bags of innovation
Thanks to the innovative use of air bags, the weirs can be raised and lowered as flood water courses downstream, helping control the depth of water through Leeds city centre and protecting properties.
The bags are manufactured using super-strong neoprene, have passed ballistic tests, and sit beneath a hinged steel weir plate produced from separate ribbed steel plate sections bolted together using 48 mm-diameter steel bolts at 400 mm centres.
Unwanted Christmas presence
With torrential rain hitting Leeds and the surrounding river Aire catchment area in the run-up to Christmas, Mr Judson and the rest of the project team were on high alert to do what they could to protect their scheme and any surrounding properties from flooding.
By the morning of Boxing Day last year however, it was clear that any hope of containing the flood water had gone.
“We had people down here at first light. It quickly became apparent that there had been a massive washout behind Knostrop lock and that the river was emptying directly into the canal system. Within 40 minutes we had people down here trying to do what we could to stabilise the situation,” he says.
Initially anything large and heavy enough that came to hand was being placed in the breach. Lumps of concrete, motorway barriers all found their way into the hole to help stem the flow enough for the team to close it off in a more orderly way before any permanent and lasting damage was done to the Knostrop lock itself.
“We managed to get some rock fill bags behind and then protect those with sheet piles,” Mr Judson says. ”The lock is very heavily monitored. At the time it was a case of doing what we could.”
Under normal flow conditions the bags are inflated, pushing the weir plates up into position and helping to control the amount of water rushing through the river channel.
If necessary, however, these bags can be simply and quickly deflated, enabling the weir plates to drop to the bed of the river and allowing water backed up behind them to flow quickly over the top.
Designed for failure
Unlike the Foss Barrier that failed on the river Ouse in York during floods at Christmas, the system ensures continued water flow despite catastrophic failure.
“It is actually a very simple system and is inherently safe,” says Leeds City Council principle project engineer Richard Dennis. “Even if the bags fail, the weir plates will lower, meaning there will be an increased flow rate anyway.”
At Crown Point weir towards the top of the scheme, the team is installing a new weir that will be skewed across the river on the same footprint as the existing permanent structure. It will consist of two central 25 m bays featuring the moveable weir, with a fish pass section at the western bank and a section of reinstated ‘heritage’ weir at the eastern bank of the new structure.
Currently the team is working behind a sheet pile cofferdam on the installation of the steel tube piles. These will anchor the base slab of the fish pass section of the weir to the weathered sandstone-bearing strata below.
Bam Nuttall Leeds Flood Alleviation Scheme IMG 7848
There are about 20 piles within each section of the weir and each has a cage of T40 reinforcing steel bars tying in to the 700 mm-thick reinforced concrete slab. The cofferdam sheets are driven to 8.5 m below mean water level and is part of a package that will see the sequence of work carefully controlled to maintain the volume of water flowing over the weir before completion.
“We need to maintain a certain ‘crest length’ of flow over the weirs,” Mr Judson says. “Environment Agency guidance means that we can only take out a third of the existing flow at any time. That is difficult when there are only two weir gates.”
To combat this issue the fish pass will be built temporarily deeper than required to maintain flow rates, before work is transferred to the eastern gate.
“We have the same issue on that side, but by that stage I will have control of the flow because the first gate has already been installed,” he says.
Work on this phase of the scheme is due to be completed in 2017 and will hopefully see residents and businesses protected from the sort of flooding that blighted Christmas 2015 for so many.
Big dig for even bigger basin
Less than 2.5 km separates the two key focus points of the Leeds FAS project, Crown Point weir upstream and the weir at Knostrop at the downstream end of the scheme.
At Knostrop Cut, a section of canal runs alongside the river Aire, which enables craft to bypass the existing Knostrop weir.
But under the project design the site team is digging out a huge section of the bank that separates the canal and river channel to form a huge basin just upstream from Knostrop weir and Knostrop lock.
This basin will act as storage for a massive volume of water during flood events and will help the management of water through Leeds city centre. This section will be allowed to fill and store storm water, before being released at manageable flow levels by the lowering of the Knostrop Weir.
To date more than 44,000 cu m of material has been dug and dredged from the Knostrop Cut using long-reach excavators sitting on the dividing bank and loading into barges which transport material to the drying station on the other side of the river.
Here the material is turned, dried and tested before being sent off for use as an inert fill at a large development scheme just outside Leeds itself.
Just as at Crown Point, the new Knostrop weir also features moveable weir plates controlled by inflatable air bags, although the design of the weir itself differs slightly by being constructed in three staggered 15 m sections across the existing weir footprint.
Again, tubular steel piles have been used to anchor the 700 mm-thick reinforced concrete slab and section walls of the weir in position.