Leeds city centre’s innovative £48M flood defence project could be replicated in other towns and cities across the UK, its designers have told NCE.
The adjustable weir system can offer a cost effective way of managing flood risk on other navigable rivers, said Arup associate director David Wilkes.
A joint venture between contractor Bam Nuttall and consultant Mott MacDonald is due to start the main construction phase of the Leeds project in January, with completion scheduled for spring 2017.
As technical advisor to Leeds City Council for the scheme, Arup developed the concept and specimen designs and helped secure planning permission.
The scheme replaces fixed weirs at Crown Point and Knostrop on the River Aire with moveable gates that allow the water level to be controlled.
This means the river can still be used by leisure craft and retain its aesthetic qualities without overflowing during peak rainfall.
Bath and Bristol enquiries
“We have had a few enquiries about the system, including from Bath and Bristol, and we are in discussions,” said Wilkes. “It is relevant for cities where rivers have a right of navigation. It is in our DNA that we will not give up a right we have to sail a boat somewhere.”
Wilkes hailed the importance of local knowledge and collaborative working as key influences on the development of the system, which is thought to be a first for this country.
“The Environment Agency had previously looked at a bigger scheme that covered 10km with a more conventional design,” said Wilkes.
“This involved big walls 2.5m to 3m high, and cost £190M.
“This idea was rejected by the government and we were asked by the council in 2010 to find a more cost effective way of reducing flood risk in Leeds.”
Arup started by deciding what was the most important area to protect.
“It helped us being based in the city,” said Wilkes. “We understand how the city works and we quickly identified the section of river we needed to tackle.”
The final scheme will extend from the city centre station to Knostrop weir, protecting 3,000 homes, 500 businesses, access to the rail services and, critically, substations and broadband hubs.
“We also knew there was a lot of apprehension about big walls going up,” said Wilkes.
“Not only would this look brutal, but allowing the water level to rise would make it difficult for drains and tributaries to discharge; you could end up chasing the problem.”
Arup’s first idea was to prevent one tributary from entering the Aire, diverting it around the city to remove a fifth of the river’s flow. This was rejected as too expensive, and requiring too much reworking of land.
Instead the consultant came up with the idea of removing the stone weirs placed in the river during the Yorkshire town’s industrial heyday to allow barges to transport goods. But this idea was rejected because it would havemade the river unnavigable.
“We needed mechanical gates that maintained the water level when it was low but, when water is rattling down from the Pennines, could be lowered to prevent floods,” said Wilkes.
“We will use a series of steel gates across the width of the river, hinged against the river bed, with a bladder - basically a Kevlar-reinforced fabric whoopee cushion - under the gate.”
The bladder will be connected to a pump that can be operated remotely to inflate and deflate the bladder, raising and lowering the gate and controlling the water level. Software will be written to allow this to happen automatically when the river level reaches a certain height, while the council will be able to override the computer if necessary.
The adjustable weirs have allowed Arup to remove half of the flood defence walls planned under the previous scheme, and to reduce the height of those remaining. The highest wall will now be 1.5m. Some will be stone, some made of glass. Hard landscaping will also be carried out.
An island at Knostrop Cut which separates the river from the Leeds and Liverpool Canal will also be removed to allow the two to merge, further assisting the flow of water in flood conditions.
A project delivery team has been established at Arup’s offices which are close to the river.
It includes staff from Arup, the council, the Environment Agency, Bam Nuttall, Mott MacDonald and consultants Mouchel.
“We are uniting the team with a common shared mission to deliver a sparkling project protecting people and property from flooding and which brings environmental and public realm enhancements,” said Wilkes.
He added that the team was charged with “delivering those works safely, on time and on budget, and [creating a scheme] considered to be successful by city residents and the wider public”.
On completion, the project will provide the city centre with protection against a one in 75 year flood event.