Two major flood barriers are rapidly becoming a reality on the UK’s east coast as efforts to provide much-needed resilience to the effects of climate change are stepped up.
The new Ipswich flood defence project reached a major milestone in November last year when the centrepiece of the scheme, a 200t, 22m long, 9m high steel gate was lifted into place across the River Orwell in Suffolk to form part of a new £71M tidal barrier.
The scale of the operation was so huge that it was lifted into place by an 800t crane, which itself was delivered to site in 22 lorries and two minibuses.
The barrier is being installed at the mouth of the New Cut River – the main channel which takes the River Orwell through Ipswich. Its installation is in response to a series of devastating floods that hit the town in recent years. The most serious occurred in December 2013, when a tidal surge wreaked havoc along the Suffolk coast.
It is hoped that when complete, the new barrier will reduce the probability of a combined fluvial and tidal flood event happening in a year to 0.33% and protect around 2,000 properties from flooding. Its installation will also unlock the council’s core development strategy, allowing new land to be released for development and growth.
Financially and geographically the most sensible position for the barrier is at the mouth of the New Cut, adjacent to the entrance to the wet docks where the river narrows significantly.
Ipswich surge 2
But as Environment Agency project manager for the scheme Andrew Usborne explains, it was not the best location from an engineering perspective.
“This is the place to put the barrier, but it’s the one place where there happens to be an Ice Age channel scoured out of the chalk,” he says. “Geotechnically it’s the worst place, but if you’d have put it further down the river then we’d have more expense with the connecting walls.”
The rotating gate will serve a dual purpose, providing velocity control to the flow of the river and stopping sediment loss during low tides, replacing an old gate a short distance away.
The barrier is being built by joint venture VBA – VolkerStevin, Boskalis Westminster and Atkins, and work started on site in November 2015. Building the housing structure which holds it in place was not an easy task.
October 2017 1 crop
To start, a 30m by 30m sheet pile cofferdam was installed in the river, diverting the water through a narrow gap created next to the bank. Then 48, 1m diameter piles were driven 50m to 60m into the ground, toeing into the chalk below. A 450mm thick blinding layer of concrete was then laid ready for a reinforced concrete base slab to be constructed on top.
This was to be the first major challenge. Overnight and into the morning of the 23 February 2017, Storm Doris, a Met Office labelled “weather bomb” storm hit. It was the same morning that the concrete was being poured for the mammoth base slab.
This is the place to put the barrier, but geotechnically it’s the worst place
The storm ripped up trees, cut off power and closed the main bridge into Ipswich. Despite this, the team managed to provide a continuous pour of around 700m3 of concrete from 7am to 3am the following morning.
Then, in November the team tackled the second major challenge on the project. The 200t gate, constructed by specialist company Hollandia, was transported from Holland in a 24 hour operation across the North Sea and lifted into place in a precision operation.
“There was a lot of holding our breath when it was lifted up and slotted in,” says VBA construction project manager Neil Dorling. “It was installed within a few hours, which was quite impressive.
Oct 2017 3
Dorling says that ensuring the hinge pins lined up with their housing was the most critical part of the operation. “The tolerance for the hinge pins, you can’t put your hands down either side, it’s incredibly tight.”
The barrier is a closed circular segment which, under normal conditions, will lie flush with the river bed, with its underside embedded in a recess. Under flood conditions, it will be rotated about its axis by hydraulic rams to create a wall against the oncoming water.
During maintenance periods, the barrier is rotated and further raised clear of the water. Access doors into the confined space allow engineers to inspect its internal structure.
The barrier can be closed within 20 minutes of an alarm being raised and it and other ancillary gates have been designed to be operated by only two workers.
It is operated from a main control room that contains the generators which power the barrier’s hydraulic rams which sit on either side of the gate. In the unlikely event of the generators failing, a number of backup systems have been included in its design to ensure the barrier continues to operate normally. The gate itself has also been designed with additional internal stiffeners to give it torsional rigidity should it need to be opened by only one ram on one side.
Further fail-safes have also been built into the design with two 1,000t crane bases built into the structure should it need removing in an emergency situation.
The gate is not the only part to the project. Earlier works included building flood protection walls on high ground on the eastern and western banks adjacent to the barrier. On the western side, the wall runs behind the ABP port’s west bank Terminal and railway line, both of which will be expected to flood should river levels rise.
The full scheme is estimated to cost £71M although it is hoping the inbuilt contingency fund will not be used and the final figure will be lower when it is completed in August this year.
Work on a near-identical but larger barrier to that being constructed at Ipswich is set to start further up the east coast, after a Bam Nuttall and Mott MacDonald joint venture landed the contract to build the £75M Boston Barrier flood defence scheme.
This has been designed to cut flood risk in any given year from a 2% to a 0.33% chance each year, for a period of 100 years. The need for a barrier was highlighted after a tidal surge flooded 579 homes in the area in the winter of 2013.
“The number of residential properties at risk of tidal flooding in Boston will rise to almost 20,000 homes in 100 years’ time as a result of climate change,” explains Environment Agency programme manager Jim Anderson.
3092164 boston barrier looking downstream gp 3to2
“Because of the high number of properties and people who are at risk of significant flooding, the proposed scheme is a priority project [for the Agency],” he adds.
The main works for the project will focus on the construction of the barrier, which will be placed in the River Witham to the south of central Boston between Black Sluice and grade II listed Maud Foster Sluice.
It will be activated to prevent flooding of the surrounding area when the tidal river level rises 5m above sea level.
As with Ipswich, the barrier is a closed circular segment which, under normal conditions, will lie flush with the river bed, with its underside embedded in a recess in its concrete base slab. Under flood conditions, it will be rotated about its axis by hydraulic rams to create a wall against the oncoming water. Larger than the similar Ipswich barrier, it will be 28m wide and will weigh 315t. As with the Ipswich barrier it will be made in Holland by Hollandia. It will be operated from a new two-storey control building being built on the nearby Port of Boston Estate.
The project will also involve construction of a pair of new vertical gates in the port to provide continuity of the line of defence to the Maud Foster Sluice.
“Of their type they will be the biggest in the UK and probably in Europe,” says Anderson.
The new barrier will be constructed in a cofferdam, with its sheet pile walls to allow it to act as a temporary flood barrier.
“When all of the other permanent works are finished, during those nine months, there will be a sheet pile cofferdam which connects into the two flood walls so we’ll have a functioning flood defence,” says Anderson. “Once the works to the new gate are complete, they’ll open up the front and the back of the cofferdam and in August 2020 we’ll have a finished defence.”
A raft of smaller structures will also be built. These include the installation of a new 1.2m high flood wall extending 430m downstream on the right bank, improved footpaths and a new flood wall and retaining wall on the left bank to improve stability of the quayside.
To construct the barrier, approximately 38,300m3 of material will be excavated in four phases. Phases one and two will be done as part of the enabling works with phases three and four undertaken near the end of construction.
The business case cost for the project is £103.2M, which will include £75M for the barrier and surrounding schemes. Costs include a contract covering technical, project and cost management support provided by a Turner Townsend and CH2M joint venture. Other costs include salaries, compensation to businesses, land costs, leases and a contingency.
The TWA and the full business case was approved in December with construction planned to start in January 2018.