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Insight | How Bam and Motts will engineer the £75M Boston Barrier

Boston barrier looking downstream

A Bam Nuttall and Mott MacDonald joint venture (JV) has been announced as the preferred bidder for the £75M Boston Barrier flood defence scheme. 

The team will carry out the detailed design and construction of the barrier and the associated works in the surrounding area. The contract award was revealed at New Civil Engineer’s Flood Management Forum 2017 held in London yesterday (November 16).

Talking exclusively to New Civil Engineer, Environment Agency (EA) programme manager Jim Anderson explained the challenges behind the construction of the complex structure.

It has been designed to cut flood risk in and around the Lincolnshire town from a 1 in 50 chance each year, to a 1 in 300 chance each year, for a period of 100 years. The need for a barrier was raised 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,” said Anderson. “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 EA).”

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 when the tidal river rises 5m above sea level to prevent flooding of the surrounding area.

The barrier is a closed half cylinder which, under normal conditions, will lie flush with the river bed, with its underside embedded in a recess below. Under flood conditions, it will be rotated about its axis by hydraulic cylinders to create a wall against the oncoming water. When installed, it will provide a 25m navigable channel. It will be 35m long and in operation will give a 10m high gate. The EA video below from 2014 gives an indication of how the barrier will operate.

It will be operated from a new two-storey control building being built on the nearby Port of Boston (PoB) Estate.

The project will also include the construction of a pair of new vertical sector 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 vertical radial sector gates in the UK and probably in Europe. They are fixed on a bearing and all of the load goes through them,” said Anderson.

Source: EnvironmentAgencyTV

The new gate will be constructed in a cofferdam, with its sheet pile walls installed to the maximum height required to act as a temporary flood barrier.

As the vertical sector gates will not be complete until after the main barrier is finished, the sheet pile walls will be used as a temporary defence allowing an operational barrier to open in December 2019 with full completion of the works in August 2020.

“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,” said 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.”

This channel will also be widened from 15.3m to 18m to allow broader vessels to moor.

A raft of smaller interventions 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 as part of the enabling works with phase three and four undertaken near the end of construction, according to the Transport and Works Act (TWA) report.

The business case for the barrier 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 are expected to be approved at the end of this month with construction planned to start in January 2018.

“The business case is a high quality document which has been engineered to make it as smooth as possible for it to go swiftly through the approvals process,” said Anderson. “We are super confident it will be given the go-ahead.

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