How the Boston Barrier scheme reduced its embodied and operational carbon

Boston, a market town in Lincolnshire with an important maritime history, also has a long history of tidal surges.

In December 2013, extensive flooding affected 800 properties across 55 streets.

Climate change predictions anticipate rising sea levels and a growth in instances of extreme weather which, without intervention, will only result in more devastation for Boston.

The Boston Barrier tidal flood defence scheme is an Environment Agency (EA)-led project that’s expected to reduce the risk of tidal flooding to more than 14,000 properties, including approximately 800 businesses.

The £100m initiative is deemed to be a “national priority project” by the EA and is fully funded by government flood and coastal erosion risk management (FCERM) grant-in-aid.

It’s part of the Boston Combined Strategy that was approved in 2008. It identified five separate phases of work to reduce the risk of tidal flooding and regenerate the town’s waterways.

The Boston Barrier is part of phase three of the strategy and consists of a tidal barrier across the river Haven, downstream of the town, with associated defence works.

The challenge – a carbon-intensive project

The centrepiece of the scheme is the primary barrier gate.

This is a 362-tonne, 26m-wide and 11m-high rising cill sector gate that is raised into position by two 55-tonne hydraulic rams to defend against tidal surges.

The forces exerted by the gate during these movements required a structure that was strong enough to form a unique design to accommodate the curvature of the barrier gate.

It also needed to meet the 100-year design life of the barrier.

The only viable solution was in-situ and precast reinforced concrete, which would have resulted in a carbon-intensive project.

The solution – choosing the right concrete mix and using 3D modelling

To make carbon savings in the barrier structure, the concrete mix incorporates 70% ground-granulated blast furnace slag (GGBS) as a cement replacement.

For every tonne of cement, about 860kg of carbon is produced, compared with 79kg per tonne of GGBS (which is a by-product of the iron industry).

In addition, GGBS is cheaper than cement and results in a dense concrete with good chemical resistance.

Another carbon-saving design decision was extensive 3D CAD modelling to enable clash detection, to verify the design and ultimately avoid rework required onsite.

The EA’s Ipswich barrier team, whose flood barrier opened in 2019, were instrumental in this decision-making.

By sharing lessons learnt and key knowledge, the Ipswich team, who had cast their gate recess onsite, suggested that prefabrication could simplify the process.

The Boston team’s solution was to generate 3D models for the 24 precast gate recess components, which were subsequently installed in three days.

Scheme contractor BAM Nuttall/Mott MacDonald JV (BMMJV) also ordered prefabricated bespoke rebar cages, which cut the base slab reinforcement installation time from two months to two weeks.

Outcome – responsible construction and production

Time savings

By pre-casting the primary gate’s recess and ordering large, prefabricated reinforcement cages, a total of seven weeks were saved on the programme’s critical path.

This also significantly improved the health, safety and wellbeing of the workforce during installation and minimised working in confined spaces.

By developing the design of the barrier’s control building, critical plant was moved onto the first floor.

This change removed the requirement for waterproofing the entire structure because the first floor is above the flood defence level. Thus, the need for piled foundations to provide resistance against potential uplift was eliminated.

In addition, the site investigation information, backed up by a settlement load test onsite, showed that the ground was suitable for shallow foundations.

This meant that approximately 70 steel tubular piles could be removed from the scope of the project.

This saved 360 tonnes of embodied carbon and £300,000 in project costs, as well as reducing construction time by four weeks and shaving three weeks off the design programme.

Energy savings

The roof of the barrier’s control building also features 32 photovoltaic (PV) panels, which provide a 9.6kW renewable power supply.

It’s anticipated that the PV panels will deliver 16% of the control building’s electricity, with a saving of £550 per year and a carbon offset of 2,000kg CO2e per year.

An air-source heat pump has also been installed, which should reduce the control building’s electricity use by 25% compared with traditional electric heaters.

This equates to annual electricity savings of £2,200 and an annual carbon offset of 4,100kg.

Alignment to UN SDGs

The UN’s Sustainable Development Goals (SDGs) were also retrospectively incorporated into the project to monitor and evaluate its successes.

The SDGs are now a focal point for the scheme, which shares learning through several routes, including the iStorm international storm surge barrier network.

By utilising SDG 12 – responsible consumption and production – various savings have been identified, including potential re-use of dredged material as backfill and as capping at a local landfill site.

This adjustment saved more than 30,000 lorry movements and was key to securing an ‘Excellent’ award under the CEEQUAL Interim Whole Team Award.

The Boston Barrier scheme provides better protection for thousands of homes from tidal flooding and will continue to do so over the next century.

Through the incorporation of pioneering design and implementation, it also provides future civil engineering schemes with a blueprint that could save millions of tonnes of carbon in the years to come.