Future of Flooding | Hexham Flood Alleviation Scheme

Raising standards

Low carbon concrete mixes being trialled on a flood alleviation scheme in Northumberland could be a key environmental solution for future projects. Sotiris Kanaris reports.

Heavy rainfall during Storm Desmond in December 2015 made the River Tyne overtop its banks north of Hexham town centre in Northumberland. The Bridge End and Tyne Mills Industrial Estates – which are on opposite sides of the river – were flooded, with 90 properties including businesses, local services and three houses impacted. There were no flood defences to protect them.

It was decided that, as the industrial estates are important contributors to the local economy, it was essential that flood defences be built to protect them in the future. This led to the £6.5M Hexham Flood Alleviation Scheme, which has just been completed.

Reinforced concrete flood walls with heights ranging from 1m to 1.5m have been constructed to protect Hexham’s industrial estates

The legacy could be much broader than just the protection of Hexham, as the Environment Agency worked with contractor Bam Nuttall and designer Arup to trial low carbon concrete mixes for the scheme.

Infrastructure project clients will be looking at this scheme when choosing their concrete mixes for permanent works in the future as the trial involves testing mixes in permanent works for the first time.

According to the project team, these concrete mixes developed by Tarmac contain only a small share of the carbon dioxide equivalent found in CEM 1 Ordinary Portland Cement.

Neither are currently covered by British Standards.

BOOSTING PROTECTION

Modelling conducted by the Environment Agency after the 2015 flood showed that the Tyne Mills Industrial estate on the south bank of the Tyne had a 2% chance of flooding in any given year. The Bridge End Industrial Estate on the north bank had a 1.33% chance of flooding.

The flood alleviation scheme involves the construction of flood defences to reduce that figure to 1% for both industrial estates.

Reinforced concrete flood walls with heights ranging from 1m to 1.5m were identified as the primary solution. The design included a 195m long flood wall to improve flood protection for the Tyne Mills Industrial Estate and a 245m long flood wall for the Bridge End Industrial Estate.

Environment Agency project executive Vanessa Collins says the initial design included an additional 100m of flood wall to protect the Tyne Mills Industrial Estate from a small watercourse called the Skinnersburn. When River Tyne’s water level rises, water backs up in the Skinnersburn, which then floods into the estate.

She explains that just before the full business case was finalised, a decision was taken to realign the flood walls and create a flood control structure, removing the need for this additional stretch of flood wall.

Bam Nuttall design manager Kevin Bulman describes the flood control structure as “essentially a culvert with a headwall on either end” with a tidal flap on the downstream end of the culvert.

KEY FACT

£6.5MCost of Hexham Flood Alleviation Scheme

He adds: “On a normal flow, the water coming down Skinnersburn will pass through the tidal flap. On a flood event when the water comes [from the Tyne] up Skinnersburn it will push the tidal flap closed and the water just cannot go further upstream than this point.”

Collins identifies the removal of this section of flood wall as one of the key contributors to the final 49% reduction in the scheme’s whole life carbon from early design phases to 544kg CO₂e.

The flood protection solution for the Bridge End Industrial Estate involved building a 245m reinforced concrete flood wall and raising a 125m embankment by up to 400mm.

“The best solution was for the Environment Agency to purchase that land and demolish the buildingEarly on in the scheme, it became apparent that the construction team would have faced space limitations at the western end of the southern riverbank where flood walls had to be constructed. The structures had to be installed between a revetment and the “Reiver Sports” building which housed a Northern Powergrid substation.

“During feasibility it became clear that the best solution was for the Environment Agency to purchase that land and demolish the building, which made it safer and easier to build the scheme,” says Collins.

The substation was relocated into a dedicated building approximately 30m from its original position within the same plot of land.

Collins adds that it is unusual for the Environment Agency to buy land, as it is usually maintaining assets on property owned by others.

All main construction works on the flood walls, embankments and flood control structure were completed in spring this year. The main construction contract was completed this summer, so the focus of works on site has now shifted to habitat restoration and landscaping.

TESTING CONCRETE

Flood wall construction may have been completed, but the Environment Agency, Bam Nuttall, Arup and Tarmac are still carrying out tests on a 27m long section used for the low carbon concrete trial.

Funding of £88,000 from the Environment Agency’s net zero carbon innovation pathway pilot fund was granted for this trial and was not part of the funding for the overall project.

Funding for the rest of the project came from a number of other sources: £1.7M from the government as economic recovery funding after the Covid-19 pandemic, £300,000 from the Northumberland regional flood and coastal committee plus £200,000 from Northumberland County Council. The rest was grant in aid from the government.

One of the new Tarmac mixes being used is Portland Limestone Ternary cement. The cementitious elements comprise 35% CEM I – which is 100% Ordinary Portland Cement – 20% limestone filler and 45% ground granulated blast furnace slag (GGBS). This mix’s carbon dioxide equivalent is 119kg/m3 3, 64% lower than standard CEM I.

Two new low carbon concrete mixes are being tested for the first time in permanent works

The other concrete mix being tested uses an alkali-activated cementitious material (AACM) comprising 95% GGBS and 5% CEM I. This mix has 100kg/m³ CO₂e, 70% less than the standard CEM I concrete.

Both mixes have undergone laboratory testing, but it is the first time they have been used for permanent works. “We did a lot of trials and testing in the batching plant before we delivered the product to site,” Bulman adds.

To test the new mixes, three 9m long flood wall sections were constructed – one using AACM; one using Portland Limestone Ternary cement; and the other using the concrete mix used on the rest of the project’s concrete structures which contains 50% CEM I and 50% GGBS. The mixes were used for the base and the wall of these elements.

These sections are all at the site of the former “Reiver Sports” building. Collins says the Environment Agency’s ownership of this plot presented an opportunity to perform the trial with the need to seek permission from another asset owner.

In addition, she says that the flood load – pressure on the structure’s surface caused by the presence of floodwater – is relatively low at the Reiver Sports site, which reduces the risk from using new products.

POSITIVE RESULTS

Since February, a series of short-term and longer-term durability tests has been carried out to assess the performance of the new concrete mixes. Bam Nuttall worked with Arup on the test specifications.

Tests for the properties of the freshly delivered low carbon concrete mixes, such as slump tests and segregation tests, have been carried out. Bulman says that the fresh properties were as requested from the concrete supplier.

The three concrete mixes belong to compressive strength class C32/40. Tests to analyse their compressive strength were carried out on days one, three, seven, 28 and 56 after the concrete was poured.

“All the results are now back and they all exceed what we need to get from both products,” Bulman says.

He adds that the results for the tensile strength of the duo of low carbon concrete mixes also met the requirements.

Tests to analyse the long-term durability of the new materials – including freeze-thaw and shrinkage tests – are still being carried out, with the final results expected in August.

“The initial feedback we’ve had from the specialist testing houses is that they are on track with the targets,” Bulman says.

“Once we have got the results, we are going to produce a technical note that’s going to incorporate all the findings from the trial,” he adds.

SHARING KNOWLEDGE

Environment Agency innovation manager Andy Powell emphasises the importance of sharing the results with the industry. He says some of the government body’s projects have benefited from the results of trials carried out by other infrastructure clients such as High Speed 2 and National Highways.

He points to the information exchange between the Hexham project team and National Highways for one of the low concrete mixes being tested, as the road operator has trialled it on a temporary haul road.

“We did a lot of trials and testing in the batching plant before we delivered the product to siteBam Nuttall Environment Agency framework manager Ruth Young adds that sharing information on successful trials can give clients and suppliers confidence when it comes to deciding whether to use low carbon materials.

If the trial of these low carbon concrete mixes proves successful, Powell says that the Environment Agency will consider them for permanent works on other projects.

The Environment Agency is also involved in the efforts to bring low carbon concrete mixes into the British standards. “We [the Environment Agency] are working up a three-year programme where we are focusing on a range of lower carbon materials. Concrete is one of those. We are looking to see what we need to do to make those ‘business as usual’ on projects or at least part of the consideration where they are suitable and are meeting the performance requirements,” says Powell.