Work to protect a coastal village from severe flooding is overcoming challenging ground conditions.
Project: Fairbourne Flood Risk Management Project
Client: Natural Resources Wales
Contract value: £4m
Main contractor: Galliford Try
Groundworks subcontractor: D Morgan
Start date: February 2013
Completion date: October 2013
While many sections of the coast around the UK are in real danger of becoming overwhelmed by surging seas and raging rivers, there is a village in north-west Wales that has a dubious claim to fame.
Said to have been originally developed as a seaside resort by flour-maker Arthur McDougall, the coastal village of Fairbourne, on the south side of River Mawddach estuary opposite Barmouth, is under threat of flooding from three different sources: the rivers Mawddach and Henddol and the sea itself.
In an effort to keep the 400 or so properties in Fairbourne safe from the advancing waters, contractor Galliford Try has teamed up with client Natural Resources Wales on a project that will reinforce existing tidal and flood defences in the village itself and further up the valley at Arthog and Fegla Fawr.
Residential and environmental protection
It is a testing project that will see the project team provide updated fluvial defence channels, create new environmental habitats and recreational areas, and develop a new area of salt marsh just to the north of the village.
In fact, one of the first tasks for the project team was to fence off the site in a bid to stop the sheep that graze these salt marshes from wandering all over it.
Since then though the project team has been able to get on with the serious business of carrying out a large muck-shift operation over alluvial deposits in an estuary. Hardly the easiest of combinations.
Galliford Try’s involvement on the scheme began back in 2011 with ECI engagement.
Since then the project has moved through the planning stages, with the contractor finally taking the design-and-build contract in early 2012.
“We thought that would be enough [borrow pit material] to cover the whole job. We didn’t think we would need to import any more material”
Paul Corner, Galliford Try
Early outline design had been carried out by consulting engineer Jacobs and Galliford Try has teamed up with design partner Black & Veatch to deliver the final project.
“Design and build closed in early 2012,” says Galliford Try project manager Ian Gemmell. “Then we had to wait until it got through the planning process before we were able to start on site. We got the all clear in February 2013.”
Some site investigation work had been carried out in spring 2012, which identified an area of firm clay overlying softer material.
It was initially planned that the team would use this area as a borrow pit, excavating as much as 20,000 cu m of material to be used in the reconstruction of the existing flood defence embankments.
“We thought that would be enough to cover the whole job. We didn’t think we would need to import any more material,” says Galliford Try site manager Paul Corner.
Poor ground handicaps job
Unfortunately further investigations following last year’s wet summer revealed that material in this area was nowhere near as good as first thought, meaning the amount of suitable fill was much smaller.
“Because of the wet summer last year we went back and carried out further site investigations,” Mr Corner explains.
“What we thought had been suitable material was in fact marginal at best. We’ve had to reduce the amount of suitable clay from that borrow pit to around 8,000 cu m.”
That level should be just enough to work on the embankment realignment, but only after a quick redesign.
Initially intended to be constructed using just one material, the embankments are now ‘zoned’ with the site-won clay material sitting at the front of the embankment and a type 2C granulated fill to the rear.
“Lime into wet clay can react after placement and cause cracks to develop. In a flood protection embankment, cracks are not what you want at all”
Paul Corner, Galliford Try
The moisture content of the site clay is the main problem for the earthworks contractor, Ellesmere Port-based D Morgan.
In a bid to drop that content, it has been creating drying mounds, helping to air the clay and speed up the draining and drying process before it can be placed in the embankments.
Moisture content tests and readings are taken every day with sand replacement tests to determine the density of the fill carried out once placed.
Lime stabilisation ruled out
With such a marginal material, might it have been considered easier, quicker and cheaper to use lime stabilisation techniques? Not according to Mr Corner.
“We had a look at lime stabilisation but dismissed it pretty quickly. Lime into wet clay can react after placement and cause cracks to develop,” he says.
“In a flood protection embankment, cracks are not what you want at all. We were also worried about applying and mixing lime immediately adjacent to a Site of Special Scientific Interest.”
The main embankment works run in two sections: one of 2 km and another running for 1 km. Both are based on a 20 m-wide footprint at its widest, battered back to a height of around 5.5 m at a 1 in 3 slope.
There is a 2.5 m-wide footpath placed on top of the embankments as part of the betterment works.
“The risk of tidal breach of the defences here in Fairbourne is high enough to push the work to the top of the priority list”
John Davies, NRW
At two sections on the longest piece of embankment, the team is completely reconstructing the structure. Here the material is being reprocessed and stockpiled as back-up fill for the rest of the scheme. In all, around a third of the material is being imported.
“All told there is approximately 35,000 cu m of fill,” Mr Gemmell says. “We will probably be importing around 12,000 cu m of stone, the rest will be site-generated.”
Each layer of fill is compacted in 200 mm-thick layers using a sheep foot roller towed behind a tracked dozer. These are thinner layers than strictly necessary, but the team is determined to make sure the compaction and density in each layer is exactly as it should be.
Fortunately the weather has been on the team’s side so far and the contract completion date of October 2013 looks well in hand.
Property owners in Fairbourne will be able to look forward to a winter season where the threat of floodwater rushing through their streets has been minimised.
Arthog’s stone pitched embankments
Further up the estuary on land around the village of Arthog, the team is working on another 1 km section of embankment-strengthening and flood protection work.
Here, however, the embankments are much lower and water has overtopped them, causing erosion and scour along them.
This has made the work slightly more problematic than back down in Fairbourne itself, with the team needing to install anti-erosion matting across the bank to help prevent any future erosion.
In addition, pitched stone from the local north Wales quarries is being laid into no-fines concrete to help widen and reinforce the crest. In all, some 3,000 sq m of stone will be placed.
“We can widen the crest at the back or front here,” Mr Corner says. “That allows us to work one side only.”
At one that is particularly prone to damage, 120 m of steel sheet piles have been installed by vibrohammer. These 5 m sheets have a 4 m toe-in and will be clad with a stone face.
Two further areas of work include Mawddach sections 1 and 2. Section 2 involves fitting badger and rabbit-proof anti-erosion netting in a SSSI, while at Mawddach section 1 a 45 m reinforced concrete flood wall with natural stone finish is being installed.
With a 2.5 m base width and standing at 1.5 m high, a 200 mm wall thickness and 200 mm pitching detail, the wall helps reduce the amount of work the team needs to carry out in the SSSI.
“Extending an earth bund in the middle of a SSI would have had a major impact and been difficult to carry out,” Mr Corner says. “We also looked at using precast concrete blocks but access is so difficult here that the in-situ design was the best solution.”
Close relationships pay dividends
Funded by the Welsh Government and the European Regional Development fund, the project is something of a triumph for client Natural Resources Wales.
It was formed in April 2013 through a merger of several bodies including the Countryside Council for Wales, Environment Agency Wales and the Forestry Commission Wales and the Fairbourne project is one of the first being carried out under its name.
“The Welsh Government has a shoreline management plan in place,” says NRW project manager John Davies. “The risk of tidal breach of the defences here in Fairbourne is high enough to push the work to the top of the priority list.
“There have been major flooding incidents here in the past, with the most recent in 2000. Since then the village has grown and we were keen to give villagers some peace of mind by reducing the possibility of any breach of the flood defences.”